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Antiresonance and bound states in the continuum in electron transport through parallel-coupled quantum-dot structures
Abstract
In this paper we make a theoretical study of electron transport through a multi-quantum-dot system, in which the peripheral quantum dots of a one-dimensional chain are embodied in the two arms of an Aharonov-Bohm interferometer. It is found that, in the absence of magnetic flux, all the even molecule states of odd-numbered quantum-dot structures decouple from the leads and in even-numbered quantum-dot systems all the odd molecule states decouple from the leads, which indicates the formation of remarkable bound states in the continuum. Meanwhile, what is interesting is that apparent antiresonance occurs in electron transport through this structure, the positions of which are accordant with all even (odd) eigenenergies of the sub-molecule of the even (odd)-numbered quantum dots without the peripheral dots. All these results are efficiently modified by the presence of magnetic flux through this system.
... If the Hubbard interaction is not very strong, we can truncate the equations of motion of the Green functions to the second order. 43 By a straightforward derivation, we find that in such an approximation, only the relevant QD and impurity levels are renormalized, i.e., ...
... By using equation-of-motion method and the theory in Ref. 43 we can work out the Green functions involved so as to the conductance. ...
... In order to analyze the quantum interference in such a situation, one has to deal with this model in the molecular-orbital representation, because the coupled structure formed by the impurity and QDs can be viewed as a new QD molecule, and just the quantum interference among its eigenstates causes the present conductance results. The detailed demonstration in Ref. 43 can help us clarify the picture of quantum interference here. We next show the conductance spectra in the case of the magnetic flux phase factor = . ...
The impurity-modulated electron transport properties in a double quantum dot (QD) Aharonov–Bohm ring are theoretically studied, by considering impurities locally and nonlocally coupled to the QDs in the ring arms, respectively. It is found that the impurities influence the electron transport in a nontrivial way: in the case of zero magnetic flux, a single-level impurity leads to the appearance of Fano line shapes in the conductance spectra, and the positions of Fano antiresonances are determined by both the impurity-QD couplings and the QD levels separated from the Fermi level; whereas when a magnetic flux is introduced with the phase factor ϕ = π the Breit–Wigner line shapes appear in the conductance curves. Compared with the local-impurity case, nonlocal impurities alter the conductance period versus the magnetic flux. In addition, when many-body effect is considered within the second-order approximation, we find the important role of the Coulomb interaction in modifying the electron transport.
... For simplicity, by means of the theory in Ref. 44 we can work out the Green functions involved so as to the conductance. Then, in Fig.9 the situation of a single-level impurity is first discussed, and by fixing t 2 at Γ we investigate the influence of the presence of t 1 on the conductance properties. ...
... In order to analyze the quantum interference in such a situation, one has to deal with this model in the molecular-orbital representation, because the coupled structure formed by the impurity and QDs can be viewed as a new QD molecule, and just the quantum interference among its eigenstates causes the present conductance results. The detailed demonstration in Ref. 44 can help us clarify the picture of quantum interference here. ...
... It can be anticipated that under the condition of the uniform couplings of multi-level nonlocal impurity to both QDs, when φ = 0 the conductance zeros are associated with the odd-numbered (evennumbered) eigenlevels of the odd-level (even-level) impurity but when φ = 2π the opposite phenomenon will come into being. 44 With regard to the many-body effect in such a case of nonlocal impurity, we take the example of single-level impurity to show the change of conductance spectra. From Fig.13 with the identical QDimpurity couplings, we can find that when Coulomb repulsions are sufficiently strong, analogous to the results of local impurity, the spectra have the opportunity to be divided into two groups. ...
The impurity-related electron transport through a double quantum dot (QD) Aharonov-Bohm (AB) interferometer is theoretically studied, by considering impurities coupled to the QDs in the interferometer arms. When investigating the linear conductance spectra \emph{vs} the impurity levels, we show that the impurities influence the electron transport in a nontrivial way, since their suppressing or enhancing the electron tunneling. A presented single-level impurity leads to the appearance of Fano lineshapes in the conductance spectra in the absence of magnetic flux, with the positions of Fano antiresonances determined by both the impurity-QD couplings and the QD levels separated from the Fermi level, whereas when a magnetic flux is introduced with the the phase factor $\phi=\pi$ the impurity-driven Breit-Wigner lineshapes appear in the conductance curves. Besides, the nonlocal impurities alter the period of conductance change \emph{vs} the magnetic flux. The multi-level impurities indeed complicate the electron transport, but for the cases of two identical local impurities coupled to the respective QDs with uniform couplings or a nonlocal impurity coupled to both QDs uniformly, the antiresonances are only relevant to the impurity levels. When many-body effect is managed within the second-order approximation, we also find the important role of the Coulomb interaction in modifying the electron transport. Comment: 17 pages, 13 figures
... It can be found that at the zero-bias limit, the interchain transmission and the transmission in Chain-U are always equal to zero. This should be attributed to the destructive quantum interference among transmission paths [31]. As for the zero-bias transmission in Chain-D, we can see that it is resonant, independent of the change of ε 0 . ...
... As a typical case, we take f a ¼ f to expand our discussion. It is obvious that in such a case, case A and case B become the same, in which the bonding molecular state (antibonding state) decouples from the leads in the case of f ¼ 0 (f ¼ p) [31]. InFig. ...
We present a comprehensive analysis about the transport properties of a quantum dot (QD) system with a side-coupled Majorana zero mode. Our calculation result shows that when the coupling manners between the two leads and QDs are identical, the local Andreev reflection and the interlead normal tunneling have the same magnitude at the zero-bias limit. Accordingly, the zero-bias conductance value is always equal to e2/2h, which is exactly one half of the resonant-tunneling conductance. This result is independent of the level number and the level distribution in the single-QD case, and in the coupled-QD case it is irrelevant to the geometry of the QD molecule. The universal transport property is a powerful evidence for the feasibility to detect the MBSs based on a QD circuit. This result also means that the QD condition is not a key factor to achieve the detection. On the other hand, if the decoupling phenomenon appears, the Majorana zero mode may play a trivial role in contributing to the conductance property.
... Refs. [1][2][3][4][5][6][7][8][9][10][11][12]. On the other hand, the time-dependent electron transport through the QD has been intensively studied due to the new effects experimentally observed in this system, e.g. ...
... which emerge in equations for functions (10), cannot be derived without decoupling procedures. Derivation of the closed set of equations for functions (10) is rather a formidable task, and in order to simplify the calculations we assume that the intra-dot Coulomb interaction is equal to the inter-dot one, U i ¼U jk (i,j,k¼1,2,3), and U ij , U i does not depend on the QDs indices. ...
We study the electron transport through the three quantum dots in a triangular configuration coupled with three electron reservoirs out of the Kondo regime. Using the equation of motion method for appropriate correlation functions the quantum dots occupancies, the nonstationary (transient) and steady-state currents are investigated for different values of the Coulomb interaction, bias voltages and inter-dot tunneling coupling. The appearance of the dark state blocking the current flow is discussed. Studying the transient properties of the system we find that the delay in the appearance of a dark state is much longer in comparison with the time needed to destruct this state.
... Another interesting phenomenon concerns the existence of localized states in the continuous bulk spectrum of a system, which are known as bound states in the continuum (BICs). BICs have been widely investigated in a wide range of physical systems, such as optical systems [69][70][71][72], plasmonicphotonic systems [73][74][75][76][77][78], acoustics [79][80][81][82][83][84], quantum dots [85][86][87][88][89], and water waves [90][91][92][93][94][95]. Recently, it has been shown that BICs can exist in topological systems [96][97][98][99]. ...
Higher-order topological phases of matter have been extensively studied in various areas of physics. While the Aubry-André-Harper model provides a paradigmatic example to study topological phases, it has not been explored whether a generalized Aubry-André-Harper model can exhibit a higher-order topological phenomenon. Here, we construct a two-dimensional higher-order topological insulator with chiral symmetry based on the Aubry-André-Harper model. We find the coexistence of zero-energy and nonzero-energy corner-localized modes. The former is protected by the quantized quadrupole moment, while the latter by the first Chern number of the Wannier band. The nonzero-energy mode can also be viewed as the consequence of a Chern insulator localized on a surface. More interestingly, the nonzero-energy corner mode can lie in the continuum of extended bulk states and form a bound state in the continuum of higher-order topological systems. We finally propose an experimental scheme to realize our model in electric circuits. Our study opens a door to further study higher-order topological phases based on the Aubry-André-Harper model.
... The radiation loss of these confined states can be eliminated by destructive inference with the continuous modes. These phenomena have been observed in many physical systems 2 , including acoustics 3-7 , electronics [8][9][10][11][12][13] , and photonics [14][15][16][17][18][19][20][21][22][23][24][25][26] . Recently, the advancement of nanofabrication technologies has triggered the rapid development of BICs in photonics, enabling real applications in the areas of sensors 27,28 , lasers 15 , and filters 29 . ...
Tuning trapped light for integrated photonics Light waves trapped within an integrated circuit could pave the way for microwave photonics and quantum information processing applications. Zejie Yu and Xiankai Sun of The Chinese University of Hong Kong demonstrated controlled interactions between sound waves and locally trapped light waves that could be used in integrated photonic circuits to reduce the amount of light loss during signal transmission. The researchers fabricated a device made of a low-refractive index, polymer waveguide placed on top of a lithium niobate film. Microwaves excited surface sound waves that changed the film’s refractive index and modulated the properties of locally trapped light moving inside the waveguide. Tuning the laser that introduces light into the device ultimately causes the lithium niobate film to switch between transparency and light absorbency.
... In contrast to this conventional wisdom, bound states in the continuum (BICs), which were first proposed by von Neumann and Wigner in 1929 [3], refer to a type of eigenstates whose wave functions are square integrable with the corresponding eigenenergy above the potential well. Such counterintuitive phenomena are of fundamental importance in quantum mechanics [4] and have been discovered in acoustics [5][6][7][8][9], electronics [10][11][12][13][14][15], and photonics [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. The advancement of nanofabrication technology has facilitated development of photonic nanostructures and devices for exploring BICs. ...
Waves that are perfectly confined in the continuous spectrum of radiating waves without interaction with them are known as bound states in the continuum (BICs). Despite recent discoveries of BICs in nanophotonics, full routing and control of BICs have not yet been explored. Here, we experimentally demonstrate BICs in a fundamentally new photonic architecture by patterning a low-refractive-index material on a high-refractive-index substrate, where dissipation to the substrate continuum is eliminated by engineering the geometric parameters. Pivotal BIC-based photonic components are demonstrated, including waveguides, microcavities, directional couplers, and modulators. Therefore, this work presents the critical step of photonic integrated circuits with BICs, and enables the exploration of new single-crystal materials on an integrated photonic platform without the fabrication challenges of patterning the single-crystal materials. The demonstrated lithium niobate platform will facilitate development of functional photonic integrated circuits for optical communications, nonlinear optics at the single photon level, as well as scalable photonic quantum information processors.
... Such counterintuitive phenomena are of fundamental importance in quantum mechanics 4 and have been discovered in acoustics [5][6][7][8][9] , electronics [10][11][12][13][14][15] , and photonics [16][17][18][19][20][21][22][23][24][25][26][27][28][29] . Although the BIC mechanism promises the confining and routing of photons without using a high-refractive-index material surrounded by low-refractive-index environment, the existing photonic BICs are all realized in the conventional architectures. ...
Waves that are perfectly confined in the continuous spectrum of radiating waves without interaction with them are known as bound states in the continuum (BICs). Despite recent discoveries of BICs in nanophotonics, full routing and control of BICs are yet to be explored. Here, we experimentally demonstrate BICs in a fundamentally new photonic architecture by patterning a low-refractive-index material on a high-refractive-index substrate, where dissipation to the substrate continuum is eliminated by engineering the geometric parameters. Pivotal BIC-based photonic components are demonstrated, including waveguides, microcavities, directional couplers, and modulators. Therefore, this work presents the critical step of photonic integrated circuits in the continuum, and enables the exploration of new single-crystal materials on an integrated photonic platform without the fabrication challenges of patterning the single-crystal materials. The demonstrated lithium niobate platform will facilitate development of functional photonic integrated circuits for optical communications, nonlinear optics at the single photon level as well as scalable photonic quantum information processors.
... One can ascertain that these states are completely localized and are decoupled from the main transmission channel. This is exactly called the BIC phenomenon [44]. ...
... In comparison with the single-QD systems, multiple QDs present more intricate quantum transport behaviors, because of the tunable structure parameters and abundant quantum interference mechanisms. 29,30 In this work, we would like to investigate the transport properties of a QD chain in the presence of sidecoupled MBSs. Our calculation shows that the zero-bias conductance is tightly dependent on the parity of QD number. ...
We investigate the transport properties of a quantum dot (QD) chain side-coupled to a pair of Majorana bound states (MBSs). It is found that the zero-bias conductance is tightly dependent on the parity of QD number. First, if a Majorana zero mode is introduced to couple to one QD of the odd-numbered QD structure, the zero-bias conductance is equal to e22h, but the zero-bias conductance will experience a valley-to-peak transition if the Majorana zero mode couples to the different QDs of the even-numbered QD structure. On the other hand, when the inter-MBS coupling is nonzero, the zero-bias conductance spectrum shows a peak in the odd-numbered QD structure, and in the even-numbered QD structure one conductance valley appears at the zero-bias limit. These results show the feasibility to manipulate the current in a multi-QD structure based on the QD-MBS coupling. Also, such a system can be a candidate for detecting the MBSs.
... At last, we would like to state that the theory in this work can be generalized to discuss the thermoelectric effect of the multi-QD structures. From the previous literature, [56][57][58] we know that multi-QD systems possess abundant quantum interference mechanism. And in these systems, the couplings between some molecular states and leads act as nonresonant channels while the states provide resonant channels for electron transmission, so the Fano effect comes into being. ...
We discuss the thermoelectric properties assisted by the Fano effect of
a parallel double quantum dot (QD) structure. By adjusting the couplings
between the QDs and leads, we facilitate the nonresonant and resonant
channels for the Fano interference. It is found that at low temperature,
Fano lineshapes appear in the electronic and thermal conductance
spectra, which can also be reversed by an applied local magnetic flux
with its phase factor φ=π. And, the Fano effect contributes
significantly to the enhancement of thermoelectric efficiency. However,
at the same temperature, the thermoelectric effect in the case of
φ=π is much more apparent, compared with the case of zero
magnetic flux. And, with the temperature increase, the thermoelectric
effect in the case of φ=π seems to be more robust. Using the
concept of Feynman path, we analyze the difference between the quantum
interferences in the cases of φ=0 and π. It is seen that in the
absence of magnetic flux the Fano interference originates from the
quantum interference among infinite-order Feynman paths, but it occurs
only between two lowest-order Feynman paths when φ=π. The
increase of temperature inevitably destroys the electron coherent
transmission in each paths. So, in the case of zero magnetic field, the
thermoelectric effect assisted by the Fano interference is easy to
weaken by a little increase of temperature.
... In addition, the conductance dip at the point of ε F =0.08t 0 almost disappears. So, the weakening of defect-AGNR coupling causes the localized state to decouple from the main transport channel of the AGNR, leading to the occurrence of bound state in continuum (BIC) phenomenon [49]. When t T further decreases, the conductance magnitude is further suppressed in the whole region, and more Fano peaks arise in the the valence-band region of the first conductance plateau. ...
Electron transport through a metallic armchair graphene nanoribbon is theoretically investigated by considering the presence of line defect. The line defect is formed by the staggered stacking of the pentagons and heptagons. Our calculation results show that the line defect mainly destroys the electron transport in the conduction-band region by inducing the abundant Fano effects in the electron transport process. Moreover, the properties of the Fano effects are tightly dependent on the width M of the nanoribbon, and the results of are completely different from those of M > 17. The spectra of the density of electron states illustrate that the line defect induces some localized quantum states, and that the different localizations of these states lead to the distinct transport results. By analyzing the influence of the structure parameters, the Fano effects are described in detail. All the results demonstrate that such a structure can be a promising candidate for electron manipulation in graphene nanoribbon.
... This can be called the phenomenon of bound state in continuum. 38 In Figs. 4(b) and 4(d), we can see that on the two sides of the Dirac point, the DOS spectra show different properties. ...
Electron transport in an armchair graphene nanoribbon is theoretically investigated by considering the presence of one line defect. It is found that different-property Fano effects occur in electron transport through such a structure, which are determined by the nanoribbon width and the coupling manner between the line defect and the nanoribbon. The spectra of the density of electron states show that the line defect induces some localized quantum states around the Dirac point, and that the different localizations of these states lead to the abundant transport results. By analyzing the influence of the structure parameters, the Fano effects are described in detail. With the obtained results, we consider such a structure to be a promising candidate for nanoswitch.
... One can ascertain that these states are completely localized and are decoupled from the main transmission channel. This is exactly called the BIC phenomenon [44]. ...
Electron transport properties in an armchair graphene nanoribbon are theoretically investigated by considering the presence of line defect. It is found that the line defect causes the abundant Fano effects and bound state in continuum (BIC) in the electron transport process, which are tightly dependent on the width of the nanoribbon. By plotting the spectra of the density of electron states of the line defect, we see that the line defect induces some localized quantum states around the Dirac point and that the different localizations of these states lead to these two kinds of transport results. Next, the Fano effect and BIC phenomenon are detailedly described via the analysis about the influence of the structure parameters. According to the numerical results, we propose such a structure to be a promising candidate for graphene nanoswitch.
PACS
81.05.Uw, 71.55.-i, 73.23.-b, 73.25.+i
Using the Anderson model Hamiltonian and the non-equilibrium Green's function method, the decoupled states and antiresonance presenting in the electronic transport through N-quantum-dot ring embodied in A-B interferometer are studied theoretically. We find that the symmetry of the coupled-dot system and the magnetic flux through the Aharonov-Bohm (A-B) interferometer are two physical mechanisms responsible for the decoupled states. Even-odd parity oscillations occur in linear conductance spectra of such a highly symmetric quantum dot ring, due to even or odd molecular state decoupling from the leads by tuning the structure parameters, i.e., the magnetic flux. The results provide a new model for the designing of the nano-device.
Optical bound states in the continuum (BICs) have recently been studied in a wide range of material systems, where light is perfectly confined in the continuous spectrum of radiating modes. In this paper, we reported periodic nonlinear metasurfaces on the etchless LiNbO3 platform, realizing the enhancement of second-order generation (SHG). All-dielectric heterogeneous metasurfaces are constructed by patterning a low-refractive-index polymer on a high refractive-index LiNbO3 film without etching. Due to BICs, light is localized in the LiNbO3 film where the excellent optical second-order nonlinearity is exploited. We demonstrated that with normal incident waves, symmetry-protected BICs are formed at near-infrared wavelength showing a vanishing linewidth in the transmission spectrum. In addition, to manifest the invisible BICs to detectable supercavity resonances at normal incidence, asymmetry is introduced into the system, degrading the symmetry-protected BICs to sharp resonances with a high Q factor. Furthermore, the second harmonic generation (SHG) of the etchless lithium niobate metasurface is studied, predicting that the SHG efficiency can exceed 10-3 with 30 MW/cm2 of the pump intensity. The proposed strategy without facing the fabrication challenge of etching single-crystal LiNbO3 film opens a new avenue for the utilization of BIC in nonlinear optics of all-dielectric heterogeneous metasurfaces.
The interaction of a discrete state coupled to a continuum is a longstanding problem of major interest in different areas of quantum and classical physics. In Hermitian models, several dynamical decoupling schemes have been suggested, in which the discrete-continuum interaction can be substantially reduced and even suppressed. In this work, we consider a discrete state interacting with a continuum via a time-dependent non-Hermitian coupling with finite (albeit arbitrarily long) duration, and show rather generally that for a wide class of coupling temporal shapes, in which the real and imaginary parts of the coupling are related each other by a Hilbert transform, the discrete state returns to its initial condition after the interaction with the continuum, while the continuum keeps trace of the interaction. Such a behavior, which does not have any counterpart in Hermitian dynamics, can be referred to as non-Hermitian pseudo decoupling. Non-Hermitian pseudo decoupling is illustrated by considering a non-Hermitian extension of the Fano–Anderson model in a one-dimensional tight-binding lattice. Such a non-Hermitian model can describe, for example, photonic hopping dynamics in a tight-binding chain of optical microrings or resonators, in which non-Hermitian coupling can be realized by fast modulation of the real and imaginary (gain/loss) parts of the refractive index of the edge microring.
Antiresonance and its application in triple-quantum-dot systems are studied theoretically using nonequilibrium Green’s function method. The presence or absence of antiresonance point can be used to determine whether quantum dot impurity exists in the system. For a tri-quantum-dot ring, the conversion between 0 and 1 for the conductance can be realized by adjusting the energy levels of quantum dots, which makes the system applicable as a quantum switch. The coupling strength can be measured according to the energy level of the antiresonance point. Once the intradot Coulomb interactions are taken into account, three new Fano antiresonances emerge in the conductance spectrum. Under the action of the magnetic field, two antiresonance points disappear simultaneously. Our work sheds lights onto the design and implementation of new quantum functional devices for electronic measurements.
We study transport properties of an arbitrary two terminal Hermitian system within a tight-binding approximation and derive the expression for the transparency in the form, which enables one to determine exact energies of perfect (unity) transmittance, zero transmittance (Fano resonance) and bound state in the continuum (BIC). These energies correspond to the real roots of two energy-dependent functions that are obtained from two non-Hermitian Hamiltonians: the Feshbach's effective Hamiltonian and the auxiliary Hamiltonian, which can be easily deduced from the effective one. BICs and scattering states are deeply connected to each other. We show that transformation of a scattering state into a BIC can be formally described as a "phase transition" with divergent generalized response function. Design rules for quantum conductors and waveguides are presented, which determine structures exhibiting coalescence of both resonances and antiresonances resulting in the formation of almost rectangular transparency and reflection windows. The results can find applications in construction of molecular conductors, broad band filters, quantum heat engines and waveguides with controllable BIC formation.
We investigate the Andreev reflection in a parallel mesoscopic circuit with Majorana bound states (MBSs). It is found that in such a structure, the Andreev current can be manipulated in a highly-efficient way, by the adjustment of bias voltage, dot levels, inter-MBS coupling, and the applied magnetic flux. Besides, the dot-MBS coupling manner is an important factor to modulate the Andreev current, because it influences the period of the conductance oscillation. By discussing the underlying quantum interference mechanism, the Andreev-reflection property is explained in detail. We believe that all the results can assist to understand the nontrivial role of the MBSs in driving the Andreev reflection.
Bound states in the continuum (BICs) are waves that remain localized even though they coexist with a continuous spectrum of radiating waves that can carry energy away. Their very existence defies conventional wisdom. Although BICs were first proposed in quantum mechanics, they are a general wave phenomenon and have since been identified in electromagnetic waves, acoustic waves in air, water waves and elastic waves in solids. These states have been studied in a wide range of material systems, such as piezoelectric materials, dielectric photonic crystals, optical waveguides and fibres, quantum dots, graphene and topological insulators. In this Review, we describe recent developments in this field with an emphasis on the physical mechanisms that lead to BICs across seemingly very different materials and types of waves. We also discuss experimental realizations, existing applications and directions for future work.
Using the Anderson model Hamiltonian and the non-equilibrium Green's function method, the decoupled states and antiresonance presenting in the electronic transport through N-quantum-dot ring embodied in A-B interferometer are studied theoretically. We find that the symmetry of the coupled-dot system and the magnetic flux through the Aharonov-Bohm (A-B) interferometer are two physical mechanisms responsible for the decoupled states. Even-odd parity oscillations occur in linear conductance spectra of such a highly symmetric quantum dot ring, due to even or odd molecular state decoupling from the leads by tuning the structure parameters, i. e., the magnetic flux. The results provide a new model for the designing of the nano-device.
Electron transport at zero temperature through T-shaped double quantum dot attached to the non-interacting leads is studied using Keldysh non-equilibrium Green's function technique. Linear conductance profile and dot occupancies are calculated for various parameters corresponding
to non-interacting as well as interacting electrons on the dots. In case of non-interacting electrons, we observe Fano-antiresonance wherein the linear conductance vanishes (despite occupancies on the dots being finite) whenever the energy level of the quantum dot not directly attached to
the leads, aligns with the Fermi energy of the electrons in the leads at zero-bias. This is understood in terms of destructive interference between several possible Feynman paths between the source and the drain. Electron–electron correlation on the dots incorporated via intradot and
interdot interaction is investigated in Hartree-Fock as well as beyond Hartree-Fock approximation. Results obtained using present decoupling scheme for Green's functions shows that the intradot interaction on the quantum dot not directly connected to the leads removes the anti-resonance point
and leads to splitting into two dips in the linear conductance profile. Results are compared with the one obtained using Hartee-Fock approximation.
Iron (Fe) nanocrystals (NCs) were epitaxially grown on silicon (Si) substrates, where interfacial alloying of Fe and Si (silicidation) was prevented using an ultrathin SiO2 film. Nanowindows (NWs) composed of Si and germanium (Ge) were introduced into this SiO2 layer. The crystallographic arrangement of the Si substrates was conveyed though the NWs, while Fe and Si atoms were not intermixed. Reactions between the epitaxial Fe NCs and Si substrate in the presence of oxygen gas were also investigated. Oxygen atoms facilitated the diffusion of Fe from NCs to Si substrates mainly through Si NWs. As a result, increase of oxygen concentration led to Si oxidation near the interface. This means Fe NCs played a role like a catalysis for Si oxidation. The interfacial reaction was changed drastically by control of nanometer-sized interfaces using Ge NWs in the ultrathin SiO2 films.
The magnetic anisotropy of Fe thin films grown on Si(111) was controlled by either oblique deposition of Fe or using an intermediate layer between the Fe film and Si substrate. By 45 degrees and 65 degrees off-normal deposition of Fe, a clear uniaxial magnetic anisotropy was observed in Pd capped Fe/Si(111) films. This uniaxial magnetic anisotropy dominated the magnetic switching behavior, as well as the magnetic hysteresis loops. In contrast to Fe/Si(111) and Fe/Pd-Si/Si(111), a 600 degrees C-annealed Fe-Si intermediate layer enhanced the coercivity (H-c) of the Fe film from 4 x 10(3) to 10 x 10(3) A/m. The simple methods of oblique deposition and suitable intermediate layer modulated the magnetic behavior of magnetic Fe thin films on Si(111) and are valuable for future applications.
The Fano effect in electron transport through a parallel-coupled multi-quantum-dot system is theoretically studied by adjusting the asymmetries of dot-lead couplings. As a result, we find that three kinds of Fano lineshapes emerge in the linear conductance spectra. Namely, when the dot-lead couplings are up-down asymmetric, two kinds of Fano effects occur with the tuning of local magnetic fluxes. The other Fano effect is observed in the case of left-right asymmetry of the dot-lead couplings, which is tightly dependent on the dot number. We then transform the Hamiltonian into molecular orbital representation and discuss the three kinds of Fano interference mechanisms in detail. It is observed that the coupling manners between the leads and molecular states are the key factors to induce the Fano antiresonance. Since the abundant Fano effects, we consider such a structure to be a candidate of a thermoelectric device. We believe that the numerical results help to understand the Fano effect of the parallel-coupled multi-quantum-dot structure.
We study the electronic transport through a four-quantum-dot (FQD) structure with a diamond-like shape through nonequilibrium Green's function theory. It is observed that the bound state in the continuum (BIC) appears in this multiple QDs system, and the position of the BIC in the total density of states (TDOS) spectrum is tightly determined by the strength of the electronic hopping between the upper QD and the lower one. As the symmetry in the energy levels in these two QDs is broken, the BIC is suppressed to a general conductance peak with a finite width, and meanwhile a Fano-type antiresonance with a zero point appears in the conductance spectrum. These results will develop our understanding of the BICs and their spintronic device applications of spin filter and quantum computing.
Tetracycline and related compounds are used extensively as broad spectrum antibiotics in the treatment of bacterial infections in ruminants. Tetracycline may cause acute pancreatitis which may result in increased serum amylase activity. However, it has been shown that administration of oxytetracycline in human results in decrease serum amylase activity. In this study changes in serum amylase activity were measured in 20 clinically healthy calves following intravenous injection of oxytetracycline hydrochloride at 10 mg/kg of body weight. Blood samples were collected at 30, 60, and 120 minutes after oxytetracycline injection. Serum amylase activity was measured using the amyloclastic assay. The activity of serum amylase was increased significantly (P < 0.05) at 30 (40.5%), 60 (35.1%), and 120 (39.3%) minutes after oxytetracycline hydrochloride administration. To the authors' knowledge this is the first study on the acute effect of tetracycline administration on serum amylase activity in calves.
Low temperature (LT: 100 K) deposition of Fe on Si(111)7×7 surface effectively reduces Fe-silicide formation at the Fe/Si interface, as compared with conventional room temperature (RT) growth. The interface condition of 5-15 monolayers (ML) LT-Fe/Si(111) remains stable at least up to 350 K. Si segregation was observed after annealing at 400 K. LT-grown Fe films also reveal a relatively flat surface morphology with a roughness of 0.4-0.6 nm. Thus, LT-Fe films were suggested as an intermediate layer for the subsequent RT-growth of Fe. We use a single domain model of magnetic anisotropy to fit the magnetic coercivity evolution of n ML RT-Fe on 5 ML LT-Fe/Si(111). Accordingly, we deduce the surface and volume-contributed magnetic anisotropy for discussion.
An exact analytical solution for the time evolution of cutoff Gaussian wave packets scattered by a double-quantum-dot Aharonov-Bohm interferometer is derived to analyze the trapping effects of the molecular states of the system. Our analysis reveals that the formation and decay of a quasistationary state at the Fano resonance produces a monochromatic emission embedded in the transmitted packet, characterized by a dominant frequency Ωav=ɛn/ℏ with a finite time duration, where ɛn is the Fano resonance energy. We demonstrate that the duration of this coherent emission can be extended by narrowing the Fano resonance with appropriate variations of the Aharonov-Bohm phase. This emission is switched off in the limit of zero width, where the localization of the associated molecular state occurs.
The quantum decay of noninteracting many particles to a common continuum is theoretically investigated in the framework of a multilevel Friedrichs-Lee model, and the role of the particle statistics on the fractional decay and Fano resonances is highlighted. In particular it is shown that, according to the Pauli exclusion principle, fractional decay and trapping found for a single particle due to destructive quantum interference among different decay paths can be inhibited for fermions, but not for bosons. Examples are provided for the decay dynamics of electrons in quantum dots side-coupled to a quantum wire.
We study theoretically the transient electron transport in the multiple quantum dots (QDs) systems forced by the suddenly applied bias voltage. Depending on the quantum dots configuration and parameters describing the considered system different beat patterns of the resulting transient current are observed. We find out that the careful inspection of the transient current beat patterns can provide the information about the inter-dot hopping amplitudes, quantum dots energy levels and their occupancies before the abrupt change of the bias voltage is made.
Spectral properties of periodic one-dimensional array of nanorings in a
magnetic field are investigated. Two types of the superlattice are considered.
In the first one, rings are connected by short one-dimensional wires while in
the second one rings have immediate contacts between each other. The dependence
of the electron energy on the quasimomentum is obtained from the Schrodinger
equation for the Bloch wave function. We have found an interesting feature of
the system, namely, presence of discrete energy levels in the spectrum. The
levels can be located in the gaps or in the bands depending on parameters of
the system. The levels correspond to bound states and electrons occupying these
levels are located on individual rings or couples of neighbouring rings and
does not contribute to the charge transport. The wave function for the bound
states corresponding to the discrete levels is obtained. Modification of
electron energy spectrum with variation of system parameters is discussed.
We discuss the thermoelectric properties assisted by the Fano effect of a
parallel double quantum dot (QD) structure. By adjusting the couplings between
the QDs and leads, we facilitate the nonresonant and resonant channels for the
Fano interference. It is found that at low temperature, Fano lineshapes appear
in the electronic and thermal conductance spectra, which can also be reversed
by an applied local magnetic flux with its phase factor $\phi=\pi$. And, the
Fano effect contributes decisively to the enhancement of thermoelectric
efficiency. However, at the same temperature, the thermoelectric effect in the
case of $\phi=\pi$ is much more apparent, compared with the case of zero
magnetic flux. By the concept of Feynman path, we analyze the difference
between the quantum interferences in the cases of $\phi=0$ and $\phi=\pi$. It
is seen that in the absence of magnetic flux the Fano interference originates
from the quantum interference among infinite-order Feynman paths, but it occurs
only between two lowest-order Feynman paths when $\phi=\pi$. The increase of
temperature inevitably destroys the electron coherent transmission in each
paths. So, in the case of zero magnetic field, the thermoelectric effect
contributed by the Fano interference is easy to weaken by a little increase of
temperature.
Using a simple optical deflection technique, we measured continuously the mechanical stress during the growth of Fe films of 0.1–1.5 nm thickness on Si(111) in ultrahigh vacuum (UHV). The stress versus coverage dependence is discussed in view of the different growth modes during the various stages of Fe deposition. The deposition of up to 0.3 nm Fe induces a compressive stress of −1 N/m. We assign this stress to the formation of a reactive Fe–Si interface layer with a silicidelike structure. Subsequent Fe deposition at 300 K leads to a small tensile stress of 0.7 N/m, whereas the deposition at 600 K induces a high tensile film stress of 18 N/m. At 600 K substrate temperature, a solid-state reaction between Fe and Si sets in, and the silicide β-FeSi2 is formed. The decrease of the atomic volume of Si by 7% in this silicide is proposed to be the cause for the tensile stress.
We present a comprehensive quantum electrodynamical analysis of the interaction between a continuum with photonic band gaps (PBGs) or frequency cut-off and an excited two-level atom, which can be either ‘bare’ or ‘dressed’ by coupling to a near-resonant field mode. A diversity of novel features in the atom and field dynamics is shown to arise from the non-Markovian character of radiative decay into such a continuum of modes. Firstly the excited atom is shown to evolve, by spontaneous decay, into a superposition of non-decaying single-photon dressed states, each having an energy in a different PBG, and a decaying component. This superposition is determined by the atomic resonance shift, induced by the spontaneously emitted photon, into or out of a PBG. The main novel feature exhibited by the decaying excited-state component is the occurrence of beats between the shifted atomic resonance frequency and the PBG cut-off frequencies, corresponding to a non-Lorentzian emission spectrum. Secondly the induced decay of a resonantly driven atom into such a continuum exhibits a cascade of transitions down the ladder of dressed states, which are labelled by decreasing photon numbers of the driving mode. Remarkably, this cascade is terminated at the dressed-state doublet, from which all subsequent transitions to lower doublets are forbidden because they fall within the PBG. This doublet then becomes an attractor state for the populations of higher-lying doublets. As a result, the photon-number distribution of the driving mode becomes strongly sub-Poissonian.
We demonstrate the trapping of a conduction electron between two identical adatom impurities in a one-dimensional semiconductor quantum-dot array system (quantum wire). Bound steady states arise even when the energy of the adatom impurity is located in the continuous one-dimensional energy miniband. The steady state is a realization of the bound state in continuum (BIC) phenomenon first proposed by von Neuman and Wigner Phys. Z. 30 465 (1929)]. We analytically solve the dispersion equation for this localized state, which enables us to reveal the mechanism of the BIC. The appearance of the BIC state is attributed to the quantum interference between the impurities. The Van Hove singularity causes another type of bound state to form above and below the band edges, which may coexist with the BIC.
It is shown that for two open quantum dots connected by a wire, “bound states in the continuum” of a single electron are formed at nearly periodic distances between the dots. This is due to Fabry-Pérot interference between quasibound states in each dot. The bound states are nonlocal, describing the electron trapped in both dots at the same time. Theoretical and numerical results show that trapped states exist even if the wire connecting the dots is relatively long.
Unprecedented Hall mobility, electron concentration and photoconductivity are demonstrated in semiconducting - thin films prepared on Si(111) surfaces by co-sputtering of iron and silicon followed by post-anneal. Characterization of the silicide as a function of the initial temperature and post-treatment shows that annealing temperatures above C are needed to obtain single phase -. Reactive deposition on substrates heated at C leads to textured films. Majority carriers are electrons in all these unintentionally doped films. Hall concentrations between and electrons and respective Hall mobilities from 290 to are measured at room temperature, involving two different conduction band minima in these two extreme cases. Only deep centres exist in the samples having the lower carrier concentration. In such a situation, raw data must be corrected for the substrate contribution to extract values which are relevant for the - film alone. Photoconductivity also takes place in these samples: at 80 K, it shows a maximum value at the direct band gap of - while at 296 K a step still appears at the same energy. Such results are a consequence of the important decrease of the residual impurity concentration in comparison to values previously published.
We have succeeded in directly measuring the Hall effect in a single-atomic layer on a Si111 crystal surface. Our four-point-probe transport measurements under magnetic field showed that the behavior of majority carriers in the surface state changed from electronlike to holelike during the structural conversion from the 3 3-Ag to 21 21-Ag, Au surface superstructure. This is due to a change in the Fermi surface caused by band folding. The results are discussed quantitatively and shown to be consistent with the electronic structure obtained by photoemission spectroscopy.
Bound states in the continuum (BIC) are shown to exist
in a single-level Fano-Anderson model with a colored interaction
between the discrete state and a structured tight-binding
continuum, which may describe mesoscopic electron or photon
transport in a semi-infinite one-dimensional lattice. The
existence of BIC is explained in the lattice realization as a
boundary effect induced by lattice truncation.
We consider a mesoscopic region coupled to two leads under the influence of external timedependent voltages. The time dependence is coupled to source and drain contacts, the gates controlling the tunnel-barrier heights, or to the gates that define the mesoscopic region. We derive, with the Keldysh nonequilibrium-Green-function technique, a formal expression for the fully nonlinear, time-dependent current through the system. The analysis admits arbitary interactions in the mesoscopic region, but the leads are treated as noninteracting. For proportionate coupling to the leads, the time-averaged current is simply the integral between the chemical potentials of the time-averaged density of states, weighted by the coupling to the leads, in close analogy to the time-independent result of Meir and Wingreen [Phys. Rev. Lett. 68, 2512 (1992)]. Analytical and numerical results for the exactly solvable noninteracting resonant-tunneling system are presented. Due to the coherence between the leads and the resonant site, the current does not follow the driving signal adiabatically: a "ringing" current is found as a response to a voltage pulse, and a complex time dependence results in the case of harmonic driving voltages. We also establish a connection to recent linear-response calculations, and to earlier studies of electron-phonon scattering effects in resonant tunneling.
It is shown that in dielectrics exhibiting a complete photonic band gap, quantum electrodynamics predicts the occurrence of bound states of photons to hydrogenic atoms. When the atomic transition frequency lies near a photonic band edge, the excited atomic level experiences an anomalous Lamb shift and splits into a doublet. One member of this doublet exhibits resonance fluorescence whereas the other level is dressed by the emission and reabsorption of near-resonant photons whose amplitude decays exponentially from the vicinity of the atom.
We report low-temperature tunneling measurements at zero magnetic field through double and triple quantum dots with adjustable interdot coupling, fabricated in a GaAs/AlGaAs heterostructure. As the coupling is increased, Coulomb blockade conductance peaks split into two (double dot) or three (triple dot) peaks each. The splitting tracks closely the measured tunnel conductance and experimentally determines the total interaction energy. Coupled double and triple dots with different gate capacitance show quasiperiodic beating.
Measurements of elastic and inelastic cotunneling currents are presented on a two-terminal Aharonov-Bohm interferometer with a Coulomb-blockaded quantum dot embedded in each arm. Coherent current contributions, even in a magnetic field, are found in the nonlinear regime of inelastic cotunneling at a finite-bias voltage. The phase of the Aharonov-Bohm oscillations in the current exhibits phase jumps of pi at the onsets of inelastic processes. We suggest that additional coherent elastic processes occur via the excited state. Our measurement technique allows the detection of such processes on a background of other inelastic current contributions and contains qualitative information about the ratio of transport and inelastic relaxation rates.
By means of the nonequilibrium Green function technique, electronic
transport through a multiple-quantum-dot system is theoretically
studied. In this system, a one-dimensional quantum dot chain between two
contacts forms a main channel for the electronic tunneling. Each quantum
dot in the chain couples laterally to a dangling quantum dot. Whenever
the energy of the incident electron is aligned with the energy levels of
the dangling quantum dots, a zero point of the electron transmission
function occurs due to the Fano antiresonance. As a result, the linear
conductance spectrum presents an insulating band around the antiresonant
point. What is interesting is that both edges of the insulating band
become steep rapidly with the increase in the numbers of quantum dots.
The many-body effect due to the intradot electron interaction on the
profile of the insulating band is also investigated by using the
equation-of-motion method of the Green functions up to the second-order
approximation. It is found that the well-defined insulating band
remains. On the basis of this feature, we propose that such a
double-quantum-dot chain can be considered as a device prototype of a
spin filter.
Iron silicide layers grown by solid phase epitaxy on Si(111) are
investigated using scanning tunnel microscope (STM), low-energy electron
diffraction and low-energy ion spectroscopy. These layers, which are
flat and homogeneous for a selected Fe thickness range, exhibit a
p(2×2) surface superstructure whatever the annealing temperature
above 600 K. Voltage-dependent STM images reveal a modification of the
silicide symmetry, from a p(1×1) towards a c(4×8) in-plane
periodicity, above 800 K. This transition is associated with an
organization of chemical species under two Si atomic planes.
We report on a general theory for analyzing quantum transport through devices in the metal-QD-metal configuration where QD is a quantum dot or the device-scattering region which contains Rashba spin-orbital and electron-electron interactions. The metal leads may or may not be ferromagnetic, and they are assumed to weakly couple to the QD region. Our theory is formulated by second quantizing the Rashba spin-orbital interaction in spectral space (instead of real space), and quantum transport is then analyzed within the Keldysh nonequilibrium Green’s function formalism. The Rashba interaction causes two main effects to the Hamiltonian: (i) it gives rise to an extra spin-dependent phase factor in the coupling matrix elements between the leads and the QD, and (ii) it gives rise to an interlevel spin-flip term, but forbids any intralevel spin flips. Our formalism provides a starting point for analyzing many quantum transport issues where spin-orbital effects are important. As an example, we investigate the transport properties of a Aharnov-Bohm ring in which a QD having a Rashba spin-orbital and electron-electron interactions is located in one arm of the ring. A substantial spin-polarized conductance or current emerges in this device due to the combined effect of a magnetic flux and the Rashba interaction. The direction and strength of the spin polarization are shown to be controllable by both the magnetic flux and a gate voltage.
We derive the quantum rate equations for an Aharonov-Bohm interferometer with two vertically coupled quantum dots embedded in each of two arms by means of the nonequilibrium Green function in the sequential tunnelling regime. Based on these equations, we investigate time-dependent resonant tunnelling under a small amplitude irradiation and find that the resonant photon-assisted tunnelling peaks in photocurrent demonstrate a combination behaviour of Fano and Lorentzian resonances due to the interference effect between the two pathways in this parallel configuration, which is controllable by threading the magnetic flux inside this device.
To measure electrical conductivity of materials in scales ranging from nanometer to millimeter, a four-point probe system was developed and installed in an ultrahigh-vacuum scanning electron microscope (UHV-SEM). Each probe, made of a W tip, was independently driven with piezoelectric actuators and a scanner in XYZ directions to achieve precise positioning in nanometer scales. The SEM was used for observing the tips for positioning, as well as the sample surface together with scanning reflection-high-energy electron diffraction capability. This four-point probe system has two kinds of special devices. One is octapole tube-type scanners for tip scanning parallel to the sample surface with negligible displacements normal to the surface. Another is a pre-amplifier which can be switched in current measurement mode between tunnel contact for scanning tunneling microscopy and direct contact for four-point probe method. The electrical resistance of a silicon crystal with a Si(111)-7×7 clean surface was measured with this machine as a function of probe spacing between 1 mm and 1μm. The result clearly showed an enhancement of surface sensitivity in resistance measurement by reducing the probe spacing.
We have determined the schematic phase diagram in detail with high reliability for Fe silicides grown by solid phase epitaxy (SPE) on a Si(111)7×7 surface at wide Fe coverage (0.2-56monolayers) and subsequent annealing temperatures from 300 to 800°C . In the SPE growth, delta-7×7 , 1×1 , bcc-Fe(111)1×1 , 2×2 , c(8×4) , 3D-2×2 (alpha-FeSi2) , 3×3-R30° , beta-FeSi2 , and fine polycrystalline phases are formed on the Si(111) surface depending on Fe coverage and annealing temperature. We have characterized the surface periodic structures and morphologies of all the above Fe silicide phases using low-energy electron diffraction and scanning tunneling microscopy. Reflection high-energy electron diffraction also has been used to determine three-dimensional structures. Based on the overall view regarding the formations and changes of Fe silicide phases on a Si(111) surface, we discuss the growth mechanisms.
Quantum-mechanical examples have been constructed of local potentials with bound eigenstates embedded in the dense continuum of scattering states. The method employed corrects and extends a procedure invented by von Neumann and Wigner. Cases are cited whereby deformation of the local potential causes the continuum bound state to move downward through the bottom of the continuum, and to connect analytically to a nodeless ground state. A doubly excited model atom is also displayed, with interactions between its two "electrons," having an infinite lifetime (in the Schrödinger equation regime). In the light of these examples, attention is focused on quantitative interpretation of real tunneling phenomena, and on the existence of continuum bound states in atoms and molecules.
Transition energies and oscillator strengths of excitons in dependence on magnetic field are investigated in types I and II semiconductor nanorings. A slight deviation from circular (concentric) shape of the type II nanoring gives a better observability of the Aharonov-Bohm oscillations since the ground state is always optically active. Kinetic equations for the exciton occupation are solved with acoustic phonon scattering as the major relaxation process, and absorption and luminescence spectra are calculated, showing deviations from equilibrium. The presence of a nonradiative exciton decay leads to a quenching of the integrated photoluminescence with magnetic field.
The dependence of charges accumulated on a quantum dot under an external voltage bias is studied. The charge is sensitive to the changes of number of filled levels and the number of conducting levels (channels). We clarify that there are two possible outcomes of applying a bias. (a) The number of conducting channels increases, but the number of filled levels decreases. (b) The number of filled levels increases or does not change while the number of conducting channels (levels) increases with the bias. In case (b), charges are generally expected to increase monotonically with the applied bias. We show, however, that this expectation may not materialize when the electron transmission coefficients depend on bias. Numerical evidences and a theoretical explanation of this negative differential capacitance, i.e., charges accumulated on a quantum dot decrease with applied bias, are presented.
Electronic transport through a mesoscopic ring consisting of coupled quantum dots is theoretically studied. First, an analytical expression about the linear conductance is obtained as a function of the quantum-dot level, which can be adjusted by a gate voltage. Then the linear conductance spectrum (the linear conductance versus the quantum-dot level) is calculated numerically. The peaks and zero points in this spectrum, due to resonance and antiresonance, respectively, are discussed in detail. The following interesting results are found for even-numbered quantum-dot rings in some specific coupling configurations with two electron reservoirs. When all the quantum-dot levels are aligned with the Fermi level, a resonant peak occurs in the spectrum if the number of quantum dots in the ring is a multiple of four. But such a peak is so fragile that a very small magnetic field perpendicular to the ring plane can change it into an antiresonant zero point. Contrarily, for any other even-numbered quantum-dot rings, such a sudden resonance-antiresonance transition does not occur. Instead, an antiresonant zero point persists in the spectrum, irrespective of whether a magnetic field is present or not. In addition, when an even-numbered quantum-dot ring couples to two electron reservoirs symmetrically, an appropriate magnetic field can completely suppress the overall linear conductance spectrum. All these results can be well explained by the decoupling effect of some eigenstates of the ring from the leads. Finally, the persistent current in a quantum-dot ring is calculated. It is found that the extrema and the zero points of the persistent current correspond to the valleys and peaks of the linear conductance, respectively.
Combined (x-ray, ultraviolet) photoelectron, electron- and bremsstrahlung-excited Auger-electron, and electron-energy-loss (EELS) spectroscopic investigations yield insight into the bonding state of silicon (i) as segregated onto Fe(100) forming a c(2×2) superstructure at equilibrium, and (ii) on Fe3Si(100), FeSi(100), and FeSi2(poly) surfaces. The slight binding-energy shifts of the Si2s, Si2p, and Fe3p core levels as compared with the pure elements (α-iron and silicon) indicate a small charge transfer from iron to silicon atoms in the silicides. For silicon in the segregated state, the bonding exhibits a predominantly homopolar character. The iron silicide valence bands show an invariable nonbonding Fe3d—derived feature and bonding iron states about 2 eV below EF. Independent of the Si bulk content, the density of states near EF is always high, reflecting the intermetallic character of the iron-silicon compounds. The Si- and Fe-induced valence states and interatomic features in the Si(L23VV) Auger transitions are evidence of the prevailing iron-silicon interaction in Fe3Si and localized silicon bonding in the monosilicide and disilicide. Segregated Si on Fe(100) at surface saturation interacts for the most part laterally, but generates some slight modification in the iron electronic structure in this configuration. The EELS spectra essentially reflect the increasing degree of valence-electron delocalization and the diminishing number of oscillating electrons in going from Fe-Si(6 at.%) to FeSi2. The results are discussed in comparison with other transition-metal silicides and with related iron compounds as well as on the basis of crystallographic data.
For the ballistic quantum transport, the conductance of each channel is quantized to a value of 2e2/h. In the presence of defects, electrons will be scattered such that the conductance will deviate from the values of the quantized conductance. We show that an antiresonance scattering can occur when an extra defect level is introduced into a conduction band. At the antiresonance scattering, exactly one quantum conductance of a one-dimensional wire disappears, in good agreement with ab initio calculations. The conductance takes a nonzero value when the Fermi energy is away from the antiresonance scattering.
Introduces a simple model of laser-induced negative ion photodetachment, incorporating the Wigner power law, in order to discuss non-perturbatively the effects of strong exciting laser fields near to threshold. The authors find that the threshold can be smeared out and shifted slightly, as might be expected. More interesting is the possibility of observing power law (t-3) and other deviations from the exponential decay of bound electron probability near to threshold.
Using scanning tunneling microscopy, solid phase epitaxial growth of FeSi2 nanodots on Si (1 1 1) sqrt(3) × sqrt(3)-R30°-B surface has been studied in the temperature range of 400-700 °C and Fe coverage of up to 0.5 monolayer. It has been found that density of nanodots formed on Si (1 1 1) sqrt(3) × sqrt(3)-R30°-B surface is essentially higher than that on Si(1 1 1)7 × 7 surface at the same temperature and coverage. Density of the 2 × 2 islands and structural defects is reduced on Si (1 1 1) sqrt(3) × sqrt(3)-R30°-B surface in comparison with that on Si(1 1 1)7 × 7 surface.
In situ Hall measurements, ex situ Hall and Seebeck coefficient temperature measurements of very thin (0.3–2.4 nm) CrSi(111) epitaxial layers with Si(111)√3×√3/30° LEED pattern are presented. The sheet p-type conductivity in CrSi(111) layers was observed from the chromium thicknesses of 0.9 nm. Chromium monosilicide layer (2.4 nm) displayed the metallic properties at room temperature by optical spectroscopy data. Sheet hole concentration was nearly constant in the temperature range of 300–500 K, but activated at high temperatures.
We discuss the interaction between two resonant states in a quantum double-well structure. The behavior of the resonant states depends on the coupling between the wells, i.e., the height and width of the barrier that separates them. We distinguish a region with resonant tunneling and a region where the two resonances repel each other. The transition between the two regions is marked by a double pole of the S matrix.
SHORTLY after the birth of quantum mechanics, von Neumann and Wigner made the remarkable proposal1 that certain spatially oscillating attractive potentials could support bound states at energies above the potential barriers (that is, spatially confined states within the continuum) by means of diffractive interference. Because of their unusual geometry, such potentials were regarded as mathematical curiosities2,3, although more recently it has been suggested that they might be found in certain atomic and molecular systems4,5. Following the observation of discrete electronic states in ultra-thin semiconductor layered structures6,7 (for example, in quantum wells), Stillinger8 and Herrick9 proposed that super-lattices might be used to construct potentials supporting these 'positive energy' bound states. Here we report direct evidence of such states in semiconductor heterostructures grown by molecular-beam epitaxy10. Infrared absorption measurements reveal a narrow, isolated transition from a bound state within a quantum well to a bound state at an energy greater than the barrier height; this state is spatially localized by Bragg reflections.
The I–V spectrum of electronic transport through a quantum dot chain is calculated by means of the nonequilibrium Green function technique. In such a system, two arbitrary quantum dots are connected with two electron reservoirs through leads. When the dot-lead coupling is very weak, a series of discrete resonant peaks in electron transmission function cause staircase-like I–V characteristic. On the contrary, in the relatively strong dot-lead coupling regime, stairs in the I–V spectrum due to resonance vanish. However, when there are some dangling quantum dots in the chain outside two leads, the antiresonance which corresponds to the zero points of electron transmission function brings about novel staircase characteristic in the I–V spectrum. Moreover, two features in the I–V spectrum arising from the antiresonance are pointed out, which are significant for possible device applications. One is the multiple negative differential conductance regions, and another is regarding to create a highly spin-polarized current through the quantum dot chain by the interplay of the resonance and antiresonance. Finally, we focus on the role that the many-body effect plays on the antiresonance. Our result is that the antiresonance remains when the electron interaction is considered to the second order approximation.
The influence of thin adsorbed films on the reflectance properties of two-phase systems is discussed. It is shown that the linear approximation to the normalized reflectivity change reduces the complicated reflectivity expressions for multiphase stratified systems to a simple form which gives direct physical insight into the properties of these systems. Several interfacial systems of physical interest are examined in detail, and the results are generalized to include attenuated total reflection, fractional coverage, and interfaces with smoothly varying optical properties.
A general formal theory of resonance reactions and scattering is developed without the use of channel radii. The approach employed allows a simple physical interpretation of the two energies, one of which must be kept fixed while the other one is varied in order for a Breit-Wigner denominator to vanish. A new result, obtained without a weak channel coupling hypothesis, is that a sharp resonance may be caused by forces which, if slightly different, would lead to a stable bound state even in the presence of strong coupling to open channels. The possibility of shapes other than the usual Breit-Wigner type is also discussed. A special formula is derived for resonances near a threshold caused by bound states just below that threshold in the same channels where the peak is seen.
How localized electrons interact with delocalized electrons is a central question to many problems in solid-state physics. The simplest manifestation of this situation is the Kondo effect, which occurs when an impurity atom with an unpaired is placed in a metal. At low temperatures a spin singlet state is formed between the unpaired localized electrons at the Fermi energy. Theories predict that a Kondo singlet should form in a single-electron transistor (SET), which contains a confirmed 'droplet' of electrons coupled by quantum-mechanical tunnelling to the delocalized electrons in the transistor's leads. If this is so, a SET could provide a means of investigating aspects of the Kondo effect under controlled circumstances that are not accessible in conventional systems: the number of electron can be changed from odd to even, the difference in energy between the localized state and the Fermi level can be turned, the coupling to the leads can be adjusted, voltage differences can be applied to reveal non-equilibrium Kondo phenomena, and a single localized state can be studied rather than a statistical distribution. But for SETs fabricated previously, the binding energy of the spin singlet has been too small to observe Kondo phenomena. Ralph and Buhrman have observed the Kondo singlet at a single accidental impurity in a metal point contact, but with only two electrodes and without control over the structure they were not able to observe all of the features predicted. Here we report measurements on SETs smaller than those made previously, which exhibit all of the predicted aspects of the Kondo effect in such a system.
We investigate resonant tunnelling through molecular states of an Aharonov-Bohm (AB) interferometer composed of two coupled quantum dots. The conductance of the system shows two resonances associated with the bonding and the antibonding quantum states. We predict that the two resonances are composed of a Breit-Wigner resonance and a Fano resonance, of which the widths and Fano factor depend on the AB phase very sensitively. Further, we point out that the bonding properties, such as the covalent and ionic bonding, can be identified by the AB oscillations.
The early stages of iron silicide formation on Si(111) were studied by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. While the initial iron interaction with Si(111) in the submonolayer regime gives rise to inhomogeneous island nucleation, deposition of 1.5 monolayers (ML) iron at room temperature and subsequent annealing at 550–600 °C leads to a flat and homogeneous film with c(8×4) surface periodicity. This c(8×4) surface reconstruction is linked to a definite film thickness and thus seems to be stabilized directly through the interface. The film is terminated by a layer of adatoms whose lateral positions form a (2×2) periodic lattice. At negative tip bias voltages, STM images show an alternating arrangement of darker and brighter adatoms corresponding to the c(8×4) supercell. While the (2×2)-periodic adatom arrangement develops in a wide temperature regime (450–600 °C) and also for thicker films, the long range ordered c(8×4) structure can be observed only for 1–2 ML Fe coverage and after high temperature annealing at about 600 °C. Then single c(8×4) domains can extend to diameters of several hundred nanometers. The atomic structure of the new phase can be derived from a CsCl (B2) structure, and a number of structural details are elucidated on the course towards the development of a complete structural model.
Electronic transport through a quantum dot chain embodied in an Aharonov-Bohm interferometer is theoretically investigated. In such a system, it is found that only for the configurations with the same-numbered quantum dots side-coupled to the quantum dots in the arms of the interferometer, some molecular states of the quantum dot chain decouple from the leads. Namely, in the absence of magnetic flux all odd molecular states decouple from the leads, but all even molecular states decouple from the leads when an appropriate magnetic flux is introduced. Interestingly, the antiresonance position in the electron transport spectrum is independent of the change of the decoupled molecular states. In addition, when considering the many-body effect within the second-order approximation, we show that the emergence of decoupling gives rise to the apparent destruction of electron-hole symmetry. By adjusting the magnetic flux through either subring, some molecular states decouple from one lead but still couple to the other, and then some new antiresonances occur.
We show within the framework of quantum-defect theory that bound states in the continuum (BIC) can occur in systems of at least three coupled Coulombic channels due to the interference of resonances belonging to different channels. As a physical realization of such BIC, we discuss the Schrödinger equation describing a hydrogen atom in a uniform magnetic field.
Localization due to space structure, rather than due to randomness, is investigated by studying the usual tight-binding model on the Sierpinski gasket. Some exact results are obtained from the decimation-renormalization-group method. It is surprising that there exist an infinite number of extended states on the Sierpinski gasket. This set of extended states forms a Cantor set. The rest of the states are exponentially localized except for two states that are localized in a power-law fashion. It can be shown that exponential localization of lengths of states on the Sierpinski gasket reveal a self-similar pattern.
Magnetic-field effects on structure-induced localization in a fractal lattice are investigated by studying the usual tight-binding model on a Sierpinski gasket in the presence of a magnetic field. We find that a magnetic field can drastically change properties of states which are localized with high degeneracy in the absence of a magnetic field. In the presence of a magnetic field, the degeneracies of those localized states are broken, and each of them becomes many extended states. Around these new extended state regions, the number of states whose localization length is larger than a certain value decreases with the logarithm of the localization length in a power-law fashion. The exponent of this power-law behavior does not depend on the strength of the magnetic field and the energy range chosen. © 1996 The American Physical Society.
We study electron localization of quasi-one-dimensional random-potential and random-magnetic-field systems with and without a uniform external magnetic field. We find that in a random-magnetic-field (RMF) system the localization length is not a monotonic decreasing function of magnetic-field randomness δ in contrast to a random-potential system in which the localization length is a monotonic decreasing function of the randomness w. We observed that, in both random-potential and RMF systems, the localization length can both increase and decrease with a uniform external magnetic field depending on the energy of a state. The crossover occurs when the electron energy is close to the band edge in a random-potential system. In a RMF system, the magnetoproperty of the localization length is complicated. A band edge effect is proposed to explain the anomalous numerical results. We also find that the inverse of the localization length of a quasi-one-dimensional disordered system peaks at certain energies inside the energy band. These peaks can be understood by the branch edge effect. © 1996 The American Physical Society.
A theoretical study is made of charged-particle motion in planar circuits made up of narrow wires, in the presence of a magnetic field of arbitrary strength, in the ballistic approximation. The basic element is a four-terminal junction of narrow wires, and its detailed properties are calculated. Its scattering probabilities strongly reflect the influence of quantum effects of the junction, including subband thresholds and virtual, resonant states, and the Hall resistance calculated from them may depart considerably from the classic wide-wire result. Physical features are related to the emergence of pinned Landau levels as the field strength increases. By suitably modifying the four-terminal method, it is extended to include elbow (``ssL'') and tee (``ssT'') junctions. The results are then used to construct linear six- and eight-terminal junctions, whose resistive properties are discussed. The Hall resistance is predicted to depend on which arms are used and on the stub spacing. The application of the method to the general linear series of junctions is then outlined. The four-terminal results are also applied to a square eight-terminal junction, to show the presence and the consequences of the Aharonov-Bohm effect in circuits with closed loops.
We describe the quantum electrodynamics of photons interacting with hydrogenic atoms and molecules in a class of strongly scattering dielectric materials. These dielectrics consist of an ordered or nearly ordered array of spherical scatterers with real positive refractive index and exhibit a complete photonic band gap or pseudogap for all directions of electromagnetic propagation. For hydrogenic atoms with a transition frequency in the forbidden optical gap, we demonstrate both the existence and stability of a photon-atom bound state. For a band gap to center frequency ratio Δω/ω0∼5%, the photon localization length ξloc≥10L, where L is the lattice constant of dielectric array. This strong self-dressing of the atom by its own localized radiation field leads to anomalous Lamb shifts and a splitting of the excited atomic level into a doublet when the transition frequency lies near a photonic band edge. We estimate the magnitude of this splitting to be 10−6 at the vacuum transition energies.
A quantum-mechanical calculation is made of ballistic transport in intersecting narrow channels of finite length. The two-dimensional (2D) semiconductor structure we study consists of perpendicular channels, one of which connects to two reservoirs of 2D electron gas. These reservoirs serve as emitter and collectors when a potential difference is applied. At a single intersection with infinite leads, there are generally bound states that are well localized to the intersection area. In a structure with finite leads to emitter and collector, such localized states or quantum dots give rise to resonant tunneling. We show that in narrow ballistic channels with few stubs the bound states couple to each other. Therefore, the states at the different intersections combine as split bound states. The splitting is N-fold if there are N intersections. Here we study conductance in structures containing a few crossed-bar or T-shaped junctions and focus on the splitting of the resonance conductance below the first subband threshold. We argue that our results are in qualitative agreement with recent measurements. We also consider the spatial distribution of currents and show that a complicated flow pattern with vortex structures appears at higher energies.
We present a multitechnique (scanning tunneling microscopy, photoelectron spectroscopy, and ion scattering spectroscopy) approach to study the formation of the Fe/Si(111) interface at room temperature. The first-deposited Fe atoms react with the surface, displacing Si atoms from their positions. The result is an amorphous layer with composition and density of states close to those of FeSi. On top of this reacted layer, crystallites of Fe with interdiffused Si grow. Upon further Fe deposition, the crystallite composition evolves to pure Fe.
A Landauer formula for the current through a region of interacting electrons is derived using the nonequilibrium Keldysh formalism. The case of proportionate coupling to the left and right leads, where the formula takes an especially simple form, is studied in more detail. Two particular examples where interactions give rise to novel effects in the current are discussed: In the Kondo regime, an enhanced conductance is predicted, while a suppressed conductance is predicted for tunneling through a quantum dot in the fractional quantum Hall regime.
The Fano effect, which arises from an interference between a localized state and the continuum, reveals a fundamental aspect of quantum mechanics. We have realized a tunable Fano system in a quantum dot (QD) in an Aharonov-Bohm interferometer, which is the first convincing demonstration of this effect in mesoscopic systems. With the aid of the continuum, the localized state inside the QD acquires itinerancy over the system even in the Coulomb blockade. Through tuning of the parameters, which is an advantage of the present system, unique properties of the Fano effect on the phase and coherence of electrons have been revealed.
Decay kinetic properties of a two-level atom near the band edges of photonic crystals (PCs) with absolute gaps are studied based on the Green's function expression for the evolution operator. The local coupling strength between the photons and an atom is evaluated by an exact numerical method. It is found that the decay behavior of an excited atom can be fundamentally changed by the variation of the atomic position: Weisskopf-Wigner and non-Weisskopf-Wigner decay phenomena occur at different atomic positions in the PCs as a result of a significant difference in the local coupling strength. Our finding implies that it is possible to engineer the luminescence spectrum by controlling the atomic position.
We have observed the Fano-Kondo antiresonance in a quantum wire with a side-coupled quantum dot. In a weak coupling regime, dips due to the Fano effect appeared. As the coupling strength increased, conductance in the regions between the dips decreased alternately. From the temperature dependence and the response to the magnetic field, we conclude that the conductance reduction is due to the Fano-Kondo antiresonance. At a Kondo valley with the Fano parameter q approximately 0, the phase shift is locked to pi/2 against the gate voltage when the system is close to the unitary limit in agreement with theoretical predictions by Gerland et al. [Phys. Rev. Lett. 84, 3710 (2000)].
The eigenvalue problem for the dressed bound-state of unstable multilevel systems is examined both outside and inside the continuum, based on the N-level Friedrichs model which describes the couplings between the discrete levels and the continuous spectrum. It is shown that a bound-state eigenenergy always exists below each of the discrete levels that lie outside the continuum. Furthermore, by strengthening the couplings gradually, the eigenenergy corresponding to each of the discrete levels inside the continuum finally emerges. On the other hand, the absence of the eigenenergy inside the continuum is proved in weak but finite coupling regimes, provided that each of the form factors that determine the transition between some definite level and the continuum does not vanish at that energy level. An application to the spontaneous emission process for the hydrogen atom interacting with the electromagnetic field is demonstrated. Comment: 10 pages, 4 figures, accepted for publication in Physical Review A
The Fano effect, which occurs through the quantum-mechanical cooperation between resonance and interference, can be observed in electron transport through a hybrid system of a quantum dot and an Aharonov-Bohm ring. While a clear correlation appears between the height of the Coulomb peak and the real asymmetric parameter $q$ for the corresponding Fano lineshape, we need to introduce a complex $q$ to describe the variation of the lineshape by the magnetic and electrostatic fields. The present analysis demonstrates that the Fano effect with complex asymmetric parameters provides a good probe to detect a quantum-mechanical phase of traversing electrons. Comment: REVTEX, 9 pages including 8 figures