The self-assembled GaInNAs quantum dot (QD) has proposed as a novel material system for long wavelength lasers on GaAs substrate. In this paper, we have investigated the thermal annealing effect on GaInNAs QDs. The increase of the PL intensity and blue shift of peak wavelength was observed by thermal annealing. For 600°C annealing, the PL intensity was increased with the increase of annealing time and maximum intensity was obtained at 2 hours. On the other hand, PL intensity was increased after 30s annealing, then decreased at longer time for 700°C annealing. A larger blue shift of peak wavelength compared to GaInNAs quantum well was observed. It is considered that the interdiffusion was enhanced in QD system due to its large strain and interface area between GaAs capping layer.
We report an experimental study of the coherent coupling between Surface Plasmon Polaritons (SPPs) and Quantum Well (QW) excitons in a hybrid metal- semiconductor nanostructure, consisting of a gallium arsenide quantum well placed in a close proximity of a metal nanoslit array. Exciton-SPP coupling is probed by low-temperature angle resolved spectroscopy. Our results give evidence of a distinct modification of the exciton dispersion relation due to interaction of the exciton with SPP fields at both interfaces of the metal film. An analysis of the experimental data within a coupled oscillator model indicates coupling strengths of several tens of meV.
We report a systematic study on 1.2 MeV Ar^8+ irradiated ZnO by x-ray diffraction (XRD), room temperature photoluminescence (PL) and ultraviolet-visible (UV-Vis) absorption measurements. ZnO retains its wurtzite crystal structure up to maximum fluence of 5 x 10^16 ions/cm^2. Even, the width of the XRD peaks changes little with irradiation. The UV-Vis absorption spectra of the samples, unirradiated and irradiated with lowest fluence (1 x 10^15 ions/cm^2), are nearly same. However, the PL emission is largely quenched for this irradiated sample. Red shift of the absorption edge has been noticed for higher fluence. It has been found that red shift is due to at least two defect centers. The PL emission is recovered for 5 x 10^15 ions/cm^2 fluence. The sample colour is changed to orange and then to dark brown with increasing irradiation fluence. Huge resistivity decrease is observed for the sample irradiated with 5 x 10^15 ions/cm^2 fluence. Results altogether indicate the evolution of stable oxygen vacancies and zinc interstitials as dominant defects for high fluence irradiation. Comment: Accepted in Physica Sattus Solidi (c)
We present calculations of the frequency-dependent spin susceptibility tensor
of a two-dimensional electron gas with competing Rashba and Dresselhaus
spin-orbit interaction. It is shown that the interplay between both types of
spin-orbit coupling gives rise to an anisotropic spectral behavior of the spin
density response function which is significantly different from that of
vanishing Rashba or Dresselhaus case. Strong resonances are developed in the
spin susceptibility as a consequence of the angular anisotropy of the energy
spin-splitting. This characteristic optical modulable response may be useful to
experimentally probe spin accumulation and spin density currents in such
systems.
We study the microwave-induced photoconductivity of a two-dimensional
electron system (2DES) in the presence of a magnetic field and a
two-dimensional modulation. The microwave and Landau contributions are exactly
taken into account, while the periodic potential is treated perturbatively. The
longitudinal resistivity exhibits oscillations, periodic in $\omega /
\omega_c$. Negative resistance states (NRS) develop for sufficiently high
electron mobility and microwave power. This phenomenon appears in a narrow
window region of values of the lattice parameter ($a$), around $a \sim l_B$,
where $l_B$ is the magnetic length. It is proposed that these phenomena may be
observed in artificially fabricated arrays of periodic scatterers at the
interface of ultraclean heterostructures. {73.20.At,05.60.-k, 72.15.Rn}
We studied the optical absorption induced by 4.7eV pulsed laser radiation on
Ge-doped a-SiO2 synthesized by a sol-gel technique. The absorption spectra in
the ultraviolet spectral range were measured during and after the end of
irradiation with an in situ technique, evidencing the growth of an absorption
signal whose profile is characterized by two main peaks near 4.5eV and 5.7eV
and whose shape depends on time. Electron spin resonance measurements performed
ex situ a few hours after the end of exposure permit to complete the
information acquired by optical absorption by detection of the paramagnetic
Ge(1) and Ge-E' centers laser-induced in the samples.
We report on recent progress in the acousto-electrical control of
self-assembled quantum dot and quantum post using radio frequency surface
acoustic waves (SAWs). We show that the occupancy state of these optically
active nanostructures can be controlled via the SAW-induced dissociation of
photogenerated excitons and the resulting sequential bipolar carrier injection
which strongly favors the formation of neutral excitons for quantum posts in
contrast to conventional quantum dots. We demonstrate high fidelity preparation
of the neutral biexciton which makes this approach suitable for deterministic
entangled photon pair generation. The SAW driven acoustic charge conveyance is
found to be highly efficient within the wide quantum well surrounding the
quantum posts. Finally we present the direct observation of acoustically
triggered carrier injection into remotely positioned, individual quantum posts
which is required for a low-jitter SAW-triggered single photon source.
Arbitrary waves incident on a solid embedded nanoparticle are studied. The acoustic vibrational frequencies are shown to correspond to the poles of the scattering cross section in the complex frequency plane. The location of the poles is unchanged even if the incident wave is nonplanar. A second approach approximating the infinite matrix as a very large shell surrounding the nanoparticle provides an alternate way of predicting the mode frequencies. The wave function of the vibration is also provided.
We study the evolution of a quantum dot controlled by a frequency-swept
(chirped), linearly polarized laser pulse in the presence of carrier-phonon
coupling. The final occupation of the exciton state is limited both due to
phonon-induced transitions between the adiabatic spectral branches and because
of phonon-assisted transitions to the biexciton state. When the biexciton shift
is large enough, the quantum dot can be modeled as a two-level system, which
corresponds to excitation with circularly polarized light. For this case, we
compare different methods of simulations: (i) a time convolutionless method,
(ii) correlation expansion and (iii) path integrals. We show that results
obtained from these methods agree perfectly at low temperatures.
It is shown that in many cases an adequate description of optical spectra of semiconductor quantum dots requires a treatment beyond the commonly used adiabatic approximation. We have developed a theory of phonon-assisted optical transitions in semiconductor quantum dots, which takes into account non-adiabaticity of the exciton-phonon system. Effects of non-adiabaticity lead to a mixing of different exciton and phonon states that provides a key to the understanding of surprisingly high intensities of phonon satellites observed in photoluminescence spectra of quantum dots. A breakdown of the adiabatic approximation gives an explanation also for discrepancies between the serial law, observed in multi-phonon optical spectra of some quantum dots, and the Franck-Condon progression, prescribed by the adiabatic approach.
We study the quantum jumps of physical quantities in a strongly correlated many electron systems based on a new p - adic functional integral approach. It is shown that a description in terms of the p - adic numbers leads to the fractal behavior and can describe the quantum jumps in the conductivity and magnetization as a function of voltage and magnetic field in nanotechnology devices and low-dimensional strongly correlated organic metals and other materials. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
We have derived a universal relaxation function for heterogeneous materials using the maximum entropy principle for nonextensive systems. The power law exponents of the relaxation function are simply related to a global fractal parameter and for large time to the entropy nonextensivity parameter q. For intermediate times the relaxation follows a stretched exponential behavior. The asymptotic power law behaviors both in the time and the frequency domains coincide with those of the Weron generalized dielectric function derived in the stochastic theory from an extension of the Levy central limit theorem. These results are in full agreement with the Jonscher universality principle and find application in the characterization of the dielectric properties of aerogels catalytic supports as well as in the problem of the relation between morphology and dielectric properties of polymer composites.
We show that optical properties of linear molecular aggregates undergo drastic changes when aggregates are deposited on a metal surface. The dipole-dipole interactions of monomers with their images can result in strong {re-structuring of both the exciton band and the absorption spectrum, depending on the arrangement of the monomer transition dipoles with respect to the surface.
Systematic ensemble photoluminescence studies have been performed on type-I
InP-quantum dots in Al0.20Ga0.80InP barriers, emitting at approximately 1.85 eV
at 5 K. The influence of different barrier configurations as well as the
incorporation of additional tunnel barriers on the optical properties has been
investigated. The confinement energy between the dot barrier and the
surrounding barrier layers, which is the sum of the band discontinuities for
the valence and the conduction bands, was chosen to be approximately 190 meV by
using Al0.50Ga0.50InP. In combination with 2 nm thick AlInP tunnel barriers,
the internal quantum efficiency of these barrier configurations can be
increased by up to a factor of 20 at elevated temperatures with respect to
quantum dots without such layers.
Ternary and quaternary cubic c-AlxIn1-xN/GaN and c-AlxGayIn1-x-y/GaN
heterostructures lattice-matched to c-GaN on freestanding 3C-SiC substrates
were grown by plasma-assisted molecular beam epitaxy. The c-AlxGayIn1-x-y alloy
permits the independent control of band gap and lattice parameter. The ternary
and quaternary films were grown at 620 C. Different alloy compositions were
obtained by varying the Al and Ga fluxes. The alloy composition was measured by
Energy Dispersive X-ray Spectroscopy (EDX) and Rutherford Backscattering
Spectrometry (RBS). X-ray reciprocal space map of asymmetric (-1-13) reflex
were used to measure the lattice parameters and to verify the lattice match
between the alloy and the c-GaN buffer.
Force-constant and positional disorder have been introduced into diamond
lattice models in an attempt to mimic the vibrational properties of a realistic
amorphous silicon model. Neither type of disorder is sufficient on its own to
mimic the realistic model. By comparing the spectral densities of these models,
it is shown that a combination of both disorders is a better representation,
but still not completely satisfactory. Topological disorder in these models was
investigated by renumbering the atoms and examining the dynamical matrix
graphically. The dynamical matrix of the realistic model is similar to that of
a positionally-disordered lattice model, implying that the short-range order in
both systems is similar.
We have measured the Anisotropic Magnetoresistance (AMR) of ultra thin (5nm)
Ga(0.95)Mn(0.05)As films. We find that the sign of the AMR can be positive or
negative, which may depend on the direction of the current with respect to the
crystal. At low temperatures, transport measurements and SQUID magnetometry
suggest that the magnetisation has a component pointing out of the plane of the
film.
We study the effect of magnetic anisotropy in a single electron transistor with ferromagnetic electrodes and a non-magnetic island. We identify the variation $\delta \mu$ of the chemical potential of the electrodes as a function of the magnetization orientation as a key quantity that permits to tune the electrical properties of the device. Different effects occur depending on the relative size of $\delta \mu$ and the charging energy. We provide preliminary quantitative estimates of $\delta \mu$ using a very simple toy model for the electrodes.
The upper critical field and flux pinning in MgB2 single crystals were investigated. The implications of these properties for technical applications are discussed and compared with transport properties of polycrystalline bulk samples and wires. In these untextured materials current percolation is important, especially at high magnetic fields. It is shown that the anisotropy of the upper critical field influences the "irreversibility line" and that the application range of MgB2 is limited by the smallest upper critical field (i.e., for the field direction perpendicular to the boron planes). Disorder, introduced by irradiation with neutrons, enhances the upper critical field, reduces the anisotropy and drastically changes flux pinning. While the enhanced Hc2 and the reduced anisotropy generally improve the transport properties of the polycrystalline samples, the contribution of the radiation-induced defects to flux pinning is small compared to the as-grown defect structure (grain boundary pinning).
The influence of surface anisotropy on the magnetization processes of maghemite nanoparticles with ellipsoidal shape is studied by means of Monte Carlo simulations. Radial surface anisotropy is found to favor the formation of hedgehog-like spin structures that become more stable as the surface anisotropy constant at the surface $k_S$ is increased form the value at the core. We have studied the change in the low temperature hysteresis loops with the particle aspect ratio and with $k_S$, finding a change in the magnetization reversal mode as $k_S$ or the particle elongation is increased.
Electron-electron interactions seem to play a surprisingly small role in the description of the integer quantum Hall effect, considering that for just slightly different filling factors the interactions are of utmost importance causing the interaction-mediated fractional quantum Hall effect. However, recent imaging experiments by Cobden et al. and Ilani et al. constitute strong evidence for the importance of electron-electron interactions even in the integer effect. The experiments report on measurements of the conductance and electronic compressibility of mesoscopic MOSFET devices that show disagreement with predictions from the single particle model. By diagonalising a random distribution of Gaussian scatterers and treating the interactions in Hartree-Fock approximation we investigate the role of electron-electron interactions for the integer quantum Hall effect and find good agreement with the experimental results.
The shot noise suppression in a sample containing a layer of self-assembled InAs quantum dots has been investigated experimentally and theoretically. The observation of a non-monotonic dependence of the Fano factor on the bias voltage in a regime where only few quantum dot ground states contribute to the tunneling current is analyzed by a master equation model. Under the assumption of tunneling through states without Coulomb interaction this behaviour can be qualitatively reproduced by an analytical expression.
The notion of artifical atom relies on the capability to change the number of carriers one by one in semiconductor quantum dots, and the resulting changes in their electronic structure. Organic molecules with transition metal atoms that have a net magnetic moment and display hysteretic behaviour are known as single molecule magnets (SMM). The fabrication of CdTe quantum dots chemically doped with a controlled number of Mn atoms and with a number of carriers controlled either electrically or optically paves the way towards a new concept in nanomagnetism: the artificial single molecule magnet. Here we study the magnetic properties of a Mn-doped CdTe quantum dot for different charge states and show to what extent they behave like a single molecule magnet.
We theoretically study the optical properties of an InAs/GaAs quantum dot
(QD) near the area of the second-order resonance between an electron confined
in the QD and two longitudinal optical phonons. We present the absorption
spectra of an inhomogeneously broadened QD ensemble and show that the minimal
model needed for an accurate description of such a system needs to account for
3-phonon states. We study also the influence of the QD height to width ratio on
the optical properties of the polaron system. The dependence of the width of
the resonance and the position of the second-order resonant feature on the
height to width ratio is presented.
We investigate the possiblity of creating directed spin-polarized currents in a two-dimensional electron gas (2DEG) subject to an asymmetric magnetic field and an external adiabatic driving. We thereby generalize concepts of quantum charge ratchets to the case with spin. Due to the Zeeman term in the Hamiltonian, spin-up and -down electrons experience different effective potentials, which can be tailored to achieve net spin currents without corresponding charge currents. We consider ballistic, coherent transport in waveguides defined in a 2DEG, where the magnetic field modulation is induced by ferromagnetic stripes on top of the 2DEG.
Graphene nanoribbons are quasi-one-dimensional meterials with finite width. Characterizing a wide class of nanoribbons by edge shape and width, we make a systematic analysis of their electronic properties. The band gap structure of nanoribbons is shown to exhibit a valley structure with stream-like sequences of metallic or almost metallic nanoribbons. Among them, all zigzag nanoribbons are metallic, and armchair nanoribbons are metallic by period of 3. We find that these stream-like sequences correspond to equi-width curves, and that the band gap of chiral and armchair nanoribbons oscillate as a function of the width. Furthermore a possible application of nanoribbons to nanoelectronics is discussed.
We report on a double quantum dot which is formed in a width-modulated etched
bilayer graphene nanoribbon. A number of lateral graphene gates enable us to
tune the quantum dot energy levels and the tunneling barriers of the device
over a wide energy range. Charge stability diagrams and in particular
individual triple point pairs allow to study the tunable capacitive inter-dot
coupling energy as well as the spectrum of the electronic excited states on a
number of individual triple points. We extract a mutual capacitive inter-dot
coupling in the range of 2 - 6 meV and an inter-dot tunnel coupling on the
order of 1.5 {\mu}eV.
We present a theoretical study of the structural and electronic properties of
graphene monolayer functionalized with boron and nitrogen atoms substituting
carbon atoms. Our study is based on the ab initio calculations in the framework
of the density functional theory. We calculate the binding energies of the
functionalized systems, changes in the morphology caused by functionalization,
and further the band gap energy as a function of the concentration of dopants.
Moreover, we address the problem of possible clustering of dopants at a given
concentration. We define the clustering parameter to quantify the dependence of
the properties of the functionalized systems on the distribution of B/N atoms.
We show that clustering of B/N atoms in graphene is energetically unfavorable
in comparison to the homogenous distribution of dopants. For most of the
structures, we observe a nonzero energy gap that is only slightly dependent on
the concentration of the substituent atoms.
In this work we derive a theory of polariton condensation based on the theory
of interacting Bose particles. In particular, we describe self-consistently the
linear exciton-photon coupling and the exciton-nonlinearities, by generalizing
the Hartree-Fock-Popov description of BEC to the case of two coupled Bose
fields at thermal equilibrium. In this way, we compute the density-dependent
one-particle spectrum, the energy occupations and the phase diagram. The
results quantitatively agree with the existing experimental findings. We then
present the equations for the linear response of a polariton condensate and we
predict the spectral response of the system to external optical or mechanical
perturbations.
For the observation of Bose-Einstein condensation, excitons in cuprous oxide
are regarded as promising candidates due to their large binding energy and long
lifetime. High particle densities may be achieved by entrapment in a stress
induced potential. We consider a multi-component gas of interacting para- and
orthoexcitons in cuprous oxide confined in a three-dimensional potential trap.
Based on the Hartree-Fock-Bogoliubov theory, we calculate density profiles as
well as decay luminescence spectra which exhibit signatures of the separation
of the Bose-condensed phases.
Transparent and conductive ZnO:Ga thin films are prepared by laser molecular-beam epitaxy. Their electron properties were investigated by the temperature-dependent Hall-effect technique. The 300-K carrier concentration and mobility were about $n_s \sim 10^{16}$ cm$^{-3}$ and 440 cm$^{2}$/Vs, respectively. In the experimental `mobility vs concentration' curve, unusual phenomenon was observed, i.e., mobilities at $n_s \sim 5\times$ 10$^{18}$ cm$^{-3}$ are significantly smaller than those at higher densities above $\sim 10^{20}$ cm$^{-3}$. Several types of scattering centers including ionized donors and oxygen traps are considered to account for the observed dependence of the Hall mobility on carrier concentration. The scattering mechanism is explained in terms of inter-grain potential barriers and charged impurities. A comparison between theoretical results and experimental data is made. Comment: 5 pages, 1 figure, conference on II-VI compounds, RevTeX
Computer simulation of the hopping charge transport in disordered organic materials has been carried out explicitly taking into account charge-charge interactions. This approach provides a possibility to take into account dynamic correlations that are neglected by more traditional approaches like mean field theory. It was found that the effect of interaction is no less significant than the usually considered effect of filling of deep states by non-interacting carriers. It was found too that carrier mobility generally increases with the increase of carrier density, but the effect of interaction is opposite for two models of disordered organic materials: for the non-correlated random distribution of energies with Gaussian DOS mobility decreases with the increase of the interaction strength, while for the model with long range correlated disorder mobility increases with the increase of interaction strength.
The method of nonequilibrium Greens functions allows for a spatial and energetical resolution of the electron current in Quantum Cascade Lasers. While scattering does not change the spatial position of carriers, the entire spatial evolution of charge can be attributed to coherent transport by complex wave functions. We discuss the hierarchy of transport models and derive the density matrix equations as well as the hopping model starting from the nonequilibrium Greens functions approach. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Using oxide substrates for functional ceramic thin film deposition beyond their usual application as chemical inert, lattice-matched support for the films represents a novel concept in ceramic thin film research. The substrates are applied as a functional element in order to controllably modify the atom arrangement and the growth mode of ceramic prototype materials such as cuprate superconductors and colossal magnetoresistance manganites. One example is the use of epitaxial strain to adjust the relative positions of cations and anions in the film and thus modify their physical properties. The other makes use of vicinal cut SrTiO3 which enables the fabrication of regular nanoscale step and terrace structures. In YBa2Cu3O7-x thin films grown on vicinal cut SrTiO3 single crystals a regular array of antiphase boundaries is generated causing an anisotropic enhancement of flux-line pinning. In the case of La-Ca-Mn-O thin films grown on vicinal cut substrates it could be demonstrated that magnetic in-plane anisotropy is achieved. Comment: 6 pages
We show how multi-partite entanglement, such as of W-state form, can be
created in branched spin chain systems. We also discuss the preservation of
such entanglement, once created. The technique could be applied to actual spin
chain systems, or to other physical systems such as strings of coupled quantum
dots, molecules or atoms.
Recently, Ferreira da Silva et al. [3] have performed a gradient pattern analysis of a canonical sample set (CSS) of scanning force microscopy (SFM) images of p-Si. They applied the so-called Gradient Pattern Analysis to images of three typical p-Si samples distinguished by different absorption energy levels and aspect ratios. Taking into account the measures of spatial asymmetric fluctuations they interpreted the global porosity not only in terms of the amount of roughness, but rather in terms of the structural complexity (e.g., walls and fine structures as slots). This analysis has been adapted in order to operate in a OpenGL flyby environment (the StrFB code), whose application give the numerical characterization of the structure during the flyby real time. Using this analysis we compare the levels of asymmetric fragmentation of active porosity related to different materials as p-Si and "porous diamond-like" carbon. In summary we have shown that the gradient pattern analysis technique in a flyby environment is a reliable sensitive method to investigate, qualitatively and quantitatively, the complex morphology of active nanostructures.
We present and discuss an algorithm to identify and characterize the long icosahedral structures (staggered pentagonal nanowires with 1-5-1-5 atomic structure) that appear in Molecular Dynamics simulations of metallic nanowires of different species subjected to stretching. The use of the algorithm allows the identification of pentagonal rings forming the icosahedral structure as well as the determination of its number, and the maximum length of the pentagonal nanowire. The algorithm is tested with some ideal structures to show its ability to discriminate between pentagonal rings and other ring structures. We applied the algorithm to Ni nanowires with temperatures ranging between 4K and 865K, stretched along the [100] direction. We studied statistically the formation of pentagonal nanowires obtaining the distributions of the maximum length and number of rings as function of the temperature. The pentagonal nanowire maximum length distribution presents a peaked shape, with peaks locate at fixes distances whose separa-tion corresponds to the distance between two consecutive pentagonal rings.
We present a solid state implementation of quantum computation, which
improves previously proposed optically driven schemes. Our proposal is based on
vertical arrays of quantum dots embedded in a mesoporous material which can be
fabricated with present technology. We study the feasibility of performing
quantum computation with different mesoporous matrices. We analyse which matrix
materials ensure that each individual stack of quantum dots can be considered
isolated from the rest of the ensemble-a key requirement of our scheme. This
requirement is satisfied for all matrix materials for feasible structure
parameters and GaN/AlN based quantum dots. We also show that one dimensional
ensembles substantially improve performances, even of CdSe/CdS based quantum
dots.
We show that $1/f$-noise in the variable range hopping regime is related to
transitions of many-electrons clusters (fluctuators) between two almost
degenerate states. Giant fluctuation times necessary for $1/f$-noise are
provided by slow rate of simultaneous tunneling of many localized electrons and
by large activation barriers for their consecutive rearrangements. The Hooge
constant steeply grows with decreasing temperature because it is easier to find
a slow fluctuator at lower temperatures. Our conclusions qualitatively agree
with the low temperature observations of $1/f$-noise in p-type silicon and
GaAs.
This article reviews recent years' progress in the low temperature analysis of standard models of spin glass order such as the Sherrington-Kirkpatrick (SK) model. Applications to CdTe/CdMnTe layered systems and explanation of glassy antiferromagnetic order at lowest temperatures stimulated us to study in detail the beautifully complex physical effects of replica symmetry breaking (RSB).We discuss analytical ideas based on highly precise numerical data which lead to the construction of relatively simple effective field theories for the SK model and help to understand the mysterious features of its exact solution. The goal is to find construction principles for the theory of interplay between frustrated magnetic order and various relevant physical degrees of freedom. The emphasis in this article is on the role of Parisi's RSB, which surprisingly creates critical phenomena in the low temperature limit despite the absence of a standard phase transition.
We have studied theoretically the effect of a tuneable lateral confinement on two-dimensional hole systems realised in III-V semiconductor heterostructures. Based on the 4x4 Luttinger description of the valence band, we have calculated quasi-onedimensional (quasi-1D) hole subband energies and anisotropic Lande g-factors. Confinement-induced band mixing results in the possibility to manipulate electronic and spin properties of quasi-1D hole states over a much wider range than is typically possible for confined conduction-band electrons. Our results are relevant for recent experiments where source-drain-bias spectroscopy was used to measure Zeeman splitting of holes in p-type quantum point contacts.
In a unified approach we consider transport properties of 1D and quasi-1D waveguides with rough surfaces. Main attention is paid to the possibility of perfect transmission of waves due to specific long-range correlations in the surface profiles. First, we show how to construct random profiles that lead to a complete transparency of waveguides with one open channel. Then, we present analytical results for many-mode waveguides. It was revealed that by a proper choice of correlations in surface profiles the transmission through such quasi-1D waveguides is described by a coset of non-interacting 1D channels with a perfect transmission along each one. The number of these conducting modes is governed by the control parameter, and can be equal to the total number of channels. Therefore, the waveguides can be completely transparent in some region of frequency of incoming waves. This unexpected phenomenon is discussed in connection with the violation of the single-parameter scaling for surface scattering.
We have used normal metal-insulator-superconductor (NIS) tunnel junction pairs, known as SINIS structures, for ultrasensitive thermometry at sub-Kelvin temperatures. With the help of these thermometers, we have developed an ac-technique to measure the electron-phonon (e-p) scattering rate directly, without any other material or geometry dependent parameters, based on overheating the electron gas. The technique is based on Joule heating the electrons in the frequency range DC-10 MHz, and measuring the electron temperature in DC. Because of the nonlinearity of the electron-phonon coupling with respect to temperature, even the DC response will be affected, when the heating frequency reaches the natural cut-off determined by the e-p scattering rate. Results on thin Cu films show a $T^{4}$ behavior for the scattering rate, in agreement with indirect measurement of similar samples and numerical modeling of the non-linear response.
We have studied the electron-phonon (e-p) interaction in thin Cu and Au films
at sub-Kelvin temperatures with the help of the hot electron effect, using
symmetric normal metal-insulator-superconductor tunnel junction pairs as
thermometers. By Joule heating the electron gas and measuring the electron and
the lattice temperatures simultaneously, we show that the electron-phonon
scattering rate follows a $T^{4}$ temperature dependence in both metals. The
result is in accordance with the theory of e-p scattering in disordered films
with vibrating boudaries and impurities, in contrast to the $T^{3}$-law
expected for pure samples, and $T^{2}$-law for static disorder.
The properties of a dilute electron gas, coupled to the lattice degrees of freedom, are studied and compared with the properties of an electron gas at half-filling, where spinless fermions with two orbitals per lattice site are considered. The simplest model which includes both the local electron-lattice interaction of the Jahn-Teller type and the electronic correlations is the $E\otimes\beta$-Jahn-Teller-Hubbard model. We analyze the formation and stability of Jahn-Teller polarons and bipolarons, respectively. Our approach is based on a hopping expansion in the strong-coupling regime. The results are compared with recently published findings for the Hubbard-Holstein model [1,2]. The special case of the Jahn-Teller-Hubbard model at half-filling is mapped on a spin-1/2 Heisenberg model with phonon-dependent coupling constants. This has been derived within a projection formalism that provides a continued-fraction representation of the Green's function. We study the exact solution for two and three particles and compare it with the effective theory on the infinite lattice with one particle per site.
The phononless hopping conductivity of a disordered system with localized states is studied in a broad range of frequencies by straightforward computer simulations taking into account Coulomb interactions. At sufficiently low temperatures, the conductivity is determined by the zero-phonon absorption of the photon by pairs of states. The laser frequency dependence of the conductivity is examined and compared with the analytical model of Efros and Shklovskii and with recent experimental data obtained on Si:P. The range of parameters is determined, for which the conductivity dependence on photon energy best reproduces the experimental results. Comment: Presented as a poster at TIDS11, to be published in pss(c)