Mikhail E. Portnoi

University of Exeter | UoE · Department of Physics and Astronomy

We show that a planar array of bipolar waveguides in graphene can be used to engineer gapped and tilted two-dimensional Dirac cones within the electronic band structure. The presence of these gapped and tilted Dirac cones is demonstrated through a superlattice tight-binding model and verified using a transfer matrix calculation. By varying the appl...

We derive the angular generation density of photoexcited carriers in gapless and gapped Bernal bilayer graphene. Exploiting the strong anisotropy of the band structure of bilayer graphene at low energies due to trigonal warping, we show that charge carriers belonging to different valleys propagate to different sides of the light spot upon photoexci...

Well-known Mott's formula links the thermoelectric power characterized by the Seebeck coefficient to conductivity. We calculate analytically the thermoelectric current and Seebeck coefficient in one-dimensional systems and show that, while the prediction of Mott's formula is valid for Dirac fermions, it is misleading for the carriers having a parab...

Supplemetary materials to our PRL paper, A. Kucherik, A. Osipov, V. Samyshkin, R. R. Hartmann, A. V. Povolotskiy, and M. E. Portnoi "Polarization-Sensitive Photoluminescence from Aligned Carbon Chains Terminated by Gold Clusters" Phys. Rev. Lett. 132, 056902 (2024).

We show that the (2+1)-dimensional massless Dirac equation, which includes a tilt term, can be reduced to the biconfluent Heun equation for a broad range of scalar confining potentials, including the well-known Morse potential. Applying these solutions, we investigate a bipolar electron waveguide in 8--$Pmmn$ borophene, formed by a well and barrier...

We synthesize a thin film composed of long carbyne chains terminated by gold clusters and study its optical properties. The presence of gold particles stabilizes longer chains and leads to their alignment. We show that the gold clusters also act as a source of electron doping, thus, changing the intensity of photoluminescence from quadratic depende...

Quasiparticles emerging in crystalline materials can possess a binary flavor known as the valley quantum number which can be used as a basis to encode information in an emerging class of valleytronic devices. Here we show that two-dimensional semimetals with tilted Dirac cones in the electronic band structure exhibit spatial separation of carriers...

We synthesize a thin film composed of long carbyne chains terminated by gold clusters and study its optical properties. The presence of gold particles stabilizes longer chains and leads to their alignment. We show that the gold clusters also act as a source of electron doping thus changing the intensity of photoluminescence from quadratic dependenc...

We employ a tight-binding model to calculate the optical selection rules of gold-terminated carbyne chains in the presence of an applied electric field. We show that both the magnitude of the edge-state gap and the strength of optical transitions across it can be tuned via the Stark effect. In the case of sufficiently long carbyne chains, the dipol...

Two-dimensional semimetals with tilted Dirac cones in the electronic band structure are shown to exhibit spatial separation of carriers belonging to different valleys under illumination. In stark contrast to gapped Dirac materials this optovalleytronic phenomenon occurs in systems with intact inversion and time-reversal symmetry that host massless...

We study the momentum alignment of photoexcited carriers and the optical control of valley population in gapless and gapped two-dimensional Dirac materials. The trigonal warping effect leads to the spatial separation of charge carriers belonging to different valleys upon linearly-polarized high-frequency photoexcitation. Valley separation in gapped...

We develop an analytic model for an ideal polyyne ring which describes the induced terahertz (THz) gap in the molecular spectrum due to the Stark effect. This simple model can also be used to describe an odd-dimered cyclocarbon which has undergone a spontaneous symmetry-breaking event (due to the Jahn-Teller effect) as an effective dipole across an...

The interband optical absorption of linearly polarized light by two-dimensional (2D) semimetals hosting tilted and anisotropic Dirac cones in the band structure is analysed theoretically. Supercritically tilted (type-II) Dirac cones are characterized by an absorption that is highly dependent on the incident photon polarization and frequency and is...

We show that if the solutions to the (2+1)-dimensional massless Dirac equation for a given one-dimensional (1D) potential are known, then they can be used to obtain the eigenvalues and eigenfunctions for the same potential, orientated at an arbitrary angle, in a 2D Dirac material possessing tilted, anisotropic Dirac cones. This simple set of transf...

We develop an analytic model for an ideal polyyne ring which describes the induced THz gap in the molecular spectrum due to the Stark effect. This simple model can also be used to describe an odd-dimered cyclocarbon which has undergone a spontaneous symmetry-breaking event (due to the Jahn-Teller effect) as an effective dipole across an ideal ring....

A concept of a middle- and far-infrared detector has been proposed. The detector is built as a planar collection of parallel graphene strips of different length and width. The feature of the detector scheme is the concurrent utilization of two different detection mechanisms: excitation in the given frequency range of low-frequency interband transit...

The interband optical absorption of linearly polarised light by two-dimensional (2D) semimetals hosting tilted and anisotropic Dirac cones in the bandstructure is analysed theoretically. Super-critically tilted (type-II) Dirac cones are characterised by an absorption that is highly dependent on the incident photon polarisation and frequency, and is...

We show that if the solutions to the (2+1)-dimensional massless Dirac equation for a given 1D potential are known, then they can be used to obtain the eigenvalues and eigenfunctions for the same potential, orientated at an arbitrary angle, in a tilted anisotropic 2D Dirac material. This simple set of transformations enables all the exact and quasi-...

We predict an optical effect associated with systems, which exhibit topologically protected states separated by a finite distance. We develop a tight-binding model to calculate the optical selection rules in linear chains of atoms of different lengths, and show the crucial importance of edge states. For long enough molecules the interband transitio...

We predict an optical effect associated with systems which exhibit topologically protected states separated by a finite distance. We develop a tight-binding model to calculate the optical selection rules in linear chains of atoms of different lengths, and show the crucial importance of edge states. For long enough molecules the interband transition...

The double-well problem for the two-dimensional Dirac equation is solved for a family of quasi-one-dimensional potentials in terms of confluent Heun functions. We demonstrate that for a double well separated by a barrier, both the energy-level splitting associated with the wave-function overlap of well states and the gap size of the avoided crossin...

We show analytically that the ability of Dirac materials to localize an electron in both a barrier and a well can be utilized to open a pseudogap in graphene's spectrum. By using narrow top gates as guiding potentials, we demonstrate that graphene bipolar waveguides can create a nonmonotonous one-dimensional dispersion along the electron waveguide,...

We show analytically that the ability of Dirac materials to localize an electron in both a barrier and a well can be utilized to open a pseudo-gap in graphene's spectrum. By using narrow top-gates as guiding potentials, we demonstrate that graphene bipolar waveguides can create a non-monotonous one-dimensional dispersion along the electron waveguid...

The double-well problem for the two-dimensional Dirac equation is solved for a family of quasi-one-dimensional potentials in terms of confluent Heun functions. We demonstrate that for a double well separated by a barrier, both the energy level splitting associated with the wavefunction overlap of well states, and the gap size of the avoided crossin...

We studied monoatomic linear carbon chains stabilised by gold nanoparticles at- tached to their ends and deposited on a solid substrate. We observe spectral features of straight chains containing from 8 to 24 atoms. Low temperature PL spectra reveal characteristic triplet ne-structures that repeat themselves for carbon chains of dier- ent lengths....

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

We use an ab initio approach to design and study a two-dimensional material—a planar array of carbon nanotubes separated by an optimal distance defined by the van der Waals interaction. We show that the energy spectrum for an array of quasimetallic nanotubes is described by a strongly anisotropic hyperbolic dispersion and formulate a model low-ener...

We theoretically investigate the problem of an electron confined to a nanohelix between two parallel gates modelled as charged wires. The double-gated nanohelix system is a binary superlattice with properties highly sensitive to the gate voltages. In particular, the band structure exhibits energy band crossings for certain combinations of gate volt...

We synthesise and deposit on a substrate monoatomic chains of carbon atoms stabilised by gold nanoparticles. Raman, absorption and photoluminescence (PL) spectra reveal resonant features of straight polyyne chains, that is significantly beyond the theoretical stability limit of 6 atoms for free-standing carbon chains. Polyyne is a direct band gap s...

We use the robust nearest-neighbor tight-binding approximation to study the same footing interband dipole transitions in narrow-bandgap carbon nanotubes (CNTs) and graphene nanoribbons (GNRs). It is demonstrated that curvature effects in metallic single-walled CNTs and edge effects in gapless GNRs not only open up bandgaps, which typically correspo...

We use an ab-initio approach to design and study a novel two-dimensional material - a planar array of carbon nanotubes separated by an optimal distance defined by the van der Waals interaction. We show that the energy spectrum for an array of quasi-metallic nanotubes is described by a strongly anisotropic hyperbolic dispersion and formulate a model...

We consider the problem of confining the famously elusive Dirac-like quasiparticles, as found in some recently discovered low-dimensional systems. After briefly surveying the existing theoretical proposals for creating bound states in Dirac materials, we study relativistic excitations with a position-dependent mass term. With the aid of an exactly-...

We consider the problem of confining the famously elusive Dirac-like quasiparticles, as found in some recently discovered low-dimensional systems. After briefly surveying the existing theoretical proposals for creating bound states in Dirac materials, we study relativistic excitations with a position-dependent mass term. With the aid of an exactly-...

We use the robust nearest-neighbour tight-binding approximation to study on the same footing interband dipole transitions in narrow-bandgap carbon nanotubes and graphene nanoribbons. It is demonstrated that curvature effects in metallic single-walled carbon nanotubes and edge effects in gapless graphene nanoribbons not only open up bang gaps, which...

In this brief review, we survey the problem of electrostatic confinement of massless Dirac particles, via a number of exactly solvable one- and two-body models. By considering bound states at zero energy, we present a route to obtain truly discrete states of massless Dirac particles in scalar potentials, circumventing the celebrated Klein tunnellin...

We outline a range of proposals on using quantum rings and nanohelices for terahertz device implementations. We show that an Aharonov-Bohm quantum ring system and a double-gated quantum ring system both permit control over the polarization properties of the associated terahertz radiation. In addition, we review the superlattice properties of a math...

In this brief review, the problem of electrostatic confinement of massless Dirac particles is surveyed via a number of exactly solvable one‐ and two‐body models. By considering bound states at zero energy, a route to obtain truly discrete states of massless Dirac particles in scalar potentials is presented, circumventing the celebrated Klein tunnel...

We show that strong light-matter coupling can be used to overcome a long-standing problem that has prevented efficient optical emission from carbon nanotubes. The luminescence from the nominally bright exciton state of carbon nanotubes is quenched due to the fast non-radiative scattering to the dark exciton state having a lower energy. We present a...

We outline a range of proposals on using quantum rings and nanohelices for terahertz device implementations. We show that an Aharonov-Bohm quantum ring system and a double-gated quantum ring system both permit control over the polarization properties of the associated terahertz radiation. In addition, we review the superlattice properties of a math...

We study the momentum alignment phenomenon and the optical control of valley population in gapless and gapped graphene-like materials. We show that the trigonal warping effect allows for the spatial separation of carriers belonging to different valleys via the application of linearly polarized light. Valley separation in gapped materials can be det...

Triangular and hexagonal multilayer phosphorene quantum dots with armchair and zigzag terminations are investigated with the orthogonal tight-binding model. The effect of increasing the number of layers is revealed. The obtained results show that in a small size multilayer quantum dot, the edge states are as sensitive to the out-of-plane external e...

This chapter is devoted to optical properties of so-called Aharonov-Bohm quantum rings (quantum rings pierced by a magnetic flux resulting in Aharonov-Bohm oscillations of their electronic spectra) in external electromagnetic fields. It studies two problems. The first problem deals with a single-electron Aharonov-Bohm quantum ring pierced by a magn...

We show that the curvature effects in quasi-metallic carbon nanotubes and edge effects in narrow-gap graphene nanoribbons not only open band gaps in the THz range but also result in giant enhancement of the transition probabilities across these gaps. This makes these nanostructures perspective candidates for sources and detectors of THz radiation.

The equilibrium geometry and electronic band structure of a planar array of carbon nanotubes are studied with the use of the Quantum Espresso code - a plane-wave realisation of the density functional theory (DFT). The many-electron correlations and van der Waals corrections are taken into account. The optimal distance between nanotubes in the array...

In this paper we study the effect of absorption peak correlation in finite length carbon nanotubes and graphene nanoribbons. It is shown, in the orthogonal {\pi}-orbital tight-binding model with the nearest neighbor approximation, that if the ribbon width is a half of the tube circumference the effect takes place for all achiral ribbons (zigzag, ar...

In this paper we study the effect of absorption peak correlation in finite length carbon nanotubes and graphene nanoribbons. It is shown, in the orthogonal π-orbital tight-binding model with the nearest neighbor approximation, that if the ribbon width is a half of the tube circumference the effect takes place for all achiral ribbons (zigzag, armcha...

We obtain exact solutions to the two-dimensional (2D) Dirac equation for the one-dimensional Pöschl-Teller potential which contains an asymmetry term. The eigenfunctions are expressed in terms of Heun confluent functions, while the eigenvalues are determined via the solutions of a simple transcendental equation. For the symmetric case, the eigenfun...

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We theoretically investigate the optical functionality of a semiconducting quantum ring manipulated by two electrostatic lateral gates used to induce a double quantum well along the ring. The well parameters and corresponding inter-level spacings, which lie in the THz range, are highly sensitive to the gate voltages. Our analysis shows that selecti...

We theoretically investigate the optical functionality of a semiconducting quantum ring manipulated by two electrostatic lateral gates used to induce a double quantum well along the ring. The well parameters and corresponding inter-level spacings, which lie in the THz range, are highly sensitive to the gate voltages. Our analysis shows that selecti...

Searching for new states of matter and unusual quasi-particles in emerging materials and especially low-dimensional systems is one of the major trends in contemporary condensed matter physics. Dirac materials, which host quasi-particles which are described by ultrarelativistic Dirac-like equations, are of a significant current interest from both a...

We propose a scheme to maintain a robust radiative ground state in semiconductor single-walled carbon nanotubes. For that purpose, we employ a microcavity operating in the regime of strong coupling between the confined electromagnetic mode of the cavity and the excitonic resonance in an ensemble of carbon nanotubes. A high value of the interband di...

We obtain exact solutions to the two-dimensional (2D) Dirac equation for the one-dimensional P\"oschl-Teller potential which contains an asymmetry term. The eigenfunctions are expressed in terms of Heun confluent functions, while the eigenvalues are determined via the solutions of a simple transcendental equation. For the symmetric case, the eigenf...

We study electronic and optical properties of single layer phosphorene quantum dots with various shapes, sizes, and edge types (including disordered edges) subjected to an external electric field normal to the structure plane. Compared to graphene quantum dots, in phosphorene clusters of similar shape and size there is a set of edge states with ene...

We study electronic and optical properties of single layer phosphorene quantum dots with various shapes, sizes, and edge types (including disordered edges) subjected to an external electric field normal to the structure plane. Compared to graphene quantum dots, in phosphorene clusters of similar shape and size there is a set of edge states with ene...

The electrons found in Dirac materials are notorious for being difficult to manipulate due to the Klein phenomenon and absence of backscattering. Here we investigate how spatial modulations of the Fermi velocity in two-dimensional Dirac materials can give rise to localization effects, with either full (zero-dimensional) confinement or partial (one-...

Analytic solutions of the quantum relativistic two-body problem are obtained for an interaction potential modeled as a one-dimensional smooth square well. Both stationary and moving pairs are considered and the limit of the {\delta}-function interaction is studied in depth. Our result can be utilized for understanding excitonic states in narrow-gap...

Analytic solutions of the quantum relativistic two-body problem are obtained for an interaction potential modeled as a one-dimensional smooth square well. Both stationary and moving pairs are considered and the limit of the δ-function interaction is studied in depth. Our result can be utilized for understanding excitonic states in narrow-gap carbon...

The electrons found in Dirac materials are notorious for being difficult to manipulate due to the Klein phenomenon and absence of backscattering. Here we investigate how spatial modulations of the Fermi velocity in two-dimensional Dirac materials can give rise to localization effects, with either full (zero-dimensional) confinement or partial (one-...

We present an analytical tight-binding theory of the optical properties of graphene nanoribbons with zigzag edges. Applying the transfer matrix technique to the nearest-neighbor tight-binding Hamiltonian, we derive analytical expressions for electron wave functions and optical transition matrix elements for incident light polarized along the struct...

We present an analytical tight-binding theory of the optical properties of graphene nanoribbons with zigzag edges. Applying the transfer matrix technique to the nearest-neighbor tight-binding Hamiltonian, we derive analytical expressions for electron wave functions and optical transition matrix elements for incident light polarized along the struct...

Understanding unwanted mutual interactions between devices at the nanoscale is crucial for the study of the electromagnetic compatibility in nanoelectronic and nanophotonic systems. Anomalous electromagnetic coupling (crosstalk) between nanodevices may arise from the combination of electromagnetic interaction and quantum entanglement. In this paper...

We show that the electromagnetic coupling at the nanoscale may be accompanied by another coupling mechanism, related to quantum entanglement. Consequently, a combined "electromagnetic-quantum" coupling is created, which stipulates long-distance and long-living interactions in electric circuits. Manifestation of this effect in electromagnetic compat...

Subjecting a nanohelix to a transverse electric field gives rise to superlattice behavior with tunable electronic properties. We theoretically investigate such a system and find Bloch oscillations and negative differential conductance when a longitudinal electric field (along the nanohelix axis) is also applied. Furthermore, we study dipole transit...

Subjecting a nanohelix to a transverse electric field gives rise to superlattice behavior with tunable electronic properties. We theoretically investigate such a system and find Bloch oscillations and negative differential conductance when a longitudinal electric field (along the nanohelix axis) is also applied. Furthermore, we study dipole transit...

We show how it is possible to trap two-dimensional massless Dirac fermions in spatially inhomogeneous magnetic fields, as long as the formed magnetic quantum dot (or ring) is of a slowly decaying nature. It is found that a modulation of the depth of the magnetic quantum dot leads to successive confinement-deconfinement transitions of vortexlike sta...