Maxim Trushin

Photothermoelectric and Photoelectric Contributions to Light Detection in Metal−Graphene−Metal Photodetectors

Graphenes high mobility and Fermi velocity, combined with its constant light absorption in the visible to far-infrared range, make it an ideal material to fabricate high-speed and ultrabroadband photodetectors. However, the precise mechanism of photodetection is still debated. Here, we report wavelength and polarization-dependent measurements of metal-graphene-metal photodetectors. This allows us to quantify and control the relative contributions of both photothermo- and photoelectric effects, both adding to the overall photoresponse. This paves the way for a more efficient photodetector design for ultrafast operating speeds.

Minimum electrical and thermal conductivity of graphene: a quasiclassical approach.

We investigate the minimum conductivity of graphene within a quasiclassical approach taking into account electron-hole coherence effects which stem from the chiral nature of low energy excitations. Relying on an analytical solution of the kinetic equation in the electron-hole coherent and incoherent cases, we study both the electrical and the thermal conductivity whose relation satisfies the Wiedemann-Franz law. We find that most of the previous findings based on the Boltzmann equation are restricted to only high mobility samples where electron-hole coherence effects are not sufficient.

Pseudospin in optical and transport properties of graphene

We show that the pseudospin, being an additional degree of freedom for carriers in graphene, can be efficiently controlled by means of the electron-electron interactions which, in turn, can be manipulated by changing the substrate. In particular, an out-of-plane pseudospin component can occur leading to a zero-field Hall current as well as to polarization-sensitive interband optical absorption.

Anisotropic current-induced spin accumulation in the two-dimensional electron gas with spin-orbit coupling

We investigate the magnetoelectric (or inverse spin-galvanic) effect in the two dimensional electron gases with both Rashba and Dresselhaus spin-orbit coupling using an exact solution of the Boltzmann equation for electron spin and momentum. The spin response to an in-plane electric field turns out to be highly anisotropic while the usual charge conductivity remains isotropic, contrary to earlier statements.

Anisotropic magnetoresistance of spin-orbit coupled carriers scattered from polarized magnetic impurities

Anisotropic magnetoresistance (AMR) is a relativistic magnetotransport phenomenon arising from combined effects of spin-orbit coupling and broken symmetry of a ferromagnetically ordered state of the system. In this work we focus on one realization of the AMR in which spin-orbit coupling enters via specific spin-textures on the carrier Fermi surfaces and ferromagnetism via elastic scattering of carriers from polarized magnetic impurities. We report detailed heuristic examination, using model spin-orbit coupled systems, of the emergence of positive AMR (maximum resistivity for magnetization along current), negative AMR (minimum resistivity for magnetization along current), and of the crystalline AMR (resistivity depends on the absolute orientation of the magnetization and current vectors with respect to the crystal axes) components. We emphasize potential qualitative differences between pure magnetic and combined electromagnetic impurity potentials, between short-range and long-range impurities, and between spin-1/2 and higher spin-state carriers. Conclusions based on our heuristic analysis are supported by exact solutions to the integral form of the Boltzmann transport equation in archetypical two-dimensional electron systems with Rashba and Dresselhaus spin-orbit interactions and in the three-dimensional spherical Kohn-Littinger model. We include comments on the relation of our microscopic calculations to standard phenomenology of the full angular dependence of the AMR, and on the relevance of our study to realistic, two-dimensional conduction-band carrier systems and to anisotropic transport in the valence band of diluted magnetic semiconductors.

Optical absorption by Dirac excitons in single-layer transition-metal dichalcogenides

We develop an analytically solvable model able to qualitatively explain nonhydrogenic exciton spectra observed recently in two-dimensional (2D) semiconducting transition-metal dichalcogenides. Our exciton Hamiltonian explicitly includes additional angular momentum associated with the pseudospin degree of freedom unavoidable in 2D semiconducting materials with honeycomb structure. We claim that this is the key ingredient for understanding the nonhydrogenic exciton spectra that was missing so far.

Anisotropic photoconductivity in graphene

We investigate the photoconductivity of graphene within the relaxation time approximation. In the presence of the inter-band transitions induced by the linearly polarized light the photoconductivity turns out to be highly anisotropic due to the pseudospin selection rule for Dirac-like carriers. The effect can be observed in clean undoped graphene samples and can be utilized for light polarization detection.

Conductivity of graphene: How to distinguish between samples with short- and long-range scatterers

Applying a quasiclassical equation to carriers in graphene, we found a way how to distinguish between samples with the domination of short- and long-range scatterers from the conductivity measurements. The model proposed explains recent transport experiments with chemically doped as well as suspended graphene.

Polarization-sensitive absorption of THz radiation by interacting electrons in chirally stacked multilayer graphene

We show that the opacity of a clean multilayer graphene flake depends on the helicity of the circular polarized electromagnetic radiation. The effect can be understood in terms of the pseudospin selection rules for the interband optical transitions in the presence of exchange electron–electron interactions which alter the pseudospin texture in momentum space. The interactions described within a semi-analytical Hartree–Fock approach lead to the formation of topologically different broken symmetry states characterized by Chern numbers and zero-field anomalous Hall conductivities.

Finite conductivity minimum in bilayer graphene without charge inhomogeneities

Boltzmann transport theory fails near the linear band crossing of single-layer graphene and near the quadratic band crossing of bilayer graphene. We report on a numerical study which assesses the role of interband coherence in transport when the Fermi level lies near the band-crossing energy of bilayer graphene. We find that interband coherence enhances conduction, and that it plays an essential role in bilayer graphene’s minimum conductivity phenomena. This behavior is qualitatively captured by an approximate theory which treats interband coherence in a relaxation-time approximation. On the basis of this short-range-disorder model study, we conclude that electron-hole puddle formation is not a necessary condition for finite conductivity in bilayer graphene at zero average carrier density.