31 research outputs found
Topological interface states -- a possible path towards a Landau-level laser in the THz regime
Volkov-Pankratov surface bands arise in smooth topological interfaces, i.e.
interfaces between a topological and a trivial insulator, in addition to the
chiral surface state imposed by the bulk-surface correspondence of topological
materials. These two-dimensional bands become Landau-quantized if a magnetic
field is applied perpendicular to the interface. I show that the energy scales,
which are typically in the 10-100 meV range, can be controlled both by the
perpendicular magnetic field and the interface width. The latter can still be
varied with the help of a magnetic-field component in the interface. The Landau
levels of the different Volkov-Pankratov bands are optically coupled, and their
arrangement may allow one to obtain population inversion by resonant optical
pumping. This could serve as the elementary brick of a multi-level laser based
on Landau levels. Moreover, the photons are absorbed and emitted either
parallel or perpendicular to the magnetic field, respectively in the Voigt and
Faraday geometry, depending on the Volkov-Pankratov bands and Landau levels
involved in the optical transitions.Comment: 7 pages, 3 figure
Volkov-Pankratov states in topological superconductors
We study the in-gap states that appear at the boundaries of both 1D and 2D
topological superconductors. While the massless Majorana quasiparticles are
guaranteed to arise by the bulk-edge correspondence, we find that they could be
accompanied by massive Volkov-Pankratov (VP) states which are present only when
the interface is sufficiently smooth. These predictions can be tested in an
s-wave superconductor with Rashba spin-orbit coupling placed on top of a
magnetic domain wall. We calculate the spin-resolved local density of states of
the VP states about the band inversion generated by a magnetic domain wall and
find that they are oppositely spin-polarized on either side of the topological
phase boundary. We also demonstrate that the spatial position, energy-level
spacing, and spin polarization of the VP states can be modified by the
introduction of in-plane electric fields.Comment: 10 pages, 8 figure
Magnetoplasmons of the tilted-anisotropic Dirac cone material (BEDT-TTF)I
We study the collective modes of a low-energy continuum model of the
quasi-two-dimensional electron liquid in a layer of the organic compound
(BEDT-TTF)I in a perpendicular magnetic field. As testified by
zero magnetic field transport experiments and \textit{ab initio} theory, this
material hosts both massless and massive low-energy carriers, the former being
described by tilted and anisotropic Dirac cones. The polarizability of these
cones is anisotropic, and two sets of magnetoplasmon modes occur between any
two cyclotron resonances. We show that the tilt of the cones causes a unique
intervalley damping effect: the upper hybrid mode of one cone is damped by the
particle-hole continuum of the other cone in generic directions. We analyse how
the presence of massive carriers affects the response of the system, and
demonstrate how doping can tune (BEDT-TTF)I between regimes of
isotropic and anisotropic screening.Comment: 14 pages, 9 figure
Magneto-optics of quasirelativistic electrons in graphene with an inplane electric field and in tilted Dirac cones in α-(BEDT TTF)2I3
Massless Dirac fermions occur as low-energy modes in several quasi-two-dimensional condensed matter
systems such as graphene, the surface of bulk topological insulators, and in layered organic semiconductors.
When the rotational symmetry in such systems is reduced either by an in-plane electric field or an intrinsic tilt
of the Dirac cones, the allowed dipolar optical transitions evolve from a few selected transitions into a wide fan
of interband transitions. We show that the Lorentz covariance of the low-energy carriers allows for a concise
analysis of the emergingmagneto-optical properties. We predict that infrared absorption spectra yield quantitative
information on the tilted Dirac cone structure in organic compounds such as α-(BEDT-TTF)2I3
Role of the Berry curvature on BCS-type superconductivity in two-dimensional materials
We theoretically investigate how the Berry curvature, which arises in
multi-band structures when the electrons can be described by an effective
single-band Hamiltonian, affects the superconducting properties of
two-dimensional electronic systems. Generically the Berry curvature is coupled
to electric fields beyond those created by the periodic crystal potential. A
potential source of such electric fields, which vary slowly on the lattice
scale, is the mutual interaction between the electrons. We show that the Berry
curvature provides additional terms in the Hamiltonian describing interacting
electrons within a single band. When these terms are taken into account in the
framework of the usual BCS weak-coupling treatment of a generic attractive
interaction that allows for the formation of Cooper pairs, the coupling
constant is modified. In pure singlet and triplet superconductors, we find that
the Berry curvature generally lowers the coupling constant and thus the
superconducting gap and the critical temperature as a function of doping. From
an experimental point of view, a measured deviation from the expected BCS
critical temperature upon doping, e.g. in doped two-dimensional
transition-metal dichalcogenides, may unveil the strength of the Berry
curvature.Comment: 14 pages, 3 figure
The quantum Hall effect in graphene - a theoretical perspective
This short theoretical review deals with some essential ingredients for the
understanding of the quantum Hall effect in graphene in comparison with the
effect in conventional two-dimensional electron systems with a parabolic band
dispersion. The main difference between the two systems stems from the
"ultra-relativistic" character of the low-energy carriers in graphene, which
are described in terms of a Dirac equation, as compared to the non-relativistic
Schr\"odinger equation used for electrons with a parabolic band dispersion. In
spite of this fundamental difference, the Hall resistance quantisation is
universal in the sense that it is given in terms of the universal constant
h/e^2 and an integer number, regardless of whether the charge carriers are
characterised by Galilean or Lorentz invariance, for non-relativistic or
relativistic carriers, respectively.Comment: 9 pages, 4 figures; brief review article for Comptes Rendus de
l'Academie des Sciences; references added with respect to previous versio
Competing Fractional Quantum Hall and Electron Solid Phases in Graphene
We report experimental observation of the reentrant integer quantum Hall
effect in graphene, appearing in the N2 Landau level. Similar to
high-mobility GaAs/AlGaAs heterostructures, the effect is due to a competition
between incompressible fractional quantum Hall states, and electron solid
phases. The tunability of graphene allows us to measure the - phase
diagram of the electron-solid phase. The hierarchy of reentrant states suggest
spin and valley degrees of freedom play a role in determining the ground state
energy. We find that the melting temperature scales with magnetic field, and
construct a phase diagram of the electron liquid-solid transition