43 research outputs found
Transfer matrix approach for the Kerr and Faraday rotation in layered nanostructures
To study the optical rotation of the polarization of light incident on
multilayer systems consisting of atomically thin conductors and dielectric
multilayers we present a general method based on transfer matrices. The
transfer matrix of the atomically thin conducting layer is obtained using the
Maxwell equations. We derive expressions for the Kerr (Faraday) rotation angle
and for the ellipticity of the reflected (transmitted) light as a function of
the incident angle and polarization of the light. The method is demonstrated by
calculating the Kerr (Faraday) angle for bilayer graphene in the quantum
anomalous Hall state placed on the top of dielectric multilayers. The optical
conductivity of the bilayer graphene is calculated in the framework of a
four-band model.Comment: 10 pages, 6 figure
Multiband theory for hexagonal germanium
The direct bandgap found in hexagonal germanium and some of its alloys with
silicon allows for an optically active material within the group-IV
semiconductor family with various potential technological applications.
However, there remain some unanswered questions regarding several aspects of
the band structiure, including the strength of the electric dipole transitions
at the center of the Brillouin zone. Using the method near
the point, including 10 bands, and taking spin-orbit coupling into
account, we obtain a self-consistent model that produces the correct band
curvatures, with previously unknown inverse effective mass parameters, to
describe 2H-Ge via fitting to {\it ab initio} data and to calculate effective
masses for electrons and holes. To understand the weak dipole coupling between
the lowest conduction band and the top valance band, we start from a spinless
12-band model and show that when adding spin-orbit coupling, the lowest
conduction band hybridizes with a higher-lying conduction band, which cannot be
explained by the spinful 10-band model. With the help of L\"owdin's
partitioning, we derive the effective low-energy Hamiltonian for the conduction
bands for the possible spin dynamics and nanostructure studies and in a similar
manner, we give the best fit parameters for the valance-band-only model that
can be used in the transport studies. Finally, using the self-consistent
10-band model, we include the effects of a magnetic field and predict the
electron and hole g-factor of the conduction and valance bands.Comment: 11 pages, 4 figure
Non-local Andreev reflection through Andreev molecular states in graphene Josephson junctions
We propose that a device composed of two vertically stacked monolayer
graphene Josephson junctions can be used for Cooper pair splitting. The
hybridization of the Andreev bound states of the two Josephson junction can
facilitate non-local transport in this normal-superconductor hybrid structure,
which we study by calculating the non-local differential conductance. Assuming
that one of the graphene layers is electron and the other is hole doped, we
find that the non-local Andreev reflection can dominate the differential
conductance of the system. Our setup does not require the precise control of
junction length, doping, or superconducting phase difference, which could be an
important advantage for experimental realization.Comment: Main text + supplementar
Tunable Berry curvature, valley and spin Hall effect in Bilayer MoS
The chirality of electronic Bloch bands is responsible for many intriguing
properties of layered two-dimensional materials. We show that in bilayers of
transition metal dichalcogenides (TMDCs), unlike in few-layer graphene and
monolayer TMDCs, both intra-layer and inter-layer couplings give important
contributions to the Berry-curvature in the and valleys of the
Brillouin zone. The inter-layer contribution leads to the stacking dependence
of the Berry curvature and we point out the differences between the commonly
available 3R type and 2H type bilayers. Due to the inter-layer contribution the
Berry curvature becomes highly tunable in double gated devices. We study the
dependence of the valley Hall and spin Hall effects on the stacking type and
external electric field. Although the valley and spin Hall conductivities are
not quantized, in MoS 2H bilayers they may change sign as a function of the
external electric field which is reminiscent of the behaviour of lattice Chern
insulators.Comment: 19 pages, 6 figure
Effective low energy Hamiltonians and unconventional Landau level spectrum of monolayer CN
We derive a low-energy effective Hamiltonians for
monolayer CN at the and points of the Brillouin zone
where the band edge in the conduction and valence band can be found. Our
analysis of the electronic band symmetries helps to better understand several
results of recent ab-initio calculations[1,2] for the optical properties of
this material. We also calculate the Landau level spectrum. We find that the
Landau level spectrum in the degenerate conduction bands at the
point acquires properties that are reminiscent of the corresponding results in
bilayer graphene, but there are important differences as well. Moreover,
because of the heavy effective mass, -doped samples may host interesting
electron-electron interaction effects.Comment: 10 pages, 5 figure
Magnetic field oscillations of the critical current in long ballistic graphene Josephson junctions
We study the Josephson current in long ballistic superconductor-monolayer graphene- superconductor junctions. As a first step, we have developed an efficient computational approach to calculate the Josephson current in tight-binding systems. This approach can be particularly useful in the long junction limit, which has hitherto attracted less theoretical interest but has recently become experimentally relevant. We use this computational approach to study the dependence of the critical current on the junction geometry, doping level, and an applied perpendicular magnetic field B. In zero magnetic field we find a good qualitative agreement with the recent experiment of Ben Shalom et al. (Reference 12) for the length dependence of the critical current. For highly doped samples our numerical calculations show a broad agreement with the results of the quasiclassical formalism. In this case the critical current exhibits Fraunhofer-like oscillations as a function of B. However, for lower doping levels, where the cyclotron orbit becomes comparable to the characteristic geometrical length scales of the system, deviations from the results of the quasiclassical formalism appear. We argue that due to the exceptional tunability and long mean free path of graphene sys- tems a new regime can be explored where geometrical and dynamical effects are equally important to understand the magnetic field dependence of the critical current
Breaking of valley degeneracy by magnetic field in monolayer MoSe2
Using polarization-resolved photoluminescence spectroscopy, we investigate
valley degeneracy breaking by out-of-plane magnetic field in back-gated
monolayer MoSe devices. We observe a linear splitting of between luminescence peak energies in
and emission for both neutral and charged excitons. The optical
selection rules of monolayer MoSe couple photon handedness to the exciton
valley degree of freedom, so this splitting demonstrates valley degeneracy
breaking. In addition, we find that the luminescence handedness can be
controlled with magnetic field, to a degree that depends on the back-gate
voltage. An applied magnetic field therefore provides effective strategies for
control over the valley degree of freedom.Comment: expanded discussion section, corrected typo in eq.