40 research outputs found
Topological phases of topological insulator thin films
We study the properties of a thin film of topological insulator material. We
treat the coupling between helical states at opposite surfaces of the film in
the properly-adapted tunneling approximation, and show that the tunneling
matrix element oscillates as function of both the film thickness and the
momentum in the plane of the film for BiSe and BiTe. As a
result, while the magnitude of the matrix element at the center of the surface
Brillouin Zone gives the gap in the energy spectrum, the sign of the matrix
element uniquely determines the topological properties of the film, as
demonstrated by explicitly computing the pseudospin textures and the Chern
number. We find a sequence of transitions between topological and
non-topological phases, separated by semimetallic states, as the film thickness
varies. In the topological phase the edge states of the film always exist but
only carry a spin current if the edge potentials break particle-hole symmetry.
The edge states decay very slowly away from the boundary in BiSe,
making BiTe, where this scale is shorter, a more promising
candidate for the observation of these states. Our results hold for
free-standing films as well as heterostructures with large-gap insulators
Proximity-induced topological phases in bilayer graphene
We study the band structure of phases induced by depositing bilayer graphene
on a transition metal dichalcogenide monolayer. Tight-binding and low-energy
effective Hamiltonian calculations show that it is possible to induce
topologically nontrivial phases that should exhibit spin Hall effect in these
systems. We classify bulk insulating phases through calculation of the Z
invariant, which unequivocally identifies the topology of the structure. The
study of these and similar hybrid systems under applied gate voltage opens the
possibility for tunable topological structures in real experimental systems.Comment: 4 pages, 4 figure
Floquet Control of Indirect Exchange Interaction in Periodically Driven Two-Dimensional Electron Systems
We present a theory for the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction
mediated by a two-dimensional (2D) electron system subjected to periodic
driving. This is demonstrated for a heterostructure consisting of two
ferromagnets laterally sandwiching the 2D metallic spacer. Our calculations
reveal new non-analytic features in the spin susceptibility. For weak
light-matter coupling, the RKKY interaction shows oscillations with a period
tunable by the light amplitude and frequency. For stronger light-matter
coupling, the interaction becomes non-oscillatory and remains purely
ferromagnetic
Proximity-Induced Superconductivity at Non-Helical Topological Insulator Interfaces
We study how non-helical spin textures at the boundary between a topological
insulator (TI) and a superconductor (SC) affect the proximity-induced
superconductivity of the TI interface state. We consider TIs coupled to both
spin-singlet and spin-triplet SCs, and show that for the spin-triplet parent
SCs the resulting order parameter induced onto the interface state sensitively
depends on the symmetries which are broken at the TI-SC boundary. For chiral
spin-triplet parent SCs, we find that nodal proximity-induced superconductivity
emerges when there is broken twofold rotational symmetry which forces the spins
of the non-helical topological states to tilt away from the interface plane. We
furthermore show that the Andreev conductance of lateral heterostructures
joining TI-vacuum and TI-SC interfaces yields experimental signatures of the
reduced symmetries of the interface states.Comment: 5 pages, 2 figure
Conductance of gated junctions as a probe of topological interface states
Energy dispersion and spin orientation of the protected states at interfaces
between topological insulators (TIs) and non-topological materials depend on
the charge redistribution, strain, and atomic displacement at the interface.
Knowledge of these properties is essential for applications of topological
compounds, but direct access to them in the interface geometry is difficult. We
show that conductance of a gated double junction at the surface of a
topological insulator exhibits oscillations and a quasi-linear decay as a
function of gate voltage in different regimes. These give the values for the
quasiparticle velocities along and normal to the junction in the interface
region, and determine the symmetry of the topological interface states. The
results are insensitive to the boundary conditions at the junction