249 research outputs found

### Spin Hall effect and Berry phase in two dimensional electron gas

The spin Hall effect is investigated in a high mobility two dimensional
electron system with the spin-orbital coupling of both the Rashba and the
Dresselhaus types. A spin current perpendicular to the electric field is
generated by either the Rashba or the Dresselhaus coupling. The spin Hall
conductance is independent of the stength of the coupling, but its sign is
determined by the relative ratio of the two couplings. The direction of spin
current is controllable by tuning the magnitude of the surface electric field
perpendicular to the two dimensional plane via adjusting the Rashba coupling.
It is observed that the spin Hall conductance has a close relation to the Berry
phase of conduction electrons.Comment: 4 paper, 1 figure

### Topological superconducting states in monolayer FeSe/SrTiO$_{3}$

The monolayer FeSe with a thickness of one unit cell grown on a
single-crystal SrTiO$_{3}$ substrate (FeSe/STO) exhibits striking
high-temperature superconductivity with transition temperature $T_{c}$ over 65K
reported by recent experimental measurements. In this work, through analyzing
the distinctive electronic structure, and providing systematic classification
of the pairing symmetry , we find that both $s$-and $p$-wave pairing with odd
parity give rise to topological superconducting states in monolayer FeSe, and
the exotic properties of $s$-wave topological superconducting states have close
relations with the unique non-symmorphic lattice structure which induces the
orbital-momentum locking. Our results indicate that the monolayer FeSe could be
in the topological nontrivial $s$-wave superconducting states if the relevant
effective pairing interactions are dominant in comparison with other
candidates.Comment: 11 pages, 4 figure

### Spin transverse force and quantum transverse transport

We present a brief review on spin transverse force, which exerts on the spin
as the electron is moving in an electric field. This force, analogue to the
Lorentz force on electron charge, is perpendicular to the electric field and
spin current carried by the electron. The force stems from the spin-orbit
coupling of electrons as a relativistic quantum effect, and could be used to
understand the Zitterbewegung of electron wave packet and the quantum
transverse transport of electron in a heuristic way.Comment: 4 pages, manuscript of invited talk on IAS Workshop on Spintronics at
Nanyang Techological University, Singapore, 200

### Quantum Anomalous Hall Effect in Flat Band Ferromagnet

We proposed a theory of quantum anomalous Hall effect in a flat-band
ferromagnet on a two-dimensional (2D) decorated lattice with spin-orbit
coupling. Free electrons on the lattice have dispersionless flat bands, and the
ground state is highly degenerate when each lattice site is occupied averagely
by one electron, i.e., the system is at half filling. The on-site Coulomb
interaction can remove the degeneracy and give rise to the ferrimagnetism,
which is the coexistence of the ferromagnetic and antiferromagnetic long-range
orders. On the other hand the spin-orbit coupling makes the band structure
topologically non-trivial, and produces the quantum spin Hall effect with a
pair of helical edge states around the system boundary. Based on the rigorous
results for the Hubbard model, we found that the Coulomb interaction can
provide an effective staggered potential and turn the quantum spin Hall phase
into a quantum anomalous Hall phase

### Topological phase in one-dimensional interacting fermion system

We study a one-dimensional interacting topological model by means of exact
diagonalization method. The topological properties are firstly examined with
the existence of the edge states at half-filling. We find that the topological
phases are not only robust to small repulsive interactions but also are
stabilized by small attractive interactions, and also finite repulsive
interaction can drive a topological non-trivial phase into a trivial one while
the attractive interaction can drive a trivial phase into a non-trivial one.
Next we calculate the Berry phase and parity of the bulk system and find that
they are equivalent in characterizing the topological phases. With them we
obtain the critical interaction strengths and construct part of the phase
diagram in the parameters space. Finally we discuss the effective Hamiltonian
at large-U limit and provide additional understanding of the numerical results.
Our these results could be realized experimentally using cold atoms trapped in
the 1D optical lattice.Comment: 7 pages, 5 figures; revised version, references added, Accepted for
publication in Physical Review

### Finite-temperature conductivity and magnetoconductivity of topological insulators

The electronic transport experiments on topological insulators exhibit a
dilemma. A negative cusp in magnetoconductivity is widely believed as a quantum
transport signature of the topological surface states, which are immune from
localization and exhibit the weak antilocalization. However, the measured
conductivity drops logarithmically when lowering temperature, showing a typical
feature of the weak localization as in ordinary disordered metals. Here, we
present a conductivity formula for massless and massive Dirac fermions as a
function of magnetic field and temperature, by taking into account the
electron-electron interaction and quantum interference simultaneously. The
formula reconciles the dilemma by explicitly clarifying that the temperature
dependence of the conductivity is dominated by the interaction while the
magnetoconductivity is mainly contributed by the quantum interference. The
theory paves the road to quantitatively study the transport in topological
insulators and other two-dimensional Dirac-like systems, such as graphene,
transition metal dichalcogenides, and silicene.Comment: 5 pages, 5 figure

### Electric-Field-Induced Resonant Spin Polarization in a Two-Dimensional Electron Gas

Electric response of spin polarization in two-dimensional electron gas with
structural inversion asymmetry subjected to a magnetic field was studied by
means of the linear and non-linear theory and numerical simulation with the
disorder effect. It was found by Kubo linear reponse theory that an electric
resonant response of spin polarization occurs when the Fermi surface is located
near the crossing of two Landau levels, which is induced from the competition
between the spin-orbit coupling and Zeeman splitting. The scaling behavior was
investigated with a simplified two-level model by non-linear method, and the
resonant peak value is reciprocally proportional to the electric field at low
temperatures and to temperature for finite electric fields. Finally numerical
simulation illustrated that impurity potential opens an enegy gap near the
resonant point and suppresses the effect gradually with the increasing strength
of disorder. This resonant effect may provide an efficient way to control spin
polarization by an external electric field.Comment: 6 pages, 5 figure

### Ordered valence bond states in symmetric two-dimensional spin-orbital systems

We consider a superexchange Hamiltonian, $H=-\sum_{}(2{\bf S}_i\cdot
{\bf S}_j-\frac 12)(2{\bf T}_i\cdot {\bf T}_j-\frac 12)$, which describes
systems with orbital degeneracy and strong electron-phonon coupling in the
limit of large on-site repulsion. In an SU(4) Schwinger boson representation, a
reduced spin-orbital interaction is derived {\it exactly}, and a mean field
theory has been developed by introducing a symmetric valence bond pairing order
parameter. In one dimension, a spin-orbital liquid state with a finite gap is
obtained. On a two-dimensional square lattice a novel type of spin-orbital
ferromagnetically ordered state appears, while spin and orbital are
antiferromagnetic. Moreover, an important relation has been found, relating the
spin and orbital correlation functions to the combined spin-orbital ones.Comment: four pages in Revtex, no figures, accepted for publication in
Physical Review Letter

### Extrinsic anomalous Hall conductivity of a topologically nontrivial conduction band

A key step towards dissipationless transport devices is the quantum anomalous
Hall effect, which is characterized by an integer quantized Hall conductance in
a ferromagnetic insulator with strong spin-orbit coupling. In this work, the
anomalous Hall effect due to the impurity scattering, namely the extrinsic
anomalous Hall effect, is studied when the Fermi energy crosses with the
topologically nontrivial conduction band of a quantum anomalous Hall system.
Two major extrinsic contributions, the side-jump and skew-scattering Hall
conductivities, are calculated using the diagrammatic techniques in which both
nonmagnetic and magnetic scattering are taken into account simultaneously. The
side-jump Hall conductivity changes its sign at a critical sheet carrier
density for the nontrivial phase, while it remains sign unchanged for the
trivial phase. The critical sheet carrier densities estimated with realistic
parameters lie in an experimentally accessible range. The results imply that a
quantum anomalous Hall system could be identified in the good-metal regime.Comment: 5 pages, 4 figure

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