arXiv subjects

Shun-Qing Shen

Publications and source records attributed to Shun-Qing Shen.

At least 109 records · Page 6Linked to original sources

arXiv2011

Theory of magnetoelectric photocurrent generated by direct interband transitions in semiconductor quantum well

A linearly polarized light normally incident on a semiconductor quantum well with spin-orbit coupling may generate pure spin current via direct interband optical transition. An electric photocurrent can be extracted from the pure spin current when an in-plane magnetic field is applied, which has been recently observed in the InGaAs/InAlAs quantum well [Dai et al., Phys. Rev. Lett. 104, 246601 (2010)]. Here we present a theoretical study of this magnetoelectric photocurrent effect associated with the interband transition. By employing the density matrix formalism, we show that the photoexcited carrier density has an anisotropic distribution in k space, strongly dependent on the orientation of the electron wavevector and the polarization of the light. This anisotropy provides an intuitive picture of the observed dependence of the photocurrent on the magnetic field and the polarization of the light. We also show that the ratio of the pure spin photocurrent to the magnetoelectric photocurrent is approximately equal to the ratio of the kinetic energy to the Zeeman energy, which enables us to estimate the magnitude of the pure spin photocurrent. The photocurrent density calculated with the help of an anisotropic Rashba model and the Kohn-Luttinger model can produce all three terms in the fitting formula for measured current, with comparable order of magnitude, but discrepancies are still present and further investigation is needed.

cond-mat.mes-hall↗

arXiv2011

Quantum spin Hall effect induced by non-magnetic and magnetic staggered potentials

We have a comparative study of the quantum spin Hall (QSH) effects induced by non-magnetic and magnetic staggered potentials respectively and show that they have the same effect in driving the topological phase transition. The result implies that both time-reversal (${\cal T}$) preserving and breaking systems can host QSH effect. We also investigate the stability of the resulting QSH effect to disorder and find that for ${\cal T}$ invariant system the edge states are always robust while those of ${\cal T}$ breaking system are also robust if there is additional symmetry in the system.

cond-mat.mes-hall↗

arXiv2011

Surface and Edge States in Topological Semi-metals

We study the topologically non-trivial semi-metals by means of the 6-band Kane model. Existence of surface states is explicitly demonstrated by calculating the LDOS on the material surface. In the strain free condition, surface states are divided into two parts in the energy spectrum, one part is in the direct gap, the other part including the crossing point of surface state Dirac cone is submerged in the valence band. We also show how uni-axial strain induces an insulating band gap and raises the crossing point from the valence band into the band gap, making the system a true topological insulator. We predict existence of helical edge states and spin Hall effect in the thin film topological semi-metals, which could be tested with future experiment. Disorder is found to significantly enhance the spin Hall effect in the valence band of the thin films.

cond-mat.mes-hall↗

arXiv2010

Surface-edge state and half quantized Hall conductance in topological insulators

We propose a surface-edge state theory for half quantized Hall conductance of surface states in topological insulators. The gap opening of a single Dirac cone for the surface states in a weak magnetic field is demonstrated. We find a new surface state resides on the surface edges and carries chiral edge current, resulting in a half-quantized Hall conductance in a four-terminal setup. We also give a physical interpretation of the half quantized conductance by showing that this state is the product of splitting of a boundary bound state of massive Dirac fermions which carries a conductance quantum.

cond-mat.mes-hall↗

arXiv2010

Z2 invariant protected bound states in topological insulators

We present an exact solution of a modifed Dirac equation for topological insulator in the presence of a hole or vacancy to demonstrate that vacancies may induce bound states in the band gap of topological insulators. They arise due to the Z_2 classification of time-reversal invariant insulators, thus are also topologically-protected like the edge states in the quantum spin Hall effect and the surface states in three-dimensional topological insulators. Coexistence of the in-gap bound states and the edge or surface states in topological insulators suggests that imperfections may affect transport properties of topological insulators via additional bound states near the system boundary.

cond-mat.mes-hall↗

arXiv2010

Crossover of Three-Dimensional Topological Insulator of Bi2Se3 to the Two-Dimensional Limit

Bi2Se3 is theoretically predicted1 2and experimentally observed2,3 to be a three dimensional topological insulator. For possible applications, it is important to understand the electronic structure of the planar device. In this work, thickness dependent band structure of molecular beam epitaxy grown ultrathin films of Bi2Se3 is investigated by in situ angle-resolved photoemission spectroscopy. An energy gap is observed for the first time in the topologically protected metallic surface states of bulk Bi2Se3 below the thickness of six quintuple layers, due to the coupling between the surface states from two opposite surfaces of the Bi2Se3 film. The gapped surface states exhibit sizable Rashba-type spin-orbit splitting, due to breaking of structural inversion symmetry induced by SiC substrate. The spin-splitting can be controlled by tuning the potential difference between the two surfaces.

cond-mat.mtrl-sci↗

arXiv2010

Magneto-electric photocurrent generated by direct inter-band transitions in InGaAs/InAlAs two-dimensional electron gas

We report observation of magneto-electric photocurrent generated via direct inter-band transitions in an InGaAs/InAlAs two-dimensional electron gas excited by a linearly polarized incident light.The electric current is proportional to the in-plane magnetic field which unbalances the velocities of the photoexcited carriers with opposite spins and consequently generates electric current from a spin photocurrent. The observed light polarization dependence of the electric current is explained microscopically by taking into account of the anisotropy of the photoexcited carrier density in wave vector space. The spin photocurrent can be extracted from the measured current and the conversion coefficient of spin photocurrent to electric current is estimated to be $10^{-3}$$\sim$$10^{-2}$ per Tesla.

cond-mat.mes-hall↗

arXiv2010

Effective continuous model for surface states and thin films of three dimensional topological insulators

Two-dimensional effective continuous models are derived for the surface states and thin films of the three-dimensional topological insulator (3DTI). Starting from an effective model for 3DTI based on the first principles calculation [Zhang \emph{et al}, Nat. Phys. 5, 438 (2009)], we present solutions for both the surface states in a semi-infinite boundary condition and in the thin film with finite thickness. An effective continuous model was derived for surface states and the thin film 3DTI. The coupling between opposite topological surfaces and structure inversion asymmetry (SIA) give rise to gapped Dirac hyperbolas with Rashba-like splittings in energy spectrum. Besides, the SIA leads to asymmetric distributions of wavefunctions along the film growth direction, making some branches in the energy spectra much harder than others to be probed by light. These features agree well with the recent angle-resolved photoemission spectra of Bi$_{2}$Se $_{3}$ films grown on SiC substrate [Zhang et al, arXiv: 0911.3706]. More importantly, we use the effective model to fit the experimental data and determine the model parameters. The result indicates that the thin film Bi$_{2}$Se$_{3}$ lies in quantum spin Hall region based on the calculation of the Chern number and the $Z_{2}$ invariant. In addition, strong SIA always intends to destroy the quantum spin Hall state.

cond-mat.mes-hall↗

arXiv2010

Electronic transport through a graphene-based ferromagnetic/normal/ferromagnetic junction

Electronic transport in a graphene-based ferromagnetic/normal/ferromagnetic junction is investigated by means of Landauer-Büttiker formulism and the nonequilibrium Green's function technique. For the zigzag edge case, the results show that the conductance is always larger than $e^{2}/h$ for the parallel configuration of lead magnetizations, but for the antiparallel configuration the conductance becomes zero because of the band-selective rule. So a magnetoresistance (MR) plateau emerges with the value 100% when the Fermi energy is located around the Dirac point. Besides, choosing narrower graphene ribbons can obtain the wider 100% MR plateaus and the length change of the central graphene region does not affect the 100% MR plateaus. Although the disorder will reduce the MR plateau, the plateau value can be still kept about 50% even in a large disorder strength case. In addition, when the magnetizations of the left and right leads have a relative angle, the conductance changes as a cosine function of the angle. What is more, for the armchair edge case, the MR is usually small. So, it is more favorable to fabricate the graphene-based spin valve device by using the zigzag edge graphene ribbon.

cond-mat.mes-hall↗

arXiv2010

Massive Dirac fermions and spin physics in an ultrathin film of topological insulator

We study transport and optical properties of the surface states which lie in the bulk energy gap of a thin-film topological insulator. When the film thickness is comparable with the surface state decay length into the bulk, the tunneling between the top and bottom surfaces opens an energy gap and form two degenerate massive Dirac hyperbolas. Spin dependent physics emerges in the surface bands which are vastly different from the bulk behavior. These include the surface spin Hall effects, spin dependent orbital magnetic moment, and spin dependent optical transition selection rule which allows optical spin injection. We show a topological quantum phase transition where the Chern number of the surface bands changes when varying the thickness of the thin film.

cond-mat.mes-hall↗

arXiv2009

Electric field modulation of topological order in thin film semiconductors

We propose a method that can consecutively modulate the topological orders or the number of helical edge states in ultrathin film semiconductors without a magnetic field. By applying a staggered periodic potential, the system undergoes a transition from a topological trivial insulating state into a non-trivial one with helical edge states emerging in the band gap. Further study demonstrates that the number of helical edge state can be modulated by the amplitude and the geometry of the electric potential in a step-wise fashion, which is analogous to tuning the integer quantum Hall conductance by a megntic field. We address the feasibility of experimental measurement of this topological transition.

cond-mat.mes-hall↗

arXiv2009

Disorder effect of resonant spin Hall effect in a tilted magnetic field

We study the disorder effect of resonant spin Hall effect in a two-dimension electron system with Rashba coupling in the presence of a tilted magnetic field. The competition between the Rashba coupling and the Zeeman coupling leads to the energy crossing of the Landau levels, which gives rise to the resonant spin Hall effect. Utilizing the Streda's formula within the self-consistent Born approximation, we find that the impurity scattering broadens the energy levels, and the resonant spin Hall conductance exhibits a double peak around the resonant point, which is recovered in an applied titled magnetic field.

cond-mat.mes-hall↗

arXiv2009

Quantum Hall effect of the surface states in topological insulator

We study the quantum Hall effect in the surface states of topological insulator in the presence of a perpendicular magnetic field in the framework of edge states. Motion of Dirac fermions will form descrete Landau levels, among which a fully saturated zero mode will have different behaviors near the boundary according to the sign of the effective mass for Dirac fermions. The Hall conductance is quantized to be n (n is an integer) in the unit of e^2/h for a positive mass, n+1 for a negative mass, and n+1/2 for massless fermions. In topological insulator the massive term to the Dirac fermions can be the Zeeman coupling in a magnetic field or be induced by the finite-size effect in an ultrathin film. For example the g-factor of Bi_2Se_3 is positive and give rise to a positive mass term for Dirac fermions. We address experimental realization of the quantum Hall effect in topological insulators.

cond-mat.mes-hall↗

arXiv2009

Detecting and Switching Magnetization of Stoner Nanograin in Non-local Spin Valve

The magnetization detection and switching of an ultrasmall Stoner nanograin in a non-local spin valve (NLSV) device is studied theoretically. With the help of the rate equations, a unified description can be presented on the same footing for the NLSV signal that reads out the magnetization, and for the switching process. The setup can be viewed as that the grain is connected to two non-magnetic leads via sequential tunneling. In one lead, the chemical potentials for spin-up and -down electrons are split due to the spin injection in the NLSV. This splitting (or the spin bias) is crucial to the NLSV signal and the critical condition to the magnetization switching. By using the standard spin diffusion equation and parameters from recent NLSV device, the magnitude of the spin bias is estimated, and found large enough to drive the magnetization switching of the cobalt nanograin reported in earlier experiments. A microscopic interpretation of NLSV signal in the sequential tunneling regime is thereby raised, which show properties due to the ultrasmall size of the grain. The dynamics at the reversal point shows that there may be a spin-polarized current instead of the anticipated pure spin current flowing during the reversal due to the electron accumulation in the floating lead used for the readout of NLSV signal.

cond-mat.mes-hall↗

arXiv2009

Incommensurate Phase of a Triangular Frustrated Heisenberg Model Studied via Schwinger-Boson Mean-Field Theory

We study a triangular frustrated antiferromagnetic Heisenberg model with nearest-neighbor interaction $J_{1}$ and third-nearest-neighbor interactions $J_{3}$ by means of Schwinger-boson mean-field theory. It is shown that an incommensurate phase exists in a finite region in the parameter space for an antiferromagnetic $J_{3}$ while $J_{1}$ can be either positive or negtaive. A detailed solution is presented to disclose the main features of this incommensurate phase. A gapless dispersion of quasiparticles leads to the intrinsic $T^{2}$-law of specific heat. The local magnetization is significantly reduced by quantum fluctuations (for S=1 case, a local magnetization is estimated as $m= \approx0.6223$). The magnetic susceptibility is linear in temperature at low temperatures. We address possible relevance of these results to the low-temperature properties of NiGa$_{2}$S$_{4}$. From a careful analysis of the incommensurate spin wave vector, the interaction parameters for NiGa$_{2}% $S$_{4}$ are estimated as, $J_{1}\approx-3.8755$K and $J_{3}\approx14.0628$K, in order to account for the experimental data.

cond-mat.str-el↗

arXiv2009

Spin-bias driven magnetization reversal and nondestructive detection in a single molecular magnet

The magnetization reversal in a single molecular magnet (SMM) weakly coupled to an electrode with spin-dependent splitting of chemical potentials (spin bias) is theoretically investigated by means of the rate equation. A microscopic mechanism for the reversal is demonstrated by the avalanche dynamics at the reversal point. The magnetization as a function of the spin bias shows hysteresis loops tunable by the gate voltage and varying with temperature. The nondestructive measurement to the orientation of giant spin in SMM is presented by measuring the fully polarized electric current in the response to a small spin bias. For Mn$_{12}$ac molecule, its small transverse anisotropy only slightly violates the results above. The situation when there is an angle between the easy axis of the SMM and the spin quantization direction of the electrode is also studied.

cond-mat.mes-hall↗

arXiv2009

Coherent Oscillations and Giant Edge Magnetoresistance in Singly Connected Topological Insulators

The topological insulators have a gap in the bulk but extended states at the edge that can carry current. We study a geometry in which such edge states will manifest themselves through periodic oscillations in the magnetoconductance of a singly connected sample coupled to leads through narrow point contacts. The oscillations occur due to quantum interference of helical edge states of electrons traveling along the circumference of the sample, and have a period of B_0=h/eA_{eff}, where A_{eff} is the effective area enclosed by the edge states of the sample. Our calculation indicates the possibility of a large change in the magnetoresistance at small B, termed giant edge magnetoresistance, which can have potential for application. The zero field conductance also exhibits oscillations as a function of the Fermi energy due to interference between edge states. The amplitude of the oscillations is governed by, and therefore can be used to investigate, the transverse width of the edge channels.

cond-mat.mes-hall↗

arXiv2009

Spin relaxation in sub-monolayer and monolayer InAs structures grown in GaAs matrix

Electron spin dynamics in InAs/GaAs heterostructures consisting of a single layer of InAs (1/3$\sim$1 monolayer) embeded in (001) and (311)A GaAs matrix was studied by means of time-resolved Kerr rotation spectroscopy. The spin relaxation time of the sub-monolayer InAs samples is significantly enhanced, compared with that of the monolayer InAs sample. We attributed the slowing of the spin relaxation to dimensionally constrained D\textquoteright{}yakonov-Perel\textquoteright{} mechanism in the motional narrowing regime. The electron spin relaxation time and the effective g-factor in sub-monolayer samples were found to be strongly dependent on the photon-generated carrier density. The contribution from both D\textquoteright{}yakonov-Perel\textquoteright{} mechanism and Bir-Aronov-Pikus mechanism were discussed to interpret the temperature dependence of spin decoherence at various carrier densities.

cond-mat.mes-hall↗