arXiv subjects

Biao Lian

Publications and source records attributed to Biao Lian.

100 records · Page 6Linked to original sources

arXiv2014

Generation and Electric Control of Spin-Coupled Valley Current in WSe2

The valley degree of freedom in layered transition-metal dichalcogenides (MX2) provides the opportunity to extend functionalities of novel spintronics and valleytronics devices. Due to spin splitting induced by spin-orbital coupling (SOC), the non-equilibrium charge carrier imbalance between two degenerate and inequivalent valleys to realize valley/spin polarization has been successfully demonstrated theoretically and supported by optical experiments. However, the generation of a valley/spin current by the valley polarization in MX2 remains elusive and a great challenge. Here, within an electric-double-layer transistor based on WSe2, we demonstrated a spin-coupled valley photocurrent whose direction and magnitude depend on the degree of circular polarization of the incident radiation and can be further greatly modulated with an external electric field. Such room temperature generation and electric control of valley/spin photocurrent provides a new property of electrons in MX2 systems, thereby enabling new degrees of control for quantum-confined spintronics devices.

cond-mat.mes-hall↗

arXiv2014

Universal Scaling of Quantum Anomalous Hall Plateau Transition

We study the critical properties of the quantum anomalous Hall (QAH) plateau transition in magnetic topological insulators. We introduce a microscopic model for the plateau transition in QAH effect at the coercive field and then map it to the network model of quantum percolation in the integer quantum Hall effect plateau transition. Generally, an intermediate plateau with zero Hall conductance could occur at the coercive field. $σ_{xx}$ would have double peaks at the coercivity while $ρ_{xx}$ only has single peak. Remarkably, this theoretical prediction is already borne out in experiment. Universal scaling of the transport coefficients $ρ_{xy}$ and $ρ_{xx}$ are predicted.

cond-mat.mes-hall↗

arXiv2014

Singlet Mott State Simulating the Bosonic Laughlin Wave Function

We study properties of a class of spin singlet Mott states for arbitrary spin S bosons on a lattice, with particle number per cite n=S/l+1, where l is a positive integer. We show that such a singlet Mott state can be mapped to a bosonic Laughlin wave function on the sphere with a finite number of particles at filling ν=1/2l. Spin, particle and hole excitations in the Mott state are discussed, among which the hole excitation can be mapped to the quasi-hole of the bosonic Laughlin wave function. We show that this singlet Mott state can be realized in a cold atom system on optical lattice, and can be identified using Bragg spectroscopy and Stern-Gerlach techniques. This class of singlet Mott states may be generalized to simulate bosonic Laughlin states with filling ν=q/2l.

cond-mat.quant-gas↗

arXiv2013

Relativistic Self-similar Dynamic Collapses of Black Holes in General Polytropic Spherical Clouds

We study the hydrodynamic self-similar mass collapses of general polytropic (GP) spherical clouds to central Schwarzschild black holes and void evolution with or without shocks. In order to grossly capture characteristic effects of general relativity (GR) outside yet close to the event horizon of a Schwarzschild black hole and to avoid mathematical complexity, we adopt the approximation of the Paczynski-Wiita gravity to replace the simple Newtonian gravity in our model formulation. A new dimensionless parameter s appears with the physical meaning of the square of the ratio of the sound speed to the speed of light $c$. Various self-similar dynamic solutions are constructed for a polytropic index $γ>4/3$. Two (for small enough $s<1$) or no (for large enough $s<1$) expansion-wave collapse solutions (EWCSs) with central event horizons exist when $γ>4/3$, representing the collapse of static singular GP spheres towards the central singularity of spacetime. Such GP spherical dynamic mass collapse is shown to be highly efficient for the rapid formation of supermassive black holes (SMBHs; mass range of $10^6-10^{10}M_{\odot}$) in the early universe or even hypermassive black holes (HMBHs; mass range of $10^{10}-10^{12}M_{\odot}$) if extremely massive mass reservoirs could be sustained for a sufficiently long time, which may evolve into hard X-ray/gamma ray sources or quasars according to their surroundings. Self-similar dynamic solutions of a GP gas are also proposed for the stellar mass black hole formation during the violent supernova explosion of a massive progenitor star, the timescale of which is estimated of $10^{-3}$ seconds. Rebound shocks travelling in supernovae are also discussed based on our self-similar shock expansion solutions.

astro-ph.SR↗

arXiv2013

Anomalous Edge Transport in the Quantum Anomalous Hall State

We predict by first-principles calculations that thin films of Cr-doped (Bi,Sb)$_2$Te$_3$ magnetic topological insulator have gapless non-chiral edge states coexisting with the chiral edge state. Such gapless non-chiral states are not immune to backscattering, which would explain dissipative transport in the quantum anomalous Hall (QAH) state observed in this system experimentally. Here we study the edge transport with both chiral and non-chiral states by Landaur-Büttiker formalism, and find that the longitudinal resistance is nonzero whereas Hall resistance is quantized to $h/e^2$. In particular, the longitudinal resistance can be greatly reduced by adding an extra floating probe even if it is not used, while the Hall resistance remains at the quantized value. We propose several transport experiments to detect the dissipative non-chiral edge channels. These results will facilitate the realization of pure dissipationless transport of QAH states in magnetic topological insulators.

cond-mat.mes-hall↗

arXiv2013

Quantum Anomalous Hall Effect with Higher Plateaus

Quantum anomalous Hall (QAH) effect in magnetic topological insulators is driven by the combination of spontaneous magnetic moments and spin-orbit coupling. Its recent experimental discovery raises the question if higher plateaus can also be realized. Here we present a general theory for QAH effect with higher Chern numbers, and show by first-principles calculations that thin film magnetic topological insulator of Cr-doped Bi$_2$(Se,Te)$_3$ is a candidate for the C=2 QAH insulator. Remarkably, whereas higher magnetic field leads to lower Hall conductance plateaus in the integer quantum Hall effect, higher magnetic moment leads to higher Hall conductance plateaus in the QAH effect.

cond-mat.mes-hall↗

arXiv2013

Synthetic Gauge Field with Highly Magnetic Lanthanide Atoms

We present a scheme for generating a synthetic magnetic field and spin-orbit coupling via Raman coupling in highly magnetic lanthanide atoms such as dysprosium. Employing these atoms offer several advantages for realizing strongly correlated states and exotic spinor phases. The large spin and narrow optical transitions of these atoms allow the generation of synthetic magnetic fields an order of magnitude larger than those in the alkalis, but with considerable reduction of the heating rate for equal Raman coupling. The effective hamiltonian of these systems differs from that of the alkalis' by an additional nematic coupling term, which leads to a phase transition in the dressed states as detuning varies. For \text{high-spin} condensates, spin-orbit coupling leads to a spatially periodic structure, which is described in Majorana representation by a set of points moving periodically on a unit sphere. We name this a "Majorana spinor helix" in analogy to the persistent spin-1/2 helix observed in electronic systems.

cond-mat.quant-gas↗

arXiv2012

The Cornerstone Of Spin Statistics Connection: The SU(2)$\times$ C $\times$ T Symmetry

We investigate the intrinsic reason for spin statistics connection. It is found that if a free field theory is rotationally (SU(2)) invariant, and has time reversal ($T$) and charge conjugation ($C$) symmetries, it obeys the spin statistics connection, except for a special case. Shr{ö}dinger equation belongs to this special case, and does not obey spin statistics connection. Further we show that if the energy spectrum of a particle $E(p)$ takes the form of a square root, namely contains branch points in the complex $p$ plane, the particle cannot belong to this special case, and must obey the spin statistics connection. This conclusion includes the relativistic particles as a particular example.

math-ph↗

arXiv2012

Searching for Non-Abelian Phases in the Bose-Einstein Condensate of Dysprosium

The recently realized Bose-Einstein condensate of Dysprosium will become a spin-8 spinor condensate at ultralow magnetic fields. In such a high spin condensate, many phases with different symmetries can exist. Among them the most interesting ones are those with non-abelian point group symmetry. In this letter we discuss the variety of symmetry phases in a spin-8 condensate resulting from numerical solutions of the Hamiltonian. We show that these symmetries can be determined uniquely from the measurements of density population on each spin component in an ultralow magnetic field, together with the measurements of the collective modes in the zero-field limit. This method can also be applied to Bose-Einstein condensate of other magnetic atoms, such as Cr and Er.

cond-mat.quant-gas↗

arXiv2011

Three-dimensional Hydrodynamic Instabilities in Stellar Core Collapses

A spherically symmetric hydrodynamic stellar core collapse under gravity is time-dependent and may become unstable once disturbed. Specifically for a homologously collapse of stellar core characterized by a polytropic exponent Γ=4/3, we examine oscillations and/or instabilities of three dimensional (3D) general polytropic perturbations. For compressible 3D perturbations, the polytropic index γof perturbations can differ from Γ=4/3 of the general polytropic hydrodynamic background flow. Our model formulation here is more general and allows the existence of internal gravity g(-)-modes and/or g(+)-modes. Eigenvalues and eigen-functions of various oscillatory and unstable perturbation modes are computed. As studied in several specialized cases of Goldreich & Weber and of Cao & Lou, we further confirm that acoustic p-modes and surface f-modes remain stable in the current more general situations. In comparison, g(-)-modes and sufficiently high radial order g(+)-modes are unstable, leading to inevitable convective motions within the collapsing stellar interior. Unstable growths of 3D g-mode disturbances are governed dominantly by the angular momentum conservation and modified by the gas pressure restoring force. We note in particular that unstable temporal growths of 3D vortical perturbations exist even when the specific entropy distribution becomes uniform and γ=Γ=4/3. Conceptually, unstable g-modes might bear conceivable physical consequences on supernova explosions, the initial kicks of nascent proto-neutron stars (PNSs) and break-ups of the collapsing core, while unstable growths of vortical perturbations can lead to fast spins of compact objects and other relevant results.

astro-ph.SR↗