arXiv subjects

Yongjin Jiang

Publications and source records attributed to Yongjin Jiang.

16 recordsLinked to original sources

arXiv2015

Topological currents in black phosphorus with broken inversion symmetry

We examine the nature of topological currents in black phosphorus when its inversion symmetry is deliberately broken. Here, the conduction and valence band edges are located at the $Γ$ point of the rectangular Brillouin zone, and they exhibit strong anisotropy along its two crystal axes. We will show below that these salient features lead to a linear transverse neutral topological currents, accompanied also by a non-linear transverse charge current at the Fermi surface. These topological currents are maximal when the in-plane electric field is applied along the zigzag crystal axes, but zero along the armchair direction.

cond-mat.mes-hall↗

arXiv2015

Magneto-electronic properties of multilayer black phosphorus

We examine the electronic properties of 2D electron gas in black phosphorus multilayers in the presence of a perpendicular magnetic field, highlighting the role of in-plane anisotropy on various experimental quantities such as ac magneto-conductivity, screening, and magneto-plasmons. We find that resonant structures in the ac conductivity exhibits a red-shift with increasing doping due to inter-band coupling, $γ$. This arises from an extra correction term in the Landau energy spectrum proportional to $n^2γ^2$ ($n$ is Landau index), up to second order in $γ$. We found also that Coulomb interaction leads to highly anisotropic magneto-excitons.

cond-mat.mes-hall↗

arXiv2014

Tunable optical properties of multilayers black phosphorus thin films

Black phosphorus thin films might offer attractive alternatives to narrow gap compound semiconductors for optoelectronics across mid- to near-infrared frequencies. In this work, we calculate the optical conductivity tensor of multilayer black phosphorus thin films using the Kubo formula within an effective low-energy Hamiltonian. The optical absorption spectra of multilayer black phosphorus are shown to vary sensitively with thickness, doping, and light polarization. In conjunction with experimental spectra obtained from infrared absorption spectroscopy, we also discuss the role of interband coupling and disorder on the observed anisotropic absorption spectra.

cond-mat.mtrl-sci↗

arXiv2013

Standard form of the scattering matrix for time reversal symmetric system

In this paper, we present the standard form of the scattering matrix of mesocopic system with spin-orbital coupling which preserves time reversal symmetry. We found some analytical structure of the scattering matrix related to the sub-matrices between arbitrary two channels. In particular, we proved that in the two-terminal mono-channel scattering problem, the transmission matrix is proportional to a SU(2) matrix. We obtained these properties through direct and elementary way and found it in agreement with polar decomposition known before.

cond-mat.mes-hall↗

arXiv2013

Quantum Hall Effects in Monolayer-Bilayer Graphene Planar Junctions

The Hall resistance of a homogeneous electron system is well known to be anti-symmetric with respect to the magnetic field and the sign of charge carriers. We have observed that such symmetries no longer hold in planar hybrid structures consisting of partly single layer graphene (SLG) and partly bilayer graphene (BLG) in the quantum Hall (QH) regime. In particular, the Hall resistance (R12xy) across the SLG and BLG interface is observed to exhibit quantized plateaus that switch between those characteristic of SLG QH states and BLG QH states when either the sign of the charge carriers (controlled by a back gate) or the direction of the magnetic field is reversed. Simultaneously reversing both the carrier type and the magnetic field gives rise to the same quantized Hall resistances. The observed SLG-BLG interface QH states, with characteristic asymmetries with respect to the signs of carriers and magnetic field, are determined only by the chirality of the QH edge states and can be explained by a Landauer-Büttiker analysis applied to such graphene hybrid structures involving two regions of different Landau level (LL) structures.

cond-mat.mes-hall↗

arXiv2013

Generation of pure bulk valley current in graphene

The generation of valley current is a fundamental goal in graphene valleytronics but no practical ways of its realization are known yet. We propose a workable scheme for the generation of bulk valley current in a graphene mechanical resonator through adiabatic cyclic deformations of the strains and chemical potential in the suspended region. The accompanied strain gauge fields can break the spatial mirror symmetry of the problem within each of the two inequivalent valleys, leading to a fnite valley current due to quantum pumping. An all-electrical measurement configuration is designed to detect the novel state with pure bulk valley currents.

cond-mat.mes-hall↗

arXiv2012

Electron pumping in graphene mechanical resonators

The combination of high frequency vibrations and metallic transport in graphene makes it a unique material for nano-electromechanical devices. In this letter, we show that graphene-based nano-electromechanical devices are extremely well suited for charge pumping, due to the sensitivity of its transport coefficients to perturbations in electrostatic potential and mechanical deformations, with the potential for novel small scale devices with useful applications.

cond-mat.mes-hall↗

arXiv2011

Connectivity of edge and surface states in topological insulators

The edge states of a two-dimensional quantum spin Hall (QSH) insulator form a one-dimensional helical metal which is responsible for the transport property of the QSH insulator. Conceptually, such a one-dimensional helical metal can be attached to any scattering region as the usual metallic leads. We study the analytical property of the scattering matrix for such a conceptual multiterminal scattering problem in the presence of time reversal invariance. As a result, several theorems on the connectivity property of helical edge states in two-dimensional QSH systems as well as surface states of three-dimensional topological insulators are obtained. Without addressing real model details, these theorems, which are phenomenologically obtained, emphasize the general connectivity property of topological edge/surface states from the mere time reversal symmetry restriction.

cond-mat.mes-hall↗

arXiv2011

Size Effects on Transport Properties in Topological Anderson Insulators

We study the size effects on the transport properties in topological Anderson insulators by means of the Landauer-Büttiker formalism combined with the nonequilibrium Green function method. Conductances calculated for serval different widths of the nanoribbons reveal that there is no longer quantized plateaus for narrow nanoribbons. The local spin-resolved current distribution demonstrates that the edge states on the two sides can be coupled, leading to enhancement of backscattering as the width of the nanoribbon decreases, thus destroying the perfect quantization phenomena in the topological Anderson insulator. We also show that the main contribution to the nonquantized conductance also comes from edge states. Experiment proposals on topological Anderson insulator are discussed finally.

cond-mat.mes-hall↗

arXiv2010

S-wave superconductivity with orbital dependent sign change in the checkerboard models of iron-based superconductors

We study three different multi-orbital models for iron-based superconductors (iron-SCs) in the solvable limit of weakly coupled square plaquettes. The strongest superconducting (SC) pairing is in the $A_{1g}$ $s$-wave channel and its development is correlated with the emergence of the next-nearest-neighbour antiferromagnetism (NNN-AFM). For the models with more than three orbitals, this study suggests that the signs of the intra-orbital pairing order parameters of the $d_{xy}$ and the $d_{xz}$ (or $d_{yz}$) orbitals must be {\it opposite}. Such sign difference stems from the intrinsic symmetry properties of inter-orbital hoppings and might, ultimately, lead to the sign-change of the SC orders between the hole Fermi pockets at the $Γ$ point and produce anisotropic or even gapless SC gaps in the electron Fermi pockets around the $M$ point in reciprocal space, as restoring back to the homogeneous limit.

cond-mat.supr-con↗

arXiv2007

Some symmetry properties of spin currents and spin polarizations in multi-terminal mesoscopic spin-orbit coupled systems

We study theoretically some symmetry properties of spin currents and spin polarizations in multi-terminal mesoscopic spin-orbit coupled systems. Based on a scattering wave function approach, we show rigorously that in the equilibrium state no finite spin polarizations can exist in a multi-terminal mesoscopic spin-orbit coupled system (both in the leads and in the spin-orbit coupled region) and also no finite equilibrium terminal spin currents can exist. By use of a typical two-terminal mesoscopic spin-orbit coupled system as the example, we show explicitly that the nonequilibrium terminal spin currents in a multi-terminal mesoscopic spin-orbit coupled system are non-conservative in general. This non-conservation of terminal spin currents is not caused by the use of an improper definition of spin current but is intrinsic to spin-dependent transports in mesoscopic spin-orbit coupled systems. We also show that the nonequilibrium lateral edge spin accumulation induced by a longitudinal charge current in a thin strip of \textit{finite} length of a two-dimensional electronic system with intrinsic spin-orbit coupling may be non-antisymmetric in general, which implies that some cautions may need to be taken when attributing the occurrence of nonequilibrium lateral edge spin accumulation induced by a longitudinal charge current in such a system to an intrinsic spin Hall effect.

cond-mat.mes-hall↗

arXiv2007

d+id'-wave Superconducting States in Graphene

We show that effective superconducting orders generally emerge at low energy in the superconducting state of graphene with conventionally defined pairing symmetry . We study such a particular interesting example, the $d_{x^2-y^2}+id'_{xy}$ spin singlet pairing superconducting state in graphene, which can be generated by electronic correlation as well as induced through a proximity effect with a d-wave superconductor. We find that effectively the d-wave state is a state with mixed s-wave and exotic $p+ip$-wave pairing orders at low energy. This remarkable property leads to distinctive superconducting gap functions and novel behavior of the Andreev conductance spectra.

cond-mat.mes-hall↗

arXiv2006

Non-equilibrium spin polarization effects in spin-orbit coupling system and contacting metallic leads

We study theoretically the current-induced spin polarization effect in a two-terminal mesoscopic structure which is composed of a semiconductor two-dimensional electron gas (2DEG) bar with Rashba spin-orbit (SO) interaction and two attached ideal leads. The nonequilibrium spin density is calculated by solving the scattering wave functions explicitly within the ballistic transport regime. We found that for a Rashba SO system the electrical current can induce spin polarization in the SO system as well as in the ideal leads. The induced polarization in the 2DEG shows some qualitative features of the intrinsic spin Hall effect. On the other hand, the nonequilibrium spin density in the ideal leads, after being averaged in the transversal direction, is independent of the distance measured from the lead/SO system interface, except in the vicinity of the interface. Such a lead polarization effect can even be enhanced by the presence of weak impurity scattering in the SO system and may be detectable in real experiments.

cond-mat.mes-hall↗

arXiv2006

Scattering wave function approach to multi-terminal mesoscopic system with spin-orbit coupling

In this paper,we present a detailed formulation to solve the scattering wave function for a multi-terminal mesoscopic system with spin-orbit coupling. In addition to terminal currents, all local quantities can be calculated explicitly by taking proper ensemble average in the Landauer-Buttiker's spirit using the scattering wave functions. Based on this formulation, we derive some rigorous results for equilibrium state. Furthermore, some new symmetry relations are found for the typical two terminal structure in which a semiconductor bar with Rashba or/and Dresselhaus SO coupling is sandwiched symmetrically between two leads. These symmetry property can provide accuracy tests for experimental measurements and numerical calculations.

cond-mat.mes-hall↗

arXiv2006

Kinetic magnetoelectric effect in a 2D semiconductor strip due to boundary-confinement induced spin-orbit coupling

In a thin strip of a two-dimensional semiconductor electronic system, spin-orbit coupling may be induced near both edges of the strip due to the substantial spatial variation of the confining potential in the boundary regions. In this paper we show that, in the presence of boundary-confinement induced spin-orbit coupling, a longitudinal charge current circulating through a 2D semiconductor strip may cause \textit{strong} non-equilibrium spin accumulation near both edges of the strip. The spins will be polarized along the normal of the 2DEG plane but in opposite directions at both edges of the strip. This phenomenon is essentially a kinetic magnetoelectric effect from the theoretical points of view, but it manifests in a very similar form as was conceived in a spin Hall effect.

cond-mat.mes-hall↗

arXiv2002

A quantum critical point in the transverse field of Mn12 system

Using exact diagonalization method,we studied the ground state of the anisotropic molecular magnets and find a critical point in the transverse field,which may divide the quantum tunneling region into two different parts. Possible ways to observe and take advantage of this point by varying the transverse field are suggested.

cond-mat.mes-hall↗