Yongjin Jiang

Tunable optical properties of multilayer 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.

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 a 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 finite valley current due to quantum pumping. An all-electrical measurement configuration is designed to detect the novel state with pure bulk valley currents.

Negative differential spin conductance in doped zigzag graphene nanoribbons

The spin dependent charge transport in zigzag graphene nanoribbons (ZGNRs) has been investigated by the nonequilibrium Green’s function method combined with the density functional theory at the local spin density approximation. The current versus voltage curve shows distinguished behaviors for symmetric and asymmetric ZGNRs, and the doping on the ZGNR edges can manipulate the spin transport. In special cases that a Be atom is substitutionally doped on one edge of the symmetric ZGNRs, one spin current shows negative differential resistance, whereas the other increases monotonically with the bias. This property might be used to design spin oscillators or other devices for spintronics.

Electron Pumping in Graphene Mechanical Resonators

The combination of high-frequency vibrations and metallic transport in graphene makes it a unique material for nanoelectromechanical devices. In this Letter, we show that graphene-based nanoelectromechanical 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.

Kinetic magnetoelectric effect in a two-dimensional 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.

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

Symmetry properties of spin currents and spin polarizations in multiterminal mesoscopic spin-orbit-coupled systems

\Gamma

On the origin of the tunnelling asymmetry in the cuprate superconductors: a variational perspective

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.

Nonequilibrium spin polarization effects in a spin-orbit coupling system and contacting metallic leads

We study theoretically some symmetry properties of spin currents and spin polarizations in a multiterminal mesoscopic spin-orbit-coupled system. Based on the scattering wave function approach, we show rigorously that no finite equilibrium terminal spin currents and/or finite equilibrium spin polarizations can survive in the equilibrium state. By use of a typical two-terminal structure as the example, we show explicitly that the nonequilibrium terminal spin currents in a multiterminal mesoscopic spin-orbit-coupled system may be nonconservative in general, and this nonconservation is intrinsic but not caused by the use of an improper definition of spin current in the calculations. We also show that the nonequilibrium lateral edge spin accumulation induced by a longitudinal charge current in a thin strip of finite length of a ballistic two-dimensional electronic system with intrinsic spin-orbit coupling may be nonantisymmetric in general, suggesting that some cautions 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 a spin Hall effect.

Connectivity of edge and surface states in topological insulators

Through variational Monte Carlo calculations on Gutzwiller projected wavefunctions, we study the quasiparticle weight for adding and removing an electron from a high-temperature superconductor. We find that the quasiparticle weight is particle–hole symmetric at sufficiently low energy. We propose to use the tunnelling asymmetry as a tool to study the mechanism of electron incoherence in high-temperature superconductors.