Bang-Fen Zhu

Landau quantization of topological surface states in Bi2Se3.

1 Department of Physics, Tsinghua University, Beijing 100084, China 2 Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 3 Microsoft Research, Station Q, University of California, Santa Barbara, CA 93106, USA 4 Department of Physics, Stanford University, Stanford CA 94305, USA 5 Physikalisches Institut, Universität Würzburg, D-97074 Würzburg, Germany * These authors contributed equally to this work. ¶ To whom correspondence should addressed. Email: xc@mail.tsinghua.edu.cn, xcma@aphy.iphy.ac.cn

Metal-to-Semiconductor Transition in Squashed Armchair Carbon Nanotubes

We investigate electronic transport properties of the squashed armchair carbon nanotubes, using tight-binding molecular dynamics and the Greens function method. We demonstrate a metal-to-semiconductor transition while squashing the nanotubes and a general mechanism for such a transition. It is the distinction of the two sublattices in the nanotube that opens an energy gap near the Fermi energy. We show that the transition has to be achieved by a combined effect of breaking of mirror symmetry and bond formation between the flattened faces in the squashed nanotubes.

Tunable Fano effect in parallel-coupled double quantum dot system

With the help of the Green function technique and the equation of motion approach, the electronic transport through a parallel-coupled double quantum dot (DQD) is theoretically studied. Owing to the interdot coupling, the bonding and antibonding states of the artificial quantum-dot molecule may constitute an appropriate basis set. Based on this picture, the Fano interference in the conductance spectra of the DQD system is readily explained. The possibility of manipulating the Fano line shape in the tunneling spectra of the DQD system is explored by tuning the dot-lead coupling, the interdot coupling, the magnetic flux threading the ring connecting dots and leads, and the flux difference between two subrings. It has been found that by making use of various tunings, the direction of the asymmetric tail of Fano line shape may be flipped by external fields and the continuous conductance spectra may be magnetically manipulated with the line shape retained. More importantly, by adjusting the magnetic flux, the function of two molecular states can be exchanged, giving rise to a swap effect, which might play a role as a qubit in the quantum computation.

Effects of electron-phonon interaction on nonequilibrium transport through a single-molecule transistor

On the basis of the nonequilibrium Greens function and nonperturbative canonical transformation for the local electron-phonon interaction (EPI), the quantum transport through a single-molecule transistor (SMT) has been investigated with particular attention paid to the joint effect of the EPI and SMT-lead coupling on the spectral function and conductance. In addition to the usual EPI-induced renormalized effects (such as the redshift, sharpening, and phonon sidebands of the SMT level), owing to improved disentangling the electron-phonon system it has been found that the profile of the spectral function of the SMT electron is sensitive to lead chemical potentials, thus can readily be manipulated by tuning the bias as well as the SMT-gate voltage. As a consequence, the broken particle-hole symmetry in this system can be clearly recognized through the phonon sidebands in the spectral function. These EPI effects also manifest themselves in the nonequilibrium transport properties of the SMT, particularly at low temperature.

Second-order nonlinear optical effects of spin currents

We show by symmetry analysis and microscopic calculation that a pure spin current has sizable second-order nonlinear optical effects. Thus spin currents can be studied by standard nonlinear optical spectroscopy.

Fano effect through parallel-coupled double Coulomb islands

By means of the nonequilibrium Green function and equation of motion method, the electronic transport is theoretically studied through a parallel-coupled double quantum dot (DQD) in the presence of on-dot Coulomb interaction U. With focus on the quantum interference in the U-dominant parallel-coupled DQD, we find two types of Fano interferences in the conductance spectra. If the one-particle DQD bonding and antibonding bands are well separated from their Coulomb blockade counterparts, the main features of Fano interference in usual DQD systems are recovered with minor revisions. The most interesting is the hybridization between the antibonding state and the Coulomb counterpart of the bonding state, which gives rises to two new channels for Fano resonance. The Fano interference in the Coulomb hybridized systems can be controlled by the electrostatic and magnetic approaches, and exhibits properties quite different from what are reported in the noninteracting Fano–Anderson model.

Power-law decay of standing waves on the surface of topological insulators

We propose a general theory on the standing waves (quasiparticle interference pattern) caused by the scattering of surface states off step edges in topological insulators in which the extremal points on the constant energy contour of surface band play a dominant role. Experimentally, we image the interference patterns on both Bi2Te3 and Bi2Se3 films by measuring the local density of states with a scanning tunneling microscope. The observed decay indices of the standing waves agree excellently with the theoretical prediction: In Bi2Se3, only a single decay index of -3/2 exists, while in Bi2Te3 with strongly warped surface band, it varies from -3/2 to -1/2 and finally to -1 as the energy increases. The -1/2 decay indicates that the suppression of backscattering due to time-reversal symmetry does not necessarily lead to a spatial decay rate faster than that in the conventional two-dimensional electron system. Our formalism can also better explain the characteristic scattering wave vectors of the standing wave caused by nonmagnetic impurities on Bi2Te3.

High‐order THz‐sideband generation in semiconductors

The optical sideband generation in semiconductors under intense THz lasers presents flat wide‐band spectra with the cutoff determined by the maximum energy‐gain of electron‐hole pairs in quantum trajectories under the THz field. The approximation based on the quantum trajectory picture agrees well with the numerical simulations.

Phonon-assisted Kondo effect in a single-molecule transistor out of equilibrium

The joint effect of the electron–phonon interaction and Kondo effect on nonequilibrium transport through a single molecule transistor is investigated by using the improved canonical transformation scheme and extended equation of motion approach. Two types of Kondo phonon-satellite with different asymmetric shapes are fully confirmed in the spectral function, and are related to the electron spin singlet or hole spin singlet, respectively. Moreover, when a moderate Zeeman splitting is caused by a local magnetic field, the Kondo satellites in the spin-resolved spectral function are found to disappear on one side of the main peak, and they disappear on the opposite side for the opposite spin component. All these peculiar signatures that manifest themselves in the nonlinear differential conductance are explained with a clear physics picture.

Effects of excitation frequency on high-order terahertz sideband generation in semiconductors

We theoretically investigate the effects of the excitation frequency on the plateau of high-order terahertz sideband generation (HSG) in semiconductors driven by intense terahertz (THz) fields. We find that the plateau of the sideband spectrum strongly depends on the detuning between the near-infrared laser field and the band gap. We use the quantum trajectory theory (three-step model) to understand the HSG. In the three-step model, an electron?hole pair is first excited by a weak laser, then driven by the strong THz field, and finally recombined to emit a photon with energy gain. When the laser is tuned below the band gap (negative detuning), the electron?hole generation is a virtual process that requires quantum tunneling to occur. When the energy gained by the electron?hole pair from the THz field is less than 3.17 times the ponderomotive energy (Up), the electron and the hole can be driven to the same position and recombined without quantum tunneling, so that the HSG will have large probability amplitude. This leads to a plateau feature of the HSG spectrum with a high-frequency cutoff at about 3.17Up above the band gap. Such a plateau feature is similar to the case of high-order harmonics generation in atoms where electrons have to overcome the binding energy to escape the atomic core. A particularly interesting excitation condition in HSG is that the laser can be tuned above the band gap (positive detuning), corresponding to the unphysical ?negative? binding energy in atoms for high-order harmonic generation. Now the electron?hole pair is generated by real excitation, but the recombination process can be real or virtual depending on the energy gained from the THz field, which determines the plateau feature in HSG. Both the numerical calculation and the quantum trajectory analysis reveal that for positive detuning, the HSG plateau cutoff depends on the frequency of the excitation laser. In particular, when the laser is tuned more than 3.17Up above the band gap, the HSG spectrum presents no plateau feature but instead sharp peaks near the band edge and near the excitation frequency.