Juntao Song

Electric-current-induced heat generation in a strongly interacting quantum dot in the Coulomb blockade regime

The heat generation by electric current flowing through a quantum dot with the dot containing both electron-electron interaction and electron-phonon interaction is studied. Using the nonequilibrium Keldysh Greens-function method, the current-induced heat generation is obtained. We find that for a small phonon frequency, heat generation is proportional to the current. However, for a large phonon frequency, heat generation is in general qualitatively different from the current. It is nonmonotonic with current, and many unique and interesting behaviors emerge. The heat generation could be very large in the Coulomb blockade region in which the current is very small due to the Coulomb blockade effect. On the other hand, in the resonant tunneling region, the heat generation is very small despite a large current, an ideal condition for device operation. In the curve of heat generation versus the bias, a negative differential of the heat generation is exhibited, although the current is always monotonously increasing with the bias voltage.

One-dimensional quantum channel in a graphene line defect

The magnetoelectric (ME) coupling of CoFeB/Ru/CoFeB synthetic antiferromagnetic (SAF) structure is studied at room-temperature when the SAF structure was deposited on the (011)- Pb(Mg1/3Nb2/3)O-3-PbTiO3 piezoelectric substrate. It was found that the magnetoelectric coupling showed butterfly like curve when measured in the absence of magnetic field but it showed hysteresis loop-like behavior as well as the exchange-bias like shift of the ME hysteresis under fixed magnetic fields of +250 Oe and +500 Oe. This exchange-bias like hysteretic ME coupling is attributed to the direct coupling of CoFeB layer with the substrate ferroelectric domain, the absence of magnetocrystalline anisotropy and the switching of 109 degrees ferroelastic domains of the substrate. We have also measured the magnetoelectric coupling of CoFe/Ru/CoFe SAF structure but observed no exchange-bias like hysteresis behavior. Our results establish another way of obtaining the non-volatile memory devices based on magnetoelectric systems

Dependence of topological Anderson insulator on the type of disorder

This paper details an investigation of the influence of different disorders in two-dimensional topological insulator systems. Unlike the phase transitions to a topological Anderson insulator induced by normal Anderson disorder, a different physical picture arises when bond disorder is considered. Using Born approximation theory, an explanation is given as to why bond disorder plays a different role in the phase transition than Anderson disorder. By comparing phase diagrams, conductance, conductance fluctuations, and the localization length for systems with different types of disorder, a consistent conclusion is obtained. The results indicate that a topological Anderson insulator is dependent on the type of disorder. These results are important for the doping processes used in the preparation of topological insulators.

Quantum pump effect induced by a linearly polarized microwave in a two-dimensional electron gas

A quantum pump effect is predicted in an ideal homogeneous two-dimensional electron gas (2DEG) that is normally irradiated by linearly polarized microwaves (MW). Without considering effects from spin-orbital coupling or the magnetic field, it is found that a polarized MW can continuously pump electrons from the longitudinal to the transverse direction, or from the transverse to the longitudinal direction, in the central irradiated region. The large pump current is obtained for both the low frequency limit and the high frequency case. Its magnitude depends on sample properties such as the size of the radiated region, the power and frequency of the MW, etc. Through the calculated results, the pump current should be attributed to the dominant photon-assisted tunneling processes as well as the asymmetry of the electron density of states with respect to the Fermi energy.

Measuring the phonon-assisted spectral function by using a nonequilibrium three-terminal single-molecular device

The electron transport through a three-terminal single-molecular transistor (SMT) is theoretically studied. We find that the differential conductance of the third and weakly coupled terminal versus its voltage matches well with the spectral function versus the energy when certain conditions are met. Particularly, this excellent matching is maintained even for the complicated structure of the phonon-assisted side peaks. Thus, this device offers an experimental approach to explore the shape of the phonon-assisted spectral function in detail. In addition, we discuss the conditions of a perfect matching. The results show that at low temperatures, the matching survives regardless of the bias and the energy levels of the SMT. However, at high temperatures, the matching is destroyed.

Transmission phase shift of phonon-assisted tunneling through a quantum dot

The influence of electron-phonon interaction on the transmission phase shift of an electron passing through a quantum dot is investigated by using scattering theory. The transmission phase versus the intradot level shows a series of phonon-induced dips. These dips are highly sensitive to the electron-phonon interaction strength lambda, and they are much more pronounced than phonon-assisted subpeaks appearing in the conductance. Phonon-induced dephasing is also studied, and the results show that the dephasing probability T(d) monotonically increases with the electron-phonon interaction strength lambda. The dephasing probability T(d)proportional to lambda(2) for small lambda but T(d)proportional to lambda at large lambda.

Quantum interference in topological insulator Josephson junctions

Using non-equilibrium Greens functions, we studied numerically the transport properties of a Josephson junction, superconductor-topological insulator-superconductor hybrid system. Our numerical calculation shows first that proximity-induced superconductivity is indeed observed in the edge states of a topological insulator adjoining two superconducting leads and second that the special characteristics of topological insulators endow the edge states with an enhanced proximity effect with a superconductor but do not forbid the bulk states to do the same. In a size-dependent analysis of the local current, it was found that a few residual bulk states can lead to measurable resistance, whereas because these bulk states spread over the whole sample, their contribution to the interference pattern is insignificant when the sample size is in the micrometer range. Based on these numerical results, it is concluded that the apparent disappearance of residual bulk states in the superconducting interference process as described in Ref. [\onlinecite{HartNautrePhys2014f}] is just due to the effects of size: the contribution of the topological edge states outweighs that of the residual bulk states.

A disorder induced field effect transistor in bilayer and trilayer graphene.

We propose using disorder to produce a field effect transistor (FET) in biased bilayer and trilayer graphene. Modulation of the bias voltage can produce large variations in the conductance when the effects of disorder are confined to only one of the graphene layers. This effect is based on the ability of the bias voltage to select which of the graphene layers carries current, and is not tied to the presence of a gap in the density of states. In particular, we demonstrate this effect in models of gapless ABA-stacked trilayer graphene, gapped ABC-stacked trilayer graphene and gapped bilayer graphene.

Topological quantum transitions in a two-band Chern insulator with n = 2

Based on a two-band Chern insulator with Chern number n = 2, we study the transport properties and the topological phase transition induced by either an external magnetic field or disorder. In this paper, a characteristic topological phase transition from n = 2 to n = 0, which is in sharp contrast to the plateau-plateau transition in the integer quantum Hall effect, is observed. This unique feature of the phase transition should be ascribed to the minimal two-band feature of this high Chern insulator. We prove this result by studying the transport properties of many different geometrical structures and the evolution of the Chern number in the presence of magnetic fields and strong disorder.

A nanowire magnetic memory cell based on a periodic magnetic superlattice

We analyse the operation of a semiconductor nanowire-based memory cell. Large changes in the nanowire conductance result when the magnetization of a periodic array of nanoscale magnetic gates, which comprise the other key component of the memory cell, is switched between distinct configurations by an external magnetic field. The resulting conductance change provides the basis for a robust memory effect, which can be implemented in a semiconductor structure compatible with conventional semiconductor integrated circuits.