Kondo physics and exact solvability of double dots systems

We study two double dot systems, one with dots in parallel and one with dots in series, and argue they admit an exact solution via the Bethe ansatz. In the case of parallel dots we exploit the exact solution to extract the behavior of the linear response conductance. The linear response conductance of the parallel dot system possesses multiple Kondo effects, including a Kondo effect enhanced by a nonpertubative antiferromagnetic RKKY interaction, has conductance zeros in the mixed valence regime, and obeys a nontrivial form of the Friedel sum rule.     

Thermoelectric properties of correlated double quantum dot system in Coulomb blockade regime

We theoretically study thermoelectric properties of a coupled double quantum dot (DQD) system coupled to normal leads using two impurity Anderson model with intra- as well as interdot Coulomb interactions. A generic formulation, which was earlier developed to study electronic properties (zero bias maximum of differential conductance and interesting partial swapping in Fano phenomena) of DQD system within Coulomb blockade regime for a non-magnetic case, is extended to investigate thermoelectric properties i.e. electrical conductance, thermoelectric power and thermal conductance of the same system, as a function of temperature by varying interdot Coulomb interaction and interdot tunneling. Interdot Coulomb interaction is found to trigger some novel features like crossover in thermoelectric power with temperature in all the configurations (series, parallel and T-shape) and a small peak in thermal conductance toward low temperatures, T≈Γ/10, in series and T-shape configurations, which is found to be missing in case of symmetric parallel configuration. The origin of these novel features is attributed to the interplay of renormalization of energy levels caused by the interdot Coulomb interaction which is interpreted in terms of local density of states and the asymmetry effects related to dot-lead couplings/interference effects.     

Tunneling between two-dimensional electron systems in a strong magnetic field

We calculate the tunnel current between two parallel two-dimensional electron systems in a strong perpendicular magnetic field. We model the strongly correlated electron systems by Wigner crystals, and describe their low-energy dynamics in terms of magnetophonons. The effects of the magnetophonons on the tunneling processes can be described by an exactly solvable independent-boson model. A tunneling electron shakes up magnetophonons, which results in a conductance peak that is displaced away from zero voltage and broadened compared with the case of no magnetic field. At low temperatures and low enough voltages the tunneling conductance is strongly suppressed, and the I–V characteristics exhibit a power-law behavior. The zero-voltage conductance is thermally activated with an activation temperature 10 K. The results are in very good agreement with experiment.     

Conductance of a quantum wire in a longitudinal magnetic field

We examine the ballistic conductance of a quantum wire in a parallel magnetic field in the presence of several impurities and derive analytic expressions for the transmission coefficient and the conductance in such a system. We show that scattering by impurities leads to a number of sharp peaks near the thresholds of the conductance quantization steps. The number of such peaks is determined by the distance between the impurities and the energy of the scattered particle. We also study the conductivity of a quantum wire in the region where the transport mechanism is diffusive. The conductivity is examined for the case in which charge carriers are scattered by randomly distributed point impurities. We study Shubnikov-de Haas oscillations in such a system. The general oscillation pattern consists of broad minima separated by irregularly spaced sharp peaks of the burst type. Zh. éksp. Teor. Fiz. 113, 1376–1396 (April 1998)     

Zero temperature conductance of parallel T-shape double quantum dots

We analyze the zero temperature conductance of a parallel T-shaped double quantum dot system. We present an analytical expression for the conductance of the system in terms of the total number of electrons in both quantum dots. Our results confirm that the system's conductance is strongly influenced by the dot which is not directly connected to the leads. We discuss our results in connection with similar results reported in the literature.     

Spin-orbit effects in a GaAs quantum dot in a parallel magnetic field

We analyze the effects of spin-orbit coupling on fluctuations of the conductance of a quantum dot fabricated in a GaAs heterostructure. Counterintuitively we argue that spin-orbit effects may become important in the presence of a large parallel magnetic field B( parallel), even if they are negligible for B( parallel) = 0. This should be manifest in the level repulsion of a closed dot, and in reduced conductance fluctuations in dots with a small number of open channels in each lead, for large B( parallel). Our picture is consistent with the experimental observations of Folk et al.     

Zero bias tunneling conductance for high field superconductors

The zero bias tunneling conductance is investigated theoretically and shown to be a promising experimental tool for detecting first order phase transitions in high field superconductors. We calculate and evaluate numerically the zero bias tunneling conductance for arbitrary spin orbital mean free path. In doing the calculations we distinguish between thin films in a parallel field and high?-value materials in a perpendicular field. In the former case anomalous properties appear for sufficient long spin orbital mean free path. Finally we determine the minimum spin orbital mean free path which is compatible with a first order phase transition and show that it is much smaller than commonly believed.     

Conductance of quantum interference transistors in parallel and in series

We theoretically study the electronic conductance G and the current–voltage characteristics of two quantum interference transistors in parallel and in series. We use two different definitions of conductance,G ∼ T and G ∼ T / R. Neither can reproduce the classical additivity law in the case of coherent transport due to quantum interference for the elements in series and quasibound states when elements are in parallel. In the case of two transistors in series, we find that the quantityT / R only qualitatively better represents the additivity law, which is probably expected because this model avoids counting the contact resistance twice. However, for the parallel configuration of transistors, the conductance is almost additive for the majority of energies when G ∼ T, except for the single-mode regime. Possible use of these configurations in digital electronics for basic logic functions is discussed.     

Spin-dependent transport induced by magnetization in zigzag graphene nanoribbons coupled to one-dimensional leads

We study spin transport in a zigzag graphene nanoribbon sample with two ferromagnetic strips deposited on the two sides of the ribbon. A tight-binding Hamiltonian was adopted to describe the sample connected to two one-dimensional leads. Our theoretical study shows that the resonance peaks of conductance for the spin-up and spin-down electrons are separated for the parallel configuration of the ferromagnetic strips, while they are not separated for the case of antiparallel configuration. This means that giant magnetoresistance can be produced at particular energies by altering the configurations of the ferromagnetic strips, and the device can be designed as a spin filter.     

Tenfold magnetoconductance in a nonmagnetic metal film

We present magnetoconductance (MC) measurements of homogeneously disordered Be films whose zero field sheet conductance ( G) is described by the Efros-Shklovskii hopping law G(T) = (2e(2)/h)exp-(T0/T)(1/2). The low field MC of the films is negative with G decreasing a factor of 2 below 1 T. In contrast the MC above 1 T is strongly positive. At 8 T, G increases tenfold in perpendicular field and fivefold in parallel field. In the simpler parallel case, we observe field enhanced variable range hopping characterized by an attenuation of T0 via the Zeeman interaction.     

Ballistic conductance of a quasi-one-dimensional microstructure in a parallel magnetic field

We discuss the behavior of the ballistic conductance of a quasi-one-dimensional microstructure in a parallel magnetic field when there is electron scattering by a single point impurity inside a channel. An exact analytic formula for the conductance is derived for a model in which the confinement potential is a parabolic well. We show that the conductance curve consists of quantization steps with sharp resonance peaks near the thresholds. Finally, we find the amplitudes and halfwidths of these peaks. Zh. éksp. Teor. Fiz. 111, 2215–2225 (June 1997)     

Aharonov-Bohm Interferometer in a T-Shaped Quantum Dot Embedded in Majorana Bound States *

We theoretically study the spin-dependent transport properties of anAharonov-Bohm (AB) interferometer composed by a T-shaped quantum dot (QD)embedded in Majorana bound states (MBS). We use the equation of motion method tocalculate the conductance across the interferometer. We note that the conductance exhibitssensitive dependence on the MBS-QD coupling strength as well as the polarization strengthof the leads when the phase factor of AB ring changes periodically. The conductance shows a transitionfrom resonance to anti-resonance when the MBS-QD coupling strength changes from small to large. Also, there is different p-dependence conductance when the leads alignment changesfrom parallel to anti-parallel. These findings suggest that such a model could be used for a sensitivedetection of MBS interactions, exploiting the high sensitivity of conductance to the AB phase in theinterferometer.     

Current and Shot Noise in a Quantum Dot Coupled to Ferromagnetic Leads in the Kondo Regime

Using the Keldysh nonequilibrium Green function technique, we study the current and shot noise spectroscopy of an interacting quantum dot coupled to two ferromagnetic leads with different polarizations in the Kondo regime. General formulas of current and shot noise are obtained, which can be applied in both the parallel （P） and antiparallel （AP） alignment cases. For large polarization values, it is revealed that the behaviour of differential conductance and shot noise are completely different for spin up and spin down configurations in the P alignment case. However, the differential conductance and shot noise have similar properties for different spin configurations in the P alignment case with the small polarization value and in the AP alignment case with any polarization value.     

Magnetotransport in a graphene monolayer with two tunable magnetic barriers

The magnetotransport property for a monolayer graphene with two turnable magnetic barriers has been investigated by the transfer-matrix method. We show that the parameters of barrier height, width, and interval between two barriers affect the electron wave decaying length, which determine the conductance with parallel or antiparallel magnetization configuration, and consequently the tunneling magnetoresistance (TMR) for the system. Interestingly, a graphene attached by two barriers with different heights can produce a resonant TMR peak at low energy region one order of magnitude larger than that for the system with two same height barriers because that the asymmetry of magnetic barriers block the electron transmission in the case of antiparallel magnetization configuration. The results obtained here may be useful in understanding of electron tunneling in graphene and in designing of graphene-based nanodevices.     

通过嵌入铁磁电极之间 Aharonov-Bohm 干涉仪自旋极化输运性质的研究

使用非平衡态格林函数方法和运动方程近似,研究了嵌入铁磁电极之间Aharonov-Bohm 干涉仪的自旋极化输运性质.在左右铁磁电极平行和反平行两种磁组态下,结合Fano因子分析和讨论了Fano 和Kondo 共振对该系统电导的影响,以及电导随自旋极化强度和磁通的变化.结果表明,自旋极化强度和磁通能有效地调节和控制电导,但电导的线形主要由磁通决定;在适当的条件下能导致大的正磁阻和负磁阻的出现.因此,该系统是一个很好的自旋阀晶体管,在自旋电子学中有潜在的应用价值. 关键词： Fano和Kondo共振 自旋极化强度 Fano因子 隧道磁阻     

Nonequilibrium transport through double quantum dots: Kondo effect versus antiferromagnetic coupling

We theoretically study the nonequilibrium transport properties of double quantum dots, in both series and parallel configurations. Our results lead to novel experimental predictions that unambiguously signal the transition from a Kondo state to an antiferromagnetic spin-singlet state, directly reflecting the physics of the two-impurity Kondo problem. We prove that the nonlinear conductance through parallel dots directly measures the exchange constant J between the spins of the dots. In serial dots, the nonlinear conductance provides an upper bound on J.     

A valley-filtering switch based on strained graphene

We investigate valley-dependent transport through a graphene sheet modulated by both the substrate strain and the fringe field of two parallel ferromagnetic metal (FM) stripes. When the magnetizations of the two FM stripes are switched from the parallel to the antiparallel alignment, the total conductance, valley polarization and valley conductance excess change greatly over a wide range of Fermi energy, which results from the dependence of the valley-related transmission suppression on the polarity configuration of inhomogeneous magnetic fields. Thus the proposed structure exhibits the significant features of a valley-filtering switch and a magnetoresistance device.     

Spin-polarized transport in zigzag graphene nanoribbons adsorbing nonmagnetic atomic chain

We theoretically study the electron transport properties in a ferromagnetic/normal/ferromagnetic tunnel junction, which is deposited on the top of a topological surface. The conductance at the parallel (P) configuration can be much bigger than that at the antiparallel (AP) configuration. Compared P with AP configuration, there exists a shift of phase which can be tuned by gate voltage. We find that the exchange field weakly affects the conductance of carriers for P configuration but can dramatically suppress the conductance of carriers for AP configuration. This controllable electron transport implies anomalous magnetoresistance in this topological spin valve, which may contribute to the development of spintronics. In addition, there shows an existence of Fabry-Perot-like electron interference in our model based on the topological insulator, which does not appear in the same model based on the two dimensional electron gas.     

The magnetoresistance effect in a nanostructure with the periodic magnetic barriers

The magnetoresistance (MR) effect is theoretically investigated in a periodic magnetically modulated nanostructure, which can be realized experimentally by depositing periodic parallel ferromagnetic strips on the top of a heterostructure. We find that there exists a significant conductance difference for electrons through the parallel (P) and antiparallel (AP) magnetization configurations, which results in a considerable magnetoresistance effect. We also find that the magnetoresistance effect depends not only on the temperature but also on the number of the periodic magnetic barriers.     

Magnetic Field Effects on Quantum-Dot Spin Valves

We study the magnetic field effects on the spin-polarized transport of the quantum dot (QD) spin valve in the sequential tunneling regime. A set of generalized master equation is derived. Based on that, we discuss the collinear and noncollinear magnetic field effects, respectively. In the collinear magnetic field case,we find that the Zeeman splitting can induce a negative differential conductance (NDC), which is quite different from the one found in previous studies. It has a critical polarization in the parallel arrangement and will disappear in the antiparallelconfiguration. In the noncollinear magnetic field case, the current shows two plateaus and their angular dependence is analyzed. Although sometimes the two current plateaus have similar angular dependence, their mechanisms are different. Our formalism is also suitable for calculating the transport in magnetic molecules, in which the spin splitting is induced not by a magnetic field but by the intrinsic magnetization.