基于自洽输运理论的介观体系动态电导的研究

基于介观体系电子动态输运的自洽理论,讨论了介观结构的动态电导.作为该理论的应用,采用一介观相干平行板电容器模型来进行研究. 结果表明:体系的动态电导与外场频率和体系费米能有关,为一复数且有有限虚部. 当外场频率较小时,动态电导随费米能的变化所呈现的特性和直流情形非常相似,但是随着外场频率的增加,两者差异就变得非常明显,体系动态电导随外场频率的变化呈现一些峰值结构. 在给定体系费米能时,动态电导随着外场频率的变化而产生振荡,并且出现了负的电导虚部,电导虚部的正负表明了体系的电容特性和电感特性. 关键词： 自洽输运理论 相干平行板电容器 电导 介观体系     

Universal transverse conductance between quantum Hall regions and (2+1) D bosonization

Using bosonization techniques for (2+1) D systems, we show that the transverse conductance for a system with general current interactions, when measured between perfect Hall regions is not renormalized at low temperatures. Our method extends two results we have recently obtained on low dimensional fermionic systems: on the one hand, the relationship between universality of Landauer conductance and universality of bosonization rules for (1+1) D systems, and on the other hand, the universal character of the bosonized topological current associated to a (2+1) D fermionic system with current interactions.     

Measurements of the dynamic conductance on SS-and NS-point contacts

Mechanically point contacts made of Ta, Al, Ag (anvil) and of Ta, Ag (needle) were investigated in the temperature range between 4.2 K and 1.3 K and in magnetic fields up to 0.8 T. For point contacts consisting of two superconducting electrodes we measured the current voltage characteristics and also the dynamic conductance. The characteristics with a negative differential conductance and the occurrence of an excess current point to the appearance of Andreev-multiple reflection at an SNS-interface in the contact region.The investigated current voltage characteristics and the dynamic conductance of typical NS-point contacts (Ta–Ag and Ta–Al) point to a microconstriction with a barrier of arbitrary strength at the interface between the normalconductor and the superconductor. We performed theoretical calculations forT=0 and also for temperatures up to the critical temperature using the model of Blonder, Tinkham and Klapwijk (BTK model). In this model the Andreev-reflection is the main scattering mechanism at the NS-interface but also elastic scattering and transmission processes are taken into consideration. The comparison of our theoretical results with the experimental results and also the determined excess current at all investigated point contacts lead to the assumption that there exist metallic contacts with very small barrier strengths at the NS-interface where the Andreev-reflection is the dominant process and the other scattering mechanisms play a subdominant role.     

Photo control of transport properties in a disordered wire: Average conductance,conductance statistics,and time-reversal symmetry

In this paper, we study the full conductance statistics of a disordered 1D wire under the application of light. We develop the transfer matrix method for periodically driven systems to analyze the conductance of a large system with small frequency of light, where coherent photon absorptions play an important role to determine not only the average but also the shape of conductance distributions. The average conductance under the application of light results from the competition between dynamic localization and effective dimension increase, and shows non-monotonic behavior as a function of driving amplitude. On the other hand, the shape of conductance distribution displays a crossover phenomena in the intermediate disorder strength; the application of light dramatically changes the distribution from log-normal to normal distributions. Furthermore, we propose that conductance of disordered systems can be controlled by engineering the shape, frequency and amplitude of light. Change of the shape of driving field controls the time-reversals symmetry and the disordered system shows analogous behavior as negative magneto-resistance known in static weak localization. A small change of frequency and amplitude of light leads to a large change of conductance, displaying giant opto-response. Our work advances the perspective to control the mean as well as the full conductance statistics by coherently driving disordered systems.     

Electron transport through a quantum dot in the presence of electron-photon and electron-phonon coupling

In this study, electron transport through a mesoscopic quantum dot system (QD-system) in the presence of electron-photon (el-pt) and electron-phonon (el-ph) interactions is discussed. The role of both of these interactions is to induce additional steps in the current, and sideband peaks in the differential conductance. By calculating the current and differential conductance of a QD-system in the presence of el-pt or el-ph coupling, we have shown that photon or phonon steps and sideband peaks are induced in the current and differential conductance whenever the applied voltage resonates with their frequency. Furthermore, additional side band peaks are induced in the differential conductance when el-pt and el-ph interactions are simultaneously included in the QD. These extra sideband peaks (ESBPs) are induced when the applied voltage and the photon frequency are in close proximity with the phonon frequency. To investigate the relationship that exists between the photon and phonon frequency in inducing ESBPs in the differential conduction, we have discussed zero applied voltage differential conductance. Under such conditions, ESBP is induced only when the photon frequency resonates with the phonon frequency. With increasing el-ph coupling amplitude, more ESBPs are induced in the differential conductance.     

NMR probing of spin excitations in the ring structure of a two-subband system

Resistively detected nuclear magnetic resonance (NMR) is observed inside the ringlike structure, with a quantized Hall conductance of 6e(2)/h, in the phase diagram of a two-subband electron system. The NMR signal persists up to 470 mK and is absent in other states with the same quantized Hall conductance. The nuclear spin-lattice relaxation time T1, is found to decrease rapidly towards the ring center. A strong dynamic nuclear polarization by the biasing current has also been observed only inside the ring. These observations are consistent with the assertion of the ringlike region being a ferromagnetic state that is accompanied by collective spin excitations.     

Photon-Assisted Conductance Structure for a Quantum Dot

We investigate theoretically the electron transport of a two-level quantum dot irradiated under a weak laser field at low temperatures in the rotating wave approximation. Using the method of the Keldysh equation of motion for nonequilibrium Green function, we examine the conductance for the system with photon polarization perpendicular to the tunnelling current direction. It is demonstrated that by analytic analysing and numerical examples, a feature of conductance peak splitting appears, and the dependence of conductance on the incident laser frequency and self-energy are discussed.     

塞曼效应对正常金属/超导/正常金属双隧道结中的相干隧穿电导的影响

在超导中通过外加塞曼磁场,研究正常金属/超导/正常金属双隧道结中的量子相干输运。同时考虑从一个正常金属电极注入一电子,从另一个正常金属电极注入一空穴,推导出系统的微分电导的一般公式。研究表明,电导谱随偏压展示振荡行为,随着温度和磁场的增大,其振荡振幅被降低,且塞曼能可导致电导峰的塞曼劈裂。在隧道极限下,超导体中会形成一系列束缚态。     

Resonant Photon-Assisted Structure of Conductance through a Quantum Wire

We investigate theoretically the electron transport properties of a two-sublevel quantum wire irradiated by a strong laser field resonant with the quasiparticle transition at low temperatures. Using the method of the Keldysh equation of motion for nonequilibrium Green function, we examine the time-averaged conductance for the system with photon polarization parallel and perpendicular to the tunnelling current direction, respectively. We demonstrate that, by analysing some numerical examples, a feature of absolute negative conductance appears in the parallel case, while the conductance shows a symmetry distributed peaks in the Derpendicular case.     

Full counting statistics for a single-electron transistor: nonequilibrium effects at intermediate conductance

We evaluate the current distribution for a single-electron transistor with intermediate strength tunnel conductance. Using the Schwinger-Keldysh approach and the drone (Majorana) fermion representation, we account for the renormalization of system parameters. Nonequilibrium effects induce a lifetime broadening of the charge-state levels, which suppress large current fluctuations.     

Dynamic conductance of carbon nanotubes

The dynamic conductance of carbon nanotubes was investigated using the nonequilibrium Green's function formalism within the context of a tight-binding model. Specifically, we have studied the ac response of tubes of different helicities, both with and without defects, and an electronic heterojunction. Because of the induced displacement currents, the dynamic conductance of the nanotubes differs significantly from the dc conductance displaying both capacitive and inductive responses. The important role of photon-assisted transport through nanotubes is revealed and its implications for experiments discussed.     

Spin-dependent Andreev reflection in a three-terminal Aharonov-Bohm interferometer with coherent indirect coupling

Within the framework of non-equilibrium Green’s functions, we investigate the spin-dependent Andreev reflection (AR) in a three-terminal Aharonov-Bohm interferometer with double quantum dot, taking account of the coherent indirect coupling via the superconducting reservoir. It is found that the time-reversal symmetry is broken by the crossed Andreev reflection (CAR) process, and moreover that the spin-value effect of the linear conductance, the spin-polarised AR current, and a pure spin current can be generated by means of the normal AR and the CAR. Expressions for the AR conductances (the transport coefficients) governing the AR properties of this system are derived analytically. The effect of the coherent indirect coupling on the conductance, the Andreev reflected tunneling magnetoresistance (ARTMR), and the spin-related current in the presence of the AR are amply analyzed. Our results indicate that the optimal properties of this system can be realised by tuning the external parameters.     

Influences of a Side-Coupled Triple Quantum Dot on Kondo Transport Through a Quantum Dot

Kondo transport properties through a Kondo-type quantum dot (QD) with a side-coupled triple-QD structure are systematically investigated by using the non-equilibrium Green's function method. We firstly derive the formulae of the current, the linear conductance, the transmission coefficient, and the local density of states. Then we carry out the analytical and numerical studies and some universal conductance properties are obtained. It is shown that the number of the conductance valleys is intrinsically determined by the side-coupled QDs and at most equal to the number of the QDs included in theside-coupled structure in the asymmetric limit. In the process of forming the conductance valleys, the side-coupled QD system plays the dominant role while the couplings between the Kondo-type QD and the side-coupled structure play the subsidiary and indispensable roles. To testify the validity of the universal conductance properties, another different kinds of side-coupled triple-QD structures are considered. It should be emphasized that these universal properties are applicable in understanding this kind of systems with arbitrary many-QD side structures.     

Nanobubbles in solid-state nanopores

From conductance and noise studies, we infer that nanometer-sized gaseous bubbles (nanobubbles) are the dominant noise source in solid-state nanopores. We study the ionic conductance through solid-state nanopores as they are moved through the focus of an infrared laser beam. The resulting conductance profiles show strong variations in both the magnitude of the conductance and in the low-frequency noise when a single nanopore is measured multiple times. Differences up to 5 orders of magnitude are found in the current power spectral density. In addition, we measure an unexpected double-peak ionic conductance profile. A simple model of a cylindrical nanopore that contains a nanobubble explains the measured profile and accounts for the observed variations in the magnitude of the conductance.     

Conductance of a 2D electron system at low frequencies

Low-frequency features of the conductance of a 2D electron system are considered. It is shown that, in addition to the parameter ωτ (ω is the external frequency and τ is the elastic relaxation time), which occurs in the frequency dependence of the 3D conductivity in the Drude approximation, the 2D conductance contains other dimensionless combinations that involve the external frequency and the 2D conductivity. The notion of the “ mobility” of a 2D system as the quantity governing the deviation of the conductance of the 2D system with ac current from its conductance under stationary conditions is introduced. Experimental data testifying to the presence of the discussed features of the 2D conductance are presented.     

正常金属/自旋三重态p波超导体结隧道谱的奇异性

利用Blonder，Tinkham和Klapwijk理论计算了正常金属/绝缘层/正常金属/自旋三重态的p波超导体结的隧道谱和平均电流.计算结果表明：在自旋三重态p波超导结的隧道谱中存在零偏压电导峰、零偏压电导凹陷和双凹陷结构，并有微分电导随偏压震荡的现象出现，在I-V曲线上出现电流台阶.这些结果在理论上支持Sr₂RuO₄的超导态是自旋三重态p波超导态. 关键词： 自旋三重态超导体 p波超导体 隧道谱     

The probe technique far from equilibrium: Magnetic field symmetries of nonlinear transport

The probe technique is a simple mean to incorporate elastic and inelastic processes into quantum transport problems. Using numerical simulations, we demonstrate that this tool can be employed beyond the analytically tractable linear response regime, providing a stable solution for the probe parameters: temperature and chemical potential. Adopting four probes: dephasing, voltage, temperature, and voltage-temperature, mimicking different elastic and inelastic effects, we provide a systematic analysis of magnetic field and gate voltage symmetries of charge current and heat current in Aharonov-Bohm interferometers, potentially far from equilibrium. Considering electron current, we prove that in the linear response regime inelastic scattering processes do not break the Onsager symmetry. Beyond linear response, even (odd) conductance terms obey an odd (even) symmetry with the threading magnetic flux, as long as the system acquires a spatial inversion symmetry. When spatial asymmetry is introduced, particle-hole symmetry assures that nonlinear conductance terms maintain certain symmetries with respect to magnetic field and gate voltage. These analytic results are supported by numerical simulations. Analogous results are obtained for the electron heat current. Finally, we demonstrate that a double-dot Aharonov-Bohm interferometer can act as a charge rectifier when two conditions are met simultaneously: (i) many-body effects are included, here in the form of inelastic scattering; and (ii) time reversal symmetry is broken.     

Conductance histograms and conducting channels in atomic-scale metallic contacts

Properties of the conductance and transport channels of atomic-scale contacts are discussed within the free electron model. In agreement with recent experiments, we find that, owing to the contribution of evanescent channels, even when the conductance is close to an integer number N of conductance quanta more than N channels may contribute significantly to the current. We show that the observation of peaks at integer multiples of G₀ in conductance histograms is not a signature of conductance quantization of individual contacts. However, the observed peaks can still be associated to the quantum nature of electron transport in metallic contacts.     

Analytical and numerical study of quantum transport in an array of nanorings: A case study with double rings

We present an analytical method to obtain an expression for electron transmission through an array of disordered nanorings (ADNRs) sandwiched between two semi-infinite metallic leads in different lead-ring coupling strengths. Our approach is based on the nearest-neighbor tight-binding approximation and transfer matrix method. First, we derive an analytical formula for electron transmission across the system. Next, we apply our approach to study of the electron transport in a perfect double nanoring (PDNR) and design a NOR gate using the magnetic fluxes inputs. The conductance, current–voltage characteristics and threshold voltage of the system are calculated in the weak and strong coupling regimes.     

Interplay of electron-electron and electron-phonon interactions in molecular junctions

In this work we consider a current carrying molecular junction with both electron-phonon and electron-electron interactions taken into account. After performing Lang-Firsov transformation and considering Markov approximations in accordance to weak coupling to the electronic leads, we obtain the master equation governing the time evolution of the reduced density matrix of the junction. The steady state of the density matrix can be used to obtain I-V characteristic of the junction in several regimes of strengths of the interactions. Our results indicate that the system can show negative differential conductance (that is, the current decreases by increasing the applied bias voltage) in some regimes as an interplay between the electron-phonon and Coulomb interactions.