Effect of lattice distortion on spin admixture and quantum transport in organic devices with spin–orbit coupling

With an extended Su–Schrieffer–Heeger model and Green's function method, the spin–orbit coupling(SOC) effects on spin admixture of electronic states and quantum transport in organic devices are investigated. The role of lattice distortion induced by the strong electron–lattice interaction in organics is clarified in contrast with a uniform chain. The results demonstrate an enhanced SOC effect on the spin admixture of frontier eigenstates by the lattice distortion at a larger SOC,which is explained by the perturbation theory. The quantum transport under the SOC is calculated for both nonmagnetic and ferromagnetic electrodes. A more notable SOC effect on total transmission and current is observed for ferromagnetic electrodes, where spin filtering induced by spin-flipped transmission and suppression of magnetoresistance are obtained.Unlike the spin admixture, a stronger SOC effect on transmission exists for the uniform chain rather than the organic lattices with distortion. The reason is attributed to the modified spin-polarized conducting states in the electrodes by lattice configuration, and hence the spin-flip transmission, instead of the spin admixture of eigenstates. This work is helpful to understand the SOC effect in organic spin valves in the presence of lattice distortion.

Influence of dopant concentration on electrical quantum transport behaviors in junctionless nanowire transistors

We discuss the random dopant effects in long channel junctionless transistor associated with quantum confinement effects. The electrical measurement reveals the threshold voltage variability induced by the random dopant fluctuation. Quantum transport features in Hubbard systems are observed in heavily phosphorus-doped channel. We investigate the single electron transfer via donor-induced quantum dots in junctionless nanowire transistors with heavily phosphorus- doped channel, due to the formation of impurity Hubbard bands. While in the lightly doped devices, one-dimensional quantum transport is only observed at low temperature. In this sense, phonon-assisted resonant-tunneling is suppressed due to misaligned levels formed in a few isolated quantum dots at cryogenic temperature. We observe the Anderson-Mott transition from isolate electron state to impurity bands as the doping concentration is increased.

Quantum transport characteristics in single and multiple N-channel junctionless nanowire transistors at low temperatures

Single and multiple n-channel junctionless nanowire transistors （JNTs） are fabricated and experimentally investigated at variable temperatures. Clear current oscillations caused by the quantum-confinement effect are observed in the curve of drain current versus gate voltage acquired at low temperatures （10 K-100 K） and variable drain bias voltages （10 mV- 90 mV）. Transfer characteristics exhibit current oscillation peaks below flat-band voltage （VFB） at temperatures up to 75 K, which is possibly due to Coulomb-blocking from quantum dots, which are randomly formed by ionized dopants in the just opened n-type one-dimensional （1D） channel of silicon nanowires. However, at higher voltages than VFB, regular current steps are observed in single-channel JNTs, which corresponds to the fully populated subbands in the 1D channel. The subband energy spacing extracted from transconductance peaks accords well with theoretical predication. However, in multiple-channel JNT, only tiny oscillation peaks of the drain current are observed due to the combination of the drain current from multiple channels with quantum-confinement effects.

Superexchange interaction enhancement of the quantum transport in a DNA-type molecule

We use the transfer matrix method and the Green function technique to theoretically study the quantum tunnelling through a DNA-type molecule. Ferromagnetic electrodes are used to produce the spin-polarized transmission probability and therefore the spin current. The distance-dependent crossover comes from the topological variation from the one- dimensional to the two-dimensional model transform as we switch on the interstrand coupling; a new base pair will present N - 1 extrachannels for the charge and spin as N being the total base pairs. This will restrain the decay of the transmission and improve the stability of the quantum transport. The spin and charge transfer through the DNA-type molecule is consistent with the quantum tunneling barrier.

Quantum transport simulation of the two-dimensional GaSb transistors

Owing to the high carrier mobility,two-dimensional(2D)gallium antimonite(GaSb)is a promising channel material for field-effect transistors(FETs)in the post-silicon era.We investigated the ballistic performance of the 2D GaSb metal-oxide-semiconductor FETs with a 10 nm-gate-length by the ab initio quantum transport simulation.Because of the wider bandgap and better gate-control ability,the performance of the 10-nm monolayer(ML)GaSb FETs is generally superior to the bilayer counterparts,including the three-to-four orders of magnitude larger on-current.Via hydrogenation,the delaytime and power consumption can be further enhanced with magnitude up to 35%and 57%,respectively,thanks to the expanded bandgap.The 10-nm ML GaSb FETs can almost meet the International Technology Roadmap for Semiconductors(ITRS)for high-performance demands in terms of the on-state current,intrinsic delay time,and power-delay product.

Observation of source/drain bias-controlled quantum transport spectrum in junctionless silicon nanowire transistor

We investigate the influence of source and drain bias voltages(V_(DS))on the quantum sub-band transport spectrum in the 10-nm width N-typed junctionless nanowire transistor at the low temperature of 6 K.We demonstrate that the transverse electric field introduced from V_(DS) has a minor influence on the threshold voltage of the device.The transverse electric field plays the role of amplifying the gate restriction effect of the channel.The one-dimensional(1D)-band dominated transport is demonstrated to be modulated by V_(DS) in the saturation region and the linear region,with the sub-band energy levels in the channel(E_(channel))intersecting with Fermi levels of the source(E_(fS))and the drain(E_(fD))in turn as V_(g) increases.The turning points from the linear region to the saturation region shift to higher gate voltages with V_(DS) increase because the higher Fermi energy levels of the channel required to meet the situation of E_(fD)=E_(channel).We also find that the bias electric field has the effect to accelerate the thermally activated electrons in the channel,equivalent to the effect of thermal temperature on the increase of electron energy.Our work provides a detailed description of the bias-modulated quantum electronic properties,which will give a more comprehensive understanding of transport behavior in nanoscale devices.

Interaction induced non-reciprocal three-level quantum transport

Besides its fundamental importance, non-reciprocity has also found many potential applications in quantum technology. Recently, many quantum systems have been proposed to realize non-reciprocity, but stable non-reciprocal process is still experimentally difficult in general, due to the needed cyclical interactions in artificial systems or operational difficulties in solid state materials. Here, we propose a new kind of interaction induced non-reciprocal operation, based on the conventional stimulated-Raman-adiabatic-passage (STIRAP) setup, which removes the experimental difficulty of requiring cyclical interaction, and thus it is directly implementable in various quantum systems. Furthermore, we also illustrate our proposal on a chain of three coupled superconducting transmons, which can lead to a non-reciprocal circulator with high fidelity without a ring coupling configuration as in the previous schemes or implementations. Therefore, our protocol provides a promising way to explore fundamental non-reciprocal quantum physics as well as realize non-reciprocal quantum device.

Preface to the Special Issue on Quantum Transport in Mesoscopic Systems

Mesoscopic systems,including nanowires,quantum dots and two-dimensional electron gases,are excellent platforms for studying emerging quantum phenomena,especially in the field of electrical transport.Quantum transport covers vast scopes of condensed matter physics,such as superconductivity,quantum Hall effect,and many investigations in mesoscopic devices.

Controlling the Directed Quantum Transport of Ultracold Atoms in an Optical Lattice with a Periodic Driving Field

We propose a new method to control the directed quantum transport of ultracold atoms in a one-dimensional optical lattice. In this proposal, the effective tunneling between the neighboring sites can be adjusted via coherent destruction of tunneling by tuning the phase of the external field, instead of using the driving field intensity or the frequency, thus the directed quantum transport of ultracold atoms can be coherently controlled in a nmch easier manner. Our proposal overcomes the major drawback of the method used by Creffield et al [Phys. Rev. Lett. 99 （2007） 110501], and can be implemented, in principle, in any one-dimensional optical lattice. Some potential applications of the scheme are also discussed.

Quantum transport through a multi-quantum-dot-pair chain side-coupled with Majorana bound states

We investigate the quantum transport properties through a special kind of quantum dot（QD） system composed of a serially coupled multi-QD-pair（multi-QDP） chain and side-coupled Majorana bound states（MBSs） by using the Green functions method,where the conductance can be classified into two kinds：the electron tunneling（ET） conductance and the Andreev reflection（AR） one.First we find that for the nonzero MBS-QDP coupling a sharp AR-induced zero-bias conductance peak with the height of e^2/h is present（or absent） when the MBS is coupled to the far left（or the other） QDP.Moreover,the MBS-QDP coupling can suppress the ET conductance and strengthen the AR one,and further split into two sub-peaks each of the total conductance peaks of the isolated multi-QDPs,indicating that the MBS will make obvious influences on the competition between the ET and AR processes.Then we find that the tunneling rate ΓLis able to affect the conductances of leads L and R in different ways,demonstrating that there exists a ΓL-related competition between the AR and ET processes.Finally we consider the effect of the inter-MBS coupling on the conductances of the multi-QDP chains and it is shown that the inter-MBS coupling will split the zero-bias conductance peak with the height of e^2/h into two sub-peaks.As the inter-MBS coupling becomes stronger,the two sub-peaks are pushed away from each other and simultaneously become lower,which is opposite to that of the single QDP chain where the two sub-peaks with the height of about e^2/2h become higher.Also,the decay of the conductance sub-peaks with the increase of the MBS-QDP coupling becomes slower as the number of the QDPs becomes larger.This research should be an important extension in studying the transport properties in the kind of QD systems coupled with the side MBSs,which is helpful for understanding the nature of the MBSs,as well as the MBS-related QD transport properties.

Single-photon scattering and quantum entanglement of two giant atoms with azimuthal angle differences in a waveguide system

We theoretically investigate coherent scattering of single photons and quantum entanglement of two giant atoms with azimuthal angle differences in a waveguide system.Using the real-space Hamiltonian,analytical expressions are derived for the transport spectra scattered by these two giant atoms with four azimuthal angles.Fano-like resonance can be exhibited in the scattering spectra by adjusting the azimuthal angle difference.High concurrence of the entangled state for two atoms can be implemented in a wide angle-difference range,and the entanglement of the atomic states can be switched on/off by modulating the additional azimuthal angle differences from the giant atoms.This suggests a novel handle to effectively control the single-photon scattering and quantum entanglement.

Electric modulation of the Fermi arc spin transport via three-terminal configuration in topological semimetal nanowires

Spin–momentum locking is a key feature of the topological surface state, which plays an important role in spintronics.The electrical detection of current-induced spin polarization protected by the spin–momentum locking in nonmagnetic systems provides a new platform for developing spintronics, while previous studies were mostly based on magnetic materials.In this study, the spin transport measurement of Dirac semimetal Cd_(3)As_(2) was studied by three-terminal geometry, and a hysteresis loop signal with high resistance and low resistance state was observed. The hysteresis was reversed by reversing the current direction, which illustrates the spin–momentum locking feature of Cd_(3)As_(2). Furthermore, we realized the on–off states of the spin signals through electric modulation of the Fermi arc via the three-terminal configuration, which enables the great potential of Cd_(3)As_(2) in spin field-effect transistors.

Number-resolved master equation approach to quantum transport under the self-consistent Born approximation

We construct a particle-number （n）-resolved master equation （ME） approach under the self-consistent Born approximation （SCBA） for quantum transport through mesoscopic systems. The formulation is essentially non-Markovian and incorporates the interplay of the multi-tunneling processes and many-body correlations. The proposed n-SCBA-ME goes beyond the scope of the Born- Markov master equation, being applicable to transport under small bias voltage, in non-Markovian regime and with strong Coulomb correlations. For steady state, it can recover not only the exact result of noninteracting transport under arbitrary voltages, but also the challenging nonequilibrium Kondo effect. Moreover, the n-SCBA-ME approach is efficient for the study of shot noise. We demonstrate the application by a couple of representative examples, including particularly the nonequilibrium Kondo system.

Adaptive Conservative Cell Average Spectral Element Methods for Transient Wigner Equation in Quantum Transport

A new adaptive cell average spectral element method(SEM)is proposed to solve the time-dependent Wigner equation for transport in quantum devices.The proposed cell average SEM allows adaptive non-uniform meshes in phase spaces to reduce the high-dimensional computational cost of Wigner functions while preserving exactly the mass conservation for the numerical solutions.The key feature of the proposed method is an analytical relation between the cell averages of the Wigner function in the k-space(local electron density for finite range velocity)and the point values of the distribution,resulting in fast transforms between the local electron density and local fluxes of the discretized Wigner equation via the fast sine and cosine transforms.Numerical results with the proposed method are provided to demonstrate its high accuracy,conservation,convergence and a reduction of the cost using adaptive meshes.

Time-dependent density functional theory for quantum transport

The rapid miniaturization of elect, ronic devices motivates research interests in quantum transport.Recently time-dependent quantum transport has become an important research topic. Here we re- view recent progresses in the development of time-dependent density-functional theory for quantum transport including the theoretical foundation and numerical algorithms. In particular, the reducedsingle electron density matrix based hierarchical equation of motion, which can be derived from Liouville---von Neumann equation, is reviewed in details. The numerical implementation is discussed and simulation results of realistic devices will be given.

Number-resolved master equation approach to quantum measurement and quantum transport

In addition to the well-known Landauer-Bfittiker scattering theory and the nonequilibrium Green＇s function technique for mesoscopic transports, an alternative （and very useful） scheme is quantum master equation approach. In this article, we review the particle-number （n）-resolved master equation （n-ME） approach and its systematic applications in quantum measurement and quantum transport problems. The n-ME contains rich dynamical information, allowing efficient study of topics such as shot noise and flfll counting statistics analysis. Moreover, we also review a newly developed master equation approach （and its n-resolved version） under self-consistent Born approximation. The application potential of this new approach is critically examined via its ability to recover the exact results for noninteracting systems under arbitrary voltage and in presence of strong quantum interference, and the challenging non-equilibrium Kondo effect.

Local Discontinuous Galerkin Methods for the 2D Simulation of Quantum Transport Phenomena on Quantum Directional Coupler

In this paper,we present local discontinuous Galerkin methods(LDG)to simulate an important application of the 2D stationary Schrödinger equation called quantum transport phenomena on a typical quantum directional coupler,which frequency change mainly reflects in y-direction.We present the minimal dissipation LDG(MD-LDG)method with polynomial basis functions for the 2D stationary Schrödinger equation which can describe quantum transport phenomena.We also give the MDLDG method with polynomial basis functions in x-direction and exponential basis functions in y-direction for the 2D stationary Schrödinger equation to reduce the computational cost.The numerical results are shown to demonstrate the accuracy and capability of these methods.

Master equation approach to transient quantum transport in nanostructures

In this review article, we present a non-equilibrium quantum transport theory for transient electron dynamics in nanodevices based on exact Master equation derived with the path integral method in the fermion coherent-state representation. Applying the exact Master equation to nanodevices, we also establish the connection of the reduced density matrix and the transient quantum transport current with the Keldysh nonequilibrium Green functions. The theory enables us to study transient quantum transport in nanostructures with back-reaction effects from the contacts, with non-Markovian dissipa- tion and decoherence being fully taken into account. In applications, we utilize the theory to specific quantum transport systems, a variety of quantum decoherence and quantum transport phenomena involving the non-Markovian memory effect are investigated in both transient and stationary scenarios at arbitrary initial temperatures of the contacts.

Wave-function approach to Master equations for quantum transport and measurement

This paper presents a comprehensive review of the wave-flmction approach for derivation of the number- resolved Master equations, used for description of transport and measurement in mesoseopie systems. The review contains important amendments, clarifying subtle points in derivation of the Master equa- tions and their validity. This completes the earlier works on the subject. It is demonstrated that the derivation does not assume weak coupling with the environment and reservoirs, but needs only high bias condition. This condition is very essential for validity of the Markovian Master equations, widely used for a phenomenological description of different physical processes.

Kondo transport through a quantum dot coupled with side quantum-dot structures

This paper investigates Kondo transport properties in a quadruple quantum dot （QD） based on the slave-boson mean field theory and the non-equilibrium Green＇s function. In the quadruple QD structure one Kondo-type QD sandwiched between two leads is side coupled to two separate QD structures： a single-QD atom and a double-QD molecule. It shows that the conductance valleys and peaks always appear in pairs and by tuning the energy levels in three side QDs, the one-, two-, or three-valley conductance pattern can be obtained. Furthermore, it finds that whether the valley and the peak can appear is closely dependent on the specific values of the interdot couplings and the energy level difference between the two QDs in the molecule. More interestingly, an extra novel conductance peak can be produced by the coexistence of the two different kinds of side QD structures.