Quantum Transport Theory Research Articles

구속 포텐셜에서 반도체 물질(Ge)의 자기장 의존성

This paper proposes an application of artificial neural networks for analyzing electricity market that has insufficient information for calculating equilibrium. Neural networks are constructed and trained on two representative cases in the electricity market. One is for calculating equilibrium price in perfect competition market and the other is for determining whether the transmission congestion occurs. The neural network uses a multilayer structure and learns with backpropagation algorithms for training. The neural networks trained in the case studies calculate the market price with a high probability and also determines an occurrence of the transmission congestion accurately. We study optical quantum transition line shapes (QTLS) showing the absorption power and quantum transition line widths (QTLW) of an electron deformation potential phonon interaction system. In order to analyze quantum transfer, the QTLW and QTLS with the magnetic field dependence was compared in two transfer processes, I.e., intra-Land level transfer process. In order to apply quantum transport theory (QTR) to a system that confines electrons by a square well confining potential, the Liouville equation projected with the Equilibrium Average Projection Scheme (EAPS) was used.

Conformation and Quantum-Interference-Enhanced Thermoelectric Properties of Diphenyl Diketopyrrolopyrrole Derivatives.

Manipulating the connectivity of external electrodes to central rings of carbon-based molecules in single molecule junctions is an effective route to tune their thermoelectrical properties. Here we investigate the connectivity dependence of the thermoelectric properties of a series of thiophene-diketopyrrolopyrrole (DPP) derivative molecules using density functional theory and tight-binding modeling, combined with quantum transport theory. We find a significant dependence of electrical conductance on the connectivity of the two thiophene rings attached to the DPP core. Interestingly, for connectivities corresponding to constructive quantum interference (CQI), different isomers obtained by rotating the thiophene rings possess the same electrical conductance while those corresponding to destructive quantum interference (DQI) show huge conductance variations upon ring rotation. Furthermore, we find that DQI connectivity leads to enhanced Seebeck coefficients, which can reach 500–700 μV/K. After including the contribution to the thermal conductance from phonons, the full figure of merit (ZT) for the CQI molecules could reach 1.5 at room temperature and it would further increase to 2 when temperature elevates to 400 K. Finally, we demonstrate that doping with tetracyanoquinodimethane can change the sign of the Seebeck coefficients by forming a charge-transfer system with the DPP.

Quench dynamics of a weakly interacting disordered Bose gas in momentum space

We theoretically study the out-of-equilibrium dynamics in momentum space of a weakly interacting disordered Bose gas launched with a finite velocity. In the absence of interactions, coherent multiple scattering gives rise to a background of diffusive particles, on top of which a coherent backscattering interference emerges. We revisit this scenario in the presence of interactions, using a diagrammatic quantum transport theory. We find that the dynamics is governed by coupled kinetic equations describing the thermalization of the diffusive and coherent components of the gas. This phenomenon leads to a destruction of coherent backscattering, well described by an exponential relaxation whose rate is controlled by the particle collision time. These predictions are confirmed by numerical simulations.

Unified bulk semiclassical theory for intrinsic thermal transport and magnetization currents

We reveal the unexpected role of the material inhomogeneity in unifying the formulation of intrinsic thermal and thermoelectric transport as well as magnetization currents. The smooth inhomogeneity leads to the position dependent local band dispersion and phase-space Berry curvature, enabling a general and rapid access to transport and magnetization currents displaying the momentum-space Berry curvature physics. Our theory does not invoke the boundary current, the thermodynamic approach to magnetization or any mechanical counterpart of statistical forces. By introducing a fictitious inhomogeneity, it applies to homogeneous samples as well, promoting the inhomogeneity to be a basic trick in semiclassical transport theories. Such a trick works regardless of the driving force of transport, e.g., temperature gradient, in contrast to the trick of fictitious gravitational field in quantum transport theories. We thus include more general mechanical driving forces and establish the Mott relation between the resulting transport thermal and electric currents, whereas this relation for these two currents was previously only known when an electric field is the driving force.

Redox Control of Charge Transport in Vertical Ferrocene Molecular Tunnel Junctions

Summary Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

Room-temperature quantum interference in single perovskite quantum dot junctions

The studies of quantum interference effects through bulk perovskite materials at the Ångstrom scale still remain as a major challenge. Herein, we provide the observation of room-temperature quantum interference effects in metal halide perovskite quantum dots (QDs) using the mechanically controllable break junction technique. Single-QD conductance measurements reveal that there are multiple conductance peaks for the CH3NH3PbBr3 and CH3NH3PbBr2.15Cl0.85 QDs, whose displacement distributions match the lattice constant of QDs, suggesting that the gold electrodes slide through different lattice sites of the QD via Au-halogen coupling. We also observe a distinct conductance ‘jump’ at the end of the sliding process, which is further evidence that quantum interference effects dominate charge transport in these single-QD junctions. This conductance ‘jump’ is also confirmed by our theoretical calculations utilizing density functional theory combined with quantum transport theory. Our measurements and theory create a pathway to exploit quantum interference effects in quantum-controlled perovskite materials.

Evaporative electron cooling in asymmetric double barrier semiconductor heterostructures

Rapid progress in high-speed, densely packed electronic/photonic devices has brought unprecedented benefits to our society. However, this technology trend has in reverse led to a tremendous increase in heat dissipation, which degrades device performance and lifetimes. The scientific and technological challenge henceforth lies in efficient cooling of such high-performance devices. Here, we report on evaporative electron cooling in asymmetric Aluminum Gallium Arsenide/Gallium Arsenide (AlGaAs/GaAs) double barrier heterostructures. Electron temperature, Te, in the quantum well (QW) and that in the electrodes are determined from photoluminescence measurements. At 300 K, Te in the QW is gradually decreased down to 250 K as the bias voltage is increased up to the maximum resonant tunneling condition, whereas Te in the electrode remains unchanged. This behavior is explained in term of the evaporative cooling process and is quantitatively described by the quantum transport theory.

An improved Green’s function algorithm applied to quantum transport in carbon nanotubes

The renormalization-decimation algorithm (RDA) of López Sancho et al. is used in quantum transport theory to calculate bulk and surface Green’s functions. We derive an improved version of the RDA for the case of very long quasi one-dimensional unit cells (in transport direction). This covers not only long unit cells but also supercell-like calculations for structures with disorder or defects. In such large systems, short-range interactions lead to sparse real-space Hamiltonian matrices. We show how this and a corresponding subdivision of the unit cell in combination with the decimation technique can be used to reduce the calculation time. Within the resulting algorithm, separate RDA calculations of much smaller effective Hamiltonian matrices must be done for each Green’s function, which enables the treatment of systems too large for the common RDA. Finally, we discuss the performance properties of our improved algorithm as well as some exemplary results for chiral carbon nanotubes.

Mean field theory of thermal energy transport in molecular junctions.

Mean field theory is applied to nonequilibrium thermal energy transport in a model molecular junction. An approximation to the total time-dependent heat current in the junction is constructed using an ensemble of Ehrenfest trajectories, and the average heat current in the steady state is obtained. The accuracy of this treatment is verified through benchmark comparisons with exact quantum mechanical results and various approximate quantum transport theories for the nonequilibrium spin-boson model. The performance of the multitrajectory Ehrenfest approach is found to be quite robust, displaying good accuracy in intermediate cases that remain elusive to many perturbative approximations and in the strong coupling limit where many methods break down. Thus, mean field theory and related trajectory-based approximate quantum dynamics methods emerge as a promising toolkit for the study of transport properties in nanoscale systems.

Anti-resonance features of destructive quantum interference in single-molecule thiophene junctions achieved by electrochemical gating.

Controlling the electrical conductance and in particular the occurrence of quantum interference in single-molecule junctions through gating effects has potential for the realization of high-performance functional molecular devices. In this work we used an electrochemically gated, mechanically controllable break junction technique to tune the electronic behaviour of thiophene-based molecular junctions that show destructive quantum interference features. By varying the voltage applied to the electrochemical gate at room temperature, we reached a conductance minimum that provides direct evidence of charge transport controlled by an anti-resonance arising from destructive quantum interference. Our molecular system enables conductance tuning close to two orders of magnitude within the non-faradaic potential region, which is significantly higher than that achieved with molecules not showing destructive quantum interference. Our experimental results, interpreted using quantum transport theory, demonstrate that electrochemical gating is a promising strategy for obtaining improved in situ control over the electrical performance of interference-based molecular devices.

Higher-order moments, cumulants, and spectra of continuous quantum noise measurements

We present general quantum mechanical expressions for higher order moments, cumulants, and spectra of continuously measured quantum systems with applications in spin noise spectroscopy, quantum transport, and measurement theory in general. Starting from the so-called stochastic master equation of continuous measurement theory, we find that the leading orders of the fluctuating detector output $z(t)$ with respect to the measurement strength $\beta$ are a white shot noise background, a constant measurement offset, and the leading order quantum noise of the measured operator $A$. Starting from quantum expressions for the multi-time moments $\langle z(t_n)\cdots z(t_1) \rangle$ we derive three- and four-time cumulants that are valid in all orders of $\beta$ covering the full regime between the weak and strong measurement limit (Zeno-limit). Intriguingly, quantum expressions for the cumulants were found that exhibit the same simple structure as those for the moments after introduction of only a slightly modified system propagator. Very compact expressions for the cumulant-based third and fourth order spectra (bispectrum and trispectrum) follow naturally. We illustrate the usefulness of higher order spectra by treating a real world two-spin system with strong hyperfine interaction.Moreover, spin noise spectroscopy is shown to have the potential for investigating the transition from weak measurements to the famous quantum Zeno regime for realistic probe laser intensities.

The Temperature Dependences of the Electron-Piezoelectric Potential Phonon Interacting System of Quasi Two Dimensional System in GaN and ZnS.

We investigated theoretically the temperature dependence of the quantum optical transition of quasi 2-Dimensional Landau splitting system, in GaN and ZnS. We apply the Quantum Transport theory (QTR) to the system in the confinement of electrons by square well confinement potential. We use the projected Liouville equation method with Equilibrium Average Projection Scheme (EAPS). Through the analysis of this work, we found the increasing properties of Quantum Transition Line Shapes (QTLSs) and the Quantum Transition Line Widths (QTLWs) of CdS and GaN with the temperature we also found that QTLW, γ(T) of ZnS < γ(T) of GaN in γ = 394 μm.

Quantum Transition Properties of Quasi Two Dimensional and Electron-Phonon Interacting System in ZnO and CdS.

We investigated theoretically the quantum optical transition properties of quasi 2-Dimensional Landau splitting system, in ZnO and CdS. We apply the Quantum Transport theory (QTR) to the system in the confinement of electrons by square well confinement potential. We use the projected Liouville equation method with Equilibrium Average Projection Scheme (EAPS). In order to analyze the quantum transition, we compare the temperature and the magnetic field dependencies of the QTLW and the QTLS on two transition processes, namely, the phonon emission transition process and the phonon absorption transition process.

Controlling observables in normal, hybrid and Josephson junctions

ABSTRACTThe theory of time-dependent quantum transport addresses the question: How do electrons flow through a junction under the influence of an external perturbation as time goes by? In this article, we invert this question and search for a time-dependent bias such that the system behaves in a desired way. Our system of choice consists of quantum dots coupled to normal or superconducting leads. We present results for junctions with normal leads where the current, the density or a molecular vibration is optimised to follow a given target pattern. For junctions with two superconducting leads, where the Josephson effect triggers the current to oscillate, we investigate what happens if the frequency of the Josephson oscillation comes close to the frequency of the molecular vibration. Furthermore we show how to suppress the Josephson oscillations by suitably tailoring the bias. In a second example involving superconductivity, we consider a Y-shaped junction with two quantum dots coupled to one superconducting and two normal leads. This device is used as a Cooper pair splitter to create entangled electrons on the two quantum dots. We maximise the splitting efficiency with the help of an optimised bias.

Theory of AC quantum transport with fully electrodynamic coupling

With the continued scaling of microelectronic devices along with the growing demand of high-speed wireless telecommunications technologies, there is increasing need for high-frequency device modeling techniques that accurately capture the quantum mechanical nature of charge transport in nanoscale devices along with the dynamic fields that are generated. In an effort to fill this gap, we develop a simulation methodology that self-consistently couples AC non-equilibrium Green functions (NEGF) with the full solution of Maxwell's equations in the frequency domain. We apply this technique to simulate radiation from a quantum-confined, quarter-wave, monopole antenna where the length $L$ is equal to one quarter of the wavelength, $\lambda_0$. Classically, such an antenna would have a narrower, more directed radiation pattern compared to one with $L \ll \lambda_0$, but we find that a quantum quarter-wave antenna has no directivity gain compared to the classical solution. We observe that the quantized wave function within the antenna significantly alter the charge and current density distribution along the length of the wire, which in turn modifies the far-field radiation pattern from the antenna. These results show that high-frequency radiation from quantum systems can be markedly different from classical expectations. Our method, therefore, will enable accurate modeling of the next generation of high-speed nanoscale electronic devices.

Photoinduced half-integer quantized conductance plateaus in topological-insulator/superconductor heterostructures

The past few years have witnessed increased attention to the quest for Majorana-like excitations in the condensed matter community. As a promising candidate in this race, the one-dimensional chiral Majorana edge mode (CMEM) in topological insulator-superconductor heterostructures has gathered renewed interests during recent months after an experimental breakthrough. In this paper, we study the quantum transport of topological insulator-superconductor hybrid devices subject to light-matter interaction or general time-periodic modulation. We report half-integer quantized conductance plateaus at $\frac{1}{2}\frac{e^2}{h}$ and $\frac{3}{2}\frac{e^2}{h}$ upon applying the so-called sum rule in the theory of quantum transport in Floquet topological matter. In particular, in a photoinduced topological superconductor sandwiched between two Floquet Chern insulators, it is found that for each Floquet sideband, the CMEM admits equal probability for normal transmission and local Andreev reflection over a wide range of parameter regimes, yielding half-integer quantized plateaus that resist static and time-periodic disorder. The $\frac{3}{2}\frac{e^2}{h}$ plateau has not yet been computationally or experimentally observed in any other superconducting system, and indicates the possibility to simultaneously create and manipulate multiple pairs of CMEMs by light. The robust half-quantized conductance plateaus, due to CMEMs at quasienergies zero or half the driving frequency, are both fascinating and subtle because they only emerge after a summation over contributions from all Floquet sidebands. Such a distinctive transport signature can thus serve as a hallmark of photoinduced CMEMs in topological insulator-superconductor junctions.

Nonadiabatic Dynamics in Single-Electron Tunneling Devices with Time-Dependent Density-Functional Theory.

We simulate the dynamics of a single-electron source, modeled as a quantum dot with on-site Coulomb interaction and tunnel coupling to an adjacent lead in time-dependent density-functional theory. Based on this system, we develop a time-nonlocal exchange-correlation potential by exploiting analogies with quantum-transport theory. The time nonlocality manifests itself in a dynamical potential step. We explicitly link the time evolution of the dynamical step to physical relaxation timescales of the electron dynamics. Finally, we discuss prospects for simulations of larger mesoscopic systems.

Charge transfer between quasi-zero-dimensional nanostructures

In the present paper, we show the main features of the theoretical formula allowing us to understand the irreversible transfer of electrons or holes, as it may be observed in some electrically conducting (conjugated) polymers. The formula reported here is derived with the use of the nonequilibrium quantum transport theory. The key role in the formula is played by the electron–phonon interaction which is included in an approximation going beyond the limits of the finite order of the perturbation calculation. The transfer formula is developed for a simple model of the stacked conjugated polymer, which is represented here by two interacting quantum dots. The numerical calculations show the dependence of the charge carrier mobility on the charge carrier concentration and on the lattice temperature. A comparison with organic field-effect transistor experiments and with molecular-scale modeling on conjugated polymers is promising.

Nonequilibrium Green’s Functions in the Atomic Representation and the Problem of Quantum Transport of Electrons Through Systems With Internal Degrees of Freedom

We develop the theory of quantum transport of electrons through systems with strong correlations between fermionic and internal spin degrees of freedom. The atomic representation for the Hamiltonian of a device and nonequilibrium Green’s functions constructed using the Hubbard operators allow overcoming difficulties in the perturbation theory encountered in the traditional approach because of a larger number of bare scattering amplitudes. Representing the matrix elements of effective interactions as a superposition of terms each of which is split in matrix indices, we obtain a simple method for solving systems of very many equations for nonequilibrium Green’s functions in the atomic representation. As a result, we obtain an expression describing the electron currents in a device one of whose sites is in tunnel coupling with the left contact and the other, with the right contact. We derive closed kinetic equations for the occupation numbers under conditions where the electron flow leads to significant renormalization of them.

Perspective: Theory of quantum transport in molecular junctions.

Molecular junctions, where single molecules are bound to metal or semiconductor electrodes, represent a unique architecture to investigate molecules in a distinct nonequilibrium situation and, in a broader context, to study basic mechanisms of charge and energy transport in a many-body quantum system at the nanoscale. Experimental studies of molecular junctions have revealed a wealth of interesting transport phenomena, the understanding of which necessitates theoretical modeling. The accurate theoretical description of quantum transport in molecular junctions is challenging because it requires methods that are capable to describe the electronic structure and dynamics of molecules in a condensed phase environment out of equilibrium, in some cases with strong electron-electron and/or electronic-vibrational interaction. This perspective discusses recent progress in the theory and simulation of quantum transport in molecular junctions. Furthermore, challenges are identified, which appear crucial to achieve a comprehensive and quantitative understanding of transport in these systems.