Multiple cotunneling in large quantum dot arrays

We investigate the effects of inelastic cotunneling on the electronic transport properties of gold nanoparticle multilayers and thick films at low applied bias, inside the Coulomb-blockade regime. We find that the zero-bias conductance, g(0)(T), in all systems exhibits Efros-Shklovskii-type variable range hopping transport. The resulting typical hopping distance, corresponding to the number of tunnel junctions participating in cotunneling events, is shown to be directly related to the power-law exponent in the measured current-voltage characteristics. We discuss the implications of these findings in light of models on cotunneling and hopping transport in mesoscopic, granular conductors.     

Density of states of amorphous GdxSi1-x at the metal-insulator transition

We have determined the electronic density of states of amorphous Gd xSi (1-x), N(GdSi)(E), in the vicinity of the metal-insulator transition by measuring the tunneling conductance dI/dV across a Gd xSi (1-x)/oxide/Pb tunnel junction at low T (T approximately 100 mK). By applying a magnetic field we can tune through the metal-insulator transition and simultaneously measure the transport and N(E) on a single sample. We find a smooth transition from a metal with strong Coulomb interactions to a developing Coulomb gap in the insulating regime. In the metallic region N(GdSi)(0) scales approximately with sigma(2).     

Violation of the Wiedemann-Franz law in a single-electron transistor

We study the influence of Coulomb interaction on the thermoelectric transport coefficients for a metallic single-electron transistor. By performing a perturbation expansion up to second order in the tunnel-barrier conductance, we include sequential and cotunneling processes as well as quantum fluctuations that renormalize the charging energy and the tunnel conductance. We find that Coulomb interaction leads to a strong violation of the Wiedemann-Franz law: the Lorenz ratio becomes gate-voltage dependent for sequential tunneling, and is increased by a factor 9/5 in the cotunneling regime. Finally, we suggest a measurement scheme for an experimental realization.     

Cotunneling-mediated transport through excited states in the Coulomb-blockade regime

We present finite-bias transport measurements on a few-electron quantum dot. In the Coulomb-blockade regime, strong signatures of inelastic cotunneling occur which can directly be assigned to excited states observed in the nonblockaded regime. In addition, we observe structures related to sequential tunneling through the dot, occurring after it has been excited by an inelastic cotunneling process. We explain our findings using transport calculations within the real-time Green's function approach, including diagrams up to fourth order in the tunneling matrix elements.     

Phase coherence in the inelastic cotunneling regime

Two quantum dots with tunable mutual tunnel coupling have been embedded in a two-terminal Aharonov-Bohm geometry. Aharonov-Bohm oscillations investigated in the cotunneling regime demonstrate coherent tunneling through nonresonant states. Visibilities of more than 0.8 are measured indicating that phase-coherent processes are involved in the elastic and inelastic cotunneling. An oscillation-phase change of pi is detected as a function of bias voltage at the inelastic cotunneling onset.     

Cotunneling through a quantum dot coupled to ferromagnetic leads with noncollinear magnetizations

Spin-dependent electronic transport through a quantum dot has been analyzed theoretically in the cotunneling regime by means of the second-order perturbation theory. The system is described by the impurity Anderson Hamiltonian with arbitrary Coulomb correlation parameter U. It is assumed that the dot level is intrinsically spin-split due to an effective molecular field exerted by a magnetic substrate. The dot is coupled to two ferromagnetic leads whose magnetic moments are noncollinear. The angular dependence of electric current, tunnel magnetoresistance, and differential conductance are presented and discussed. The evolution of a cotunneling gap with the angle between magnetic moments and with the splitting of the dot level is also demonstrated.     

Shell filling and exchange coupling in metallic single-walled carbon nanotubes

We report the characterization of electronic shell filling in metallic single-walled carbon nanotubes by low-temperature transport measurements. Nanotube quantum dots with average conductance approximately (1-2)e(2)/h exhibit a distinct four-electron periodicity for electron addition as well as signatures of Kondo and inelastic cotunneling. The Hartree-Fock parameters that govern the electronic structure of metallic nanotubes are determined from the analysis of transport data using a shell-filling model that incorporates the nanotube band structure and Coulomb and exchange interactions.     

Spin-dependent tunneling and Coulomb blockade in ferromagnetic nanoparticles

In this paper we review studies on spin-dependent transport in systems containing ferromagnetic nanoparticles. In a tunnel junction with a nanometer-scale-island, the charging effect leads to an electric current blockade phenomenon in which a single electron charge plays a significant role in electron transport, resulting in single-electron tunneling (SET) properties such as Coulomb blockade and Coulomb staircase. In a tunnel junction with a ferromagnetic nano-island and electrode, it was expected that the interplay of spin-dependent tunneling (SDT) and SET, i.e., spin-dependent single-electron tunneling (SD-SET), would give rise to remarkable tunnel magnetoresistance (TMR) phenomena. We investigated magnetotransport properties in both sequential tunneling and cotunneling regimes of SET and found the enhancement and oscillation of TMR. The self-assembled ferromagnetic nanoparticles we have employed in this study consisted of a Co–Al–O granular film with cobalt nanoparticles embedded in an Al–O insulating matrix. A _Co36Al22O42${\mathrm{Co}}_{36}{\mathrm{Al}}_{22}{\mathrm{O}}_{42}$ film prepared by a reactive sputtering method produced a TMR ratio reaching 10% and superparamagnetic behavior at room temperature. The TMR ratio exhibited an anomalous increase at low temperatures but no indication of change with bias voltage. In Section 4, we show that the anomalous increase of the MR provided evidence for higher-order tunneling (cotunneling) between large granules through intervening small granules. We emphasize that the existence of higher-order tunneling is a natural consequence of the granular structure, since broad distribution of granule size is an intrinsic property of granular systems. In Section 5, we concentrate on SD-SET properties in sequential tunneling regimes. We fabricated two types of device structures with Co–Al–O film using focused ion-beam milling or electron-beam lithography techniques. One had a granular nanobridge structure: point-shaped electrodes separated by a very narrow lateral gap filled with the Co–Al–O granular film. The other had a current-perpendicular-to-plane (CPP) geometry structure: a thin Co–Al–O granular film sandwiched by ferromagnetic electrodes with the current flowing in the direction perpendicular to the film plane through a few Co particles. We found the enhancement and oscillation of TMR due to spin-dependent SET in sequential tunneling regimes. In Section 6, we report experimental evidence of a spin accumulation effect in Co nanoparticles leading to the oscillation of TMR with alternate sign changes. Furthermore, we discovered that the spin relaxation time in the nanoparticles is unprecedentedly enhanced up to the order of more than hundreds of nanoseconds, compared to that evaluated from the spin-diffusion length of ferromagnetic layers in previous CPP-GMR studies, i.e., the order of tens of picoseconds.     

Unconventional transport in the "hole" regime of a si double quantum dot

Studies of electronic charge transport through semiconductor double quantum dots rely on a conventional "hole" model of transport in the three-electron regime. We show that experimental measurements of charge transport through a Si double quantum dot in this regime cannot be fully explained using the conventional picture. Using a Hartree-Fock (HF) formalism and relevant HF energy parameters extracted from transport data in the multiple-electron regime, we identify a novel spin-flip cotunneling process that lifts a singlet blockade.     

Imaging and tunneling spectroscopy of gold nanocrystals and nanocrystal arrays

Scanning tunneling microscopy (STM) and spectroscopy (STS) have been used to determine the structural and electronic properties of thiol-passivated 29000 amu gold nanocrystals, both individually and in spontaneously formed quasi-two-dimensional arrays. Experiments were performed at temperatures of 300 K, 77 K, and 8 K. Even at room temperature, tunneling through these 1.7 nm nanocrystals is shown to give rise to a Coulomb blockade. At cryogenic temperatures, the spectroscopy of the nanocrystals in arrays and in isolation shows an incremental charging effect (the Coulomb staircase) and evidence is found for quantization of the electronic states. Received: 10 September 1998 / Received in final form: 16 February 1999     

压缩应变载荷下氮化镓隧道结微观压电特性及其巨压电电阻效应

电子器件可控性研究在日益追求器件智能化和可控化的当今社会至关重要. 基于第一性原理和量子输运计算, 本文研究了压缩应变载荷对氮化镓(GaN)隧道结基态电学性质和电流输运的影响, 在原子尺度上窥视了氮化镓隧道结的微观压电性, 验证了其内在的巨压电电阻(GPR)效应. 计算结果表明, 压缩应变载荷可以调节隧道结内氮化镓势垒层的电势能降、内建电场、电荷密度和极化强度, 进而实现对隧道结电流输运和隧穿电阻的调控. 在-1.0 V的偏置电压下, -5%的压缩应变载荷将使氮化镓隧道结的隧穿电阻增至4倍. 本研究展现了氮化镓隧道结在可控电子器件中的应用潜力, 也展现了应变工程在调控电子器件性能方面的光明前景.     

Are "pinholes" the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic pinholes in the tunnel barrier. We have studied the subgap current in superconductor-insulator-superconductor (SIS) and superconductor-insulator-normal-metal (SIN) junctions. In Al/AlO(x)/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlO(x)/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical Keldysh-Green function theory to quantify single- and two-particle tunneling and find good agreement with experiment over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.     

Mesoscopic fluctuations of tunneling and cotunneling in quantum dots

Recent measurements of mesoscopic tunneling and cotunneling fluctuations in Coulomb blockaded ballistic quantum dots are presented. The statistics and parametric fluctuations (as a function of magnetic field) of Coulomb blockade peak heights are found to be consistent with random-matrix-theory predictions. Mesoscopic fluctuations of elastic cotunneling, measured in the valleys between blockade peaks, are also presented along with a semiclassical explanation of the observed enhancement of the magnetic field scale of cotunneling fluctuations compared to resonant tunneling fluctuations.     

Nonlinear transport near a quantum phase transition in two dimensions

The problem of nonlinear transport near a quantum phase transition is solved within the Landau theory for the dissipative insulator-superconductor phase transition in two dimensions. Using the nonequilibrium Schwinger round-trip Green function formalism, we obtain the scaling function for the nonlinear conductivity in the quantum-disordered regime. We find that the conductivity scales as E2 at low fields but crosses over at large fields to a universal constant on the order of e(2)/h. The crossover between these two regimes obtains when the length scale for the quantum fluctuations becomes comparable to that of the electric field within logarithmic accuracy.     

Cotunneling current through a two-level quantum dot coupled to magnetic leads: the role of exchange interaction

The cotunneling current through a two-level quantum dot weakly coupled to ferromagnetic leads is studied in the Coulomb blockade regime. The cotunneling current is calculated analytically under simple but realistic assumptions as follows: (i)?the quantum dot is described by the universal Hamiltonian, (ii)?it is doubly occupied, and (iii)?it displays a fast spin relaxation. We find that the dependence of the differential conductance on the bias voltage is significantly affected by the exchange interaction on the quantum dot. In particular, for antiparallel magnetic configurations in the leads, the exchange interaction results in the appearance of interference-type contributions from the inelastic processes to the cotunneling current. Such dependence of the cotunneling current on the tunneling amplitude phases should also occur in multi-level quantum dots weakly coupled to ferromagnetic leads near the mesoscopic Stoner instabilities.     

Correlated tunneling in intramolecular carbon nanotube quantum dots

We investigate correlated electronic transport in single-walled carbon nanotubes with two intramolecular tunneling barriers. We suggest that below a characteristic temperature the long-range nature of the Coulomb interaction becomes crucial to determine the temperature dependence of the maximum G(max) of the conductance peak. Correlated sequential tunneling dominates transport yielding the power law G(max) proportional, variant T(alpha(end-end)-1), typical for tunneling between the ends of two Luttinger liquids. Our predictions are in agreement with recent measurements.     

Transport across an Anderson quantum dot in the intermediate coupling regime

We describe linear and nonlinear transport across a strongly interacting single impurity Anderson model quantum dot with intermediate coupling to the leads, i.e. with tunnel coupling Γ of the order of the thermal energy k _B T. The coupling is large enough that sequential tunneling processes (second order in the tunneling Hamiltonian) alone do not suffice to properly describe the transport characteristics. Upon applying a density matrix approach, the current is expressed in terms of rates obtained by considering a very small class of diagrams which dress the sequential tunneling processes by charge fluctuations. We call this the “dressed second order” (DSO) approximation. One advantage of the DSO is that, still in the Coulomb blockade regime, it can describe the crossover from thermally broadened to tunneling broadened conductance peaks. When the temperature is decreased even further (k _B T < Γ), the DSO captures Kondesque behaviours of the Anderson quantum dot qualitatively: we find a zero bias anomaly of the differential conductance versus applied bias, an enhancement of the conductance with decreasing temperature as well as universality of the shape of the conductance as function of the temperature. We can without complications address the case of a spin degenerate level split energetically by a magnetic field. In case spin dependent chemical potentials are assumed and only one of the four chemical potentials is varied, the DSO yields in principle only one resonance. This seems to be in agreement with experiments with pseudo spin [U. Wilhelm, J. Schmid, J. Weis, K.V. Klitzing, Physica E 14, 385 (2002)]. Furthermore, we get qualitative agreement with experimental data showing a cross-over from the Kondo to the empty orbital regime.     

Transport properties of granular metals at low temperatures

We investigate transport in a granular metallic system at large tunneling conductance between the grains, g(T)>1. We show that at low temperatures, Tg(T)delta) behavior where conductivity is controlled by the scales of the order of the grain size. In three dimensions we predict the metal-insulator transition at the bare tunneling conductance g(C)(T)=(1/6pi)ln((E(C)/delta), where E(C) is the charging energy of a single grain. Corrections to the density of states of granular metals due to the electron-electron interaction are calculated. Our results compare favorably with the logarithmic dependence of resistivity in the high-T(c) cuprate superconductors indicating that these materials may have a granular structure.     

Charge transport in Weyl semimetals

We study transport in Weyl semimetals with N isotropic Weyl nodes in the presence of Coulomb interactions or disorder at temperature T. In the interacting clean limit, we determine the conductivity σ(ω,T) by solving a quantum Boltzmann equation within a "leading log" approximation and find it to be proportional to T, up to logarithmic factors arising from the flow of couplings. In the noninteracting disordered case, we compute the Kubo conductivity and show that it behaves differently for ω < T and ω > T: in the former regime we recover a previous result, of a finite dc conductivity and a Drude width vanishing as NT(2); in the latter, we find that σ(ω,T) vanishes linearly with ω with a leading term as T → 0 equal to the clean, free-fermion result: σ(0)((N))(ω,T = 0) = Ne(2)/12h|ω|/v(F). We compare our results to transport data on Y(2)Ir(2)O(7) and comment on the possible relevance to recent experiments on Eu(2)Ir(2)O(7).     

MgO单晶势垒磁性隧道结的第一性原理计算和实验研究

在MgO单晶势垒磁性隧道结中发现的室温高隧穿磁电阻现象,是近些年自旋电子学以及磁性隧道结磁电阻材料研究中的又一重大突破.本文主要评述和介绍2001年以来MgO单晶势垒磁性隧道结第一性原理计算和实验上的重要进展,以及介绍利用Layer-KKR第一性原理计算方法研究的Fe(001)/MgO/Fe、Fe(001)/FeO/MgO/Fe、Fe(001)/Mg/MgO/Fe、Fe(001)/Co/MgO/Co/Fe和Fe(001)[MgO/Fe/MgO/Fe等基于单晶MgO(001)单势垒及双势垒磁性隧道结材料的电子结构和自旋相关输运性质研究的最新进展.这些第一性原理定量计算的结果,不仅从物理上增强了对MgO单晶势垒磁性隧道结的电子结构和自旋相关输运特性的了解,而且对于研究新型室温磁电阻隧道结材料及其在自旋电子学器件中的广泛应用,具有一定的参考价值.