Co-tunneling of the charge through a 2-D electron island

A double barrier Single Electron Transistor is realized in two dimensions by confining the 2-D electron gas of a GaAs/GaAlAs heterojunction to a small island by means of Schottky gates. Two gates provide adjustable tunnel barriers and a central gate controls the electron number in the island. The island has small single-particle energy level spacing and forms a metallic island. Periodic conductance oscillations characteristic of Coulomb blockade are observed when the central gate voltage is varied. The ability to vary the tunnel conductance allows us to study the basic physics of the Coulomb blockade: our results show that the quantum charge fluctuation mechanism which limits the tunneling blockade at low temperature is of second order in tunnel barrier transparencies in agreement with the charge Macroscopic Quantum Tunneling (q-MQT) or co-tunneling model.     

Conductance oscillations and transport spectroscopy of a quantum dot

Results are reported for low temperature measurements of the conductance through small regions of a two-dimensional electron gas (2 DEG). An unconventional GaAs heterostructure is used to form a 2 DEG whose density can be tuned by the gate voltage applied to its conductive substrate. Electron beam lithography is used to pattern a narrow channel in the 2 DEG interrupted by two constrictions, defining a small 2 DEG island between them. The conductance is found to oscillate periodically with the gate voltage, namely with electron density. Calculations of the capacitance between the substrate and the island show that the period of oscillation corresponds to adding one electron to the island. The oscillatory behavior results primarily from the discreteness of charge and the Coulomb interaction between electrons. However, the observed temperature dependence of these oscillations requires a more sophisticated treatment which includes the quantized electron energy levels as well. The magnetic field dependence of the oscillations allows us to extract the discrete energy spectrum of the quantum dot in the quantum-Hall regime.     

Coulomb blockade in an open quantum dot

We report the observation of Coulomb blockade in a quantum dot contacted by two quantum point contacts each with a single fully transmitting mode, a system thought to be well described without invoking Coulomb interactions. Below 50 mK we observe a periodic oscillation in the conductance of the dot with gate voltage, corresponding to a residual quantization of charge. From the temperature and magnetic field dependence, we infer the oscillations are mesoscopic Coulomb blockade, a type of Coulomb blockade caused by electron interference in an otherwise open system.     

单电子三势垒隧穿结I-V特性研究

在正统理论的基础上，提出了单电子三势垒隧穿结模型的主方程，并用线性方程组解法求出了其稳态解.通过数值模拟，得到了该系统的I-V特性曲线.发现其有别于双势垒隧穿结的情况，在传统库仑台阶的平台处曲线存在波纹状结构，分析得出这是由于第二个库仑岛上的电子数变化对I-V曲线的影响.此外，研究了各物理参数对I-V曲线的影响，发现三结系统可以降低对温度的要求，并应用Fermi能级处的能级间隔估算出出现库仑台阶现象的最高温度Tmax，为相关单电子器件的参数选择提供了理论依据. 关键词： 正统理论 库仑台阶 主方程 隧穿概率     

Non-equilibrium effects and self-heating in single-electron Coulomb blockade devices

We present a comprehensive investigation of non-equilibrium effects and self-heating in single electron transfer devices based primarily on the Coulomb blockade effect. During an electron trapping process, a hot electron maybe deposited in a quantum dot or metal island, with an extra energy usually of the order of the Coulomb charging energy, which is much higher than the temperature in typical experiments. The hot electron may relax through three channels: tunneling back and forth to the feeding lead (or island), emitting phonons, and exciting background electrons. Depending on the magnitudes of the rates in the latter two channels relative to the device operation frequency and to each other, the system may be in one of three different regimes: equilibrium, non-equilibrium, and self-heating (partial equilibrium). In the equilibrium regime, a hot electron fully gives up its energy to phonons within a pump cycle. In the non-equilibrium regime, the relaxation is via tunneling with a distribution of characteristic rates; the approach to equilibrium goes like a power law of time (frequency) instead of an exponential. This channel is plagued completely in the continuum limit of the single-electron levels. In the self-heating regime, the hot electron thermalizes quickly with background electrons, whose temperature T_e is elevated above the lattice temperature T_ol. We have calculated the coefficient in the well-known T⁵ law of energy dissipation rate, and compared the results to experimental values for aluminum and copper islands and for a two-dimensional semiconductor quantum dot. Moreover, we have obtained different scaling relations between the electron temperature, the operation frequency and device size for various types of devices.     

Aharonov-Casher-effect suppression of macroscopic tunneling of magnetic flux

We suggest a system in which the amplitude of macroscopic flux tunneling can be modulated via the Aharonov-Casher effect. The system is an rf SQUID with the Josephson junction replaced by a Bloch transistor--two junctions separated by a small superconducting island on which the charge can be induced by an external gate voltage. When the Josephson coupling energies of the junctions are equal and the induced charge is q = e, destructive interference between tunneling paths brings the flux tunneling rate to zero. The device may also be useful as a qubit for quantum computation.     

单电子晶体管通断图及其分析

通过研究单电子晶体管在电路中的静电能量的变化,得到了它的阻塞、导通情况与其两端偏压和控制栅压之间的关系,从而给出了它的通断图.并且发现单电子晶体管的对外特性主要由其对外的总电容决定;而单电子岛两个隧道结电容的不同主要反映在通断图上由隧道结电容所决定的晶体管阻塞、导通边界线上,这两条边界线同时也与外电路有关 关键词： 单电子晶体管 库仑阻塞 隧穿     

Finite frequency quantum noise in an interacting mesoscopic conductor

We present a quantum calculation based on scattering theory of the frequency-dependent noise of current in an interacting chaotic cavity. We include interactions of the electron system via long range Coulomb forces between the conductor and a gate with capacitance C. We obtain explicit results exhibiting the two time scales of the problem, the cavity's dwell time tau(D) and the RC time tau(C) of the cavity in relation to the gate. The noise shows peculiarities at frequencies of the order and exceeding the inverse charge relaxation time tau(-1) = tau(D)(-1) + tau(C)(-1).     

Auxiliary-level-assisted operations with charge qubits in semiconductors

We present a new scheme for rotations of a charge qubit associated with a singly ionized pair of donor atoms in a semiconductor host. The logical states of such a qubit proposed recently by Hollenberg et al. [16] are defined by the lowest two energy states of the remaining valence electron localized around one or another donor. We show that an electron located initially at one donor site can be transferred to another donor site via an auxiliary molecular level formed upon the hybridization of the excited states of two donors. The electron transfer is driven by a single resonant microwave pulse in the case where the energies of the lowest donor states coincide or by two resonant pulses in the case where they differ from each other. Depending on the pulse parameters, various one-qubit operations—including the phase gate, the NOT gate, and the Hadamard gate—can be realized in short times. Decoherence of an electron due to the interaction with acoustic phonons is analyzed and shown to be weak enough for coherent qubit manipulation to be possible, at least in proof-of-principle experiments on one-qubit devices.     

Full current statistics in the regime of weak coulomb interaction

We evaluate the full statistics of the current via a Coulomb island that is strongly coupled to the leads. This strong coupling weakens Coulomb interaction. We show that in this case the effects of the interaction can be incorporated into the renormalization of transmission eigenvalues of the scatterers that connect the island and the leads. We evaluate the Coulomb blockade gap in the current-voltage characteristics, the value of the gap being exponentially suppressed as compared to the classical charging energy of the island.     

Ground state energy of two dimensional electrons in strong magnetic fields: Small clusters in fractional occupations

We diagonalize numerically the Hamiltonian for a two-dimensional system of up to seven interacting via Coulomb force electrons in a strong magnetic field using the symmetric gauge. The effect of positive charge background is taken into account. We find significant downward cusps for certain values of the total angular momentum corresponding to fractional occupation of the lowest Landau level in agreement with results obtained by using Landau gauge. This commensurate energy for fractional occupation is interpreted in terms of the “closest configuration” in the space of single particle angular momentum.     

Kondo resonance assisted thermoelectric transport through strongly correlated quantum dots

We theoretically studied the thermoelectric transport properties of a strongly correlated quantum dot system in the presence of the Kondo effect based on accurate numerical evaluations using the hierarchical equations of motion approach. The thermocurrent versus gate voltage shows a distinct sawtooth line-shape at high temperatures. In particular, the current changes from positive(hole charge) to negative(particle charge) in the electron number N = 1 region due to the Coulomb blockade effect. However,at low temperatures, where the Kondo effect occurs, the thermocurrent's charge polarity reverses, along with a significantly enhanced magnitude. As anticipated, the current sign can be analyzed by the occupation difference between particle and hole.Moreover, the characteristic turnover temperature can be further defined at which the influences of the Coulomb blockade and Kondo resonance are in an effective balance. Remarkably, the identified characteristic turnover temperature, as a function of the Coulomb interaction and dot-lead coupling, possessed a much higher value than the Kondo temperature. When a magnetic field is applied, a spin-polarized thermocurrent can be obtained, which could be tested in future experiments.     

Coulomb effects in a nanoscale SINIS junction

We study a system of two superconductors connected by a small normal grain. We consider the modification of the Josephson effect by the Coulomb interaction on the grain. Coherent charge transport through the junction is suppressed by Coulomb repulsion. An optional gate electrode may relax the charge blocking and enhance the current leading to the single Cooper pair transistor effect. Temperature dependences of critical current and of the minigap induced in the normal grain by the proximity to superconductor are studied. Both temperature and Coulomb interaction suppress critical current and minigap, but their interplay may lead to the nonmonotonic and even reentrant temperature dependence.     

Effect of the Coulomb scattering on graphene conductivity

The effect of the Coulomb scattering on graphene conductivity in field-effect transistor structures is discussed. Interparticle scattering (electron-electron, hole-hole, and electron-hole) and scattering on charged defects are taken into account in a wide range of gate voltages. It is shown that an intrinsic conductivity of graphene (purely ambipolar system, where both electron and hole densities exactly coincide) is defined by a strong electron-hole scattering. It has a universal value independent of the temperature. We give an explicit derivation based on the scaling theory. When there is even a small discrepancy in the electron and hole densities caused by the applied gate voltage, the conductivity is determined by both a strong electron-hole scattering and a weak external scattering: on the defects or phonons. We suggest that the density of the charged defects (occupancy of defects) depends on the Fermi energy to explain the sublinear dependence of conductivity on a fairly high gate voltage observed in the experiments. We also eliminate the contradictions between the experimental data obtained in the deposited and suspended graphene structures regarding the graphene conductivity. The text was submitted by the authors in English.     

Coulomb blockade electrometer with a high-T c island

We report on a successful attempt to fabricate a Coulomb blockade electrometer consisting of an ultrasmall YBa₂Cu₃O_7−δ (YBCO) island coupled to two gold electrodes through a high-resistance native surface tunnel barrier. A third electrode placed near the island was used as an electrostatic gate. Spectra typical for tunneling into the YBCO superconductor were reproducibly measured. At temperatures below 0.5 K the low-bias conductance was suppressed by the Coulomb blockade. The blockade could be periodically varied by the gate potential. An external magnetic field of up to 5 T strongly influenced the transport via the island but without any change in the period of the Coulomb oscillations. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 2, 112–117 (25 January 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Smearing of the coulomb blockade by resonant tunneling

We study the Coulomb blockade in a grain coupled to a lead via a resonant impurity level. We show that the strong energy dependence of the transmission coefficient through the impurity level can have a dramatic effect on the quantization of the grain charge. In particular, if the resonance is sufficiently narrow, the Coulomb staircase shows very sharp steps even if the transmission through the impurity at the Fermi energy is perfect. This is in contrast to the naive expectation that perfect transmission should completely smear charging effects.     

Single-electron computing without dissipation

The possibility of performing single-electron computing without dissipation in an array of tunnel-coupled quantum dots is studied theoretically, taking the spin gate NOT (inverter) as an example. It is shown that the logical operation can be implemented at the stage of unitary evolution of the electron subsystem, although complete switching of the inverter cannot be achieved in a reasonable time at realistic values of model parameters. The optimal input magnetic field is found as a function of the interdot tunneling energy and intradot Coulomb repulsion energy. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 275–279 (25 August 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Ab initio description of high-temperature superconductivity in dense molecular hydrogen

We present a first-principles study of the electron-phonon interaction and the prediction of the superconducting critical temperature in molecular metallic hydrogen. Our study is able to single out the features which drive the system towards superconductivity: mainly, a rich and complex Fermi surface and strongly coupled phonon modes driving the intra- or intermolecular charge transfer. We demonstrate that in this simple system, a very high superconducting critical temperature can be reached via electron-phonon and Coulomb electron-electron interactions.     

Fast and accurate single-island charge pump: implementation of a cooper pair pump

We introduce a Cooper pair "sluice" for the implementation of a frequency-locked current source. The device consists of two mesoscopic SQUIDs and of a single superconducting island with a gate. We demonstrate theoretically that it is possible to obtain a current as high as 0.1 nA at better than ppm accuracy via periodically modulating both the gate charge and the effective Josephson coupling. We find that the device is tolerant against background charge noise and operates well even in a dissipative environment. The effect of the imperfect suppression of the Josephson coupling and the finite operating frequency are also investigated.