介观金属双环系统中的持续电流和量子能谱

基于电荷的不连续性,对处于外磁场中的介观双环系统进行量子化.假设系统在电荷表象中具有变换的对称性,通过求解电流和Hamilton算符的本征值方程,给出介观金属环互感系统中的量子电流和能谱关系;分析和研究了介观金属环中量子电流和能谱的性质.结果表明,持续电流和量子能谱不仅与外磁场、介观双环参数有关,而且还明显地依赖于电荷的量子化性质.     

介观多环耦合系统中的量子电流增强效应

基于电荷具有离散性的事实对介观电路进行量子化，以介观三环为例研究多环耦合系统中的量子电流增强效应.结果表明，量子电流增强效应不仅存在于耦合双环系统中，而且在金属多环系统中也存在. 关键词： 介观电路 耦合系统 量子电流 增强效应     

磁场对介观耦合金属环中持续电流的影响

在考虑电荷是量子化的基础上，研究了外加磁场对介观耦合金属环中持续电流的影响.结果表明：由于存在耦合，介观金属环中总是存在附加的持续电流，附加的持续电流与电路参数及耦合系数有关.当外加磁通量按正弦规律变化时，介观耦合金属环中出现倍频与分频效应. 关键词： 介观耦合金属环 持续电流 磁场     

Possibility of persistent voltage observation in a system of asymmetric superconducting rings

The possibility of observing persistent voltage in superconducting rings of different arm widths is experimentally investigated. It was previously found that switching of the arms between superconducting and normal states by an AC current induces DC voltage oscillation in the magnetic field with a period corresponding to the flux quantum inside the ring. We used systems with a large number of asymmetric rings connected in series to investigate the possibility of observing this quantum phenomenon near the superconducting transition, where thermal fluctuations lead to switching of ring segments without an external influence and the persistent current is much smaller than in the superconducting state.     

Quantum theory of the interaction of Josephson junctions with non-classical microwaves

We present a study of the interaction between Josephson junctions in circular superconducting rings and non-classical microwaves, treating both quantum mechanically. A Hamiltonian that describes both inductive and capacitive coupling between the two systems is derived within the external field approximation. Other Hamiltonians which go beyond the external field approximation, and describe explicitly the interaction of the quantum circuit that produces the non-classical microwaves with the Josephson junction circuit, are also presented. A comparison between current experiments which use classical electromagnetic fields and the proposed experiments that use non-classical microwaves, is made.     

Quantum Switch and Efficient Spin-Filter in a System Consisting of Multiple Three-Quantum-Dot Rings

Using the non-equilibrium Keldysh Green's function technique, we investigate electron transport properties of a system consisting of multiple three-quantum-dot rings. The conductance as a function of the electron energy is numerically calculated. An antiresonance point emerges in the conductance spectra and evolves into a welldefined insulating band with the increasing number of three-quantum-dot rings. The position of the well-defined insulating band can be modulated by varying the tunneling coupling strength between adjacent three-quantumdot rings. When an external magnetic flux is introduced, several to 100% spin-polarized windows will occur due to the Zeeman splitting. These results strongly suggest that this device may realize multiple functions including quantum switch and efficient spin filtering.     

两端线型双量子点分子Aharonov-Bohm干涉仪电输运

设计一个两端线型双量子点分子Aharonov-Bohm (A-B)干涉仪. 采用非平衡格林函数技术, 理论研究无含时外场作用下的体系电导和引入含时外场作用下的体系平均电流. 在不考虑含时外场时, 调节点间耦合强度或磁通可以诱导电导共振峰劈裂. 控制穿过A-B干涉仪磁通的有无, 实现了共振峰电导数值在0与1之间的数字转换, 为制造量子开关提供了一个新的物理方案. 同时借助磁通和Rashba自旋轨道相互作用, 获得了自旋过滤. 当体系引入含时外场时, 平均电流曲线展示了旁带效应. 改变含时外场的振幅, 实现了体系平均电流的大小与位置的有效控制, 而调节含时外场的频率, 则可以实现平均电流峰与谷之间的可逆转换. 通过调节磁通与Rashba自旋轨道相互作用, 与自旋相关的平均电流亦得到有效控制. 研究结果为开发利用耦合多量子点链嵌入A-B 干涉仪体系电输运性质提供了新的认知. 上述结果可望对未来的量子器件设计与量子计算发挥重要的指导作用.     

铁磁/半导体/铁磁结构的双量子环自旋输运的特性

研究了与铁磁/半导体/铁磁结构相关的双量子环自旋输运的规律,研究结果表明：总磁通为零条件下,铁磁电极磁化方向反平行时,双量子环与单量子环相比提高了自旋电子透射概率的平均值.铁磁电极磁化方向平行时,双量子环对提高自旋向下电子平均透射概率的效果更明显;双量子环受到Rashba自旋轨道耦合作用影响时,自旋电子的平均透射概率明显高于单量子环,即使再加上外加磁场的影响,透射概率较高这一特征依然存在;双量子环所含的δ势垒具有阻碍自旋电子输运的作用,随δ势垒强度Z的增大透射概率 关键词： 双量子环 Rashba自旋轨道耦合 透射概率 δ势垒')" href="#">δ势垒     

含杂质的介观结构中的持续电流

采用量子波导理论,研究了介观结构中杂质对透射几率和持续电流的影响。结果发现持续电流的大小不仅依赖于结构中环的尺度,而且也受到杂质的影响,杂质势既可以产生电流放大也可压制持续电流的放大。通过调节杂质的势来控制持续电流。     

Four-Electron Systems in a Coupled Double-Layer Quantum Dots

Making use of the method of few-body physics, the energy spectrum of a four-electron system consisting in a vertically coupled double-layer quantum dot as a function of the strength ofa magnetic field is investigated. Discontinuous ground-state transitions induced by an external magnetic field are shown. We find that, in the strong coupling case, the ground-state transitions depend not only on the external magnetic field B but also on the distance d between double-layer quantum dots. However, in the case of weak coupling, the ground-state transitions occur in the new sequence of the values of the magic angular momentum. Hence, the interlayer separation d and electron-electron interaction strongly affect the ground state of the coupled quantum dots.     

Cyclic hydrocarbons: nanoscopic (π)-SQUIDs?

A nonperturbative method for calculating persistent currents in molecules and nanoscopic quantum rings is presented. Starting from the extended Hubbard model on a ring threaded by an Aharonov-Bohm flux, a feedback term through which the current can generate magnetic flux is added. Another extension of the Hamiltonian describes the energy stored in the internally generated field. This model is evaluated using exact diagonalization and an iterative scheme to find the minima of the free energy with respect to the current. The magnetic properties due to electron delocalization of conjugated hydrocarbons like benzene [magnetic anisotropy, magnetic susceptibility exaltation, nucleus-independent chemical shift (NICS)] — that have become important criteria for aromaticity — can be examined using this model. A possible novel mechanism for a permanent orbital magnetic moment in quantum rings analogous to the one in π-SQUIDs is found in the framework of the proposed model. The quantum rings must satisfy two conditions to exhibit this kind of permanent orbital magnetic moment: a negative Drude weight and an inductivity above the critical level.     

Magnetic flux and strain effects on electron transport in a linear array of nanoscopic rings

Electron transport through a linear array of nanoscopic rings with six quantum dot sites per ring is investigated in the presence of an external magnetic flux producing an Aharonov-Bohm phase shift effect. A tight-binding model is employed to analytically calculate the transmission as a function of electron energy, external flux, and inter-site coupling parameters. Current vs. voltage relationships of the ring system are computed using a standard scattering theory of transport and shown to modulate between semiconductor and ohmic characteristics. System parameters are adjusted in order to study the effects of a longitudinal strain on the transmission properties of the linear multiple-ring array. Longitudinal strain is modeled with a Slater-Koster type theory and is demonstrated to affect the transmission properties primarily by narrowing the transmission bands and opening up additional bandgaps in the band structure. In addition, a universal resonant transmission condition as a function of flux is extended to show that the application of strain causes the resonant transmission peaks to converge towards one-half of a flux quantum.     

Effect of quantum entanglement on Aharonov-Bohm oscillations, spin-polarized transport and current magnification effect

We present a simple model of transmission across a metallic mesoscopic ring. In one of its arm an electron interacts with a single magnetic impurity via an exchange coupling. We show that entanglement between electron and spin impurity states leads to reduction of Aharonov-Bohm oscillations in the transmission coefficient. The spin-conductance is asymmetric in the flux reversal as opposed to the two-probe electrical conductance which is symmetric. In the same model, in contradiction to the naive expectation of a current magnification effect, we observe enhancement as well as suppression of this effect depending on the system parameters. The limitations of this model to the general notion of dephasing or decoherence in quantum systems are pointed out.     

Contradiction between the results of observations of resistance and critical current quantum oscillations in asymmetric superconducting rings

Magnetic field dependences of critical current, resistance, and rectified voltage of asymmetric (half circles of different widths) and symmetrical (half circles of equal widths) aluminum rings close to the super-conducting transition were measured. All these dependences are periodic magnetic field functions with periods corresponding to the flux quantum in the ring. The periodic dependences of critical current measured in opposite directions were found to be close to each other for symmetrical rings and shifted with respect to each other by half the flux quantum in asymmetric rings with ratios between half circle widths of from 1.25 to 2. This shift of the dependences by a quarter of the flux quantum as the ring becomes asymmetric makes critical current anisotropic, which explains the effect of alternating current rectification observed for asymmetric rings. Shifts of the extrema of the periodic dependences of critical current by a quarter of the flux quantum directly contradict the results obtained by measuring asymmetric ring resistance oscillations, whose extrema are, as for symmetrical rings, observed at magnetic fluxes equal to an integer and a half of flux quanta.     

Tunnel magnetotransport in heterostructures with quantum rings

A tunnel current through a heterostructure whose barrier contains quantum rings is calculated. The plane of the rings is parallel to the barrier interface. In a magnetic field perpendicular to this plane, a tunnel current at a fixed bias experiences Aharonov-Bohm oscillations under the variation of magnetic flux through a ring; however, these oscillations are not strictly periodic.     

Proposal for quantum gates in permanently coupled antiferromagnetic spin rings without need of local fields

We propose a scheme for the implementation of quantum gates which is based on the qubit encoding in antiferromagnetic molecular rings. We show that a proper engineering of the intercluster link would result in an effective coupling that vanishes as far as the system is kept in the computational space, while it is turned on by a selective excitation of specific auxiliary states. These are also shown to allow the performing of single-qubit and two-qubit gates without an individual addressing of the rings by means of local magnetic fields.     

Tunable-Frequency Lasing on Thin Semiconductor Quantum Rings

Doklady Physics - It is been established that thin semiconductor quantum rings in an external magnetic field have unique selection properties: by choosing the type of heterostructure and the...     

Spin-polarized current through a lateral double quantum dot with spin–orbit interaction

We study the spin-polarized current through a vertical double quantum dot scheme. Both the Rashba spin–orbit (RSO) interaction inside one of the quantum dots and the strong intradot Coulomb interactions on the two dots are taken into account by using the second-quantized form of the Hamiltonian. Due to the existence of the RSO interaction, spin-up and spin-down electrons couple to the external leads with different strengths, and then a spin polarized current can be driven out of the middle lead by controlling a set of structure parameters and the external bias voltage. Moreover, by properly adjusting the dot levels and the external bias voltages, a pure spin current with no accompanying charge current can be generated in the weak coupling regime. We show that the difference between the intradot Coulomb interactions strongly influences the spin-polarized currents flowing through the middle lead and is undesirable in the generation of the net spin current. Based on the RSO interaction, the structure we propose can efficiently polarize the electron spin without the usage of any magnetic field or ferromagnetic material. This device can be used as a spin-battery and is realizable using the present available technologies.     

Quantum rings in tilted magnetic fields

The electronic states of semiconductor quantum rings (QRs) under tilted magnetic fields are studied in the framework of the effective mass and envelope function approximations. For an axial field, the orbital Zeeman contribution prevails leading to the well-known Aharanov–Bohm spectrum, but it slowly decreases as the magnetic field direction declines. For an in-plane field, only the diamagnetic shift survives and it leads to the formation of double quantum well solutions, this result being relevant for experimental techniques which use in-plane magnetic fields to determine the spin of QR ground states. We also investigate the magnetic response of partially overlapped QRs, which are characteristic of high-density samples of self-assembled rings, and find that the spectrum is quite sensitive to ring coupling.     

Analytic study of electron transmission through serial mesoscopic metallic rings

We investigate the electron transmission through a structure of serial mesoscopic metallic rings coupled to two external leads. A set of analytical expressions based on the quantum waveguide transport and the transfer matrix method are derived and used to discuss the effects of geometric configurations on transmission probabilities. It is found that in the contact ring case the existence of an applied magnetic flux is necessary to create transmission gaps, while in the non-contact ring case transmission gaps always appear irrespective of whether there is an applied magnetic flux or not. The transmissions for periodic rings with a defect ring and periodic rings built by two sorts of rings are also briefly studied. It is also found that the transmission periodicity with wave vector must be ensured by the commensurability of two characteristic lengths, i.e., of the half perimeter of a ring and the connecting wire between two adjacent rings. The special points of wave vector and magnetic flux which give rise to the transmission resonance and antiresonance are analyzed in detail.