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.     

Collective quantum tunneling of strongly correlated electrons in commensurate mesoscopic rings

We present a new effect that is possible for strongly correlated electrons in commensurate mesoscopic rings: the collective tunneling of electrons between classically equivalent configurations, corresponding to ordered states possessing charge and spin density waves (CDW, SDW) and charge separation (CS). Within an extended Hubbard model at half filling studied by exact numerical diagonalization, we demonstrate that the ground state phase diagram comprises, besides conventional critical lines separating states characterized by different orderings (e.g. CDW, SDW, CS), critical lines separating phases with the same ordering (e.g. CDW-CDW) but with different symmetries. While the former also exist in infinite systems, the latter are specific for mesoscopic systems and directly related to a collective tunnel effect. We emphasize that, in order to construct correctly a phase diagram for mesoscopic rings, the examination of CDW, SDW and CS correlation functions alone is not sufficient, and one should also consider the symmetry of the wave function that cannot be broken. We present examples demonstrating that the jumps in relevant physical properties at the conventional and new critical lines are of comparable magnitude. These transitions could be studied experimentally e.g. by optical absorption in mesoscopic systems. Possible candidates are cyclic molecules and ring-like nanostructures of quantum dots. Received 27 November 2000     

The weak localization correction to the polarization and persistent currents in mesoscopic metal rings

We re-examine the effect of electron-electron interactions on the persistent current in mesoscopic metal rings threaded by an Aharonov-Bohm flux. The exchange contribution to the current is shown to be determined by the weak localization correction to the polarization. We explicitly calculate the contribution from exchange interactions with momentum transfers smaller than the inverse elastic mean free path to the average current, and find that it has the same order of magnitude as the canonical current without interactions.     

Two interacting particles in a disordered chain IV: Scaling of level curvatures

The curvatures of two-particle energy levels with respect to the enclosed magnetic flux in mesoscopic disordered rings are investigated numerically. We find that the typical value of the curvatures is increased by interactions in the localised regime and decreased in the metallic regime. This confirms a prediction by Akkermans and Pichard (Eur. Phys. J. B 1, 223 (1998)). The interaction-induced changes of the typical curvatures at different energies and disorder strengths exhibit one-parameter scaling with a conductance-like single parameter. This suggests that interactions could influence the conductance of mesoscopic systems similarly. Received 24 August 1998     

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     

Effect of dephasing on the current statistics of mesoscopic devices

We investigate the effects of dephasing on the current statistics of mesoscopic conductors with a recently developed statistical model, focusing, in particular, on mesoscopic cavities and Aharonov-Bohm rings. For such devices, we analyze the influence of an arbitrary degree of decoherence on the cumulants of the current. We recover known results for the limiting cases of fully coherent and totally incoherent transport and are able to obtain detailed information on the intermediate regime of partial coherence for a varying number of open channels. We show that dephasing affects the average current, shot noise, and higher order cumulants in a quantitatively and qualitatively similar way, and that consequently shot noise or higher order cumulants of the current do not provide information on decoherence additional or complementary to what can be already obtained from the average current.     

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.     

Impurity spin dynamics and quantum coherence in mesoscopic rings

We investigate the effects of impurity spin dynamics on quantum coherence in mesoscopic metallic rings. Spin relaxation induces temporal current fluctuations which are closely related to persistent currents in such systems. The persistent current is suppressed in the presence of magnetic impurities. We discuss spin-polarisation effects, spin-glass ordering, and the Kondo effect which are shown to restore the current even in the presence of magnetic scattering.     

Studying quantum current magnification in mesoscopic coupling metallic rings

We propose a quantum Hamiltonian and obtain the relationship between external magnetic field and currents of three mesoscopic coupling rings. The impact of coupling on quantum current magnification in the system is studied. It is found that the quantum current magnification effect strongly depend on both the external magnetic flux and the coupling factors. As a result, by taking use of coupling rings, we can design some circuits to transfer and communicate signals.     

Persistent currents in mesoscopic rings and conformal invariance

The effect of point defects on persistent currents in mesoscopic rings is studied in a simple tight-binding model. Using an analogy with the treatment of the critical quantum Ising chain with defects, conformal invariance techniques are employed to relate the persistent current amplitude to the Hamiltonian spectrum just above the Fermi energy. From this, the dependence of the current on the magnetic flux is found exactly for a ring with one or two point defects. The effect of an aperiodic modulation of the ring, generated through a binary substitution sequence, on the persistent current is also studied. The flux-dependence of the current is found to vary remarkably between the Fibonacci and the Thue-Morse sequences. Received: 4 March 1998 / Revised: 20 April 1998 / Accepted: 30 April 1998     

Rapid communication Fabrication of nano-dot- and nano-ring-arrays by nanosphere lithography

We have applied the method of nanosphere lithography to fabricate arrays of nanometer-scale gold and cobalt particles. The individual cobalt particles were found to be in a single domain state as verified by magnetic force microscopy. By tuning the preparation conditions, we also successfully fabricated arrays of mesoscopic gold rings for the first time with potential application for persistent current experiments.     

A new vortex state with non-uniform vorticity in superconducting mesoscopic rings

A study is presented of the superconducting states in mesoscopic rings. On the basis of self-consistent solution of the Ginzburg-Landau equations, a new kind of vortex states with non-uniform vorticity is found for some cases to be thermodynamically more stable, than the solution with unique winding number for the whole ring. There are indications that the solution with non-uniform vorticity concerns a metastable state of a superconducting mesoscopic ring.     

Geometric-symmetry consideration on magneto-conductance oscillations in purely ballistic quantum rings

We report an analysis of the magneto-conductance oscillations in two-dimensional mesoscopic ring structures with different geometric symmetry. In particular, we demonstrate that the h/2e magneto-conductance oscillations may be absent or present in purely ballistic quantum rings depending on their geometric symmetry. A detailed analysis of the results is carried out by expanding the transmission amplitudes with backscattering terms. The role of interchannel scattering, which is an essential element of multichannel ballistic transport, is stressed.     

Electronic border states in mesoscopic metallic cavities: their contribution to the orbital magnetism

We present a study of persistent currents (PC) in mesoscopic two-dimensional metallic cavities. The cavity is simulated with a gate potential that constrains the electrons to be in the regions of the sample. Varying the profile of the gate potential we can change the size and the geometry of the sample. We study non-single connected samples for which the typical current, I_typ, is substantially enhanced at half-filling, due to the existence of electronic border states near the Fermi level for this filling. These states have a large angular momentum and its response to the magnetic external flux is paramagnetic. We discuss the relevance of our results in relation to the, as yet, unexplained discrepancies between measurements of large values of PC in mesoscopic metallic rings and theoretical predictions.     

Superpositions and Entanglement of Mesoscopic High-order Squeezed Vacuum States in Cavity QED

We propose a scheme for generating the superpositions and the entanglement between the mesoscopic high-order squeezed vacuum states by considering the multi-photon interaction of N two-level atoms in a cavity with high quality factor, assisted by a strong driving field. In terms of specific choices of the cavity detuning, many multiparty entangled states between the atoms and the mesoscopic high-order squeezed vacuum states and among the high-order squeezed vacuum states of the cavity modes can be generated, including the macroscopic “Schr?dinger cats” of the mesoscopic high-order squeezed vacuum states, the entanged states of the macroscopic “Schr?dinger cats”, and so on. Our scheme is achievable within the current techniques in the cavity QED.     

Measurement-induced decoherence in electronic interferometry at nanoscale

We introduce a theoretical formalism describing a wide class of ‘Which Path’ experiments in mesoscopic/nanoscopic transport. The physical system involves a mesoscopic interferometer (e.g. an Aharonov-Bohm ring with embedded dots or a side-coupled quantum dot) which is electrostatically coupled to a nearby quantum point constriction. Due to the charge sensing effect the latter acts as a charge detector. Therefore the interference pattern can be monitored indirectly by looking at the current characteristics of the detector as shown in the experimental work of Buks et al. [E. Buks, R. Schuster, M. Heiblum, D. Mahalu, V. Umansky, Nature (London) 391 (1998) 871]. We use the non-equilibrium Green-Keldysh formalism and a second order perturbative treatment of the Coulomb interaction in order to compute the relevant transport properties. It is shown that in the presence of the Coulomb interaction the current through the detector exhibits oscillations as a function of the magnetic field applied on a single-dot AB interferometer. We also discuss the dependence of the visibility of the Aharonov-Bohm oscillations on the gate potential applied to the dot.     

Spin-flip mesoscopic transport through a quantum dot coupled to carbon nanotube terminals

We investigate mesoscopic transport through a system that consists of a central quantum dot (QD) and two single-wall carbon nanotube (SWCN) leads in the presence of a rotating magnetic field. The spin-flip effect is induced by the rotating magnetic field, and the tunnelling current is sensitively related to the spin-flip effect. We present the calculations of charge and spin current components to show the intimate relations to the SWCN leads. Zeeman effect is important when the applied magnetic field is strong enough. The current characteristics are quite different when the source-drain bias is zero (eV=0) and nonzero (eV≠0). The asymmetric peak and valley of spin current versus gate voltage exhibit Fano resonance. Multi-resonant peaks of spin current versus photon energy ħω reflect the structure of CN quantum wires, as well as the resonant photon absorption and emission effect. The matching-mismatching of channels in the CN leads and QD results in novel spin current structure by tuning the frequency.     

Phase slips in Aharonov–Bohm oscillations

We give a possible mechanism for periodically occurring phaseslips in Aharonov–Bohm oscillations of mesoscopic rings.