双能态自旋-晶格声子耦合量子隧道系统的非经典能态和量子相干耗散

采用关联表象变分波函数方案, 介入三个非经典关联效应, 求解有限温度双能态自旋-晶格声子耦合量子隧道系统的非经典态, 着重研究化解由于粒子自旋-单声子相互作用引起的量子涨落导致双能态系统的退相干性量子耗散. 这三个非经典关联效应是: 1) 声子位移-粒子自旋 (σ_z)间强非绝热关联; 2) 声子压缩态效应及其伴随发生的单声子相干态-声子压缩态两过程相干效应; 3) 由关联表象导致的声子位移(U_D)与声子压缩(U_S)的表象关联非绝热修正. 结果表明: 由于引入粒子自旋-双声子相互作用, 大幅度地增强了声子场压缩态, 特别是更进一步极大幅度地增强了非经典压缩-相干态效应. 因此, 由粒子自旋-单声子相互作用产生的Debye-Walle相干弹性散射效应导致量子隧道项(-Δ₀σ_x)的强烈指数衰减及其伴随严重的量子相干损失的极大幅度的抑制, 并且自旋-晶格声子耦合量子隧道系统的非经典态能量大幅度降低. 关键词： 非经典能态 量子隧穿相干损失 自旋-双声子相互作用 压缩相干态效应     

Tunneling, dissipation, and superfluid transition in quantum Hall bilayers

We study bilayer quantum Hall systems at total Landau level filling factor nu=1 in the presence of interlayer tunneling and coupling to a dissipative normal fluid. Describing the dynamics of the interlayer phase by an effective quantum dissipative XY model, we show that there exists a critical dissipation sigma(c) set by the conductance of the normal fluid. For sigma>sigma(c), interlayer tunnel splitting drives the system to a nu=1 quantum Hall state. For sigma相似文献   

Out-of-equilibrium admittance of single electron box under strong Coulomb blockade

We study admittance and energy dissipation in an out-of-equilibrium single electron box. The system consists of a small metallic island coupled to a massive reservoir via single tunneling junction. The potential of electrons in the island is controlled by an additional gate electrode. The energy dissipation is caused by an AC gate voltage. The case of a strong Coulomb blockade is considered. We focus on the regime when electron coherence can be neglected but quantum fluctuations of charge are strong due to Coulomb interaction. We obtain the admittance under the specified conditions. It turns out that the energy dissipation rate can be expressed via charge relaxation resistance and renormalized gate capacitance even out of equilibrium. We suggest the admittance as a tool for a measurement of the bosonic distribution corresponding collective excitations in the system.     

Macroscopic quantum tunneling in a d-wave high-TC Bi2Sr2CaCu2O8 + delta superconductor

While Josephson-junction-like structures intrinsic to the layered cuprate high temperature superconductors offer an attractive stage for exploiting possible applications to new quantum technologies, the low energy quasiparticle excitations characteristically present in these d-wave superconductors may easily destroy the coherence required. Here we demonstrate for the first time the feasibility of macroscopic quantum tunneling in the intrinsic Josephson junctions of a high temperature superconductor Bi(2)Sr(2)CaCu(2)O(8 + delta), and find it to be characterized by a high classic-to-quantum crossover temperature and a relatively weak quasiparticle dissipation.     

Decoherence and entanglement in coherent quantum tunneling of magnetization with dissipation

We study the coherent quantum tunneling of magnetization, for example, in a biaxial molecular magnet with dissipation of the environment which results in the suppression of the tunneling and therefore the decoherence of superposition of macroscopic quantum states in terms of the general spin-boson model. The degree of entanglement between the magnet and the environment is evaluated explicitly with the help of reduced density matrix. We show an interesting relation that the degree of entanglement approaches maximum value when the coherent tunneling is suppressed completely.     

Phase coherence of the electron and hole in a ferromagnetic film in proximity with a superconductor

The scattering matrix approach between the clean and dirty limits is developed for the study of tunneling spectra in a ferromagnetic film in proximity to a superconductor. The minigap and the damped oscillation from ``0" to ``π" state in tunneling conductance are attributed to the phase coherence of the electrons and the corresponding Andreev-reflected holes in the ferromagnetic film. The calculated results provide a reasonable explanation for the behavior observed in recent experiments.     

分子磁体与环境的纠缠和退相干

本文研究了双轴分子磁体在耗散环境中的相干量子隧穿,作为环境的声子库抑制了相干量子隧穿,从而引起分子磁体中薛定谔猫态的退相干. 而环境内部声子之间的相互作用会导致分子磁体与热库之间退耦合,于是对退相干有一定的抑制作用. 在绝热近似和非绝热近似下,借助于约化密度矩阵计算了超Ohmo耗散中分子磁体与环境之间的纠缠度,当纠缠达到最大时,相干隧穿被完全抑制.     

Dissipative tunneling through a parabolic potential in the Lindblad theory of open quantum systems

By using the Lindblad theory for open quantum systems, an analytical expression of the tunneling probability through an inverted parabola is obtained. This penetration probability depends on the environment coefficients. It is shown that the tunneling probability increases with the dissipation and the temperature of the thermal bath. Received 15 October 1999 and Received in final form 5 January 2000     

Decay of a Bose-Einstein condensate in a dissipative lattice – the mean-field approximation and beyond

The dynamical evolution of a Bose-Einstein condensate in an open optical lattice is studied. Based on the Bose-Hubbard model we rederive the mean-field limit for the case of an environmental coupling including dissipation and phase-noise. Moreover, we include the next order correlation functions to investigate the dynamical behavior beyond mean field. We observe that particle loss can lead to surprising dynamics, as it can suppress decay and at the same time restore the coherence of the condensate. These behavior can be used to engineer the evolution, e.g. in the form of a stochastic resonance-like response, to inhibit tunneling or to create stable nonlinear structures of the condensate.     

On the theory of layered high-temperature superconductors

Assuming that charge carriers form a Fermi liquid state, we study a model for layered high-temperature superconductors with unretarded intralayer and interlayer pairing. Guided by band structure calculations and inverse photoemission experiments, we adopt a tight binding band with nearest and next-nearest neighbors hopping within the sheets and weak interlayer hopping. The gap equations are solved numerically, without imposing a cutoff energy, characteristic to phonon mediated superconductivity. On this basis we calculate the gap parameters,T _c , the tunneling conductance, infrared absorption and the coherence length for various band fillings =1/2–x by introducing excess holes of concentrationx. Assuming the interlayer coupling strength to be smaller than the intralayer one, our main results are as follows:T _c is dominated by the intralayer properties, reaching a maximum atx0.3, where strong coupling features appear. In the presence of interlayer pairing, the gap becomes anisotropic perpendicular to the layers, and standard BCS-behavior is modified. In particular the BCS-square root singularity in the density of states and in the tunneling conductance is replaced by van Hove singularities characterizing the anisotropic gap. In particular, we investigate the anisotropy of the tunneling conductance for specular and diffuse tunneling for a junction parallel or perpendicular to the layers, infrared absorption, as well as the coherence length, parallel and perpendicular to the layers.     

Macroscopic quantum tunneling in d-wave YBa2Cu3O7-delta Josephson junctions

The escape rate from the zero voltage state in a superconducting Josephson junction (JJ) is determined by the temperature, but it saturates at low temperature due to macroscopic quantum tunneling (MQT). Complications due to d-wave symmetry in a high temperature superconductor, like low energy quasiparticles and an unconventional current-phase relation, may influence the escape rate. We report, for the first time to our knowledge, the observation of MQT in a YBa(2)Cu(3)O(7-delta) grain boundary biepitaxial JJ. This proves that dissipation can be significantly reduced by a proper junction configuration, which is of significance for quantum coherence.     

New features of Wehrl entropy and Wehrl PD of a single Cooper-pair box placed inside a dissipative cavity

In this paper, we discuss the coherence of a single Cooper-pair box irradiated by a single-mode quantized field in a coherent state inside a phase-damped cavity. Analytic results under certain parametric conditions are obtained, by means of which we analyze the influence of dissipation on the Wehrl entropy and Wehrl phase distribution. An interesting relation between the coherence and the dissipation effect is observed.     

Calculation of quantum escape rates from a metastable well

The quantum-mechanical decay of a metastable state of a system coupled to a heat bath environment is studied. A functional integral method is presented allowing for the calculation of decay rates at finite temperatures and in the presence of dissipation. Analytical methods for high and intermediate temperatures are combined with an accurate numerical method for low temperatures where the system decays by quantum tunneling. Explicit results are given for a system with a cubic potential and frequency-independent damping.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Tunneling resonances and coherence in an optical lattice

The center-of-mass quantization of atoms trapped in a gray optical lattice is observed to manifest itself in the steady-state properties of the atoms. Modulations in the lifetime and macroscopic magnetization as a function of an applied B field are attributed to quantum mechanical tunneling resonances and are shown to exist only under conditions which afford spatial coherence of the trapped atoms over several lattice wells and coherence times that exceed the tunneling period.     

Dissipative death of quantum coherences in a spin system

We investigate the influence of weak dissipation on the dynamics of a kicked spin system. Compared to its classical limit the quantum system is strongly affected by small dissipation. We present the attenuation of two different intrinsically quantum phenomena, recurrencies and tunneling. We find agreement of analytical perturbative estimates and numerical results. We further show that dissipation acts quite differently on the quantum evolution depending on whether we are in a classically chaotic or regular domain.     

运用动态腔环境实现对原子自发辐射过程的调控

本文从理论上讨论了运用动态的腔环境实现对处于激发态的两能级原子自发辐射过程的调控.研究发现,周期性地改变与原子相互作用的腔环境导致电磁模式之间相互作用,产生电磁模式密度重新分配,并使得原子与腔环境之间的能量交换的频率以及能量耗散的速度发生改变;当腔环境的变化频率和原子、环境之间交换能量的过程保持相对一致时可以实现稳定的相干性演化,衰减效应受到显著的抑制,进而可以通过环境变化操纵相干态的演化. 关键词： 自发辐射 动态环境 量子调控     

Localization stabilized by noise

We present a model which describes a quantum two-state system interacting with the environment represented by stochastic noise. We show that coherent tunneling between the two states survives if the interaction with the environment is weak. On the contrary, a strong interaction destroys quantum coherence and the system randomly jumps from one state to the other. Moreover, the jump probability rate becomes extremely small for very strong noise. The model is relevant for understanding the quantum properties of some mesoscopic systems.     

Persistence of structure over fluctuations in biological electron-transfer reactions

In the soft-wet environment of biomolecular electron transfer, it is possible that structural fluctuations could wash out medium-specific electronic effects on electron tunneling rates. We show that beyond a transition distance (2-3 A in water and 6-7 A in proteins), fluctuation contributions to the mean-squared donor-to-acceptor tunneling matrix element are likely to dominate over the average matrix element. Even though fluctuations dominate the tunneling mechanism at larger distances, we find that the protein fold is "remembered" by the electronic coupling, and structure remains a key determinant of electron transfer kinetics.     

Coincidence of checkerboard charge order and antinodal state decoherence in strongly underdoped superconducting Bi2Sr2CaCu2O8 + delta)

The doping dependence of nanoscale electronic structure in superconducting Bi(2)Sr(2)CaCu(2)O(8 + delta) is studied by scanning tunneling microscopy. At all dopings, the low energy density-of-states modulations are analyzed according to a simple model of quasiparticle interference and found to be consistent with Fermi-arc superconductivity. The superconducting coherence peaks, ubiquitous in near-optimal tunneling spectra, are destroyed with strong underdoping and a new spectral type appears. Exclusively in regions exhibiting this new spectrum, we find local "checkerboard" charge ordering of high energy states, with a wave vector of Q = (+/- 2pi/4.5a(0),0); (0, +/- 2pi/4.5a(0)) +/- 15%. Surprisingly, this spatial ordering of high energy states coexists harmoniously with the low energy Bogoliubov quasiparticle states.     

Dynamics and dissipation induced by single-electron tunneling in carbon nanotube nanoelectromechanical systems

We demonstrate the effect of single-electron tunneling (SET) through a carbon nanotube quantum dot on its nanomechanical motion. We find that the frequency response and the dissipation of the nanoelectromechanical system to SET strongly depends on the electronic environment of the quantum dot, in particular, on the total dot capacitance and the tunnel coupling to the metal contacts. Our findings suggest that one could achieve quality factors of 10(6) or higher by choosing appropriate gate dielectrics and/or by improving the tunnel coupling to the leads.