有效质量法调控原子玻色-爱因斯坦凝聚体的双阱动力学

操控原子玻色-爱因斯坦凝聚体在双势阱中的动力学通常是通过改变势阱深度来实现,本文提出了一种基于调节原子有效质量的控制方案,可以在不改变双阱势的前提下操控凝聚体的双阱动力学.利用双模近似,本文解析地导出了超冷原子在双阱势中的隧穿强度和相互作用强度对有效质量的依赖关系,并基于平均场近似数值模拟了在有效质量调节下的凝聚体动力学演化,展示了隧穿振荡和自束缚等典型的双阱动力学行为.此外,本文的研究还发现,借助负有效质量效应,这一方案甚至可以等效地实现对负散射长度原子凝聚体双阱动力学行为的操控.     

冷原子穿越激光束的量子隧穿时间

原子通过激光冷却技术能够被制备在低温状态，这时冷原子云会展现出量子力学的波动性.研究了一束冷原子入射到一个蓝失谐的激光束上所表现出的量子力学隧穿效应.蓝失谐的激光束相对于冷原子而言等效于一个量子力学势垒.根据二能级模型，在理论上分析了具有内部结构的原子矢量物质波穿过激光束的量子力学反射与透射，特别是对原子穿越激光束所需的时间——量子隧穿时间进行了详细的研究.量子力学波动性使得冷原子穿越一个激光束时明显地展现出与经典粒子(热原子)不同的结果. 关键词： 冷原子 原子光学 量子隧穿     

Experimental realization of strong effective magnetic fields in an optical lattice

We use Raman-assisted tunneling in an optical superlattice to generate large tunable effective magnetic fields for ultracold atoms. When hopping in the lattice, the accumulated phase shift by an atom is equivalent to the Aharonov-Bohm phase of a charged particle exposed to a staggered magnetic field of large magnitude, on the order of 1 flux quantum per plaquette. We study the ground state of this system and observe that the frustration induced by the magnetic field can lead to a degenerate ground state for noninteracting particles. We provide a measurement of the local phase acquired from Raman-induced tunneling, demonstrating time-reversal symmetry breaking of the underlying Hamiltonian. Furthermore, the quantum cyclotron orbit of single atoms in the lattice exposed to the magnetic field is directly revealed.     

Experimental realization of strong effective magnetic fields in optical superlattice potentials

We present the experimental generation of large effective magnetic fields for ultracold atoms using photon-assisted tunneling in an optical superlattice. The underlying method does not rely on the internal structure of the atoms and, therefore, constitutes a general approach to realize widely tunable artificial gauge fields without the drawbacks of near-resonant optical potentials. When hopping in the lattice, the accumulated phase shift by an atom is equivalent to the Aharonov–Bohm phase of a charged particle exposed to a staggered magnetic field of large magnitude, on the order of one flux quantum per plaquette. We study the ground state of this system and observe that the frustration induced by the magnetic field can lead to a degenerate ground state for non-interacting particles. We provide a local measurement of the phase acquired by single particles due to photon-assisted tunneling. Furthermore, the quantum cyclotron orbit of single atoms in the lattice exposed to the effective magnetic field is directly revealed.     

Superfluid fermi gas in a 1D optical lattice

We calculate the superfluid transition temperature for a two-component 3D Fermi gas in a 1D tight optical lattice and discuss a dimensional crossover from the 3D to quasi-2D regime. For the geometry of finite size discs in the 1D lattice, we find that even for a large number of atoms per disc the critical effective tunneling rate for a quantum transition to the Mott insulator state can be large compared to the loss rate caused by three-body recombination. This allows the observation of the Mott transition, in contrast to the case of Bose-condensed gases in the same geometry.     

Direct observation of the quantum tunneling of single hydrogen atoms with a scanning tunneling microscope

Single hydrogen atoms were imaged on the Cu(001) surface by scanning tunneling microscopy (STM). The vibrations of individual H and D atoms against the surface were excited and detected by inelastic electron tunneling spectroscopy (STM-IETS). Variable temperature measurements of H atom diffusion showed a transition from thermally activated diffusion to quantum tunneling at 60 K. Regimes of phonon-assisted and electron-limited quantum tunneling were observed. The thermal diffusion rate of D atoms varied over 7 orders of magnitude between 80 and 50 K with no transition to quantum tunneling down to a thermal hopping rate of 4x10(-7) s(-1).     

Tunneling Dynamics of the Halves of a Double-Well Trap Containing a Bose-Einstein Condensate

In this paper, we have studied tunneling dynamics of the halves of a double-well trap containing a Bose-Einstein condensate. It is found that there exist step structure and macroscopic quantum self-trapping of population difference of atoms, and exist Shapiro-like steps of atomic tunneling current. Both the population difference and the atomic tunneling current depend strongly on the total number of atoms and the initial phase difference.     

Anisotropy of the EPR line shape of dislocation acceptor centers in semiconducting type Ic diamonds

The shape anisotropy of the EPR lines of broken carbon bonds in a dislocation core with an edge component in natural semiconducting type Ic diamonds has been investigated. It has been found that electrically active acceptor centers are formed at dislocation steps, jogs, or kinks. The distance between the paramagnetic centers is determined.     

Tunneling Dynamics of the Halves of a Double-Well Trap Containing a Bose-Einstein Condensate

In this paper, we have studied tunneling dynamics of the halves of a double-well trap containing a Bose-Einstein condensate. It is found that there exist step structure and macroscopic quantum self-trapping of populationdifference of atoms, and exist Shapiro-like steps of atomic tunneling current. Both the population difference and theatomic tunneling current depend strongly on the total number of atoms and the initial phase difference.     

Magnetoexciton in semiconductor concentric double rings

We investigate theoretically the magnetoexciton states in semiconductor concentric quantum double rings using the multi-band effective mass theory. We find that a perpendicular magnetic field can lead to oscillations in the exciton energy which appear as kinks in the magneto-photoluminescence (PL) spectra as the magnetic field increases. The spatial distribution of the exciton over the rings depends sensitively on the thicknesses of the inner and outer rings. The tunneling coupling between the inner and outer rings and the heavy-hole and light-hole mixing results in different anticrossing behaviors. Exciton can be converted into a spatially separated type-II exciton by tuning the thickness, the inner and/or outer ring radius and the magnetic field. We show that this type I–type II transition is reflected in the oscillator strength of the PL spectrum which will be the experimental signature that will provide us with information about the spatial distribution of the exciton.     

Magnetoexciton in semiconductor concentric double rings

We investigate theoretically the magnetoexciton states in semiconductor concentric quantum double rings using the multi-band effective mass theory. We find that a perpendicular magnetic field can lead to oscillations in the exciton energy which appear as kinks in the magneto-photoluminescence (PL) spectra as the magnetic field increases. The spatial distribution of the exciton over the rings depends sensitively on the thicknesses of the inner and outer rings. The tunneling coupling between the inner and outer rings and the heavy-hole and light-hole mixing results in different anticrossing behaviors. Exciton can be converted into a spatially separated type-II exciton by tuning the thickness, the inner and/or outer ring radius and the magnetic field. We show that this type I–type II transition is reflected in the oscillator strength of the PL spectrum which will be the experimental signature that will provide us with information about the spatial distribution of the exciton.     

STM tunneling through a quantum wire with a side-attached impurity

The STM tunneling through a quantum wire (QW) with a side-attached impurity (atom, island) is investigated using a tight-binding model and the non-equilibrium Keldysh Green function method. The impurity can be coupled to one or more QW atoms. The presence of the impurity strongly modifies the local density of states of the wire atoms, thus influences the STM tunneling through all the wire atoms. The transport properties of the impurity itself are also investigated mainly as a function of the wire length and the way it is coupled to the wire. It is shown that the properties of the impurity itself and the way it is coupled to the wire strongly influence the STM tunneling, the density of states and differential conductance.     

Competing classical and quantum effects in shape relaxation of a metallic island

Pb islands grown on a Si substrate transform at room temperature from a flattop facet geometry into an unusual ring shape. The volume-preserving mass transport is catalyzed by the electrical field from the tip of a scanning tunneling microscope. The ring morphology results from the competing classical and quantum effects in the shape relaxation. The latter is enhanced by the large anisotropy of the effective mass, and leads to a sequential strip-flow growth on alternating strips of the same facet defined by substrate steps, showing its dynamical impact on the stability of a nanostructure.     

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.     

Localized molecular constraint on electron delocalization in a metallic chain

An artificial quantum structure consisting of a single CO molecule adsorbed on a Au chain was assembled by manipulating single Au atoms on NiAl(110) at 12 K with a scanning tunneling microscope (STM). The CO disrupts the delocalization of electron density waves in the chain, as it suppresses the coupling between neighboring chain atoms. The possibility to specify the CO position on the chain allows controlled modification of the electronic properties in a quantum system. Inelastic electron tunneling spectroscopy with the STM provides vibrational characterization of the adsorbed CO.     

冷原子物理中的一维少体问题

作为构成量子多体系统的基本单元,一维少体系统的研究不仅可以在理论上为多体系统的量子关联及动力学等性质提供更为基本的理解,也可以为实验上制备多体系统提供更加方便和功能更加全面的方法.本文回顾了冷原子物理中一维少体系统最新的实验和理论进展.首先介绍了少体实验中实现的谐振子势阱中确定原子数的精确制备,亚稳态势阱和双阱系统中原子的隧穿,以及强相互作用下等效自旋链的实验结果.然后深度解析了理论研究方面,特别是基于精确可解模型的一些重要结果,包括亚稳态势阱中相互作用原子的隧穿概率,以及相应实验上常见势阱的能谱分析、密度分布、隧穿动力学以及强相互作用极限下的有效自旋链模型等.     

Fluctuations and decoherence in chaos-assisted tunneling

We study quantum dynamical tunneling between two symmetry-related islands of stability in the phase space of a classically chaotic system. The setting for these experiments is the motion of carefully prepared samples of cesium atoms in an amplitude-modulated standing wave of light. We examine the dependence of the tunneling dynamics on the system parameters and indicate how the observed features provide evidence for chaos-assisted (three-state) tunneling. We also observe the influence of a noisy perturbation of the standing-wave intensity, which destroys the tunneling oscillations, and we show that the tunneling is more sensitive to the noise for a smaller value of the effective Planck constant.     

Tunneling Radiation of Vector Particles from a Quantum Correction Black Hole

The purpose of this paper is to discuss the Hawking radiation of vector particles from a quantum correction black hole by the mean of quantum tunneling. In order to achieve this purpose, based on the Proca field equation and WKB approximation, the quantum tunneling method is used to calculate the tunneling rate and Hawking temperature of the black hole. According to the analysis of the consequences, we find that the tunneling rate and Hawking temperature are related to the quantum parameter besides the horizon radius and mass of the black hole. Furthermore, when the results are compared with those of scalar particles and fermions of the black hole, no difference is found. Therefore, the tunneling rate and Hawing temperature of the black hole do not change with the type of radiation particles.

     

Isotope effects in the diffusion of hydrogen

Isotope effects in diffusion of hydrogen atoms are investigated theoretically. It is shown that isotope effect is reduced by a nonadiabatic effect of the heat bath so that the classical‐quantum crossover temperature and quantum tunneling rate lose their mass dependence. On the other hand, isotope effect is reversed in classical hopping rate under strong spatial confinement at the barrier top. These results indicate that isotope effects can be the means of observing influences of many degrees of freedom characterizing environment in diffusion process. This revised version was published online in August 2006 with corrections to the Cover Date.