玻色-费米混合气体的非线性Landau-Zener隧穿

在有相互作用的非线性两模系统中, 通过对相互作用参数的调节, 研究了该系统中玻色-费米混合气体的Landau-Zener隧穿现象. 研究发现, 其中某一组分的自相互作用会影响另一组分粒子的隧穿, 使得隧穿出现临界现象. 关键词： 玻色-费米混合气体 Landau-Zener隧穿 量子绝热隧穿     

Landau-Zener tunneling of Bose-Fermi mixture in double-well

The nonlinear Landau-Zener tunneling of a Bose-Fermi mixture in a double-well potential is studied in the present paper. The effect of interaction parameters on bosonic and fermionic tunneling probability is studied for the mixture of ⁴⁰K-⁸⁷Rb. The tunneling phenomena of the system can be controled by adjusting sweeping rate, intraspecies interaction, interspecies interaction and the numbers of bosons and fermions. It is noted that there are three different regions in phase diagram: self-trapping (ST), complete tunneling (CT) and incomplete tunneling (ICT).     

The tunneling phenomena of the Fermi superfluid gases in a double-well potential by manipulating the s-wave scattering length

In the present work, we have investigated tunneling dynamics of superfluid Fermi gas in a double-well potential in deep BEC regime and in the unitarity regime by adjusting the scattering length or the interaction parameter y. The scattering length a _sc or y could affect the quantum transition dramatically. At certain regime, the complete population transfer between two modes can be obtained. However, at some other regimes, the quantum transition can be completely blocked.     

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.     

Hydrogen tunneling modes in α-Mn suppressed by elastic stresses

Behavior of the tunneling mode of hydrogen in MnH_0.04 and MnH_0.07 under high pressures in sapphire anvils was studied by the method of incoherent inelastic neutron scattering (INS). It is established that the INS peak corresponding to the hydrogen tunneling in a double-well potential disappears at a pressure of 0.8 GPa in a quasi-hydrostatic regime, while being retained without visible changes under pure hydrostatic conditions. An analogous, albeit weaker, suppression of the tunneling mode takes place upon grinding of a freshly prepared sample. The effect of suppression of the hydrogen tunneling modes by applied inhomogeneous elastic stresses is explained by a shift of the energy levels in the adjacent wells caused by the static displacements.     

Electron transport in granular amorphous silicon dioxide films with ferromagnetic nanoparticles placed in a magnetic field

Electron transport in amorphous silicon dioxide films with embedded nanoparticles (Co, Nb, Ta) was studied. The mean number of localized states in the interparticle tunneling channel was derived from the temperature dependence of conductivity for various grain concentrations under the assumption of the electron transport being governed by resonance tunneling in a chain of localized states between grains. To confirm the assumption of the inelastic character of tunneling, the dependences of the magnetoresistance on grain concentration, temperature, and magnetic field were studied. Accepting the single-orbital model, where the intergrain tunneling magnetoresistance is determined by s-s tunneling, it was found that the existence of weakly split localized states in the tunneling channel results in a lack of magnetoresistance saturation in strong magnetic fields. The combined effect of a decrease in the s-s tunneling coefficient and of growth in the probability of inelastic electron spin scattering with increasing length of the chain of localized states between particles in which the electron is tunneling accounts for the characteristic temperature-concentration dependences of the magnetoresistance. The experimental observation of these features provides an argument for the electron transport in a-SiO₂(Co,Nb,Ta) structures being governed by inelastic resonance tunneling through intergrain localized states.     

费米-费米散射长度对费米超流气体在幺正极限区域的隧穿现象影响

运用双势阱模型通过调节费米-费米散射长度研究了超流费米气体在幺正极限区域的隧穿现象. 研究发现费米-费米散射长度对量子隧穿效应有显著的影响. 在确定的区域, 可以得到在双势阱中完全的量子隧穿现象, 而在另一些区域, 这种隧穿就完全消失. 由于在实验室中超流费米气体的散射长度是可以调节的, 所以在实验上可以实现通过调节散射长度来控制超流费米气体的宏观隧穿现象.     

High-frequency response and the possibilities of frequency-tunable narrow-band terahertz amplification in resonant tunneling nanostructures

The characteristics of the high-frequency response of single- and double-well resonant tunneling structures in a dc electric field are investigated on the basis of the numerical solution of a time-dependent Schrödinger equation with open boundary conditions. The frequency dependence of the real part of high frequency conductivity (high-frequency response) in In_0.53Ga_0.47As/AlAs/InP structures is analyzed in detail for various values of the dc voltage V _dc in the negative differential resistance (NDR) region. It is shown that double-well three-barrier structures are promising for the design of terahertz-band oscillators. The presence of two resonant states with close energies in such structures leads to a resonant (in frequency) response whose frequency is determined by the energy difference between these levels and can be controlled by varying the parameters of the structure. It is shown that, in principle, such structures admit narrow-band amplification, tuning of the amplification frequency, and a fine control of the amplification (oscillation) frequency in a wide range of terahertz frequencies by varying a dc electric voltage applied to the structure. Starting from a certain width of the central intermediate barrier in double-well structures, one can observe a collapse of resonances, where the structure behaves like a single-well system. This phenomenon imposes a lower limit on the oscillation frequency in three-barrier resonant tunneling structures.     

Tunneling of trapped-atom Bose condensates

We obtain the dynamics in number and phase difference, for Bose condensates that tunnel between two wells of a double-well atomic trap, using the (nonlinear) Gross-Pitaevskii equation. The dynamical equations are of the canonical form for the two conjugate variables, and the Hamiltonian corresponds to that of a momentum-shortened pendulum, supporting a richer set of tunneling oscillation modes than for a superconductor Josephson junction, that has a fixed-length pendulum as a mechanical model. Novel modes include ‘inverted pendulum’ oscillations with an average angle of π; and oscillations about a self-maintained population imbalance that we term ‘macroscopic quantum self-trapping’. Other systems with this phase-number nonlinear dynamics include two-component (interconverting) condensates in a single harmonic trap, and He³B superfluids in two containers connected by micropores.     

Resonant scattering of light in a near-black-hole metric

We show that low-energy photon scattering from a body with radius R slightly larger than its Schwarzschild radius r _s resembles black-hole absorption. This absorption occurs via capture resulting in one of the many long-lived, densely packed resonances that populate the continuum. The lifetimes and density of these meta-stable states tend to infinity in the limit r _s →R. We determine the energy-averaged cross section for particle capture into these resonances and show that it is equal to the absorption cross section for a Schwarzschild black hole. Thus a non-singular static metric may trap photons for arbitrarily long times, making it appear completely ‘black’ before the actual formation of a black hole.     

Interaction of two-level tunneling system with phonons

Properties of two-level tunneling system which interacts with lattice vibrations are discussed in the framework of the polaron theory and the phase space approach. The decrease of the dynamic distortion of the lattice with increasing ratio η of the rigid-lattice tunneling matrix element to the small polaron binding energy is calculated in the intermediate coupling regime 0 < η < 2. A relatively sharp transition from the distorted to the undistorted state of the lattice takes place when η crosses the value 2. The expressions for the renormalized tunneling matrix element and for the longitudinal relaxation time T₁ are derived. A two-level tunneling model in which the double-well potential is asymmetric is also investigated.     

Nonmagnetic impurity effects and neutron scattering spectrum in iron pnictides

We study the effect of local impurity and the neutron scattering spectrum based on the five-orbital model obtained by the first principle calculation for iron pnictides. We find that the interband impurity scattering is induced by the complex multiorbital structure. This fact means that the fully-gapped sign-reversing s-wave state, which is predicted by spin-fluctuation theories, is very fragile against impurities. The result suggests a reasonable possibility that the fully-gapped s-wave state without sign reversal (s₊₊-wave) would be realized in dirty iron pnictides. We also find that broad peak structure observed in the neutron scattering measurements can be explained by the s₊₊-wave state.     

二维周期光子晶格中的非线性Landau-Zener隧穿

在二维周期光子晶格中研究了Kerr型非线性Landau-Zener隧穿. 首先在六方晶系光子晶格中推导出非线性三能级Landau-Zener隧穿模型, 在特殊初值条件下将其转化为非线性二能级模型. 对于三能级隧穿模型, 选择特殊初值, 研究了隧穿率随参数的变化规律. 关键词： 光子晶格 Kerr型非线性 Landau-Zener隧穿     

A controllable double-well optical trap for cold atoms (or molecules) using a binary <Emphasis Type="Italic">π</Emphasis>-phase plate: experimental demonstration and Monte Carlo simulation

We experimentally demonstrate a practical scheme to form a controllable double-well optical dipole trap for cold atoms (or cold molecules), and give some experimental results as well as the fabrication method of a binary π-phase plate. The dependence of the double-well characteristics on the phase etching error of the π-phase plate and the evolution of the double-well optical trap from two wells to a single one are studied both theoretically and experimentally, and the experimental results are consistent with the theoretical prediction. Furthermore, the dynamic process of loading and splitting of cold ⁸⁷Rb atoms from a standard magneto-optical trap (MOT) into our controllable double-well one are studied by Monte Carlo simulations. Our study shows that the loading efficiency of cold atoms from the standard MOT into our single-well trap can reach 100%, and the relative atomic density will be reduced from 1.0 to ∼0.5 during the evolution of our double-well trap, in which the temperature of cold atoms is reduced from 20 μK to ∼15 μK. In final, some potential applications of our controllable double-well optical trap in atom and molecule optics are briefly discussed.     

Dimensional crossover in two-dimensional Bose-Fermi mixtures

We investigate the equilibrium properties of boson-fermion mixtures consisting of a Bose condensate and spin-polarized Fermi gas confined in a harmonic two-dimensional (2D) trap using mean-field theory. Boson-boson and boson-fermion coupling constants have a logarithmic dependence on the density because of the two-dimensional scattering events when the s-wave scattering lengths are on the order of mixture thickness. We show that this modifies the density profiles significantly. It is also shown that the dimensional crossover stabilizes the mixture against collapse and spatial demixing is observed for the case of a negative boson-fermion scattering length.     

The effect of an external electric field on the optical properties of a quantum-dot molecule with a resonant state of the D 2 − center

The effect of an external electric field on the probability of the electron radiative transition from a resonant u-state to the localized g-state of the D ₂ ^? center in the presence of dissipative tunneling has been investigated in the model of zero-radius potential. It is shown that the probability of radiative transition increases by about two orders of magnitude in the case of the external electric field intensity for which the initially asymmetric double-well oscillator potential simulating a quantum-dot molecule becomes symmetrical.     

Superconductors containing impurities with crystal-field split energy levels

We investigate theoretically the problem of a superconducting matrix containing paramagnetic rare earth impurities with crystal-field split energy levels. There are two competing mechanisms which change the superconducting transition temperatureT _c. One is inelastic charge scattering of conduction electrons from the aspherical part of the 4f charge distribution, which leads to an increase inT _c similar to that of optical phonons. The other and often predominant mechanism comes from the exchange interaction, which depressesT _c and can be very effective even among non-magnetic levels via off-diagonal matrix elements. Crystalline fields serve to alter the effectiveness of the two kinds of scattering depending upon the symmetry character of the low-lying levels, and in favorable cases one may study separately the effects of the two types of scattering by adding different impurities to a given host. We find that crystal-field levels at energies quite high compared tok _B·T_c can still have an important effect onT _c. It is shown that the crystalline-field splitting should be directly observable as structure in the tunneling characteristics.     

Phase transitions in the atomic, electronic, and magnetic subsystems of LaSrMnO films versus the synthesis temperature

The evolution of the cluster structure in amorphous LaSrMnO films as synthesis temperature T _s increases from 20 to 300°C is considered. Two order-disorder phase transitions with different scale parameters are observed. One of them, the aggregation of disordered atoms into small (～20 Å) amorphous clusters at T _s = 100°C, shows up as a sharp increase in the intensity of diffuse X-ray scattering (diffuse halo 1) with a simultaneous suppression of incoherent (background) scattering. At T _s > 150°C, disordering dominates (I _incoh = I _max) until the next stage of ordering sets in at T _s = 250?300°C. At this stage, the crystalline phase forms from large (>100 Å) crystalline clusters. This amorphous-crystalline phase transition is characterized by the appearance of Debye lines and a reduction of the halo intensity. The structural phase transition to long-range order is accompanied by a decrease in the LaSrMnO resistivity from 10¹⁰ to 10 Ω cm and a change from the tunneling mechanism of conductivity involving metallic clusters (which is typical of granulated systems) to the hopping mechanism with a hop variable length following the Mott law ρ ～ exp(T ^?1/4). In the magnetic subsystem, the paramagnetic-ferromagnetic phase transition occurs.     

Widths of 4f antiprotonic levels in the oxygen region

It is shown that the widths of the 4f antiprotonic levels in ¹⁴N, ^16,17,18O, ¹⁹F and ²³Na are all well described in terms of the lower moments of t-matrices based on the Dover-Richard N N̄ potential. However, the lowest S-wave moment 〈t_s〉 - related to the NN̄ scattering length — is by itself not sufficient with the higher moment 〈r²t_s〉 and the P-wave moment 〈t_p〉 both playing important roles.