Superfluid vs Ferromagnetic Behavior in a Bose Gas of Spin-1/2 Atoms

We study the thermodynamic phases of a Bose gas of spin-1/2 atoms in the Hartree-Fock approximation. Our main result is that, for repulsive or weakly attractive inter-component interaction strength, the superfluid and ferromagnetic phase transitions occur at the same temperature. For strongly attractive inter-component interaction strength, however, the ferromagnetic phase transition occurs at a higher temperature than the superfluid phase transition. We also find that the presence of a condensate acts as an effective magnetic field that polarizes the normal cloud. We finally comment on the validity of the Hartree-Fock approximation in describing different phenomena in this system.     

Superfluid Properties of One-Component Fermi Gas with an Anisotropic p-Wave Interaction

We investigate superfluid properties and strong-coupling effects in a one-component Fermi gas with an anisotropic p-wave interaction. Within the framework of the Gaussian fluctuation theory, we determine the superfluid transition temperature T _c, as well as the temperature T ₀ at which the phase transition from the p _x -wave pairing state to the p _x +ip _y -wave state occurs below T _c. We also show that while the anisotropy of the p-wave interaction enhances T _c in the strong-coupling regime, it suppresses T ₀.     

Collective Excitations of Trapped Superfluid Fermi Gases in a BCS-BEC Crossover with Arbitrary Atom Numbers

We investigate the collective excitations of an axially-symmetric harmonically trapped superfluid Fermi gas in the BCS-BEC crossover, focusing on the dependence of the frequency of the collective modes on the atom number. Starting from a phenomenological equation of state valid for the whole BCS-BEC crossover regime and the time-dependent Gross-Pitaevskii equation of the condensed fermionic superfluid, we obtain analytically the chemical potential and ground-state wavefunction of the paired fermions with arbitrary atom numbers by use of variational method. We solve the generalized Bogoliubov-de Gennes equations numerically and discuss the properties of low-lying excitations when the system consists with Fermi atoms of several hundreds or thousands. Our numerical results reveal some new properties of the small system in the excited states.     

Thermodynamic Behavior of Spin-1 Heisenberg Chain: a Comparative Study

Journal of Superconductivity and Novel Magnetism - In this work, we present a comparative study of thermodynamic quantum equilibrium observables of spin-1 Heisenberg chain. As a theoretical...     

The Hysteresis Behavior of a Spin-1 Blume-Capel Nanoisland

The magnetic properties of a spin-1 Blume-Capel nanoisland are investigated by the use of the effective field theory based on the probability distribution technique. The influence of the random crystal field and the system parameters on the hysteresis behavior and on the magnetic properties of the nanoisland is examined. The results show a number of characteristic behaviors, such as the appearance of double, triple, quadruple, and even quintuple hysteresis loops for appropriate values of the system parameters.     

Sound Velocity and Eigenfrequencies of a Disk-Shaped Harmonically Trapped Superfluid Fermi Gas in the BCS-BEC Crossover

The analytical expressions for the zero-temperature sound velocity and low-energy eigenfrequencies of a disk-shaped harmonically trapped three-dimensional superfluid Fermi gas are derived on the basis of the hydrodynamic theory in the regimes along the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) crossover. Theoretically, by plotting the sound velocity as a function of 1/(k _F a), it finds that there is an obvious flat behavior for the sound velocity in a disk-shaped trap around unitarity. However, there is no such flat behavior for a cigar-shaped Fermi gas at the same superfluid regime. Here, k _F is the Fermi wave vector and a is the s-wave scattering length.     

Quartz Tuning Forks and Acoustic Phenomena: Application to Superfluid Helium

Immersed mechanical resonators are well suited for probing the properties of fluids, since the surrounding environment influences the resonant characteristics of such oscillators in several ways. Quartz tuning forks have gained much popularity in recent years as the resonators of choice for studies of liquid helium. They have many superior properties when compared to other oscillating bodies conventionally used for this purpose, such as vibrating wires. However, the intricate geometry of a tuning fork represents a challenge for analyzing their behavior in a fluid environment—analytical approaches do not carry very far. In this article the characteristics of immersed quartz tuning fork resonators are studied by numerical simulations. We account for the compressibility of the medium, that is acoustic phenomena, and neglect viscosity, with the aim to realistically model the oscillator response in superfluid helium. The significance of different tuning fork shapes is studied. Acoustic emission in infinite medium and acoustic resonances in confined volumes are investigated. The results can aid in choosing a quartz tuning fork with suitable properties for experiments, as well as interpreting measured data.     

Superfluid Density in the BCS-BEC Crossover Regime of a Two-Component Fermi Gas at Finite Temperatures

We investigate the superfluid carrier density ρ _s in the BCS-BEC crossover regime of a superfluid Fermi gas at finite temperatures. Including pairing fluctuations within the gaussian fluctuation approximation, we determine the superfluid order parameter Δ, Fermi chemical potential μ, as well as ρ _s , in a consistent manner. In the weak-coupling regime, the normal fluid density ρ _n ≡ρ−ρ _s is dominated by quasi-particle excitations associated with the dissociation of Cooper pairs. As one approaches the BCS-BEC crossover regime, effects of pairing fluctuations on ρ _n become strong. In the strong-coupling BEC regime, ρ _n is shown to be dominated by Bogoliubov phonon in a molecular Bose superfluid.     

Kinetic Theory of a Spin-1/2 Bose-Condensed Gas

We derive a kinetic theory for a spin-1/2 Bose-condensed gas of two-level atoms at finite temperatures. The condensate dynamics is described by a generalized Gross–Pitaevskii equation for the two-component spinor order parameter, which includes the interaction with the uncondensed fraction. The noncondensate atoms are described by a quantum kinetic equation, which is a generalization of the spin kinetic equation for spin-polarized quantum gases to include couplings to the condensate degree of freedom. The kinetic equation is used to derive hydrodynamic equations for the noncondensate spin density. The condensate and noncondensate spins are coupled directly through the exchange mean field. Collisions between the condensate and noncondensate atoms give rise to an additional contribution to the spin diffusion relaxation rate. In addition, they give rise to mutual relaxation of the condensate and noncondensate due to lack of local equilibrium between the two components.     

Phase Diagrams and Magnetic Properties of a Spin-3/2 Blume-Capel Nanoparticle

Within the framework of Monte Carlo simulation, the critical behavior and the magnetic properties of a spherical nanoparticle with the presence of a crystal field are studied. The particle is formed by a ferromagnetic core of spin-3/2 surrounded by a ferromagnetic shell of spin-3/2 with antiferromagnetic interface coupling located on a simple cubic lattice. The ground-state phase diagrams are obtained in the plane of the crystal field versus the shell coupling and the interface coupling versus the crystal field. The effects of the shell coupling and the crystal field on both the critical and compensation temperatures are investigated. The phase diagrams exhibit a rich variety of behaviors, which include the first- and second-order phase transitions, the multiphase critical point, the isolated critical point, and the compensation point.     

Population Dynamics of a Spin-1 Bose Gas at Finite Temperatures

We study population dynamics of a trapped spin-1 Bose gas above the Bose-Einstein transition temperature. Starting from the semiclassical kinetic equation for a spin-1 gas, we derive coupled rate equations for the populations of internal states. Solving the rate equations, we study the dynamical evolution of spin populations. We also estimate the characteristic timescale in which the system reaches equilibrium. Finally, we briefly discuss the extension of the theory to Bose-condensed phase below the Bose-Einstein transition temperature, and discuss the static equilibrium condition for the spin-1 system.     

NMR Properties of a Possible Non-unitary State of Superfluid 3He in Aerogel

No Heading We discuss the NMR properties of a non-unitary state which has recently been proposed by Fomin as a candidate for the A-like phase of superfluid ³He in aerogel. The new state is robust against the anisotropic scattering by the aerogel strands. We solve Leggettts equation for the robust state. Two transverse resonances as well as a longitudinal resonance are obtained. We find that in the pulsed-NMR the free induction frequency depends on the tipping angle as (1 + cos ).PACS numbers: 67.57.Lm, 67.57.Pq,     

Structural and Superfluid Properties of the 4He Monolayer on a C28 Molecule

We have performed path-integral Monte Carlo calculations to study the adsorption of N ⁴He atoms on a single C₂₈ fullerene molecule. Radial density distributions show a layer-by-layer growth of ⁴He with the first adlayer being located at a distance of 5.3 Å from the center of C₂₈. The monolayer is found to show a commensurate structure at N=16 with each of the 16 adsorption sites on the molecular surface being occupied by one ⁴He atom. As more helium atoms are adsorbed beyond N=16, the adlayer is in a mixed state consisting of 4 localized atoms at the hexagonal faces and the other atoms delocalized over the pentagonal faces. Another structurally-ordered state is observed at N=32, where the helium layer shows the same crystalline structure with an icosahedral symmetry as observed for 32 ⁴He atoms on a C₆₀ molecule. It is found that more ⁴He atoms can be squeezed into the first layer to disrupt this icosahedral structure when enough ⁴He atoms are added in the second layer. Finally we observe the reentrant superfluid response of the monolayer with superfluidity being quenched completely at the ordered states of N=16 and 32.     

Trapping Effect of Periodic Structures on the Thermodynamic Properties of a Fermi Gas

We report the thermodynamic properties of an ideal Fermi gas immersed in periodic structures such as penetrable multilayers or multitubes simulated by one (planes) or two perpendicular (tubes) external Dirac comb potentials, while the particles are allowed to move freely in the remaining directions. In contrast to what happens to the bosonic chemical potential, which is a constant for T<T _c , a non decreasing with temperature anomalous behavior of the fermionic chemical potential is confirmed and monitored as the tube bundle goes from 3D to 1D when the wall impenetrability overcomes a threshold value. In the specific heat curves dimensional crossovers are very noticeable at high temperatures, where the system behavior goes from 3D to 2D for multilayers or from 3D to 1D for multitubes, as the wall impenetrability is increased.     

Spin Compensation Temperatures in the Monte Carlo Study of a Mixed Spin-3/2 and Spin-1/2 Ising Ferrimagnetic System

The magnetic properties of mixed-spin S ^′ = 3/2 and S = 1/2 ferrimagnetic system have been studied by Monte Carlo simulations. The ground-state phase diagrams of mixed spin-3/2 and spin-1/2 Ising system are given. The critical and compensation temperatures have been found with different values of reduced exchange interactions. The variation of total magnetization with reduced exchange interactions of mixed spins is given with different temperatures and crystal fields. We have also given the variation of total magnetization with crystal field for different reduced exchange interactions and different temperatures. The magnetic hysteresis cycle is found to have different values of reduced exchange interactions, temperatures, and crystal fields. The multiple hysteresis and the superparamagnetic phase are established.     

富集10B的Ce:Li6Lu(10BO3)3晶体生长及其闪烁性能

为了提高中子探测效率, 以富集¹⁰B的H₃¹⁰BO₃为原料, 通过提拉法生长了富集¹⁰B的Ce:Li₆Lu(¹⁰BO₃)₃晶体。X射线激发发射光谱测试表明: 其发光峰位于360~480 nm, 属于Ce³⁺离子典型的5d - 4f跃迁发光, 其闪烁发光效率为BGO晶体的3.9倍。在350 nm紫外光和¹³⁷Cs所发出的662 keV的γ射线激发下测得的衰减时间分别为21.0 ns 和31.7 ns, 在¹³⁷Cs辐射源激发下所测得的相对光输出是CsI(Tl)晶体的20%, 能量分辨率为9.7%。在慢化²⁵²Cf中子源激发下可以观测到明显的中子全能峰, 其能量分辨率为33%。上述研究结果表明, Ce:Li₆Lu(¹⁰BO₃)₃晶体具有较高的闪烁效率、快的衰减时间和良好的中子探测效率, 是一种具有应用前景的中子探测用闪烁晶体。     

Zero-Temperature Study of a Tetrameric Spin-1/2 Chain in a Transverse Magnetic Field

We consider an alternating Heisenberg spin-1/2 antiferromagnetic–ferromagnetic chain with the space-modulated dominant antiferromagnetic exchange and anisotropic ferromagnetic coupling (tetrameric spin-1/2 chain). The zero-temperature effect of a symmetry breaking transverse magnetic field on the model is studied numerically. It is found that the anisotropy effect on the ferromagnetic coupling induces two new gapped phases. We identified their orderings as a kind of the stripe antiferromagnetic phase. As a result, the magnetic phase diagram of the tetrameric chain shows five gapped quantum phases, and the system is characterized by four critical fields which mark quantum phase transitions in the ground state of the system with the changing transverse magnetic field. We have also exploited the well-known bipartite entanglement (name as concurrence) and global entanglement tools to verify the occurrence of quantum phase transitions and the corresponding critical points.     

Optical and Electrical Properties of Polycrystalline ZnSe:Li

ZnSe:Li ceramics were prepared by hot-pressing powder mixtures of ZnSe and LiOH at 1050°C under a pressure of 30 MPa. The luminescence spectra of the ceramics are dominated by a red emission centered at 630 nm. With increasing Li concentration, the intensity and width of the red band increase, and the band shifts to shorter wavelengths. The nature of the emission centers and the effect of the doping level on their parameters are discussed. The electrical properties of the ZnSe:Li ceramics can be understood in terms of percolation in heterogeneous systems.     

Li‐Rich Li[Li1/6Fe1/6Ni1/6Mn1/2]O2 (LFNMO) Cathodes: Atomic Scale Insight on the Mechanisms of Cycling Decay and of the Improvement due to Cobalt Phosphate Surface Modification

Lithium‐rich Li[Li_1/6Fe_1/6Ni_1/6Mn_1/2]O₂ (0.4Li₂MnO₃‐0.6LiFe_1/3Ni_1/3Mn_1/3O₂, LFNMO) is a new member of the xLi₂MnO₃·(1 ? x)LiMO₂ family of high capacity–high voltage lithium‐ion battery (LIB) cathodes. Unfortunately, it suffers from the severe degradation during cycling both in terms of reversible capacity and operating voltage. Here, the corresponding degradation occurring in LFNMO at an atomic scale has been documented for the first time, using high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM), as well as tracing the elemental crossover to the Li metal anode using X‐ray photoelectron spectroscopy (XPS). It is also demonstrated that a cobalt phosphate surface treatment significantly boosts LFNMO cycling stability and rate capability. Due to cycling, the unmodified LFNMO undergoes extensive elemental dissolution (especially Mn) and O loss, forming Kirkendall‐type voids. The associated structural degradation is from the as‐synthesized R‐3m layered structure to a disordered rock‐salt phase. Prior to cycling, the cobalt phosphate coating is epitaxial, sharing the crystallography of the parent material. During cycling, a 2–3 nm thick disordered Co‐rich rock‐salt structure is formed as the outer shell, while the bulk material retains R‐3m crystallography. These combined cathode–anode findings significantly advance the microstructural design principles for next‐generation Li‐rich cathode materials and coatings.     

Incommensurate Filling of Ultracold Spin-1 Atoms in Optical Superlattice with a Weak Magnetic Field

The ground states of ultracold spin-1 atoms trapped in an optical superlattice in a weak magnetic field with incommensurate filling of three atoms in one double-well are obtained. It is shown that the ground-state diagrams of the reduced double-well model are remarkably different for the antiferromagnetic and ferromagnetic atoms. These novel spin-states can be controlled easily and exactly by modulating the tunneling parameter and the quadratic Zeeman energy, which may be a tool for the study of spin-entanglement.