匀强磁场与简谐势阱中的低维荷电自旋-1玻色气体

采用截断求和法和半经典近似,以二维理想玻色气体为例,研究了磁场和简谐势阱中低维荷电自旋-1玻色子的相变及磁性质．结果表明,电荷-磁场和自旋-磁场作用的竞争导致玻色-爱因斯坦凝聚临界温度随磁场的增大先略微上升后缓慢下降．截断求和法能够有效的改进半经典近似的不足．最后,讨论了磁化强度由抗磁性到顺磁性的转变及自旋因子临界值随磁场和温度的变化．     

Spin caloritronics in noncondensed Bose gases

We consider coupled spin and heat transport in a two-component atomic Bose gas in the noncondensed state. We find that the transport coefficients show a temperature dependence reflecting the bosonic enhancement of scattering and discuss experimental signatures of the spin-heat coupling in spin accumulation, spin separation, and total dissipation. Close to the critical temperature for Bose-Einstein condensation, we find that the spin-heat coupling is strongly reduced, which is also reflected in the spin caloritronics figure of merit that determines the thermodynamic efficiency of spin-heat conversion.     

Spin Hall magnetoresistance in Nb/Y3Fe5O12 hybrids

We investigate the spin Hall magnetoresistance (SMR) in niobium (Nb) attached to Y₃Fe₅O₁₂ near the superconducting critical temperature (T_c) of Nb. The SMR vanishes after cooling the sample below T_c, and recovers if the temperature is raised. When a magnetic field larger than the critical field of Nb is applied, the SMR re‐emerges with an enhanced magnitude even if the temperature is below T_c. The experimental results demonstrate that the SMR could be completely suppressed by the coupling between superconducting condensation and spin–orbit interaction in superconductors. In addition to the fundamental physics on the charge–spin interactions in superconductors, our work adds a different dimension to superconducting spintronics. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Probing anomalous longitudinal fluctuations of the interacting Bose gas via Bose-Einstein condensation of magnons

The emergence of a finite staggered magnetization in quantum Heisenberg antiferromagnets subject to a uniform magnetic field can be viewed as Bose-Einstein condensation of magnons. Using nonperturbative results for the infrared behavior of the interacting Bose gas, we present exact results for the staggered spin-spin correlation functions of quantum antiferromagnets in a magnetic field at zero temperature. In particular, we show that in dimensions 1相似文献   

Soft spin wave near nu=1: evidence for a magnetic instability in Skyrmion systems

The ground state of the two-dimensional electron gas near nu=1 is investigated by inelastic light scattering measurements carried down to very low temperatures. Away from nu=1, the ferromagnetic spin wave collapses and a new low-energy spin wave emerges below the Zeeman gap. The emergent spin wave shows soft behavior as its energy increases with temperature and reaches the Zeeman energy for temperatures above 2 K. The observed softening indicates an instability of the two-dimensional electron gas towards a magnetic order that breaks spin rotational symmetry. We discuss our findings in light of the possible existence of a Skyrme crystal.     

Collapse of a Bose gas: Kinetic approach

We have analytically explored the temperature dependence of critical number of particles for the collapse of a harmonically trapped attractively interacting Bose gas below the condensation point by introducing a kinetic approach within the Hartree?CFock approximation. The temperature dependence obtained by this easy approach is consistent with that obtained from the scaling theory.     

Low temperature electron spin resonance of the Kondo ion in a heavy fermion metal: YbRh2Si2

We report an electron spin resonance (ESR) study on single crystals of the heavy fermion metal YbRh2Si2 which shows pronounced non-Fermi liquid behavior related to a close antiferromagnetic quantum critical point. It is shown that the observed ESR spectra can be ascribed to a bulk Yb3+ resonance. This is the first observation of ESR of the Kondo ion itself in a dense Kondo lattice system. The ESR signal occurs below the Kondo temperature (T(K)) which thus indicates the existence of large unscreened Yb3+ moments below T(K). We observe the spin dynamics as well as the static magnetic properties of the Yb3+ spins to be consistent with the results of nuclear magnetic resonance and magnetic susceptibility.     

Influence of capillary condensation on the near-critical solvation force

We argue that in a fluid, or magnet, confined by adsorbing walls which favor liquid, or the (+) phase, the solvation (Casimir) force in the vicinity of the critical point is strongly influenced by capillary condensation which occurs below the bulk critical temperature T(c). At T slightly below and above T(c), a small bulk field h<0, which favors gas, or the (-) phase, leads to residual condensation and a solvation force which is much more attractive (at the same large wall separation) than that found exactly at the critical point. Our predictions are supported by results obtained from density-matrix renormalization-group calculations in a two-dimensional Ising strip subject to identical surface fields.     

Electronic conductivity in 1D Co spin chain single crystal

We have measured the transport and magnetic properties of the model spin chain single crystal Ca₃Co₂O₆ in high pulsed fields. A crossover between 1D and 3D transport is observed, with the opening of a Coulomb gap below the order temperature of the individual chains. The samples show changes in the variable range hopping transport dimensionality and gap; the magnetic field suppresses the gap, inducing a significant reduction of the resistance, while dimensionality seems governed by temperature. At fields associated with the levels of magnetization of the frustrated triangular lattice formed by the Co lines, and below the critical magnetic temperature, a further increase of the conductivity is observed. This effect is associated with planar magnetic states perpendicular to the lines.     

Bose-Einstein condensation of the magnetized ideal Bose gas

We study the charged non-relativistic Bose gas interacting with a constant magnetic field but which is otherwise free. The notion of Bose-Einstein condensation for the three-dimensional case is clarified, and we show that although there is no condensation in the sense of a phase transition, there is still a maximum in the specific heat which can be used to define a critical temperature. Although the absence of a phase transition persists for all values of the magnetic field, we show how as the magnetic field is reduced the curves for the specific heat approach the free field curve. For large values of the magnetic field we show that the gas undergoes a “dimensional reduction” and behaves effectively as a one-dimensional gas except at very high temperatures. These general features persist for other spatial dimensions D and we show results for D = 5. Finally we examine the magnetization and the Meissner-Ochsenfeld effect.     

Spin-drag Hall effect in a rotating Bose mixture

We show that in a rotating two-component Bose mixture, the spin drag between the two different spin species shows a Hall effect. This spin-drag Hall effect can be observed experimentally by studying the out-of-phase dipole mode of the mixture. We determine the damping of this mode due to spin drag as a function of temperature. We find that due to Bose stimulation there is a strong enhancement of the damping for temperatures close to the critical temperature for Bose-Einstein condensation.     

The full diagonal model of a Bose gas

This paper is the final one in a series in which we investigate some models of an interacting Bose gas using Varadhan's large deviation version of Laplacian asymptotics; in it we study the equilibrium thermodynamics of the full diagonal model of a Bose gas. We obtain a formula expressing the pressure, in the thermodynamic limit, as the supremum of a functional over the space of positive bounded measures. We analyse this formula for a large class of interaction kernels and show that there is a critical temperature below which there is Bose-Einstein condensation.     

Condensation of charged particles of a Bose-Einstein gas in a quantizing magnetic field

We determine the critical temperature of a degenerate bosonic gas of charged particles in the quantum limit for a magnetic field. We further determine the concentrations of the bosons that condense at the zero Landau level at temperatures below the critical temperature. We show that the degeneration of a bosonic gas can be suppressed when the magnetic field values are much greater than when particles begin to occupy the energy level having a nonzero value of the Landau quantum number.Brest Pedagogical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 73–77, December, 1992.     

Spin texture and magnetoroton excitations at nu=1/3

Neutral spin texture (ST) excitations at nu=1/3 are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin flips. At low magnetic fields, the ST energy is below that of the magnetoroton minimum. With increasing in-plane magnetic field these mode energies cross at a critical ratio of the Zeeman and Coulomb energies of eta(c)=0.020+/-0.001. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magnetoroton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at nu=1/3.     

Quantum-classical reentrant relaxation crossover in Dy2Ti2O7 spin ice

We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through measurements of the ac magnetic susceptibility. While the characteristic spin-relaxation time (tau) is thermally activated at high temperatures, it becomes almost temperature independent below T(cross) approximately 13 K. This behavior, combined with nonmonotonic magnetic field dependence of tau, indicates that quantum tunneling dominates the relaxational process below that temperature. As the low-entropy spin ice state develops below T(ice) approximately 4 K, tau increases sharply with decreasing temperature, suggesting the emergence of a collective degree of freedom for which thermal relaxation processes again become important as the spins become strongly correlated.     

Bose–Einstein condensation versus Dicke–Hepp–Lieb transition in an optical cavity

We provide an exact solution for the interplay between Bose–Einstein condensation and the Dicke–Hepp–Lieb self-organization transition of an ideal Bose gas trapped inside a single-mode optical cavity and subject to a transverse laser drive. Based on an effective action approach, we determine the full phase diagram at arbitrary temperature, which features a bi-critical point where the transitions cross. We calculate the dynamically generated band structure of the atoms and the associated suppression of the critical temperature for Bose–Einstein condensation in the phase with a spontaneous periodic density modulation. Moreover, we determine the evolution of the polariton spectrum due to the coupling of the cavity photons and the atomic field near the self-organization transition, which is quite different above or below the Bose–Einstein condensation temperature. At low temperatures, the critical value of the Dicke–Hepp–Lieb transition decreases with temperature and thus thermal fluctuations can enhance the tendency to a periodic arrangement of the atoms.     

Lamellar-like structures in ferrofluids placed in strong magnetic fields

We consider a ferrofluid system consisting of magnetic particles interacting with a magnetic dipole–dipole interaction. We study the strong magnetic field regime where all magnetic dipoles are completely polarized in the direction of the magnetic field. We introduce a lattice gas model that serves to describe space ordering phenomena in such systems. It is found that, within mean field theory, this model predicts a second order phase transition to a phase with inhomogeneous lamellar-like ordering below a certain critical temperature.     

Collective phenomena in cold indirect excitons

Due to their long lifetimes, indirect excitons can cool to below the temperature of quantum degeneracy. This gives an opportunity to experimentally study cold composite bosons. Both theoretically predicted phenomena and phenomena that have not been anticipated were observed in a cold gas of indirect excitons. In this contribution, we overview our studies of cold indirect excitons over the past decade, presenting spontaneous coherence and condensation of excitons, spatially modulated exciton state, long-range spin currents and spin textures, and exciton localization–delocalization transitions.     

Magnetic properties of the charge ordered Nd0.75Na0.25MnO3

Nd_0.75Na_0.25MnO₃ polycrystalline ceramic is prepared via sol-gel process and its magnetic properties and electron spin resonance (ESR) spectra have been investigated experimentally. As the compound is cooled from room temperature, a charge-ordered state first develops below 170 K. A high magnetic field melts the charge ordered state and stabilizes a ferromagnetic (FM) state below 170 K. A field induced transition, analogous to a spin flip transition, is observed between 40 and 170 K. The critical temperature for spin flip increases with increasing temperature. Below 130 K, the compound tends to be intrinsically inhomogeneous, i.e. FM clusters and paramagnetic domains coexist in this system at least, which is confirmed by ESR measurements. When the external magnetic field is zero, long range FM interaction is not developed in this system; however, a tendency of re-entrant FM transition is observed in this compound.     

Magnetic properties of charged spin-1 Bose gases with ferromagnetic coupling

The magnetic properties of a charged spin-1 Bose gas with ferromagnetic interactions are investigated within mean-field theory. It is shown that a competition between paramagnetism, diamagnetism and ferromagnetism exists in this system. It is shown that diamagnetism, being concerned with spontaneous magnetization, cannot exceed ferromagnetism in a very weak magnetic field. The critical value of reduced ferromagnetic coupling of the paramagnetic phase to ferromagnetic phase transition [Formula: see text] increases with increasing temperature. The Landé-factor g is introduced to describe the strength of the paramagnetic effect which comes from the spin degree of freedom. The magnetization density [Formula: see text] increases monotonically with g for fixed reduced ferromagnetic coupling [Formula: see text] as [Formula: see text]. In a weak magnetic field, ferromagnetism makes an immense contribution to the magnetization density. On the other hand, at a high magnetic field, the diamagnetism tends to saturate. Evidence for condensation can be seen in the magnetization density at a weak magnetic field.