Tetsuro Nikuni

Bose-Einstein condensation of dilute magnons in TlCuCl3.

The recent observation [A. Oosawa et al., J. Phys. Condens. Matter 11, 265 (1999)] of the field-induced Neel ordering in the spin-gap magnetic compound TlCuCl3 is interpreted as a Bose-Einstein condensation of magnons. A Hartree-Fock-type calculation based on this picture is shown to describe the temperature dependence of the magnetization well.

Dynamics of Trapped Bose Gases at Finite Temperatures

Starting from an approximate microscopic model of a trapped Bose-condensed gas at finite temperatures, we derive an equation of motion for the condensate wavefunction and a quantum kinetic equation for the distribution function for the excited atoms. The kinetic equation is a generalization of our earlier work in that collisions between the condensate and non-condensate (C12) are now included, in addition to collisions between the excited atoms as described by the Uehling–Uhlenbeck (C22) collision integral. The continuity equation for the local condensate density contains a source term Γ12which is related to the C12collision term. If we assume that the C22collision rate is sufficiently rapid to ensure that the non-condensate distribution function can be approximated by a local equilibrium Bose distribution, the kinetic equation can be used to derive hydrodynamic equations for the non-condensate. The Γ12source terms appearing in these equations play a key role in describing the equilibration of the local chemical potentials associated with the condensate and non-condensate components. We give a detailed study of these hydrodynamic equations and show how the Landau two-fluid equations emerge in the frequency domain ωτμ ≪ τμis a characteristic relaxation time associated with C12collisions. More generally, the lack of complete local equilibrium between the condensate and non-condensate is shown to give rise to a new relaxational mode which is associated with the exchange of atoms between the two components. This new mode provides an additional source of damping in the hydrodynamic regime. Our equations are consistent with the generalized Kohn theorem for the center of mass motion of the trapped gas even in the presence of collisions. Finally, we formulate a variational solution of the equations which provides a very convenient and physical way of estimating normal mode frequencies. In particular, we use relatively simple trial functions within this approach to work out some of the monopole, dipole and quadrupole oscillations for an isotropic trap.

Quantum Fluctuations and Magnetic Structures of CsCuCl3 in High Magnetic Field

A theoretical interpretation is given on the magnetization process of CsCuCl 3 showing a small jump for the external field applied parallel to the c -axis. It is shown that quantum fluctuations are so important in this S =1/2 triangular antiferromagnet that they can change the ground-state spin structure. The observed magnetization jump is successfully explained as a spin flop process caused by the quantum effect.

Dynamical Instability of a Condensate Induced by a Rotating Thermal Gas

We study surface modes of the condensate in the presence of a rotating thermal cloud in an axisymmetric trap. By considering collisions that transfer atoms between the condensate and the noncondensate, we find that m>0 modes, which rotate in the same sense as the thermal cloud, damp less strongly than m<0 modes, where m is the polarity of the excitation. We show that above a critical angular rotation frequency, equivalent to the Landau stability criterion, m>0 modes become dynamically unstable, leading to the possibility of vortex nucleation. We also generalize our stability analysis to treat the case where the stationary state of the condensate already possesses a single vortex.

TWO-FLUID HYDRODYNAMICS FOR A TRAPPED WEAKLY INTERACTING BOSE GAS

We derive coupled equations of motion for the condensate (superfluid) and noncondensate (normal fluid) degrees of freedom in a trapped Bose gas at finite temperatures. Our results are based on the Hartree-Fock-Popov approximation for the time-dependent condensate wave function, and thermodynamic local equilibrium for the noncondensate atoms. In the special case of a uniform weakly interacting gas, our hydrodynamic equations are shown to be consistent with the two-fluid equations of Landau. The collective modes in a parabolically trapped Bose gas include the analog of the out-of-phase second-sound mode in superfluid

Theory of Magnetic Structures of CsCuCl3 in Transverse Magnetic Field

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Longitudinal Spin Waves in a Dilute Bose Gas

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Hydrodynamic Damping in Trapped Bose Gases

At zero field CsCuCl 3 is known to form a helical spin structure along the c -axis and the 120° structure in the c -plane. This paper discusses theoretically the effect of a magnetic field applied perpendicular to the c -axis, which breaks the axial symmetry. First the classical ground state is studied. The transverse magnetic field deforms the helical spin structure and causes a continuous phase transition from the helical (i.e. incommensurate) spin structure to a commensurate state, as the field increases. Detailed studies are made of the helix pitch and of the nature of the incommensurate spin structure as functions of the field. The commensurate state has a nontrivial degeneracy, which is lifted by quantum (and thermal) fluctuations. Therefore the 1/ S expansion is applied to determine the spin structure of the commensurate state. From these studies possible spin structures are suggested for each region of field strength.

Two-Fluid Dynamics for a Bose-Einstein Condensate out of Local Equilibrium with the Noncondensate

We present a kinetic theory for a dilute noncondensed Bose gas of two-level atoms that predicts the transient spin segregation observed in a recent experiment. The underlying mechanism driving spin currents in the gas is due to a mean-field effect arising from the quantum interference between the direct and exchange scattering of atoms in different spin states. We numerically solve the spin Boltzmann equation, using a one-dimensional model, and find excellent agreement with experimental data.

Fluctuation-induced phase in in a transverse magnetic field: theory

Griffin, Wu and Stringari have derived the hydrodynamic equations of a trapped dilute Bose gas above the Bose-Einstein transition temperature. We give the extension which includes hydrodynamic damping, following the classic work of Uehling and Uhlenbeck based on the Chapman-Enskog procedure. Our final result is a closed equation for the velocity fluctuations δvwhich includes the hydrodynamic damping due to the shear viscosity θ and the thermal conductivity κ. Following Kavoulakis, Pethick and Smith, we introduce a spatial cutoff in our linearized equations when the density is so low that the hydrodynamic description breaks down. Explicit expressions are given for θ and κ, which are position-dependent through dependence on the local fugacity when one includes the effect of quantum degeneracy of the trapped gas. We also discuss a trapped Bose-condensed gas, generalizing the work of Zaremba, Griffin and Nikuni to include hydrodynamic damping due to the (non-condensate) normal fluid.