Condensate Order Parameter Research Articles

Dynamics of the BCS-BEC Crossover in a Degenerate Fermi Gas

We study the short-time dynamics of a degenerate Fermi gas positioned near a Feshbach resonance following an abrupt jump in the atomic interaction resulting from a change of magnetic field. We investigate the dynamics of the condensate order parameter and pair wave function for a range of field strengths. When the jump is sufficient to span the BCS to Bose-Einstein condensation crossover, we show that the rigidity of the momentum distribution precludes any atom-molecule oscillations in the entrance channel dominated resonances observed in 40K and 6Li. Focusing on material parameters tailored to the 40K Feshbach resonance at 202.1 G, we comment on the integrity of the fast sweep projection technique as a vehicle to explore the condensed phase in the crossover region.

Spontaneous Soliton Formation and Modulational Instability in Bose-Einstein Condensates

The dynamics of an elongated attractive Bose-Einstein condensate in an axisymmetric harmonic trap is studied. It is shown that density fringes caused by self-interference of the condensate order parameter seed modulational instability. The latter has novel features in contradistinction to the usual homogeneous case known from nonlinear fiber optics. Several open questions in the interpretation of the recent creation of the first matter-wave bright soliton train [Nature (London) 417, 150 (2002)]] are addressed. It is shown that primary transverse collapse, followed by secondary collapse induced by soliton-soliton interactions, produces bursts of hot atoms at different time scales.

The strange-quark chemical potential as an experimentally accessible ‘order parameter’ of the deconfinement phase transition for finite baryon density

We consider the change of the strange-quark chemical potential in the phase diagram of nuclear matter, employing the Wilson loop and scalar quark condensate order parameters, mass-scaled partition functions and enforcing flavour conservation. Assuming the region beyond the hadronic phase to be described by massive, correlated and interacting quarks, in the spirit of lattice and effective QCD calculations, we find the strange-quark chemical potential to change sign: from positive in the hadronic phase, to zero upon deconfinement, to negative in the partonic domain. We propose this change in the sign of the strange-quark chemical potential to be an experimentally accessible order parameter and a unique, concise and well-defined indication of the quark-deconfinement phase transition in nuclear matter.

Bose condensates from the point of view of laser physics

The dynamic analogy between the Bose condensate of photons (laser), Bose condensate of atoms, and Bose condensate of Cooper pairs in a superconductor is discussed. It is pointed out that the coherent state of Bose condensates of different types can appear only due to stimulated transitions.The equations are discussed that combine into a unified dynamic system the order parameter of the Bose condensate and the concentration of atoms beyond the condensate for an ensemble of atoms captured by a trap, as well as the order parameter of Cooper pairs, the concentration of free quasi-particles, and the densities of phonons and photons in a semiconductor.

Finite-temperature simulations of the scissors mode in Bose-Einstein condensed gases.

The dynamics of a trapped Bose-condensed gas at finite temperatures is described by a generalized Gross-Pitaevskii equation for the condensate order parameter and a semiclassical kinetic equation for the thermal cloud, solved using N-body simulations. The two components are coupled by mean fields as well as collisional processes that transfer atoms between the two. We use this scheme to investigate scissors modes in anisotropic traps as a function of temperature. Frequency shifts and damping rates of the condensate mode are extracted, and are found to be in good agreement with recent experiments.

Pseudospin and the Kaplan-Nelson model for kaon condensation

We apply the pseudospin method to the minimal Kaplan-Nelson model of the charged kaon condensation. We show that the pseudospin coherent state, with the pseudospin as a favorable order parameter for charged boson condensation, significantly improves the energy of the dense matter in the condensed phase.

Two-dimensional field theory description of a disoriented chiral condensate.

We consider the effective $1+1$-dimensional chiral theory describing fluctuations of the order parameter of the Disoriented Chiral Condensate (DCC) which can be formed in the central rapidity region in a relativistic nucleus-nucleus or nucleon-nucleon collisions at high energy. Using $1+1$-dimensional reduction of QCD at high energies and assuming spin polarization of DDC one can find the Wess-Zumino-Novikov-Witten (WZNW) model at level $k=3$ as the effective chiral theory for the one-dimensional DDC. Some possible phenomenological consequences are briefly discussed.

Superconductivity in the liquid-dimer valence-bond state.

Introducing an unambiguous prescription which converts singlet dimers into quasidipoles, we describe the low-energy excitations in the liquid-dimer state as fluctuations of the average dipole moment. The exchange of these fluctuations leads to a long-range interaction between holes in this state. This interaction favors the two-particle Bose condensate and destroys the order parameter of the one-particle Bose condensate even at zero temperature.

SPACE-TIME PROPAGATION OF CONFINED GLUONS

We assume that there is gluon condensate in the zero-momentum mode in the QCD ground state. A lowest-order calculation in terms of a condensate order parameter leads to a dynamical mass for gluons via the Schwinger mechanism and a gluon propagator with no on-mass-shell singularities — that is, the gluon is a "nonpropagating mode" in the gluon condensate. We transform our momentum-space propagator into coordinate space and find that the propagator has essentially the same delta-function light-cone singularities as the free propagator. However, in contrast to a theory without confinement, we show that the propagator exhibits exponential decay, both for time-like and space-like propagation. In this manner, we obtain a space-time characterization of the confinement phenomenon in terms of an order parameter of the condensate.

SU(2) gauge invariance and order parameters in strongly coupled electronic systems.

We show that the spin-\textonehalf{} Heisenberg model (the strong-coupling limit of the Hubbard model) is also the strong-coupling limit of an SU(2) lattice gauge theory with fermions. The local SU(2) gauge symmetry is manifest. The role of this gauge invariance is investigated in both the Hamiltonian and path-integral formulations. Off half-filling, our results reveal the existence of a lattice SU(2) bosonic matrix field which is a natural candidate for a condensate order parameter.

Phenomenological potentials for QCD order parameters.

We discuss an effective potential for QCD from a phenomenological point of view. The potential is expressed in terms of order parameters of the gluon and chiral condensates and a scale is set by using the value of obtained from QCD sum-rule studies. We point out that in the case of nontopological soliton model of hadrons, the quantity which plays the role of the ''bag constant'' is not a renormalization-group invariant and therefore should not be identified with the energy density of the vacuum. Our model also provides a simple characterization of the mass matrix coupling scalar gluonium and scalar quarkonium. Further, the structure of the model is such that the dependence of the condensate order parameters on temperature and baryon density (as determined in lattice-gauge studies) may be readily parametrized. We also discuss the results of certain nuclear-physics experiments in terms of the dependence of QCD order parameters on baryon density.

Time-dependent gross-Pitaevskii equation at T≠0

A dynamical equation for the order parameter of superfluid helium at finite temperatures is derived quantum mechanically. This is done in the framework of the generalized theory of brownian motion by Mori. The statistical condensate (order parameter) is viewed as if it is a brownian particle in a fluctuating medium. An equation for the distribution has been derived. This equation allows a systematic derivation of the kinetic equation for the order parameter at finite temperatures, which is a generalization of the Landau-Khalatnikov equation for the superfluid order parameter.