Coexistence of Bose condensation and pairing in Boson mixtures

We consider the problem when there are two kinds of Bosons with an attraction between them. We find the system to consist of two Bose condensates with an additional pairing order between the Bosons. The properties of this state are discussed

Critical velocities and the effect of steady and oscillating rotations on solid He4

We apply our recently developed model of a Bose condensate of quantum kink wave in solid He4 to understand recent torsional oscillator experimental results of the citical velocities and the effect of the steady and oscillating rotations at around 0.1 degree K. When the D.C. rotation is present we find a decrease of the Q factor given by $Q^{-1} \propto f_{sf}\times \Omega_{D}/\omega_{TO}$ where $f_{sf}$ is the superfluid fraction; $\Omega_{D}$, the D. C. angular rotation velocity, $\omega_{TO}$, the torsional oscillator oscillating frequency. We estimate the AC critical velocity $\Omega_A^{crit}$ as that required to generate a kink wave of wavevector $2\pi/L_d$ where $L_d$ is the distance between nodes of the dislocation network. We generalize this to include a steady rotation and find a D. C. critical velocity $\Omega_D^{crit} \propto (\Omega_{A}^{crit})^{1/2}$. Estimates for both the steady and the oscillating critical velocities are in order of magnitude agreement with experimental results. We have also examined an alternative mechanism of kink tunnelling through a node in the dislocation networm and find that there is also a dependence on the torsional oscillator frequency: $\Omega_D^{crit}=[\Omega_A^{crit} \omega_{TO}2\pi]^{1/2}.$ The DC critical velocity $\Omega_D^{crit}$ is ten times higher than the experimental value

'Electron' and 'photon' emerging from supersymmetric neutral particles: A possible realization in ultracold Bose-Fermi atom mixture

We show that the 'electron' and 'photon' can emerge from a supersymmetric Hubbard model which is a non-relativistic theory of the neutral particles. The Higgs boson and 'photon' may not appear in the same phase of the phase diagram. In a Mott insulator phase of the boson, the 'electron' and 'photon' are stablized by an induced Coulomb interaction between 'electrons'. This emergent mechanism may be 'realized' in an ultracold Bose-Fermi atom mixture except the long range Coulomb interaction is repalced by a nearest neighbor one. We suggest to create 'external electric field' so that the 'electron' excitation can be observed by measuring the linear density-density response of the 'electron' gas to the 'external field' in the time flying experiment of the mixture. The Fermi surface of the 'electron' gas may also be expected to be observed in the time flying.Comment: 4 pages, 1 figur

Electron-like and photon-like excitations in an ultracold Bose-Fermi atom mixture

We show that the electron-like and photon-like excitations may exist in a three-dimensional Bose-Fermi Hubbard model describing ultracold Bose-Fermi atom mixtures in optical lattices. In a Mott insulating phase of the Bose atoms, these excitations are stabilized by an induced repulsive interaction between 'electrons' if the Fermi atoms are nearly half filling. We suggest to create 'external electric field' so that the electron-like excitation can be observed by measuring the linear density-density response of the 'electron' gas to the 'external field' in a time-of-flight experiment of the mixture. The Fermi surface of the 'electron' gas may also be expected to be observed in the time-of-flight.Comment: Supersede cond-mat/051033

Quantum Spinodal Decomposition in Multicomponent Bose-Einstein Condensates

We investigate analytically the non-equilibrium spatial phase segregation process of a mixture of alkali Bose-Einstein condensates. Two stages (I and II) are found in analogy to the classical spinodal decomposition. The coupled non-linear Schr\"odinger equations enable us to give a quantitative picture of the present dynamical process in a square well trap. Both time and length scales in the stage I are obtained. We further propose that the stage II is dominated by the Josephson effect between different domains of same condensate different from scenarios in the classical spinodal decomposition. Its time scale is estimated.Comment: revte

Non conservation of the magnetization current across magnetic hetero-structures

We show that when the magnetizations on opposite sides of a junction are not collinear, the magnetization current is not conserved as the junction is crossed. Thus the usual treatment of this problem needs to be modified. We argue that this is due to an implicit assumption of an external torque that is required to clamp the magnetization in place. The physical consequence of this is explored

Soliton and 2D domains in ultra-thin magnetic films

We show that many two dimensional domain patterns observed in Monte Carlo simulations can be obtained from the many soliton solutions of the imaginary time Sine Gordon equation. This opens the door to analytic physical understanding of the micromagnetics in ultra-thin films.Comment: 9 pages, 3 PostScript figure

Generalization of conformal mapping to scattering of electromagnetic waves from surfaces: An example of a triangle

We discuss a way to exploit the conformal mapping to study the response of a finite metallic element of arbitrary shape to an external electromagnetic field at finite frequencies. This provides a simple way to study different physics issues and provides new insights that include the issue of vorticity and eddy current, and the nature of the divergent electric field at the boundaries and at corners. The nature of the resonance can be directly addressed and clarified. We study an example of an equilateral triangle and found good agreement with results obtained with traditional numerical techniques

Internal dipolar field and soft magnons in periodic nanocomposite magnets

We study spin wave excitations in a three-dimensional nanocomposite magnet of exchange coupled hard (SmCo$_5$) and soft (FeCo) phases. The dipolar interaction splits the spin wave energies into the upper and lower branches of the spin wave manifold. When the amount of the soft phase is increased the energy of low-lying spin excitations is considerably softened due to two reasons: (i) the low- lying mode locked into the soft phase region with a spin wave gap at ${\bf k}= 0$ which scales approximately proportional to the anisotropy constant of the soft phase and (ii) the internal dipolar field which comes from magnetic charges forming at hard-soft boundaries with normals parallel to the magnetization displaces the spin wave manifold toward the lower energies. With adding more soft phase the spin wave gap closes and the system moves to another ground state characterized by the magnetization mismatch between spins of the hard and soft phases.Comment: 15 pages, 4 figure

Stable and unstable regimes in Bose-Fermi mixture with attraction between components

A collapse of the trapped boson- fermion mixture with the attraction between bosons and fermions is investigated in the framework of the effective Hamiltonian for the Bose system. The properties of the $^{87}$Rb and $^{40}$K mixture are analyzed quantitatively at $T= 0$. We find numerically solutions of modified Gross- Pitaevskii equation which continuously go from stable to unstable branch. We discuss the relation of the onset of collapse with macroscopic properties of the system. A comparison with the case of a Bose condensate of atomic $^7Li$ system is given.Comment: 7 pages, 5 figure