No Arabic abstract
We numerically investigate some properties of unbalanced St{u}ckelberg holographic superconductors, by considering backreaction effects of fields on the background geometry. More precisely, we study the impacts of the chemical potential mismatch and St{u}ckelberg mechanism on the condensation and conductivity types (electrical, spin, mixed, thermo-electric, thermo-spin and thermal conductivity). Our results show that the St{u}ckelbergs model parameters $C_{alpha}$ and $alpha$ not only have significant impacts on the phase transition, but also affect the conductivity pseudo-gap and the strength of conductivity fluctuations. Moreover, the effects of these parameters on a system will be gradually reduced as the imbalance grows. We also find that the influence of $alpha$ on the amplitude of conductivity fluctuations depends on the magnitude of the both $C_{alpha}$ and $deltamu/mu$ in the electric and thermal conductivity cases. This results in that increasing $alpha$ can damp the conductivity fluctuations of an unbalanced system in contrast to balanced ones.
Solitons are important nonperturbative excitations in superfluids. For holographic superfluids, we numerically construct dark solitons that have the symmetry-restored phase at their core. A central point is that we include the gravitational back-reaction of the matter fields, which becomes important at low temperatures. We study in detail the properties of these solitons under variation of the back-reaction strength via tuning the gravitational constant. In particular, the depletion fraction of the particle number density at the core of the solitons is carefully investigated. In agreement with the probe-limit analysis, the depletion fraction shows the same qualitative behavior as in Bogoliubov-de Gennes (BdG) theory, even if the back-reaction is included. We find that the depletion decreases with increasing back-reaction strength. Moreover, the inclusion of back-reaction enables us to obtain the effective energy density of solitons within holography, which together with an evaluation of the surface tension leads to a simple physical explanation for the snake instability of dark solitons.
We study the behavior of holographic entanglement entropy (HEE) for imbalanced holographic superconductors. We employ a numerical approach to consider the robust case of fully back-reacted gravity system. The hairy black hole solution is found by using our numerical scheme. Then it is used to compute the HEE for the superconducting case. The cases we study show that in presence of a mismatch between two chemical potentials, below the critical temperature, superconducting phase has a lower HEE in comparison to the AdS-Reissner-Nordstrom black hole phase. Interestingly, the effects of chemical imbalance are different in the contexts of black hole and superconducting phases. For black hole, HEE increases with increasing imbalance parameter while it behaves oppositely for the superconducting phase. The implications of these results are discussed.
We report the experimental observation of St{u}ckelberg oscillations of matter waves in optical lattices. Extending previous work on Landau-Zener tunneling of Bose-Einstein condensates in optical lattices, we study the effects of the accumulated phase between two successive crossings of the Brillouin zone edge. Our results agree well with a simple model for multiple Landau-Zener tunneling events taking into account the band structure of the optical lattice.
We investigate analytically the properties of the Weyl holographic superconductor in the Lifshitz black hole background. We find that the critical temperature of the Weyl superconductor decreases with increasing Lifshitz dynamical exponent, $z$, indicating that condensation becomes difficult. In addition, it is found that the critical temperature and condensation operator could be affected by applying the Weyl coupling, $gamma$. Moreover, we compute the critical magnetic field and investigate its dependence on the parameters $gamma$ and $z$. Finally, we show numerically that the Weyl coupling parameter $gamma$ and the Lifshitz dynamical exponent $z$ together control the size and strength of the conductivity peak and the ratio of gap frequency over critical temperature $omega_{g}/T_{c}$.
We investigate analytically the asymptotic critical behavior at large chemical potential of the conformal field living at the AdS boundary of a four-dimensional spacetime Einstein gravity. The threshold values of the chemical potential for the appearance of condensate states are discrete, equal spacing, with the gap approaches zero logarithmically in the limit $Trightarrow 0$. Numerical results surprisingly show that, the result apply even for states with low quantum number, as low as for the first or second excited states of the condensate, especially on the liquid side of the black hole van der Waals - like phase transition. We postulate that, at the exact limit $T = 0$ where the gap is zero, all excite states of the condensate are activated above a finite chemical potential, suggesting a new quantum phase transition as a function of the chemical potential.