Showing papers by "Rong-Gen Cai published in 2013"

Constraining the anisotropic expansion of the Universe

Abstract: We study the possibly existing anisotropy in the accelerating expansion Universe with the Union2 type Ia supernovae data and gamma-ray burst data We construct a direction-dependent dark energy model and constrain the anisotropy direction and strength of modulation We find that the maximum anisotropic deviation direction is (l, b) = (126 degrees, 13 degrees) [or, equivalently, (l, b) = (306 degrees, -13 degrees)], and the current anisotropy level is g(0) = 0030(+0010)(-0030) (1 sigma confidence level with Union2 data) Our results do not show strong evidence for the anisotropic dark energy model We also discuss potential methods that may distinguish the peculiar velocity field from the anisotropic dark energy model

Strong gravitational lensing in a black-hole spacetime dominated by dark energy

Abstract: We study the influence of phantom fields on strong field gravitational lensing. Supposing that the gravitational field of the supermassive central object of the Galaxy is described by a phantom black hole metric, we estimate the numerical values of the coefficients and observations and find that the influence of the phantom fields is somewhat similar to that of the electric charge in a Reissner-Nordstr\"om black hole, i.e., the deflect angle and angular separation increase with the phantom constant $b$. However, other observations are contrary to the Reissner-Nordstr\"om case and show the effects of dark energy, such as (i) compressing the usual black hole and more powerfully attracting photons, (ii) making the relativistic Einstein ring larger than that of the usual black hole, and (iii) not weakening the usual relative magnitudes, which will facilitate observations.

Competition between the s-wave and p-wave superconductivity phases in a holographic model

Abstract: We build a holographic superconductor model with a scalar triplet charged under an SU(2) gauge field in the bulk. In this model, the s-wave and p-wave condensates can be consistently realized. We find that there are totally four phases in this model, namely, the normal phase without any condensate, s-wave phase, p-wave phase and the s+p coexisting phase. By calculating Gibbs free energy, the s+p coexisting phase turns out to be thermodynamically favored once it can appear. The phase diagram with the dimension of the scalar operator and temperature is drawn. The temperature range for the s+p coexisting phase is very narrow, which shows the competition between the s-wave and p-wave orders in the superconductor model.

Competition between the s-wave and p-wave superconductivity phases in a holographic model

Abstract: We build a holographic superconductor model with a scalar triplet charged under an SU(2) gauge field in the bulk. In this model, the s-wave and p-wave condensates can be consistently realized. We find that there are totally four phases in this model, namely, the normal phase without any condensate, s-wave phase, p-wave phase and the s+p coexisting phase. By calculating Gibbs free energy, the s+p coexisting phase turns out to be thermodynamically favored once it can appear. The phase diagram with the dimension of the scalar operator and temperature is drawn. The temperature range for the s+p coexisting phase is very narrow, which shows the competition between the s-wave and p-wave orders in the superconductor model.

Entanglement entropy in holographic p-wave superconductor/insulator model

Abstract: We continue our study of entanglement entropy in the holographic superconducting phase transitions. In this paper we consider the holographic p-wave superconductor/insulator model, where as the back reaction increases, the transition is changed from second order to first order. We find that unlike the s-wave case, there is no additional first order transition in the superconducting phase. We calculate the entanglement entropy for two strip geometries. One is parallel to the super current, and the other is orthogonal to the super current. In both cases, we find that the entanglement entropy monotonically increases with respect to the chemical potential.

A Holographic P-wave Superconductor Model

Abstract: We study a holographic p-wave superconductor model in a four dimensional Einstein-Maxwell-complex vector field theory with a negative cosmological constant. The complex vector field is charged under the Maxwell field. We solve the full coupled equations of motion of the system and find black hole solutions with the vector hair. The vector hairy black hole solutions are dual to a thermal state with the U(1) symmetry as well as the spatial rotational symmetry breaking spontaneously. Depending on two parameters, the mass and charge of the vector field, we find a rich phase structure: zeroth order, first order and second order phase transitions can happen in this model. We also find "retrograde condensation" in which the hairy black hole solution exists only for the temperatures above a critical value with the free energy much larger than the black hole without hair. We construct the phase diagram for this system in terms of the temperature and charge of the vector field.

A holographic model of SQUID

Abstract: We construct a holographic model of superconducting quantum interference device (SQUID) in the Einstein-Maxwell-complex scalar theory with a negative cosmological constant. The SQUID ring consists of two Josephson junctions which sit on two sides of a compactified spatial direction of a Schwarzschild-AdS black brane. These two junctions interfere with each other and then result in a total current depending on the magnetic flux, which can be deduced from the phase differences of the two Josephson junctions. The relation between the total current and the magnetic flux is obtained numerically.

Generalized formalism in gauge-invariant gravitational perturbations

Abstract: By the use of the gauge-invariant variables proposed by Kodama and Ishibashi, we obtain the most general perturbation equations in the (m + n)-dimensional spacetime with a warped product metric. These equations do not depend on the spectral expansions of the Laplace-type operators on the n-dimensional Einstein manifold. These equations enable us to have a complete gauge-invariant perturbation theory and a well-defined spectral expansion for all modes, and the gauge invariance is kept for each mode. By studying perturbations of some projections of Weyl tensor in the case of m = 2, we define three Teukolsky-like gauge-invariant variables and obtain the perturbation equations of these variables by considering the perturbations of the Penrose wave equations in the (2 + n)-dimensional Einstein spectime. In particular, we find the relations between the Teukolsky-like gauge-invariant variables and the Kodama-Ishibashi gauge-invariant variables. These relations imply that the Kodama-Ishibashi gauge-invariant variables all come from the perturbations of the Weyl tensor of the spacetime.

Wilson Line Response of Holographic Superconductors in Gauss-Bonnet Gravity

Abstract: We study the Wilson line response in the holographic superconducting phase transitions in the Gauss-Bonnet gravity. In the black brane background case, the Little-Parks periodicity is independent of the Gauss-Bonnet parameter, while in the anti-de Sitter soliton case, there is no evidence for the Little-Parks periodicity. We further study the impact of the Gauss-Bonnet term on the holographic phase transitions quantitatively. For a given Wilson line, a bigger Gauss-Bonnet coefficient makes the phase transition easier for the black brane background while harder for the soliton background. For a given chemical potential, the response to the Wilson line is less sensitive for a bigger Gauss-Bonnet coefficient in the black brane case while the response is more sensitive in the soliton case. And more, a bigger Gauss-Bonnet coefficient makes the superconducting state more stable for both the black brane and the soliton backgrounds. DOI: 10.1103/PhysRevD.87.026002

Entanglement Entropy in Holographic P-Wave Superconductor/Insulator Model

Abstract: We continue our study of entanglement entropy in the holographic superconducting phase transitions. In this paper we consider the holographic p-wave superconductor/insulator model, where as the back reaction increases, the transition is changed from second order to first order. We find that unlike the s-wave case, there is no additional first order transition in the superconducting phase. We calculate the entanglement entropy for two strip geometries. One is parallel to the super current, and the other is orthogonal to the super current. In both cases, we find that the entanglement entropy monotonically increases with respect to the chemical potential.

Entanglement entropy in a holographic p-wave superconductor model

Abstract: In a recent paper, arXiv:1309.4877, a holographic p-wave model has been proposed in an Einstein-Maxwell-complex vector field theory with a negative cosmological constant. The model exhibits rich phase structure depending on the mass and the charge of the vector field. We investigate the behavior of the entanglement entropy of dual field theory in this model. When the above two model parameters change, we observe the second order, first order and zeroth order phase transitions from the behavior of the entanglement entropy at some intermediate temperatures. These imply that the entanglement entropy can indicate not only the occurrence of the phase transition, but also the order of the phase transition. The entanglement entropy is indeed a good probe to phase transition. Furthermore, the "retrograde condensation" which is a sub-dominated phase is also reflected on the entanglement entropy.

The Entropy of a Dielectric Black Hole

Abstract: In a dielectric black hole background, photons will be radiated via Hawking evaporation mechanism. In this paper, we calculate the entanglement entropy associated with a static dielectric black hole by employing 't Hooft's brick-wall model. It is found that the lowest energy of radiated particles is coordinate dependent. The resulted entanglement entropy is composed of three parts: a parameter independent leading constant term $\frac{1}{12}$, a logarithmic correction term and some series terms. The convergency of the series terms is also discussed.