分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-08
摘要： Within massive gravity, we construct a gravity dual for the insulator/metal phase transition and colossal magnetoresistance effect found in some manganese oxides materials. In the heavy graviton limit, a remarkable magnetic-field-sensitive DC resistivity peak appears at the Curie temperature, where an insulator/metal phase transition happens and the magnetoresistance is scaled with the square of field-induced magnetization. We find that metallic and insulating phases coexist below the Curie point and the relation with the electronic phase separation is discussed.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-08
摘要： A real antisymmetric tensor field was introduced to realize a holographic magnetic ordered phase in our previous papers. However, a more careful analysis shows there is a vector ghost in the model. In this paper we present a modified Lagrangian density for the antisymmetric tensor, which is ghost free and causality is well defined, and keeps all the significant results in the original model qualitatively. We show this modified Lagrangian density could come from the dimensional compactification of p-form field in string/M theory. For static curved space-time, we also prove that this modified model is ghost free and does not violate causality. This new model offers a solid foundation for the application of antisymmetric tensor field in holographic duality, especially for the spontaneous magnetization.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-08
摘要： By combining a holographic p-wave superconductor model and a holographic ferromagnetism model, we study the coexistence and competition of ferromagnetism and p-wave superconductivity. It is found that the results depend on the self-interaction of magnetic moment of the complex vector field and which phase appears first. In the case where the ferromagnetic phase appears first, if the interaction is attractive, the system shows the ferromagnetism and superconductivity can coexist at low temperatures. If the interaction is repulsive, the system will only be in a pure ferromagnetic state. In the case where the superconducting phase appears first, the attractive interaction will lead to a magnetic p-wave superconducting phase at low temperatures. If the interaction is repulsive, the system will be in a pure p-wave superconducting phase or a ferromagnetic phase when the temperature is lowered.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-08
摘要： In this paper we build a holographic model of paramagnetism/antiferromagnetism phase transition, which is realized by introducing two real antisymmetric tensor fields coupling to the background gauge field strength and interacting with each other in a dyonic black brane background. In the case without an external magnetic field and in low temperatures, the magnetic moments condense spontaneously in an antiparallel manner with the same magnitude and the time reversal symmetry is also broken spontaneously (if the boundary spatial dimension is more than 2, spatial rotational symmetry is broken spontaneously as well), which leads to an antiferromagnetic phase. In the case with the weak external magnetic field, the magnetic susceptibility density has a peak at the critical temperature and satisfies the Curie-Weiss law in the paramagnetic phase of antiferromagnetism. In the strong external magnetic field case, there is a critical magnetic field B-c in the antiferromagnetic phase: when the magnetic field reaches Bc, the system will return into the paramagnetic phase by a second order phase transition.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-08
摘要： Coupling an antisymmetric tensor field to the electromagnetic field in a dyonic Reissner-Nordstrom-anti-de Sitter black hole background, we build a holographic model for the paramagnetism/ferromagnetism phase transition. In the case of zero magnetic field, the time reversal symmetry is broken spontaneously and spontaneous magnetization happens at low temperatures. The critical exponents are in agreement with the ones from mean field theory. In the case of nonzero magnetic field, the model realizes the hysteresis loop of a single magnetic domain and the magnetic susceptibility satisfies the Curie-Weiss law.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-08
摘要： The gravitational collapse of a massless scalar field with a self-interaction term lambda phi(4) in anti-de Sitter space is investigated. We numerically investigate the effect of the self-interaction term on the critical amplitudes, forming time of apparent horizon, stable island, and energy transformation. The results show that a positive A suppresses the formation of black hole, while a negative A enhances the process. We define two susceptibilities to characterize the effect of the self-interaction on the black hole formation, and find that near the critical amplitude, there exists a universal scaling relation with the critical exponent alpha approximate to 0.74 for the time of black hole formation.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-08
摘要： A holographic description on the antiferromagnetic quantum phase transition (QPT) induced by the magnetic field and the criticality in the vicinity of the quantum critical point have been investigated numerically recently. In this paper, we show that the properties of QPT in this holographic model are governed by a CFT dual to the emergent AdS(2) in the IR region, which confirms that the dual boundary theory is a strong coupling theory with dynamic exponent z = 2 and logarithmic corrections appearing. We also compare them with the results from the Hertz model by solving the RG equation at its upper critical dimension and with some experimental data from pyrochlores Er2-2xY2xTi2O7 and BiCoPO5.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-08
摘要： We propose a gravity dual of antiferromagnetic quantum phase transition induced by magnetic field and study the critical behavior around the quantum critical point. It turns out that the boundary critical theory is a strong coupling theory with dynamic exponent z = 2 and that the hyperscaling law is violated and logarithmic corrections appear near the quantum critical point. Some novel scaling relations are predicated, which can be tested by experiment data in the future. We also make some comparison with experimental data on low-dimensional magnets BiCoPO5 and pyrochlores Er2-2xY2xTi2O7.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-08
摘要： In the last years it has been shown that some properties of strongly coupled superconductors can be potentially described by classical general relativity living in one higher dimension, which is known as holographic superconductors. This paper gives a quick and introductory overview of some holographic superconductor models with s-wave, p-wave and d-wave orders in the literature from point of view of bottom-up, and summarizes some basic properties of these holographic models in various regimes. The competition and coexistence of these superconductivity orders are also studied in these superconductor models.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-08
摘要： We study in detail the phase structure of a holographic p-wave superconductor model in a five dimensional Einstein-Maxwell-complex vector field theory with a negative cosmological constant. To construct complete phase diagrams of the model, we consider both the soliton and black hole backgrounds. In both two cases, there exist second order, first order and zeroth order phase transitions, and the so-called "retrograde condensation" also happens. In particular, in the soliton case with the mass of the vector field being beyond a certain critical value, we find a series of phase transitions happen such as "insulator/superconductor/insulator/superconductor", as the chemical potential continuously increases. We construct complete phase diagrams in terms of temperature and chemical potential and find some new phase boundaries.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-08
摘要： We study the P - V criticality and phase transition in the extended phase space of charged Gauss-Bonnet black holes in anti-de Sitter space, where the cosmological constant. appears as a dynamical pressure of the system and its conjugate quantity is the thermodynamic volume of the black holes. The black holes can have a Ricci flat (k - 0), spherical (k = 1), or hyperbolic (k = -1) horizon. We find that for the Ricci flat and hyperbolic Gauss-Bonnet black holes, no P - V criticality and phase transition appear, while for the black holes with a. spherical horizon, even when the charge of the black hole is absent, the P - V criticality and the small black hole/large black hole phase transition will appear, but it happens only in d = 5 dimensions; when the charge does not vanish, the P - V criticality and the small black hole/large phase transition always appear in d = 5 dimensions; in the case of d >= 6, to have the P - V criticality and the small black hole/large black hole phase transition, there exists an upper bound for the parameter b = (alpha) over tilde vertical bar Q vertical bar(-2/(d-3)), where (alpha) over tilde is the Gauss-Bonnet coefficient and Q is the charge of the black hole. We calculate the critical exponents at the critical point and find that for all cases, they are the same as those in the van der Waals liquid-gas system.
分类： 物理学 >> 基本粒子与场物理学 提交时间： 2016-05-08
摘要： In this paper we investigate in some detail the holographic ferromagnetic phase transition in an AdS(4) black brane background by introducing a massive 2-form field coupled to the Maxwell field strength in the bulk. In two probe limits, one is to neglect the back reaction of the 2-form field to the background geometry and to the Maxwell field, and the other to neglect the back reaction of both the Maxwell field and the 2-form field, we find that the spontaneous magnetization and the ferromagnetic phase transition always happen when the temperature gets low enough with similar critical behavior. We calculate the DC resistivity in a semi-analytical method in the second probe limit and find it behaves as the colossal magnetic resistance effect in some materials. In the case with the first probe limit, we obtain the off-shell free energy of the holographic model near the critical temperature and compare with the Ising-like model. We also study the back reaction effect and find that the phase transition is always second order. In addition, we find an analytical Reissner-Norstrom-like black brane solution in the Einstein-Maxwell-2-form field theory with a negative cosmological constant.
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