First and second order quantum phase transitions in multi-band superconductors

In multi-band and inter-metallic materials superconductivity can be destroyed by applying external pressure in these systems. In many cases the critical temperature is driven continuously to zero, the superconducting to normal transition being associated with a superconducting quantum critical point (SQCP). In this paper we propose a model for this type of SQCP based on the increase of hybridization as pressure is applied in the material. We study a two-band superconductor with hybridization V between these bands. We use a BCS approximation and include both inter- and intra-band attractive interactions. We show that for negligible inter-band interactions, as hybridization increases there is a second order phase transition from a superconductor to a normal state at zero temperature at a critical value of the hybridization V_c. This SQCP can be reached by pressure, since this external parameter controls hybridization in the system. We also find discontinuous transitions at zero temperature and the appearance of a gapless superconducting (GS) phase in a certain range of hybridization in the case of inter-band interactions being dominant.     

A model ferromagnet with first and second order phase transitions

A model ferromagnet with pair and four spin interaction strengths in the ratio x is shown to have classical behaviour for x > 3, and discontinuous magnetization for x ? 3, the point x = 3 corresponding to a tricritical point.     

Hysteresis loop signatures of phase transitions in a mean-field model of disordered Ising magnet

The multiplicity of long-lived states in frustrated disordered magnets makes the task to experimentally deter-mine which of them has the lowest free energy (and thus what thermodynamic phase the sample is in) seem rather hopeless. Nevertheless here we show in the framework of Landau-type phenomenological model that signatures of the mean-field equilibrium phase transitions in such highly nonequilibrium systems may be found in the evolution of the hysteresis loop form. Thus the sequence of transitions from spin-glass to mixed phase and to ferromagnetic one results in the changes from inclined hysteresis loop to that with the developing vertical sides and to one with the perfectly vertical sides. Such relation between loop form and the location of global minimum may hold beyond the mean-field approximation and can be useful in the real experiments and Monte-Carlo simulations of the problems involving rugged potential landscape.     

Efficiency of the microcanonical over-relaxation algorithm for vector spins analyzing first and second order transitions

We simulate vectorial spin systems solely with the microcanonical over-relaxation algorithm where the temperature is calculated by a formula of Nurdin and Schotte. We show that this procedure is the most efficient local algorithm besides the nonlocal cluster algorithm not only for first order transitions but also for second order ones. A comparison is made with the Metropolis, heat bath, multicanonical and the Creutzs demon algorithms. We study, using these algorithms, the frustrated J1-J2 model on a cubic lattice for XY, Heisenberg and O(4) spins. These models have a breakdown of symmetry Z3 SO(N)/SO(N-1) for the number N = 2,3,4 of spin components leading to transitions of first order. We show that they are strongly first order. Then, to test the over-relaxation update for second order transitions, we study a ferromagnet on a cubic lattice and a frustrated antiferromagnet on a stacked triangular lattice. We finally point out the advantages and the flaws of the over-relaxation procedure.     

Application of the generalized Landau ansatz to second order phase transitions

The ansatz ofAmit et al. with expansion coefficients which can be nonanalytic functions of either the temperature or the order parameter is used to predict the generalized susceptibility (compressibility) and the surface tension belowT _c and the critical isotherm atT _c from the specific heat, the order parameter at the coexistence curve, and the coherence length. The results agree within 50% with experiments on different phase transitions in ferromagnets, classical gases, quantum gases, binary mixtures, and superfluid helium. The ansatz can be written in two different versions; their results differ only by logarithmic factors.     

Calculation of the specific heat for the first order,tricritical and second order phase transitions in NH4C1

Abstract

This work presents our calculation for the specific heat C _v under an Ising model that uses our Raman frequencies of the _1/7TA (93cm ¹) and _1/5TO (144cm^?1) modes for NH₄C1. The specific heat calculation has been performed for first-order (P = 0kbar), tricritical (P=1.6kbar) and second-order (P = 2.8kbar) phase transitions in the NH₄C1 crystal. Our calculated C _v values are in good agreement with the experimentally observed Cp data from the literature for NH₄C1. This indicates that the NH₄C1 crystal can be adequately described in the lattice region by means of the Ising model studied here.     

Intersubband transitions from the second subband in a coupled quantum wells structure

Asymmetrical coupled quantum wells structures with energy separation between the first two subbands of the order of 10–50 meV are key structures in the design of optically pumped intersubband lasers. In these structures the population of the second subband is not negligible and intersubband transitions from the second to higher excited subbands can be observed. In this work we investigate the temperature dependency of the intersubband transitions from the second subband in an asymmetrical coupled quantum wells structure. We show that this approach provides a direct way to measure the energy separation between the second subband and the Fermi energy which is a crucial parameter in the design of optically pumped intersubband lasers.     

Absence of first order electronic transitions in liquid metals

High pressure shock wave data on a wide variety of metals indicates that electronic transitions are continuously distributed in the liquid phase and accompanied by melting maxima. A qualitative explanation for this behavior is suggested.     

Hysteresis in vortex glass transitions of the first order

The distribution of relaxation times in a glass is very broad. As a result the free energy of a glass slowly approaches its equilibrium value but never reaches it. This slow relaxation explains why, in the hysteresis cycle, the melting tempetature is higher than the solidification temperature.

We estimate the width of this hysteresis for a general type of transition into the glassy state. We apply these estimates to the special case of the vortex melting transition and find good agreement with recent experiments. We show how strong disorder changes the character of the vortex melting transition and evaluate the position of the tricritical point.     

Disclinations and first order transitions in 2D melting

We present a unified analysis of two-dimensional defect melting involving both dislocations and disclinations. This permits the study of the feedback between the two types of defects and explains the first order of the melting transition.     

Thermal properties of He II according to a Landau-theory of phase transitions of second order

A special type of Landau theory of second order phase transitions allows the incorporation of spatial fluctuations of the order parameter in a consistent way. The resulting theory describes the thermal properties of He II nearT _λ in agreement with experiment.     

The ferromagnetic transition of iron according to a Landau-theory of phase transitions of second order

The properties of iron at its ferromagnetic transition point are described by a type of Landau-theory developed for He II. The theory includes spatial fluctuations of the spontaneous magnetisation which lead to a logarithmic singularity of the specific heat nearT _c in agreement with measurements byKraftmakher andRomashina.     

NANOGrav results and dark first order phase transitions

The recent NANOGrav evidence of a common-source stochastic background provides a hint to gravitational waves(GW) radiation from the Early Universe. We show that this result can be interpreted as a GW spectrum produced from first order phase transitions(FOPTs) around a temperature in the keV-MeV window. Such a class of FOPTs at temperatures much below the electroweak scale can be naturally envisaged in several warm dark matter models such as Majoron dark matter.     

Continuous phase transitions which should be first order

It is demonstrated that phase transitions predicted to be first order by the Landau symmetry arguments can be continuous for a finite range of the physical parameters. For transitions with a rank-two tensor order parameter it is shown that fluctuations in a phase associated with the anisotropy of the tensor result in a renormalization of the cubic term. This leads to continuous transitions when the coefficient of the bare cubic term is small compared to the coefficient of the quartic term. It is shown that the argument can be extended to more general cases when the order parameter belongs to a multidimensional representation of the broken symmetry group.     

From the first to the second quantized string theory

Starting from the hamiltonian formulation of the bosonic string we calculate the Feynman transition amplitude D. Inverting D we get the kinetic operator and thus the free field action.