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Journal of Physics A Mathematical and General

Static and dynamic critical behaviour of displacive phase transition at Tcin 1/n expansion

Arno Holz, J.T.N. Medeiros

Published Jul 1, 1975• 5 citations
Publisher

Abstract

Quantum effects on the static critical behaviour of an isotropic n-vector model undergoing a displacement-type phase transition are studied at T c and to O(1/n). For T c not=0 there is no quantum effect on eta , and for T c =0 one gets eta =0 for d>3, and for 2<d<3 eta is due to quantum fluctuations, implying that it assumes a different value than in the classical case. The dynamical behaviour of the system violates the dynamic scaling hypothesis.

DOI
10.1088/0305-4470/8/7/014

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Quantum effects on the static critical behaviour of an isotropic n-vector model undergoing a displacement-type phase transition are studied at T c and to O(1/n). For T c not=0 there is no quantum effect on eta , and for T c =0 one gets eta =0 for d>3, and for 2<d<3 eta is due to quantum fluctuations, implying that it assumes a different value than in the classical case. The dynamical behaviour of the system violates the dynamic scaling hypothesis.
The renormalization group approach to critical phenomena is developed for quantum mechanical problems with non-commuting operators. Applying the theory to the spin 1 / 2 Ising model with a transverse field Gamma it is found that the critical exponents are those of the Ising model with Gamma =0 if the transition occurs at T>0. However, for transitions at T=0 the critical behaviour of the d-dimensional transverse system corresponds to that of the (d+1)-dimensional Ising model with Gamma =0, in agreement with series expansion predictions. At T=0 the dynamic, as well as static, critical behaviour is given by mean field theory for d>3.
We study Bose-Einstein condensates at the low-energy limit and show that their collective dynamics exhibit interesting quantum dynamical behavior. The system undergoes a dynamical quantum phase transition after a sudden quench into a properly distributed static force, provided its dispersion relation is linear. We corroborate the occurrence of a dynamical phase transition by calculating the Fisher zeros of the Loschmidt amplitude and showing that they cross the real time axis in the thermodynamic limit. A connection is established between the order of nonanalycity in the return rate function and the spectral density of the force. Furthermore, it is shown that a logarithmic or power-law scaling holds at the critical times depending on the displacement spectrum. The scaling behaviors are studied for three different cases. Eventually, a scheme for the quantum simulation of such a dynamical phase transition and its verification is proposed. We show that by taking into account the environmental effects the behavior remains observable.
The Landau-Ginsburg free-energy functional (effective Hamiltonian) is derived for compressible Ising systems where the Ising spins are coupled with quantum phonon fields and a transverse field is present. In the zero transverse field case, the phonon dynamical modes also disappear from the effective Hamiltonian, which then compares with those studied by Sak and De Moura et al. in the non-quantum compressible Ising systems. In the presence of dynamics in the spin system (compressible transverse Ising system), the phonon modes get effectively coupled in the Hamiltonian and while at non-zero temperatures this does not affect the critical behaviour (which remains the same as in the non-quantum case), the presence of these dynamical modes effectively increases the lattice dimensionality by unity at zero-temperature transitions (T c =0) where one observes the critical behaviour corresponding to a d+1 dimensional non-quantum compressible Ising model. The critical behaviour of a cooperative Jahn-Teller phase transition when T c to 0 is also discussed.
Quantum Phase Transitions Quantum phase transitions, in contrast to temperature-driven critical phenomena, occur due to a competition between different parameters describing the basic interactions of the system. Their specific feature is the quantum character of the critical fluctuations. This implies, through the uncertainty principle, that energy fluctuations and time are coupled. Then at zero temperature time plays a crucial and fundamental role, the static properties being entangled with the dynamics (Continentino, 1994a; Sondhi et al ., 1997; Sachdev, 1999). In this book we are mainly interested in quantum phase transitions that occur in electronic many-body systems and how scaling concepts can be useful to understand their properties close to these transitions (Continentino, 1994a). Even though a similar approach has been used in the case of interacting bosons (Fisher et al ., 1989) the fermionic problem has its own idiosyncrasies and difficulties. For example, there is no natural order parameter associated with the localisation transition in the electronic case, while bosons at zero temperature are either localised or superfluid so that the superfluid order parameter can be used to distinguish between both phases. If the results of the study of quantum phase transitions were restricted to zero temperature, this would be an interesting but purely academic area of research. What is really exciting about this subject is the effect of quantum critical points (QCP) in the finite temperature phase diagram of actual physical systems, even far away from the QCP (Freitas, 2015). As we will show, there is a special line in this phase diagram, the quantum critical trajectory , where the temperature dependence of the thermodynamic and transport properties is governed by the quantum critical exponents, i.e. those associated with the QCP. This and the observed crossover effects induced by temperature in the non-critical side of the phase diagram of a material with a QCP are sufficient to make the study of quantum phase transitions an inevitable subject. We start this chapter by introducing the scaling theory of quantum critical phenomena, since scaling concepts are used throughout this book.
We study dynamic properties of the critical phenomena of the two-dimensional kinetic Ising model above and below the critical temperature Tc. In particular, we study the critical slowing-down (T > Tc) and the symmetry breaking (T ≪ Tc) through the temperature- and size-dependences of relaxation time of magnetization and we compare the dependences with those of static quantities such as the susceptibility. We find intrinsic dynamical critical behaviour on both sides of Tc with a critical region as wide as the one in the static critical behaviour. We also discuss the dynamic properties as an eigenvalue problem of the time-evolution operator (the Liouville operator) and thier scaling properties.
We show that quantum fluctuations display a singularity at thermal critical points, involving the dynamical z exponent. Quantum fluctuations, captured by the quantum variance [Frérot et al., Phys. Rev. B 94, 075121 (2016)PRBMDO2469-995010.1103/PhysRevB.94.075121], can be expressed via purely static quantities; this in turn allows us to extract the z exponent related to the intrinsic Hamiltonian dynamics via equilibrium unbiased numerical calculations, without invoking any effective classical model for the critical dynamics. These findings illustrate that, unlike classical systems, in quantum systems static and dynamic properties remain inextricably linked even at finite-temperature transitions, provided that one focuses on static quantities that do not bear any classical analog-namely, on quantum fluctuations.