XY-like critical behavior of the thermodynamic and transport properties ofYBa2Cu3

Abstract

The heat capacity, magnetization, and electrical conductivity of single-crystal samples of YBa2Cu307 "have been measured and are shown to support the existence of an intermediate critical regime in the vicinity of T"governed by the XY-like critical exponent v= -.Clear evidence is found for

Critical dynamics, duality, and the exact dynamic exponent in extreme type-II superconductors

“…4 Model A dynamics is purely relaxational and gives the value z ≈ 2.015 for the dynamic critical exponent in three dimensions. Model F, on the other hand, features a conserved density coupled to a spin-wave mode and gives the exact value z = d/2 for d ∈ (2,4]. For the dynamical universality class of extreme type II superconductors, Monte Carlo simulations give z ≈ 3/2, 5,6 which would be consistent with model F dynamics and therefore with superfluid 4 He dynamical universality class.…”

Upper critical field and fluctuation conductivity in the critical regime of doped SmFeAsO

“…This nonanalyticy is removed if interactions between fluctuations are taken into consideration, which is necessary in the so called critical regime, that is in a region of the T c -H phase diagram close to T c (H). The temperature range in which critical fluctuations should be observed is proportional to ξ 0 -6 in the 3D case, that is an experimentally accessible range of the order of 1K for high-T c superconductors 18 . In a magnetic field this range is expected to increase further as ∼H 2/3 or ∼H 1/2 in 3D and 2D, respectively 19 .…”

Critical scaling of the ac conductivity for a superconductor aboveTc

“…It is natural then to ask: If scaling and universality exist in the critical region, to which universality class does the transition belong? From observations of the effects of critical superconducting fluctuations on thermodynamic properties, such as the penetration depth [1,2], magnetic susceptibility [3][4][5], specific heat [3,6] and thermal expansivity [7] a consensus is emerging that the zero-field normalsuperconducting transition is in the static universality class of the three-dimensional, complex order-parameter (3D XY) model. In contrast, the effect of critical fluctuations on transport properties, such as the conductivity, depends on the nature of the dynamics near T c and is much less explored.…”

“…In these theories the dynamic exponent z associated with the growth of the characteristic orderparameter time-scale near T c appears in the conductivity and takes the value z = 2. By examining the deviation of z from 2 inside the critical region through linear d.c. [3,4,[11][12][13][14], non-linear d.c. [15][16][17] and linear a.c. [18] conductivity measurements, the dynamic universality class can, in principle, be determined. Currently, however, there is much variation in the measured values for z and the dynamic universality class of the zerofield normal-superconducting transition remains uncertain.…”