3DXYand Lowest Landau Level Fluctuations in DeoxygenatedYBa2Cu3

Abstract

Conductivity measurements reflect vortex solid melting in YBa 2 Cu 3 O 72d films. Field-independent glass exponents n g Ӎ 1.9 and z g Ӎ 4.0 describe the transition T g ͑H͒ for 0 , H # 26 T. At low fields, 3D XY exponents n XY Ӎ 0.63 and z XY Ӎ 1.25 are also observed, with z XY smaller than expected. These compete with glass scaling according to multicritical theory. A predicted power-law form of T g ͑H͒ is observed for 0.5T c , T g , T c . For T g , 0.5T c , 3D XY scaling fails, but a selfconsistent lowest Landau level analysis becomes possible, obtaining T c2 ͑H͒ with positive curvature.[S0031-9007(97)02912-8] PACS numbers: 74.25.Bt, 74.25.Dw, 74.40. + k, The nature of fluctuations near the superconducting to normal state transition in high-temperature superconductors (HTSCs) is still a matter of controversy. Several distinct fluctuation types and regions have been proposed, e.g., 3D XY fluctuations at low fields, lowest Landau level (LLL) fluctuations at high fields, and glasslike fluctuations (for disordered HTSCs) near the finite-field transition T g ͑H͒. However, experimental analyses based upon the different scaling theories lead to conflicting results. This situation is most evident for competing 3D XY and LLL fluctuations, both of which are supported experimentally, in the same region of the phase diagram, in spite of being incompatible [1].The 3D XY transition is driven by phase fluctuations of a complex order parameter (OP) which fall into the universality class of the l transition in 4 He. The zerofield, "intermediate" (nonelectrodynamic) phase fluctuations of the HTSCs are thought to be of this type [2]. At T T c (and H 0), these fluctuations diverge in size, driving the resistive phase transition. Recent experimental evidence supporting this picture is found in specific heat [3,4], magnetization [4,5], penetration depth [6], and current-voltage (J-E) measurements [4,7]. The finite-field transition T g ͑H͒, which is similarly driven by phase fluctuations of the OP, joins smoothly to T c ϵ T g ͑H 0͒. However, the glass and 3D XY fluctuations exhibit distinct scaling functions and exponents [2].Fluctuations of the OP amplitude occur near the upper critical (mean-field) temperature T c2 ͑H͒. These fluctuations drive the Cooper pair density to zero, but do not correspond to a true transition; superconducting order vanishes at the slightly lower temperature T g ͑H͒. In the lowfield region, the distinction between OP amplitude and phase fluctuations results in the dominance of 3D XY critical behavior near T T c . At high fields, this distinction is not present, yielding a different type of behavior, most conveniently described in terms of the Ginzburg-Landau LLL approximation, with its corresponding scaling theory [8,9]. Experimental evidence in support of this behavior is found in specific heat [10 -12], magnetization [10,12], and J-E characteristics [10,13]. A crossover is expected between the low-field (3D XY ) and high-field (LLL) behaviors, and its clarification is fundamental in the investigation of H...

Absence of three-dimensionalXYbehavior in the magnetic penetration depth ofYBa2Cu3O<

“…Studies of critical dynamics in applied fields include measurements of heat capacity, 11,12 magnetization, 13 and I-V characteristics. [14][15][16] All of these properties are found to be well described by scaling functions consistent with a 3D XY critical region several degrees wide. There is, however, no clear sense of the manner in which the width of the critical region varies as a function of field magnitude.…”

Critical scaling of the ac conductivity for a superconductor aboveTc

“…In three dimensions z = 3/2 for model F [30,36] which, although not observed in the a.c. conductivity data [18], is seen in some d.c. conductivity experiments [4,12,14] and simulations [37]. Another extension of the present theory is to consider a non-zero magnetic field, with the aim of examining the crossover from the zero-field critical scaling of the 3D XY-model to the lowest-Landau-level scaling which obtains in high fields [13,38].…”

“…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.…”

Dynamic critical exponent of two-, three-, and four-dimensionalXYmodels with relaxational and resistively shunted junction dynamics

“…37 However, from the scaling of the magnetoconductivity of a thick YBCO-123 film z = 1.25(5) was found in Ref. 36, whereas a similar experiment reported z ≈ 2 in Ref. 55.…”