Anisotropic dynamical scaling near the vortex-glass transition of twinnedYBa2Cu3

Abstract

The complex electrical conductivity o (u) is determined from the linear ac susceptibility between 3 Hz and 3 MHz measured in fields between 0.4 and 12T parallel and perpendicular to the c axis of a twinned crystal. Dynamical scaling of ] o [ and o "/o' reveals ordering temperatures Ts(B) and field-independent scaling functions by using the variable (T,z Ts)/ -[T Ts[. F-or B J c, the critical exponents v and z agree with values obtained on films indicating isotropic vortex-glass fluctuations, whereas those for B [[ c indicate elongated fluctuations in the c direction mediated by correlated pinning via the edges of twin boundaries. PACS numbers: 74.25.Nf, 74.60.ae, 74.72.Bk Based on early indications and suggestions for a glassy vortex state in a granular high-T, superconductor [1], Fisher et aL [2,3] argued that in the absence of a driving current j this state represents a thermodynamic equilibrium phase emerging from a vortex liquid below some transition line Ts(B). In clear distinction to the predictions of the flux creep [4] and thermally activated flux flow [5] models, which yield genuine superconductivity,i.e. , zero resistance p(j -+ 0), only at zero ternperature, the vortex glass (VG) is characterized by p = 0 for all temperatures below T~(B). Convincing evidence for the existence of this continuous phase transition has been provided by a number of scaling analyses of nonlinear voltage-current isotherms for YBazCus07 s (YBCO) [6 -14]. The principal scaling variable of the VG transition is the glass correlation length 8 ] T -Ts [ " describing the diverging size of glassy islands and of lakes of vortex fluid above and below Tg(B), respectively, and also the increasing lifetime of these fluctuations r P.The present physical interest in the VG transition is focused on the not yet well understood effect of the possible types of disorder. The original "isotropic" VG model of Fisher et aL [2,3] considered weak pinning to point defects which, due to charge doping, is intrinsic for all high-T, materials. According to subsequent mean-field (MF) work by Dorsey et al. [15], (i) the VG transition may arise from a random spatial distribution of T, and (ii) critical fluctuations lead to real exponents larger than the MF limits,~F = 1/2 and zMF = 4 . Fisher et aL [3] also indicated the possibility of anisotropic vortex fluctuations, characterized by a superdivergence of the correlation length parallel to B, E, Et/&, with 0.5 ( t, ' ( 1.More recently, Nelson and Vinokur [16] considered the case of columnar pinning, either implemented artificially or originating from the mosaic of twin planes intrinsically present in bulk YBCO. Based on the Bose glass (BG) model and ignoring point defects they predicted a transition at TBG to the vortex glass, characterized by ( = TABLE

Percolative vortex motion in high-temperature superconductors

“…The ratio of the critical exponents, t/sϭ2.0Ϯ0.4, observed experimentally in YBa 2 Cu 3 O 7Ϫ␦ single crystals 8,9 and ceramics 38 is consistent with the critical exponents for bond percolation on a 3D lattice, 24 t/sϭ2.60Ϯ0.14. This is particularly surprising since the percolating vortices in superconductors are line objects, whereas point particles are assumed in the simulations of bond percolation models.…”

“…16 A semiquantitative agreement was found for ac susceptibility measurements on Bi 2 Sr 2 CaCu 2 O 8 single crystals 17 and for vibrating reed measurements on YBa 2 Cu 3 O 7Ϫ␦ single crystals and films. 18 Reed et al 8 and Kötzler et al 9,10 showed that the extended TAFF model was in quantitative agreement with the frequency-dependent ac susceptibility of Y-1:2:3 single crystals and films near T c , whereas the ac susceptibility at lower temperatures was in agreement with the existence of critical fluctuations around a vortex glass transition temperature T g .…”

“…How- ever, the longitudinal correlation length of vortices in hightemperature superconductors is quite small 47 and therefore the treatment of individual vortex segments as point particles might be justified. The agreement between the calculated and measured ratio of the critical exponents t/s in Y-1:2:3 twinned single crystals 8,9 and ceramics 38 indicates the presence of two pinning mechanisms with significantly different activation energies. The different pinning mechanisms may be related to pinning by point defects and to pinning by extended defects.…”

“…3,4 Near the transition between the glassy and the liquid vortex phases the electronic conductivity obeys scaling laws 1,2 and far above the transition the motion of the vortex liquid is diffusive. [5][6][7] The existence of a scaling regime for the linear, frequency-dependent conductivity [8][9][10] ( ) and the nonlinear dc conductivity 11,12 ( j) is firmly established. Though these experimental findings in transport quantities support the existence of a vortex glass transition, measurements of thermodynamic variables have not shown an indication of a phase transition.…”

Surface superconductivity in niobium for superconducting RF cavities

“…For a short cylinder with heigth h = 2a the radius must be replaced by a/ √ 2 2 . The complex conductivity σ(ω) is obtained from the measured susceptibility χ(ω) by means of an inversion routine [24]. The average Ohmic resistivity at 10 Hz and T > T c is found to be ρ n = (0.5±0.1) nΩm, confirming the purity of the samples and the specification of the manufacturers of RRR ≃ 300.…”