Dissipative and Hall quantum creep inYBa2Cu3

Abstract

From dynamic relaxation measurements of the decay of supercurrents in YBa 2 Cu 3 O 7Ϫ␦ films with thicknesses between 1.2 and 150 nm we determine the correlation length L c ͑0͒ ͑the length of the tunneling vortex segment at Tϭ0 K͒ to be 2.2 nm and show that quantum creep occurs in a transition regime where Hall tunneling is as important as dissipative tunneling.Soon after the discovery of high-T c superconductivity Yeshurun and Malozemoff 1 reported on the existence of giant flux creep in high-temperature superconductors. Giant flux creep which has been found in all layered superconductors arises from the thermally activated motion of vortices from one metastable configuration to a neighboring one. The probability for such a hopping process is proportional to exp͓ϪU( j,T,B e )/kT͔, where U( j,T,B e ) is the activation energy which depends on the current j, external field B e ϭ 0 H e and temperature T. At low temperature U( j,T,B e )/kT diverges and the hopping probability vanishes. However, several experiments demonstrated that substantial relaxation was still present in the millikelvin regime. One of the most convincing proofs of the existence of this quantum creep was given by Fruchter et al.2 who found a temperature independent relaxation rate below 1 K in an YBa 2 Cu 3 O 7Ϫ␦ single crystal. Evidence for the existence of quantum creep has also been found in many other investigations on high-T c superconductors, 3-7 heavy fermions, 3 organic superconductors, 4 and Chevrel phases. 8In contrast to thermally activated flux motion which depends essentially on the height of the activation energy barrier, quantum creep depends on the time spent under the energy barrier during the tunneling of a vortex segment of length L c .9 Quantum creep is, therefore, inherently related to the dynamics of a vortex segment, which according to Kopnin et al. 10,11 is described by the following equation of motion:where v ជ v is the vortex velocity, ⌽ 0 the flux quantum, j s the current density, and ẑ a unit vector parallel to the vortex. The first term on the left-hand side of Eq. ͑1͒ represents the viscous drag and the second term is the Hall contribution. Until recently quantum creep experiments have always been interpreted by assuming that dissipative effects were dominant, i.e., ӷ␣. However, Feigel'man et al. 12 proposed recently that high-T c superconductors might be in the superclean limit and that quantum creep is essentially determined by the Hall term in Eq. ͑1͒. Their proposition was based on an estimate of n ͑0͒ by using a linear extrapolation of n (TуT c ) data which leads to a low-temperature resistivity n ͑0͒Ϸ10 ⍀ cm and a mean free path lϷ70 nm, which is indeed much larger than E F /⌬, where is the coherence length, E F the Fermi energy, and ⌬ the superconducting gap.The purpose of this paper is to show that at low temperatures the high-T c superconductor YBa 2 Cu 3 O 7 is ͑i͒ neither in the purely dissipative regime nor in the superclean limit, but in an intermediate regime where both Hall and dissipative terms contr...

Thermally assisted quantum vortex tunneling in the Hall and dissipative regime

“…In the experiment of Ref. [8], the relaxation rate is Q(0)/Q 0 ∼ 2.3 (2.0) in the YBCO (BiSCCO) system.…”

“…( 14) and ( 15), the Hall coefficient α is reduced from its pure value πhn s due to the dissipative effect. Although Ao et al suggested that the Hall coefficient is originated from the topological property and thus not renomalized, [21] it seems that at least some aspects of the experimental behavior [8] can be understood on the basis of the renomalization of the Hall coefficient. Therefore, it is meaningful to take α and η to be two parameters determined by the magnitude of ω 0 τ r .…”

“…Hence, the dynamical magnetization relaxation rate Q defined as k B T /U is expected to vanish at T = 0. However, many experiments [5][6][7][8][9][10] have demonstrated that the relaxation rate does not disappear at sufficiently low temperatures, which leads to the existence of quantum tunneling of vortices trapped in the pinning potential.…”

“…Many experimental results have been interpreted within the two frameworks. Recently, however, van Dalen et al [8] observed experimentally that the vortex tunneling in HTSC may occur in the intermediate regime between the purely dissipative tunneling and the superclean Hall tunneling. Feigel'man et al [12] and Morais Smith et al [13] studied the problem in the two regimes, but they only obtained the qualitative results based on the scaling analysis of the action.…”

Magnetic Relaxation of Superconducting Quantum Dot and Tunneling of Electron in a Magnetic Field

“…Feigel'man et al 23 proposed that the Magnus force is dominant in clean superconductors, while other authors 24,25,26 argued that the vortex tunneling may occur in an intermediate regime. As long as we consider the HTSCs, the dissipative term must not be crucial because of small coherence lengths (as well as vortex cores).…”