Yuri Eltsev

First-order melting transition observed from resistivity measurements in ultra-pure YBa2Cu3O7−δ single crystals with high twin boundary density

R(T) measurements have been performed on optimally and overdoped heavily twinned high-purity YBa2Cu3O7-delta single crystals, under a magnetic field B oriented parallel to the twin boundary planes (B parallel to c). The characteristic feature attributed to the flux line lattice melting transition has been observed in the overdoped case for B greater than or equal to 5 T. The first-order transition was not found in the optimally doped sample, even when B was tilted with respect to the c-axis. Differences between these samples are discussed.

Density of states fluctuations in the c-axis magnetoconductivity of YBa2Cu3O7−δ

The magnetoresistivity up to 12 T of YBa2Cu3O7−δ has been measured with B//I//c fromTc+3 K toTc+40 K. A change of sign was observed, from positive nearTc to negative at temperatures above approximatelyTc+10 K. This previously not reported change of sign can be explained by recent fluctuation theories that include fluctuations in the normal density of states (DOS).

Resistivity Studies by Multiterminal Transport Measurements on Single Crystal YBa2Cu3O7−δ

Multiterminal transport studies provide a powerful technique to investigate vortex structure and correlations in the vicinity of the vortex solid-to-liquid transition in single crystals of high temperature superconductors. Recent results are described, including new contact configurations, allowing both longitudinal and transverse vortex correlations to be studied in one crystal during a single temperature sweep at constant field.

Studies of dynamic critical behaviour in YBa2Cu3O7−δ single crystals

Up to now, the value of the dynamic critical exponent z is still an open question. We have studied this exponent in the intermediate (XY) critical regime in zero field for a set of high quality YBa2Cu3O7-delta Single crystals in terms of Fisher-Fisher-Huse (FFH) dynamic scaling, and using previously obtained values for the critical exponents cu of the de critical conductivity fluctuations. In this regime (i.e. the absence of Coulomb interactions), one would expect model-E dynamics (z = 1.5) to apply with the conserved charge-density conjugate to the phase fluctuations. However, we find the dynamic critical exponent to be z = 2, which is in agreement with the suggestion of FFH that due to plasma fluctuations, fully relaxational dynamics, model A (z = 2), should be appropriate.

Vortex liquid properties in optimally doped and oxygen-deficient YBa2Cu3O7−δ single crystals

Abstract The electrical anisotropy, γ =( m c / m ab ) 1/2 , can easily be varied by changing the oxygen content in single crystals of YBa 2 Cu 3 O 7− δ (YBCO), thus making this system useful as a test system for vortex dynamics in high-temperature superconductors (HTSC). Here, some recent results are reviewed. Measurements of the c -axis resistivity with the magnetic field B ∥ ab -plane is shown to be a powerful tool in studying vortex velocity correlations both parallel and perpendicular to the applied field. In optimally doped crystals, we find evidence that the in-plane vortex lattice melts at T m into a disentangled vortex liquid, which looses its correlation along the c -axis at a higher temperature T br > T m . For weakly disordered crystals and magnetic fields B ∥ c -axis, a relation between the vortex glass line and the magnetic field dependence of the characteristic energy, U *, in the pinned vortex liquid is proposed. This approach is shown to give support to our idea of describing the vortex glass resistivity in the liquid regime by a modified glass correlation length, ξ g = ξ 0 | k B T / U 0 −1| − ν , which depends on the average pinning energy, U 0 , in the system [A. Rydh, O. Rapp, M. Andersson, Phys. Rev. Lett. 83 (1999) 1850].

In-plane anisotropy and chain contribution from the magnetoresistivity of YBa2Cu3O7−δ

Abstract Magnetoresistivity measurements were made in untwinned YBa 2 Cu 3 O 7−δ above T c for B ∥ c and B ∥ ab , with I a or b . A fluctuation theory that allows for an in-plane coherence length anisotropy (γ =ζ b ζ a was used to determine γ. The contribution to the magnetoconductivity from the CuO-chains was also analyzed.

Different estimates of the anisotropy from resistive measurements in high-Tc superconductors

The anisotropy is one key parameter in understanding the physics of high temperature superconductors (HTSC). In addition to quantifying the variation of physical properties in different crystal directions, it contributes to strongly enhanced thermal fluctuations, and thereby to the rich variety of new phenomena observed in HTSC. Some examples are the existence of a mixed state solid-to-liquid phase transition (which depending on the amount of disorder can be a melting transition or a glass transition), the sometimes extreme broadening of the resistive transition in magnetic fields, and the strong decrease of the critical current, even far below T c for increasing temperature or magnetic field (see e.g. Cohen and Jensen, 1997).

First Order-Like Out-of-Plane Resistive Transition in YBa2Cu3O7−δ for B Exactly Parallel to ab-Planes

Melting transition of a flux line lattice//ab-planes has been studied in twinned YBa2Cu3O7−δsingle crystals using transport measurements along thec-axis. In contrast to previously studied geometry with field and current in planes (W.K. Kwok et al., Phys. Rev. Lett.72, 1088 (1994)), a step-likec-axis resistive transition has been found for exact field alignment alongab-planes. The angular dependence of ρcwith rotation of the crystal around thec-axis was studied. The results show that below the resistive step, the pinning force is anomalously increased when the field is aligned alongac(bc)-crystal facets.

Determination of In-Plane Anisotropy from Magnetoresistivity Measurements on YBa2Cu3O7−δ

AbstractThe magnetoresistance of six untwinned YBa2Cu3O7−δcrystals has been measured above TcforI⊥B‖c. Fluctuation theories allowing for an in-plane coherence length anisotropy (ξa≠ ξb) were used in the analyses. The results suggest that the anisotropy, γ = ξb/ξa, is considerably lower than that expected from the resistivity ratio, i.e.,