Björn Lundqvist

Vortex Dynamics in a Thin Film of Tl2Ba2CuO6

The vortex dynamics in a Tl2Ba2CuO6(Tl-2201) thin film with a critical temperature Tc= 80.8 K has been studied by electrical resistivity measurements for magnetic fields 0≤B≤12 T. The vortex glass line, as determined from the disappearance of linear resistivity, was found to be well described by Bg(T) = B0[(1−T/Tc)/(T/Tc)]α, with B0≈ 0.65 T and α ≈ 1.9. The glass line of Tl-2201 is observed to be located below the one of Tl2Ba2CaCu2O8+δ(Tl-2212) in a magnetic phase diagram based on a reduced temperature scale T/Tc, thus suggesting a higher anisotropy for Tl-2201.

Vortex dynamics for in- and out-of-plane magnetic fields in oxygen deficient single crystals of YBa2Cu3O7−δ

The resistive transition into a glassy vortex state in oxygen deficient single crystals of YBa2Cu3O7-delta (YBCO) has been studied for magnetic fields 0 less than or equal to B less than or equal t ...

Density of states fluctuations in the magnetoresistivity of YBa2Cu3O7−δ

Abstract The effects of fluctuations in the density of states (DOS) have been studied. A regularisation method was used to calculate the otherwise divergent DOS term. It was found that the DOS term can explain an observed change of sign in the c-axis magnetoconductivity but that its effect on in-plane magnetoconductivity is less significant.

Vortex liquid-to-solid transition in underdoped YBa2Cu3O7−δ single crystals with B∥ab

Abstract A nearly magnetic field independent vortex liquid-to-solid transition has been observed in both the ab -plane and in the c -axis resistivity of underdoped YBa 2 Cu 3 O 7− δ single crystals for magnetic fields B ∥ ab -plane. These observations suggest that intrinsic pinning caused by the superconducting layers do neither lead to a system of completely decoupled superconducting layers nor to a smectic vortex phase at high fields. A possible explanation may include a transition driven by changes in elastic properties (e.g., the shear modulus) of the vortex system.

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].

Field independent vortex solid-to-liquid transition for B‖ab in oxygen deficient single crystals of YBa2Cu3O7−δ

Abstract Single crystals of YBa 2 Cu 3 O 7−δ provide a suitable test system for vortex dynamics in high temperature superconductors, since the electrical anisotropy γ = ( m c / m ab ) 1 2 can easily be varied over a large range by changing the oxygen content. In this paper the vortex solid-to-liquid transition, as determined from a vortex-glass analysis of the resistivity, has been studied for magnetic fields 0 B ≤12 T parallel with the ab plane of oxygen deficient single crystals of YBCO. Strikingly, for high magnetic fields a field independent vortex solid-to-liquid transition is observed. A possible explanation for this phenomenon involving a crossover to a two-dimensional (2D) system is suggested.

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).

Magnetoresistivity of untwinned single crystals of YBa2Cu3O7-δ

Abstract The magnetoconductivity of untwinned single crystals has been measured for in-plane current and various orientations of the magnetic field. Some results and preliminary analyses in terms of fluctuation theories are presented.

Vortex dynamics in oxygen deficient single crystals of YBa2Cu3O7−δ

Abstract In YBa 2 Cu 3 O 7−δ (YBCO), oxygen deficiencies in the CuO chains change the coupling between the charge carriers in adjacent superconducting CuO 2 planes and thereby the electrical anisotropy, γ = (m c /m ab ) 1/2 . Such crystals therefore provide a simple test system for systematic studies of the influence of anisotropy on vortex dynamics. We have studied the vortex glass transition of YBCO single crystals with δ in the range of 0.07 to 0.27, corresponding to γ between 9 and 18, for magnetic fields 0 c -axis. Fits of the glass transition lines to B ∞ (1-T/T c ) α show large deviations for a combination of high anisotropy and high magnetic fields.