D. Reefman

Numerical simulations on two dimensional vortex lattices

Abstract We have studied the dynamics of a two-dimensional vortex lattice with the use of a molecular dynamics method for systems of up to 2000 vortices. The interaction potential is taken to be of the Pearl form and temperature is taken into account via a white Gaussian noise source. We find that the low temperature state is predominantly a triangular array of two-dimensional vortices. Long range order is absent. Defects are present at very low temperatures already, and possibly cause at higher temperatures Kosterlitz-Thouless (dislocation mediated) melting of the lattice. With a London penetration depth of 1250 A, we calculate the field distribution at zero temperature and the T -dependent magnetic field time-correlation function. The results compare well with NMR experiments on Tl in Tl 2 Ba 2 CaCu 2 O 8 in the temperature range below the melting temperature T m .

Single crystal 205Tl NMR in the normal and superconducting state of the high-Tc superconductor Tl2Ba2CaCu2O8: A study of the Knight and chemical shift

Abstract We have performed 205 Tl NMR in c -axis aligned single crystals of Tl 2 Ba 2 CaCu 2 O 8 ( T c ≈101 K ) in a field of 4.7 T. There are two Tl-lines with, at room temperature, distinct intensities, chemical and Knight shifts. One line arises from the normal (N) Tl-sites in the Tl-double layers and mimics the Cu-nuclear spin. The other one arises from impurity (I) Tls replacing part of the Ca-sites within the Cu double layer, i.e. the same locations as for the Y-ions in YBa 2 Cu 3 O 7 . Because it appeared possible to follow the dependence of the shifts of the two lines on temperature ( T ) down to T / T c ≤0.1, (for H ∥ c and H ∥ a, b ) the chemical and the Knight shifts could be determined accurately. The T -dependences of the Knight shift of the N(I)-line in the present sample with about 10% Ca-replacement are analogous to those of Cu(Y) in YBa 2 Cu 3 O 6.6 : the shift starts to decrease already above T c without an abrupt change at T c . Only below T / T c ≈0.6 do the resonance lines start to broaden.

Specific heat of the local-pair superconductor

Abstract The temperature dependence of the specific heat C ( T ) of the local-pair (LP) superconductor is calculated in the mean-field-random-phase-approximation, using the mapping of the LP model at constant carrier density onto the anisotropic Heisenberg pseudo-spin system at constant magnetization. Both the 3D and quasi 2D (highly anisotropic) LP superconductors are investigated, both for a small carrier concentration n . Below T c , C ( T ) obeys power laws with different exponents in the 3D and quasi 2D cases, and rises steeply as T approaches T c . Above T c the model predicts a linear T -dependence for C ( T ) for the quasi 2D case in a substantial temperature range before C ( T ) reaches its classical value ≈ n . Such a contribution to C ( T ) above T c has been reported in some experiments. The model may also account for the small values for entropy at T c found in the experiments in the superconducting oxides. For temperatures much bigger than the LP-band width, we find C ( T )∼ T −2 in both cases, in agreement with ref. [5] (R. Micnas et al., Rev. Mod. Phys. 62 (1990) 113).

Langevin dynamics simulation of vortices in a layered superconductor

Abstract We study the dynamics of the vortex lattice with a Langevin dynamics method in a layered superconductor, for systems of up to 4180 vortices, subject to periodic boundary conditions. The interaction between the pancake vortices in each layer is of magnetic origin, and the accompanying long-range interaction is dealt with exactly. Temperature is taken into account by means of a gaussian noise source. Depending on the value of the magnetic field (applied perpendicular to the layers) we observe layer decoupling prior to lattice melting. We calculate the temperature dependence of the positional and angular correlation functions, and several time dependent correlation functions. The obtained results are compared to simulations for a single layer. Some effects of pinning are addressed briefly. In the simulations, we also obtain the NMR spin-relaxation rates due to thermal motion of the vortices. The relevance of this relaxation mechanism for existing systems is discussed, and the effects of weak pinning on the melting are addressed briefly.

Electronic structure and properties of the superconductor Pb2SrxLa2−xCu2O6+δ

The electronic structure of the cuprate superconductor Pb2SrLaCu2O6 has been calculated within the local density approximation with the augmented spherical wave method. The results feature a band crossing the Fermi level, being mostly of copper oxide character, with a slight admixture of lead oxide. The admixture of Pb character in the conduction band is in agreement with NMR measurements.

Monte Carlo study of the local pair superconductor

Abstract The local Pair (LP) superconductor is investigated by means of a Monte Carlo (MC) method. The ground state phase-diagram in the density versus repulsion/transfer integral plane is calculated, and compared to analytical (RPA) results. The phase diagram features the superconducting phase, a charge-ordered phase and a region where superconductivity and charge ordering coexist. Also, the finite temperature phase diagram and the temperature dependences of the specific heat and the superconducting order parameter are presented.

Tunneling characteristics of the local-pair superconductor

Abstract Although local-pair superconductivity is in principle gapless, we show that the inclusion of a very thin non-superconducting layer at the surface of the local-pair (LP) superconductor is sufficient to produce a (pseudeo) gap in the tunneling spectrum, which makes the tunneling characteristics similar to that expected for conventional BCS superconductor-to-metal tunneling junctions. The origin of the pseudo-gap is completely different, however. It arises as soon as the local pairs Bose-condense below Tc in the bulk LP-superconductor, since then the depairing of these local pairs in the surface layer has to involve (+k, -k) fermions. Accordingly the temperature dependence of the gap is quite different from that predicted from BCS theory.

Local-pair approach to high-Tc superconductivity

Some results for the ordering specific heats of the local pair superconductor obtained by a Monte Carlo method are presented. The λ-type anomaly agrees well with experiment. Furthermore, an explanation of the observed anomalous temperature dependences of the NMR Knight shift and T1 of the superconducting cuprates is given on basis of a model of spinless holes, moving in an antiferromagnetic background of localized copper spins.

Monte Carlo study of the NMR properties of the quasi-2D Heisenberg antiferromagnet doped with real space pairs

Abstract A Monte Carlo study of a model of real space pairs (RSPs), moving in a quasi-2D Heisenberg antiferromagnet, is presented. It is found that, in a system without (Coulomb) repulsion between the RSPs, the system of RSPs may phase-separate or remain homogeneous, depending on the RSP density and on the ration of the antiferromagnetic (AF) coupling parameter J to the RSP bandwidth. In the first case, the RSPs become static, whereas in the second case the RSPs remain dynamic. The effects of the doping on the antiferromagnetic correlation length are presented, together with the NMR Knight shift K and spin lattice relaxation time (1/( T 1 T )), and the specific heat. It turns out that the interaction of dynamic holes with the AF system suppresses the growth of the AF correlation length substantially. Finally, a comparison with recent analytical work on the same model, and with experimental results on high- T c superconductors, is presented. It turns out that the model describes the experimentally observed behavior of 1/( T 1 T ) and K quite well.

Anomalies in the superconducting properties of the misfit layer compound (BiS)1.08TaS2

Superconducting weak link junctions of (BiS)1.08TaS2 single crystals are investigated using a novel weak link production technique. Previous and present measurements of the superconducting properties of (BiS)1.08TaS2 indicate the presence of two different critical temperatures at which transitions take place. Experimental observations are presented supporting the assertion that a transition from bulk (T<0.75 K) to 1D superconductivity (0.75 K<T<1.05 K) takes place. Differential resistance measurements of the (BiS)1.08TaS2 weak link junctions indicate the presence of superconducting fluctuations up to temperatures as high as 2.4 K, three times the bulk transition temperature. Furthermore, in these measurements a rich peak structure is observed at low temperatures.