High Temperature Superconductors GR/K22211, Final Report

• Introduction

We have carried out neutron scattering studies, laboratory measurements and NMR work to examine and attempt to understand various features of the superconducting state in a range of materials. Our work over the last four years has been extremely successful generating 76 publications, including 18 Phys. Rev. B, 1 Nature and 8 Phys. Rev. Letts. articles. The conclusions from our experimental work have been presented at several prestigious international conferences including the Mott Lecture in 1996 and Vortices in Exotic Systems (in honour of A. A. Abrikosov) 1998.

 

• High Temperature Superconductors (Neutron Studies)

In collaboration with Ted Forgan and co-workers at the University of Birmingham we have used small angle neutron scattering (SANS) to investigate the flux line lattice (fll) in a number of the high temperature superconductors. In the YBa2Cu3O7 system we have elaborated on the melting phenomena, showing that for "untwinned" single crystals in the mixed state, the pancake vortices decouple as soon as the fll melts. We have also shown that there are field dependent fll reorientation transitions as the angle between the applied magnetic field and the c axis of the crystal changes. These experiments will help us to understand the intrinsic properties of these materials and force a re-evaluation of conclusions drawn from data collected on heavily twinned samples.

 

• High Temperature Superconductors (NMR Studies)

We have used NMR to study the normal state pseudo-gap in the cuprate superconductors. These studies have been carried out in collaboration with Jeff Tallon and Grant Williams of the New Zealand Institute for Industrial Research and Development, Lower Hutt. In under doped samples we have shown that the hole dependence of the gap scales in the same fashion for materials with one or two CuO₂ layers. We have shown the normal gap has d-wave symmetry and that the gap energies scale with Tc. In overdoped Y _1-xCa_xBa₂Cu₃O_7-d we were able to model our NMR data by assuming an energy dependent DOS with a peak pinned to Fermi level that grows with overdoping. We have studied impurity effects in YBa₂ (Cu1-xM_x)₄O₈ where M = Zn or Ni. We have shown that the suppression of Tc is the same for both impurities. This implies that previous suggestions of a local suppression of the antiferromagnetic correlations at the Zn sites or a local moment on the Ni are not responsible for the reduction in T_c. NMR and NQR experiments have been performed to clarify the situation.

 

(3) "p-wave" Superconductivity in Ruthenates

Our studies of the fll using SANS have been extended to encompass a range of new materials. We have studied the magnetic field dependence of the fll in single crystals of Sr₂ RuO₄. These experiments, which were carried out with Ted Forgan, have revealed the presence of a real square lattice, providing strong evidence in favour of p-wave superconductivity in this material. These measurements were performed in applied magnetic fields of a similar strength to those used for decoration experiments and in themselves represent a significant advance for the SANS imaging technique.

 

(4) The Superconducting State in Heavy Fermion Materials

Along with Andrew Huxley, (CEA, Grenoble), we have studied the formation, quality and morphology of the fll in single crystals of the heavy fermion superconductors CeRu₂ and UPt₃. In CeRu₂ the fll can be described as a collection of rigid bundles of vortices. The average diameter of a bundle decreases for and becomes comparable with the penetration depth at fields where magnetisation data shows there is a "peak effect" (PE). A memory of field history on passing through the PE regime has also been found. These observations are consistent with collective weak pinning theory.

 

(5) Thermomagnetic effects, disorder and the "Peak Effect" in NbSe2

We have used a number of techniques to examine the pinning properties of the layered superconductor NbSe₂. This work has been carried out with A. K. Grover (TIFR, Bombay) and Shobo Bhattacharya (NEC, Princeton). We have observed changes from very weak pinning at low temperatures to strong, history dependent pinning near T_c or Hc2 that are correlated with the extent of the PE. We have studied the reversible / irreversible transitions and the associated changes in the fll which can be correlated to changes in the effective pinning strength. We have been able to develop a generic phase diagram of the various disordered states that can be expected to form near T_c in a pure and weakly disordered anisotropic superconductor.

 

(6) RNi₂B₂Cmagnetic superconductors

We have studied the interplay between magnetism and superconductivity in the rare earth nickel borocarbides. This work has been carried out in collaboration with a number of co-workers including Igor Yanson (Verkin Inst., Ukraine), Richard Doyle (IRC, Cambridge) and V. Kogan (Ames Lab., Iowa). We have investigated the morphology of the fll in non magnetic YNi₂B₂C. Our data has shown there is a square to hexagonal transition in the fll and that at lower fields the distorted hexagonal lattice exhibits a reorientation transition. These observations have proved to be an important test of "non-local" models of superconductivity. We have constructed phase diagrams and determined the magnetic structures of several members of this family of compounds including Ho and TmNi₂B₂C. In the case of HoNi₂B₂ C we have shown that just below T_c (T<9K) the Ho ions order in along period c-axis spiral, while at low temperatures (T<5K) there is a commensurate magnetic ordering. At intermediate temperatures (5<T<6K) there is a coexistence of a- and c-modulation. This modulated state has a drastic effect on the superconductivity causing a rapid decrease in H _c2. We have also studied the fll in the magnetic members of this series of compounds. For HoNi₂B₂C, we have combined neutron measurements with conventional magnetometry and micro Hall probe data to study the pinning in this material. We have shown that bulk pinning is limited to the "re-entrant" region of the H-T phase diagram between 5 and 6K.

 

Publications

To view a list of the publications resulting from the work supported by this grant please

click here.

 

Contact address

Don M^cK. Paul, Physics Department, University of Warwick, Coventry, CV4 7AL, UK.

Tel. +44 (0)24 76523603, e-mail phrje@warwick.ac.uk