4PO8-20 The study of the electronic properties of sodium tungsten bronze

Nadir N. Garif'yanov, Kazan Physical-Technical Institute, Kazan 420029 Russia

Presenting Author: N.N. Garif'yanov

Nonstoichiometric sodium tungsten bronze Na_xWO₃ is one of the first superconducting metalooxide. Previously we showed (see for example [1-2]) that sodium tungsten bronzes and high-T_c systems are very similar. Here we present the results of measurements of magnetic susceptibility, electron paramagnetic resonance (EPR) and resistivity of sodium tungsten bronze. We observed unusual phase transition at 2 K in single crystals of sodium bronze with sodium content x=0.2 doped by small impurity of Gd. This phase transition is haracterized hysterezis behaviour of magnetic susceptibility at temperature just above 2K and accompanied by significant sudden rise in the intensity of the EPR signal and small resistivity drop at 2 K. This effect was not observed in single crystals of sodium tungsten bronze with another sodium content. The possible reasons of such behaviour are discussed.

[1] I.A. Garifullin, N.N. Garif'yanov et al., JETP Lett.(USA), 1991, Vol.54, No.7, p.376.

[2] E.L. Vavilova, I.A. Garifullin, N.N. Garif'yanov et al., JETP Lett. (USA), 1993, Vol.58, No.8, P.625.

4PO8-21 Superconductivity and Crystal Structure in Partially Oxidised Nb Powder

Nianhua Peng 1,2, J.T.S. Irvine 2, J.W.L. Dobson 2, Yunhua Shi 3, and W.Y. Liang 3. 1 Surrey Centre for Research in Ion Beam Applications, University of Surrey, GU2 5XH, United Kingdom. 2 School of Chemistry, University of St Andrews, St Andrews KY16 9ST, United Kingdom. 3 IRC in Superconductivity, University of Cambridge, Cambridge CB3 0HE, United Kingdom.

Presenting Author: N. Peng

Superconductivity has been observed at temperatures in range 6-9K for a number of strontium niobate perovskite samples. Although samples were apparently phase pure by X- ray powder diffraction, neutron diffraction revealed the presence of small amounts of Nb metal impurities. In order to test whether this impurity was responsible for the superconducting phase transition, Nb powder samples with a small degree of oxygen inclusion were prepared by high temperature solid state reaction of Nb and Nb₂O₅ in a flowing Ar atmosphere. A linear relation is observed between superconducting transition temperature and lattice unit cell parameter refined from high resolution neutron powder diffraction data, in accord with early data. T_c decreases from 9 to 6.5K as a increases from 3.300 to 3.308 Å. This correlation is observed both for these oxygen-doped Nb samples and for the minority Nb-like impurity in strontium niobates that had exhibited superconducting phase transitions.

4PO8-22 Superconductivity in Granular Platinum

Alexander Schindler, Reinhard König, Thomas Herrmannsdörfer, and Hans F. Braun, Physikalisches Institut, Experimentalphysik V, Universität Bayreuth, D-95440 Bayreuth, Germany

Presenting Author: A. Schindler

Superconductivity in platinum has been found in granular samples but not yet in the bulk material. The reported transition temperatures as well as the critical magnetic fields, both parameters refering to intergranular superconductivity, strongly depend on the packing fraction ƒ of the platinum compacts: we found 0.62 mK £ T_c £ 1.38 mK and 6.6 m T £ B_c £ 67 m T for 0.8 ³ ƒ ³ 0.5.

Our current investigations concentrate on the separation of inter- and intragranular superconductivity in this system. The sharp onset of diamagnetic behaviour, observed at T ~ 1.9 mK for all samples, might be interpreted in terms of the intragranular transition into the superconducting state. The strong dependence of the ac susceptibility on the ac excitation field in the intergranular regime reflects the temperature dependent intergranular critical current density j_c. Samples with different packing fractions display significantly different critical current densities: for ƒ = 0.50 and 0.67 we obtain j_c(B = 0, T = 0) @ 1 A/cm² and 0.1 A/cm², respectively. The temperature dependence of j_c, however, shows strong positive curvature over a wide temperature range for both samples.

These results, consideration of supercurrent limiting mechanisms, and further experiments on diluted as well as on thermally treated samples should contribute to a better understanding of the origin and the type of superconductivity in granular platinum.

4PO8-23 A superconducting high pressure CaSi₂ phase with T_c~14K

S. Sanfilippo 1, M. Nunez-Regueiro 1, M. Affronte 2, O. Laborde 1, P. Bordet 3, M. Hanfland 4, D. Levi 4, S. Lefloch 3, and A. Palenzona 5, and G.L. Olcese 5. 1 C.N.R.S., C.R.T.B.T., B.P.166, 38042 Grenoble. 2 I.N.F.M. and Universita di Modena, 41100 Modena. 3 C.N.R.S., Lab. de Cristallographie, B.P.166, 38042 Grenoble, France. 4 E.S.R.F., B.P.220, 38043 Grenoble, France. 5 I.N.F.M. and Universita di Genova, 16146,Italy.

Presenting Author: M. Nunez-Regueiro

We have searched for higher T_c's by applying an external pressure to the CaSi₂ family of compounds. By resistive measurements, we have found that, starting from the trigonal TR6 phase, superconductivity with T_c ~ 4K shows up when an external pressure P*10GPa is applied and above 140Kbar a T_c ~ 14K, among the highest ever found in silicon based materials, is observed. Synchrotron radiation measurements under pressure show that the trigonal CaSi₂ phase transforms to a new phase with hexagonal symmetry at about 9GPa. Moreover the lattice parameters dramatically shrink at about 16GPa suggesting a collapse to the AlB₂ -type structure at the highest pressure. Noting that the starting trigonal phase has sp₃ (diamond-like) Si planes while the A AlB₂ -type structure has sp₂ (graphite-like) layers, it turns out that this situation is close to the appearance of superconductivity in the sp2 simple hexagonal phase of pure Si which was found to be superconducting with T_c =8.2K under high pressure (15GPa). Measuremetns on the tetragonal CaSi₂ phase are also described. We discuss the appearence of superconductivity in CaSi₂ on the basis of the available results of band structure calculations and by comparing the differents Debye temperatures measured on the different phases.

4PO8-24 Novel Superconducting States in Heavy-Fermion Superconductors

S. Dzhumanov and U.T. Kurbanov, Institute of Nuclear Physics, 702132, Tashkent, Uzbekistan

Presenting Author: S. Dzhumanov

The puzzling superconducting (SC) properties (e.g. splitting of the SC transition, complex H-T phase diagram with three A, B and C phases, kinks on the temperature dependences of the lower and upper critical magnetic fields, and change of these features with pressure and magnetic field) of heavy-fermion superconductors (HTSC) cannot be properly understood within the standard theories of superconductivity. Although many experimental data are interpreted in terms of Ginzburg-Landau theory and modified BCS-like theories, these analyses are still inconclusive. As these theories give no insight into underlying SC mechanisms. Here exotic SC states in HFSC and their changes under the magnetic field and pressure are studied beyond the standard BCS-like Fermi-liquid approach within the novel two-stage Fermi-Bose-liquid model of superconductivity [1]. According to this model the BCS-like pairing of fermions with their mean-field order parameter is not yet SC transition and plays only a precursor role in this transition. It is shown that the specific feature of the one-component mean-field SC order parameter of attracting three-dimensional (3d) heavy triplet cooperons is responsible for novel SC states in HFSC. In particular, the presence (in a weak intercooperon coupling) and absence (in a strong intercooperon coupling) of the kink-like behavior of SC order parameter (its kink-like behavior is caused by the first-order phase transition pair condensation-to-single particle condensation of attracting 3d triplet cooperons) resolve the above long-standing puzzles observed experimentally.

[1] S. Dzhumanov, Physica C235-240, 2269 (1994).

4PO8-25 Superconductivity and non--Fermi--liquid behaviour close to the Quantum Critical Point

H. Wilhelm 1, S. Raymond 1, D. Jaccard 1, F. Bouquet 1, Y. Wang 1, A. Junod 1, O. Stockert 2, and H.V. Loehneysen 2. 1 DPMC, University Geneva, Quai Ernest-Ansermet 24, 1211 Geneva 4, Switzerland. 2 Physikalisches Institut, University Karlsruhe, 76128 Karlsruhe, Germany.

Presenting Author: H. Wilhelm

In Ce-based intermetallics like CeRu₂Ge₂, CePd₂Si₂ or CeCu_6-xAu_x quantum critical phenomena have been observed near the antiferromagnetic (AFM) instability. It can be reached either by pressure or doping. CeRu₂Ge₂ shows a non--Fermi--liquid (NFL) behaviour in a small pressure interval around the critical pressure P_c [1] whereas a superconducting state in coexistence with an AFM order was observed in CePd₂Si₂ close to P_c [2]. In CeCu_6-xAu_x critical phenomena occur close to the critical concentration x_c=0.1 [3]. NFL behaviour is induced by hydrostatic pressure on CeCu₅Au well below Pc. Just before Pc is reached evidence of superconductivity wasfound [4].

We will present transport and AC-calorimetric measurements under high pressure up to approximately 11 GPa and 8 GPa, respectively, which revealed pressure-induced superconductivity in CePd₂Si₂ and CeCu₅Au on the verge of magnetic order and confirm the thermodynamic origin of the phase transitions in CeRu₂Ge₂ detected by electrical resistivity [1] and thermoelectrical power.

[1] H. Wilhelm et al., Phys. Rev. B 59, 3651 (1999).

[2] N.D. Mathur et al., Nature 394, 39 (1998).

[3] O. Stockert et al., Phys. Rev. Lett. 80, 5627 (1998).

[4] H. Wilhelm et al., to appear in: Rev. High Pressure Sci. Technol.

4PO8-26 Magnetic relaxation study in the superconducting mixed state of CeRu₂

E. Lähderanta 1, S.B. Roy 2, S. Chaudhary 2, and P. Chaddah 2. 1 Wihuri Physical Laboratory, University of Turku, 20014 Turku, Finland. 2 Low temperature Physics Group, Centre for Advanced Technology, Indore 452013, India.

Presenting Author: E. Lähderanta

The vortex matter phase transition in the intermediate valence compound CeRu₂ has recently drawn much attention [1]. Results for the remanent moment and relaxation in the superconducting mixed state of single crystal samples of CeRu₂ are presented. The temperature dependence of the remanent moment shows upon warming the sample a decrease which has a change of slope dMrem/dT at about 4 K, independent of the strength of the magnetizing field. In the temperature range below 3.7 K the behaviour of dM_rem/dT depends strongly on this field. Possible explanation of the observed behaviour is a change in the flux-dynamics of CeRu₂ and the so called "fish-tail" effect (sharp increase of the magnetic irreversibility at high fields). The relaxation data are analyzed using the Maley-plot method.

[1] S. Chaudhary, S.B. Roy, P. Chaddah, Physica B (Procedings of the 22nd International Conference on Low Temperature Physics, LT22, Finland (1999)).

4PO8-27 Investigation of the Superconducting Gap in CeRu₂

Andriy V. Moskalenko 1, Yurii G. Naidyuk 1, Igor K. Yanson 1, Christof Geibel 2, Yoshichika Onuki 3, Masato Hedo 3, Yoshihiko Inada 3, Yoshinori Haga 4, and Etsuji Yamamoto 4. 1 Institute for Low Temperature,47 Lenin Ave., 310164, Kharkiv, Ukraine. 2 Max-Planck-Institut fuer Chemische Physic fester Stoffe Bayeurther Str.40, Haus 16, D-01187, Dresden, Germany. 3 Graduate School of Science, Osaka University Toyonaka 560, Japan. 4 Advanced Science Research Center, Atomic Energy Research Institute of Japan Tokai, Ibaraki 319-11, Japan.

Presenting Author: A.V. Moskalenko

We have investigated both singlecrystalline and polycrystalline CeRu₂. The polycrystalline CeRu₂ has residual resistivity ratio (RRR) 14, residual resistivitry r₀ = 31.5 mWcm and critical temperature T_c = 6.25 K. The single crystal was grown by the Czochralski pulling method in a tetra-arc furnace and has RRR = 120, r₀ = 1 mWcm and critical temperature T_c = 6.3 K. The experiments were held at temperature 1.7 K varying magnetic field up to 6T. The applied magnetic field was parallel to the current flow. The samples were investigated by the point-contact spectroscopy method. Point contacts (PC) were prepared by touching the CeRu₂ freshly broken surface with the edge of a Cu or Ag electrode. The differential resistance dV/dI was recorded vs. bias voltage V using a standard lock-in amplifier technique, modulating the direct current I with a small ac component.The PC investigations of CeRu₂ were done to study superconducting gap both in single and polycrystals in magnetic field. The measured dV/dI (V) curves of PC’s correspond fairly well to the Blonder, Tinkham and Klapwijk model. It allowed us to obtain the magnetic field dependencies of the superconducting gap D. The gap decreases approximately linearly in magnetic field both for single- and polycrystals of CeRu₂. The gap vanishes around B_c = 3.5 T close to the irreversibility field. However, a shallow minimum at zero voltage bias as a residual superconducting feature presents in the PC characteristics up to the upper critical field Hc2. The gap value in zero magnetic field is up to 30% greater for the single crystal than in polycrystalline CeRu₂. Superconducting properties of CeRu₂ strongly depend on the quality of sample. The characteristic irreversibility field coincides with the phase transition between the solid and the liquid vortex state of the bulk material. It appears that the superconducting features of the spectra vanish in the liquid vortex state due to weak pinning. Then even a tiny current concentrated in the contact can move the vortices there and drive the junction into a resistive state.