POSTER SESSION 4PO8: Non-Cuprates and Non-Superconducting Oxides

POSTER SESSION 4PO8: Non-Cuprates and Non-Superconducting Oxides

Thursday, Feb. 24, 1:45 p.m. – 3:45 p.m., Hall D (GRB)

4PO8-1 Conventional mechanisms for "exotic" superconductivity

Lev P. Gor'kov 1,2, Daniel F. Agterberg 1, and Victor V. Barzykin 1. 1 National High Magnetic Field Lab, 1800 E. Paul Dirac Dr., Tallahassee, FL 32310. 2 L.D. Landau Institute for Theoretical Physics, Chernogolovka, 142432, Russia.

Presenting Author: L.P. Gor'kov

We consider the pairing state due to the usual BCS mechanism in substances of cubic and hexagonal symmetry where the Fermi surface forms pockets around several points of high symmetry. We find that the symmetry imposed on the multiple pocket positions could give rise to a multidimensional nontrivial superconducting order parameter. The time reversal symmetry in the pairing state is broken. We suggest several candidate substances where such ordering may appear, and discuss means by which such a phase may be identified. We discuss the effect of isotropic impurity scattering on the exotic superconducting states that arise from this mechanism. We find that while these non s-wave states are suppressed by non-magnetic impurities, the suppression is much weaker than would be expected for unconventional superconductors with isotropic non-magnetic impurity scattering.

4PO8-2 The Impact of Hyperfine Interaction on Superconductivity

Thomas Herrmannsdoerfer, Physikalisches Institut, Universitaet Bayreuth,D-95440 Bayreuth, Germany

Presenting Author: T. Herrmannsdoerfer

In order to study the influence of the weakest magnetic contribution in matter, i.e. the nuclear magnetism, on superconductivity, measurements of the superconducting critical field have been performed down to micro Kelvin temperatures. So far, data were taken on the modellike type-I superconducting metals AuAl₂, Al, AuIn₂, and Sn which have been chosen in respect to their destinctive strength of hyperfine interaction. In all systems the nuclear magnetism is observed to compete with the superconducting critical field at ultralow temperatures. However, a comparison of all results now opens the possibility to distinguish between Cooper pair breaking contributions due to electromagnetic and hyperfine interaction. Besides reductions of the superconducting critical field due to nuclear paramagnetism at T < 5mK in all investigated materials, a strong competition and finally a coexistence of nuclear ferromagnetic order and superconductivity in the nuclear magnetic ordered phase (T < 0.04 mK) of the strongly hyperfine coupled as well as strongly interacting nuclear spin system AuIn_2 has also been observed [1]. This project is the first effort to study the interplay of nuclear magnetism and superconductivity.

[1] S. Rehmann, T. Herrmannsdoerfer, and F. Pobell, Phys. Rev. Lett. 78, 1122 (1997).

4PO8-3 Retarded interelectron interaction and optical properties of unconventional superconductors

Oleg V. Dolgov, Max-Planck-Institute, 70569 Stuttgart, Germany, and Holger J. Kaufmann, IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom

Presenting Author: O.V. Dolgov

Effects of the strong interelectron interaction via intermediate bosons on quasiparticle and optical properties of metals in the normal state were analyzed. Microscopic expressions for the optical conductivity are used to determine the spectra of these bosons. The application of mentioned approach to ruthenides and systems with heavy-fermions is discussed.

4PO8-4 Density-Functional-Theory Calculations of the Superconductor: Strontium Ruthenate

Keshav N. Shrivastava, Seiichi Takami, Momoji Kubo, and Akira Miyamoto, Department of Materials Chemistry, Graduate School of Engineering,Tohoku University, Aoba-yama 07, Sendai 980-8579, Japan

Presenting Author: K.N. Shrivastava

The density-functional-theory, (DFT), has been used to minimize the energy of a cluster of ruthenium atoms as determined from the Schrodinger equation in the local density approximation (LDA). The oxygen atoms are placed on the bridge site of a few layers of ruthenium atoms. The positions of the O atoms and those of the Ru atoms in the top two layers are varied to find a minimum in the energy. The bond lengths of oxygen to first layer of Ru atoms and second layer of Ru atoms are determined. The Ru-O stretching vibrational mode perpendicular to the surface is found at about 64 to 67 meV. The DFT calculations show that hcp site of oxygen is favoured by more than 0.2 eV over the fcc site. The ruthenium atom along with four atoms in a plane plays an important role in determining superconductivity in the Sr doped ruthenium oxide. The d_xy orbitals of ruthenium are split from d_yz and d_zx orbitals so that the two-dimensional superconductivity occurs in the d_xy orbital with tunneling perpendicular to the xy plane.

4PO8-6 Ultrasonic attenuation in Sr₂RuO₄

Christian Lupien 1, Louis Taillefer 1, Y. Maeno 2, and Z. Mao 2. 1 Department of Physics, University of Toronto. 2 Department of Physics, Kyoto University.

Presenting Author: C. Lupien

Ultrasound attenuation is a technique that is sensitive to the anisotropy of the superconducting order parameter. As was demonstrated in the heavy fermion compound UPt₃ it can reveal the existence of nodes in the gap and shed light on their structure. In the perovskite superconductor Sr₂RuO₄, there is growing evidence for a p-wave order parameter but few experiments have probed the associated anisotropy directly. We present preliminary ultrasound attenuation data measured on a single crystal of Sr₂RuO₄.

4PO8-7 Optical conductivity of the p-wave superconductor Sr₂RuO₄

Balazs Dora 1, Attila Virosztek 1,2, and Kazumi Maki 3. 1 Department of Physics, Technical University of Budapest, H-1521 Budapest, Hungary. 2 Research Institute for Solid State Physics and Optics, P.O. Box 49, H-1525 Budapest, Hungary. 3 Department of Physics and Astronomy, University of Southern California, Los Angeles, CA 90089-0484.

Presenting Author: K. Maki

It is now generally believed that the newly discovered superconductivity in Sr₂RuO₄ is of spin-triplet p-wave, and is known to be extremely sensitive to impurities. We calculate here the frequency dependent conductivity in p-wave superconductor with the order parameter d (k)=d d (k₁+ik₂), and the scattering is due to impurities in the unitary limit. Here d is the unit vector indicating the triplet pairing like in 3He-A, and k is the quasiparticle wave vector within the a-b plane. As in most of the unconventional superconductors, we can not use the simplifications introduced by Mattis-Bardeen or Abrikosov et al. At low temperatures (T<0.1T_c), the conductivity has a remarkable peak at zero frequency reflecting the island of the density of states introduced at the Fermi surface due to impurities. This feature will provide another test of p-wave superconductivity in Sr₂RuO₄.

4PO8-8 Eutectic systems containing the superconductor Sr₂RuO₄

Z.Q. Mao 1,2 and Y. Maeno 1,2. 1 Department of Physics, Kyoto University, Kyoto 606-8502, Japan. 2 Core Research for Evolutional Science and Technology , Japan Science and Technology Corporation (CREST-JST), Kawaguchi, Saitama 332-0012, Japan.

Presenting Author: Z. Mao

Sr₂RuO₄ is a spin-triplet p-wave superconductor with intrinsic T_c0 of 1.5 K. We have prepared crystals with T_c almost equal to T_c0 by optimizing crystal growth conditions with a floating-zone method. In that process, we observed two intriguing eutectic solidification systems: (1) lamellae of Ru metal embedded in the Sr₂RuO₄ matrix, and (2) epitaxial-like growth of SrRuO₃ (a ferromagnetic metal with Curie temperature of 160 K) on Sr₂RuO₄.

The Sr₂RuO₄-Ru composite structure occurs only in the core region of crystal rods, because the surface of molten-zone always has active evaporation of Ru and only the liquid inside reaches the eutectic concentration. More interestingly, the embedding of Ru lamellae in the main phase of Sr₂RuO₄ leads to an enhancement in T_c up to 3 K.

For Sr₂RuO₄- SrRuO₃ solidification, the SrRuO₃ also appears only in the core region of crystal rods. The c-axis of SrRuO₃ has the same orientation as that of Sr₂RuO₄. The volume of SrRuO₃ core is controllable: the SrRuO₃ layer can have the thickness of as much as 2 mm in some cases. This configuration may prove interesting in the study of tunneling effect along the c-axis between a spin-triplet superconductor and a ferromagnetic metal.

4PO8-9 Measurements of magnetoresistivity and magnetization of Sr₂RuO₄

Jun Ho Kim 1, N.R. Dilley 1, R.P. Dickey 1, A. Amann 1, C. Sirvent 1, M.B. Maple 1, Hazuki Kawano 2, and Pengcheng Dai 2. 1 Department of Physics and Institute for Pure and Applied Physical Sciences, University of California, San Diego, La Jolla, CA 92093-0360. 2 Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6393.

Presenting Author: J. Kim

We report measurements of magnetoresistivity, dc magnetization, and ac magnetic susceptibility on several single crystalline samples of Sr₂RuO₄(T_c = ~1K). Magnetoresistivity was measured in the temperature range 0.1 ~ 4K, and in applied magnetic fields up 6 tesla. The magnetic field and current were applied both parallel and perpendicular to the c-axis of the crystal. From the measurements, we determined the superconducting upper critical field, H_c2(T). We observed that in magnetic fields above H_c2, and for transport currents both in the ab-plane and along the c-axis, Sr₂RuO₄ exhibited a metallic resistivity down to 0.1K. We have also studied the magnetic phase diagram by measuring dc magnetization M(H) in the temperature range 0.4 ~ 2K and in applied fields up to 60mT along the c-axis of the crystal using a Faraday magnetometer. We discuss the resistively and magnetically determined superconducting phase diagrams along with ac susceptibility data.