ORAL SESSION 4C6: Non-Cuprates I

Thursday, Feb. 24, 3:45 p.m. – 6:15 p.m., Room 302A (GRB)

Chairs: A. Bishop (Los Alamos Nat'l. Lab), J. Dow (Arizona St. U)

4C6-1 Field-Temperature Phase Diagram of Sr₂RuO₄

Y. Maeno 1,2, T. Akima 1, H. Yaguchi 1,2 , Z.Q. Mao 1,2 , S. Nishizaki 1, and T. Ishiguro 1,2. 1 Department of Physics, Kyoto University, Kyoto 606-8502, Japan. 2 CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan.

Presenting Author: Y. Maeno

[3] D.F. Agterberg, Phys. Rev. Lett. 80 (1997) 5184.

4C6.2 Testing the Symmetry of Unconventional Superconductors beyond the Cuprates and beyond d-wave

D.J. Van Harlingen, Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, IL 61801

Presenting Author: D.J. Van Harlingen

The discovery that the high temperature superconducting cuprates exhibit d-wave pairing has nucleated intense study of the mechanisms and implications of unconventional order parameter symmetry. A wide range of materials have emerged that are also suspected to have unconventional symmetry, including the heavy fermion superconductors, that exhibit two distinct superconducting phase transitions, the organic superconductors, thought to be d-wave, and the ruthenates, thought to be p-wave with a complex order parameter, corresponding to a state with spontaneously-broken time reversal symmetry. In addition, it has been recently recognized that superconductors with highly-anisotropic order parameters are fragile to scattering and can nucleate subdominant superconducting phases characterized by different, perhaps complex, order parameter symmetries, near interfaces, inside vortex cores, and in the presence of impurities. In this talk, I will discuss experimental strategies for determining the symmetry of unconventional superconducting materials, focusing in particular on Josephson interferometer techniques that directly probe the phase anisotropy of the order parameter. I will report the status of experiments being carried out in our group and others to determine the symmetry of magnetically-doped cuprates and non-cuprate systems such as the heavy fermion superconductor UPt₃, the organic superconductor k -(ET)₂Cu[N(CN)₂]Br, and the ruthenate superconductor Sr₂RuO₄.

4C6.3 Pseudo gap picture for the normal state of Sr₂RuO₄

Y. Liu 1, R. Jin 1, and F. Lichtenberg 2. 1 Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA. 2 Center for Electronic Correlations and Magnetism, Augsburg University, D-86135 Augsburg, Germany.

Presenting Author: Y. Liu

The normal-state magnetoresistance (MR) of superconducting Sr₂RuO₄ single crystals has been measured. The transverse and longitudinal in-plane MR were found to be positive with an unusual linear field dependence above a threshold field at low temperatures, a result not expected from the free-electron model or standard band calculation. The analysis of such linear behavior in MR and other experimental results has led to the picture of a novel coherent state characterized by a spin pseudo gap in the quasi-particle excitation spectrum for Sr₂RuO₄. Its possible connection with the presence of a quantum critical point in Sr₂RuO₄ will be discussed.

4C6.4 Unconventional Normal-State Properties and Superconductivity in Heavy-Fermion Metals

Frank Steglich 1, N. Sato 2, O. Trovarelli 1, C. Geibel 1, P. Gegenwart 1, C. Langhammer 1, M. Lang 1, M. Grosche 1, G. Sparn 1, P. Thalmeier 1, and T. Dahm 3. 1 Max-Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany. 2 Department of Physics, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan. 3 Institut für Theoretische Physik, Universität Tübingen, D-72076 Tübingen, Germany.

Presenting Author: F. Steglich

Heavy-fermion (HF) superconductors which show a non-Landau Fermi-liquid normal state have recently attracted much interest. Here we focus on two compounds which are superconductors already at ambient pressure: CeCu₂Si₂ and CeNi₂Ge₂. For comparison, we also discuss the non-superconducting isostructural compound YbRh₂Si₂ exhibiting pronounced non-Fermi-liquid (NFL) effects at low temperatures. The interrelation between a NFL normal state and the underlying spinfluctuations on the one hand and HF superconductivity on the other has yet to be clarified. In another class of HF metals, superconductivity forms out of a heavy Landau-Fermi-liquid state coexisting with long-range antiferromagnetic (AF) order. We discuss inelastic neutron-scattering measurements on the compound UPd₂Al₃ which exhibits local-moment AF order below T_N = 14.3 K and HF superconductivity below 2 K. These results reveal a magnetic-exciton branch of localized 5f electrons at approx. 1meV which is strongly coupled to the itinerant heavy quasiparticles. It is, therefore, proposed that this material is the first magnetic-exciton-mediated superconductor.

4C6.5 Why Broken Time Reversal Symmetry Occurs in Unconventional Superconductors?

Manfred Sigrist, Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan

Presenting Author: M. Sigrist

In recent years a number of superconductors have been found which very likely realize phases with broken time reversal symmetry T. They are identified by their magnetic properties, for example in m SR experiments. Two well-known examples amoung the heavy Fermion superconductors are U_1-xTh_xBe₁₃ and UPt₃ which both exhibit superconducting double transitions. Below the second phase transition T-violation occurs. It can be shown that among all instabilities possible within an unconventional superconducting a T-violating one is most favored by the gain of condensation energy. This is not only true for second transitions in the bulk but also at surfaces and interfaces, as probably observed in high-temperature superconductors.

In contrast, in Sr₂RuO₄ a T-violating phase occurs immediately at the onset of superconductivity. We will show that in this quasi-two-dimensional Fermi liquid the stabilizing mechanism involves very likely mechanisms of magnetic origin, such as spin-orbit coupling effects and spin fluctuations.

4C6.7 High-T_c Superconductivity in Electron-Doped Layer Structured Metal Nitride Halides

Shoji Yamanaka. Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan. CREST, Japan Science and Technology Corporation (JST)

Presenting Author: S. Yamanaka

New high-transition temperature (high-T_c) superconductors based on layered metal nitrides have been developed. Layer structured crystals b -MNCl (M = Zr, Hf) have the SmSI type structure; the M-N double layers occupy the SmS positions and are sandwiched between the close-packed chlorine layers. The layers with a sequence of [Cl-M-N-N-M-Cl] are stacked with each other by a weak van der Waals interaction. The crystals are characterized as semiconductors with a band gap of larger than 3 eV. The nitride layers can be electron-doped by intercalation of lithium between the chloride layers. On intercalation the layers gave a metallic behavior and became superconductors with T_c as high as 13 and 25.5 K for b -ZrNCl and b -HfNCl, respectively. The T_c for the b -HfNCl intercalation compound is higher than that observed in any intermetallic compound (T_c = 23.2 K of Nb₃Ge), and suggests that the layered nitride structures may offer transition temperatures comparable to those observed in layered copper oxide structures. The intercalation systems have many varieties on the way of structural modifications. The effects of chemical modifications to the transition temperatures will be presented, such as effects of doping levels, types of doping species, interlayer separation (effect of co-intercalation), substitution of halogens, substitution of metals, and types of layer structured polymorphs.

4C6.8 Electrical Quadrupole Interaction in RNi_nB_nC (n=1,2) Compounds as Obtained From ⁵⁷Fe Mössbauer Spectroscopy, c’/a-ratio and the Occurrence of Superconductivity

E.M. Baggio-Saitovitch 1, D.R. Sánchez 1, and H. Micklitz 2. 1 Centro Brasileiro de Pesquisas Físicas, Rua Xavier Sigaud 150 Urca, CEP 22290-180, Rio de Janeiro, Brazil. 2 II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany.

Presenting Author: E.M. Baggio-Saitovich

The ⁵⁷Fe Mössbauer effect (ME) spectra of RNiBC (R=Gd, Tb, Dy, Ho, Er and Y) compounds doped with 1 at % ⁵⁷Fe, taken between 2 K and 300 K, reveal one quadrupole doublet (quadrupole splitting ½ D E_Qô ) which becomes magnetically broadened below 24 K and 4.6 K for TbNiBC and ErNiBC, respectively.

A linear relationship is obtained for all RNiBC compounds from the plot of the measured ½ D E_Qô values versus the ratio c’/a (c’ is the spacing of the R-C planes between which the (Ni(Fe)-B)₂ layer is located). It coincides with the linear relationship between ½ D E_Qô and the c’/a-ratio obtained in earlier ⁵⁷Fe ME experiments on RNi₂B₂C compounds [1]. This finding is a clear indication that the electronic structures of Ni(Fe) in RNiBC and RNi₂B₂C are very similar. At the same time the c’/a-ratio may be a key for understanding why several of the collinear antiferromagnetically ordered RNi₂B₂C compounds are superconducting while in the case of RNiBC only LuNiBC has been reported to be a superconductor [2]: all superconducting compounds have a c’/a-ratio ³ 1.48. Only for such a large c’/a-ratio the Ni-B bonding angle and the resulting density of states seems to be favorable for superconductivity. This picture is supported by early band structure calculations [3].

[1] Z. Zeng et al., Phys. Rev. B 55, 3087 (1997).

[2] L. Gao et al., Phys. Rev. B 50, 9445(1994).

[3] L.F.Mattheiss, Phys. Rev. B 49, 13279(1994).

4C6.9 Field-Induced Crossover in La_0.7Pb_0.3MnO₃

Ann K. Heilman 1, Y.Y. Xue 1, Bernd Lorenz 1, C.W. Chu 1, M. Gospodinov 2, and S.G. Dobreva 2. 1 Texas Center for Superconductivity, University of Houston, 3201 Cullen Blvd., Suite 202, Houston, TX . 2 Laboratory of Crystal Growth, Institute of Solid State Physics, Bulgarian Academy of Sciences, Sofia, 1784, Bulgaria.

Presenting Author: A.K. Heilman

A field-induced crossover is observed both resistively and magnetically in a single crystal of La_0.7Pb_0.3MnO₃, a compound with a second-order ferromagnetic transition that exhibits colossal magnetoresistance (CMR). The resistivity (r ) data observed in high magnetic fields above 1.0 T show a universal decrease with the magnetization (M) and little dependence on temperature. This decrease follows an exponential correlation of the form r = r ₀exp(-M/M₀). In contrast, below about a field of 1.0 T, r is independent of M. The critical exponents, as determined by an analysis of the magnetization data at fixed temperatures using the Arrott-Noakes and Kouvel-Fisher methods, have different values depending on whether the high field data or the low field data are analyzed. The results are consistent with the interpretation that at low fields, the compound behaves as a collection of magnetic clusters with significant antiferromagnetic contributions and mean-field-like inter-cluster interactions, whereas, at high fields the compound behaves as a system of individual spins with short-range interactions. Around the Curie temperature, T_c, the crossover between these two regions is accompanied by the colossal magnetoresistance.