4PO8-80 to 4PO8-85

4PO8-80 Zero-Bias Anomalies in Normal Metal/ Charge Density Wave Interfaces

J.P. McCarten 1, J.H. Miller, Jr. 2,3, X. Xu 2, I. Pirtle 2, J.R. Liu 2, and W.K. Chu 2,3. 1 Department of Physics, Clemson University, Clemson, SC 29634-1911. 2 Texas Center for Superconductivity at the University of Houston. 3 Department of Physics, University of Houston, 4800 Calhoun Rd., Houston, TX 77204, USA.

Presenting Author: J.P. McCarten

In many reduced dimensional conductors, such as the oxides K_0.3MoO₃ and Rb_0.3MoO₃, and the transition-metal trichalcogenides NbSe₃, TaS₃ and NbS₃, the ground state is a charge density wave (CDW). These strongly correlated electron systems exhibit collective electron transport. We report on the observation of zero-bias anomalies in the differential resistance of NbSe₃ crystals near an interface between strongly pinned and weakly pinned regions of a CDW, where current is being converted from normal metallic to CDW current. Our results are similar to the reported zero-bias anomalies across an interface between normal metal (gold) and blue bronze (K_0.3MoO₃). We also observe a dramatic drop in the temperature-dependent zero-bias resistance when the temperature is cooled below about three quarters of the Peierls transition temperature (T_P), suggesting the existence of a new phase transition. At least two additional experiments also indicate a phase transition at 3/4T_P.

The origin of the zero-bias anomalies and apparent phase transition is unknown at present. We are currently exploring several hypotheses, such as a condensation of solitons near the interface, possible Andreev-type reflections, and other scenarios. The application of a magnetic field and other experiments can potentially shed additional light on the observed behavior.

4PO8-82 Temperature and Field Dependence of Resistivities and Anisotropy of Layered Manganites La_2-2xSr_1+2xMn₂O₇

C.C. Almasan 1, C.L. Zhang 1, G.A. Levin 1, and J.S. Gardner 2. 1 Department of Physics, Kent State University, Kent, OH 44242, USA. 2 Los Alamos National Laboratory, Los Alamos, NM 87545, USA.

Presenting Author: C.C. Almasan

The in-plane r _ab and out-of-plane r _c resistivities of single crystals of La_1.2Sr_1.8Mn₂O₇ and LaSr₂Mn₂O₇ have been measured by a multiterminal technique. We pay special attention to the resistive anisotropy r _c/r _ab which reflects the ratio of phase coherence lengths in the respective directions. In La_1.2Sr_1.8Mn₂O₇, both resistivities dramatically decrease below the Curie temperature T_c = 120 K. Anisotropy changes with temperature nonmonotonically and displays a peak at T_c. This maximum indicates that the in-plane phase coherence length of the electrons is coupled with the correlation length of the spin waves. With increasing magnetic field, the maximum in resistivity and anisotropy becomes less pronounced and its location shifts to higher temperatures, approaching 300 K in a field of 14 T. In LaSr₂Mn₂O₇, a paramagnetic-antiferromagnetic transition takes place at the Néel temperature T_N = 170 K. Both r _c and r _ab display a broad peak close to T_N. Magnetic field suppresses the magnitude of the peaks, but does not shift their position. The anisotropy exhibits a more complex behavior reflecting the antiferromagnetic and charge ordering transitions, which take place at different temperatures.

This research was supported at KSU by the National Science Foundation under Grant No. DMR-9801990

4PO8-83 YNi₂B₂C: Possible Anisotropic Pressure Dependence of the Superconducting Transition Temperature

D.R. Sánchez 1, E. M. Baggio-Saitovitch 1, and S.L. Bud’ko 2. 1 Centro Brasileiro de Pesquisas Físicas, Rua Xavier Sigaud 150, Urca, CEP 22290-180, Rio de Janeiro, Brazil. 2 Ames Laboratory and Dept. of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA.

Presenting Author: D.R. Sanchez

The family of quaternary borocarbides RNi₂B₂C (R - rare earth), which attracts a great deal of attention in the last years, has a tetragonal structure of the ThCr₂Si₂ type with the alternating R-C and (Ni-B)₂ layers. Despite this layered structure anisotropic measurements of superconducting critical field, normal state resistivity and band structure calculations, strongly suggest a three-dimensional nature of the physical properties of the borocarbides. In addition band structure calculations have shown that superconductivity in the quaternary borocarbides is particularly sensitive to the NiB₄ tetrahedral geometry and that favorable superconducting properties are expected for the compounds where the NiB₄ tetrahedra (i.e. for example the c/a ratio) are nearly "ideal" (LuNi₂B₂C, YNi₂B₂C) [1].

The YNi₂B₂C compound becomes superconducting below 16 K, while the nonmagnetic LaNi₂B₂C does not show a superconducting transition down to 2.5 K. In the present work we attempt to link chemical pressure in the (Y_1-xLa_x)Ni₂B₂C, and hydrostatic pressure in YNi₂B₂C with the variation in T_c. The results indicate that the observed hydrostatic pressure effect on YNi₂B₂C is possibly a consequence of the balance between positive in-plane and negative out-of-plane uniaxial pressure derivatives. They suggest also that c-uniaxial pressure is detrimental for superconductivity in this system.

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

4PO8-84 Supression of the metal-to-semiconductor transition in Bismuth Cobaltates: Can cobaltates superconduct?

S.M. Loureiro 1, D.P. Young 1, R. Jin 2, Y. Liu 2, P. Bordet 3, Y. Qin 4, H. Zandbergen 4, M. Godinho 5, M. Núñez-Regueiro 6, B. Batlogg 7, and R.J. Cava 1. 1 Dept. of Chem. and Mat. Inst., Princeton NJ 08544, USA. 2 Dep. Physics, Penn State U., University Park, PA 16802, USA. 3 Lab. de Cristallographie, BP 166, 38042 Grenoble Cedex 09, France. 4 Delft U. of Technology, 2628 Al Delft, The Netherlands. 5 Dep. Física, FCUL, Campo Grande, 1700 Lisboa, Portugal. 6 CRTBT, BP 166, 38042 Grenoble Cedex 09, France. 7 Lucent Technologies, Murray Hill, NJ 07974, USA.

Presenting Author: S.M. Loureiro

Even though superconducting cuprates can be formed from a large variety of elements and crystallize in different structure types, they share several commonalties. The same functional blocks, presence of (CuO₂) planes where superconductivity is thought to occur, highly hybridized Cu 3d-O 2p bands at E_F, and the appearance of superconductivity where the AFM-insulator to PM-metal transition occurs. In the cuprates, Cu²⁺ is d⁹ with the t_2g level completely filled and the 9th electron occupying the d_x2-y2 orbital, which is half-filled. This fact is considered important for the appearance of superconductivity.

Can superconductivity occur in other non-Cu based compounds having half-filled e_g level d-bands? In order to investigate this fact we have synthesized and studied Bi₂M₃Co₂O_y compounds (M=Ca, Sr, and Ba). These are analogous to the superconductor Bi₂Sr₂CaCu₂O_y with (CoO₂) planes instead of (CuO₂) planes. These compounds are either semiconductors or have metal-to-semiconductor transitions at Temperatures up to 80K. Through Pb-doping we completely suppressed this transition and render them fully metallic, although not superconducting. Due to its strong chemical and structural similarity with the Bi-2212 cuprates, this system provides a unique opportunity for studying which electronic features are really inherent to the superconducting cuprates.

4PO8-85 Possible RT Superconductor from PbCO₃ 2PbO-Ag₂O System

Danijel Djurek 1, Zvonko Medunic 1, Anton Tonejc 2, and Matija Paljevic 3. 1 A. Volta Applied Ceramics (AVAC), Zagreb, Croatia, 2 Faculty of Sciences, Physics Department, Zagreb, Croatia, 3 Rudjer Boskovic Institute, Zagreb, Croatia

Presenting Author: Danijel Djurek

In the course of past decade we have investigated mixtures of lead oxides PbO and PbO and Pb₃O₄ with Ag₂O or Ag₂CO₃ fired in oxygen atmosphere at 330-340 °C. Many samples exhibited resistive transitions characteristic for superconductivity (SC) and extended up to room temperature (RT). In dozen of cases resistive transitions were accompanied with microwave absorption (MA) in modulated magnetic fields. The resistive transitions as well as characteristic MA hysteresis gradually smeared by heating-cooling cycles.

Careful x-ray investigation of fired Pb₃O₄-Ag₂O (atomic Pb/Ag=5 system exhibiting MA revealed the traces of oxi-carbonate PbCO₃ PbO (PCP, CO₂/PbO=1/2) and we concluded that, in order to induce resistive transitions, carbon must be added to Pb-Ag-O system. PCP consists of alternative PbCO₃-PbO-PbCO₃-PbO . . . layers and partial substitution of Pb by Ag in PbCo₃ layers may introduce a set of novel metals. Attempts to induce metallic phases from PCP-Ag₂O (Ag₂CO₃) system in combined O₂-CO₂ atmosphere at 340-350 °C failed so far and an alternative approach was introduced by the use of mixed PbCO₃ 2PbO (PC₂P, CO₂/PbO=1/3) and Ag₂O (Ag₂CO₃) (Pb/Ag=5) powders at 330 °C in CO₂ atmosphere. The subsequent annealing at 340-350 °C in combined CO2-O2-O2 atmosphere results in three types of phases. For higher CO2 pressures oxi-carbonates with CO₂/PbO > 1/2 are formed and compounds are insulators. Otherwise, for low CO₂ pressures PbCO₃ layers release CO₂ with subsequent formation of Ag-doped intermediate (Bystrom) phases Pb_1-xAg_xO₁₊₈ (0.40 ( d ) 0.55) which are metallic but not superconducting. Finally, samples carrying SC properties appear for intermediate pressures of CO₂ and optimal O₂ pressure, which may be attributed to Pb_1-xAg_xCO₃ PbO₁₊₈ (0.25 (x (0.33).

The highest transition temperature achieved is 335-340 K. The resistivity at 22 °C, slightly below threshold current density of 150 A/cm², is three orders of magnitude less than that of silver and magnetic properties are in favour of superconductivity. In spite of possible well known difficulties connected with interpretations of poliphasic samples we believe that observed properties are related to superconductivity appearing above R. T.