4PO8-30 ZrNi₂In – New Superconducting Heusler Alloy

Andrzej J. Zaleski 1, Dariusz Kaczorowski 1, and V.I. Zaremba 2. 1 Institute of Low Temperature and Structure Research, P.O.Box 1410, 50-950 Wroclaw 2, Poland. 2 Department of Inorganic Chemistry, Lviv State University, 290 005 Lviv, Ukraine.

Presenting Author: A.J. Zaleski

Superconductivity was already detected in 18 Heusler alloys i.e. ternary intermetallics forming a large class of cubic compounds having the composition TX₂Z, where T and Z are transition or noble metal atom and Z is sp element.

In the paper we present the results of resistivity, dc and ac susceptibility and specific heat measurements of polycrystalline ZrNi₂In. Critical temperature of the compound (defined as the onset of magnetic transition) was equal to T_c=9K, surprisingly high for this class of compounds (the maximum value of critical temperature was previously obtained for YPd₂Sn and was equal to T_c=4.64 K).

From the dc magnetization measurements lower and upper critical fields were evaluated and their temperature dependence was obtained. From the slope of dH_c2/dT superconducting parameters of the material were calculated.

4PO8-31 Superconductivity in Zn doped CuIr₂S₄

Guanghan Cao, Hideaki Kitazawa, Hiroyuki Suzuki, Takao Furubayashi, Kazuto Hirata, and Takehiko Matsumoto, National Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

Presenting Author: G. Cao

The thiospinel compound CuIr₂S₄ exhibits the metal-insulator (M-I) transition at 230K accompanied by the structural transition. In spite of many investigation, the mechanism of the anomalous M-I transition is still an open question. The Zn doped polycrystals Cu_1-xZn_xIr₂S₄ (x < 0.6) were synthesized to examine the effects of excess electrons supplied by replacing Cu with Zn. The M-I transition temperature decreases initially with increasing x. The insulating phase disappears around x = 0.3 and the superconducting phase appears at the same time. By resistivity, magnetic susceptibility and specific heat measurements, we have confirmed the superconductivity is originated from a bulk. The transition temperature T_c decreases with increasing x (T_c = 2.8 K for x = 0.3 and T_c = 2.2 K for x = 0.5). This corresponds to the dicrease of the state density with increasing x, as evaluated from the magnetic susceptibility in the metallic state. The resistivity with x > 0.3 shows unusual temperature dependence in the nomal state. We will discuss the origin of the newly discovered superconductor as well as that of the M-I transition.

4PO8-32 Metal-insulator transition and superconductivity in the thiospinel, Cu_1-xZn_xIr₂S₄ studied by Cu-NMR

Atsushi Goto, Tadashi Shimizu, Guanghan Cao, Hiroyuki Suzuki, Hideaki Kitazawa and Takehiko Matsumoto, National Research Institute for Metals, Tsukuba, Ibaraki 305-0047, Japan

Presenting Author: A. Goto

Metal-insulator transition (M-I-T) and superconductivity in the thiospinel, Cu_1-xZn_xIr₂S₄ have been investigated by means of ⁶³Cu-NMR. This system is known to show first order M-I-T with lowering T. The transition temperature (T_c) decreases with increasing x, and the metallic phase is stabilized down to low T for x>0.3. We find that in the region with 0<x<0.3, a uniform metallic phase above T_c is split into two regions below T_c, each representing metallic and insulating phases, respectively. The Knight shift and 1/T1 data have revealed that the insulating region is non-magnetic, as that in the parent compound, CuIr₂S₄. These features make clear contrast to those in the case of the Se substitution into the S sites, where the M-I-T occurs uniformly. In the region with x>0.3, superconductivity appears at low T (<3K). We observe a coherence peak in 1/T1 just below the superconducting transition temperature. The same mechanism as that in the isostructural compound, CuRh₂S₄ is expected to be responsible for this superconductivity.

4PO8-33 Superconducting Properties of CuS_2-xSe_x under High Pressure

Yoshihiko Takano 1, Naoaki Uchiyama 2, Shinji Ogawa 2, Nobuo Mori 3, Yoshihide Kimishima 4, Shunichi Arisawa 1, Akira Ishii 1, Takeshi Hatano 1, and Kazumasa Togano 1. 1 National Research Institute for Metals. 1-2-1 Sengen, Tsukuba 305-0047, Japan. 2 Dept. of Appl. Sci, Tokyo Denki Uni. 3 Institute for Solid State Physics, Univ. of Tokyo. 4 Dept. of Physics, Yokohama National Univ.

Presenting Author: Y. Takano

Copper dichalcogenide compounds CuS_2-xSe_x with pyrite structure have been synthesized at a pressure of 5GPa with a cubic anvil press. Electrical resistivity was measured between room temperature and 0.5K using a ³He refrigerator. All the samples showed metallic behavior and exhibited superconductivity below 1.5-2.5K. Magnetic susceptibility was measured with a SQUID magnetometer under various pressures. Nearly hydrostatic pressure was generated in the sample space filled with a pressure-transmitting medium of a 1:1 mixture of fluorinert FC70 and FC77. With increasing pressure, superconducting transition temperature of CuSe₂ was reduced. The relation between the pressure and substitution effect of superconducting transition temperature will be presented.

4PO8-34 NMR Study on Metal-Insulator Transition of Spinel CuIr₂(S_1-xSe_x)₄ and (Cu_1-xNi_x)Ir₂S₄

K. Kumagai 1, S. Tsuji 1, K. Kakuyanagi 1, R. Endoh 2, and S. Nagata 2. 1 Division of Physics, Graduate School of Science, Hokkaido University, Sapporo 060-0081, Japan. 2 Department of Materials Science and Engineering, Muroran Institute of Technology, Muroran 050-8585, Japan.

Presenting Author: K. Kumagai

The metal-insulator transition (MIT) is discovered at T_MI ~ 226K in CuIr₂S₄ [1]. The MIT of CuIr₂S₄ is accompanied with a structural change from the cubic to tetragonal symmetry with a volume contraction of 0.7%. In order to elucidate the origin of the MIT in the Cu-spinels, it is important to clarify the valence state and magnetic properties in the insulating state. We have investigated the metal-insulator transition (MIT) in CuIr₂(S_1-xSe_x)₄ and (Cu_1-xNi_x)Ir₂S₄ by Cu-NMR. The decreases of the nuclear spin-lattice relaxation rate and the core-polarization contribution to the Knight shift show clearly the existence of the MIT in the range of x<0.8 for CuIr₂(S_1-xSe_x)₄. The transition temperature, T_MI, in CuIr2(S1-xSex)4 changes linearly with x³, indicating the predominant atomic size effect on the MIT.

Recently, superconductivity is discovered near the boundary of metal-insulator transition for (Cu_1-xNi_x)Ir₂S₄ [2]. Substitution of Cu site by a small amount of Zn or Ni induces the region of metallic phase. We will discuss the change of magnetic nature associated with the MIT in terms of our NMR results for CuIr₂(S_1-xSe_x)₄ and (Cu_1-xNi_x)Ir₂S₄.

[1] S. Nagata, et al., Physica B 194-196 (1994) 1077.

[2] T. Furubayashi, et al., private communication.

4PO8-35 Crystal Growth and Study of the Metal-Insulator Transition of BaCo_0.9Ni_0.1S_2-y

W.J. Zhu, Y.S. Song, and P.H. Hor, Department of Physics and Texas Center for Superconductivity,

University of Houston, Houston, TX 77204-5932

Presenting Author: W.J. Zhu

BaCo_0.9Ni_0.1S_2-y single crystals have been grown by the self-flux method. The nickel content of crystals tends to be higher than the starting composition. The metal-insulator transition was observed at about 200 K (warming up) and 160 K (cooling down). Although the normal magnetic states for the ab plane and the c-axis are quite different, both magnetic susceptibilities drop concurrently at transition. Crystals break easily after thermally cycled through M-I transition. Anisotropic conductivity and magnetic properties will be presented.

4PO8-36 On the Minimum of T_c in Superconducting Solid Solutions Pb_1-xSn_xMo₆S₈

Mark O. Rikel 1,2 and Victor Tsebro 2. 1 Applied Superconductivity Center, University of Wisconsin, Madison, WI 53705, USA. 2 Lebedev Physics Institute, Russian Academy of Sciences, Moscow, 117924 Russia.

Presenting Author: M.O. Rikel

The T_c(x) and lattice parameters of (Pb_1-xSn_x)_nMo₆S₈ single-phase (n = 1) and multiphase (n = 2) samples were measured. We estimated the positions of the tie lines between the Chevrel phase (CP) and liquid (Pb, Sn) in the Pb-Sn-Mo-S system and found that the x = 0.5 compound is in equilibrium with the (Pb + 87±5% Sn) melt at 1050°C. Based on this result, we reexamined all existing T_c(x) data on this system and showed that they can be presented as a universal D T_c/T_c(x) curve (D T_c is the deviation from the linear behavior) with the minimum at the equiatomic x = 0.5 composition.

A strong and almost unique correlation between D T_c/T_c(x), mean-square lattice distortions, and disorder contribution to resistivity was found for different CP-based solid solutions. Our results indicate that, in Chevrel phases, the influence of disorder on T_c prevails over the charge-carrier density effects.

4PO8-37 T_c enhancement in binary Mo₆Se₈ by doping

Octavio Peña 1, Anita Corrignan 2, Christelle Hamard 2, Francoise Le Berre 2, and Andrzej Wojakowski 3. 1 LCSIM - UMR 6511 CNRS, Université de Rennes 1, France. 2 LCSIM-UMR 6511, Université de Rennes 1, France. 3 Inst. Low Temp. and Struct. Research, 50950 Wroclaw, Poland.

Presenting Author: O. Peña

An important T_c increase has been systematically observed in the Chevrel-phase binary compound Mo₆Se₈ (T_c = 6.45 K) after insertion of very small quantities of ions at the origin site of the rhombohedral crystalline structure. The amount of inserted atoms is limited to 10 % of the regular vacancies. Several important features will be reported and discussed: i) the T_c increase can be as large as 0.7 K, reaching a saturation limit at 7.15 - 7.2 K in the doped specimens. Absolutely no degradation of the superconducting properties was ever observed; ii) a threshold value for the ionic radius (r ~ 1.05Å) is observed, with a systematic increase of T_c for larger-sized ions ; no increase of T_c was observed for ionic radii below this value; iii) the T_c saturation value is reached much quicker, as a function of doping, for ions having the largest ionic radii; iv) rare-earth elements, in particular highly magnetic ions such as divalent Eu (gJ= 7) or trivalent Ce (gJ = 2.14), can be inserted without destroying superconductivity.

Experimental data will concern more than 20 different dopants, including rare-earth, alkaline-earth and transition elements. Some anomalous cases (In, Cu or Ag) will be also discussed.

4PO8-38 Magnetic Field Induced Reentrant Superconductivity

C.A.R. Sa de Melo, School of Physics, Georgia Institute of Technology, Atlanta, GA, 30332-0430

Presenting Author: C.A.R. Sa de Melo

New experimental results in organic superconductors [1] suggest the possibility of a striking magnetic field induced reentrant superconducting phase at high magnetic fields [2]. This seems to be the first experimental system that might exhibit reentrant superconductivity in a magnetic field without the presence of magnetic ions, as in the case of magnetic superconductors. [3] This new reentrant phase is explained theoretically as the result of triplet pairing and of a magnetic field dimensional reduction that takes advantage of the anisotropy of these superconductors. [4] In addition, when there is competition between singlet and triplet pairing, the superconductors can be singlet at low magnetic fields and triplet at high magnetic fields. This unusual phase diagram has two magnetic field regimes which are separated by a first order transition line where there is a clear discontinuity in the spin susceptibility from smaller values (singlet case) to larger values (triplet case) at low temperatures. [5]

[1] I.J. Lee, M.J. Naugthon, G.M. Danner and P.M. Chaikin, Phys. Rev. Lett. 78, 3555 (1997).

[2] D. Jerome, Nature 387, 235 (1997).

[3] O. Fischer, H. W. Meul, M. G. Karkut, G. Remenyi, U. Welp, and K. Maki, Phys. Rev. Lett 55, 2972 (1985).

[4] A.G. Lebed, JETP Lett. 44, 114 (1986); N. Dupuis, G. Montambaux, and C.A.R. Sa de Melo, Phys. Rev. Lett. 70, 2613 (1993).

[5] C.A.R. Sa de Melo, J. Superc. 12, 459 (1999).

4PO8-39 Competition Between Superconductivity and Magnetism in 2-d Organic Metals

S.L. Kakani, Institute of Physics, 4G45, Shastri Nagar Extension, Bhilwara-311001, India

Presenting Author: S.L. Kakani

In organic Superconducting systems which are quasi two dimensional, charge density waves (CDW) and superconductivity may occur. The T_c of the materials which present superconductivity and CDW or SDW is low, but the transition from the normal state to one of these states present a great interest [1,2]. Considering the following model Hamiltonian:

H=S a b _ip[e _i(p)d a b - m b *s _{Za b} H] C⁺_ipa C_ipa + 1/2S _ijlmV_ijlm S a b _pp’qC⁺_ip+qa C⁺_jp’qb C_mp’b C_lp,a m *b S a b _ij[H_ij(Q)s a _Zb S _pC⁺_ip+qa C_jpb

the first term describes the electronic conduction band in normal phase, the second term is phonon mediated electron-electron interection responsible for superconductivity and the last term represents the antiferromegnatic ordering of ions. Following equation of motion method and Green's function technique, we have derived equation for superconductivity order parameter, critical temperature, density of states, free energy and phase transition [3]. The model is applied to 2D organic superconductor k-(BedT-TTF)₂Cu(NCS)₂. The model explains coexistence of SDW state and superconductivity state in the system.

[1] S.L.Kakani, Superconductivity: Key Problems, Arihant Publishers, Jaipur India (1996)

[2] Yu. V. Sushko & K. Andres, Phys. Rev. B47,330 (1993)

[3] Anil Surana, R.K. Paliwal & S.L. Kakani, J. Superconductivity, to be published.