4PO8-40 Superconducting properties of k -(BEDT-TTF)₂Cu(NCS)₂ organic superconductor from the reversible magnetization

Hyeong-Jin Kim 1, Jae-Hyuk Choi 1, Mun-Seog Kim 1, Sung-Ik Lee 1, and Jin-Tae Kim 2. 1 National Creative Research Initiative Center for Superconductivity and Department of Physics, Pohang University of Science and Technology,Pohang 790-784, Republic of Korea. 2 Korea Research Institute of Standards and Science,

P.O. Box 102, Yusong, Taejon 305-600, Republic of Korea.

Presenting Author: H.-J. Kim

Organic superconductor k -(BEDT-TTF)₂Cu(NCS)₂ with T_c @ 9.5 K has a highly anisotropic nature similar to the high-T_c superconductor.

Temperature dependence of magnetization was measured in the field range of 1 kOe £ H £ 5 kOe perpendicular to the conducting layers.

Near T_c, the reversible magnetization deviates from the mean-field behavior due to the positional distortion of vortices, which is reflected by the crossover of the magnetization curves at T @ 8.75 K.

Far below T_c where the fluctuation is diminished, the magnetization was analyzed using Hao and Clem's model and the thermodynamic parameters, such as the Ginzburg-Landau parameter k , the critical fields [H_c(0) and H_c2(0)] and the characteristic lengths [x (0) and l (0)], were obtained.

4PO8-41 Structural Study on Double Honeycomb Lattice Superconductors: A_xZrNCl and A_xHfNCl (A; Alkali Metals and Organic Molecules)

Shin-ichi Shamoto 1, Keigo Iizawa 1, Takeshi Koiwasaki 2, Masahiro Yasukawa 2, Shoji Yamanaka 2, Oleg Petrenko 3, Steve M. Bennington 3, Hajime Yoshida 4, Kenji Ohoyama 4, Yasuo Yamaguchi 4, Yasuhiro Ono 1, Yuzuru Miyazaki 1, and Tsuyoshi Kajitani 1. 1 Department of Applied Physics, Faculty of Engineering, Tohoku University, Sendai 980-8579, Japan. 2 Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Higashi-hiroshima 739-8527, Japan. 3 ISIS Science Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom. 4 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.

Presenting Author: S. Shamoto

Layered nitride chloride superconductors, A_xZrNCl (T_c ~15K) and A_xHfNCl (T_c ~20-25K) (A;alkali metals and organic molecules) have been found to have double honeycomb lattice as a conducting plane, which has an unique 2-dimensional band structure. The conduction band, which is partially filled by electrons supplied from alkali metals, has 3-fold nesting condition, although any structural phase transition in both compounds has not been observed yet. Larger density of states is expected for A_xHfNCl than for A_xZrNCl from the distance of the nearest neighbor Zr or Hf atoms, which have main contribution on their conduction bands. This is consistent with results about Pauli paramagnetic susceptibility of these compounds.

The transition temperature of Na_xHfNCl (T_cmax=20K) is somewhat lower than that of Li_xTHF_yHfNCl (T_cmax=25.5K), while the difference of T_c between Na_xZrNCl (T_cmax=15K) and Li_xTHF_yZrNCl (T_cmax=15K) can be ignored. If some distortion by CDW reduced T_c from 25.5K to 20K, a pressure would increase T_c due to lattice hardening. As for Na_xHfNCl (x~0.3, T_c=20K), however, a pressure effect on T_c was -0.013(6) K/kbar, leaving this phenomenon quite puzzling. Recent results of phonon measurement of Na_xHfNCl on MARI at ISIS will also be presented.

*4PO8-42 Pressure dependence of the transition temperature of the layered superconductors Y₂C₂Br_2-xI_x

R.W. Henn 1, K. Ahn 1, H.-A. Krug von Nidda 2, R.K. Kremer 1, and A. Simon 1. 1 Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany. 2 Universität Augsburg, Institut für Physik, Universitätsstraße 1, D-86159 Augsburg, Germany.

Presenting Author: R.W. Henn

Depending on the halogen constitution x, the superconductors Y₂C₂Br_2-xI_x exhibit transition temperatures T_c ranging from 5.05K for x=0 to 11.6K for x=1.5. A further increase of the I content reduces T_c to 9.97K for Y₂C₂I₂ (x=2). This finding was explained by the existence of a peak in the electronic density of states (DOS) close to the Fermi level: with increasing iodine content the Fermi energy is shifted with respect to this peak in the DOS due to a volume size effect. The change in the density of states at the Fermi energy N(eF) is reflected by a change in the transition temperature. If this explanation holds, then the effect on T_c induced by an increase of the unit cell volume should be reversed by applying hydrostatic pressure.

We present a study of the pressure dependence of T_c of samples of Y₂C₂Br_2-xI_x determined by SQUID magnetometry. T_c increases with hydrostatic pressure at a rate of 1.1K/GPa for Y₂C₂I₂, while the T_c of Y₂C₂Br₂ decreases at a rate of -0.2K/GPa. For x=1.5 close to the maximum T_c in the series Y₂C₂Br_2-xI_x the transition temperature is nearly independent of the applied pressure.

These observations give further support to the above suggestion, that in the Y₂C₂(Br,I)₂ series the Fermi energy is shifted over a peak in the DOS with increasing iodine content.

4PO8-43 Influence of Annealing Temperature on the Superconducting Properties of Scandium Boride Carbides

Zhanna M. Tomilo, Nina A. Prytkova, Paul V. Molchan and Sergey N. Ustinovich, Laboratory of Superconducting Materials Physics, Institute of Solid State and Semiconductor Physics, National Academy of Sciences of Belarrus, Minsk, 220072, Belarus

Presenting Author: Z.M. Tomilo

Superconducting polycrystalline samples of the Sc-Ni-B-C have been prepared by arc-melting in argon atmosphere by two-stage procedure [1]. Since ScNi₂B₂C compound is though to be metastable [1], the investigation of influence of annealing temperature on phase composition and superconducting characteristics of these samples was carried out. The as-melted samples were wrapped with Ta foil, placed into a quartz tube and annealed in the temperature range 400 to 1000° C. Annealing time was 24 hours as a rule; in some cases we used more prolonged annealing.

It may be concluded that annealing at 600° C results in the improvement of the superconducting characteristics of the scandium boride carbides samples as compared with the as-melted ones.

The superconducting stability properties of the as-melted ScNi₂B₂C samples were carried out. Phase composition and superconducting properties of scandium boride carbides samples are presented.

[1] H.C. Ku, C.C. Lai, Y.B. You, J.H. Shich, W.Y. Guan. Phys. Rev. B. 1994, V.50, N1, p.351-352.

4PO8-45 Superconductivity in Clean and Disordered Quarternary Nonmagnetic Carbides

Stefan-Ludwig Drechsler 1, Helge Rosner 1, Klaus Koepernik 2, Ingo Opahle 3, Helmut Eschrig 1, Sergey Shulga 4, Jens Freudenberger 1, Guenther Fuchs 1, Karl-Hartmut Mueller 1, Konstantin Nenkov 1, Ldwig Schultz 1, Andreas Kreyssig 2, Dieter Lipp 3, Alexander Gladun 3, F. Gegenwart 2, and T. Chichorek 2. 1 Inst. f. Festkoerper- u. Werkstofforsch. Dresden (IFW Dresden), D-01171 Dresden, Germany (IFW Dresden). 2 Max-Planck-Institut-Chemie u.Physik Fester Stoffe, D-01187 Dresden, Germany. 3 Technische Universitaet Dresden. 4 Inst. of Spectroscopy, RAS, Troitsk, Russia.

Presenting Author: S.-L. Drechsler

Disorder effects due to substitutions at the rare earth sites R, and at the transition metal sites T,in R_1-xR'_xC(T_1-yT'_yB)₂ compounds, where R,R'= Y,Lu,La, T=Ni,Pt,Co, are studied both theoretically and experimentally. Total and partial densities of states N(E) are calculated within the LDA-LCAO coherent potential approximation. Isoelectronic substitutions at R sites lead to nearly linear interpolations for N(0) and for the normalized jump of the electronic specific heat c_p at the critical temperature T_c, whereas T_c itself, the upper critical field H_c2(T), its positive curvature near T_c [1,2], and the TH^1-b -dependence of c_p in the mixed state are sensitive to the strength of disorder. Well-defined samples put the borocarbides into or close to the clean limit of type II superconductors. The disorder effect from T-sites is strong. The values of d²H_c2/dT²>0 near T_c, H_c2(0), and b provide a convenient measure of the disorder.

4PO8-46 Thermal Conductivity of RNi₂B₂C (R=Er, Tm, Gd) Systems

Shixun Cao, Katsuhiko Nishimura, and Katsunori Mori, Faculty of Engineering, Toyama University, 3190 Gofuku, Toyama 930-8555, Japan

Presenting Author: S.X. Cao

In order to investigate the interaction between electron and phonon in the borocarbide intermetallic superconductors, the temperature dependence of electrical resistivity and thermal conductivity have been measured for RNi₂B₂C (R=Er, Tm, Gd) systems under various applied magnetic fields. The measurements were carried out in the temperature range from 2 to 20 K and in the magnetic field up to 5 Tesla. Samples of the borocarbide superconducting compounds are prepared by arc-melting method. Powder X-ray diffraction (XRD) measurement shows that these samples are predominantly single phase of centered-tetragonal structure.

The compound ErNi₂B₂C has T_c=10.5 K, and shows reentrant behavior of superconductivity near T_N=5.5 K under applied magnetic fields of 1.0 to 1.5 Tesla. The temperature dependence of thermal conductivity of RNi₂B₂C (R=Er, Tm) shows a change in slope at T_c. In zero magnetic field, the thermal conductivity of ErNi₂B₂C shows a small peak at T_N=5.5 K, and it disappears under applied magnetic fields. At temperatures below T_c, the value of thermal conductivity in an applied magnetic field is larger than that in zero field for the borocarbide superconducting compounds.

4PO8-47 Thermal Conductivity of Rare-Earth Nickel Borocarbides

Brian D. Hennings 1, K.D.D. Rathnayaka 1, Donald G. Naugle 1, and P.C. Canfield 2. 1 Department of Physics, Texas A&M University, College Station, TX 77843 USA. 2 Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011 USA.

Presenting Author: B.D. Hennings

YNi₂B₂C and LuNi₂B₂C are relatively high T_c superconductors (T_c ~ 15K) for quaternary inter-metallic compounds. Recent neutron scattering experiments show anomalous phonon behavior below T_c for both YNi₂B₂C (Kawano, et. al. [1]) and LuNi2B2C (Dervenagas, et. al. [2]). Thermal conductivity measurements reveal both phonon and electron scattering mechanisms, and thus may provide information on the electron-phonon interaction and new phonon modes below T_c. Preliminary measurements indicate a strong enhancement below T_c in the thermal conductivity of LuNi₂B₂C, similar to the weaker enhancement below T_c in the thermal conductivity of YNi₂B₂C reported by Sera, et. al. [3]. Results and analysis will be presented.

[1] Kawano, et. al., Phys. Rev. Lett. 77, 4628, (1996).

[2] Dervenagas, et. al., Phys. Rev. B 52, R9839, (1995).

[3] Sera, et. al., Phys. Rev. B 54, 3062, (1996).

Work supported by the Robert A. Welch Foundation and the Texas Center for Superconductivity at the University of Houston (TCSUH).

4PO8-48 Superconductivity in RNi₂B₂C (R = Er, Dy, Ho, Y) Films Grown via Pulsed Laser Ablation

James M. Eyhorn and Donald G. Naugle, Department of Physics, Texas A&M University, College Station, TX 77843

Presenting Author: J.M. Eyhorn

Thin films of RNi₂B₂C (R = Er, Dy, Ho and Y) have been fabricated using the technique of Pulsed Laser Ablation. The films were deposited in vacuum onto MgO substrates held at temperatures ranging from 625 ° C to 800 ° C. No post deposition annealing was done. The ablation targets were made by arc-melting NiB, C and the appropriate rare-earth. The arc-melted buttons were then annealed, polished flat, and silver epoxied to the target holder. Resistance vs. temperature data from 20 K to 0.35 K will be presented. The superconducting transition temperature (T_c) is strongly affected by deposition temperature, as is the normal state resistance. The topography and structure have been investigated by atomic force microscopy, scanning electron microscopy and x-ray diffraction. The results will be discussed.

4PO8-49 A Photemission Study of the Valence Band of LaNi₂B₂C and Y_1-xLu_xNi₂B₂C (x=0, 0.5, 0.7)

E. Alleno 1, C. Teodorescu 2, N. Franco 2, E. Tominez 1, and C. Godart 1. 1 LCMTR-CNRS, GLVT, 2-8 rue Henri Dunant, 94320 Thiais, France. 2 LURE, Université Paris-Sud, Bâtiment 209D, 91405 Orsay Cedex, France.

Presenting Author: E. Alleno

YNi₂B₂C and LuNi₂B₂C are two intermetallic superconductors which belong to the RNi₂B₂C familly (R=rare-earth) with respective transition temperature T_c = 15.5K and 16.6K. In contrast to these rather « high » T_c, YNi₂B₂C with non magnetic La is not superconducting. According to band structure calculations, difference in the density of states (N(e )) at the Fermi level (e _f)could account for these different superconducting behaviours. Therefore we performed photoemission spectroscopy (PES) at the valence band of these compounds with constant electronic density, to check experimentally if there was a correlation between T_c and the observed PES intensity near ef. The spectra (hn=32eV and hn=80eV) of Y_(1-x)Lu_xNi₂B₂C (x= 0, 0.5, 0.7, T_c = 15K, 14.5K, 15.5K) and LaNi₂B₂C (non superconducting) display the same features i.e., a clear Fermi edge and a large peak at -1.3eV and a shoulder at -2.3 eV with only minor intensity differences. There is therefore no correlation between the observed intensity at the Fermi edge and T_c. We believe this imply N(e _f) is nearly constant in these compounds and that T_c is mainly driven by the electron-phonon interaction.