4PO8-50 Superconducting properties of LuNi₂B₂C films and tunnel junctions

Giuseppe Grassano 1, Carlo Ferdeghini 1, Maria Iavarone 2, Ruggero Vaglio 2, and Fabio Canepa 3. 1 INFM-Dipartimento di Fisica,Universita` di Genova, via Dodecanneso 33, 16146 Genova. 2 INFM-Dipartimento di Scienze Fisiche, Universita’ di Napoli Federico II, Piazzale Tecchio 80, I-80125, Napoli, Italy. 3 INFM-Dipartimento di Chimica,Universita` di Genova, via Dodecanneso 35, 16146 Genova.

Presenting Author: M. Iavarone

LuNi₂B₂C films have been prepared by a Pulsed Laser Deposition technique in high vacuum condition using a multitarget deposition system. A special set-up for the in situ interchanging of shadow masks has been developed in order to deposit the different patterned layers. This allows the device fabrication without breaking the HV condition and preserving the quality of the interfaces. The deposition parameters of LuNi₂B₂C single layers have been optimised to reach good superconducting, structural and morphological properties with high reliability. We report also the study of the growth of different materials used as buffer and/or barrier layers. New results on the properties of superconducting SIS (N) Josephson/tunnel junctions based on borocarbide thin films are also presented and discussed. The study of the Josephson tunnel characteristics allows to clarify open questions on the superconducting behavior of these materials.

4PO8-51 Superconductivity, magnetic ordering, and its interplay in HoNi₂¹¹B₂C

Armin Dertinger 1, Andreas Kreyssig 2, Clemens Ritter 3, Michael Loewenhaupt 2, and Hans F. Braun 1. 1 Physikalisches Institut, Experimentalphysik V, Universität Bayreuth, D-95440 Bayreuth, Germany. 2 Institut für Angewandte Physik, Technische Universität Dresden, D-01062 Dresden, Germany. 3 Institut Max von Laue - Paul Langevin, 156 X, F-38042 Grenoble, France.

Presenting Author: A. Dertinger

We investigated the low-temperature properties of polycrystalline HoNi_2+x¹¹B_2-xC samples by means of AC-susceptibility measurements and neutron powder diffraction in the temperature range of 1.5 to 10 K and zero DC-magnetic field. By varying the value of x between -0.1 and 0.1, one has access to samples with a large variety of different chemical compositions ranging from boron- to nickel-rich which are not available for single crystals at the moment. This offers an ideal possibility to study the interplay between the complex set of magnetic structures and the occurence of superconductivity.

Within the homogeneity range of HoNi₂¹¹B₂C we find samples showing superconductivity with a transition temperature of 8.7 K, co-existing with the antiferromagnetic order of the holmium-moments below about 5 K, samples showing re-entrant behaviour with a relatively low superconducting transition temperature of 6.8 K, as well as samples remaining in a non-superconducting state down to at least 1.5 K, which exhibit antiferromagnetism below the magnetic ordering temperature. Correlated with the changes in the electronic properties, we observe a strong effect of the variation of the chemical composition on both the evolution and the volume fractions of the three different magnetic structures.

4PO8-52 Magnetic and Transport Properties of (Lu_1-xYb_x)Ni₂B₂C

Shi Li 1, M.C. de Andrade 1, R.P. Dicky 1, A. Amann 1, M.B. Maple 1, K.D.D. Rathnayaka 2, D.G. Naugle 2, S.L. Bud'ko 3, P.C. Canfield 3, and W.P. Beyermann 4. 1 Department of Physics, University of California, San Diego, La Jolla, CA 92093-0319. 2 Department of Physics, Texas A&M University College Station, TX 77843-4242. 3 Ames Laboratory and Department of Physics, Iowa State University, Ames, IA 50011. 4 Department of Physics, University of California, Riverside, CA 92521.

Presenting Author: S.L. Li

The pseudo-quaternary Lu_1-xYb_xNi₂B₂C (0 £ x £ 1) compounds exhibit a systematic progression from high temperature superconductivity (T_c =16.3 K, x = 0) to heavy fermion behavior (x = 1). We have studied single crystals of Lu_1-xYb_xNi₂B₂C by means of magnetic susceptibility c(T), magneto resistance R(T), and thermoelectric power S(T) measurements. The single crystals of Lu_1-xYb_xNi₂B₂C were prepared by a flux growth method. The magnetic susceptibility c(T) measured from 2 K to 300 K with magnetic field applied parallel to the ab-plane or the c-axis, showed Curie-Weiss behavior at high temperatures and a non-magnetic behavior at low temperatures, separated by a characteristic temperature T₀. The characteristic temperature T₀ decreases with increasing x and is always much larger than the superconducting critical temperature T_c. The R(T) data were measured from 100 mK to 300 K and revealed an evolution from superconducting, to Kondo scattering, and to Kondo lattice behavior as x increases from 0 to 1. The T_c vs x curve does not indicate re-entrant superconductivity down to 100 mK, and has a shape that suggests a pair-weakening mechanism is responsible for the suppression of superconductivity. The S(T) data from 2 K to 300 K correlate well with the electrical resistivity measurements.

4PO8-53 (T_c,TN)-x Phase Diagram of Er_1-xDy_xNi₂B₂C System

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

Presenting Author: S.X. Cao

The variations of the superconducting transition temperature T_c and the magnetic transition temperature T_N as functions of the Dy concentration x in samples of Er_1-xDy_xNi₂B₂C are systematically investigated by the measurements of electrical resistivity, magnetization, and specific heat. The measurements were carried out in the temperature range from 2 to 20 K. 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 pure compound ErNi₂B₂C has T_c>T_N, and Dy_xNi₂B₂C has T_c<T_N. Therefore, this dilution series allows the investigation of the cross-over region for which T_c < T_N. A second magnetic phase transition T_m is observed for Er_1-xDy_xNi₂B₂C compounds with x=0.29, 0.30, and 0.31, with a concomitant suppression of T_c. The similar second magnetic phase transition Tm and suppression of T_c are also observed for the compounds with x=0.6, 0.7, and 0.8. These results provide the (T_c,T_N)-x phase diagram of Er_1-xDy_xNi₂B₂C system.

4PO8-54 The angle-dependence of the upper critical field of HoNi₂B₂C thin films

Kerstin Haese, Bernhard Holzapfel, and Ludwig Schultz, IFW Dresden, Helmholtzstr. 20, Dresden, 01069, Germany

Presenting Author: K. Haese

For the magnetic superconductor HoNi₂B₂C we report on the out-of-plane angle-dependence of the upper critical field H_c2. We observed a strong H_c2 anisotropy in polycrystalline, c-axis oriented thin films of this material. Especially, the shape of the resistive transitions shows an angle dependence in a magnetic field, if the field is applied in a certain angle towards the (001) direction. This angle dependence results from different magnetic structures, which forms at a given temperature and the behavior is, therefore, also temperature dependent. At 2K an angle dependent magnetoresistance above the upper critical field could be observed, which is not present at 5K and 6K. On the other hand at 5K and 6K a systematical narrowing of the resistive transition with increasing deviation of the magnetic field from the (001) direction occurs. In the absence of a magnetic field the sample shows the superconducting transition at 8.8K and no reentrant behavior. We discuss the influence of the magnetic structures of the Ho moments and compare it with the also measured out-of-plane angle-dependence of the upper critical field of nonmagnetic YNi₂B₂C thin films.

4PO8-55

L.-S. Hsu, National Chang-Hua University of Education, Department of Physics, Chang-Hua 55058, Taiwan

Presenting Author: L.-S. Hsu

The electronic structures of five polycrystalline YNi_2-xCo_xB₂C 9x=0, 0.05, 0.1, 0.15) absorption thresholds, and Ni (Co) K edges. The Ni (Co) 2p core-level spectra yield the absorption spectra are compared with band-structure calculations. The VB satellite is resonantly 23 for the pure compound and roughly 0.2 for the x-0.15 compound. These values are constant, and the bonding characters between the Ni (Co) and B atoms are determined.

4PO8-56 Time-Correlated Tunneling of Solitons in Charge and Spin Density Waves

J.H. Miller, Jr. 1,2, A.W. Beckwith 1,2, E. Prodan 3, and C. Ordoñez 1. 1 Department of Physics , University of Houston, Houston, TX. 2 Texas Center for Superconductivity, University of Houston, 4800 Calhoun Rd., Houston, TX 77204-5932, USA. 3 Department of Physics, Rice University, 6100 Main, Houston, TX 77005-1892 USA.

Presenting Author: J.H. Miller, Jr.

Charge and spin density waves are strongly correlated electron systems observed in reduced dimensional conductors, including oxides (K_0.3MoO₃ and Rb_0.3MoO₃), transition-metal trichalcogenides (NbSe₃, TaS₃, and NbS₃), and Bechgaard salts. A density wave can collectively transport an electric current when an applied field exceeds a sharp depinning threshold.

We consider a model in which an electric field induces quantum nucleation of kink-antikink pairs in a pinned charge or spin density wave. Pair nucleation events, prevented by Coulomb blockade below a pair creation threshold, become correlated in time above threshold, by analogy to time-correlated single-electron tunneling. The model provides a natural explanation for the observed: (1) lack of significant density wave phase polarization below threshold in NbSe3 and other materials, (2) narrow band noise, (3) coherent oscillations, and (4) mode-locking at high drift frequencies.

4PO8-57 Formation of Large and Nearly Small Strong-Coupling Bipolarons and Their Relevance to the High-T_c Superconductivity

S. Dzhumanov, A.A. Baratov, U.T. Kurbanov, and B. Yavidov, Institute of Nuclear Physics, 702132, Tashkent, Uzbekistan

Presenting Author: S. Dzhumanov

We present a complete quantitative pairing theory of dressed carriers (i.e. polarons) describing the formation of large delocalized (D) and nearly small localized (L) bipolarons in the underdoped high-T_c superconductors (HTSC). Using different varitional wave functions we have calculated the ground state of an interacting system with two carriers and crystal lattice in the real (r)-space within the continuum model and adiabatic approximation taking into account the combined effect of the short- and long-range electron-lattice interactions in absence and presence of interelectron correlation. The binding energies and characteristic sizes of r-space D and L bipolarons and their accurate formation conditions (or stability regions) are determined as a function of the ratio of high-frequency dielectric constant to static dielectric constant. The phase diagrams for D and L bipolaronic states are presented. The above continuum theory of r-space (bi)polarons is extended to defect systems. At low doping, D and L (bi)polaronic states are formed as the discrete energy levels or in-gap states in the charge-transfer gap of cuprate insulators in accordance with experimental data. At large doping, these r-space bipolarons will convert into k-space ones (i.e. strong-coupling Cooper pairs) which later on are condensed at different temperatures and total doping dependence of T_c has two inverse-parabolic form sometimes with asymmetric maximums as it frequently is observed in doped HTSC.

4PO8-58 An Optimal Phonon Approach to the Spin Peierls Model with Non Adiabatic Phonons

Barry A. Friedman, Physics Department, Sam Houston State University, Huntsville, TX 77341

Presenting Author: B.A. Friedman

We have investigated the spin Peierls model treating the lattice quantum mechanically using an optimal phonon method. For small systems, the optimal phonon method is compared to truncation techniques. There is good agreement between the two methods, however the optimal phonon method is superior in that it achieves greater basis reduction and the optimal phonons are relatively insensitive to system size. Using the optimal phonons from small systems and the density matrix renormalization group a soliton-antisoliton pair is studied. Our calculation resolves a factor of four discrepancy between adiabatic theories and experiment.

4PO8-59 Interaction effects on persistent currents in the presence of magnetic frustration

Hyeonjin Doh, Juyeon Yi, and Sung-Ik Lee, Physics Dept., Pohang University of Science and Technology, Pohang, 790-784, Korea

Presenting Author: H. Doh

The persistent current in Hubbard model specially in the presence of magnetic frustration is investigated. The system of electrons confined to a cylindrical geometry is subjected to a solenoid flux threading its center as well as a magnetic field perpendicular to the face. Here the perpendicular magnetic field introduces frustration f=p/q which is the flux per plaquette in units of the flux quantum. The persistent current is decreased by on-site Coulomb repulsion. The effects of the frustration for odd number of q is enhanced the Coulomb interaction, but it is opposite for the even number of q.