2PO9-100 Structure and transport properties of novel fullerides A_xC₇₀ (A=Ba and Sm)

X.H. Chen 1, Z. Sun 1, L.Z. Cao 1, D.H. Chi 2, T. Takenobu 2, and Y. Iwasa 2. 1 Structural Research Laboratory and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China. 2 Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan.

Presenting Author: X.H. Chen

A series of new stable fullerides A_xC₇₀ (A=Sm, Ba) has been synthesized using solid state reaction. The LeBail refinement on high resolution synchrotron X-ray diffraction data shows that A₃C₇₀ (A=Sm, Ba) has a monoclinic structure with lattice parameters a=14.88(1) Å, b=10.0(1) Å, c=10.93(1) Å, and b =96.16(2) for A=Sm; a=23.55(1) Å, b=23.49(1), c=24.51(1) Å, and b =94.03(2) for A=Ba, respectively. The structure of Ba₃C₇₀ is derived from the A15 structure adopted by Ba₃C₆₀. Double of the lattice parameters for Ba₃C₇₀ relative to the A15 structure could arise from the same Ba cation vacancy ordering. The low symmetry relative to C₆₀ fullerides could be the intrinsic characterization and an important factor for the absence of superconductivity for C₇₀ fullerides. The transport mechanism is different x in the Sm_xC₇₀ system. The temperature-dependence resistivity can be explained by the three dimensional variable-range hopping mechanism for the samples A₃C₇₀ (A=Ba, Sm); while the transport properties for the samples Sm₆C₇₀ and Sm₉C₇₀ appear to be dominated by weak localization (WL) theory. It suggests that there exists at least one metallic phase in the Sm_xC₇₀ system.

2PO9-101 Local structure, percolation and insulator-conductor transition in doped HTS

Kirill Mitsen and Olga Ivanenko, Solid State Physics Dept., Lebedev Physical Institute, 117924, Moscow, Russia

Presenting Author: O.M. Ivanenko

In this work we are making the conclusion, that the transition from insulator to conductor state in HTS under doping passes through the particular intermediate concentration region when the negative U-centers originate. This concentration region coincides with HTS range. As proposed -U-centers are formed on the neighbour cations when the definite spatial arrangement of the localized doped carriers is realized. These -U-centers are the pair acceptors and the transfer of electron pairs from oxygen ions into -U-centers give rise the free hole carriers. The conductivity (and superconductivity) appears at the threshold of -U-center percolation.

2PO9-102 Hole Localization and Superconductivity in La₂CuO_4+d

Shusuke Yomo 1, Kazuhiro Soga 1, Zu-Gang Li 2, Pei-Herng 2, and Nobuo Mori 3. 1 Div. of Fundamental Studies and Dep. of Electronic and Info. Eng., Hokkaido Tokai University, Minami-ku, Sapporo 005-8601, Japan. 2 Dep. of Phys. and Texas Center for Superconductivity at the University of Houston, Houston, TX 77204-5932, USA. 3 Inst. for Solid State Physics, University of Tokyo, Kashiwanoha, Kashiwa 277-8581, Japan.

Presenting Author: S. Yomo

Hall coefficient of the oxygen-intercalated La214 cuprate was measured at room temperature and 77.4 K for d = 0.015-0.110. The Hall number defined as a quantity inversely proportional to the Hall coefficient was suppressed in the excess oxygen region above d = 0.030, contrary to the expected increase of hole carriers with oxygenation in the system. This demonstrates the existence of localized holes in the superconducting phases. This is consistent with the resistivity and the thermopower measurements. The d -dependence of the Hall number is discussed with the appearance of the electronically phase-separated superconducting phases.

2PO9-103 Estimations of the dislocation migration activation energies for HTSC ceramics from temperature dependence modeling of the measured velocity of the solitonic Wave of Change of Reflection and Conduction (WCRC).

E.M. Kudriavtsev 2, V.I. Emel’yanov 1, V.V. Krivov 2, and S.D .Zotov 2. 1 Moscow State University, Moscow, Russia. 2 Lebedev Physical Institute of RAS, Moscow, Russia.

Presenting Author: E.M. Kudriavtsev

Wave of Change of Reflection and Conduction (WCRC) presents a new solitonic transport phenomena. It is excited by IR laser pulse after threshold ~ 10 kW/cm² and has the following features: 1) the WCRC signal is of one sign, 2) its velocity U is nearly constant, 3) WCRC reflects from sample surfaces without changing the velocity.

Measured WCRC velocity temperature dependence, U(T), of ceramic HTSC samples YBaCuO, NdCeCuO and LaSrCuO in the region 200K < T < 400K [Physica C, 234-240(1994)1439] is going up with T- increasing for YBaCuO, NdCeCuO (and some other materials) but it goes down for LaSrCuO, which has also anomalous T_c dependence of sound velocity.

Measured U(T) dependence was analyzed by help of model of slow wave of dislocation recombination developed by V.I. Emel’yanov. The comparison of experimental and theoretical data gives the possibility of dislocation migration activation energies evaluation for studied materials. These values are the following: 0.001 eV; 0.02 eV and 0.04 eV for YBaCuO, NdCeCuO and LaSrCuO, respectively.

The indicated values are of interest to mechanical properties estimation of ceramic samples. Authors do not know other data for mentioned dislocation migration energy values. Suggested method could be useful for other materials evaluation including perspective HTSC ceramics.

2PO9-104 Influence of microwave irradiation on excitation of non-Josephson oscillation (NJO) in superconducting Sn films with PSCs and SNS structures

Oleksiy B. Agafonov 1, Georgii E. Churilov 1, Dmytro A. Dikin 1,2, Vitaly M. Dmitriev 1,3. 1 B.Verkin Institute for Low Temperature Physics and Engineering of National Academy of Sciences of Ukraine, 47 Lenin Ave., 310164 Kharkov, Ukraine. 2 Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA. 3 International Laboratory of High Magnetic Fields and Low Temperatures, 95 Gajowicka Str., 53-421 Wroclaw, Poland.

Presenting Author: Oleksiy B. Agafonov

In the superconducting thin films of some metals (e.g. Al, Sn) at certain conditions (some temperature and dc current density range) the low frequency (compared to Josephson frequency) oscillation of electric potential takes place. This phenomenon is called the non-Josephson oscillation (NJO). It is strongly related to the nonequilibrium quasiparticle dynamics in phase-slip centers (PSC), rising in thin films at dc transport current values exceeding the critical current value. We studied experimentally NJO excitation and its characteristics in tin (Sn) thin films both with PSCs and with artificial structures of Superconductor - Normal metal -Superconductor (SNS) type under the influence of the external electromagnetic field of wide frequency range. It was found that when the frequency of the field was of the order of 1010 Hz, the NJO excitation could be observed only in the very narrow field level range and only at the condition that SNS have been transformed to SS'S by the field influence. We have observed how an application of the external field changed gradually parameters of PSC and SNS, and how it led to NJO excitation at different, compared to unperturbed case, conditions (dc current, microwave field and temperature values).