The superconducting and normal-state properties of the "electron-doped" copper oxides are remarkably different from the "hole-doped" ones. The reasons for these differences (e.g. s- vs. d-wave symmetry, \rho \propto T^2 vs. \rho \propto T,...) are presently unknown. A comprehensive study of the normal-state properties of NCCO (Re = Nd) and PCCO (Re = Pr) is an important prerequisite for understanding these differences. From resistivity, Hall effect, thermoelectric power (TEP) and Nernst effect(X. Jiang et al, to be published), it can be shown that ReCCO is best described by a two-carrier model, where a positive component (hole-like) seems to be coincident to the appearance of superconductivity. We present a comparison of transport properties as a function of Ce and O content, which leads us to the conclusion that oxygen acts primarly as an impurity scatterer for the charge carriers and has little effect on the carrier concentration. We find a sudden change in the trend of transport properties as the Ce content crosses x = 0.15 .
[O10.02] Anisotropic Transport in Detwinned YBCO and PBCO Crystals
V.M. Browning, M.S. Osofsky, E.F. Skelton, S.B. Qadri (Naval Research Laboratory)
The most widely studied of the high temperature superconductors are the "123" cuprates of which YBa_2Cu_3O_7-d (YBCO) is a member. These materials are unique in that their structure consists of both Cu-O planes (common to all of the high T_c materials) and one-dimensional Cu-O chains. Although isostructural with the superconducting 123 materials, PBCO stands alone as the only rare-earth 123 material that is non-superconducting and non-metallic. It is well established that the Cu-O chains manifest themselves as an anisotropy in the a-b plane resistivity of the superconducting 123 materials, however the role of the Cu-O chains in PBCO is not yet well understood. Using detwinned YBCO and PBCO crystals we have conducted a study of the various transport properties (resistivity, magnetoresistance, Hall effect and magnetoresistance) in these two very different materials. These measurements will be presented and their implications for the role of Cu-O chains in the transport properties will be discussed.
[O10.03] Orbital Magnetoresistance in LaSrCuO Thin Films
Fedor Balakirev, Igor Trofimov, S. Guha, Peter Lindenfeld (Department of Physics, Rutgers University)
Measurements of resistivity, Hall effect, and magnetoresistance have been made from T_c to 350 K on c--axis oriented thin--film specimens of La_2-xSr_xCuO_4 with different values of x, and one specimen that also contains Nd. The specimens were grown on LaSrAlO_4 substrates by laser ablation, and shown by x--ray diffraction to be single--crystal films with their c--axis perpendicular to the film plane. The orbital magnetoresistance as a function of temperature is quite similar in all specimens, and does not seem to be simply related to the resistivity or the Hall angle, \Theta_H. The linear dependence of cot \Theta_H on T^2 is followed, except for an upturn at low T.
[O10.04] Anisotropic Resistivity of RNi_2B_2C (R = Ho, Er, Lu)
I.R. Fisher, J.R. Cooper (IRC in Superconductivity, University of Cambridge, UK.), P.C. Canfield (Ames Laboratory, Iowa State University, USA.)
Results are presented for the in-plane and c-axis resistivity of single crystals of RNi_2B_2C (R = Lu, Er, Ho). The c-axis resistivity is equal to the in-plane value, with only small deviations below 150 K. The in-plane resistivity of both magnetic (R = Er, Ho) and nonmagnetic (R = Lu) compounds have the same temperature dependence, with \rho _a of the magnetic compounds displaced upwards by a few \mu Ømegacm. A negative longitudinal magnetoresistance that scales as B/T in the normal-state of the magnetic compounds clearly shows that this constant difference in the resistivity of the magnetic and nonmagnetic compounds is due to spin-disorder scattering.
[O10.05] Electronic Transport in Ion-damaged NCCO Thin Films
S.I. Woods, A.S. Katz, J. Herrmann, S.S. Lau, M.B. Maple, R.C. Dynes (Department of Physics, University of California at San Diego; La Jolla, CA 92093-0319)
We have studied the temperature dependence of electronic transport in thin films of Nd_1.85Ce_0.15CuO_4-\delta as a function of ion damage. The samples are c-axis oriented laser-ablated 1000 Åfilms patterned into 100 micron wide Hall bars. The films are sequentially damaged with doses of 200keV Ne^+ ions. We have measured resistivity, carrier density, and magnetoresistance at several levels of damage to investigate how they evolve as superconductivity is destroyed.
[O10.06] Insulator-to-metal crossover near optimum doping in the normal state of the high-Tc compound LaSrCuO
G.S. Boebinger, Y. Ando, A. Passner (Bell Labs, Lucent Technologies, Murray Hill, NJ 07974), T. Kimura, M. Okuya, J. Shimoyama, K. Kishio (Appl Chem, Univ of Tokyo, Tokyo, JAPAN), K. Tamasaku, N. Ichikawa, S. Uchida (SRC, Univ of Tokyo, Tokyo, JAPAN)
The high-T_c cuprates exhibit a common evolution as the carrier concentration is increased from the undoped insulator: the insulating phase gives way to the superconducting phase and, eventually, to a metallic phase upon sufficient overdoping. The low-temperature behavior of the crossover from insulator-to-metal (I-M) is hidden by the superconducting phase. We utilize a 61-T pulsed magnetic field to suppress superconductivity and study this I-M crossover in the low-temperature normal-state resistivity as a function of carrier concentration in La_2-xSr_xCuO_4 single crystals. These experiments reveal an insulator-to-metal (I-M) crossover(G.S. Boebinger, et al, Phys. Rev. Lett, to be published Dec, 1996) for both in-plane resistivity \rho_ab and c-axis resistivity at a Sr concentration near optimum doping (x \simeq 0.16). All underdoped samples (x <0.16) show low-temperature insulating behavior, even in samples with linear-T \rho_ab above T_c and apparently large k_Fl \simeq 10.
[O10.07] Two Dimensional Conduction, and Coexistence of Localization and Superconductivity in Insulating Cuprates
C. C. Almasan , G. A. Levin, T. Stein (Kent State University), S. H. Han, D. A. Gajewski, M. B. Maple (University of California at San Diego)
We report measurements of out-of-plane (\rho_c) and in-plane (\rho_ab) normal-state resistivities of single crystals of insulating PrBa_2Cu_3O_7-\delta and superconducting Y_0.47Pr_0.53Ba_2Cu_3O_7-\delta (T_c\approx 18\;K) using a flux transformer method. In the latter specimen, the onset of superconductivity was suppressed by a magnetic field of 9\; T. We have found that the anisotropy \rho_c/\rho_ab of both samples increases monotonically at low temperatures with no signs of saturation. The temperature dependence of \rho_c/\rho_ab for PrBa_2Cu_3O_7-\delta is very well described by \rho_c/\rho_ab=a+bT^-2/3 over two decades of temperatures (1.9\; K\leq T< 200\; K). The same relationship holds for Y_0.47Pr_0.53Ba_2Cu_3O_7-\delta over more than one decade of T (1.9\; K\leq T< 70\; K). Both the absence of saturation and the T^-2/3 dependence of \rho_c/\rho_ab indicate a 2-dimensional (2-D) conduction. This means that the average in-plane hopping distance of the localized charge carriers increases with decreasing T according to Mott's øverlineR\sim T^-1/3 law, while the elementary step in the c-direction remains T independent, equal to the spacing between the bilayers. Our results also represent the first direct observation of hopping transport in a superconducting material, suggesting that pairing takes place between localized states. The work at KSU was supported by NSF under Grant No. DMR-9601839, and at UCSD by DOE under Grant No. DE-FG03-86ER-45230.
[O10.08] Planar tunneling in superconducting YBa_2Cu_3-xZn_xO_7 thin films as a function of crystallographic orientation.
E. Paraoanu, M. Covington, L. H. Greene (Dept. of Physics, University of Illinois at Urbana-Champaign)
Superconducting YBa_2Cu_3-xZn_xO_7 thin films are grown in four different crystallographic orientations. Resistivity vs. temperature measurements show that Zn strongly suppresses the critical temperature, T_c: As the Zn concentration is increased from x=0 to x=0.1, T_c decreases from 90K to 40K. Also with increased Zn concentration, the resistive transition broadens and the residual resistivity increases. Tunneling measurements into (103)-oriented films show that as x increases the Gap-Like Feature (GLF) scales in energy with T_c, indicating the superconducting state is being probed. We also find that the magnitude of both the GLF and the Zero Bias Conductance Peak (ZBCP) are reduced with increasing Zn concentration. A detailed study of the dependence of the ZBCP on applied magnetic field and temperature, for different Zn concentrations and different crystallographic orientations, will be presented.
Supported by ^\astThe National Science Foundation (NSF DMR 94-21957) and ^The National Science Foundation through the Science and Technology Center for Superconductivity (NSF DMR 91-20000). ^\ddaggerPresent address: NIST, Boulder, CO.
[O10.09] Anomalous Field Effect in Ultra-Thin Films of Metals Near the Superconductor-Insulator Transition
Gloria Martinez-Arizala, C. Christiansen, A.M. Mack, N. Markovic, A.M. Goldman (University of Minnesota)
\hyphenationpola-ri-ty \hyphenationsu-per-con-ducting
A field effect conductance modulation experiment has been performed on a sequence of nominally homogeneous ultrathin films of metals with thicknesses that traverse the superconductor- insulator transition. The conductance as a function of gate voltage, G(V_G), increases for either polarity of V_G for films on the insulating side of the transition, and decreases in a similar fashion for films below the transition region. The data implies a connection between the insulating and the superconducting states. A qualitative interpretation of these results suggesting the existence of localized Cooper pairs and superconducting fluctuations in the insulating state will be discussed.