4PO5-30 Infrared Study of NdGdCuO₄ Crystal-Field Excitations

S. Jandl 1, Z. Ichalalène 1, P. Richard 1, V. Nekvasil 2, C.T. Lin 3, and M. Cardona 3. 1 Département de Physique, Université de Sherbrooke, Sherbrooke, Québec, Canada. 2 Institut of Physics, Czech Academy of Sciences Cukrovarnicka 10, 16200 Prague 6, Czech Republic. 3 Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany.

Presenting Author: S. Jandl

Infrared active crystal-field (CF) excitations, between the ⁴I_9/2 and ⁴I_11/2 manifold electronic levels of the Nd³⁺ ions, have been studied in NdGdCuO₄ at 4.2 K. Influence of the Gd³⁺ substitution on the infrared active CF excitations has been determined by comparison with the CF excitations of the parent undoped Nd₂CuO₄ and cerium doped Nd_1.85Ce_0.15CuO₄ compounds. The observed intermultiplet transitions are associated with two inequivalent Nd³⁺ ion sites in NdGdCuO₄. The unperturbed Nd³⁺ ion site G 6® G 6 and G 6® G 7 CF excitations similar to those observed around 1995, 2006, 2013, 2077, 2383 and 2414 cm^-1 in Nd₂CuO₄ and Nd_1.85Ce_0.15CuO₄ are slightly shifted to higher energies due to the a and c unit cell parameters contraction following the Nd³⁺ substitution by Gd³⁺. The perturbed Nd³⁺ ion site CF excitations in NdGdCuO₄ correspond to the observed CF excitations around 1975 and 2364 cm^-1 in Nd_1.85Ce_0.15CuO₄. They indicate the presence of low symmetry distortions in NdGdCuO₄ similar to the ones introduced by cerium doping in Nd₂CuO₄.

4PO5-31 Laser Irradiation Effects in YBaCuO Amorphous Films

J. Konopka 1,2, S.J. Lewandowski 1, A. Abal'oshev 1, P. Gierlowski 1, A. Klimov 1, V.D. Okunev 1,3, Z.A. Samoilenko 1,3, V.N. Varyukhin 3. 1 Institute of Physics PAN, Al. Lotników 32/46, 02-668 Warszawa, Poland. 2 IMST, Karl Friedrich Gauß Str. 2, Kamp Lintfort, Germany. 3 Donetsk Physico-Technical Institute, 340114 Donetsk, Ukraine.

Presenting Author: J. Konopka

The effect of laser irradiation on coupling energy of O and Cu atoms in YBaCuO thin films was investigated by measuring the changes in structure, optical transmittivity and conductivity of irradiated films. Photon energies of h=2.44 eV, h =2.15 eV (Cu vapor green laser) and h=5.01 eV (KrF excimer laser) were used for film irradiation.

Irradiation by lower photon energies enhanced the growth of the dielectric phase by destroying the metallic clusters. With charge transfer gap E_g = 1.75 eV for O^2p Cu^3d transition in the insulator, the photon energy in this case is sufficient to generate a bounded exciton. By contrast, UV irradiation influences mainly the clusters with metallic conductivity.

4PO5-32 Influence of Pr on electronic structure of plane oxygen in PrBa₂Cu₃O₇ studied by soft X-ray emission spectroscopy

N. Yamada 1, E. Kabasawa 1, J. Nakamura 1, Y. Tezuka 2, H. Yamazaki 1, S. Shin 2, M. Fujisawa 2, J.Z. Liu 3, R.N. Shelton 3, T. Iri 1, R.C.C. Perera 4, M.M. Grush 5, T. A. Callcott 5, and D.L. Ederer 6. 1 Department of Applied Physics and Chemistry, University of Electro-Communications, Chofu-shi, Tokyo 182-8585, Japan. 2 Synchrotron Radiation Laboratory, lnstitute of Solid State Physics, University of Tokyo, 3-2-1 Midori-cho, Tanashi-shi, Tokyo 188, Japan. 3 Department of Physics, University of California, Davis, CA 95616. 4 Lawrence Berkeley Laboratory, Berkeley, CA 94720. 5 Department of Physics, University of Tennessee, Knoxville, TN 37996. 6 Department of Physics, Tulane University, New Orleans, LA 70118.

Presenting Author: N. Yamada

Among the RBa₂Cu₃O₇ (R: rare earth element) structure high temperature oxide superconducting compounds, most of which have 90 K superconducting transition temperature, PrBa₂Cu₃O₇ is nonmetallic and nonsuperconducting. Fehrenbacher and Rice have presented an attractive model for this puzzle of high temperature superconductivity in cuprates. Then, Liechtenstein and Mazin made a band theoretical calculation for ABa₂Cu₃O₇ (A= Y, Pr, Nd) based on the Fehrenbacher and Rice model, and showed the drastic change of the electronic structure on doping Y with Pr. In previous conference, we presented the polarization dependence of the x-ray absorption spectra (XAS) near oxygen absorption edge in the specimens and could get information of the unoccupied electronic states of O(1) chain, O(2,3) in-plane and O(4) apical oxygens. We present in this report the occupied electronic states of O(1) chain, O(2,3) in-plane oxygens from the measurements of O-Ka x-ray emission spectra in PrBa₂Cu₃O₇ and YBa₂Cu₃O₇ single crystals. Polarization and excitation energy dependence of the O-Ka x-ray emission spectra (XES) was observed in both compounds. The evidence of hybridization between the Pr 4f and Opp orbitals was observed in the spectrum of O(2,3) in plane of PrBa₂Cu₃O₇, which was predicted by Fehrenbacher and Rice. The spectra are compared with the band theoretical calculation by Liechtenstein and Mazin.

4PO5-33 Polarization Analysis of X-ray Absorption Spectra in PrBa₂Cu₄O₈ Single Crystal: Hole State near O-1s Absorption Edge

Y. Ohura 1, S. Horii 2, S. Suzuki 1, E. Kabasawa 1, N. Yamada 1, H. Yamazaki 1, J. Nakamura 1, S. Shin 2, T. Iri 1, J.Z. Liu 3, and R.N. Shelton 3. 1 Department of Applied Physics and Chemistry, University of Electro-Communications, Chofu-shi Tokyo182-8585, Japan. 2 The Institute for Solid State Physics, The University of Tokyo, Minato-ku Tokyo 106-8666, Japan. 3 Department of Physics, University of California, Davis, CA 95616.

Presenting Author: Y. Ohura

PrBa₂Cu₃O₇ (Pr123) is unique in having semiconductivity and not superconductivity, and having an isostructure with superconducting YBa₂Cu₃O₇ (Y123). PrBa₂Cu₄O₈ (Pr124) has homologous structure of PrBa₂Cu₃O₇ and is also non-superconducting as PrBa₂Cu₃O₇ compound. In order to understand the mechanism of disappearance of superconductivity by Pr in these compounds, polarization analysis of x-ray absorption spectroscopy (XAS) of PrBa₂Cu₄O₈ near the O-1s absorption edge was performed at KEK-PF BL-19B. XAS spectra directly reflect a hole state density, and it is possible to pick up the signal from specific site with combining the polarization analysis. XAS spectra for E//a- and b-axes of untwined PrBa₂Cu₄O₈ single crystal were measured, and the unoccupied states of plane and chain sites were deduced from these analysis. The spectrum for E//a has one broad component above Fermi level, and is very similar to the one of PrBa₂Cu₃O₇. This component is specified to be upper Hubbard band and Fehrenbacher-Rice state. On the other hand, the spectra for E//b shows sharp peak at the lower energy region than E//a spectra. The present results were compared with the previous results of polycrystalline YBa₂Cu₄O₈ and PrBa₂Cu₄O₈ samples and single-crystalline PrBa₂Cu₃O₇ and YBa₂Cu₃O₇ samples.

4PO5-35 The underdoped-overdoped crossover in YBa₂Cu₃O_x

Jürgen Röhler, Universität zu Köln, D-50937 Köln, Germany

Presenting Author: J. Roehler

The optimum doped state of YBa₂Cu₃O_x (T_c=92 K at x=6.92) exhibits several remarkable electronic and structural properties related to its physics along the c-axis. Normal state transport properties along c, e.g. the electric resistance r _c, are reduced, hence the large 2D anisotropy. On the other hand the superconducting anisotropy is strongly enhanced as evidenced by a clear minimum in the coherence length along c, x _c. Among the structural properties the doping induced c-axis lattice strain changes its sign. The static out-of-plane displacements of the planar Cu₂ atoms (dimpling) shows a maximum in their doping dependency, wheras the Raman shift of the out-of-plane O2,3 in-phase mode (A_1g) exhibits a significant kink. The structural changes around x_opt do not lift the orthorhombic symmetry, but deform the elastic strain pattern from underdoped "a -ortho" to overdoped "b -ortho". Overdoping expands the c-axis and compresses strongly the a-axis, but affects only weakly the b-axis. Hence the overdoped orthorhombicity increases by compression along a, instead by expansion along b (the underdoped case).

We discuss possible mechanisms driving the changes of the electronic and atomic structures at x_opt: the destructive role of the van Hove singularity at E_F for the superconductivity , and the correlations of the planar charge ordering effects with the out-of-plane degrees of freedom, and interlayer couplings.

4PO5-36 Magnetic ordering and superconductivity concomitance in La₂CuO_4+x single crystals

V. Pomjakushin 1, A. Balagurov 2, and A. Zakharov 3. 1 Laboratory for Neutron Scattering, ETH Zurich and Paul Scherrer Institute, CH-5232 Villigen PSI. 2 Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, 141980, Dubna, Moscow Region. 3 MAX-Lab Lund University, Box 118 S-221 00 Lund, Sweden.

Presenting Author: V. Pomjakushin

The superconducting La₂CuO_4+x (x~0.04) single crystals with low oxygen mobility was studied by µSR and neutron diffraction. Despite the fact that all crystals are inside the miscibility gap (0.01<x<0.06), only crystals with x>0.03 show a macroscopic phase separation (PS) at T_PS=250K according to neutron diffraction data. The crystals with x~0.03 remain in single phase down to helium temperatures. All crystals undergo a transition to an ordered low temperature magnetic state (LTMS) below T_LTMS=8-25K. In addition, the PS-crystals show antiferromagnetism (AFM) with high Neel temperatures T_N>100K. The transition to the LTMS was observed only by µSR as the increasing (or appearance) of the AFM-fraction of the crystal, but the LTMS was not detected in neutron diffraction as the respective commensurate AFM Bragg peaks. T_LTMS is also revealed as a maximum in the temperature dependence of the longitudinal muon spin relaxation rate measured in LF-µSR experiment. The LTMS-transition is always close to the onset of superconductivity T_c (T_LTMS~T_c), not depending on presence of the macroscopical phase separation. These effects are treated as an additional microscopic phase separation which occurs in the vicinity of T_c.

4PO5-37 Phase Separation in La₂CuO_4+d (0.01 £ d £ 0.055)

Ling Chen, J.Q. Li, and Z.X. Zhao, National Laboratory for Superconductivity, Center for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China

Presenting Author: L. Chen

A phase separation (PS) is revealed by low-temperature TEM investigation in La₂CuO_4+d prepared by electrochemical oxidation. It is found that materials with 0.01 £ d £ 0.055 partially go a phase transition from B_mab to F_mmm. When temperature is lower than 50K, the diffraction intensity of weak satellite spots of (0kl) (k+l=2n+1) in B_mab space group decreases sharply. The crystal structure can be depicted approximately as F_mmm. This kind of PS is induced by lattice distortion originated from rotation of CuO₆. The inhomogeneity in PS is intrinsic.

4PO5-38 A Spectroscopic Study of YBa₂Cu₃O_7-y and Ba₂Sr₂CaCu₂O₈ Single Crystals by Tunneling Spectroscopy

B. Susla 1, M. Bode 2, B.V. Veal 3, M. Kaminski 1. 1 Institute of Physics, Pozna University of Technology, Piotrowo 3, 60-965 Pozna, Poland. 2 Institute of Applied Physics University of Hamburg, Jungiusstr. 11, D-20355 Hamburg, Germany. 3 Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 USA.

Presenting Author: B. Susla

Tunneling spectroscopy is one of the most powerful spectroscopic technique for the study of low-energy inelastic excitations as well as the electronic density of states in cuprate superconductors. Experimental method provides some information about the electronic structure, coupling mechanism of superconductors, the size of the energy gap, phonon density of state, and magnitude of the electron-phonon coupling. In this study the point contact tunneling spectra of the Ba₂Sr₂CaCu₂O₈ single crystals using gold (Au), lead (Pb) and niobium (Nb) points, were measured for a variety of point penetration depths. The scanning tunneling spectroscopy (STS) experiments were performed in a UHV - system with a base pressure below p = 5x10^–11 mbar. In order to determine the dI/dV signal by the lock-in technique, the feedback circuit was switched of and an ac component (U_mod=5mV, n =325 Hz) was applied between the scanning tunneling microscope tip and the surface. The point-contact spectroscopy measurements at 4.2 K reveal Andreev reflection with a superconducting energy gap structure as well as clear evidence for electron-phonon coupling. We discuss the tunneling spectra, which showed zero-bias conductance at 77 K. We present also results from STS in low temperature, which reveal systematic differences in the electronic structure.

4PO5-39 A Rigorous Treatment of Tunneling in NS-Junctions*

Xin-Zhong Yan 1,2 and Chia-Ren Hu 1. 1 Dept. of Physics, Texas A&M Univ., College Station, TX 77843-4242. 2 Inst. of Physics and Ctr. for Condens. Matter Phys., Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China.

Presenting Author: X.-Z. Yan

On the basis of Green's-function method of non-equilibrium systems (i.e., the Keldysh formalism), we develop a theory of single particle tunneling in normal-metal-superconductor (NS) junctions. The tunneling current in a junction under a finite voltage is expressed in terms of the renormalized non-equilibrium Green's functions. Based on a tunneling Hamiltonian, we calculate the conductance of an NS junction. We will also compare the present treatment with the well-known scheme by Blonder, Tinkham, and Klapwijk.

*Work supported by Texas Higher Education Coordinating Board and the Texas Center for Superconductivity at the University of Houston.