4PO5-40 Zero-Bias Anomaly of the Tunneling Spectroscopy on Different Crystal Orientation of YBa₂Cu₃O₇ Epitaxial Thin Films

Hong-Wen Cheng, B.C. Yao, C.C. Chi, and M.K. Wu, Material Science Center and Department of Physics, National Tsing Hua University, Hsingchu, Taiwan 300, R.O.C.

Presenting Author: H.-W. Cheng

We report the tunneling spectroscopy on (100), (110) and (001) surfaces of YBCO epitaxial thin films using planar junctions at various temperatures and magnetic fields. The magnetic fields were applied perpendicular and parallel respectively to the surfaces of the epitaxial YBCO thin films up to 6 teslas. Both the experimental spectra of YBCO (100) and (110) films show zero-bias conductance peak (ZBCP). When tunneling through the (100) YBCO films, we also observed a narrower peak (FWHM @ 0.5 meV, which could not be explained by the theory of Andreev Bound States) on the top of another broader one (FWHM @ 10 meV) at zero-bias voltage. At 4.2 K, the splitting of zero-bias conductance peak predicted by theory was not observed even when the perpendicularly applied magnetic field is up to 6 Teslas. Both the ZBCP of (100) and (110) YBCO films will be suppressed by the applied magnetic fields.

4PO5-41 Electron structure and phonon spectra anomalies in oxide HTS

A.I. Golovashkin 1, N.V. Anshukova 1, L.I. Ivanova 2, and A.P. Rusakov 2. 1 P.N.Lebedev Physical Institute RAS, Moscow, Russia. 2 Moscow Steel and Alloys Institute, Moscow, Russia.

Presenting Author: A.I. Golovashkin

The new results on the HTS phonon spectra with anomalous softening of longitudinal high frequency optical phonon W_LO were analyzed. It was shown that these data can be understood as a consequence of charge superstructure ordering in oxygen sublattice in addition to metal sublattice ordering. Such oxygen ordering can be short range and dynamical. The influence of ordering and doping on the electron spectrum and HTS Fermi surface was considered. It was shown that such a charge and spin ordering (CDW and SDW) produces the doubling of lattice periods and new Brillouin zone. Hole doping leads to the Fermi surface, which has sides parallel to [100] directions in addition to [110] ones. These results agree with experimental data. From our analysis, the phonon spectra anomalies may be associated with the nesting conditions between the congruent parts of Fermi surface.

4PO5-42 ¹⁷O Hyperfine Anomalies in La₂CuO₄

R.E. Walstedt and S-W. Cheong, Department of Physics, Rutgers University, Piscataway, 08855, USA

Presenting Author: R.E. Walstedt

Recent results are presented for the ¹⁷O hyperfine (HF) tensor for planar oxygen sites in La₂CuO₄. This high-T_c host compound exhibits a rare hyperfine anomaly in yielding oxygen hyperfine couplings in the weakly ferromagnetic ordered state which differ sharply from those found in the paramagnetic state. Further evidence of unconventional behavior is found in the paramagnetic state HF tensor results reported in this poster: HF asymmetry around the Cu-O bond axis can only be explained within the confines of known HF interactions if a significant level of p-bonding is present in La₂CuO₄. In current theoretical models the possibility of p bonding has been largely ignored. Such effects may underlie the anomalous and poorly understood ¹⁷O nuclear relaxation properties of the doped, superconducting phase. An alternative explanation of these results might be that the anomalous anisotropy represents a precursor of the much larger and dramatically different HF interaction which sets on in the ordered state.

4PO5-43 Phase Separation in Ca Substituted YBCO

Dimitrios Palles 1, Dimitrios Lampakis 1, Grit Bottger 2, Efthymios Liarokapis 1, and Emanuel Kaldis 3. 1 Department of Physics, National Technical University of Athens, GR-15780 Athens, Greece. 2 Laboratory for Neutron Scattering ETH & PSI, 5232 Villigen PSI, Switzerland. 3 I.R. Laboratorium fur Festkorperphysik, ETH, 8093 Zurich, Switzerland.

Presenting Author: E. Liarokapis

A micro-Raman spectroscopic study in the temperature range of 5-300K and at room temperature hydrostatic pressures up to 4GPa has been carried out on Y_1-xCa_xBa₂Cu₃O_7-d polycrystalline superconducting compounds with x=0.00-0.20 and d =0.045-0.005. Apart from the five strong Raman active phonons (4A_g + 1B_1g), additional structures have been observed in the y(cc)y (402cm^-1) and the y(xx)y (300 and 322cm^-1) scattering configurations with increasing intensities with the Ca content.

The modes that appear in the out-of-phase B1g phonon of O_pl are related with the disorder introduced by the Ca substitution for Y and to a lcoal mode induced by the Ca atoms. The behavior of the spectral profile of the O_pl in-phase Ag symmetry phonon indicates that this phonon depends on the amount of oxygen and not the concentration of the Ca subsitution introduced carriers. There seem to exist two oxygen content differentiated phases which are present also in the pure oxygen overdoped YBCO. The insensitivity of the O_pl in-phase mode frequency combined with the behavior of the O_apex phonon, can be attributed to the excess charge distribution away from the CuO₂ planes. A phase separation seems to be induced for hydrostatic pressures above 3GPa possibly related to the Ca atom cluster formation.

4PO5-44 Phase transitions at low oxygen concentration in YBa2Cu3Ox

Presenting Author: E. Liarokapis

Dimitrios Palles 1, Nikolaos Poulakis 1, Efthymios Liarokapis 1, Kazik Conder 2, and Emanuel Kaldis 3. 1 Department of Physics, National Technical University of Athens, GR-157 80 Athens, Greece. 2 Laboratorium fur Festkorperphysik ETH, Hongerberg, Switzerland. 3 i.R. Laboratorium fur Festkorperphysik, ETH, 8093 Zurich, Switzerland.

A micro-Raman systematic spectroscopic study at room temperature has been carried out on YBa₂Cu₃O_x polycrystalline compounds in the whole range from the antiferromagnetic insulating to the metallic state (6<x<7). The evolution of the spectral characteristics mainly in the y(cc)y and the y(xx)y scattering configurations was examined on several microcrystallites of each sample as a function of the oxygen content.

The O_apex phonon appears sensitive to the local chain oxygen environment as modified with the oxygen content. The behavior of the O_p1 in-phase A_g symmetry phonon in the range 6.00<x<6.40 can be described by steplike changes corresponding to chain superstructure phases, usually separated by miscibility gaps, as has been previously done for 6.40<x<7.00. In analogy to the martensitic phase transformation observed in the underdoped-overdoped separation line, x@ 6.95, from the sudden frequency shift of the O_p1 in-phase phonon, there appears a phase transition in the range 6.10<x<6.15, the existence of which is additionally supported by the behavior of other phonons.