4PO6-30 First-Principles Study of Oxygen Ordering in YBa₂Cu₃O_7-x

Claudia Ambrosch-Draxl 1, Pavel A. Korzhavyi 2, and Börje Johansson 2. 1 Institute of Theoretical Physics, University Graz, Universitätsplatz 5, 8010 Graz, Austria. 2 Condensed Matter Theory Group, Uppsala University, S-75121 Uppsala, Sweden.

Presenting Author: C. Ambrosch-Draxl

The concentration and ordering of oxygen vacancies in YBa₂Cu₃O_7-x governs the superconducting transition temperature. With the aim of establishing a relationship between the site occupancy within the Cu-O chains and the electronic structure, we have performed first-principles total energy calculations for YBa₂Cu₃O_6.5 compounds considering various patterns of vacancy ordering. Supercells containing up to four formula units are taken into account. All calculations have been carried out by means of the full-potential LAPW method.

The effective interactions of oxygen atoms within the Cu-O chains are evaluated from the total energies using the cluster expansion technique. The most stable configuration for x=0.5 is found to be the experimentally observed ortho-II structure. This finding is confirmed by Monte-Carlo simulations for a rigid lattice using the obtained pair interactions. Thermal disorder effects as well as the ordering at other oxygen concentrations are studied. The influence of vacancy ordering on the electronic total and partial densities of states is discussed in detail.

4PO6-31 Possible Origin of Coexistence of Charge Ordering with Local Tilt Disorder

Shin-ichi Shamoto, Department of Applied Physics, Faculty of Engineering, Tohoku University, Sendai 980-8579, Japan

Presenting Author: S. Shamoto

In a soft ionic lattice system, metals with good Fermi surface nesting condition undergo charge density wave transitions, since cations or anions can easily adjust their positions. On the other hand, in a strong covalent bond network system such as cuprates, it is difficult to make a periodic charge density wave by ions, since the displacement of ions along covalent bonds costs much energy. However, even in this kind of materials there are structural phase transitions accompanied by the change of bond angles, e.g., the tilting of CuO₆ octahedra, resulting in a modification of transfer integral between the nearest neighbor transition metal atoms, although the change is expected to be small. In the vicinity of metal-insulator transition in strongly correlated electron system, however, such a small change could affect the electronic ground state seriously, if there was any periodic tilt ordering which is discovered as a local tilt disorder by recent PDF analysis. This tilt ordering will produce a wave of transfer integral by which the electronic states could result in charge ordering. We will discuss the possibility together with our periodic tilt ordering model.

4PO6-32 Probe of the chain oxygen order in underdoped YBa₂Cu₃O_6.44 by means of nuclear Spin Echo Double Resonance (SEDOR) of the Cu(1) chain copper

N. Poulakis 1, K. Conder 2, E. Kaldis 2, and Y. Berthier 1. 1 Laboratoire de Spectrométrie Physique, Université Joseph Fourier Grenoble I, CNRS (UMR C5588), B.P. 87, 38402 St.-Martin d’Hères, France. 2 Laboratorium für Festköperphysik, E.T.H. Zürich, CH-8093 Zürich, Switzerland.

Presenting Author: N. Poulakis

It has been proposed that the inhomogeneity of the charge and spin distribution in CuO₂ layers recently observed in underdoped 1-2-3 high T_c superconductors may be associated with the presence of the ortho-III type order in the chain layers [1].

Using a double resonance technique (SEDOR) on the NQR spectrum of Cu(1), we probe the existing ordering patterns in the chain oxygen sublattice of an underdoped YBa₂Cu₃O_6.44 compound with T_c » 40 K. The 30-32 MHz frequency region of the spectrum consists of three sharp lines which correspond to ⁶³Cu(1) in 3d¹⁰ state (empty chains) with different environments (neighbouring chains). A broad structure, with at least three prominent peaks observed in the 18-24.5 MHz region, is associated with the two ^63,65Cu isotopes in 3d⁹ configuration located on Cu(1) sites with more than two neighbouring oxygen atoms.

The different neighbouring relationships are well resolved using SEDOR technique. Moreover, this technique gives a good insight of the statistical distribution between full, empty and broken chains in the structure. Finally, we discuss the possible influence of such ordering pattern on the charge and spin distribution in the CuO₂ layers.

[1] M.A. Teplov et al., Journal of Supercond. 12, 113 (1999)

4PO6-33 Local electronic structure in pure and doped La₂CuO₄

Peter F. Meier, Samo Plibersek, Erich P. Stoll, and H.-U. Suter, Physics Institute, Winterthurerstr. 190, CH-8057 Zurich, Switzerland

Presenting Author: P.F. Meier

The local electronic structure of the La₂CuO₄ crystal has been calculated using the density functional method. Although emphasis has been directed towards the CuO₂ planes in the crystal the out-of-plane apical oxygens are well represented in all the clusters chosen. Each cluster is embedded in a background potential of ions of bare pseudo-potentials appropriate to La₂CuO₄. The clusters are CuO₆, Cu₂O₁₁ and Cu₅O₂₆ where all the copper atoms lie in the CuO₂ planes.

Modifications to the cluster have enabled the effects of doping with strontium and excess oxygen to be modelled. Where appropriate different spin states have been studied, for example, closed shell singlet states and the lower energy triplet states. The suitability of the clusters have been measured by the ability of the calculations to reproduce the electric field gradients at the copper atoms which can be derived from nuclear quadrupole spectroscopic methods. In addition on-site and transferred hyperfine fields have been calculated and localised excitations (polarons and spin bags) are studied as a function of the doping concentrations. Some structural distortions after doping have been investigated, in particular the anti-Jahn-Teller distortions of the oxygen environment of the copper atoms when the neighbouring La³⁺ is replaced by Sr²⁺.

4PO6-34 Investigation of the magnetoresistance behavior in high pulsed magnetic fields up to 35T in thick films YBa₂Cu₃O_x and YBa₂Cu₃O_x (5% Ag-doped) near temperature superconductivity transition

Efrim Broide, Seperator Ltd., Kiryat Weizmann Science Park, Bldg. 13A, Ness-Ziona 70400, Israel, and M. Yakunin, Institute of Metal Physics, Russian Academy of Science, Ekaterinburg, Russia.

Presenting Author: E. Broide

The influence of pulsed magnetic fields up to 35T on samples YBa₂Cu₃O_x and YBa₂Cu₂O_x (5% Ag-doped) thick films produces after electromagnetic separation HTSC 1-2-3 powders was investigated. The field was generated in the multiturned copper wire coil with a semisinusoidal pulse duration of about 10ms. To measure the magnetoresistivity the sample voltage under the constant current regime was made to an accuracy of 0.5x10^-6 V and minimal time interval of 100 ns.

We discovered a nonlinear behavior in the magnetoresistance of YBa₂Cu₃O_x after measurements with current greater than 1 A/cm² at the temperature 67.4K. However, in the specimens with YBa₂Cu₃O_x (5% Ag-doped) we observed a linear plot of magnetoresistivity and magnetic field at currents less 20A/cm² at the 77K.

In our view the difference in behavior of the two types of samples is a function of the resistivity of granular contacts in polycrystal thick films YBa₂Cu₃O_x and YBa₂Cu₃O_x (5% Ag-doped).

It is found the experimental effect of the temperature T_c rising in the end of SC-transition at 77K in pulsed magnetic field under 10 pulses 32T in YBa₂Cu₃O_x thick films with shortage of the oxygen 6-6.8%. The thickness of films was 5Omkm they ones was produced by heating of the powders with organic binding materials. Before development T_c was 74K, after the pulses the T_c has grown to 80K. The resistance transition with the current 1 mA in specimen was investigated.

It is reasonable suppose, that observed effect is caused by martensite nature of the transition tetra-orthorombic structure in YBa₂Cu₃O_x, thermodynamical instability of this compound near temperature SC-transition in high magnetic field and as a result, of the displaced deformation.

The final products (thick films), which were produced from the enriched powder, revealed larger critical currents (about an order of magnitude), compared to those obtained from virgin (non-enriched) mixture.

[1] P.J.M. van Bentum, H. Van Kempen et al. Physica B 155 (1989) 160-163.

[2] C. Van Haesendonck, J.P. Locquet, et al. Physica B 155 (1989) 149-155.

[3] P. Rodrigues, Jr. Chiveder, P. Pureur, and S. Reich Physica C 211 (1993) 13-21.

4PO6-35 Effects of the Target-to-Substrate Angle on Off-Axis Sputter Deposition and EPR Studies of Near-Surface Magnetic Properties of YBCO Thin Films

D.E. Pugel, M.B. Salamon, and L.H. Greene, Department of Physics, University of Illinois Urbana-Champaign, 1110 W. Green St., Urbana, IL, 61801

Presenting Author: D.E. Pugel

We have determined the dependence of target-to-substrate angle on the elemental concentration of c-axis YBCO thin films. Away from the standard off-axis position, energy distributions of sputtered elements vary spatially within the sputter plume as a consequence of the angular dependence of thermalization. These effects increase with target-to-substrate angle and hence, with larger film area. Standard materials characterization techniques as well as angle-dependent Rutherford Backscattering Spectrometry (RBS) demonstrate that c-axis films grown at angles which deviate from the standard off-axis geometry produce nominally Y(123) in the bulk with dramatic changes in the surface morphology and deposition rate [1].

Several thin film planar tunneling experiments are consistent with a broken-time-reversal symmetry (BTRS) state. To compliment tunneling measurements, we have developed a technique to measure electron paramagnetic resonance (EPR) effects of the near-surface region of a superconductor. Preliminary data are consistent with the spontaneous formation of magnetic moments at low temperature on YBCO thin films and may prove to be an important confirmation of BTRS [2].

[1] Pugel, DE, Greene, LH, Appl. Phys. Lett., 75, 1589, (1999).

[2] We acknowledge the Illinois EPR Center, the Center for the Microanalysis of Materials (DOE DEFG02-91-ER45439) and the Science and Technology Center for Superconductivity (NSF-DMR91-20000).

4PO6-36 Large distinction between underdoped and overdoped La_2-xSr_xCuO₄ single crystals as observed from magnetic and transport measurement: A possible evidence for macroscopic phase separation in overdoped high temperature superconductors

H.H. Wen 1, X.H. Chen 2, W.L. Yang 1, Y.M. Ni 1, and Z.X. Zhao 1. 1 National Laboratory for Superconductivity, Institute of Physics and Center for Condensed Matter Physics, P.O. Box 603, Beijing 100080, P.R. China. 2 Physics Department, University of Sciences and Technology of China, Hefei 230026, P.R. China.

Presenting Author: H.H. Wen

Magnetic and DC tranpsort properties have been extensively investigated on one underdoped (x=0.092) and one overdoped (x=0.24) La_2-xSr_xCuO₄ single crystals both have roughly the same quality and the same critical transition temperature T_c=26 K. Large distinctions have been found for these two samples. Firstly, the temperature dependent magnetization measurement shows that there is only one transition for the underdoped sample, but two transitions for the overdoped one. Detailed investigations show that one of the transitions for the overdoped sample is close to the solitary one of the underoped sample and is identical to the upper critical field for both samples. The second transition for the overdoped sample is corresponding to a bulk phase coherence transition. Secondly, the irreversible line H_irr(T) is close to the upper critical field H_c2(T) for the underdoped sample, but H_irr(T) is far below H_c2(T) for the overdoped one. Therefore on the underdoped sample we observed a pronounced second peak effect which is contrasted with a weak second peak effect on the overdoped sample. Thirdly, dc transport measurement shows that, by increasing the magnetic field, the R(T) curves are not broadened so much but parallely shift to lower temperature region. This is in sharp contrast with the overdoped sample in which the R(T) curves are strongly broadened by the magnetic field. The large distinctions mentioned above strongly suggest that the overdoped sample can separate into superconduting clusters and good metallic regions. The superconductivity appeared in the clusters for an overdoped sample may have the same origin as in the underdoped region.

4PO6-37 LT-STS Study on YBCO

Murakami Hironaru 1, Qi Yang 1, Sakai Kazuo 1, Ito Toshimichi 1, and Hidehito Asaoka 2. 1 Department of Electrical Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan. 2 Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-1195, Japan.

Presenting Author: M. Hironaru

STM/STS observation was carried out on a YBCO single crystal surface. To get a reliable spectrum characteristic without surface degradation, a field etching treatment was in situ applied to the crystal surface by applying pulse voltages between the sample and a tungsten probe-tip at 4.2K. After the treatment, fairy stable STM images and reproducible tunneling spectra were observed. These spectra showed clear superconducting gap structures and an asymmetrical dip and bump structure as detected on BSCCO.

4PO6-38 Minibands in the Electron Spectrum due to Phase Coherent Transport

Serhii Shafranjuk, Institute of Magnetism NASU, 36b Vernadsky aven., UA-252680, Kyiv-142, Ukraine

Presenting Author: S. Shafranjuk

The phase coherent electron transport across the multilayered normal heterostructure embedded between two superconducting electrodes S is considered. The heterostructure consists of a sequence where the normal layers N are eparated by potential barriers I. Inside the composite quantum well created in such SNb S-junction b is the number of normal N layers), the interference between the ordinary and Andreev reflections results in the minigaps of prohibited energy in the electron excitation spectrum. Consequently, the minigaps are present also in the electron density of states that can be observed as periodic dip series in the tunneling conductivity. We find that the non-magnetic impurities and the interface's asperities affect the interference pattern smearing it out. The dc magnetic field applied in parallel to the junction, shifts the minigap's position in an oscillatory way. The model is suggested to interpret a number of known experiments made on composite junctions.

*4PO6-39 Local Probing of O-delta in HgBa₂CuO_4+d

J.G. Correia 1, J.P. Araujo 2, S.M. Loureiro 3, P. Toulemonde 4, S. Le Floch 4, P. Bordet 4, J.J. Capponi 4, W. Troeger 5, B. Ctortecka 5, T. Butz 5, H. Haas 6, R. Gatt 7, J.G. Marques 8, and J.C. Soares 8. 1 Div. EP, CERN, Geneva, Switzerland. 2 Physics Dept., Porto University, Portugal. 3 Chemics Dept., Princeton University, USA. 4 CNRS, Laboratoire de Cristallographie, Grenoble, France. 5 Physics Dept., Leipzig University, Germany. 6 Physics Dept., Hahn-Meitner-Institut, Berlin, Germany. 7 Physics Dept., Wisconsin University, Madison, USA. 8 Physics Dept., Instituto Tecnologico e Nuclear, Sacavém & Centro de Fisica Nuclear, Lisbon University, Portugal.

Presenting Author: J.P. Araujo

The family of mercury-based copper oxides, Hg₁Ba₂Ca_n-1Cu_nO_2n+2+d , shows high critical temperatures up to T_c=164 K at 30 Gpa in the Hg-1223 (n=3) compound. All members crystallize with a tetragonal lattice that however, shows to be particularly disordered at the Hg planes. In spite of intense studies, it is not understood how Hg atoms interact with point defects like the non-stoichiometric oxygen, O-delta, which goes to the Hg planes and regulates T_c.

In this work the Perturbed Angular Correlation (PAC) local technique was applied to measure the electric field gradients (EFG) at Hg sites in the Hg1201 (n=1) compound. High quality pellets were implanted at ISOLDE/CERN with very low doses of the 199m^Hg radioactive isotope. The PAC experiments were performed during annealing at 490 K, under argon or oxygen flow, to change the O-delta doping concentration. The results show that the EFG parameters are highly sensitive to the O-delta content that is clearly distinguished from other point-like oxygen-defect complexes. First principle calculations (FLAPW) of the EFGs were performed in undoped and oxygen doped Hg1201 that show an excellent agreement with the experimental results. The local behavior of the Hg atoms in highly and poor oxygen doped Hg1201 was studied at low temperatures. Below 100 K the asymmetry parameter of the EFG becomes non-zero showing that the charge distribution near the O₂-Hg-O₂ apical chain becomes non-axially symmetric at low temperature in these samples.