POSTER SESSION 4PO5: Neutron Diffraction/NMR/Spectroscopy

Thursday, Feb. 24, 1:45 p.m. – 3:45 p.m., Hall D (GRB)

*4PO5-1 Dispersionless spin-gap in a clean high-T_c superconductor: evidence for spin-charge separation

B. Lake 1,2,3, G. Aeppli 4,3, T.E. Mason 1,2, A. Schroder 5, D.F. McMorrow 3, K. Lefmann 3, M. Isshiki 6, M. Nohara 6, H. Takagi 6, and S.M. Hayden 7. 1 Dept. of Physics, University of Toronto, Toronto, ON M5S 1A7, Canada. 2 Oak Ridge Natl. Lab., Oak Ridge, TN 37831, USA. 3 Dept. of Condensed Matter Physics and Chemistry, Risø Natl. Lab., DK 4000 Roskilde, Denmark. 4 N.E.C. Research, 4 Independence Way, Princeton, NJ 08540, USA. 5 Dept. of Physics, University of Karlsruhe, D-76128 Karlsruhe, Germany. 6 ISSP, University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106-8666, Japan. 7 H.H. Wills Physics Lab., University of Bristol, Bristol BS8 1TL, United Kingdom.

Presenting Author: B. Lake

The magnetic excitations of a high quality sample of La_2-xSr_xCuO₄ (x=0.163) have been studied using inelastic neutron scattering. A clean dispersionless energy gap was found. This result contrasts with the findings of, for example, photoemission where the gap was demonstrated to be a function of wavevector and to have the dispersion characteristic of a d-wave superconductor. The technique of photoemission spectroscopy is sensitive to the pairing of the charges, in contrast to neutron scattering which is sensitive only to the spins of these paired charges. Given the overwhelming evidence for d-wave superconductivity in cuprate superconductors we see our data not as evidence against d-wave superconductivity but as proof that the spin excitations in the superconducting state do not parallel the charge excitations in the manner assumed by conventional d-wave theories.

4PO5-2 Neutron diffraction of the vortices near the entanglement transition in (K,Ba)BiO₃

Isabelle Joumard 1, Jacques Marcus 1, and Thierry Klein 1, and Robert Cubitt 2. 1 LEPES-CNRS, 25 Avenue des Martyrs, BP 166, 38 042 Grenoble, France. 2 ILL, Avenue des Martyrs, BP 156, 38 042 Grenoble, France.

Presenting Author: I. Joumard

In contrast to other high T_c oxides, the (K,Ba)BiO₃ superconductor (T_c~30K) presents the advantage of having a very simple cubic i.e. isotropic structure. It could thus be used to prove the existence of a phase transition in the vortex solid avoiding further complications related to the anisotropy [1]. Indeed, we have shown by neutron diffraction the existence of a vortex lattice at low field and an anomalous decrease of the intensity of the diffraction peaks which can be attributed to incoherent wandering of the vortex lines about the field direction. When the amplitude of the mean displacement reaches approximately 30% of the intervortex spacing (B~0.6T), dislocations become energetically favorable and proliferate into the sample.

We show here that the orientational order is partially lost above 0.4T (the diffraction pattern turns into a diffraction ring) but can be recovered by oscillating the applied field during the cooling process. In striking contrast with previous measurements on Sr₂RuO₄ [2] where only longitudinal order was affected by oscillating the applied field, these oscillations have no effect on the width of the rocking curves (longitudinal order) in (K,Ba)BiO₃.

[1] I. Joumard, J. Marcus, T. Klein and R. Cubitt, Phys. Rev. Lett., 82, 4930, (1999).

[2] T.M. Riseman et al., Nature, 396, 242, (1998).

4PO5-3 Local structural evidence for charge inhomogeneities in the CuO₂ planes of YBa₂Cu₃O_6+x (x=0.25, 0.45, 0.65, 0.94)

M. Gutmann 1, S.J.L. Billenge 1, J. Johnson 2, G.H. Kwei 3, and E.L. Brosha 3. 1 Physics Dept., Michigan State University, East Lansing, MI 48824, USA. 2 Argonne National Laboratory, Argonne, IL 60439, USA. 3 Los Alamos National Laboratory, NM 87545, USA.

Presenting Author: M.J. Gutmann

We have investigated the local structure of YBa₂Cu₃O_6+x (x=0.25, 0.45, 0.65, 0.94) using the pair-distribution function (PDF) analysis of time-of-flight diffraction data. In contrast to earlier XAFS work we find no evidence for a split of the apical oxygen O(4). Instead a slightly better fit over the average crystallographic model results when the in-plane Cu site is split along the c-direction. This result can be interpreted in terms of an inhomogeneous charge distribution in the CuO₂ planes resulting from doping. This is similar to observations of the local structure in La_2-xSr_xCuO₄ [see abstract by Bozin et al.]. Preprints from this work are available from our web-site http://www.pa.msu.edu/cmp/billinge-group. We will also present results from other HTSC systems.

4PO5-4 Magnetic ordering in the superconducting mixed Ruthenium-Copper oxide Sr₂Y(Ru_1-xCu_x)O₆

D.Y. Chen 1, M.K. Wu 2, C-H. Du 1, P.D. Hatton 1, F.Z. Chien 3, and C. Ritter 4. 1 Department of Physics, University of Durham, Durham, DH1 3LE, United Kingdom. 2 Materials Science Center, National Tsing Hua University, Hsinchu, Taiwan. 3 Department of Physics, University of Tamkang, Taiwan. 4 Institut laue Langevin, Grenoble, France.

Presenting Author: N. Parkinson

A series of mixed Ruthenium-Copper oxides of general formulae Sr₂Y(Ru_1-xCu_x)O₆ x=0.05, 0.1 and 0.15 were prepared by high temperature synthesis. Transport and specific heat measurements at low temperatures showed these materials to superconducting with a T_c of approximately 24K and to have magnetic transitions. High-resolution neutron powder diffraction studies have been undertaken to determine the crystallographic structure and magnetic ordering present in the samples. The materials were found to be single phase P21/n with a=5.7602Å, b=5.7795Å, c=8.1506Å and a =90.313° for x=0.05 samples. The structure is related to that of Sr₂YRuO₆ determined by Battle et al with additional Ruthenium-Copper ordering. In addition to the structural peaks, antiferromagnetic peaks were observed at low scattering angles at temperatures below 28K confirming the magnetic order.

4PO5-5 Frequency Evolution of Neutron Peaks Below T_c: Commensurate and Incommensurate Structure in LaSrCu and YBaCuO

Ying-Jer Kao 1, Qimiao Si 2, and Kathryn Levin 3. 1 Department of Physics, University of Chicago, Chicago, IL 60637, USA. 2 Department of Physics, Rice University, Houston, TX 77251,USA. 3 James Franck Institute, University of Chicago, Chicago, IL 60637, USA.

Presenting Author: Y.-J. Kao

We study the evolution of the neutron cross-section with variable frequency omega and fixed T below and above T_c in two different cuprate families. Our calculations incorporate the effects of the appropriate Fermi surface shapes as well as antiferromagnetic exchange enhancements, both in the presence of d-wave pairing. For omega near the gap energy D , both optimal LaSrCuO and slightly underdoped YBCO exhibit incommensurate peaks with positions near those of normal state LaSrCuO. Among other findings reasonably consistent with experiment, we see (i) peak sharpening below T_c in LaSrCuO, (ii) a continuous decrease in incommensurability, as the resonance at ~2D is approached in underdoped YBaCuO and (iii) the re-emergence of incommensurate peaks above the resonance frequency. Finally we find a new commensurate peak in LaSrCuO (not yet seen) which is present over a very narrow range of omega and appears to derive from the interplay of d-wave pairing and Van Hove effects.

4PO5-7 Low energy charge fluctuations in the presence of the pseudo spin gap in YBa₂Cu₄O₈

Andreas Suter 1, Miha Mali 2, Josef Roos 2, and Detlef Brinkmann 2. 1 Los Alamos National Laboratory, Los Alamos NM 87545, USA. 2 Physik-Institut, Universität Zürich, CH-8057 Zürich, Switzerland.

Presenting Author: A. Suter

We present results from a ¹⁷O nuclear magnetic resonance study in the stoichiometric super-conductor YBa₂Cu₄O₈. A double irradiation method enables us to show that, below around 180 K » T*, the spin-lattice relaxation rate of plane oxygen is not only driven by magnetic, but also signifi-cantly by quadrupolar fluctuations, i.e. low-frequency charge fluctuations. From the temperature dependence as well as from the magnitude of the spin-lattice relaxation, we conclude that this low-frequency charge fluctuations are not of simple phononic or simple electron-like quasiparticle origin. In the superconducting state, on lowering the temperature, the quadrupolar relaxation diminishes faster than the magnetic one. On the other side, we do not observe any quadrupolar contributions in the spin-lattice relaxation of plane copper. These findings show that, with the opening of the pseudo spin gap, a charge degree of freedom of mainly oxygen character is present in the electronic low-energy excitation spectrum. Different possibilities of charge degrees could explain our findings: (i) Formation of spatially strongly correlated holes. (ii) Anomalous strong electron-phonon coupling. These findings suggest that a simple magnetic single spin fluid model is not sufficient to describe the low-energy excitations for temperatures T<T*.

4PO5-8 YBa₂Cu₄O₈ Double Chain Charge Fluctuations Detected via Apex Oxygen Quadrupolar Relaxation

M. Mali 2, A. Suter 1, J. Roos 2, D. Brinkmann 2, Hugo Keller 2, and J. Karpinski 3. 1 Los Alamos National Laboratory, Los Alamos, NM 87545, USA. 2 Physik-Institut, Universität Zürich, CH-8057 Zürich, Switzerland. 3 Laboratorium für Festkörperphysik, Eidgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland.

Presenting Author: M. Mali

We report a ¹⁷O NMR study of the apex oxygen in YBa₂Cu₄O₈. By a recently developed NMR double irradiation method [1] we show that the spin-lattice relaxation rate of apex oxygen is predominantly of quadrupolar origin and is driven by charge fluctuations in the double chain. The temperature dependence of this quadrupolar relaxation is different from the temperature dependence of the magnetic relaxation of double chain copper and oxygen. This indicates a possible spin-charge separation of the charge carrier quasi-particles in the double chain quasi-one dimensional conductor [2].

[1] A. Suter, M. Mali, J. Roos, and D. Brinkmann, (cond-mat/9906216), (1999).

[2] F. Raffa, M. Mali, A. Suter, A. Yu. Zavidonov, J. Roos, and D. Brinkmann, Phys. Rev. B 60, 3636 (1999).

4PO5-9 The NMR of High Temperature Superconductors without Anti-Ferromagnetic Spin Fluctuations

Jamil Tahir-Kheli, First Principles Research, Inc., 8391 Beverly Blvd., #171, Los Angeles, CA 90048

Presenting Author: J. Tahir-Kheli

A microscopic theory for the NMR anomalies of the planar Cu and O sites in superconducting LaSrCuO is presented that quantitatively explains the observations without the need to invoke anti-ferromagnetic spin fluctuations on the planar Cu sites (in press, Journal of Physical Chemistry) and its inability to explain the recent (S. Yu et al., in press PRL) measurements of the Cu-O and Cu-Cu indirect nuclear-nuclear couplings. The theory is derived from the recently published ab-initio band structure (PRB 58, 12323 (1998), PRB 58, 12307 (1998), J. Phys. Chem, in press, PRL submitted) calculations that correct LDA computations tendency to overestimate the self-coulomb repulsion for the half-filled Cu d_x2-y2 orbitals for these ionic systems. The new band structure leads to two bands at the Fermi level with holes in the Cu d_z2 and apical O p_z orbitals in addition to the standard Cu d_x2-y2 and planar O ps orbitals. This band structure is part of a new theory for the cuprates that explains a broad range of experiments and is based upon the formation of Cooper pairs comprised of a k­ electron from one band and a -k¯ electron from another band (Interband Pairing Model).