Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V12: Superconducting Ruthenides and Borides |
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Sponsoring Units: DCMP Chair: James Knight, University of South Carolina Room: LACC 402A |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V12.00001: Search for complex order parameter symmetry in Sr$_2$RuO$_4$ by Josephson interferometry experiments Francoise Kidwingira, Dale Van Harlingen, Yoshiteru Maeno We present the results of Josephson interferometry measurements on single crystals of the unconventional superconductor Sr$_{2}$RuO$_{4}$ designed to determine the order parameter symmetry by mapping out the phase anisotropy. A number of experiments suggest that this material has odd pairing symmetry and may exhibit an in-plane complex order parameter of the form p$_{x}$+ip$_{y}$. To test this, we have carried out measurements of the critical current vs. magnetic field in single junctions and dc SQUIDs fabricated between a conventional superconductor and single edge faces or corners of the Sr$_{2}$RuO$_{4 }$crystal. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V12.00002: de Haas-van Alphen studies of electron doped Sr$_2$RuO$_4$ Peter D.A. Mann, Christoph Bergemann, Naoki Kikugawa, Stephen R. Julian, Andrew P. Mackenzie, Yoshiteru Maeno We present the results of a highly sensitive de Haas-van Alphen (dHvA) rotation study of La$_x$Sr$_{2-x}$RuO$_4$, for $x = 0.02$, 0.06 and 0.10. By substituting Sr$^{2+}$ with La$^{3+}$, we are effectively doping the unconventional superconductor Sr$_2$RuO$_4$ with electrons without gross distortions to the crystal structure. According to band structure calculations, the resultant rigid band shift should push the Fermi level towards a van Hove singularity at $x = 0.20$, which has important consequences for theories of spin-fluctuation mediated superconductivity. In this study, due to extremely high quality samples and a low noise experimental set-up, we have been able to extract information about changes in the Fermi surface warping, electronic effective masses and spin susceptibility up to $x = 0.10$. The experimental runs were carried out in fields up to 18\,T, and the samples were rotated in the [001] to [100] plane. We have determined the field-dependent dHvA amplitude and compared it with numerically calculated results from a model Fermi surface. Effective masses were calculated from the temperature dependence of the oscillation amplitudes. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V12.00003: Fermi Surface Topology of $Ca_{2-x}Sr_{x}RuO_4$ Determined by Angle-Resolved Photonelectron Spectroscopy Hongbo Yang, Shancai Wang, A.K.P. Sekharan, Ziqiang Wang, Hong Ding, S. Souma, H. Matsui, T. Sato, T. Takahashi, Chenxi Lu, Jiandi Zhang, R. Jin, D. Mandrus, E. W. Plummer We report angle-resolved photoelectron spectroscopy results of the Fermi surface of $Ca_{1.5}Sr_{0.5}RuO_{4}$, which is at the boundary of magnetic/orbital instability in the phase diagram of the Ca-substituted Sr ruthenates. Three $t_{2g}$ energy bands and the corresponding Fermi surface sheets are observed, which are also present in the Ca-free $Sr_2RuO_4$.We find that while the Fermi surface topology of the $\alpha, \beta (d_{yz,zx})$ sheets remains almost the same in these two materials, the $gamma (d_{xy})$ sheet exhibits a holelike Fermi surface in $Ca_{1.5}Sr_{0.5}RuO_4$ in contrast to being electronlike in $Sr_2RuO_4$. Our observation of all three volume conserving Fermi surface sheets clearly demonstrates the absence of orbital-selective Mott transition, which was proposed theoretically to explain the unusual transport and magnetic properties in $Ca_{1.5}Sr_{0.5}RuO_{4}$. We will also report resent ARPES results on the samples with different Ca concentration. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V12.00004: Angular dependence of magnetoacoustic interaction in Sr$_{2}$RuO$_{4}$. A. Suslov, O. Svitelskiy, X. Zhang, J. Crow, D. Shulyatev For the first time magnetoacoustic phenomena in the ultrasound velocity and attenuation in layered superconductor Sr$_{2}$RuO$_ {4}$ were studied while samples were rotated in magnetic field. The samples were grown at the NHMFL by a floating zone technique. The superconductive transition observed in zero magnetic field at T$_c$ = 1.2 K by ac susceptibility and ultrasonic measurements proves the high quality of the single crystals. The measurements were performed in magnetic fields up to 33 T at temperatures down to 45 mK in the ultrasonic frequency range from 30 to 350 MHz. The frequencies of acoustic de Haas - van Alphen oscillations perfectly agree with known structure of the Fermi surface of this crystal. A strong anisotropy of sound attenuation in the normal state has been discovered. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V12.00005: Search for Time-Reversal Symmetry Breaking Effects in Sr$_2$RuO$_4$ Using Scanning Magnetic Microscopy Per Bj\"ornsson, Kathryn Moler, Yoshi Maeno Strontium ruthenate (Sr$_2$RuO$_4$) is a spin-triplet superconductor ($T_c$ = 1.5 K) which is thought to have a two-component time reversal symmetry breaking (TRSB) order parameter. We have imaged patterned samples of this material using sub-micron Hall probes in search of magnetic fields that are expected to be caused by spontaneous currents at the edge of a TRSB superconductor. Because of the localization of the currents to edges, both high spatial resolution and high sensitivity to magnetic fields are necessary in order to provide strong tests of the theory. We will compare our measurement results with theoretical models for possible magnetic field distributions in superconductors with TRSB order parameters. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V12.00006: Symmetry dependent excitations in pure and titanium doped Sr$_2$RuO$_4$ Tim Kidd, Tonica Valla, Han Jin Noh, John Rameau, Peter Johnson, Yoshiteru Maeno While Sr$_{2}$RuO$_{4}$ is well known for its exotic superconductivity, the normal state properties of the system are also of great interest. The system is close to both ferro- and anti-ferromagnetic instabilities, and the system displays both Fermi liquid and anomalous transport properties at different temperature regimes. Doping with small amounts of titanium has a strong effect on the system, radically changing the low temperature transport properties as well as transforming the ground state into a spin density wave with short range order. The electronic structure that gives rise to such interesting behavior is quite complex, with three bands that can be divided into both quasi-2D and quasi-1D subsets according to their orbital symmetry. Measurements of individual bands using photoemission spectroscopy reveal that the scattering mechanisms for each dimensional subset are quite different, with each contributing to different aspects of the normal state transport. From measurements of the electronic self-energy, the strength and energy scale of the interactions in the 2D band show little change with Ti doping, while the 1D bands present a more complicated picture. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V12.00007: Suppressed mixed-state dissipation in magnesium diboride films Milind Kunchur, Daniel Arcos, Gabriel Saracila, David Christen, James Thompson We have investigated dissipative transport in magnesium diboride films at magnetic fields approaching the upper-critical value and currents crossing the depairing limit. The depairing current and the upper-critical field behave in a reasonable manner, conforming to the conventional ideas for these phase boundaries. However the flux motion at high fields and currents experiences a drastic suppression, such that the value of applied field has little effect on the resistive transition shape or current-resistance chanracteristic, other than to cause an overall shift due to pair-breaking. We discuss these results and the possible role of the double-barreled spatial distribution of quasiparticles around a vortex in a two-band superconductor. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V12.00008: Negative thermal expansion of MgB$_{2}$ below T$_{c}$ - violation of the Ehrenfest relation J. Neumeier, T. Tomita, J.S. Schilling, A. Serquis, V.F. Nesterenko, S.S. Indrakanti The thermal expansion coefficient $\alpha$ of MgB$_{2}$ is observed to change from positive to negative upon cooling through the superconducting transition temperature $T_{c}$. The bulk Gr\"uneisen parameter, which is proportional to $\alpha$, changes sign at $T_{c}$ as well. The jumps in $\alpha$ and the heat capacity at $T_c$ are used to estimate its pressure dependence, $dT_{c}$/$dP$; comparison to the measured value reveals violation of the Ehrenfest relation by 42\%. The results suggest anomalous coupling between the lattice and superconducting electrons via low-energy phonons. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V12.00009: Low-Energy Charge Excitation in Single Crystal MgB$_{2}$ Y.Q. Cai, P. Chow, H. Ishii, C.C. Chen, K.S. Liang, C.T. Chen, S. Tsuda, S. Shin, Y. Takano, K. Togano, H. Kito, C.C. Kao The charge dynamics of MgB$_{2}$ has been studied at 300K along the {\em c}-axis using inelastic X-ray scattering (IXS) with 65 and 250 meV resolution at 9.886 keV on the Taiwan IXS beamline at SPring-8. [1] An unusual charge excitation has been observed which shows periodic dispersion with momentum transfer {\em q} that follows: $\omega = \omega_{0} - 2{\gamma}{\rm cos}(qc)$, with $\omega_{0} = 3.55$ eV, $\gamma = 0.49$ eV, and $c = 3.52$ {\AA} the lattice constant along the {\em c}-axis. This charge excitation persists even at momentum transfers greater than $q_{c}$, where the conventional bulk plasmon excitation has decayed into the single-particle continuum. This feature may be linked with the predicted sharp collective mode at $q < 0.6$ {\AA}$^{-1}$ [2], but the behavior at higher $q$ is not accounted for. Possible origins and implications of this feature are discussed. This work was supported by the National Science Council of Taiwan. [1] Y.Q. Cai, et al., AIP Conf. Proc. 705, 340 (2004). [2] W. Ku, et al., Phys. Rev. Lett. 88, 057001 (2002). [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V12.00010: The de Haas--van Alphen effect in the superconducting state of MgB$_2$. A. Carrington, J. Fletcher, S.M. Kazakov, J. Karpinski The de Haas-van Alphen (dHvA) signal arising from orbits on the $\pi$ Fermi surface sheet of the two-gap superconductor MgB$_2$ has been observed in the vortex state below $H_{c2}$. An extra attenuation of the dHvA signal, beyond those effects described in the conventional Lifshitz-Kosevich expression, is seen due to the opening of the superconducting gap. Our data show that the $\pi$ band gap is still present up to $H_{c2}$. Using current theories of dHvA oscillations in the superconducting state we extract estimates for the evolution of the $\pi$ band gap with magnetic field. Contrary to results for other materials, we find that the most recent theories dramatically underestimate the damping in MgB$_2$. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V12.00011: Relationship between superconductivity and calculated \textit{$\sigma $}--band conductivity in MgB$_{2}$ Pablo de la Mora, Gustavo Tavizon Four different relaxation times were needed to adjust the electrical conductivity and Hall coefficient to experimental values in MgB$_{2}$. Al-doping was analyzed in the \textit{rigid band approximation}; this permitted a detailed study of the partial substitution of magnesium for aluminium (Mg$_{1{\-}x}$Al$_{x}$B$_{2})$. Other substitutions such as AB$_{2}$ (A=Be, Sc, Zr, Nb and Ta) were also discussed. In (Mg$_{1{\-}x}$Al$_{x}$B$_{2})$ $T_{c}$ diminishes with Al content, the other compounds are not superconductors or have a low $T_{c}$. In this work it was found that with electron doping, such as Al substitution, the \textit{$\sigma $}{\-}band conductivity decreases in a very similar way as T$_{c}$ and the corresponding bands become less anisotropic. \textit{$\sigma $}{\-}band contribution for BeB$_{2}$ and ScB$_{2}$ at $E_{F}$ is very small and the anisotropy is much lower. For Zr, Nb and Ta there are no \textit{$\sigma $}{\-}bands at $E_{F}$. These results give a clear connection between superconductivity and the character of the \textit{$\sigma $}-band, band conductivity, and band anisotropy. This gives a plausible explanation for the diminution of $T_{c}$ with different doping of MgB$_{2}$ [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V12.00012: Effects of He$^{+}$ ion irradiation on the two-band superconductivity of MgB$_{2}$ by point-contact spectroscopy Wan Kyu Park, Bernd Wilken, Karen Parkinson, Laura Greene, Brian Moeckly, John Rowell An unresolved issue in MgB$_{2}$ is whether the two superconducting gaps can be merged into a single gap as T$_{c}$ is reduced. It has been predicted that several effects, including enhanced interband scattering or a smeared density of states, can reduce T$_{c}$. Our transport measurements on MgB$_{2}$ thin films un-irradiated and irradiated with 1 MeV He$^{+}$ ions (doses: 1x10$^{15}$ -- 1x10$^{17}$ ions/cm$^{2})$ show that $\rho _{40K}$ increases and that T$_{c}$ decreases roughly linearly with dose and $\rho _{40K}$, similar to other work [R. Gandikota \textit{et al}., cont-mat/0410655]. Point-contact spectroscopy on MgB$_{2}$ irradiated with 1x10$^{16}$ ions/cm$^{2}$, with T$_{c}$ reduced to 36.3 K from 39.3 K, shows that 2$\Delta $/k$_{B}$T$_{c}$ increases from 1.43 to 1.76 (decreases from 4.13 to 3.90) for the smaller (larger) gap, so the gaps tend to merge. Detailed measurements and analyses based on the existing models [$e.g.$, J. M. Rowell, SST \textbf{16} R17 (2003), and J. Kortus \textit{et al}., cond-mat/0411667] will be presented. We acknowledge Pavel Krasnochtchekov and Robert Averback and support by the DoE DEFG02-91ER45439, through the FSMRL and the Center for Microanalysis of Materials. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V12.00013: Acoustic Studies of the Superconductor Magnesium Diboride Kenneth Pestka II, Julian Maynard MgB2 is currently of great interest because this simple two element compound is a superconductor with the highest transition temperature for a Bardeen-Cooper-Schieffer (BCS) type superconductor. As for other BCS superconductors, important quantities to measure through the superconducting transition include the ultrasound velocity and attenuation. Measuring these quantities for MgB2 is difficult because good single crystal samples are very small. Recently we have succeeded in making such measurements, using resonant ultrasound spectroscopy (RUS) for small samples[1], on high quality crystals grown epitaxially on SiC. We have also completed some preliminary measurements on small crystalline samples obtained from J. Karpinski. [1] J. D. Maynard, Physics Today,49,26-31 (1996), ``Resonant Ultrasound Spectroscopy.'' [Preview Abstract] |
Thursday, March 24, 2005 1:51PM - 2:03PM |
V12.00014: Electron scattering dependence of dendritic magnetic instability in superconducting MgB2 films Zuxin Ye, Qiang Li, Yufeng Hu, A.V. Pogrebnyakov, Y Cui, X.X. Xi, J.M. Redwing, Qi Li We studied magnetic stability in both ultra-pure and carbon-doped MgB$_{2}$ films using magneto-optical imaging, transport and bulk magnetization measurements. In the carbon-doped MgB$_{2}$ film, dendritic flux-jumps were observed at low temperature as reported in previous experiments. However, a remarkably stable flux penetration was observed in the ultra-pure MgB$_{2}$ film, clearly showing the classic behavior of the critical state model. Such different behaviors indicate that the electron scattering ultimately controls the magnetic stability of MgB$_{2}$ films. [Preview Abstract] |
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V12.00015: The different development of the anisotropic upper critical field in MgB$_2$ by aluminum and carbon doping M. Angst, S.L. Bud'ko, R.H.T Wilke, P.C. Canfield The temperature dependence of the upper critical field, $H_{c2} $, for both field directions in partially substituted polycrystalline MgB$_2$ was determined. Whereas the suppression of $T_c$ is similar for aluminum and carbon substituted samples, $H_{c2}$ is affected by the substitution in profoundly different ways. In the case of Al substitution changes can tentatively be described by intrinsic effects (shift of the Fermi level). In the C substituted samples, $H_{c2}$ is increased drastically, and extrinsic effects (disorder) have to play a major role. The strong contrast between the two substitutions is discussed, taking into account three relevant scattering rates. [Preview Abstract] |
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