Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S13: uSR and NMR in Superconductors |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Steve Anlage, University of Maryland Room: LACC 402B |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S13.00001: Electronic phase separation in La$_{2-x}$Sr$_x$CuO$_{4+y}$ H.E. Mohottala, B.O. Wells, J.I. Budnick, H.A. Hines, Ch. Niedermayer, A.R. Moodenbaugh, F.C. Chou We have performed a combination of $\mu $SR and bulk magnetization experiments on oxygen-intercalated single crystals of \textit{La}$_{2-x}$\textit{Sr}$_{x}$\textit{CuO}$_{4+y}$, with various Sr contents x. All of the samples were both superconducting with $T_{C}$\textit{$\sim $40 K} and magnetic with a spin density ordering temperature $T_{M}$\textit{$\sim $40 K}. Interpretation of our results suggests that the superconducting and magnetic states arise from separate regions of the sample and comprise separate phases. The coexistence of these phases implies a phase separation that is purely electronic. In fact, we have seen no evidence for a structural phase separation in x-ray diffraction studies. The magnetic region is not superconducting and appears to be closely related to the anomalous, 1/8 hole doped, weakly superconducting versions of \textit{La}$_{2-x}M_{x}$\textit{CuO}$_{4}$. While the superconducting regions are harder to characterize, there is evidence that these regions are completely non-magnetic. This work reveals the subtleties of a more complex relationship between magnetism and superconductivity in the copper oxides than has previously been assumed. This work was partially supported by the US-DOE through contract DE-FG02-00ER45801 and the Cottrell Scholar Program of the Research Corporation. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S13.00002: Muon Spin Rotation Study of Field Induced Magnetism in Overdoped $\mathrm{La_{2-x}Sr_xCuO_4}$ G.J. MacDougall, J. Rodriguez, C.R. Wiebe, G.M. Luke, A.T. Savici, P.L. Russo, Y.J. Uemura, S. Wakimoto, R.J. Birgeneau The interplay between magnetism and superconductivity in the cuprates has been a topic of extensive research over the last two decades. Muon spin rotation ($\mu$SR) is an ideal probe of these properties because it is a volume sensitive local probe of magnetism and superconductivity. A rich diversity of phenomena has been discovered and explored using this technique in different parts of the electronic phase diagram. Unfortunately, such studies have not been extended to the highly overdoped region, largely due to the lack of high quality single crystals. Our group has recently obtained a series of highly overdoped crystals of $\mathrm{La_{2-x}Sr_xCuO_4}$ and performed transverse muon spin rotation (TF-$\mu$SR) studies as a function of magnetic field. A large, field-induced signal relaxation was seen with unusual temperature and doping dependences, extending well into the non-superconducting regime. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S13.00003: Muon spin rotation measurements of the vortex core size in single-gap and multi-gap type-II superconductors Fergal Callaghan, Mikko Laulajainen, Christina Kaiser, Jeff Sonier, Roger Miller We present muon spin rotation ($\mu$SR) measurements of the vortex core size in the single-gap superconductor V$_3$Si and the two-gap superconductor NbSe$_2$. Both temperature and applied magnetic field strongly affect the electronic structure of the vortex cores in these type-II materials, which in turn affects the spatial extent of the cores themselves. Increasing temperature causes excitations of the localized quasiparticle core states, whereas increasing magnetic field results in a delocalization of the core states. The presence of two gaps at the Fermi surface in NbSe$_2$ leads to an unusually large core size at low magnetic fields, with the ratio of the low-field to high-field core sizes being approximately equal to the ratio of the magnitudes of the two energy gaps. We show that the measured behavior of the vortex core size in these materials is fully explained in terms of these effects, and a comparison to recent theoretical work is made. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S13.00004: Penetration depth, symmetry breaking, and gap nodes in superconducting $\bf PrOs_4Sb_{12}$ Lei Shu, D.E. MacLaughlin, R.H. Heffner, G.D. Morris, O.O. Bernal, F. Callaghan, J.E. Sonier, A. Bosse, J.E. Anderson, N.A. Frederick, W.M. Yuhasz, M.B. Maple The vortex-state field distribution in the filled-skutterudite heavy-fermion superconductor PrOs$_4$Sb$_{12}$, in which there is evidence for time-reversal-symmetry (TRS) breaking, has been studied using transverse-field muon spin relaxation (TF-$\mu$SR)\@. The superconducting-state TF-$\mu$SR relaxation rate $\sigma_s(T)$, a measure of the vortex-lattice field distribution width, is found to be nearly constant below ${\sim}T_c/2$. Our results suggest $\lambda(T) \approx \rm const.$ at low temperatures, consistent with a nonzero gap for quasiparticle excitations. Surface penetration-depth measurements in zero static field yield $\lambda(T) - \lambda(0) \propto T^2$, which suggests point nodes in the gap. A similar discrepancy is found in the TRS-breaking superconductor~Sr$_2$RuO$_4$, but not in a number of non-TRS-breaking superconductors, conventional and unconventional. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S13.00005: NMR study of Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ ($x=0.17$) Guoqing Wu, W.G. Clark, S.E. Brown, F. Zamborszky, H. Balci, R.L. Greene Recent studies of the electron-doped high-temperature superconductor Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ (PCCO) show very unusual physical properties at the doping level $x=0.17$. Transport experiments are consistent with the existence of a quantum critical point in the normal state. Within the superconducting state, specific heat measurements in a magnetic field are interpreted as evidence for a transition in superconducting order parameter symmetry. We report the $^ {63,65} $Cu-NMR spectrum and spin dynamics of PCCO single crystals with $x=0.17$ and contrast their properties with crystals of $x=0.15 $. The anisotropic Knight shifts are dominated by the Pr$^{3+}$ moments and change little between the samples. The spin lattice relaxation rates are affected by the doping, as are the linewidths at low temperatures. \\ $^\ast$This work is supported at UCLA by NSF Grants DMR-0334869 (WGC) and 0203806 (SEB). [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S13.00006: NMR Studies of Plutonium-based Superconductors N.J. Curro, T. Caldwell, E.D. Bauer, L.A. Morales, M.J. Graf, Y. Bang, J.D. Thompson, A.V. Balatsky, J.L. Sarrao The intermetallic plutonium-based compound PuCoGa$_5$ exhibits the highest superconducting transition temperature for a heavy- fermion compound (18.5K), and may bridge the gap between the high temperature superconducting transition metal oxides, and the Ce and U-based heavy fermion superconductors. The unusual magnetic and electronic behavior of the plutonium may play a significant role in the pairing mechanism. We have performed extensive Ga and Co NMR investigations of the electric field gradient (EFG), the Knight shift and the spin lattice relaxation rate in both the normal and superconducting states, and our results suggest an unconventional pairing symmetry. Time dependent studies of the EFG suggest the presence of significant changes to the lattice due to the radioactive decay of the Pu. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S13.00007: Vortex vibrations and NMR T$_2$ relaxation in YBCO Rachel Wortis, Ting Lu The similarity of spin echo decay rates for planar and apical oxygen in the vortex state of YBCO suggest that T$_2$ is dominated by vortex motion. We present a calculation of T$_2$ assuming overdamped harmonic vibrations in an anisotropic 3D superconductor with the field applied parallel to the c axis. The resulting field, temperature and position dependence is compared with available data. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S13.00008: Re-examination of the interpretation of NMR spin-lattice relaxation measurements in cuprates Peter Fritz Meier, Anne-Christine Uldry \newcommand {\ybco}[1]{YBa$_2$Cu$_3$O$_{#1}$} \newcommand {\ybcoE}{YBa$_2$Cu$_4$O$_{8}\:$} \newcommand {\lasco}{La$_{2-x}$Sr$_{x}$CuO$_{4}\:$} We have examined measurements of the nucleus spin-lattice relaxation rates on Cu, planar O and Y reported for optimally doped \ybco{7} in the temperature range between 100 and 300 K. In a representation appropriate to anisotropic materials there is no striking different temperature dependence between the three sites. We have analyzed all the data with the model of fluctuating magnetic fields, factorizing the temperature dependence into a product of two temperature-dependent terms $V(T)$ and $\tau_{\mathit{eff}}(T)$. $V(T)$ contains the static antiferromagnetic spin-spin correlations and their changes in temperature which determine the influence of the hyperfine interaction energies. The effective relaxation time $\tau_{\mathit{eff}}(T)$ reflects the dynamics of the spins. We present a fit of the model parameters to the data and compare the model predictions extrapolated to higher temperatures with experiments. The model predicts two independent contributions to $\tau_{\mathit{eff}}$ which will be discussed in detail. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S13.00009: Comprehensive analysis of NMR spin-lattice relaxation data in underdoped cuprates Anne-Christine Uldry, Peter Fritz Meier \newcommand {\ybco}[1]{YBa$_2$Cu$_3$O$_{#1}$} \newcommand {\ybcoE}{YBa$_2$Cu$_4$O$_{8}\:$} \newcommand {\lasco}{La$_{2-x}$Sr$_{x}$CuO$_{4}\:$} We present an analysis of NMR and NQR spin-lattice relaxation measurements reported for \ybco{6.63}, \ybcoE and \lasco in terms of a model of fluctuating magnetic fields. The model parameters obtained by fitting the data vary among the different compounds and depend on the doping level. We compare these changes and discuss the physics behind them. The model generally fits the data well. In particular, high accuracy NQR measurements on the Cu relaxation in \ybcoE are fitted with excellent precision. The model provides also a simple explanation of the observed temperature and doping dependence of $^{63}T_{1c}^{-1}$ in \lasco, which have been measured up to high temperatures. It is shown that the data provide evidence for a crossover from the relaxation dominated by quasi-particle scattering to one governed by local spin fluctuations. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S13.00010: Temperature dependence of nuclear spin shifts in cuprates Samuel Renold, Anne-Christine Uldry, Peter Fritz Meier There exists by now an extended set of data on NMR spin shifts, $^k\! K_{\alpha,spin}$, measured for various nuclear species $k$ and directions $\alpha$ of the external field in a variety of cuprates. We present a phenomenological model that fits the temperature dependence $^k\! K_{\alpha,spin}(T)$ in the normal state of most of the published data in terms of a few parameters (including the pseudo gap) surprisingly well. The dependence of these parameter values on the compound and the doping level is presented and discussed. In particular, the pseudo gap temperature is compared to corresponding values deduced from the nuclear spin-lattice relaxation rates. Furthermore, we comment on the orbital shifts and their relationship to the measured values of $^k\! K_{\alpha,spin}$ at lowest temperatures. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S13.00011: Properties of the N layer inside of a SINIS sandwich-type Josephson junction Ivan Nevirkovets, Oleksandr Chernyashevskyy, John Ketterson We report characteristics of a multi-terminal SINIS device which has electrical leads connected to the middle N layer; here S, I, and N denote a superconductor (Nb), an insulator (AlO$_{x})$, and a normal metal (Al) respectively. Specifically, we studied properties of the middle (10-20 nm thick) N layer in dependence of Josephson current passing through all layers and on an injection current passing through one of the individual (NIS) junctions. The unperturbed I-V curve of the N film displays a small phase-coherent current near zero voltage, which can be suppressed by a very weak magnetic field (applied in parallel to the layers). In the last case, the state of the N film may be regarded as normal; at the same time, a sufficiently high supercurrent can flow between the S electrodes. An injection current (lower than the Josephson current) from one junction induces additional phase-coherent contribution to the I-V curve. We consider possible explanation of the observed behavior. [Preview Abstract] |
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