### Session A9: Superconductivity: Thermodynamic and Doping Effects

 Monday, March 5, 2007 8:00AM - 8:36AM A9.00001: YBCO at the border between antiferromagnetism and sperconductivity Invited Speaker: Douglas Bonn Monday, March 5, 2007 8:36AM - 8:48AM A9.00002: Investigation of the M\"{o}ssbauer spectrum of RuSr$_2$GdCu$_2$O$_8$ as a function of Temperature shows that there is only one type Ru site D. Coffey , G. Harmon , B. Graves , N. Miller , M. DeMarco , B. Dabrowski , S. Kolesnik , M. Maxwell , S. Toorongian , M. Haka A sample of RuSr$_2$GdCu$_2$O$_8$ was prepared with enriched $^{99}$Ru which allows us to study the temperature dependence of the M\"{o}ssbauer spectrum up 145K. The sample magnetically orders at 138K and has a transition to superconductivity at 8.7K with an onset at $\sim$13K. The spectrum at 4.2K was fit with a single-site fit. The hyperfine field is 59.4K with isomer shift which indicates that the charge state of the Ru ion is close to +5. The strength of the electric quadrupole interaction is 0.36 mm/sec with $\eta=0.2$. This spectrum is essentially identical to that found for a sample prepared with the natural $^{99}$Ru abundance. At 146K, above the magnetic transition temperature, the spectrum is fit with a pure electric quadrupole interaction of the same magnitude as at 4.2K with the same isomer shift. Monday, March 5, 2007 8:48AM - 9:00AM A9.00003: Magnetic Penetration Depth in Overdoped Tl-2201 Superconductors Jess H. Brewer , Scott Stubbs , Darren Peets , Ruixing Liang , Walter Hardy , Doug Bonn , Peter Russo , Jeff Sonier Studies of the magnetic penetration depth $\lambda_{ab}$ {\it via\/} the $\mu^+$SR lineshape in the vortex state has revealed a great deal about {\sl under\/}doped cuprate superconductors, including the original confirmation of $d$-wave superconductivity. However, {\sl over\/}doped cuprates have been neglected, partly due to the difficulty of doping sufficiently to decrease $T_c$, and partly because the overdoped materials are thought to be ordinary Fermi liquid'' superconductors, about which many presume we already know everything. In the belief that we may {\sl not\/} know everything about these materials, the UBC group has set out to grow high quality crystals of Tl$_2$Ba$_2$CuO$_{6+\delta}$ (Tl-2201), which can be made {\sl very\/} overdoped, to the point of $T_c \to 0$. We have now used $\mu^+$SR lineshape studies to measure $\lambda_{ab}$ as a function of $T$ and $H$ for crystal mosaics with $T_c$'s of 72, 60 and 46 K. As expected, $\lambda_{ab}^{-2}(T=0)$ continues to increase with doping beyond optimal doping, but then decreases again with higher doping. We also find a strong dependence on the applied field $H$. The low-$T$ behavior of $\lambda_{ab}^{-2}(T)$ is again strongly linear, as expected for a $d$-wave superconductor. Monday, March 5, 2007 9:00AM - 9:12AM A9.00004: Nodeless $d$-wave superconducting pairing in antiferromagnetic underdoped Pr$_{2-x}$Ce$_x$CuO$_{4-\delta}$ Tanmoy Das , R.S. Markiewicz , A. Bansil Experimental results concerning the superconducting pairing symmetry have been contradictory in electron doped cuprates. In particular, penetration depth measurements appear to indicate the presence of an s-wave and/or a d-wave gap in different doping regimes [1]. Here, we discuss the doping and $T$-dependence of the penetration depth in Pr$_{2-x}$Ce$_x$CuO$_{4-\delta}$ (PCCO) and provide a natural explanation for the occurrence of a nodeless superconducting gap and a nonmonotonic gap variation with maximum gap near hot-spots in the underdoped system [2]. Despite the presence of a $d_{x^2-y^2}$ pairing gap, we find a crossover of the low-$T$ penetration depth from a nodeless behavior in the underdoped case to a linear-in-T behavior (characteristic of $d$-wave) as doping increases and a nodal Fermi surface pocket emerges. The present results support the coexistence of antiferromagnetism and superconductivity in the electron doped cuprates [3]. Work supported in part by the USDOE. \newline [1] M.-S. Kim {\it {et. al.,}} PRL, {\bf{91}}, 087001 (2003). \newline [2] T. Das {\it {et. al.,}} PRB, {\bf{74}}, 020506(R) (2006). \newline [3] Y. Dagan {\it {et. al.,}} PRL, {\bf{92}}, 167001 (2004). Monday, March 5, 2007 9:12AM - 9:24AM A9.00005: Small T$^{-1}_{1}$ peak near T$_{c}$ in unconventional BCS superconductors David Parker , Stephan Haas It is usually believed that a coherence peak just below T$_{c}$ in the nuclear spin lattice relaxation rate T$_{1}^{-1}$ in superconducting materials is a signature of conventional s-wave pairing. We demonstrate that {\bf any} unconventional superconductor obeying BCS pure-case weak-coupling theory should show a small T$_{1}^{-1}$ coherence peak near T$_{c}$ ,generally with a height between 3 and 15 percent greater than the value at T$_{c}$. It is due to impurity scattering, magnetic effects, gap anisotropy and other effects that this peak has not been commonly observed. Monday, March 5, 2007 9:24AM - 9:36AM A9.00006: A Practical Algorithm for Fitting Magnetic Moment Data for Superconducting Thin Films and Multilayers in Parallel Magnetic Fields Sergiy Kryukov , Wentao Xu , Lance De Long Superconducting thin films and multilayers in DC magnetic fields applied nearly parallel to the film plane can yield spurious magnetization data dominated by extreme shape anisotropy and strong diamagnetism of a confined supercurrent. This situation may lead to highly reproducible discontinuities or apparently random instabilities'' when measured with SQUID magnetometers such as the ubiquitous Quantum Design MPMS, which requires samples to behave as an ideal point-dipole. We have devised an accurate multipole fitting routine for the raw SQUID voltmeter output that eliminates spurious contributions to the axial dipole moment from transverse (off-axis dipole) or non-point-dipole axial magnetizations. We demonstrate this method as applied to Nb/Ni multilayers and Nb thin films and foils of various thicknesses that probe the influence of supercurrent confinement on the non-dipole response. Monday, March 5, 2007 9:36AM - 9:48AM A9.00007: Magnetic Instabilities along the Superconducting Phase Boundary of Nb/Ni Multilayers Wentao Xu , Amish Joshi , Sergiy Kryukov , Lance De Long , Elvira Gonzalez , Elena Navarro , Javier Villegas , Jose Vicent We report vibrating reed and SQUID magnetometer data that exhibit prominent cusps or oscillations of the SC onset temperature, $\vert \Delta$T$_{C}$(H)$\vert \quad \approx$ 0.01 to 0.7 K, for a [Nb(23nm)/Ni(5nm)]$_{5}$ multilayer (ML) in DC magnetic fields applied nearly parallel to the ML plane. The vibrating reed data exhibit additional structures below T$_{C}$ that may mark multiple SC transitions or vortex lattice rearrangements within the ML. This striking behavior would then pose new challenges for theoretical and experimental investigations of SC/FM interfaces that involve pi phase shifts'' of the SC order parameter and exotic (LOFF'') pairing states. Alternatively, the anomalies may signal dynamical instabilities within a confined, strongly anisotropic Abrikosov vortex lattice. Monday, March 5, 2007 9:48AM - 10:00AM A9.00008: X-ray Structural Studies of HgBa$_2$CuO$_{4+\delta}$ G. Chabot-Couture , J. N. Hancock , L. Lu , A. Bianconi , F. Bridges , Z. Islam , H. Oyanagi , Y.-C. Cho , Y. Li , G. Yu , X. Zhao , M. Greven In recent years, there has been mounting evidence for electronic and structural inhomogeneities in the cuprate high-temperature superconductors (HTSC). From stripe phases found in lanthanum-based cuprates to the oxygen-order-driven lattice modulations in YBa$_2$Cu$_3$O$_{7-\delta}$ and to the nanoscale electronic density-of-states patches" in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and other cuprates, these inhomogeneities appear to have significant effects on the electronic, transport, and spectroscopic properties of these systems. Of all the high-T$_\textrm{c}$ materials, HgBa$_2$CuO$_{4+\delta}$ has the highest transition temperature among single-layer compounds and one of the simplest structures. Consequently, it may be the perfect candidate system to help separate the effects of extrinsic structural inhomogeneities from those that are universal and intrinsic to HTSC. To begin to address this issue, we have grown sizable, high-quality crystals of HgBa$_2$CuO$_{4+\delta}$ and carried out two structural studies: a diffuse x-ray scattering experiment, showing evidence for short-range structural displacement modulations, and a polarized extended x-ray absorption fine-structure (EXAFS) experiment on the temperature-dependent local structure around the copper and mercury atoms. Monday, March 5, 2007 10:00AM - 10:12AM A9.00009: Microscopic origin of the oxygen reduction process and its impact on superconductivity in electron-doped copper oxides Hye Jung Kang , Pengcheng Dai , Branton J. Campbell , Peter J. Chupas , Stephan Rosenkranz , Peter L. Lee , Qingzhen Huang , Shiliang Li , Seiki Komiya , Yoichi Ando The oxygen reduction process is one of the unique processes in the electron-doped high temperature copper oxides. Superconductivity is induced when the electron-doped as grown samples are annealed in the oxygen reduced atmosphere. Many experiments show that a small amount of oxygen reduction affects the mobility of charge carriers and it suppresses the long range antiferromagnetic order especially at high doping level. However, the detailed microscopic process of oxygen reduction and its effect on superconductivity are still unknown. Our x-ray and neutron scattering data, combined with chemical and thermo-gravimetric analysis measurements in the electron-doped Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4}$ show that the microscopic process of oxygen reduction is to remove Cu deficiencies in the as-grown materials and to create oxygen vacancies in the stoichiometric CuO$_{2}$. Our results indicate that the role of annealing is to repair disorder in the CuO$_{2}$ plane induced by Cu deficiencies and to provide itinerant electrons for superconductivity. Monday, March 5, 2007 10:12AM - 10:24AM A9.00010: X-ray diffuse scattering experiments from bismuth based high T$_c$ superconductors M. Izquierdo , S. Megtert , P. A. Albouy , J. Avila , M. A. Valbuena , G. Gu , J. S. Abell , G. Yang , M.C. Asensio , R. Comes A detailed X-ray diffuse scattering study of the recently found two dimensional (2D) displacive short range order (SRO) superstructure, with doubled periodicity along the orthorhombic a direction from the high T$_c$ superconductors Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$(BISCO-2212) is reported. The investigation has been extended to high and low temperatures for optimally doped crystals, to crystals with different doping levels and to the one layer compound Bi$_2$Sr$_2$CaCuO$_ {6+\delta}$ (Bi-2201). The most striking feature is that both, the intensity of the diffuse scattering and the extent of the 2DSRO vary with doping as the critical temperature, T$_c$. These findings show that these short range ordering features are of importance for a better understanding of high Tc materials, at least those from the BISCO family. Monday, March 5, 2007 10:24AM - 10:36AM A9.00011: Low-temperature Specific Heat in underdoped YBa$_{2}$Cu$_{3}$O$_{7}$ Single Crystals Hai-Hu Wen , Yue Wang , Hong Gao , Lei Shan One single crystal of YBa$_{2}$Cu$_{3}$O$_{7}$ has been post-annealed into six different doping levels in the underdoped region with T$_{c}$ ranging from 30 K to 92 K. The low temperature specific heat has been measured on these samples down to 100 mK with magnetic field applied along c-axis and a-b plane. By subtracting the specific heat measured in these two different field directions, we have successfully removed the Schottky anomaly and obtained the field induced increase of the specific heat coefficient due to Doppler shift effect in d-wave superconductors. It is found that even for the very underdoped sample (T$_{c}$=30 K), the quasiparticle density of states is always increased by applying a magnetic field. This is similar to our earlier results in very underdoped LaSrCuO single crystals leading to the expectation for a Fermi arc ground state for the pseudogap state. In addition, the field increased part $\Delta \gamma$=AH$^{0.5}$ becomes smaller towards underdoping, indicating a larger nodal gap slope. The implications of our results on the mechanism of cuprate superconductors are discussed based on the Fermi arc picture of normal state. The YBCO single crystal was provided by Prof. Xin Yao at Shanghai Jiaotong University, China. . Monday, March 5, 2007 10:36AM - 10:48AM A9.00012: Thermodynamic properties of cuprate superconductors: Singularities and Pseudogaps. James Storey , Jeffery Tallon , Grant Williams We have calculated the entropy and superfluid density of Bi-2212 from an ARPES-derived rigid energy dispersion and a model for the normal-state pseudogap. Their detailed doping and temperature dependence is found to closely mimic the experimentally measured data, thus indirectly validating the ARPES data. The doping level at which the Fermi level crosses a van Hove singularity (vHs) is determined and found to agree with that inferred from ARPES. The superfluid density is found to be linear in $T$ at the vHs crossing. Surprisingly, the doping dependence of $T_{c}$ seems to be unaffected by crossing the singularity. Monday, March 5, 2007 10:48AM - 11:00AM A9.00013: Confined vortex loops in superconductors with a magnetic core Mauro Doria , Antonio Romaguera , Milorad Milosevic , Francois Peeters A magnetic moment inside an extreme type II superconductor gives rise to confined vortex states in its neighborhood. We show how the presence of such states can be tracked down in the I-V characteristic curve, thus providing a simple method to their detection. The Lorentz force causes their growth and periodic evolution, passing through a long range ordered state (bulk) or an external vortex pair state (sub-micron particle). For a small magnetic moment there are exactly three confined vortex loops whereas for a large more elaborate vortex arrangements arise are possible. Their onset from the upper critical field core in sets of threes is proved to be energetically favorable over the growth of just one or two confined vortex loops independently of boundary conditions. All our results are derived in the context of the time-dependent and of the time-independent Ginzburg-Landau theory applied to a superconductor with periodic (bulk) and superconductor-insulator (sub-micron particle) boundary conditions, using three independent numerical methods.