Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J13: Defects and Other Structure in Superconductors |
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Sponsoring Units: DCMP Chair: Don Gubser, Naval Research Laboratory Room: LACC 402B |
Tuesday, March 22, 2005 11:15AM - 11:51AM |
J13.00001: Impact of inhomogeneities on various properties of novel superconductors and magnetic oxides Invited Speaker: Novel superconducting compounds such as cuprates and borocarbides are in the ``pseudogap state'' at T$>$Tc: they display anomalous diamagnetism, an energy gap, a ``giant'' proximity Josephson effect, etc. The compound is intrinsically inhomogeneous because of the effects of doping and pair-breaking. The phase diagram is characterized by three energy scales (T*$>$Tc*$>$Tc;res). When cooled below T*, the system begins to display phase separation, i.e., coexistence of metallic and insulating phases as well as other possible effects, e.g., a CDW gap.Below Tc* superconducting ``clusters'' embedded into the normal metallic matrix appear, leading to diamagnetism, pairing gap, a.c.losses, and the ``giant'' proximity effect. The Meissner and resistive transitions are split, and the ``intrinsic'' critical temperature (Tc*) greatly exceeds Tc;res. Further cooling towards Tc=Tc;res. increases the volume taken up by the superconducting ``islands'' and eventually results in a percolation transition at Tc, so that a macroscopic coherent dissipationless phase forms. The percolation transition is similar to that in manganites [L.Gor'kov, V.Kresin, Phys. Rep. 400, 149 (2004)]. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J13.00002: Electron Ising Nematic in High Tc Superconductors E.W. Carlson, K.A. Dahmen, E.H. Fradkin, S.A. Kivelson There is theoretical and experimental evidence that electronic liquid crystal phases can arise in strongly correlated electronic systems. The electron nematic breaks the rotational, but not the translational, symmetry of the host crystal. In the cuprates, Cu-O bonds are often favorable directions for such charge-ordered states. Then the electron nematic has Ising symmetry, corresponding to orienting the easy and hard local directions of conduction along the Cu-O bond directions. Quenched disorder in the host material acts as a random field. Using this mapping to the random field Ising model, we construct a random resistor network model which provides a direct connection between local ordering and local transport properties. Interesting behavior reminiscent of this model has been reported in recent experiments on the cuprates: switching noise with slow time dynamics was observed in the resistivity of YBCO nanowires (Bonetti et al., PRL 2004), and magnetic hysteresis at intermediate field strengths has been reported in LSCO (Panagopoulos et al., PRB 2004). [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J13.00003: Superconducting Fluctuations in 2D Riley Crane Using microwave cavities over a broad frequency range, we have performed a comprehensive temperature-dependent study of the AC fluctuation superconductivity in two-dimensional disordered InOx films. A number of distinct regions of superconducting fluctuations can be observed. Various scaling relations concerning these regions can be extracted. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J13.00004: Exploring the influence of out-of-plane impurities on the electronic structure of BSCCO-2212 K. McElroy, Jinho Lee, H. Eisaki, S. Uchida, J. C. Davis Nonstoichiometric atoms (NSA) are universally found in the High-Tc superconductors. BSCCO-2212 (the most studied of these compounds on the nanoscale) shows inhomogeneity (K. M. Lang et al Nature 2001) and the effects of weak scattering of low energy quasiparticles in much of its phase diagram (K. McElroy et al, Nature 2003; cond-mat/0406491). However, the interaction of these nanoscale low energy phenomena with the NSA is unknown. We report on scanning tunneling misroscope (STM) experiments that identify these NSA and characterize their effects on the low energy electronic structure. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J13.00005: Atomic arrangement and charge distribution in YBCO tilt grain boundaries J.K. Bording, J.W. Halley, Y. Zhu It is well known that the critical current, J$_{c}$, in high-Tc superconductors is reduced at grain boundaries. Recent high resolution holography experiments show the [100] tilt grain boundaries in YBa$_{2}$Cu$_{3}$O$_{7-x}$ to have an excess negative charge localized at the boundary dislocation core. Upon doping with Ca, this charge is reduced and the critical current increased. To shed light on this behavior at an atomic scale, we carried out Tight Binding (TB) calculations of these boundaries. Our TB scheme is charge self consistent to allow charge transfer typical for ionic materials. We present the arrangement of atoms and charge in YBCO tilt grain boundaries as determined by a combination of TB calculations, recent high resolution Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy measurements. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J13.00006: Transport Properties of Amorphous Tantalum Thin Films Near the Superconductor-Insulator Transition Yongguang Qin, Brian Gross, Jongsoo Yoon In amorphous superconducting thin films, the superconducting transition temperature continuously decreases with decreasing film thickness, and eventually the film becomes an insulator. This is known as the superconductor-insulator transition (SIT). Because the SIT is a phase transition between two different zero temperature ground state, it is an example of quantum phase transition. We observe such a SIT in amorphous tantalum thin films occurring at a sheet resistance of $\sim h^2/2e$ when the film thickness is $\sim 1nm$. We report detailed study on the transport characteristics of amorphous tantalum films at the vicinity of the superconductor-insulator transition. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J13.00007: Cooperative doping mechanism of YBCO grain boundaries A. Franceschetti, S.J. Pennycook, S.T. Pantelides A major impediment to large-scale applications of high-Tc superconductors has been the low critical crurrent across grain boundaries. Grain-boundary doping, in particular by Ca, was shown to increase the critical current, but an atomic-scale understanding of the underlying mechanism is still lacking. Here we report first-principles, density-functional calculations of Ca-impurity and O-vacancy formation energies in YBCO. Using biaxial strain to mimic the effect of local strain at the grain boundaries, we find that Ca segregates at Cu or Ba sites, depending on the sign and magnitude of the local strain field. In bulk YBCO, Ca$^{2+}$ ions are known to substitute for Y$^{3+}$ ions, thereby providing additional holes. However, the doping mechanism proposed for bulk YBCO does not apply to grain-boundary doping, as Ca substitutes for Cu or Ba atoms. Instead, we propose that Ca doping provide strain relief at the grain boundary, thereby reducing the propensity of O vacancies to segregate, and effectively restoring high hole concentrations. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J13.00008: Investigation of Current Characteristics in YBCO RABITS Tape Kris Barraca, Gukseon You, Jeremy Young, L. Wang, Chuhee Kwon Using Variable Temperature Scanning Laser Microscopy (VTSLM), the current transport properties of high temperature superconducting YBCO RABITS tape are studied. Voltage response images are taken to map the critical temperature (Tc*), as well as the critical current (Jc*) of the YBCO RABITS sample. These maps are used to investigate the way the current flowed through the YBCO RABITS film. They showed that the current flows through the YBCO RABITS film by percolation. The size of percolation cluster is about 50- 150 micrometers. Some high voltage response areas in temperature scanning are related to the lower value of Tc* and Jc*. This means that some defects result in the crowded current in those areas. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J13.00009: X-ray diffraction study of charge stripe order in La$_{1.875-x}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$ Hiroyuki Kimura, Yukio Noda, Hideto Goka, Masaki Fujita, Kazuyoshi Yamada, Masaichiro Mizumaki, Naoshi Ikeda, Hiroyuki Ohsumi The charge stripe order in La$_{1.875-x}$Ba$_{0.125- x}$Sr$_{x}$CuO$_{4}$ with $0.05\leq x\leq 0.10$ and its relevance with high-$T_{\rm c}$ superconductivity have been investigated by synchrotron X-ray diffraction. For $x=0.05$, as temperature decreases, incommensurate superlattice peaks associated with the stripe order appear just below the structural phase transition temperature $T_{\rm d2}$, indicating the strong relevance between the formation of the charge stripe order and the structural phase transition. However, in $x=0.075$ and 0.09, the superlattice peaks emerge far above $T_{\rm d2}$ as a short range correlation, indicating a precursor of charge ordering. Furthermore, temperature dependences of the superlattice peak intensity, correlation length, and incommensurability for $x=0.05$ are different from those for $x=0.075$ and 0.09. These results suggest that the transition process into the charge stripe order strongly correlates with the order of the structural phase transitions. A quantitative comparison of the structure factor associated with the charge order have been also made for all the samples. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J13.00010: Magnetic stripe formation of in-plane c-axis aligned YBCO thin films Chong Wang, Q. Y. Chen, Hye-Won Seo, Wei-Kan Chu, Tom Johansen The magnetic anisotropy of YBCO (110) thin films, with c-axis aligned in-plane, has been investigated by Magneto-Optical Imaging (MOI) method. The (110) YBCO thin films were fabricated by magnetron sputtering on SrTiO$_{3}$ substrates. The MOI measurement yields a stripe pattern of vortex-penetration deep into the film in the [110] direction. This pattern is superimposed on that predicted by the Bean model. The stripe is interpreted in the context of micro- or nano-irregularity on the sample edge. This stripe geometry is analyzed as the limiting case for a large assembly of parabolic contours typically found in an isotropic sample, such as c-axis oriented ones, in the presence of a region of non-superconducting irregularity. [Preview Abstract] |
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