Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S9: Superconductive Tunneling |
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Sponsoring Units: DMP Chair: Myron Salamon, University of Texas at Dallas Room: Colorado Convention Center Korbel 1D |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S9.00001: An asymmetric SQUID for measurement of ultra-small Josephson junctions D.F. Sullivan, J.R. Anderson, C.J. Lobb, F.C. Wellstood Ultra-small Josephson junctions offer a variety of potential applications, as well as an opportunity to probe the Josephson effect at the nanoscale. Such junctions, however, are susceptible to fluctuations in the phase difference, $\gamma _{1}$, across the junction, which leads to a suppression of the critical current I$_{01}$. The relevant energies which govern the physics of Josephson junctions are the charging energy E$_{C}$, the Josephson coupling energy E$_{J}$, and the thermal energy k$_{B}$T. Small junctions have E$_{C}$/E$_{J} \quad >>$ 1, while large junctions, with stable critical currents, have E$_{C}$/E$_{J} \quad <<$ 1. A potential method for stabilizing the phase across a small junction will be presented, which entails shunting it with an additional capacitance C$_{1 }$and incorporating it in a SQUID loop with another junction having a much larger critical current I$_{02}$. The SQUID loop inductance, L, couples $\gamma _{1}$ to the\textit{ stable} phase difference $\gamma _{2}$ of the large junction. Thus, by properly choosing L and C$_{1}$, the uncertainty in $\gamma _{1 }$should be reduced, allowing a precise measurement of I$_{01}$. In addition to the theoretical arguments behind this approach, experimental data incorporating these ideas will be presented. This work was supported by the National Science Foundation. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S9.00002: The interplay of the gap, the magnetic resonance, and the van Hove singularity Giorgio Levy, Christophe Berthod, Oystein Fischer The characteristic features of the tunneling spectra in the Bi-based HTS are a $d$-wave like gap structure, strong and often asymmetric coherence peaks, and an asymmetric dip-hump structure at higher energy. Hoogenboom \textit{et al.} [1] analysed the spectra of the two-layer compound Bi2212 and showed that all of these properties can be understood assuming $d$-wave superconductivity, a band structure as measured by ARPES, and an interaction of the quasiparticles with the magnetic resonant mode. In particular the asymmetric dip-hump results in this model from the interplay of the gap, the mode and the van Hove singularity present in the band structure. Here we analyse new data for the three-layer compound Bi2223. Unlike in Ref.~[1], we perform full unconstrained least-square fits in order to determine the various parameters of the model directly from the experimental data. This allows us to determine the doping dependence of the gap and of the magnetic resonance energy. [1] B. W. Hoogenboom, C. Berthod, M. Peter, \O. Fischer, and A. A. Kordyuk, Phys. Rev. B \textbf{67}, 224502 (2003). [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S9.00003: Scanning Tunneling Spectroscopy of Bi$_{2}$Sr$_{2}$CuO$_{6+x}$ Kamalesh Chatterjee, M.C. Boyer, W.D. Wise, Ming Yi, Takeshi Kondo, E.W. Hudson Scanning tunneling microscopy has revealed many interesting spectral features of the high temperature superconductors, including the nature of atomic scale defects like single atom impurities and magnetic vortices and the existence of inhomogeneity. Most of these studies have focused on the bilayer compound Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ (Bi-2212). Here we present scanning tunneling microscopy results from its single layer relative, Bi$_{2}$Sr$_{2}$CuO$_{6+x}$ (Bi-2201), comparing and contrasting these measurements with previously reported results from Bi-2212. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S9.00004: Temperature-Dependent STM Studies of the High Temperature Superconductor BSCCO Ming Yi, Kamalesh Chatterjee, M.C. Boyer, W.D. Wise, Takeshi Kondo, E.W. Hudson Even as the relationship between the superconducting and pseudogap states in high temperature superconductors remains mysterious, scanning tunneling microscopy has revealed a number of similarities and differences between spectroscopy in the two phases. Unfortunately, until now spectra from the same position have not been reported. Here we present results of spectroscopy on identifiable atoms, followed from 4 K to well above the superconducting transition temperature. In particular we will focus on implications for the relationship between the superconducting and pseudogap phases in BSCCO. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S9.00005: Point-contact spectroscopy of the pure and Cd-doped heavy-fermion superconductor CeCoIn$_{5}$ W. K. Park, L. H. Greene, J. L. Sarrao, J. D. Thompson, L. D. Pham, Z. Fisk Point-contact spectroscopy has been performed on pure and Cd-doped CeCoIn$_{5}$. Conductance spectra of CeCoIn$_{5}$ along three different orientations exhibit consistent features: i) background asymmetry; ii) Andreev reflection signal over similar energy scales ($\sim $1 meV) and of similarly reduced magnitudes (10 - 13{\%}) at zero-bias.$^{1,2}$ Comparison with the extended Blonder-Tinkham-Klapwijk model calculations indicate the first spectroscopic evidence for $d_{x2-y2}$ symmetry of the superconducting order parameter.$^{1,2}$ A two-fluid model$^{3}$ will be discussed to explain the asymmetry and the reduced Andreev reflection. Cd-doped (10{\%}) CeCoIn$_{5}$ exhibits intriguing conductance behaviors as a function of temperature and magnetic field, undergoing antiferromagnetic and superconducting transitions: a broad zero-bias peak below $T_{N}$ and two competing conductance channels below $T_{c}$. 1.$^{ }$W.K. Park \textit{et al}., PRB \textbf{72}, 052509 (2005); cond-mat/0507353; cond-mat/0606535. 2.$^{ }$W.K. Park and L.H. Greene, PRL \textbf{96}, 259702 (2006). 3. S. Nakatsuji, D. Pines and Z. Fisk, PRL \textbf{92}, 016401 (2004). Work supported by the U.S. DoE DEFG02-91ER45439 through the FSMRL and the CMM at UIUC, by NSF-DMR-0503360 at UCD {\&} UCI, and performed at LANL under auspices of the U.S. DoE, office of Science. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S9.00006: Superconducting gap anisotropy in LuNi$_{2}$B$_{2}$C by point-contact spectroscopy Xin Lu, Wan Kyu Park, Laura H. Greene, Jung-Dae Kim, Sunmog Yeo, Sung-IK Lee The superconducting gap anisotropy in non-magnetic members of the intermetallic borocarbide family still remains controversial. Several scenarios have been proposed including the s+g pairing symmetry and multi-band/multi-gap superconductivity. In order to address this issue, especially the puzzling existence of point nodes along $a$- and $b$-axis, we apply the point-contact spectroscopy technique to investigate the superconducting gap structure of single crystals LuNi$_{2}$B$_{2}$C ($T_{c}\sim $16.5 K) along three different crystallographic orientations. \textit{ab}-plane surfaces are prepared by embedding and polishing crystals and their orientations are confirmed by X-ray diffraction. Our preliminary conductance data, analyzed by the one-band Blonder-Tinkham-Klapwijk model, show anisotropic gap values, $\sim $1.6 meV and $\sim $2.6 meV, along (001) and (110) directions, respectively. We will discuss the possible origin for the gap anisotropy. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S9.00007: What is local about the Local Density Of States? Reza Jamei, John Robertson, Eun-Ah Kim, Alan Fang, Aharon Kapitulnik, Steven Kivelson While the influence of impurities on the local density of states (LDOS) in a metal is notoriously non-local due to interference effects, low order moments of the LDOS in general can be shown to depend only on the local structure of the Hamiltonian. Specifically, we show that an analysis of the spatial variations of these moments permits one to ``work backwards'' from scanning tunneling microscopy (STM) data to infer the local structure of the underlying effective Hamiltonian. Applying this analysis to STM data from the high temperature superconductor, BSCCO, we find that the variations of the electro-chemical potential are remarkably small (i.e., the disorder is, in a sense, weak) but that there are large variations in the local magnitude of the d-wave gap parameter. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S9.00008: Tunneling DOS of Superconductor / Strong Ferromagnet Bilayers Paul SanGiorgio, Malcolm Beasley, Serge Reymond, Jun Hyung Kwon, Tesu Kim, Kookrin Char We report tunneling density of states (DOS) studies of superconductor (Nb) / strong ferromagnet (CoFe, Ni) bilayers along with quantitative comparisons with calculations made with the Usadel equation. Since both CoFe and Ni are quite strong ferromagnets, we expect theoretically that the DOS we observe as a function of ferromagnet thickness, $d_F$, should be similar. Instead, we find that the Nb/CoFe superconducting DOS exhibits a scaling behavior with a characteristic length of $0.4$~nm, whereas the Nb/Ni superconducting DOS does not scale. Further, the Nb/Ni bilayers have a ``double peak'' DOS for $d_F = 1.5 - 3.0$~nm and for $d_F = 3.5 - 4.0$~nm, we report the first observation of an ``inverted'' DOS in a strongly ferromagnetic material. Various modifications to the basic Usadel equation will be compared to the experimental data. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S9.00009: Node-like excitations in superconducting PbMo$_6$S$_8$ probed by scanning tunneling spectroscopy Gilles Santi, C\'edric Dubois, Alexander Petrovic, {\O}ystein Fischer We present the first scanning tunneling spectroscopy study on the Chevrel phase PbMo$_6$S$_8$, an extreme type II superconductor with a coherence length only slightly larger than in high-$T_c$ cuprates. Tunneling spectra measured on atomically flat terraces are spatially homogeneous and show well-defined coherence peaks. The low-energy spectral weight, the zero bias conductance and the temperature dependence of the gap are incompatible with a conventional isotropic $s$-wave interpretation, revealing the presence of low-energy excitations in the superconducting state. We show that our data are consistent with the presence of nodes in the superconducting gap. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S9.00010: Superconducting and vortex properties of the $\beta$-pyrochlore KOs$_2$O$_6$ probed by scanning tunneling spectroscopy C\'edric Dubois, Gilles Santi, {\O}ystein Fischer The pyrochlore superconductor KOs$_2$O$_6$ was studied by scanning tunneling spectroscopy in both the Meissner and vortex states. In view of the controversy concerning the gap symmetry in this material, several symmetry scenarii were tested against our measured spectra. We find that a very anisotropic (40\%) s-like gap accounts best for the measured data. This could be interpreted as the signature of a singlet-triplet mixed state allowed by the absence of inversion symmetry in this compound. Vortices were observed for both magnetic fields considered (2 and 6~T) and were arranged in a hexagonal lattice. From the decay of the zero bias conductance away from the vortex cores, we obtain coherence lengths around 3--4~nm, in line with previous estimates based on $H_{\mathrm{c2}}$. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S9.00011: Doping-dependent effect of competing orders (CO) on low-energy quasiparticle (QP) excitations in cuprate superconductivity (SC) Andrew Beyer, Ching-Tzu Chen, Nai-Chang Yeh There is general consensus from experimental and theoretical studies of cuprate superconductors that CO with energies close to the SC gap exist in the cuprates and that at times they can coexist with SC in the ground state. Clarifying the exact role of CO requires both theoretical insight into the microscopic physics and sensitive experimental tools to determine the QP properties. We present an experimental and theoretical investigation of the low-energy QP excitations from coexisting CO and SC in hole-type Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{x}$ and YBa$_{2}$Cu$_{3}$O$_{x }$and in electron-type La$_{0.1}$Sr$_{0.9}$CuO$_{2}$. Our studies involve numerical simulations using a microscopic model of coexisting SC/CO and realistic bandstructures to fit experimental QP tunneling spectra to extract doping dependent CO and SC parameters. We suggest that the low-energy pseudogap is associated with CO being either charge-density waves or disorder-pinned spin-density waves but not d-density waves. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S9.00012: The duality of the density orderings in the high temperature superconductor Kangjun Seo, Jiangping Hu, Handong Chen We study a d-wave superconductor with possible orders in both the particle-particle and the particle-hole channels using the Bogoliubov-deGennes technique. In the superconducting phase, a duality exists in the particle-particle and particle-hole ordering channels. A small pair density localization generates the d-wave checkerboard density order(DWCB) in the particle-hole channel and the extended s-wave density order(PDW) in the particle-particle channel. The mixed state of DSC with DWCB and PDW with 4$a \times 4a$ periodicity can explain the checkerboard modulation observed in FT-STS from STM as well as the characteristic features such as non-dispersive Fermi arc in the pseudogap state. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S9.00013: Coulomb Interaction-induced Checkerboard Patterns in Disordered Cuprates Degang Zhang We study the effect of the Coulomb interaction on the local density of states (LDOS) and its Fourier component in disordered cuprates. It is shown that the Coulomb interaction suppresses strongly the maximum value of the LDOS induced by the dopant impurity at each energy and expands significantly the Friedel oscillation in real space. The existence of the Coulomb interaction with a moderate strength yields an energy- dependent checkerboard LDOS modulation around the impurity, which is very different from that produced by pure quasiparticle interference. The orientation and transformation of the checkerboard pattern with energy and the relations among the modulation vectors, dopings and the bias voltages agree qualitatively with the recent STM experiments. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S9.00014: Atomic Scale Imaging of Quasiparticle Lifetimes in Bi2Sr2CaCu2O8+d J.W. Alldredge, Jinho Lee, K. McElroy, K. Fujita, M. Wang, J.A. Slezak, H. Eisaki, S. Uchida, J.C. Davis Using a d-wave superconductor model with the addition of a $\Gamma _{1}+\Gamma _{2}$*Energy term we are able to relate the observed quasiparticle spectrum to two $\Gamma $ parameters which give us the quaisparticle lifetime on the atomic scale. The quasiparticle lifetime is related to classic impurity atoms (zinc) as well as local suppression of the superconducity at low dopings. The quasiparticle lifetime is shown to change both in spatial distribution and in value as a function of doping. We compare our measured lifetimes to ARPES data and to residual conductance measurements. [Preview Abstract] |
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