Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session V10: Proximity Effect, Electron-doped Cuprates, STM |
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Sponsoring Units: DCMP Chair: Thomas Lemberger, Ohio State Universtiy Room: Morial Convention Center RO8 |
Thursday, March 13, 2008 11:15AM - 11:27AM |
V10.00001: Superconducting proximity effect and Majorana fermions at the surface Liang Fu, Charles Kane A strong topological insulator is an insulating material in which spin-orbit interaction inverts the band gap at an odd number of time reversed pairs of points in the Brillouin zone. These materials have topologically protected gapless spin-split surface states, whose Fermi arc is characterized by a Berry's phase of $\pi$. We study the proximity effect between an s-wave superconductor and the surface states of a strong topological insulator. The resulting two dimensional state resembles a spinless $p_x+ip_y$ superconductor, but does not break time reversal symmetry. This state supports Majorana bound states at vortices. Such bound states obey non-Abelian statistics and have been studied in the context of topological quantum computing. We show that linear junctions between superconductors mediated by the topological insulator form a non chiral 1 dimensional wire for Majorana fermions, and that circuits formed from these junctions provide a method for creating, manipulating and fusing Majorana bound states. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V10.00002: Giant proximity effect in a phase-fluctuating superconductor Dominic Marchand, Lucian Covaci, Mona Berciu, Marcel Franz When a tunneling barrier between two superconductors is formed by a normal material that would be a superconductor in the absence of phase fluctuations, the resulting Josephson effect can undergo an enormous enhancement. We establish this novel proximity effect by a general argument as well as a numerical simulation and argue that it may underlie recent experimental observations of the giant proximity effect between two cuprate superconductors separated by a barrier made of the same material rendered normal by severe underdoping. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V10.00003: Search for Proximity Effect in the Local Pairing Temperature of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ Colin Parker, Aakash Pushp, Kenjiro K. Gomes, Abhay Pasupathy, Genda Gu, Shimpei Ono, Yoichi Ando, Ali Yazdani The proximity effect is seen when a superconductor is in contact with a metal. The pairing gap in the superconductor is reduced near the interface while superconducting correlations are induced in the metal. Recent results in high-Tc superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ indicate that the pairing gap closes inhomogeneously in space, producing a unique state just above Tc in these compounds where non-superconducting regions are in contact with regions where the pairing gap can still be measured. We will present detailed scanning tunneling measurements that map the local density of states in the same area of the sample from the low temperature fully gapped regime to the high temperature regime where most gaps have closed. We find that the temperature that the gap closes locally is sensitive to the gaps in the surrounding region. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V10.00004: Superconducting Proximity Effect in Thin Semiconducting Films Michael Vissers, Soren Flexner, Paul Welander, Kevin Inderhees, Tim McArdle, Jim Eckstein We report results using a novel 3 terminal device structure that provides two independent resistance measurements that we use to examine the influence of the superconducting proximity effect on both the transport properties of the thin film N layer by quantifying its sheet resistance, Rs, as well as independently measuring the junction conductance across the N-S boundary, Gc. When the N layer is a degenerate semiconductor the changes in these quantities are large. Gc increases much more than the factor of 2 that Andreev Reflection or BTK theory predicts, and both Gc and Rs exhibit reentrance as temperature decreases. We interpret these effects as being due to a transition between a phase fluctuating and phase stiff proximity effect in the N layer. This manifests itself by moving the N-S electrical boundary into the semiconductor, increasing Gc, while simultaneously removing volume available to normal transport forcing the measured Rs to increase. This work was supported by the DOE BES at the F. Seitz Materials Research Laboratory at the University of Illinois, Urbana. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V10.00005: Charge leakage in superconducting $\rm La_{2}CuO_{4}-La_{2-x}Sr_{x}CuO_{4}$ superlattices S. Smadici, P. Abbamonte, J. Lee, S. Wang, C.D. Cavellin, A. Gozar, G. Logvenov, I. Bozovic Ability of grow crystalline high-temperature superconductor (HTS) superlattices using molecular beam epitaxy has opened new avenues of research. Scanning tunneling microscopy measurements have identified a pronounced influence of the dopant atoms on microscopic properties of bulk HTS, therefore raising the question of whether the holes can be spatially separated in an oxide heterostructure from the disordered doped layers in a way analogous to semiconductor modulation doping. We used resonant soft x-ray scattering (RSXS) to answer this question for $\rm La_{2}CuO_{4}-La_{2-x}Sr_{x}CuO_{4}$ superlattices. For a $\rm 15\times [2\times LCO-4\times LSCO]$ superlattice with x=0.36, the measured hole amplitude modulation at the O $K$ edge shows a relatively weak localization of the doped holes to the LSCO layers. By using the interference between ``structural" and resonant scattering at the Cu $L_{3}$ edge, the c-axis stress of the LSCO sublattice was also observed. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V10.00006: Long range proximity effect in High Tc Josephson NanoJunctions: a quantitative study Jerome Lesueur, Nicolas Bergeal, Marco Aprili, Takis Kontos, Martin Sirena, Giancarlo Faini, Jean-Pierre Contour Proximity effect (PE) between a superconductor S and a normal metal N has been a powerful tool to study conventional superconductors. In High Tc (HTc) compounds, low quality interfaces and poor Fermi wave-vector match with most of metals considerably reduce the PE, and make really difficult its study. We have designed Josephson NanoJunctions in which two S reservoirs are coupled through an N layer at a nanoscale, where N is a lightly disordered HTc, whose Tc has been reduced by ion irradiation. In these SNS junctions, Cooper pairs propagate through the N layer by PE. In this situation with no metallurgical interfaces within the same material, we have shown that a long range PE takes place, which can be quantitatively described by the diffusive Usadel equations. The Josephson coupling temperature can be computed. The role of the order parameter symmetry will be also discussed. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V10.00007: The Superconducting State of Pr$_{2-x}$Ce$_x$CuO$_4$ : Tunneling study Yoram Dagan, Roy Beck, Richard Greene We report a tunneling study between the electron-doped high T$_c$ cuprate superconductor Pr$_{2-x}$Ce$_x$CuO$_4$ and Lead as a function of doping, temperature and magnetic field. The temperature dependence of the gap follows the BCS prediction. Our data fits a nonmonotonic $d$-wave order parameter for the whole doping range studied. From our data we are able to conclude that the electron-doped cuprate Pr$_{2-x} $Ce$_x$CuO$_4$ is a weak coupling, BCS superconductor in the dirty limit. Phys. Rev. Lett. 99, 147004 (2007) [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V10.00008: Local tunneling spectroscopy and infrared spectroscopy of the electron-doped cuprate Sm$_{2-x}$Ce$_{x}$CuO$_4$ A. Zimmers, Y. Noat, T. Cren, W. Sacks, D. Roditchev, B. Liang, R. L. Greene, R. P. S. M. Lobo, N. Bontemps We present infrared and local tunneling spectroscopy of the electron-doped cuprate Sm$_{2-x}$Ce$_{x}$CuO$_4$. In STM, at optimal doping x=0.15, a clear signature of the superconducting gap is observed with an amplitude ranging from place to place and from sample to sample ($\Delta\sim$~3.5-6meV). Another spectroscopic feature is simultaneously observed at high energy above $\pm$50meV. Its energy scale and temperature evolution is found to be compatible with previous photoemission and optical experiments. If interpreted as the signature of antiferromagnetic order in the samples, these results could suggest the coexistence on the local scale of antiferromagnetism and superconductivity on the electron-doped side of cuprate superconductors. Using optical spectroscopy, we analyzed the effects of the normal state gap opening (the higher energy gap seen in STM) and phonon structure as a function of temperature and doping from the underdoped to the metallic composition. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V10.00009: Spin correlations and magnetic excitation spectrum of electron-doped Nd$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$ Eugene Motoyama, Guichuan Yu, Yuan Li, Inna Vishik, Patrick Mang, Martin Greven, Owen Vajk, Klaudia Hradil, Richard Mole One of the most intriguing issues in the field of high-$T_c$ superconductivity is the electron-hole asymmetry: the hole- or electron-doping of the parent Mott insulators leads to superconductors with differing properties. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the antiferromagnetic phase extends much further with doping, appearing to overlap with the superconducting phase. Our inelastic neutron scattering measurements have shown the possibility that in the archetypical compound Nd$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$, genuine long-range antiferromagnetism and superconductivity do not co-exist (Motoyama {\it et al.}, Nature {\bf 445}, 186 (2007)). However, some uncertainty remains, due to the inhomogeneity of Ce concentration $x$ in the large single crystals. Here we report new results using improved homogeneity. In addition to the implications for the phase diagram using energy-integrated measurements, we discuss the impact of improved crystals on the (energy-resolved) magnetic excitation spectrum in the superconducting state. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V10.00010: EXAFS study of the role of apical oxygen on superconductivity in Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$ S. Rosenkranz, D. Haskel, M. Balasubramanian, S. Heald, S. Li, P. Dai, Y. Ando One of the longstanding puzzles surrounding high-T$_C$ in cuprates concerns the apparent asymmetry between electron and hole doping. Whereas hole doping quickly induces superconductivity, electron doping alone in materials such as R$_{2}$CuO$_{4}$ is insufficient and superconductivity is only achieved after a high temperature, low-oxygen annealing. While it was believed that this annealing process removes small amounts of oxygen in apical positions assumed to induce localization of the doped electrons, this scenario is incompatible with Raman, infrared- transmission, and ultrasound studies. In contrast, based on synchrotron X-ray and neutron diffraction studies combined with chemical and thermo-gravimetric analysis measurements, we recently showed that the annealing process alleviates minor Cu- deficiencies present in the as-grown sample. Here we present EXAFS studies on powder and single-crystal samples of as-grown and annealed Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$. Our results are consistent with no change in the occupation of apical oxygen between superconducting and as-grown samples, providing further evidence that the main effect of the annealing process is to repair defects in the superconducting planes due to Cu- deficiencies present in as grown samples. \\ Work supported by US DOE BES-DMS DE-AC02-06CH11357 [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V10.00011: High pressure study on Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-\delta }$ single crystals Costel R. Rotundu, Viktor V. Struzhkin, Alexander Goncharov, Richard L. Greene Transport measurements to 2.5 GPa on electron-doped cuprates revealed modest changes in physical properties, if any [1]. Room temperature synchrotron diffraction on powder samples show that the non-superconducting parent Pr$_{2}$CuO$_{4}$ exhibits a gradual structural transition from T' to T phase when subject to pressures greater than 15 GPa [2]. Pr$_{2-x}$Ce$_{x}$CuO$_{4}$ (PCCO) is superconducting for a Ce doping range 0.12 -- 0.2 (T' structure). Both pressure and Ce doping have the same shrinkage effect on the lattice constants. Here, we report high pressure ac susceptibility, resistivity and Raman shift data on x = 0.15 single crystals to 30 GPa. References: [1] C. Murayama \textit{et al.}, Nature, \textbf{339}, 293 (1989) [2] H. Wilhelm \textit{et al.}, Science and Technology of High Pressure, Proceedings of AIRAPT-17, pp. 740-743, Universities Press, Hyderabad, India (2000) [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V10.00012: Recent ARPES results on electron-doped high-Tc superconductors and comparison to their hole-doped counterparts Pierre Richard, Zihui Pan, Madhab Neupane, Yiming Xu, Patrick Fournier, Shiliang Li, Pengcheng Dai, Ziqiang Wang, Hong Ding Since the CuO$_{2}$ planes of cuprates, where high-temperature superconductivity occurs, can be doped either by holes or electrons, it appears important to establish similarities between these two types of doping in order to have an overview of high-temperature superconductors, especially for the electronic structure. Hence, although much less studied by ARPES than their hole-doped counterpart, the electronic structure of the electron-doped cuprates is believed to provide essential hints towards the understanding of high-Tc superconductivity. We present our recent ARPES results on electron-doped cuprates, focusing on various ranges of energy. We compare the results to those obtained on commonly studied hole-doped cuprates. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V10.00013: Point contact tunnelling as a probe for the superconducting properties of Niobium RF cavities T. Proslier, J. Moore, J. Zasadzinski, M. Pellin, J. Norem Niobium, with its very high H$_{C1}$, has been used in superconducting RF cavities for accelerator systems for 40 years with continuous improvement. The quality of cavities (Q) is governed by the surface impedance \textbf{R}$_{BCS}$, which depends on the quasiparticle gap, \textbf{$\Delta $}, and the superfluid density, \textbf{n}$_{S}$. Both of these parameters are seriously affected by surface imperfections (metallic phases, dissolved oxygen, magnetic impurities). Surface treatments of Nb cavities improved the Q factor, but are not understood from a fundamental point of view. Point contact tunneling spectroscopy is an ideal, surface sensitive probe as the quasiparticle current measures \textbf{$\Delta $}${\rm u}$ We present some measurements on SRF cavity-grade Nb using point contact tunneling demonstrating that the nature of the Nb oxide has a significant effect on $\Delta $. Atomic Layer Deposition was used to grow an alumina oxygen diffusion barrier on Nb providing control of the Nb oxidation layer composition with subsequent annealing. Point contact tunneling on these samples help to unravel the complicated effect of Nb surface oxidation on\textbf{${\rm g}\Delta $}. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V10.00014: Scanning Tunneling Microscopy of Defect-Induced Superstructure in 2H-NbSe$_{2}$ Hui Wang, Jonghee Lee, Michael Dreyer, Barry Barker Scanning tunneling microscopy and spectroscopy measurements were performed on pure 2H-NbSe$_{2}$ with defects introduced by tip-sample interaction at 4K. Domains of a new superstructure with a lattice constant equals $\sqrt {13} $a$_{0}$ instead of 3a$_{0}$ were observed around the defects. Closer to the defects we also observed the disordered phase of this structure. Both of them are stable under the STM measurements at 4K. Atomically resolved microscopy and spectroscopy studies suggest a 2H to 1T phase transition induced by surface atom sliding. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V10.00015: STM Investigation of Bosonic Modes and a Superconducting Gap in the Electron Doped Cuprate Pr$_{1-x}$LaCe$_{x}$CuO$_{4}$ Francis Niestemski, Shankar Kunwar, Sen Zhou, Shiliang Li, Hong Ding, Ziqiang Wang, Pengcheng Dai, Vidya Madhavan While continual effort in scanning tunneling microscopy (STM) remains focused on the hole-doped cuprates less attention has been given to the equally important electron-doped side of the phase diagram. ~We use a variable temperature UHV STM to investigate the electron-doped cuprate superconductor Pr$_{1-x}$LaCe$_{x}$CuO$_{4}$. ~We explore temperatures 2 K and higher and find a superconducting gap that disappears above Tc. ~We also find satellite features anti-correlated to the gap which we associate with bosonic modes. ~We relate our findings to neutron scattering results and discuss how this electron doped superconductor differs from more familiar hole-doped cuprates. [Preview Abstract] |
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