Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P9: Superconductivity: Synthesis |
Hide Abstracts |
Sponsoring Units: DMP Chair: Doug Kirven, Sigma-K Room: Colorado Convention Center Korbel 1D |
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P9.00001: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P9.00002: Superconducting properties of the hexagonal layered molybdenum carbide $\eta $-Mo$_{3}$C$_{2}$ K. Yamaura, Q. Huang, M. Akaishi, E. Takayama-Muromachi Superconductivity of $\eta $-Mo$_{3}$C$_{2}$ ($T_{c}$=8.5K) was reported in 1960s, while detailed superconducting and structure properties remained uncertain probably because those were complicated somewhat by carbon non-stoichiometry, partially thermal decomposition, and so on. Recently, we found the degree of problems is fairly reduced by employing high-pressure method, resulting in a distinct sample quality-improvement, which allowed us to conduct neutron diffraction, magnetic susceptibility, and specific-heat measurements on a polycrystalline form of $\eta $-Mo$_{3}$C$_{2}$ [1]. A significant layered character was found in the structure, which comprises edge-sharing CMo$_{6}$ octahedra sheets and $\sim $50{\%} carbon occupied blocks. Magnetic characterization revealed the Ginzburg-Landau parameter of $\eta $-Mo$_{3}$C$_{2}$ is $\sim $26, which is close to that for the comparable $T_{c}$ compound Li$_{2}$Pd$_{3}$B ($\sim $21), but less than a half of that for MgCNi$_{3}$ ($\sim $54). \newline \newline [1] K. Yamaura et al., PRB 74, 184510 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P9.00003: Origin of Superconductivity in the Y-Sr-Ru-O System Eduard Galstyan, Yuyi Xue, Milko Iliev, Yanyi Sun, Paul Chu The origin of the superconductivity (SC) in the perovskite-based Sr$_{2}$YRuO$_{6}$ system with minor Cu doping on Ru ion sites has attracted much interest among researchers in recent years. Addressing the question of the nature of SC, J. D. Dow and D. R. Harshman proposed a model in which SC condensation occurs in the SrO plane. While the model appears to be in disagreement with many other research works, the exact reason of SC in the Y-Sr-Ru-O system is still uncertain. In this work we provide experimental evidence that the SC behavior of these materials is attributed to a minor impurity phase with the stoichiometry of YSr$_{2}$Cu$_{3}$O$_{7-\delta }$ (YSCO). We show that both the SC and the YSCO grains appear only when the synthesis temperature is higher than a local melting temperature. In addition, the YSCO grains, which are thermodynamically unstable under ambient pressure, appear within dense surroundings, where the melting indications are obvious. Stresses during solidification seem to play roles and may also explain the earlier reports of trace SC in YSCO samples synthesized at ambient pressure. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P9.00004: Growth and superconductivity of single crystals La$_{2-x}$Ba$_{x}$CuO$_{4}$ G. D. Gu, J. S. Wen, Z. J. Xu, G. Y. Xu, J. M. Tranquada, Q. Li, A. R. Moodenbaugh, K. Yamada In 1986, Bednorz and Muller made a great discovery of the high temperature superconductivity in La$_{2-x}$Ba$_{x}$CuO$_{4}$ cuprate materials. Since the discovery of the superconductivity in high temperature superconducting oxide La$_{2-x}$Ba$_{x}$CuO$_{4 }$, a large number of groups have attempted to grow the single crystals. However, no single crystal La$_{2-x}$Ba$_{x}$CuO$_{4 }$with x$>$0.11 has been successfully grown. In this work, the effects of the growth condition and the compositions of a feed rod on the crystal growth of La$_{2-x}$Ba$_{x}$CuO$_{4 }$has been studied by an infrared image floating zone method. The experimental result shows that a planar solid-liquid growing interface tends to break down into a cellular interface when the growth velocity is more than 1 mm/h. When the planar solid-liquid growing interface break down into a cellular interface, the single crystal size decreases abruptly and the as-grown rod is not single phase. The large single crystals of La$_{2-x}$Ba$_{x}$CuO$_{4 }$with x=0 to 0.165 has been successfully grown. The single crystals of La$_{2-x}$Ba$_{x}$CuO$_{4 }$with x=0 to 0.165 up to 6 mm diameter and 150 mm length have been grown. The superconductivity transition temperature T$_{c}$ of as-grown single crystals of La$_{2-x}$Ba$_{x}$CuO$_{4 }$(x=0 to 0.165) have been measured. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P9.00005: Refined crystal growth and characterization of the high-T$_{c}$ superconductor HgBa$_{2}$CuO$_{4+\delta }$ Yuan Li, Neven Barisic, Guillaume Chabot-Couture, Yong-Chan Cho, Gertjan Koster, Guichuan Yu, Xudong Zhao, Martin Greven Among the high-T$_{c}$ superconductors, HgBa$_{2}$CuO$_{4+\delta }$ (Hg1201) is one of the most desirable systems for experimental study due to its relatively simple structure and high T$_{c}$. For quantitative experimental work, it is necessary to grow sizable, high-quality crystals, and to obtain fine oxygen/doping control. Here we report on our most recent improvements in the growth and characterization of Hg1201, leading to further improved sample quality. Our new results include charge transport, magnetic susceptibility, and x-ray photoelectron spectroscopy (XPS) measurements. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P9.00006: Carrier density control and phase diagram of Li$_x$ZrNCl superconductors Yasujiro Taguchi, Atsushi Kitora, Yoshihiro Iwasa We succeeded in synthesizing a series of Li$_x$ZrNCl samples with controlled doping level $x$ ($0 \le x \le 0.3$) which are confirmed to be of single phase by means of synchrotron x-ray diffraction measurements. We found that $T_c$ rapidly increases upon reducing Li concentration below $x$=0.12 to reach the maximum value of 15.2 K at $x$=0.06, and that a superconductor-to-insulator transition (SIT) is encountered at $x$=0.05 due to the Anderson localization effect. Such an increase in $T_c$ on the verge of SIT seems to be difficult to explain by the conventional theory, but may be indicative of the charge fluctuation contribution to superconductivity in low-carrier-density systems. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P9.00007: Strain induced morphological instability of epitaxial YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ films deposited on LAO and STO. Priya.V. Chinta, O. Lozano, P. Wadekar, Q.Y. Chen*, X.M. Wang, J.R. Liu, W.K. Chu, H.W. Seo, L.W. Tu, H.M. Huang, Y.L. Cheng, C.P. Sun, H.D. Yang Surface morphological instabilities in superconducting YBCO films have not yet been fully understood because of the intractably involved underlying driving forces. In this work we attempted to pin-point the lattice mismatch effect on the evolution of surface morphology for YBCO films on LaAlO$_{3}$ and SrTiO$_{3}$ substrates grown epitaxially by DC magnetron sputtering or laser ablation. The initial root-mean-square roughness of the samples was about 10-15 nm. This value was reduced to 1-3 nm after the samples were subjected to a 30-keV (Ar)$_{n}^{+}$ gas cluster ion beam (GCIB) sputtering at right angle to a dose of 2$\times $10$^{16}$/cm$^{2}$. Controlled annealing of these smoothened films in flowing O$_{2}$ atmosphere was then conducted at different temperatures and time periods, upon which consistent surface roughening (SR) was observed. This SR is attributed to the elastic strain of lattice-mismatch between the film and substrate. The effects of interface coherency on such phenomenon will be discussed. *Also with NSYSU [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P9.00008: Disordered superconducting films in strong magnetic fields Yonatan Dubi, Yigal Meir, Yshai Avishai Experimental studies of magneto-resistance in disordered superconducting thin films reveal an abundance of unexpected results, such as a huge peak in the magneto-resistance on the insulating side of the superconductor-insulator transition which evolves as the field is tilted, and traces of superconducting correlations that survive well above the transition. Recently, a theory that accounts for these effects was suggested, in which it is postulated that (i) well-separated SC islands are formed in the disordered film, and (ii) their size and strength diminishes with magnetic field. In this work we present extensive numerical calculations which support these conjectures, by means of a locally self-consistent solution of the BdG equations in the presence of disorder and tilted magnetic field. Simple phenomenological arguments from percolation theory are then used to explain various experimental findings, such as the non- monotonic magneto-resistance in parallel field and the relation between the critical field and the magnetic field tilt angle. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P9.00009: Investigation of tunneling density of state of FSF trilayers. Jun Hyung Kwon, Jewook Park, Seong Kook Choi, Kookrin Char, P.G. SanGiorgio, M.R. Beasley Unconventional superconductivity such as $\pi $-state and long range triplet superconductivity may arise in an FSF trilayer structure, depending on the relative directions of the magnetization of two ferromagnetic layers. In order to observe such unconventional states by measuring tunneling density of state(DOS) of FSF structure, we fabricated Al(12nm)/AlOx/CoFe(2,5nm)/Nb(20,30,40nm)/NiFe(10nm) using stencil mask method in a cross-strip geometry. In order to easily change magnetization direction of the magnetically soft NiFe layer, we further etched the last NiFe layer into a square shape by ion beam milling. We will present magnetic force microscope(MFM) image of the NiFe layer as its magnetization direction is switched to different directions. Density of state measurement of FSF structure with different thicknesses of the CoFe layer will be able to probe the unconventional superconductivity with appropriate magnetization directions of the two F layers. Our latest efforts to measure such state will be presented. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P9.00010: Magnetic Field Enhanced Insulating Behavior in Thin Films with Local Cooper Pairing K. H. Sarwa B. Tan, Kevin A. Parendo, Z. Ovadyahu, A. M. Goldman The effects of a perpendicular magnetic field on insulating amorphous indium oxide thin films exhibiting local superconductivity have been investigated. At very low temperatures the application of relatively low magnetic fields produces a giant positive magnetoresistance that increases with decreasing temperature. This suggests that the ground state in zero field may be a Cooper pair insulator. At nonzero temperatures a magnetic field enhances this insulating behavior. The low temperature I-V characteristics exhibit strong nonlinearities with threshold voltages for enhanced conduction whose magnitudes are independent of both temperature and magnetic field. This behavior indicates a connection with Cooper pairing and might be associated with the depinning of a charge structure. Although X-ray analyses characterize the films being amorphous, AFM images exhibit significant roughness that might be responsible for the formation of regions with a high local density of Cooper pairs. This work was supported in part by the National Science Foundation under grant NSF/DMR-0455121. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P9.00011: Electrical transport properties of ultrathin superconducting Pb films. R.P. Panguluri, M.M. Ozer, J.R. Thompson, H.H. Weitering, B.E. Nadgorny We present electrical transport properties of metallic ultra thin epitaxially grown Pb (111) films on Si (111) substrate. We observed a reduced superconducting transition temperature from bulk Pb using electrical resistivity measurements and deduced the temperature dependence of out-of-plane critical magnetic fields from the sheet resistance R as a function of the applied magnetic field. These results are consistent with M. M. Ozer et al.,$^{1}$ obtained by magnetic techniques. We identified the mean field critical temperature and current densities from I-V curves in zero magnetic field. We explored the possible presence of Kosterlitz-Thouless transition (T$_{KT})$ in this system. We discuss these results based on the Ginzburg-Landau Coloumb-Gas (GLCG) model for 2D vortex fluctuations. 1. M. M. Ozer, J. R. Thompson, and H. H. Weitering, Nature Physics, \textbf{2}, 173 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P9.00012: The effect of magnetic field on superconductivity in ultrathin amorphous Pb films with paramagnetic impurities. Ashwani Kumar, H. Jeffrey Gardner, Peng Xiong We report on a systematic study of the effect of applied magnetic fields on superconductivity in ultrathin amorphous Pb films containing various amounts of paramagnetic impurities. The Pb film, along with a 1 nm thick Sb buffer layer, was quench-condensed onto a Si substrate with pre-deposited Au contacts in a modified dilution refrigerator. Cr impurities were then deposited onto the Pb film by heating a NiCr wire at a fixed current. Both the Pb thickness (thus its T$_{C})$ and the Cr density can be varied, and electrical measurements can be performed at each step in perpendicular magnetic fields up to 8 T, all\textit{ in situ}. The reduction of the Pb T$_{C}$ with increasing Cr density is well described by the Abrikosov-Gorkov theory. The application of perpendicular magnetic fields did not result in any suppression of the pair-breaking effect by the Cr impurities, i.e., field enhanced superconductivity, on several samples covering a wide range of Pb thicknesses and Cr densities. The pronounced reentrant behavior found in the magnetic field-tuned transitions in pure Pb films$^{1}$ was progressively suppressed by increasing Cr impurities. $^{1}$ J.S. Paker et al., Europhys. Lett. 75, 950 (2006). [Preview Abstract] |
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