Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W25: Novel Superconductors I |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Taner Yildirim, NIST/University of Pennsylvania Room: D166 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W25.00001: Parallels and contrasts in the pairing mechanism between cuprate and organic superconductors Thomas Maier The organic superconductors share many characteristics with the cuprates, such as the existence of antiferromagnetism, unconventional superconductivity and Mott insulating behavior. In addition, despite their complexity, their physics can be approximated by a single-band Hubbard model on a two-dimensional lattice. Here, we will present dynamic cluster quantum Monte Carlo simulations of a half-filled ``dimer'' Hubbard model on an anisotropic triangular lattice, which find a transition from an antiferromagnetic phase to a d-wave superconducting phase with increasing frustration. In particular, we will discuss commonalities and differences in the nature of the superconducting behavior between the dimer Hubbard model of the organic compounds and the standard Hubbard model of the cuprate materials. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W25.00002: Can simple hydrocarbon molecular solids superconduct? A theoretical investigation of superconductivity in K$_{3}$Picene Xuhui Luo, Serdar Ogut, Taner Yildirim Unlike many well established high temperature (T$_{c})$ superconductors such as cuprates, M$_{3}$C$_{60}$, MgB$_{2}$ and iron-pnictides, the possibility of superconductivity in molecular hydrocarbon solids remains a controversial issue. This topic became active again by a recent study reporting superconductivity up to T$_{c} \sim $ 17 K in potassium doped Picene [1], a wide-bandgap semiconducting solid hydrocarbon. However, there is no theoretical study about possible mechanism of superconductivity. In this talk, we present a detailed first-principles study of the electron-phonon (el-ph) coupling in doped organic molecular solids. Due to large system size, the calculation of el-ph coupling using the standard linear response theory is not feasible. Hence, we have developed a finite-displacement method where both the phonon energies and el-ph coupling can be easily calculated for large systems. We have tested our code for well-established superconductors such as K$_{3}$C$_{60}$ and MgB$_{2}$. As a comparison, we have also studied the el-ph coupling in alkali-doped Pentacene, a similar well-studied hydrocarbon in which no superconductivity has been observed. We discuss the effect of charge transfer as well as pressure on T$_{c }$for solid Picene and make predictions for future possible experiments. \\[0pt] [1] Mitsuhashi et al. Nature, 464, p. 76 (March, 2010). [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W25.00003: Electronic structure and magnetic properties of the hydrocarbon K$_{3}$picene superconductor near the metal-insulator transition Minjae Kim, B.I. Min, Geunsik Lee, Hee Jae Kwon, Y.M. Rhee, Ji Hoon Shim Superconductivity has recently been observed in K-doped picene, K$_{3}$picene, which is a first organic superconductor in the hydrocarbon system with high transition temperature Tc=18K [1]. We have investigated the electronic structures and magnetic properties of K$_{3}$picene by density-functional theory. We have shown that the metal-insulator transition (MIT) is driven in K$_{3}$picene by 5{\%} volume enhancement with a formation of local magnetic moment. Active bands for superconductivity near the Fermi level (E$_{F}$ ) are found to have hybridized character of LUMO and LUMO+1 picene molecular orbitals. Fermi surfaces of K$_{3}$picene manifest neither prominent nesting feature nor marked two-dimensional behavior. By estimating the ratio of the Coulomb interaction $U $and the band width $W $of the active bands near E$_{F}$ , $U/W$, we have demonstrated that K$_{3}$picene is located in the vicinity of the Mott transition. \\[4pt] [1] R. Mitsuhashi \textit{et. al}. Nature. 464, 76 (2010) [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W25.00004: Slow-Motion $^{1}H$ NMR Study of $\kappa$-(ET)$_{2}$Cu[N(CN)$_{2}$]Br J.C. Gezo, Tak-Kei Lui, R.W. Giannetta, C.P. Slichter, J.A. Schleuter The recent discovery of an anomalous Nernst signal in the pseudogap phase of organic superconductor $\kappa$-(ET)$_{2}$Cu[N(CN)$_{2}$]Br suggests the presence of magnetic flux vortices above T$_{c}$.\footnote{M. S. Nam et al, Nature 449, 584-587 (2007)} We report spin-locked and line-narrowed proton NMR data on the pseudogap phase of this material. These experiments are sensitive to magnetic fluctuations, and probe far slower timescales ($10^{-6}$-$10^{-3}$s) than the previously explored NMR parameter space. Other experiments\footnote{C. H. Recchia et al, Phys. Rev. Lett. 78, 3543-3546 (1997)} have suggested that vortices leave an NMR fingerprint at these low frequencies. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W25.00005: NMR Study of $\kappa$-$\mathrm{(ET)_{2}Cu[N(CN)_{2}]Br}$ Tak-Kei Lui, J.C. Gezo, R.W. Giannetta, C.P. Slichter, J.A. Schlueter We report measurements of the $\mathrm{^{13}C}$ spin-lattice relaxation rate, $(T_{1}T)^{-1}$ of $\kappa$-$\mathrm{(ET)_{2}Cu[N(CN)_{2}]Br}$, an organic superconductor with $T_{C}=11.9\,\mathrm{K}$. The shape of the $(T_{1}T)^{-1}$versus $T$ graph suggests the existence of a finite spin singlet-triplet gap\footnote{C. P. Slichter, Experimental Evidence for Spin Fluctuations in High Temperature Superconductors, in \emph{Strongly Correlated Electronic Materials}: A Los Alamos Symposium, pp. 427-479, ed. K.S. Bedell \emph{et al}., Addison-Wesley (1994)}. Knight shifts were measured to search for a two-component electronic system. We also report measurements using an ``S-shape'' pulse sequence\footnote{L. R. Becerra, C. A. Klug, C. P. Slichter, and J. H. Sinfelt, J. Phys. Chem. \textbf{97}, 12014 (1993)} that inverts half of the absorption line, permitting one to study slow motions and spin density waves. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W25.00006: 13C NMR measurements of Zeeman limited superconductivity in $\kappa $-(BEDT-TTF)2Cu(NCS)2 Jeffrey Wright, James Brugger, John Schlueter, Reizo Kato, Stuart Brown The class of superconductors formulated by the BEDT-TTF molecule offers a unique ability to study superconductivity which is Zeeman limited, utilizing their qausi-2D layered structure. The critical field H$_{c2}$, which usually limits superconductivity due to orbital screening currents, can be suppressed by aligning the applied magnetic field parallel to the conducting layers. In this orientation, the field penetrates in the form of Josephson vortices, and the dominant effect of the magnetic field is from the Zeeman interaction with quasiparticles. We present 13C NMR measurements of the spin lattice relaxation rate as a function of applied field on a single crystal of $\kappa $-(BEDT-TTF)2Cu(NCS)2 after aligning the field in this orientation. A quadratic dependence is observed: R(B) $\sim $ B$^{2}$, which gives clear evidence of k-space nodes in the superconducting gap. Extending these measurements to field strengths near and above the Pauli limit, we observe a phase transition within the superconducting state at B=21.5T, and we comment on the compatibility of these results with the sought after FFLO state. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W25.00007: Upper Critical Field in the Molecular Organic Superconductor (DMET)$_{2}$I$_{3}$ Pashupati Dhakal, Harukazu Yoshino, Jeong Il Oh, Koichi Kikuchi, Michael J. Naughton We report the temperature dependence of the upper critical magnetic field in the quasi-one-dimensional molecular organic superconductor (DMET)$_{2}$I$_{3}$, for magnetic field applied along the intrachain, interchain, and interplane directions. The upper critical field tends to saturation at low temperature for field in all directions and does not exceed the Pauli paramagnetic limit. Thus the superconductivity appears to be conventional spin singlet, in contrast to the status of the isostructural Bechgaard salts. Furthermore, we will discuss a magnetic field-induced dimensional crossover effect in the normal metallic state leading to a magnetoresistance minimum for the field parallel to the most conducting plane. This result will be useful to understand the nature of superconductivity in the molecular organic conductors. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W25.00008: London penetration depth of $\lambda$-(BETS)$_2$GaCl$_4$ superconductor Jason Murphy, H. Kim, K. Cho, M.A. Tantar, A. Kobayashi, H. Kobayashi, R. Prozorov After many years of study, it is still unclear whether the superconductivity of organic charge transfer salts is conventional or not. In particular, symmetry of the superconducting gap is an important unresolved question. The gap anisotropy and nodal structure can be probed via precision measurements of London penetration depth, $\lambda(T)$, for which tunnel-diode resonator has been developed over past 20 years. Here we report $\lambda(T)$ in $\lambda$-(BETS)$_2$GaCl$_4$ crystals ($T_c \sim 5$~K ) measured down to 400 mK. We find that different samples exhibit different widths of superconducting transition, $\Delta T_c$. It is known for organic superconductors that cooling rate from room temperature is important for the properties probed at the low temperatures. We discuss observed variation of $\Delta T_c$ with the cooling rate as well as sample size and aspect ratio. Results are compared with thermal conductivity measurements [1]. \\[4pt] [1] M. A. Tanatar, T. Ishiguro, H. Tanaka, H. Kobayashi, Phys.Rev. B {\bf 66}, 134503 (2002). [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W25.00009: Thermodynamic study of the superconducting gap structure of (TMTSF)$_2$ClO$_4$ Shingo Yonezawa, Yoshiteru Maeno, Klaus Bechgaard, Denis Jerome We have studied the superconducting (SC) gap structure of the quasi-one-dimensional molecular conductor (TMTSF)$_2$ClO$_4$ based on our high-resolution heat capacity measurement. We developed a new calorimeter, which allowed us field-angle resolved calorimetry using one single crystal weighing as small as 76~$\mu$g. From our results, we investigate the SC gap structure in the $k$-space as well as in the spin space. Comparison between the SC phase diagram deduced from the present results and those obtained from previous resistivity measurements are also discussed. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W25.00010: Superconducting energy gap features of MgB$_{2}$ thin films on different substrates and orientations Wenqing Dai, Qi Li, Ke Chen, Xiaoxing Xi We report a detailed study of tunneling spectra of MgB$_{2}$/I/Pb planar junctions with MgB$_{2}$ films on various substrates and of various doping levels. Planar trilayer junctions were fabricated using MgB$_{2}$ films with native oxide barrier grown by the Hybrid Physical-Chemical Vapor Deposition technique. Both $\pi $ and $\sigma $ bands contribute to the tunneling spectra of tilted-axis films on MgO (211) substrate and mainly $\pi $ band was observed on c-axis MgB$_{2}$ films on SiC (0001), MgO (111) and c-sapphire substrates. We observed $\sigma $ gap value of $\sim $7.9 meV in MgB$_{2}$ films on SiC substrates which display higher T$_{c}$ due to the lattice strain. This is larger than 7.4 meV on unstrained substrates. However, the $\pi $ gap value of all samples is $\sim $2.3 meV. We concluded that the strain in MgB$_{2}$ films on SiC substrates mainly affects the $\sigma $ band of MgB$_{2}$. In addition, small amount of nitrogen gas was added during film growth to introduce more scattering in MgB$_{2}$ films. We systematically studied the change of two gaps with nitrogen doping from the tunneling spectra of MgB2/I/Pb junctions. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W25.00011: Measurements of Multiple Gap Substructure in MgB$_2$ Below 1 Kelvin Steven Carabello, Joseph Lambert, Roberto Ramos The two superconducting energy gaps of magnesium diboride (MgB$_2$) are well established. First-principles calculations have also predicted substructure within the sigma- and pi-band gaps. However, due to anticipated intraband impurity scattering, there is controversy as to whether these finer structures can be observed in real samples. Prior experimental evidence above 1 Kelvin has provided evidence supportive of these features. We have performed tunneling spectroscopy experiments on MgB$_2$/insulator/Pb Josephson junctions on SiC substrates, at temperatures as low as 20 mK. By measuring differential conductance at low temperatures, and by using extremely clean MgB$_2$ thin films, we have resolved features within the energy gaps to under 1 meV. We report results of these experiments, which are in remarkable agreement with theoretical predictions for this substructure. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W25.00012: Time Dependent Transport Properties of Superconducting MgB$_{2}$ Murat Olutas, Atilgan Altinkok, Atilla Kilic, Kivilcim Kilic The time dependent transport properties of polycrystalline superconducting MgB$_{2}$ sample was investigated in details by means of current-voltage (I -- V curves) measurements with different current sweep rates (dI/dt) and transport relaxation (V -- t curves) measurements. The corresponding measurements were carried out as functions of temperature (T), transport current (I) and external magnetic field (H). Upon cycling of the transport current, it was observed that the hysteresis effects in I -- V curves are negligible. Further, for slow or fast sweep rates (dI/dt) of the transport current, the measured voltage dissipation at a given transport current is approximately stable and does not change much with time. Analysis of the I -- V curves shows that the pinning potential is also practically independent of dI/dt. The time evolution of sample voltage reveals that the increase in sample voltage is quite a sharp and non-linear. The V -- t curves were interpreted in terms of re-organization of flux lines and suppression of the effective superconducting order parameter along the sample as the time progresses. The quenched state created in V -- t curves was correlated to the supercooling of flux lines. Finally, the experimental results of polycrystalline MgB$_{2}$ sample were compared to previous similar studies on superconducting Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-x}$ and Bi$_{1.7}$Pb$_{0.3}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{x}$ polycrystalline samples. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W25.00013: Measurement of Hc1 in MgB2 thin films and multilayer structures by a microwave absorption technique Chenggang Zhuang, Ke Chen, Som Tyagi, Xiaoxing Xi For superconducting RF applications, Gurevich suggested a route to enhance the vortex penetration field, Hc1, and thermal breakdown field by a multilayer structure consisting of alternating insulator and superconductor layers with thicknesses smaller than the penetration depth. We have measured Hc1 of MgB2 thin films and multilayer structures by measuring the microwave absorption of the sample at 9.3 GHz in a TE102 rectangular cavity under an applied magnetic field. The magnetic fluxon penetration into the sample as the applied field is increased to greater than Hc1 leads to an increase in the microwave absorption. Preliminary results indicate an enhancement of Hc1 in the MgB2 thin films from the bulk value, consistent with Gurevich's thickness effect model, which is very promising for RF applications of MgB2. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W25.00014: Magnetic force microscopy study of the penetration depth in MgB$_{2}$ thin films Jeehoon Kim, Leonardo Civale, Evgueni Nazaretski, Nestor Haberkorn, Josh Thibodaux, Ilya Vekhter, Brian Moeckly, Joe D. Thompson, Roman Movshovich We performed magnetic force microscopy (MFM) investigations of superconducting vortices in thin films of MgB$_{2}$. Our MFM instrument has the capability of scanning multiple samples. Prior to imaging vortices in MgB$_{2}$, vortex imaging in a Nb thin film was performed to characterize the cantilever's tip. This procedure allows extraction of the penetration depth with only one fitting parameter. Images of two MgB$_{2}$ films with different thickness were taken as a function of temperature, together with periodic checks of the condition of the magnetic tip via imaging of vortices in the Nb reference sample, during a single cooldown. The temperature dependent penetration depth determined by MFM will be compared to that obtained via SQUID magnetometry on the same samples. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W25.00015: Metastable Vortex Lattice Phases in MgB$_2$ C. Rastovski, K. Schlesinger, P. Das, M.R. Eskildsen, L. DeBeer-Schmitt, N. Zhigadlo, J. Karpinski We present small-angle neutron scattering (SANS) studies of vortex lattice (VL) metastability in MgB$_2$. Three different VL phases are observed, all of which have a triangular symmetry. At low fields the VL is aligned with the crystalline $a$-axis. At intermediate fields the VL rotates away from the $a$-axis, leading to two degenerate domain orientations. Once the rotation reaches $30^{\circ}$ a single domain, high field VL is reformed, now aligned along the $a^*$-axis. Metastable configurations are obtained when crossing the equilibrium VL transition lines by cooling or heating in a constant field. At any given field and temperature the equilibrium VL can be obtained by inducing vortex motion. We have explored the details of how the metastable VL transitions to the ground state, and established that the equilibrium VL phase propagate from the edge of the sample towards the center as the magnetic field is reduced. We have successfully prepared samples with a coexistence of metastable and ground state VL domains, and explored how large field changes are necessary to completely suppress the metastable VL phase. The SANS results are compared to measurements of the critical current obtained from magnetization measurements. This work was supported by DOE BES award no. DE-SC0005051. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700