Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A25: Superconductivity: Phases and Phase Transitions |
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Sponsoring Units: DCMP Chair: Thomas Lemberger, The Ohio State University Room: D166 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A25.00001: Two-dimensional Quantum Critical Point in Underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ Thomas Lemberger, Jie Yong, Andrew McCray, Muntaser Naamneh, Amit Kanigel, Mohit Randeria Underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ films with T$_{c}$'s from 5 to 70 K are fabricated by sputtering and Pulsed Laser Deposition (PLD). Temperature dependences of superfluid densities are measured to study the superconductor-to-insulator quantum phase transition. Sputtered films, which tend have higher dopings, show superfluid densities that are weakly linear in T at low-T and drop dramatically where Kosterlitz-Thouless-Berezinski theory predicts, assuming that individual CuO$_{2 }$bilayers are uncoupled. However, our PLD films, which are more underdoped than the sputtered films, have superfluid densities that are roughly linear from low T to T$_{c}$. Also, There is no indication of thermal critical behavior near T$_{c}$. Underdoped YBCO crystals also lack critical behavior, even though critical behavior is strong in optimally doped and moderately underdoped samples. Near the superconductor-to-insulator phase transition, T$_{c}$ and n$_{s}$(0) have a linear relationship that mimics that of ultrathin, two-dimensional films of Ca-doped YBa$_{2}$Cu$_{3}$O$_{7-\delta }$, thereby indicating a 2-D quantum critical point at low doping. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A25.00002: Magnetoresistance Peak in Quench Deposited Ultra-Thin Amorphous Bismuth Films Yen-Hsiang Lin, Allen Goldman A magnetoresistance peak in perpendicular magnetic field has been found on the insulating side of the thickness-tuned superconductor-insulator (SI) transition of quench-deposited amorphous bismuth films. The presence of a peak suggests the presence of local superconductivity in these insulating films. Arrhenius type conduction and non-linear I-V characteristics are also observed in the peak regime. The magnitude of this magnetoresistance peak increases substantially with decreasing temperature and with increasing film thickness. The dependence of the peak magnetic field on temperature and thickness may help to explain the underlying mechanism of the magnetoresistance peak. [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A25.00003: Chemical Activity in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ Acrosst the Transition to Superconductivity Juana V. Acrivos Changes in the Gibbs free enthalpy, chemical activity across the transition temperature to superconductivity T$_{c}$ in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ is described by enhanced element X-Ray absorption XAS and diffraction XRD [HKL] reflections. Critical oscillations in the index of refraction within the XAS line width ($\pm $ 2.5eV) at the Ba L2, L3 and Y K-edges observed $\sim $30K above T$_{c}\approx $93K in [HKL] reflections indicates their activity. Enhanced absorbance A versus T obtains the activation enthalpy and entropy: $\Delta $H$^{\ne }_{>}$=-220 meV, $\Delta $S$^{\ne }_{>}$=-2 meV/K (121\underline {$>$}T\underline {$>$}93K) for mixed normal and superconducting phases, which compensates the reported O atom ordering activation energy near T$_{c}$ by 50 meV. The activation needed to mix differently ordered superconducting phases: $\Delta $H$^{\ne }_{<}$= -67 meV, $\Delta $S$^{\ne }_{<}$ = -1 meV/K (60K\underline {$<$}T\underline {$<$}93K) indicates lattice ordering persists to 60K. Enhanced XRD scattering induced near the transition to superconductivity in 3D solids indicates that the role of 2D reactive [HKL] planes is similar to the chemical activity of reactive linear bonds in molecular reactions. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A25.00004: Unitarity in periodic potentials and correlated s-wave Cooper pair insulators Predrag Nikolic, Zlatko Tesanovic We explore the emergence of novel universal regimes and correlated states in strongly interacting band insulators. Lattice potentials introduce Cooper, exciton and inter-valley channels for scattering resonances, which can be studied in the BCS-BEC framework. This is revealed by characterizing a large number of renormalization group fixed points. The superfluid-insulator transition is found to be pair-breaking in the weak-coupling BCS limit, while it belongs to the bosonic mean-field or XY universality class in the strong-coupling BEC limit as fermionic excitations remain gapped. The latter leads to correlated bosonic Mott insulators of Cooper pairs, and is the only option in two dimensions. Such an insulator may break lattice symmetries, but even if it doesn't it can be sharply distinguished from the band insulator out of equilibrium. The models we study can be realized with ultra-cold gases of alkali atoms tuned to a broad Feshbach resonance in an optical lattice. We discuss possible consequences for cuprate superconductors, where antinodal pair dynamics has certain features in common with our simple s-wave picture. [Preview Abstract] |
Monday, March 21, 2011 8:48AM - 9:00AM |
A25.00005: Cooper pair localization in a-Bi thin films near the superconductor-insulator transition S.M. Hollen, H.Q. Nguyen, E. Rudisaile, J. Shainline, G. Fernandes, J.M. Xu, J.M. Valles, Jr. Ultrathin films near the Superconductor-Insulator Transition (SIT) can exhibit Cooper pair transport in their insulating phase. This Cooper Pair Insulator state is achieved in amorphous Bi films patterned with a nanohoneycomb array of holes. We will present evidence from a number of experiments on these substrates supporting that 1) thickness variations, which result in variations in $T_{c}$ and $\Delta$, serve to localize the Cooper pairs; 2) the weak links between these superconducting islands control the SIT. Finally, we will discuss our most recent experiments that aim to characterize this Cooper pair insulator state and confirm the role of the thickness variations in the localization of Cooper pairs. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A25.00006: Study of granular two-band superconducting films: existence of a zero-temperature metallic phase Bojun Yan, Tai-Kai Ng A variational approach is used to study the zero-temperature phase transition of two-band granular superconducting films. For s+(-) superconductors with strong enough disorder, we show the plausible existence of a metallic phase between the superconducting and insulator phases which is absent in normal single band granular superconducting films. We propose that the metallic phase may be observed in granular films of pnictide superconductors. Novel possibilities such as charge 2e metal and ``topological metal'' are also discussed. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A25.00007: Phase diagram of electrostatically doped SrTiO3 Yeonbae Lee, Steve Snyder, Jack Hellerstedt, Colin Clement, Laura Kinnischtzke, Joseph Kinney, Allen Goldman We report on the properties of electrostatically doped SrTiO3 over broad ranges of temperature and carrier concentration. Electrostatic doping has been carried out with the use of an electric double layer transistor employing an ionic liquid as a gate dielectric. The result is an apparent carrier-density dependent metal insulator transition that may be associated with the reduction of the density of thermally excited carriers in the conduction band derived from shallow states in the band gap. This results in a phase diagram that is analogous to that found for cuprate superconductors, however, with superconductivity appearing at much lower temperatures. In addition for doping levels short of those inducing superconductivity, an anomalous Hall effect is observed, suggesting the appearance of ferromagnetism near the boundary between the insulating and superconducting regimes of the doping layer. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A25.00008: Thermodynamic signature for the phase transition at the pseudogap temperature in underdoped YBCO (6.56) Victor Fanelli, Scott Riggs, Arkady Shekhter, Yoko Suzuki, Jonathan Betts, Albert Migliori, Greg Boebinger, Brad Ramshaw, Ruixing Liang, Walter Hardy, Doug Bonn The physics of the pseudogap, and its connection to the strange metal phase remain poorly understood. The outstanding problem is whether the apparent crossover between these two regimes is a thermodynamic phase boundary. We performed high precision resonant ultrasound spectroscopy measurement on de-twinned monocrystals of underdoped YBCO (6.56) in a broad temperature range up to 300 K. We find a compelling thermodynamic signature for the phase transition at the pseudogap temperature T = 270 K. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A25.00009: Signature of Aslamazov-Larkin fluctuation Hall conductivity in Tantalum Nitride films above their superconducting transition temperature Nicholas Breznay, Mihir Tendulkar, Aharon Kapitulnik, Karen Michaeli, Alexander Finkel'stein We have studied the Hall effect in superconducting Tantalum Nitride films. We find a large contribution to the Hall conductivity near the superconducting transition, which we can track to temperatures well above Tc and magnetic fields well above the upper critical field, Hc2(0). This contribution arises from Aslamazov-Larkin superconducting fluctuations, and we find quantitative agreement between our data and theoretical analysis based on time dependent Ginzburg-Landau theory. We will also remark on the appearance of a sign change in the Hall effect and on the high field fluctuation conductivity in superconducting Tantalum and Indium Oxide thin films. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A25.00010: Magnetic Phase Diagram of the electron-doped high-$T_{c}$ superconductor Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ M.K. Chan$^1$, E.M. Motoyama$^2$, G. Yu$^1$, Y. Li$^2$, J.P. Carlo$^3$, T.J. Williams$^4$, S.K. Kim$^3$, T. Goko$^{3,4}$, Y.J. Uemura$^3$, G.M. Luke$^4$, M. Greven$^1$ An intriguing issue in high-$T_{c}$ superconductivity is the phase diagram asymmetry with respect to electron and hole-doping. The antiferromagnetic phase extends further with electron doping and appears to overlap with superconductivity. Our prior results suggested that genuine long-range antiferromagnetic order and superconductivity do not co-exist in Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ [Motoyama \textit{et al.} Nature 445, 186 (2007)]. However, some uncertainty remained due to Ce concentration inhomogeneity in large single crystals. Here we report neutron scattering and $\mu $SR measurements on crystals with improved Ce homogeneity. Inelastic neutron scattering indicates that genuine long-range antiferromagnetic order indeed disappears within a small doping window around x =0.12. Meanwhile, $\mu $SR measurements show that static magnetic order persists up to x = 0.14, where bulk superconductivity first unambiguously appears. Our results suggest a possible first-order phase transition in a narrow region of the phase diagram, between x = 0.12 and x =0.14, characterized by clusters of short-range static magnetic order and traces of superconductivity. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A25.00011: Multiple Phase Transitions: Phase Diagram with Interacting Phase Boundaries of Different Order Bohdan Andraka, Pradeep Kumar, Avadh Saxena We present a thermodynamic discussion of the consequences of interacting phase boundaries. The particular focus here is when the superconducting phase boundaries are of different order in a phase diagram in the magnetic field-temperature (H-T) plane. Thus depending on the form of the dominant interaction, we derive thermodynamic observables such as specific heat, superfluid density (as could be measured by lower critical field) and thermal expansion as a function of field and temperature and especially their discontinuities at the phase boundaries. We suggest that these considerations have relevance for the superconducting transition and the phase diagram in PrOs4Sb12. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A25.00012: Properties and behavior of superconductors exhibiting a Fulde-Ferrell-Larkin-Ovchinnikov phase William A. Coniglio, Charles C. Agosta The body of data on the Ful\-de\--Fer\-rell\--Lar\-kin\--Ov\-chin\-ni\-kov (FFLO) state in 2d organic superconductors has grown to a critical mass where we may begin studying the boundaries of the FFLO phase in detail. In some very clean layered superconductors, when a magnetic field is aligned exactly parallel to the conducting layers, a superconducting phase develops at fields above the Pauli paramagnetic limit $H_p$ and temperatures below about $T_c/3$. The phase is widely ascribed to FFLO behavior. We focus on the superconductors $\kappa$-(ET)$_2$Cu(NCS)$_2$, $\beta^{\prime\prime}$-(ET)$_2$SF$_5$CH$_2$CF$_2$SO$_3$, and $\lambda$-(BETS)$_2$GaCl$_4$, which have been studied by rf penetration depth and other techniques. We have probed the boundaries of the FFLO phase using alignment angle to tune the amount of spin-orbit scattering and temperature to control the degree of Pauli paramagnetic limiting. Using our data collected in pulsed magnetic fields at low temperature, we have gained new understanding about the behavior of the state and the conditions necessary for it to develop. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A25.00013: Exploration of the pressure-induced superconducting phase in rare-earth tritellurides ($R$Te$_{3}$) Diego A. Zocco, James J. Hamlin, M. Brian Maple, Jiun-Haw Chu, Ian R. Fisher It has recently been reported that the low-dimensional rare-earth tritellurides $R$Te$_{3}$ ($R$ = La-Nd, Sm, Gd-Tm) enter an unidirectional, incommensurate charge-density-wave (CDW) state when cooled below a temperature $T_{CDW1}$ $\sim$ 450 - 250 K, which decreases with increasing rare earth atomic number, due to the effect of chemical pressure. For the heavier $R$ ($i. e.$, Dy-Tm), a second CDW appears at $T_{CDW2}$ $<$ $T_{CDW1}$, orthogonal to the first one. We have recently found that the application of external pressure induces a superconducting (SC) state in TbTe$_3$ at low temperatures, coexisting with the two CDWs and the local moment rare-earth magnetism. In this talk, we present the results of experiments we have performed on these materials at high pressures and very low temperatures, to help develop an understanding of the origin of the superconducting state. [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A25.00014: Violation of Onsager reciprocity in underdoped cuprates? Victor Yakovenko, Chandra Varma, Aharon Kapitulnik One of the canons of condensed matter physics is the Onsager reciprocity principle for systems in which the Hamiltonian commutes with the time-reversal operator. Recent results of measurements of the Nernst coefficient [1] in underdoped $\rm YBa_2Cu_30_{6+x}$, together with the measurements of the anisotropy of conductivity and the inferred anisotropy of the thermopower, imply that this principle is violated [2]. The probable violation and its temperature dependence are shown to be consistent with the loop-current phase which has been directly observed in other experiments. The violation is related directly to the magneto-electric symmetry of such a phase in which an applied electric field generates an effective magnetic field at right angle to it and to the order parameter vector, and vice versa.\\[4pt] [1] R. Daou \emph{et al.}, Nature {\bf 463}, 519 (2010).\\[0pt] [2] C. M. Varma, V. M. Yakovenko, A. Kapitulnik, arXiv:1007.1215 [Preview Abstract] |
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