Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J23: Superconductivity: Fluctuation Phenomena |
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Sponsoring Units: DCMP Chair: Michelle Johannes, Naval Research Laboratory Room: D165 |
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J23.00001: Systematic determination of the superconducting fluctuation regime in the cuprates with torque magnetometry Guichuan Yu, Yuan Li, Ruihua He, Xudong Zhao, Martin Greven Among the most important temperature scales in the high-$T_{c }$cuprates are those associated with the normal state pseudogap and the superconducting (SC) gap. Early Nernst effect and torque magnetometry measurements appeared to suggest an onset of SC fluctuations well above $T_{c}$ in La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO). Recently, stripe correlations were found to contribute to the observed large Nernst signal in LSCO, leading to a reinterpretation of the earlier Nernst data. We report a systematically torque magnetometry study of the SC fluctuation regime in the simple model compound HgBa$_{2}$CuO$_{4+\delta }$, which has the highest $T_{c}$ (97 K) among all single-layer cuprates, as well as initial results for LSCO. We demonstrate that the SC fluctuation regime is narrow and that it closely tracks T$_{c}$, which implies that the higher temperature scale observed in LSCO does indeed not result from SC fluctuations. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J23.00002: Superconducting fluctuation regime in HgBa$_{2}$CuO4$_{+\delta }$ revealed by microwave measurements Neven Barisic, Mihael Grbic, Antonije Dulcic, Yuan Li, Xudong Zhao, Guichuan Yu, Martin Greven, Miroslav Pozek There have been many attempts to measure the fluctuations preceding the superconducting long-range order in the cuprates with various experimental techniques, yet the onset temperature of the fluctuation regime has proven difficult to determine. We used a novel approach to microwave conductivity measurements in order to elucidate the phase diagram of the cuprates. Initial measurements were performed on the single-layer material HgBa$_{2}$CuO$_{4+\delta }$. From c-axis data for a sample close to optimal doping, we clearly discern the opening of the pseudogap at T*=185 K, the appearance of the superconducting fluctuations at the much lower temperature T'=105 K, and the transition to the superconducting state at T$_{c}$=94.3 K. Our result implies that the superconducting fluctuations extend only to about 10 K above T$_{c}$. Using the same approach, a narrow fluctuation regime is also found in other cuprates. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J23.00003: Superconducting phase diagram and fluctuations in SmFeAsO$_{0.85}$F$_{0.15}$ single crystals U. Welp, C. Chaparro, W.-K. Kwok, A. Rydh, N.D. Zhigadlo, J. Karpinski, S. Weyeneth We use micro-calorimetry to investigate the anisotropic phase diagram and effects of superconducting fluctuations in sub-micro-gram single crystals of SmFeAsO$_{0.85}$F$_{0.15}$. Our measurements reveal that SmFeAsO$_{0.85}$F$_{0.15}$ is characterized by a large anisotropy of $\Gamma \sim $8 and a short in-plane Ginzburg-Landau coherence length of $\xi _{ab}$(0) $\sim $ 1.3 nm. These materials parameters promote strong superconducting fluctuations which are seen in the zero-field specific heat as clear upwards curvature in C/T at temperatures below T$_{c}$ = 49.5 K and long tails above T$_{c,}$. The resulting anomaly is cusp-shaped with height of $\Delta $C/T$_{c}$ = 24 mJ/moleK$^{2}$, which can be fitted with 3D-Gaussian fluctuations. The transition shows pronounced broadening in magnetic fields applied along the c-axis. The field evolution in fields higher than 3 T is well described in the frame of 3D lowest-Landau-level scaling of fluctuations using an upper critical field slope of -4.4 T/K. We will compare these characteristics to the behavior of other members of the FeAs-family. This work was supported by DOE-BES under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J23.00004: A dynamical study of phase fluctuations and their critical slowing down in amorphous superconducting films Wei Liu, Minsoo Kim, Ganapathy Sambandamurthy , Peter Armitage We report a comprehensive study of the complex AC conductance of amorphous superconducting InO$_x$ thin films. Using a novel broadband microwave ``Corbino'' spectrometer we measure the explicit frequency dependency of the complex conductance and the phase stiffness over a range from 0.21 GHz to 15 GHz at temperatures down to 350 mK. Dynamic AC measurements are sensitive to the temporal correlations of the superconducting order parameter in the fluctuation range above $T_c$. Among other aspects, we explicitly demonstrate the critical slowing down of the characteristic fluctuation rate on the approach to the superconducting state and show that its behavior is consistent with vortex-like phase fluctuations and a phase ordering scenario of the transition. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J23.00005: Superconducting fluctuations in high-Tc cuprate superconductors Brigitte Leridon Experimental results on electrical transport in various high-Tc cuprate superconductors are shown (namely in YBa$_{2}$Cu$_{3}$O$_{6+x}$, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$, La$_{1-x}$Sr$_{x}$CuO$_{4})$. The part of the conductivity associated to superconducting fluctuations in the vicinity of the transition -- also named paraconductivity - is extracted using different techniques for evaluating the normal state conductivity. Whenever possible, the conductivity measured under high pulsed field (50T) is used for the analysis. The results are compared to other experimental probes such as the Nernst effect, and are discussed from one compound to the other. It is shown that some straightforward conclusions can be drawn from relatively simple observations. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J23.00006: Pairing associated with a single quantum critical energy in superconducting electron-doped cuprates Kui Jin, Nicholas Butch, Kevin Kirshenbaum, Paul Bach, Johnpierre Paglione, Richard Greene Though a comprehensive study of magnetotransport on electron-doped La$_{2-x}$Ce$_{x}$CuO$_{4}$ thin films, we show that an envelope of spin fluctuations yielding non Fermi liquid behavior ($\rho =\rho _{0}$ +AT) surrounds the superconducting dome in the overdoped region (x =0.15 to 0.21). This behavior survives to zero temperature over a range of fields exceeding the upper critical field. For example, the resistivity of x = 0.15 is linear in temperature over three decades down to 20 mK at 7.5 T. We demonstrate that all of the relevant energy scales in this system: those determining superconducting pairing, spin correlations, and the Fermi liquid metallic state, emanate from one common critical point at the end of the superconducting dome. These observations suggest that the superconductivity pairing is associated with spin fluctuations and with a single quantum critical energy in electron-doped cuprates. This work was partially supported by NSF-DMR 0653535. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J23.00007: Shot noise measurements in mesoscopic N-S-N structures Martin Stehno, D.J. Van Harlingen Nonlocal subgap transport in mesoscopic superconductor-hybrid devices has received attention as a possible route towards creating and detecting entangled electron pairs in solid state devices. We study local and nonlocal transport in multi-terminal Cu/Al structures with transparent interfaces and separation between contacts comparable to the coherence length in the superconductor. The current shot noise in the two branches of the device is measured simultaneously and compared to the shot noise in a single contact. We discuss cross-correlations expected from Crossed Andreev Reflection and Elastic Co-tunneling processes, non-equilibrium transport in the superconductor, and device heating. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J23.00008: Transport noise crossovers in disordered electron nematics Wan-Ju Li, Benjamin Phillabaum, Erica Carlson, Karin Dahmen Recently, low-frequency transport noise in underdoped YBCO was shown to exhibit an enhancement below 250K, consistent with fluctuations associated with a symmetry-breaking collective electronic state [1]. We discuss these results in relation to crossovers associated with the development of local electronic nematic order. Using a mapping of disordered electron nematics to random anisotropic resistor networks, we predict the thermal evolution of the noise power in transport associated with the crossover to local electron nematic behavior. \\[0pt] [1] Caplan et al., Phys. Rev. Lett. {\bf 104}, 177001 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J23.00009: Vortex Noise in Thin Nb Films on a Triangular Anti-Dot Lattice Tanner Schulz, Liwen Tan, Beth Stadler, E. Dan Dahlberg Thin Nb films are deposited on a periodic, triangular, anti-dot lattice with a lattice constant of 100nm. The lattice serves as pinning sites where superconducting vortices are trapped. The vortex density is set by an external field. At a vortex density commensurate with the pinning lattice transport measurements show an increase in current density. Interstitial vortex pinning produces similar current features at integer multiples of the matching field. We examine the voltage noise spectra as the applied field and DC current bias are varied. Noise signals appear above a field dependent threshold current and show minima at the matching fields. The noise is due to vortex motion in a pinning potential that varies with vortex density and driving forces. We use our results to study vortex motion and compare our signals to existing vortex noise models. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J23.00010: Inelastic scattering effects in current noise for a one dimensional Landauer system Manohar Kumar, Zheng P. Baardman, Roel H.M. Smit, Jan M. van Ruitenbeek Generally, current shot noise is measured at low bias currents, and it reflects the transmission probability of the electrons. Here we present the first measurement at bias currents above the phonon energy of the system, $i.c.$ a chain of Au atoms. The onset of phonon emission processes is signaled by an abrupt jump in differential conductance which results from the change in the transmission probability of the electrons due to phonon excitation. One should expect a sign of this change to be visible in shot noise. Indeed, a distinct signature in the current shot noise signal is observed due to inelastic scattering as a linear deviation from the Levitov- Lesovik classical shot noise. Surprisingly, we have observed that the deviation of noise from the classical noise at the phonon frequency is either positive or negative, depending on whether the transmission is above or below 0.96G0. These observations agree with recent predictions of a sign change in the phonon-induced correction to the noise [1-3], but the point of cross-over is higher than predicted. References: \textit{1. Federica Haupt et.al., PRL 103, 136601 (2009)} \textit{2. R. Avriller et. al., PRB 80 041309 (2009)} \textit{3. T.L.Schmidt et al., PRB 80 041307(R), (2009)} [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J23.00011: AC conductivity across the disorder driven superconductor insulator transition Yen Lee Loh, Karim Bouadim, Nandini Trivedi, Mohit Randeria The superconductor-insulator transition (SIT) is defined, at the most fundamental level, in terms of electromagnetic response. The Mattis-Bardeen theory for conventional superconductors becomes inadequate near the disorder-tuned SIT, where phase fluctuations become important. We present AC conductivity results obtained using determinant quantum Monte Carlo simulations, which include both quantum and thermal phase fluctuations. We find unexpected low-energy weight in the AC conductivity especially near the SIT, and we identify possible sources of this weight. We comment on implications for experiments [1,2]. \\[4pt] [1] R. Vald\'es Aguilar et al., Phys. Rev. B 82, 180514 (2010)\\[0pt] [2] I. Hetel et al., Nature Physics 3, 700-702 (2007) [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J23.00012: Spectroscopic probes of the disorder-driven superconductor-insulator transition Mohit Randeria, Karim Bouadim, Yen-Lee Loh, Nandini Trivedi In spite of decades of research, the mechanism of the disorder- driven superconductor- insulator transition (SIT) and the nature of the insulator are not understood. We use quantum Monte Carlo simulations that treat, on an equal footing, inhomogeneous amplitude variations and phase fluctuations, a major advance over previous theories. The energy gap in the density of states survives across the transition, but coherence peaks exist only in the superconductor. A characteristic pseudogap persists above the critical disorder and critical temperature, in contrast to conventional theories. Surprisingly, the insulator has a two-particle gap scale that vanishes at the SIT, despite a robust single-particle gap. Our predictions are testable with scanning probe experiments. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J23.00013: Fast vortices in the cuprates? A vortex plasma model analysis of the THz conductivity and diamagnetism in $La_{2-x} Sr_x CuO_4 $ Lucas Bilbro, Rolando Valdes Aguilar, Gennady Logvenov, Oshri Pelleg, Ivan Bozovic, N.P. Armitage We present measurements of the fluctuation superconductivity in an underdoped thin film of$La_{1.905} Sr_{0.095} CuO_4 $ using time-domain THz spectroscopy. We compare our results with the measurements of diamagnetism in a similarly doped crystal of $La_{2-x} Sr_x CuO_4 $. Through a vortex-plasma model, we show that if the fluctuation diamagnetism originates in vortices, then we necessarily obtain an anomalously large vortex diffusion constant, more than 100 times larger than estimates from the Bardeen-Stephen model. [Preview Abstract] |
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