Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q33: Superconductivity: Response to Electromagnetic Fields and Proximity Effect |
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Sponsoring Units: DCMP Chair: Art Hebard, University of Florida Room: 403 |
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q33.00001: Magnetic field dependence of the infrared transmittance of superconducting NbTiN D.B. Tanner, J. Hwang, X. Xi, H. Zhang, G.L. Carr Superconductivity may be destroyed by raising the temperature of the superconductor above the transition temperature or by increasing an applied magnetic field above the upper critical field. We have studied the behavior of key microscopic properties, the superconducting energy gap and the superfluid density, through far-infrared magnetospectroscopy measurements on thin-film Nb$_{0.5}$Ti$_{0.5}$N. The measurements were performed at the National Synchrotron Light Source, Brookhaven National Laboratory. As temperature is increased, the gap and the superfluid density are reduced, both reaching zero at $T_{c}.$ The behavior with field is different. Over much of the range between 0 and 10 T, the gap is almost unchanged, while the superfluid density is reduced, roughly following the area not in vortices in the film. Only near the highest field does the superconducting gap become reduced. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q33.00002: Strong correlation effects and optical conductivity in electron doped cuprates Tanmoy Das, R.S. Markiewicz, A. Bansil We demonstrate that most features ascribed to strong correlation effects in various spectroscopies such as angle-resolved photoemission spectroscopy (ARPES) and optical spectra of the cuprates are captured by a calculation of the self-energy incorporating effects of spin and charge fluctuations[1]. The self-energy is calculated over the full doping range of electron-doped cuprates from half-filling to the overdoped system. The spectral function reveals four subbands, two widely split incoherent bands representing the remnant of the split Hubbard bands, and two additional coherent, spin- and charge-dressed in-gap bands split by a spin-density wave, which collapses at the AFM quantum critical point (QCP) in the overdoped regime. The transition between the in-gap states leads to pseudogap features in the mid-infrared region of the optical spectra, where the incoherent features persist to high doping even above the QCP, producing a remnant Mott gap. Notably, our results are also in good accord with variational cluster and quantum Monte Carlo calculations. Work supported in part by the USDOE. \\[3pt] [1] Tanmoy Das, R. S. Markiewicz, and A. Bansil, arXiv:0807.4257. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q33.00003: A broadband microwave study of the superconducting~fluctuations in 2D InOx thin films Wei Liu, Minsoo Kim, Tailung Wu, Sambandamurthy Ganapathy, Peter Armitage We apply a broadband microwave `Corbino' spectrometer covering the~range from 10MHz to 20GHz to the study of 2D disordered~superconducting InOx thin films.~ Explicit frequency dependency of~the superfluid stiffness and conductivity are obtained down to 270mK.~The AC measurements are sensitive to different time scales of the~superconducting fluctuations.~ A number of fluctuation regimes are~investigated (gaussian fluctuations, vortex proliferation) as we cool~the sample into the low-temperature Kosterlitz-Thouless-Berezinskii- like phase.~We discuss our results in terms of prevailing scenarios~for fluctuation superconductivity and make connection to other~experimental results. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q33.00004: Time-resolved terahertz photoconductivity of insulating cuprates Amir Farahani, Jesse Petersen, Ruixing Liang, J. Steven Dodge We use a visible pump, terahertz probe technique to study the photoconductivity of the undoped cuprates. We use ultrafast optical pulses ($E_{\mathrm{pump}} = 3.1$~eV) to create photocarriers in high quality single crystals of Sr$_2$CuO$_2$Cl$_2$ and YBa$_2$Cu$_3$O$_6$, and time-domain terahertz spectroscopy to probe the resulting photoconductivity. We observe a rapid onset of photoconductivity followed by a non-exponential relaxation on a picosecond timescale. This dynamics is independent of photocarrier concentration over the range of 0.2 to 1.7 percent excitations per copper atom. Assuming a quantum efficiency of unity, we infer a mobility of $\sim$0.2 cm$^2$/Vs, significantly lower than the Hall mobility in chemically doped systems \footnote{Y. Ando {\em et al.} PRL {\bf 87} 017001 (2001)}. As the fluence is increased, there is a weak decrease in the photoconductivity amplitude. We also measured the frequency dependence of the photoconductivity in the terahertz range, and observe an increase in photoconductivity with frequency up to 600 GHz, suggesting polaronic effects. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q33.00005: Magnetic field-induced modification of superfluid density and interplane spectral weight in YBa$_2$Cu$_3$O$_y$ Andrew LaForge, Willie Padilla, Kenneth Burch, Zhiqiang Li, Alexander Schafgans, Kouji Segawa, Yoichi Ando, Dimitri Basov We report on the interlayer infrared response of YBa$_2$Cu$_3$O$_y$ in an applied magnetic field. This study explores both the underdoped ($y$ = 6.67 and 6.75) and optimally doped ($y$ = 6.95) regions of the phase diagram, and includes data for fields applied both parallel to the $c$ axis and to the CuO$_2$ planes in this anisotropic superconductor. A sum rule analysis reveals that magnetic fields $H \parallel c$ eliminate the high-frequency contribution to the superfluid density, returning the system to a more BCS-like energy scale [1]. For fields $H \parallel$ CuO$_2$, however, the high-energy component scales with the superfluid density, and the anomalous scheme of condenstate formation is maintained, at least in underdoped $y$=6.67 and 6.75 samples. This behavior is discussed in relation to the change of electronic kinetic energy and the suppression of interplane phase coherence. [1] A. D. LaForge \emph{et al.}, Phys. Rev. Lett. \textbf{101}, 097008 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q33.00006: A Terahertz Conductivity Study of Pseudogap Phase in Underdoped LSCO. Lucas Bilbro, Ivan Bozovic, N. Peter Armitage Using Terahertz Time Domain Spectroscopy (TTDS), we investigate the pseudogap phase of the high-temperature cuprate superconductors. We measure the frequency and temperature dependence of the complex conductivity for a number of underdoped thin films of La$_{2-x}$Sr$_{x}$CuO$_{4}$ at frequencies well below the superconducting gap. A number of issues are investigated, including evidence for a non-zero finite frequency superfluid stiffness in regions outside the superconducting phase boundary. We discuss the extent of the fluctuation regime and the impact of these fluctuations on the physics of the pseudogap. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q33.00007: Scaling of the superfluid density in severely underdoped \boldmath YBa$_2$Cu$_3$O$_{6+y}$\unboldmath C.C. Homes, W.A. Huttema, P.J. Turner, D.M. Broun, Ruixing Liang, W.N. Hardy, D.A. Bonn Microwave impedance techniques have been used to determine the critical temperature ($T_c$), the in-plane superfluid density ($\rho_{s0}$), and the dc conductivity ($\sigma_{dc}$) just above $T_c$ in a highly-underdoped sample of YBa$_2$Cu$_3$O$_{6+y}$ for $y\simeq 0.333$. In this state the sample may be annealed to yield different levels of chain oxygen order and electronic doping in the copper-oxygen planes, resulting in a range of $T_c \simeq 2 - 17$~K.\footnote{D. M. Broun {\it et al.}, Phys. Rev. Lett. {\bf 99}, 237003 (2007).} The linear relation between $\rho_{s0}$ and $T_c$ is not observed, instead $\rho_{s0}\propto T_c^2$. However, the results do follow the more general scaling relation $\rho_{s0}/8 \simeq 4.4\,\sigma_{dc}T_c$,\footnote{C. C. Homes {\it et al.}, Phys. Rev. B {\bf 72}, 134517 (2005).} extending the validity of this relation for the in-plane data by an order of magnitude. In addition, these new results now provide a region of overlap between the scaling observed in the copper-oxygen planes, and perpendicular to the planes along the poorly-conducting {\it c} axis. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q33.00008: Real-time photoinduced quasi-particle relaxation of superconductors Jianmin Tao, Jian-Xin Zhu Ultrafast optical phenomena are of fundamental importance in the investigation of electronic dynamics of metals and superconductors [1]. By considering a model Hamiltonian with electron-boson coupling of a superconductor exposed to a time-dependent laser field, we calculate the current density, which can be expressed in terms of the quasi-particle density matrices. The time evolution of these density matrices is derived within a mean-field approximation using the equation-of-motion approach and is numerically investigated with Runge-Kutta method. we discuss the consequence of the $d$-wave pairing symmetry in the quasi-particle relaxation process. \\[3pt] [1] R. D. Averitt and A. J. Taylor, J. Phys: Condensed Matter 14, R1357 (2002). [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q33.00009: Raman scattering from the CaC6 superconductor in the presence of disorder Aleksej Mialitsin, Jun Sung Kim, Reinhard Kremer, Girsh Blumberg Polarized Raman scattering has been performed on CaC$_6$ single crystal superconductor. We identify two of the three Raman active E$_g$ phonon modes at 440 and 1508\,cm$^{-1}$ expected for the $R\overline{3}m$ space group of CaC$_6$. These first order scattering modes appear along with the D and G bands around 1300\,cm$^{-1}$ and 1600\,cm$^{-1}$ that are similar in origin to the corresponding bands in plain graphite. The intensities of the D and G bands in CaC$_6$ correlate with degree of disorder. The D band arises from the double resonant Raman scattering process; its frequency shifts as a function of excitation energy with $\sim$\,35\,cm$^{-1}$/eV. The double resonant Raman scattering probes phonon excitations with finite wave vector $\emph{\textbf{q}}$. We compare experimental results to from-first-principles calculations. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q33.00010: Possibility of p-wave triplet pairing in Nb/Ni bilayers Wenjian Lu, Kookrin Char, Y.K. Bang, P. SanGiorgio, M.R. Beasley We have experimentally investigated the density of states (DOS) in Nb/Ni (S/F) bilayers and found the anomalous double peak structure. In order to analyze the measured DOS data, we propose a theory model in which $p$-wave triplet pairing correlations are induced by an inhomogeneous magnetization in the ferromagnet. The induced triplet component penetrates into the ferromagnet over a long length (much larger than a characteristic length scale $\xi_F=\sqrt{D/h}$, where $D$ is the diffusion coefficient and $h$ exchange energy). We numerically calculate the DOS of Nb/Ni bilayers based on Eilenberger equation with various parameters and discuss the unusual sub-gap structure in the DOS. We find a good qualitative and quantitative agreement between the model calculations and our measurements and therefore suggest the possibility of $p$-wave triplet correlations in Nb/Ni bilayers. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q33.00011: 1D Chain of Interacting Majorana Bound States at the Edge of a Topological Insulator Vasudha Shivamoggi, Joel Moore We study a realization of a 1d chain of Majorana bound states that consists of alternating ferromagnetic and superconducting regions at the edge of a quantum spin hall insulator. Each boundary between a ferromagnetic and superconducting region supports a Majorana bound state, and the pair-wise interaction energies have previously been calculated in the weakly interacting limit. By adjusting the phases of the order parameters in these regions, it is possible to create a Majorana bound state localized at each interface. In the limit of well separated Majorana fermions, the system can be mapped to the transverse field Ising model. To reach the random critical Majorana chain studied by Bonesteel and Yang, the phases of the ferromagnetic and superconducting order parameters must be drawn from essentially the same random distribution. We examine factors in an experimental system that will move the system away from the critical point, such as Coulomb interactions and breaking of the duality between the ferromagnetic and superconducting regions. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q33.00012: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q33.00013: Superconducting Proximity Effect in Thin Semiconducting Films Michael Vissers, Kevin Inderhees, Tim McArdle, Stephanie Law, Paul Goldbart, Laura Greene, Jim Eckstein We report results using a novel 3 terminal device to study the influence of the superconducting proximity effect on the sheet resistance of the N-layer, Rs, as well as the junction conductance across the N-S boundary, Gc. When the N-layer is a degenerate semiconductor the changes in these quantities are large. In samples with large Gc, we find that the N-layer sheet resistance below the SC transition increases as T decreases. We can also extract the actual J-E relationship by modeling the spatial current profile beneath the injector electrode. We interpret these effects as being due to a phase stiff proximity effect in the N layer moving the N-S electrical boundary into the semiconductor as the temperature is lowered, thereby removing volume available to normal transport, and causing 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] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q33.00014: I-V studies of proximity-induced superconductivity in long nanowires Haidong Liu, Zuxin Ye, K.D.D. Rathnayaka, Wenhao Wu Recently we have observed an anomalous long-range superconducting proximity effect in superconducting Zn and Sn nanowires in contact with bulk film electrodes of Au, Sn, or Pb. With electrodes having a higher transition temperature, nanowires up to 60 micron in length are observed to superconduct at the transition temperature of the electrodes. With Au electrodes, superconductivity in the nanowires is largely suppressed. These samples are fabricated by electroplating nanowires into porous membranes where the electrode-nanowire contacts were made via a self-contacting technique. In this talk, measurements of the I-V characteristics at various temperatures and magnetic fields will be discussed to further understand the observed proximity effect. The most striking feature observed in the dc I-V characteristics is a reproducible step-like feature, identical to that observed in superconducting whiskers and micro-bridges. Such multiple steps are attributed to the successive establishment of localized phase slip centers (PSCs) along a nanowire since some sections of a nanowire have smaller critical currents. With our data, we estimate that the length of a typical PSC is 5-10 microns. These results suggest that the observed proximity effect occurs along the entire length of a nanowire, rather than only at the nanowire-electrode interfaces [Preview Abstract] |
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