Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session J41: Response to Electromagnetic Fields (Optical, Raman) |
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Sponsoring Units: DMP Chair: Oleg Shpyrko, University of California, San Diego Room: F152 |
Tuesday, March 16, 2010 11:15AM - 11:27AM |
J41.00001: Infrared conductivity of superconducting NbTiN in a magnetic field D.B. Tanner, Xiaoxiang Xi, J. Hwang, G.L. Carr The optical conductivity of thin-film Nb$_{0.5}$Ti$_{0.5}$N in applied magnetic fields has been estimated from the results of far-infrared transmission and reflection measurements. The measurements were performed at the National Synchrotron Light Source, Brookhaven National Laboratory at fields up to 10 T. The combined measurements have been analyzed to give the real and imaginary parts of the conductivity. In turn, these quantities allow the magnetic-field dependence of the superconducting energy gap and the superfluid density to be estimated. The effect of pairbreaking by the field on the gap will be discussed. [Preview Abstract] |
Tuesday, March 16, 2010 11:27AM - 11:39AM |
J41.00002: Dependence of the quasiparticle recombination rate on the superconducting gap and T$_{C}$ G.L. Carr, Xiaoxiang Xi, J. Hwang, H. Tashiro, D.H. Reitze, D.B. Tanner The relaxation of excess quasiparticles in a BCS superconductor is known to depend on quantities such as the quasiparticle {\&} phonon density of states, and their coupling (Kaplan et al, \textit{Phys. Rev.} B \textbf{14} 4854, 1976). Disorder or an applied field can disrupt superconductivity, as evidenced by a reduced T$_{C}$. We consider some simple modifications to the quasiparticle density of states consistent with a suppressed energy gap and T$_{C}$, leading to changes in the intrinsic and effective (measured) rates for excess quasiparticles to recombine into pairs.~ We review some results for disordered MoGe and discuss the magnetic-field dependence of the recombination process. [Preview Abstract] |
Tuesday, March 16, 2010 11:39AM - 11:51AM |
J41.00003: Measurements of the quasiparticle recombination process in a type II superconductor with field-suppressed energy gap Xiaoxiang Xi, J. Hwang, H. Zhang, D.H. Reitze, C.J. Stanton, D.B. Tanner, G.L. Carr Our pump-probe studies of excess quasiparticle relaxation in the type II superconductor NbTiN show a relaxation rate proportional to the excess quasiparticle number density, as expected for bimolecular recombination driven by a large excess quasiparticle population. Application of a magnetic field parallel to the sample surface is found to slow significantly the quasiparticle recombination process. We compare our results with a model of recombination that considers two candidate magnetic field effects: quasiparticle spin polarization (paramagnetism) and finite pair lifetime effects, with the latter appearing to be the more important. [Preview Abstract] |
Tuesday, March 16, 2010 11:51AM - 12:03PM |
J41.00004: Dynamical Response of the Abrikosov Vortex Lattice in superconductor with an alternative oscillating field Fareh PeiJen Lin The time-dependent Ginzburg Landau Equation is solved analytically to find the response function of the dense vortex system, the order parameter $\Psi$, when the vortices in a superconductor are forced into motion by an alternative transport current results from an sinusoidal electric field with amplitude $E$ and frequency $\Omega$. Base on the solution, motion and configuration of the moving vortices, temporal current density, energy dissipation and other dynamical properties are studied. With large input signal, response function can go beyond simple the ohmic behavior and higher harmonic mode is generated. Analog to static superconducting-normal phase transition line, the dynamical phase transition surface in 4-dimensional space, $T,H,E$ and $\Omega$, are determised. Comparison with optical measurement on $NbN$ is made. [Preview Abstract] |
Tuesday, March 16, 2010 12:03PM - 12:15PM |
J41.00005: Microwave spectroscopy of d-wave quasiparticles in highly underdoped YBCO Wendell Huttema, Patrick Turner, Ruixing Liang, Walter Hardy, Doug Bonn, David Broun In the underdoped cuprates, photoemission and scanning-tunneling spectroscopies reveal that low energy quasiparticle excitations are confined to ``Fermi arcs" on the Brillouin zone diagonals. We have used microwave spectroscopy to probe these excitations with high resolution in high purity YBa$_2$Cu$_3$O$_{6+y}$. We have carried out detailed measurements of the superfluid density and conductivity as a function of doping, using a novel in-situ oxygen-ordering technique to tune doping in a single sample, in the range $T_c=20$~K to $T_c=0$~K. These measurements complement and extend the other spectroscopic measurements and show that nodal $d$-wave quasiparticles continue to exist as superconductivity is suppressed. We show that as the superconducting transition temperature $T_c$ is tuned to zero: the zero temperature superfluid density goes to zero; the maximum superconducting gap goes to zero; the gap slope softens; and Fermi liquid corrections cause the electrical current carried by the quasiparticles to decrease smoothly to zero. [Preview Abstract] |
Tuesday, March 16, 2010 12:15PM - 12:27PM |
J41.00006: Signatures of Fermi Surface Reconstruction in Raman Spectra of Underdoped Cuprates J.P.F. LeBlanc, J.P. Carbotte, E.J. Nicol Applying a phenomenological model of the pseudogap state, due to Yang, Rice and Zhang [1], we have calculated the Raman B$_{1g}$ and B$_{2g}$ spectra as a function of temperature, as well as doping, for the underdoped cuprates. Motivated by recent angle-resolved photoemission experiments (ARPES) [2], we discuss changes in intensity and peak position brought about by the presence of a pseudogap and the implied Fermi surface reconstruction which are elements of this model. Our calculations capture the main qualitative features revealed in the extensive data set on HgBa$_2$CuO$_{4+\delta}$.\\[4pt] [1] K.Y Yang, T.M. Rice and F.-C. Zhang, PRB 73 17541 (2006).\\[0pt] [2] Kondo et al. Nature 457, 296 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 12:27PM - 12:39PM |
J41.00007: Optical conductivity from cluster DMFT: Formalism and application to high-Tc superconductors Nan Lin, Emanuel Gull, Andrew Millis The optical conductivity of the one-band Hubbard model is calculated using the ``dynamical cluster approximation'' implementation of dynamical mean field theory for parameters appropriate to high-temperature copper-oxide superconductors. We present a scheme for obtaining the vertex corrections and show their significant contributions especially to the conductivity of undoped and lightly doped materials where it strongly affects the above-gap absorption. For parameters $U=9t$ and $t'=-0.3t$ relevant to high temperature superconductors, at densities of one electron per site an insulating state is found with a gap $\sim$ 1.8eV similar in magnitude to that observed in La$_2$CuO$_4$. On doping, the gap is rapidly destabilized and absorption appears at low frequencies, again in a manner consistent with the measurements. The mid-infrared feature observed at intermediate dopings is shown to arise from a pseudogap structure in the density of states. [Preview Abstract] |
Tuesday, March 16, 2010 12:39PM - 12:51PM |
J41.00008: Exchange boson dynamics in the cuprates Thomas Timusk, Jing Yang, Jungseek Hwang, Jules Carbotte, Ewald Schachinger The electron-boson spectral density function $I^{2}\chi(\Omega)$ responsible for carrier scattering of high temperature superconductors can be calculated from the optical scattering rate using a maximum entropy technique. Published data on a range of high temperature superconductors (with two notable exceptions) show a peak at an energy ($\Omega_{r}$) and a broad high frequency background. The energy of the peak is proportional to the superconducting transition temperature $\Omega_{r}\approx$ 6.3$k_{B}T_{c}$ for a large range of materials with $T_{c}$'s ranging from of 30 K for LSCO up to 130 K for HgBa$_{2}$Ca$_{2}$Cu$_{3}$O$_{8+\delta}$. This relationship in the charge channel compares well with a similar one in the spin channel where the frequency of the magnetic resonance seen by polarized neutron scattering is also found to be proportional to $T_c$, $\Omega_r^{neutron}\approx$ 5.4$k_{B}T_{c}$. The amplitude of both the optical resonance and the magnetic neutron peak decrease strongly with increasing temperature. [Preview Abstract] |
Tuesday, March 16, 2010 12:51PM - 1:03PM |
J41.00009: Distinguishing between spin-fluctuation and phonon-mediated $d$-wave superconductivity in cuprates by high-pressure Raman scattering Xiaojia Chen, Jian-Xin Zhu, Viktor V. Struzhkin, Alexander F. Goncharov, Cheng-Tian Lin, Russell J. Hemley, Ho-kwang Mao Determining the nature of interaction responsible for the Cooper-pair formation in high-$T_{c}$ cuprates remains one of the grand challenges in modern condensed matter physics. The most probable candidates are lattice vibrations (phonons) and spin fluctuation modes. Recently, it has been argued that Raman scattering in $B_{1g}$ symmetry may serve as a probe to distinguish between phonon-mediated and magnetically mediated d-wave superconductivity. Here we report the results of electronic Raman scattering measurements in Bi-based bilayer and trilayer superconductors at high pressures and at temperatures around 12 K. As a clean and effective tool, pressure enhances $T_{c}$ and thus increases the pairing interaction in these materials. Meanwhile, we find that pressure also brings about the change of the $B_{1g}$ mode. The observed evolution of $B_{1g}$ modes with pressure sheds important insight on the pairing mechanism of high-$T_{c}$ superconductivity. [Preview Abstract] |
Tuesday, March 16, 2010 1:03PM - 1:15PM |
J41.00010: Superconducting fluctuation of La$_{2-x}$Sr$_x$CuO$_4$ thin films probed by THz conductivity measurement Daisuke Nakamura, Yuki Shibuya, Yoshinori Imai, Atsutaka Maeda, Ichiro Tsukada In high-$T_c$ cuprate superconductor, Nernst signal appears far above $T_c$ in the underdoped region, which was suggested as the manifestation of vortex-like precursor of the superconducting transition.\footnote{Z. A. Xu {\it et al.}, Nature {\bf 406}, 486 (2000).} On the other hand, we previously investigated the superconducting fluctuation by measuring the microwave complex conductivity of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) thin films, and found a different analytical result that the superconducting fluctuation exists up to $T \sim 2 T_c$ at most, even at the infinite frequency.\footnote{H. Kitano {\it et al.}, Phys. Rev. {\bf B73}, 092504 (2006); T. Ohashi {\it et al.}, Phys. Rev. {\bf B79}, 184507 (2009).} To resolve this discrepancy, we directly measured the THz conductivity on LSCO thin films with various doping level ($x$ = 0.07, 0.12, 0.15, 0.225). We found that the imaginary part of the complex conductivity increases rapidly from the temperature about $T \sim 2 T_c$ at most, which is consistent with the result of our microwave measurement. This result suggests that the appearance of large Nernst signal in high-$T_c$ superconductor is not caused by superconducting fluctuation. [Preview Abstract] |
Tuesday, March 16, 2010 1:15PM - 1:27PM |
J41.00011: THz conductivity of BaCo$_{0.2}$Fe$_{1.8}$As$_{2}$ and Fe(Se,Te) superconductor A. Maeda, D. Nakamura, T. Katase, H. Hiramatsu, H. Hosono, Y. Imai, T. Akiike, R. Tanaka, I. Tsukada Ac complex conductivity is investigated for Fe-based superconductors, BaCo$_{0.2}$Fe$_{1.8}$As$_{2}$ (122) with $T_c=$17 K and Fe(Se,Te) (11) with $T_c=$10 K. For the 122 material, typical features of a superconductor were seen in the frequency dependence of conductivity, $\sigma$. In particular, we find a structure corresponding to superconductivity gap, 2$\Delta$, whose magnitude is 3meV at lowtemperatures, leading to 2 $\Delta/k_BT_c=$ 4.1. Althogh the material is known to be a multi-band superconductor, this value is in good agreement with the smaller gap found in an ARPES measurement[1]. For the 11 material, we have not suceeded in obtaining a clear gap structure, probablly because of lower $T_c$, at present. We are now trying to measure microwave conductivity by a broadnabd technique[2], the result of which will be presented. \\[4pt] [1] K. Terashima {\it et al.}, PNAS 106 (2009) 7330. \\[0pt] [2] H. Kitano {\it et al.}, RSI 79 (2008) 074071, {\it ibid}, PRB 73 (2006) 174522, T. Ohashi {\it et al.}, PRB79 (2009) 184507. [Preview Abstract] |
Tuesday, March 16, 2010 1:27PM - 1:39PM |
J41.00012: Microwave Conductivity of YBa$_2$Cu$_3$O$_{6.5}$ in the Low Temperature Limit Jordan Baglo, James Day, Jake Bobowski, Shun Chi, Pinder Dosanjh, Rinat Ofer, Brad Ramshaw, Lynne Semple, Ruixing Liang, Walter Hardy, Doug Bonn With adaptation for dilution refrigerator operation, the unique capabilities of bolometric microwave spectroscopy have been introduced to the millikelvin temperature regime. Using this technique we have measured the microwave conductivity of ultra-high purity Ortho-II YBa$_2$Cu$_3$O$_{6.5}$ down to below 100 mK, over a continuous frequency range from 0.5 to 20 GHz. The resulting spectra are analyzed in the context of a phenomenological lineshape model for the quasiparticle conductivity. The evolution of the spectra from the low temperature limit sheds new light on the low-energy charge dynamics of YBa$_2$Cu$_3$O$_{6.5}$. [Preview Abstract] |
Tuesday, March 16, 2010 1:39PM - 1:51PM |
J41.00013: ABSTRACT WITHDRAWN |
Tuesday, March 16, 2010 1:51PM - 2:03PM |
J41.00014: Ultrafast photoconductivity of undoped cuprates J. Steven Dodge, Amir Farahani, Jesse Petersen, Ruixing Liang, Ivan Bozovic Using a visible pump-THz probe technique, we studied the ultrafast transient photoconductivity of the insulating cuprate La$_2$CuO$_4$, and compared it with earlier measurements of Sr$_2$CuO$_2$Cl$_2$ and YBa$_2$Cu$_3$O$_6$. In all these compounds, we observe a rapid onset of photoconductivity that is followed by a non-exponential relaxation on a picosecond time scales, the dynamics of which are independent of photocarrier concentration ranging from 0.2 to 1.5 percent. The photoconductivity decay is qualitatively similar to the decay of the photoinduced gap absorption in Sr$_2$CuO$_2$Cl$_2$,\footnote{J. S. Dodge, arXiv:0910.5048} indicating a common origin for the two effects. Assuming a quantum efficiency of unity, the estimated peak mobility for all three compounds is within 0.1-0.4~$\mathrm{cm}^2/\mathrm{V}\cdot\mathrm{s}$; this is lower than the Hall mobility in chemically doped systems with similar carrier concentrations,\footnote {Ando et al, Phys. Rev. Lett. 87, 017001 (2001)} but orders of magnitude larger than earlier DC photoconductivity results.\footnote{Thio et al, Phys. Rev. B 42, 10800 (1990)} The similarity of the peak photoconductivity across three different compounds indicates that it is an intrinsic feature of the copper oxide plane. [Preview Abstract] |
Tuesday, March 16, 2010 2:03PM - 2:15PM |
J41.00015: The Persistance of Superconducting Fluctuations in Underdoped LSCO Lucas Bilbro, R. Valdes Aguilar, G. Logvenov, I. Bozovic, N.P. 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 at terahertz frequencies in LSCO for dopings effectively covering the entirety of the underdoped regime. A number of issues are investigated, including evidence for a non-zero finite frequency superfluid stiffness in regions outside the superconducting phase boundary. We find a much smaller extent of superfluid stiffness above Tc as compared to the prevailing interpretation of Nernst and diamagnetic responses. We discuss the significance of our measurements regarding the extent of the fluctuation regime and the impact of these fluctuations on the physics of the pseudogap. [Preview Abstract] |
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