Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X25: Superconductivity: Optical, Raman and Other Spectroscopies |
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Sponsoring Units: DCMP Chair: David Tanner, University of Florida Room: D166 |
Thursday, March 24, 2011 2:30PM - 2:42PM |
X25.00001: How can we relate the critical temperature and the superconducting gap amplitude in cuprate superconductors? Alain Sacuto, Seabstien Blanc, Yann Gallais, Maximilien Cazayous, Marie Aude Measson, J.S. Wen, Z.J. Xu, Genda Gu We explore the superconducting state of hole-doped cuprates by electronic Raman scattering as a function of both temperature and doping level. We observe a loss of coherent quasi-particles in the anti-nodal region and show that coherent Bogoliubov quasiparticles are confined around the nodes. This contrasts to conventional superconductors where superconductivity develops uniformly along the normal-state Fermi surface. We define the fraction of coherent Fermi surface, f$_{c}$ around the nodes for which quasi-particles are well defined and superconductivity sets in. We establish that T$_{c} \quad \propto $ f$_{c}\Delta _{max}$. $\Delta _{max}$ is the maximum amplitude of the d-wave superconducting gap. This new relation differs from the standard BCS theory and gives us some clues for increasing T$_{c}$ in the cuprates. S. Blanc et al. Phys. Rev. B \textbf{82}, 144516 (2010); S. Blanc et al. Phys. Rev. B \textbf{80}, 140502 (2009). [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X25.00002: Raman response in density wave materials Elizabeth Nowadnick, Alexander Kemper, Brian Moritz, Thomas Devereaux Raman spectroscopy, which uses different incoming and outgoing light polarizations to measure different areas of the Brillouin zone, allows researchers to probe the nature of charge and spin density wave gaps. We present calculations of the Raman response for two density wave materials: rare earth tri-tellurides in the charge density wave state and the iron pnictides in the spin density wave state. Both of these materials have phase diagrams which can be further understood by clarifying the nature of the density wave state. For example, in the tri-tellurides, either one or two charge density wave gaps are present depending on the type of rare earth element in the compound. In the pnictides, which we treat with a multiband model, superconductivity coexists with or is in close proximity to a spin density wave state. We discuss what can be learned from our calculations and compare to experimental results. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X25.00003: Far-infrared spectroscopy of magnetic-field-induced pairbreaking in superconducting thin films Xiaoxiang Xi, J. Hwang, C. Martin, D.B. Tanner, G.L. Carr A magnetic field will break the time-reversal symmetry of the superconducting condensate pairing, giving rise to a pair-breaking effect. This pairbreaking has been confirmed by our recent far-infrared transmission and reflection measurement of a superconducting NbTiN thin film in an in-plane magnetic field. The complex optical conductivity was extracted, and the optical gap was obtained from its real part. Comparison with the pair-breaking theory of Abrikosov and Gor'kov yields good quantitative agreement, confirming directly the theory's validity for the optical conductivity. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X25.00004: Optical properties of the pseudogap state in deeply underdoped cuprates Adam Pound, Jules Carbotte, Elisabeth Nicol Recent optical measurements of deeply underdoped cuprates have revealed that a coherent Drude response persists well below the end of the superconducting dome in the phase diagram[1]. We show that this observation is consistent with the resonating valence bond spin-liquid model proposed by Yang, Rice, and Zhang[2]. Within this model, we analyze the three elements that cause the overall reduction in optical conductivity in the approach to the Mott insulating state: a Gutzwiller factor associated with increased correlations, which causes a reduction in the coherent part of the carrier Green's function; a shrinking of the Fermi surface defining the hole Luttinger contours; and an increase in optical effective mass. We show that each of these elements yields qualitative agreement with various experimental observations. Finally, we show how the increased magnitude of the pseudogap at low doping modifies the microwave conductivity and the Wiedemann-Franz law. \\[4pt] [1] W.J. Padilla et al., Phys. Rev. B 72, 060511 (2005)\\[0pt] [2] K.-Y. Yang, T.M. Rice, F.-C. Zhang, Phys. Rev. B 73, 174501 (2006) [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X25.00005: Point group sensitive probes of the pseudogap electronic structure in Bi2212 J.P. Hinton, J.D. Koralek, J. Orenstein, I. Firmo, M. Hamidian, K. Fujita, J.C. Davis We combine optical transient grating spectroscopy (TGS) and spectroscopic imaging scanning tunneling microscopy (SI-STM) to study the pseudogap electronic structure in the underdoped cuprate superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$. In TGS a pair of 50 fs pump pulses at 800 nm coincident on the sample surface generate a sinusoidal variation in the index of refraction. This index grating is phase sensitively probed, allowing us to clearly resolve two components in the optical response below Tc. We attribute one of the components to a coherent nonlinear optical process, whose properties are sensitive to the point group symmetry of the pseudogap electronic structure. We compare the results of these optical experiments with recent analysis of SI-STM data (M. J. Lawler \textit{et al} \textbf{Nature 466}$, $347 (2010)) which measures the amplitude of peaks at various reciprocal lattice vectors in the Fourier transform of atomically resolved images of the pseudogap electronic structure. The symmetry properties of the SI-STM Bragg amplitudes provide additional evidence relevant to the point group of the pseudogap electronic structure. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X25.00006: Evidence for symmetry breaking in the pseudogap phase of the single-layer Cuprate Pb-Bi2201 J.D. Koralek, J. Hinton, J. Orenstein, R.-H. He, M. Hashimoto, Z.-X. Shen, H. Karapetyan, A. Kapitulnik, H. Eisaki We use time-resolved optical spectroscopy, combined with angle resolved photoemission, and polar Kerr effect measurements, to study the single-layer Cuprate superconductor Pb$_{0.55}$Bi$_{1.5}$Sr$_{1.6}$La$_{0.4}$CuO$_{6+\delta }$ (Pb-Bi2201). Near optimal doping this material has convenient temperature scales with a $T_{c}$ of 38K and $T^{\ast }$ of 130K, allowing signals associated with the superconducting and pseudogap phases to be clearly separated in the raw data. The unusual time dependence of the pseudogap signal is suggestive of a coherent nonlinear optical process which is sensitive to changes in the electronic point group symmetry. This nonlinear signal turns on at $T^{\ast }$ and persists to low temperature. Angle resolved photoemission and polar Kerr effect measurements performed on the same batch of samples reveal the opening of a particle-hole asymmetric gap and the onset of Kerr rotation, both with strikingly similar temperature dependence to the nonlinear optical signal. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X25.00007: BCS-BEC Crossover Approach to the Optical Conductivity in high $T_c$ Superconductors Dan Wulin, Hao Guo, Chih-Chun Chien, Kathryn Levin We address the finite frequency $\omega$ conductivity in the cuprates. We presume that the pseudogap arises from stronger-than-BCS attraction, which leads to non-condensed pairs above and below $T_c$. Our theoretical formalism, which is consistent with gauge invariance and the transverse f-sum rule, yields a mid infrared peak associated with the energy needed to break pairs. It also leads to a situation in which very high $\omega$ spectral weight participates in the formation of the condensate. These observations, along with others reported here are consistent with experiment. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X25.00008: Kerr effect measurements in the pseudo-gap regime of LBCO and Pb-BSCO using high resolution Sagnac Hovnatan Karapetyan, Vikram Nathan, Ruihua He, Makoto Hashimoto, Zhi-Xun Shen, Aharon Kapitulnik, Hiroshi Eisaki, Jake Koralek, Jamie Hinton, Joe Orenstein, John Tranquada, Genda Gu, Markus Huecker Polar Kerr effect in several high-Tc superconductors systems was measured at zero magnetic field with high precision using a cryogenic Sagnac fiber interferometer with zero-area. We observed non-zero Kerr rotations of order $\sim 1 \mu$rad appearing near the pseudogap temperature $T^*$, and marking what appears to be a true phase transition. In this talk we will review our work on $\mathrm{YBa_2Cu_3O}_{6+x}$, $\mathrm{La_{1.875}Ba_{0.125}CuO_4}$ and $\mathrm{Pb_{0.55}Bi_{1.5} Sr_{1.6}La_{0.4}CuO_{6+\delta}}$. In particular, in Pb-BSCO we observe an emergence of Kerr signal that coincides with ARPES data showing an abrupt change at $T^*$ from a relatively simple one- band metal into a state with profoundly-altered electronic structure. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X25.00009: DC Transport in Pseudogapped Superconductors: The Role of the Fermi Arcs Hao Guo, Benjamin M. Fregoso, Dan Wulin, Chih-Chun Chien, Kathryn Levin We examine the dc conductivity $\sigma$ in a d-wave pseudogapped high $T_c$ superconductor for a range of different hole doping concentrations and temperatures $T$. Our approach is based on treating the cuprates as mid-way between BCS and Bose Einstein condensation and our correlation functions are demonstrably consistent with gauge invariance and the transverse f-sum rule. Studies of the $\omega \rightarrow 0$ dc conductivity below $T_c$ lead to a peak structure (observed experimentally) while above $T_c$ we show that pseudogap effects manifest themselves in the resistivity primarily through a depression in the effective carrier number with decreasing $T$. We discuss related implications for resistivity vs $T > T_c$ experiments and demonstrate that the trends with hole doping are compatible with the data, while the role of the Fermi arcs appears overall to be secondary. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X25.00010: Ultrafast transient grating and pump probe measurements in optimally doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ thin films Darius Torchinsky, Fahad Mahmood, David Hsieh, James McIver, A. Bollinger, I. Bozovic, Nuh Gedik We have performed pump probe and transient grating measurements on high-T$_c$ thin films of optimally doped La$_{2-x}$Sr$_{x}$CuO$_{4}$. In these experiments, a pair of femtosecond pulses are interfered on the sample generating a sinusoidal intensity modulation that in turn induces a density grating of photoexcitations. The resulting change in reflectivity allows time-resolved optical measurement of the separate effects of recombination and diffusion. We describe the temperature and excitation density dependence of these measurements and discuss their implications on the nature of superconductivity in the cuprates. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X25.00011: Ultrafast quasiparticle dynamics of La$_{2-x}$Sr$_x$CuO$_4$ probed by time-resolved THz spectroscopy Alex Frenzel, Daniel Pilon, Anthony Bollinger, Ivan Bozovic, Nuh Gedik We have studied picosecond quasiparticle recombination dynamics in the superconducting state of the cuprate superconductor La$_{2-x}$Sr$_x$CuO$_4$. After excitation by a 1.5 eV optical pulse, the optical conductivity in the range 0.5 - 2 THz is measured at varying time delays using coherent time-domain terahertz spectroscopy. We show that the conventional two-fluid model, which successfully describes the optical conductivity in YBCO, is unable to accurately reproduce our results. At optimal doping, we observe a weak dependence on excitation density in the recovery rate at low fluence. We comment on the recovery rate of the superconducting state in terms of the bimolecular recombination dynamics described by the phenomenological Rothwarf-Taylor model. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X25.00012: Optical conductivity in dynamic Hubbard model Giang Bach, Jorge Hirsch, Frank Marsiglio The Dynamic Hubbard model is a candidate to capture the physics of two-band Hubbard models, such as the enhancement of critical Hubbard $U$ for the Mott transition. A pseudo-spin $1/2$ auxiliary field, which modifies the Coulomb $U$ interaction based on the on-site occupancy of electrons, breaks the electron-hole symmetry normally associated with the Hubbard model. The dependence of optical conductivity on the number of particles also reveals the effect of the pseudo-spin on the spectral weight distribution as a function of frequency. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X25.00013: Optical and DC conductivity in the two-dimensional $t-t'-t''$ Hubbard model near the antiferromagnetic quantum critical point Dominic Bergeron, Andr\'e-Marie S. Tremblay We calculate the conductivity of the two-dimensional Hubbard model with second and third nearest neighbor hoppings $t'$ and $t''$ for dopings in the vicinity of the antiferromagnetic quantum critical point (QCP) using the two-particle self-consistent approach. This approach is non-perturbative and was benchmarked against quantum Monte Carlo calculations from weak to intermediate coupling. We include vertex corrections that are the analogs of the Maki-Thompson and the Aslamazov-Larkin terms in the theory of paraconductivity, but for antiferromagnetic fluctuations. With these corrections the f-sum rule is satisfied and important effects in DC and optical conductivity are obtained. In the pseudogap regime induced by antiferromagnetic correlations, the resistivity increases with vertex corrections. This effect is stronger on the hole-doped side where the system changes from metallic to insulating. This is opposite to what is observed when $t'=t''=0$. On the non-magnetic side of the QCP, the resistivity decreases with vertex corrections. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X25.00014: Exact nonequilibrium model for time-resolved photoemission spectroscopy of an electronic charge density wave insulator at zero temperature Wen Shen, James Freericks We exactly solve the nonequilibrium problem of electrons moving in a lattice potential that corresponds to a checkerboard ordered charge density wave at zero temperature. The exact solution can be found in arbitrary dimensions by calculating a series of two-by-two evolution operators with the Trotter formula. We examine how the charge density wave responds to being excited into nonequilibrium by a large electric field femtosecond pulse. We find that the order parameter is rapidly reduced (but not to zero) and then rings with an oscillation frequency given by the potential scattering energy U. The density of states shows evidence of gap closing for short times, which then reforms for long times. We discuss the implications of the solution of this model for the nonequilibrium melting of charge density waves observed in recent experiments. [Preview Abstract] |
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