Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W9: Superconductivity: Optical and Raman Spectroscopy |
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Sponsoring Units: DMP Chair: David Tanner, University of Florida Room: Colorado Convention Center Korbel 1D |
Thursday, March 8, 2007 2:30PM - 2:42PM |
W9.00001: Evolution of the Spin Susceptibility of High-$T_c$ Superconductors. Thomas Timusk, Jungseek Hwang, Ewald Schachinger, Jules Carbotte We demonstrate that a new tool, a model independent numerical Eliashberg inversion of the optical self-energy, based on maximum entropy considerations can be used to extract the magnetic excitation spectra of high-transition-temperature superconductors. In Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ we explicitly show that the magnetic mode that dominates the self-energy at low temperatures directly evolves out of a smooth transfer of spectral weight to the mode from the continuum just above it. This redistribution starts already at 200 K in optimally doped materials but is much weaker in overdoped samples. This provides evidence for the magnetic origin of the superconductivity and presents a challenge to theories of the spin susceptibility and to neutron scattering experiments in high-temperature superconductors. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W9.00002: Intraband Optical Spectral Weight in the presence of a van Hove singularity: application to Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Frank Marsiglio, Fabrizio Carbone, Alexey Kuzmenko, Dirk van der Marel While the Kubo sum rule is often applied to the entire optical spectral weight to learn about the bare plasma frequency, the so-called ``Kubo single band sum rule'' is used to determine the optical spectral weight corresponding to intraband transitions in the valence band. We use a tight binding band with further than nearest neighbour hopping to explore the range of superconductivity-induced changes that are possible. We find that changes of both signs can occur within a conventional BCS framework. Using a band structure determined for Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, we find that a straightforward BCS calculation of the optical spectral weight cannot account for the experimental observations. Including a scattering rate collapse phenomenologically, however, gives a very good description of the change in optical spectral weight at $T_c$ as a function of doping. [Preview Abstract] |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W9.00003: Hall conductivity spectral weight in electron and hole doped cuprates H. D. Drew The optical conductivity quasiparticle spectral weight in optimally electron and hole doped cuprates is suppressed to about 25{\%} of value predicted by band stucture.$^{1}$ This suppression is due to Mott-Hubbard correlations caused by strong coulomb interactions. These correlations inter the longitudinal conductivity and the Hall conductivity differently. We have investigated the Fermi-liquid like behavior of the electron doped Pr$_{1.82}$Ce$_{0.18}$CuO$_{4}$ (slightly overdoped) and optimally hole doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O $_{8+\delta }$ using a spectral weight analysis of the Hall conductivity. In both materials the Hall conductivity spectral weight was found to be suppressed to about 10{\%} of the band value. This more substantial suppression of the Hall spectral weight addresses fundamental questions concerning the effects of Mott and antiferromagnetic correlations on the transport properties of strongly correlated materials. $^{1 }$ A. J. Millis, \textit{et. al.}, Phys. Rev. B72, 224517 (2005). [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W9.00004: High energy scales in electronic self-energy imaged by optical conductivity E.J. Nicol, J. Hwang, T. Timusk, A. Knigavko, J.P. Carbotte We use a new technique to directly extract an estimate of the quasiparticle self-energy from the optical conductivity which can be easily related to both theory and angle-resolved photoemission spectroscopy (ARPES) experiments. In the high $T_c$ cuprate Bi-2212 we find evidence for a new high energy scale at 900 meV in addition to the two previously well known ones at roughly 50 and 400 meV. The intermediate scale at 400 meV has been recently seen in ARPES as a large kink which optics finds to be weaker and shifted. In YBCO, the three energy scales are shifted to lower energy relative to Bi-2212 and we observe the emergence of a possible fourth high energy feature at 600 meV. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W9.00005: Optical and thermodynamic properties of Hg-1201 E. van Heumen, R. Lortz, F. Carbone, A.B. Kuzmenko, D. van der Marel, X. Zhao, G. Yu, Y. Cho, N. Barisic, M. Greven, C.C. Homes, S.V. Dordevic We present optical spectra and specific heat measurements of the optimally doped, single layer cuprate superconductor HgBa$_{2}$CuO$_{4}$ (T$_{c}$ = 97 K). Optical spectra have been obtained with a high temperature resolution allowing us to track small changes in the integrated spectral weight. We find that the low frequency spectral weight shows an extra increase when the system enters the superconducting state, indicating that the kinetic energy of the charge carriers decreases in the superconducting state. This is consistent with our earlier observations on other optimally doped cuprates$^{1,2}$. From specific heat measurements on the same sample we estimate the change in internal energy and compare this with the estimated changes of the kinetic energy. We find a change in internal energy $\Delta $U$\approx $0.1 meV per copper and $\Delta $W$\approx $0.1 meV per copper for the kinetic energy. [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W9.00006: Optical conductivity of single plane cuprate superconductor HgBa$_2$CuO$_4$ R.P.S.M. Lobo, N. Bontemps, J. Hwang, J. Yang, T. Timusk, D. Colson, A. Forget We investigated the ab-plane infrared and visible spectra of a HgBa$_2$CuO$_4$ single crystal close to optimal doping ($T_c = 90$~K) from 100 to 40000 cm$^{-1}$. Data as a function of temperature (down to 30 K) was limited to frequencies below 10000 cm$^{-1}$. The low frequency scattering rate has a linear frequency dependence. Under 120~K a supplementary small drop below 1000 cm$^{-1}$ suggests the presence of a pseudogap. This is the same frequency at which the optical conductivity shows a clear loss of spectral weight in the superconducting state. The low frequency effective mass is temperature dependent and increases from 1.5 at room temperature to 2.5 just above $T_c$. We will compare our results to other single plane cuprates. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W9.00007: Visible Pump-THz Probe Spectroscopy of the Undoped Cuprate Sr$_2$CuCl$_2$O$_2$ Jesse Petersen, J. Steven Dodge, Ruixing Liang We present experimental results on the mobility of photoexcited carriers in Sr$_2$CuCl$_2$O$_2$, an undoped cuprate. We use ultrafast laser pulses to excite photocarriers in the antiferromagnetic insulating lattice. We then probe the low-frequency dynamical conductivity of this nonequilibrium state with time-domain terahertz spectroscopy. Our current measurements place an upper bound on the mobility that is consistent with Hall mobility measurements.\footnote{Y. Ando {\em et al.} PRL {\bf 87} 017001 (2001)} [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W9.00008: Small Magnetic Fields Arrest the Josephson Plasma Resonance in La$_{2-x}$Sr$_{x}$CuO$_{4 }$for x=1/8 Alexander Schafgans, Andrew LaForge, Sasa Dordevic, Seiki Komiya, Yoichi Ando, Dimitri Basov We report on a study of the far infrared interlayer response in a La$_{2-x}$Sr$_{x}$CuO$_{4}$ (La214) crystal at the x=1/8 doping. A magnetic field up to 8 Tesla, applied perpendicular to the CuO$_{2}$ planes, is found to completely suppress the Josephson plasma resonance (JPR) in sharp contrast to the mild suppression of the JPR if the field is applied along the planes. We suggest that this anomalous sensitivity of the JPR feature to modest fields for H$\vert $c-axis is due to the interaction of in-plane charge inhomogeneities with vortices that form in the CuO$_{2}$ planes. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W9.00009: Order parameter of the paired hole states in Sr$_{14}$Cu$_{24}$O$_{41}$ studied by optics and UV-resonance Raman scattering Andrivo Rusydi, B. Schulz, R. Rauer, I. Mahns, H. Eisaki, Y. Fujimaki, S. Uchida, P. Abbamonte, M. R\"ubhausen The order parameter of the paired hole states of hole Wigner crystal (HC) in the self-doped spin (S)=1/2 two-leg ladders of Sr$_{14}$Cu$_{24}$O$_{41}$ (SCO) is studied with optics and UV-resonance Raman scattering. We observe a pair breaking excitation of the holes (2$\Delta _{h-h})$ at 200 meV which can be attributed to the rungs of the ladders. The intensity of the 2$\Delta _{h-h}$ peak as function of temperature matches very well with the formation of HC at about 250 K ($T_{HC})$. The energy of 2$\Delta _{h-h}$ is temperature independent, even at its transition temperature indicating a remaining short range order with a strongly decreased volume fraction. The order parameter of the paired hole states develops in a non-mean-field fashion and 2$\Delta _{h-h }$/k$_{b}$T$_{HC}$ is about 11, i.e. in the strong coupling limit. Our optics studies also show low- and high-spin transitions along the legs and rungs below 130 K. Our measurements confirm theoretical predictions of the existence of the paired hole states outlining the strong local pairing of the holes. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W9.00010: Ultrafast Observation of the Coexistence of Antiferromagnetism and Superconductivity in the High-T$_{c}$ Superconductor Tl-2223 Elbert E.M. Chia, Jian-Xin Zhu, D. Talbayev, A.J. Taylor, R.D. Averitt, In-Sun Jo, Kyu-Hwan Oh, Sung-Ik Lee In high-T$_{c}$ superconductors (HTSC), it is commonly believed that Cooper pairing occurs via antiferromagnetic spin fluctuations. These spin fluctuations can be shown to exist if the competing ground state to the superconducting (SC) state is antiferromagnetism (AFM). It reveals itself when, for example, the SC state is destroyed or suppressed using an externally applied magnetic field. Ultrafast spectroscopy has been widely used in probing the relaxation dynamics of photoexcited quasiparticles in correlated electron systems, and in particular, cuprate HTSCs. However, no such measurements have been taken for the regime where AFM and SC might possibly coexist. We report the first ultrafast relaxation measurements in such a coexistence phase in the HTSC Tl$_{2}$Ba$_{2}$Ca$_{2}$Cu$_{3}$O$_{y}$. Without applying any external magnetic field, we see a qualitative change in the relaxation dynamics below $\sim $40 K, which is suggestive of an entry into the AFM+SC coexistence phase. To quantitatively explain our data, we combined a coupled model describing the time-evolution of quasiparticles and high-frequency phonons in the presence of a gap in the density of states, and a mean field model that gives rise to a decrease in the SC gap as one enters the coexistence state. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W9.00011: Electron-phonon coupling in SrTi$_{1-x}$Nb$_{x}$O$_{3}$ D. van Mechelen, P. Armitage, C. Grimaldi, A. Kuzmenko, J. Teyssier, D. van der Marel One of the major questions in the physics of high temperature is, to what extent electron-phonon coupling is important for the transport anomalies and for the superconductivity itself. One of the difficulties in addressing this issue for the cuprates, is the complexity of these materials, which are doped Mott-insulators, anti-ferromagnetic, striped, etcetera. In order to separate out the electron-phonon coupling we have studied the perovskite SrTi$_{1-x}$Nb$_{x}$O$_{3}$ with 0.0002 $<$ x $<$ 0.02. The lowest unoccupied bands of pristine SrTiO$_{3 }$are Ti 3d bands of t$_{2g}$ character, which become occupied with electrons upon substituting Nb for Ti. Here we report THz, infrared and optical spectra, DC resistivity and Hall effect. The infrared spectra at 7 K reveal a very narrow (less than 2 meV) Drude peak, the spectral weight of which reveals a strong mass-enhancement (m*/m$\sim $10) at the lowest carrier concentration, which diminishes gradually to m*/m$\sim $2 as we increase the carrier concentration. The mass enhancement, the doping dependence thereof, and several other features in the infrared spectra, indicate that electron-phonon coupling is strong. The trend as a function of doping furthermore suggests the evolution from a phonon-renormalized Fermi-liquid toward a gas of small polarons in the limit of x$\to $0. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W9.00012: First demonstration of a superconducting detector cooled by solid-state refrigerators. N.A. Miller$^{1,2}$, J.A. Beall$^{1}$, D.J. Benford$^{3}$, T.C. Chen$^{4}$, J.A. Chervenak$^{3}$, W.D. Duncan$^{1}$, F. Finkbeiner$^{5}$, G.C. Hilton$^{1}$, K.D. Irwin$^{1}$, S.H. Moseley$^{3}$, G.C. O'Neil$^{1,2}$, D.R. Schmidt$^{1}$, L.R. Vale$^{1}$, R.F. Silverberg$^{3}$, J.N. Ullom$^{1}$ We have successfully cooled a Transition-Edge Sensor (TES) using solid-state refrigerators based on Normal metal/Insulator/Superconductor (NIS) tunnel junctions. The cooling mechanism is the preferential tunneling of the highest energy (hottest) electrons through the biased NIS junctions. We describe the cooling performance, temperature noise, and energy resolution of the NIS-cooled TES. In particular, we show that the NIS refrigerators introduce no detectable noise in the TES operation. NIS refrigerators can cool from temperatures near 0.3 K to below 0.1 K. Combining a pumped $^{3}$He system with NIS refrigerators provides a compact, lightweight alternative to adiabatic demagnetization refrigerators and dilution refrigerators. Bath temperatures near 0.1 K are desirable for state-of-the-art sensors for astronomy and materials analysis, as well as for a wide range of basic science applications. $^{1}$National Institute of Standards and Technology (NIST) -- Boulder $^{2}$University of Colorado at Boulder $^{3}$NASA/Goddard Space Flight Center $^{4}$Global Science and Technology $^{5}$SSAI [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W9.00013: Fast, Single-photon Detection with Superconducting Niobium Nanowires Anthony Annunziata, Aviad Frydman, Michael Rooks, Daniel Prober We present recent measurements of the quantum efficiency, counting rate, and dark count rate for single-photon detectors based on superconducting niobium nanowires at 337 nm and 470 nm wavelengths for several detector geometries. From this data it is shown that the reset time (and therefore the single-photon counting rate) of these detectors is not dependent on the kinetic inductance of the niobium wire, as is the case for other detectors of this type made from niobium-nitride. The counting rate approaches 1 GHz even for very large area (100 $\mu $m$^{2})$ detectors. A phenomenological model of detection is presented that suggests the ability to resolve the number of photons absorbed during individual detection events. Preliminary data is shown that supports this assertion. These detectors have a variety of potential applications ranging from VLSI circuit diagnostics to quantum communication and single molecule spectroscopy. This work is supported by NSF and IBM research. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W9.00014: Antenna-Coupled Superconducting Bolometers for Studying Dynamics with Terahertz Spectroscopy Daniel Santavicca, Matthew Reese, Alan True, Charlie Schmuttenmaer, Daniel Prober We report microwave and terahertz characterizations of antenna-coupled hot electron bolometers designed for laboratory-based terahertz spectroscopy. The active element is a superconducting niobium microbridge, and the incident signal is coupled to the microbridge by a planar double-dipole antenna. These devices combine sub-nanosecond response with high sensitivity and the ability to operate below saturation when viewing a room temperature background. The optimum small signal responsivity is 4.4 x 10$^{4}$ V/W, obtained at a bath temperature T$_{b}\approx $ 0.9T$_{c}$. The corresponding saturation power is 7 nW. The saturation power increases and the responsivity decreases as the bath temperature is lowered. The measured noise equivalent power is 2.0 x 10$^{-14}$ W/(Hz)$^{1/2}$, near the predicted thermal fluctuation limit. The unique combination of speed and sensitivity demonstrated by these detectors will enable new measurements of dynamic processes in the far-infrared on millisecond to nanosecond timescales. [Preview Abstract] |
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