Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U1: High Tc Transport |
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Sponsoring Units: DCMP Chair: Marcel Franz, UBC Room: LACC 152 |
Thursday, March 24, 2005 8:00AM - 8:36AM |
U1.00001: New measurements of the magnetic penetration depth in YBCO by Gd ESR, Hc, and microwave cavity perturbation. Invited Speaker: In superconductors where local electrodynamics are valid, the superfluid density, or more correctly the superfluid stiffness, is directly related to the London penetration depth. For the cuprate superconductors this fundamental property has proven to be difficult to establish reliably, in \textit{any} region of the phase diagram. Materials issues contribute to this difficulty, as do technical problems associated with the various techniques that are used, problems that are exacerbated by the large anisotropies encountered in the cuprates. Here I will describe our efforts to measure the absolute superfluid stiffness in YBCO over the doping range from slightly overdoped (Tc = 88K) to severely underdoped (Tc = 5K), using a variety of techniques, including the novel technique of zero field ESR in Gd-doped samples, measurements of the lower critical field Hc1, and a new cavity perturbation method. These and other measurements give a new picture of the relation between the superfluid stiffness and Tc. Also, the \textit{slope} of the temperature dependence of the superfluid stiffness drops rapidly for the highly underdoped samples. The implication of these results for models of superconductivity will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 9:12AM |
U1.00002: Evidence of a nodeless superconducting gap in PrCeCuO from magnetic penetration depth measurements Invited Speaker: We have measured the inverse-squared magnetic penetration depth, $\lambda ^{-2}$(T), at 50 kHz of films of the electron-doped cuprate superconductor Pr$_{2-x}$Ce$_{x}$CuO$_{4-\delta }$ over a range of Ce dopings, 0.124 $\le $ x $\le $ 0.144, that extends from underdoped to overdoped. The maximum T$_{C}$ was 24 K at x = 0.131. The films were grown by mbe on SrTiO$_{3}$ substrates that had been buffered with a thin layer of the insulating parent compound, Pr$_{2}$CuO$_{4-\delta }$, to obtain the cleanest possible films. Resistivity decreased smoothly and monotonically with doping. We used a two-coil mutual inductance technique to determine the film conductivity $\sigma $ down to about 0.5 K, and we obtained $\lambda ^{-2}$ from $\sigma _{2}$ in the usual fashion. We found that $\lambda ^{-2}$(T) was flat at low temperatures. That is, $\lambda ^{-2}$(T)/$\lambda ^{-2}$(0) changed by less than the experimental noise of 0.15{\%} over a factor of 3 or more change in T. Fits to the low-T data yield minimum a gap value, $\Delta _{min}$(0)/k$_{B}$T$_{C}$, that is unity near optimal doping and decreases with over- and underdoping. This talk will compare our results with other penetration depth measurements that find quadratic behavior at low T, consistent with a d-wave gap and with phase sensitive measurements. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:48AM |
U1.00003: Universal scaling relation in high-temperature superconductors Invited Speaker: Superconductivity at elevated temperatures in the copper-oxide materials has proven to be one of the great challenges in condensed matter physics. Despite 18 years of intensive study, the nature of the superconductivity in these systems is still not agreed upon. Scaling laws express a systematic and universal simplicity among complex systems in nature. We have recently observed a scaling relation in the high-temperature superconductors\footnote{C.C. Homes {\it et al.}, Nature {\bf 430}, 539 (2004)} between the strength of the superconducting condensate $\rho_s$ (a measure of the number of carriers in the superconducting state $n_s$), the critical temperature $T_c$, and the dc conductivity $\sigma_{dc}$ just above the critical temperature: $\rho_s \simeq 35\,\sigma_{dc}\,T_c$. This scaling relation does not depend on the crystal structure, type of dopant, nature of the disorder, or direction. Interestingly, values for the elemental superconductors Nb and Pb also fall close to this line. However, it may be shown from spectral weight arguments that these points correspond to systems in the BCS “dirty” limit (the scattering rate $1/\tau$ is larger than the isotropic energy gap $2\Delta$); in the extreme dirty limit, the scaling relation $\rho_s \simeq 65\,\sigma_{dc}\,T_c$ is recovered. The implications of the clean and dirty-limit approaches within the copper-oxygen planes are discussed. The superconductivity perpendicular to the planes is often described within a BCS framework by the Josephson effect, which interestingly also yields $\rho_s \simeq 65\,\sigma_{dc}\,T_c$, where the superfluid density and the dc conductivity are now taken along the {\it c} axis. Despite the fact that the BCS model considers an isotropic energy gap, and the cuprates are considered to be {\it d}-wave in nature with nodes, the scaling behavior of the dirty-limit and the Josephson effect is in agreement with experimental observations. This suggests that electronic inhomogeneities may play a crucial role in the nature of superconductivity in these materials. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:24AM |
U1.00004: An Infrared Probe of the Nodal Metal State in High-Tc Superconductors Invited Speaker: The focus of this talk will be on the electromagnetic response of the nodal metal state which is initiated with only few holes doped in parent antiferromagnetic systems and extends up the pseudogap boundary in the generic phase diagram of cuprates. The key spectroscopic signature of the nodal metal is the two- component conductivity: the Drude mode at low energies followed by a resonance in mid-IR. The former can be attributed to the response of coherent quasiparticles residing on the Fermi arcs. The microscopic origin of the mid-IR band is yet to be understood. A combination of transport and infrared data uncovers fingerprints of the Fermi liquid behavior in the response of the nodal metal. The comprehensive nature of the infrared conductivity data sets for YBCO and LSCO systems allows us to critically re-evaluate common approaches to the interpretation of the optical data. Specifically, I will re- examine the role magnetic excitations in generating electronic self energy effects through the analysis of the infrared data for underdoped YBCO in magnetic field. [Preview Abstract] |
Thursday, March 24, 2005 10:24AM - 11:00AM |
U1.00005: Thermal and charge transport in low-doped cuprates at very low temperature Invited Speaker: Thermal and charge transport measurements were performed in the normal and superconducting states of ultra-pure samples of low- doped YBCO down to very low temperature. The normal ground state, whether accessed by varying doping or applying a magnetic field, is shown to be metallic. Upon cooling towards T=0, the thermal conductivity exhibits a finite residual linear term and the resistivity increases by only a modest amount, in stark contrast to what is observed in LSCO. The continuity of the residual linear term upon leaving the superconducting state points to a normal state with a nodal excitation spectrum. By directly comparing charge and heat conductivities as T $\rightarrow$0 we are able to perform a preliminary test of the Wiedemann-Franz law in underdoped cuprates. [Preview Abstract] |
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