Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N38: Transport Properties of High-Tc Superconductors |
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Sponsoring Units: DCMP Chair: Patrick Fournier, University of Sherbrooke Room: Baltimore Convention Center 341 |
Wednesday, March 15, 2006 8:00AM - 8:12AM |
N38.00001: Anomalous dimensional crossover in critical microwave-conductivity fluctuations of superconducting La$_{2-x}$Sr$_{x}$CuO$_{4}$ thin films Haruhisa Kitano, Takeyoshi Ohashi, Atsutaka Maeda, Ichiro Tsukada We demonstrate that there are two dimensional crossover lines separating the phase diagram of La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO) into three regions with different universality classes, by using a dynamic scaling analysis of the microwave complex conductivity. For underdoped LSCO from x=0.07 to x=0.14, we show clear evidence for the 2D-\textit{XY} universality class (the BKT transition in the nearly decoupled CuO$_{2}$ planes), while the 3D-\textit{XY} universality class is observed for nearly optimally doped region (x=0.15, 0.16). Surprisingly, for overdoped LSCO with x$>$0.17, we found that the critical behavior strongly suggested the 2D universality class, in contrast to the reduction of anisotropic properties with hole-doping. We discuss the implication of these results in terms of the effect of the quantum critical fluctuations. [Preview Abstract] |
Wednesday, March 15, 2006 8:12AM - 8:24AM |
N38.00002: Finite Size Effects in YBCO Films in Zero and Non-Zero Field Su Li, Hua Xu, M. C. Sullivan, Steven M. Anlage, C. J. Lobb The phase transition in high Tc superconductors in a magnetic field has been intensely studied. However, only a few papers have discussed finite-size effects (Phys. Rev. B \textbf{69}, 214524 (2004)). Neglecting finite size effects can cause misinterpretation of the experimental data leading to incorrect critical exponents. We will report results of DC transport measurements on YBa2Cu3O6.95 films in zero field (less than 50 nT) as well as in fields up to 6 T. The results will be analyzed in terms of field-induced finite size effects and thickness-induced finite size effects. [Preview Abstract] |
Wednesday, March 15, 2006 8:24AM - 8:36AM |
N38.00003: Fluctuations at the superconductor/normal phase transition of YBCO thin films Hua Xu, Su Li, Christopher Lobb, Steven Anlage The zero-field phase transition of high $T_c $ superconductors has been studied by a number of techniques, such as penetration depth, magnetic susceptibility, specific heat and thermal expansion, which reveal information about the static, properties of fluctuations. Transport properties (such as the conductivity) which probe the dynamics near $T_c $ are less explored, and a wide range of critical exponents were reported experimentally. We investigated fluctuation effects of $YBa_2 Cu_3 O_{7-d} $~(YBCO) around~$T_c $ by doing frequency-dependent microwave conductivity measurements and dc current-voltage characteristics on the same film. For each experiment the scaling behavior of the data was investigated. The critical exponents $\nu $ and z from the two different experiments will be extracted and compared. We also investigated YBCO frequency-dependent conductivity fluctuation effects for different powers or currents and for different film thickness. The finite size effects in both the microwave and DC voltage current experiments will also be discussed. (This work was supported by NSF grant number DMR-0302596) [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N38.00004: Transport and Noise Properties of High Temperature Superconductor Nanostructures D.S. Caplan, X. Zhai, J.N. Eckstein, D.J. Van Harlingen The study of transport and noise properties of high-$T_c $ superconductor nanostructures provides a sensitive probe of their local electronic structure and may give insight into the proposed mechanisms for the superconductivity and the nature of the anomalous normal phases. We discuss a novel technique for fabricating nanostructures based on the growth of cuprate films on substrates pre-patterned by Focused Ion Beam etching. We report measurements in underdoped YBCO and BSCCO nanostructures fabricated by this technique that are designed to understand anomalous switching noise observed in the pseudogap phase above $T_c$. Our goal is to test if these signal show the existence of dynamical domains characterized by inhomogeneous conductivity or by anisotropic charge stripes. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N38.00005: Thermoelectric power as evidence for a Quantum Phase Transition in electron-doped cuprates Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ . Pengcheng Li, R.L. Greene, K. Behnia We report magnetic field driven normal state thermoelectric power (S) measurement in electron-doped cuprate system Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ as a function of doping (x from 0.11 to 0.19) down to 2K. Consistent with the normal state Hall effect$^{a}$, S in the underdoped region (0.11-0.15) is negative. S changes sign at certain temperatures in overdoped samples (0.16-0.18), which supports the picture of a spin density wave rearrangement of the Fermi surface$^{b}$. More significantly, both S and S/T at 2K (at 9T) increase dramatically from x=0.11 to 0.16, and then saturate in the overdoped region. This kink around x=0.16 is similar to the previous Hall effect result$^{a}$ in Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$. Our results are further evidence for antiferromagnetism to paramagnetism quantum phase transition in electron-doped cuprates. a. Y. Dagan et al, Physical Review Letters, 92 (16) 167001, 2004 b. A. Zimmers et al, Europhysics Letters 70 (2) 225, 2005 [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N38.00006: Anomalous Nernst Effect in High-$T_c$ Superconductors by Layer Decoupling Jose P. Rodriguez The extended vortex-liquid phase found in high-$T_c$ superconductors at temperatures and magnetic fields that lie above the vortex-lattice melting line can be attributed to weak inter-layer coupling. We compute the diamagnetic contribution to the equilibrium magnetization there, $M$, using the corresponding uniformly frustrated $XY$ model, given in terms of the phase of the superconducting order parameter[1]. A high-temperature expansion that is valid in the vicinity of the mean-field phase transition yields (i) that $-M$ increases monotonically with the external magnetic field, and (ii) that it vanishes as the mean-field phase transition is approached from below. Further, we show (iii) that the assumption of a direct dependence between $M$ and the Peltier transport coefficient results in an anomalous Nernst signal inside of the vortex-liquid phase. These results are compared to recent experimental determinations of the equilibrium magnetization and of the Nernst effect in the vortex-liquid phase of high-$T_c$ superconductors[2]. \newline \newline [1] J.P. Rodriguez, PRB {\bf 66}, 214506 (2002); {\bf 69}, 069901(E) (2004). \newline [2] Y. Wang, et al., PRL, in press (arXiv: cond-mat/0503190). [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N38.00007: Thermoelectric transport near the pair breaking quantum phase transition out of a $d$-wave superconductor Daniel Podolsky, Ashvin Vishwanath, Joel Moore, Subir Sachdev We study electric, thermal, and thermoelectric conductivities in the vicinity of a $z=2$ superconductor-diffusive metal transition in two dimensions, both in the high and low frequency limits. We find violation of the Wiedemann-Franz law, with a Lorentz ratio {\it below} the Sommerfeld value (more charge than heat transport). In addition, the dc thermoelectric conductivity $\alpha$ does not vanish at low temperatures, in contrast to Fermi liquids. We introduce a Langevin equation formalism to study critical dynamics over a broad region surrounding the quantum critical point. [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N38.00008: Competing orders in LSCO probed by heat transport Shiyan Li, D. G. Hawthorn, Louis Taillefer, K. Yamada We elucidate the nature of the thermal metal-to-insulator transition in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) [1] through measurements of the thermal conductivity $\kappa$ performed very close to the transition, down to temperatures as low as 50 mK and in magnetic fields $H$ up to 17 T. For a single crystal with x = 0.15, a monotonic increase in the residual linear term $\kappa_0$/$T$ is observed up to 17 T, as expected for a d-wave superconductor. For a crystal with x = 0.144, however, we observe an initial increase in $\kappa_0$/$T$ at low field, followed by a decrease when $H$ exceeds a critical field $H^*$. This result is consistent with recent neutron scattering measurements on a similar sample [2], which show that static spin-density-wave (SDW) order is not present in zero field, but sets in at a critical magnetic field $H^*$, and then co-exists/competes with superconductivity (SC) for $H > H^*$. Taken together, these two measurements reveal that the SC phase gives way to a phase which is both magnetic and insulating, whether by increasing magnetic field or by decreasing doping. Using low-energy quasiparticle transport, we map out the $T$ = 0 field-doping ($H-x$) phase diagram of LSCO. [1] D.G. Hawthorn et al., Phys. Rev. Lett. 90, 197004 (2003); X.F. Sun et al., Phys. Rev. Lett. 90, 117004 (2003). [2] B. Khaykovich et al., Phys. Rev. B 71, 220508(R) (2005). [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N38.00009: Existence of a boson mode in underdoped YBCO Nicolas Doiron-Leyraud, S.Y. Li, M. Sutherland, L. Taillefer, R. Liang, D.A. Bonn, W.N. Hardy We have examined the underdoped region of the cuprate phase diagram via a study of heat transport at temperatures down to 50 mK in samples of YBCO with a hole concentration in the vicinity of 5\%. The measured thermal conductivity was found to be well described by a sum of three terms, $\kappa(T) = aT + bT^{\alpha} + cT^3$, which we associate with, respectively, fermionic quasiparticles, phonons, and a new bosonic mode. By comparing data taken at different doping levels on the same sample it was possible to track the evolution of the $T^3$ term. Its coefficient was found to increase with decreased doping, i.e., with proximity to magnetism. We speculate that this term may be associated with the transport of heat by magnons, as observed recently by Li et al. [1] in the undoped cuprate material Nd$_2$CuO$_4$. This would suggest that long range magnetic order in YBCO appears at a doping level very close to that at which superconductivity ends. [1] S.Y. Li et al., Phys. Rev. Lett. {\bf 95}, 156603(2005) [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N38.00010: Low-Temperature Normal State Hall Effect in High-$T_{c}$ La$_{2-x}$Sr$_{x}$CuO$_{4}$ Fedor Balakirev, Jonathan Betts, Albert Migliori, Ichiro Tsukada, Yoichi Ando, Gregory Boebinger We report Hall effect measurements in the normal state of the high-$T_{c}$ superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$. The Hall resistivity was measured by suppressing superconductivity in 60T magnetic field, thus revealing the normal-state behavior in the low temperature limit. The carrier concentration is varied from overdoped to underdoped regimes by partially substituting La with Sr in a set of thin film samples. We find a discontinuity in the doping dependence of the Hall coefficient suggestive of a phase transition near optimal doping. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N38.00011: Effect of Disorder Outside the CuO$_{2}$ Planes on $T_{c}$ of Copper Oxide Superconductors. Kazuhiro Fujita, Kenji Kojima, Hiroshi Eisaki, Shin-ichi Uchida The effect of disorder on the superconducting transition temperature $T_{c}$ of cuprate superconductors is examined. Disorder is introduced into the cation sites in the plane adjacent to the CuO$_{2}$ planes of single-layer systems, Bi$_{2}$Sr$_{1.6}$Ln$_{0.4}$CuO$_{6+y}$. Disorder is controlled by changing rare earth (Ln) ions with a different ionic radius with the doped carrier density kept constant. We show that this type of disorder works as weak scatterers in contrast to the in-plane disorder produced by Zn, but remarkably reduces $T_{c}$ , suggesting novel effects of disorder on high-$T_{c}$ superconductivity. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N38.00012: Mixed State $c-$axis Resistivity of Y$_{0.54}$Pr$_{0.46}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ Single Crystals T. Katuwal, V. Sandu, C.C. Almasan, B.J. Taylor, M.B. Maple We report temperature $T$, magnetic field $H$, and angle $\theta$ dependent out-of-plane resistivity $\rho_{c}$ measurements on $Y_{0.54}Pr_{0.46}Ba_{2}Cu_{3}O_{7-\delta}$ single crystals. We performed these measurements in order to investigate the origin of the large $\rho_{c}$ of layered superconductors like cuprates and of its T, H, and $\theta$ dependence. The $\rho_{c}(T, H, \theta)$ data are very well fitted by the Ambegaokar$-$ Halperin expression [V. Ambegaokar and B. I. Halperin, Phys. Rev. Lett. 22, 1364 (1969)] for temperatures up to the critical temperature $T_{c}$ and applied magnetic field up to 14 T. This implies that in the underdoped cuprates the layered structure can be depicted as stacks of Josephson junctions. We calculated the value of the critical current density $J_{c}$ at different temperatures by using the above model and the values of the fitting parameters. Both the magnitude and $T$ dependence of $J_{c}$ are consistent with previous reports. This result supports the applicability of the model and indicates that the mixed state $c-$axis dissipation is mainly due to the Josephson effect. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N38.00013: C-axis Resistivity and Magnetoresistance of the Electron-doped Cuprate Pr$_{1.85}$Ce$_{0.15}$CuO$_{4}$ Weiqiang Yu, B. Liang, R. L. Greene C-axis resistivity and magnetoresistance have been studied extensively in the hole-doped high temperature superconductors. Observations, such as a resistivity upturn and associated negative magnetoresistance (n-MR), were attributed to the pseudogap. Recently similar phenomena were reported in the electron-doped superconductor Sm$_{1.85}$Ce$_{0.15}$CuO$_{4}$ (SCCO), and a universal Zeeman splitting of a spin gap (pseudogap) state was proposed $^1$. Here we report transport properties of Pr$_{1.85}$Ce$_{0.15}$CuO$_{4}$ (PCCO) ($T_C\approx 25K$) single crystals for comparison. Our c-axis n- MR can be explained by superconducting fluctuations due to the Aslamazov-Larkin (AL) process and the fluctuating electronic density of states (FDOS) above $H_{C2}$. We find that PCCO does not follow the Zeeman scaling behavior as reported for SCCO. This work is supported by NSF (Grant DMR 0352735). $^1$ T. Kawakami et al., Phys. Rev. Lett. 95, 017001 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 10:36AM - 10:48AM |
N38.00014: Resistivity and Hall effect measurements in Pr$_{2-x}$Ce$_{x}CuO$_{4-y }$ up to 60T R.L. Greene, Pengcheng Li, F. Balakirev We report resistivity and Hall effect measurements in the electron-doped cuprate system Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ in magnetic field up to 60T. We found negative magnetoresistance (MR) in the underdoped region for all magnetic field values, similar to the low field data reported previously$^{a}$. The MR becomes positive at high field in the optimal doped (x=0.15) sample at low temperature. Most surprisingly, we observed a substantial magnetic field dependence of the Hall coefficient at high field (above $\sim $40T) in optimal doped and overdoped samples (from x=0.15 to 0.19) in a certain temperature range. A spin density wave induced Fermi surface reconstruction model can be used to explain this phenomenon. We also report for the first time the parallel upper critical field (H//ab plane) for Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$. (a. Y. Dagan et al., Physical Review Letters 94 (5) 11 2005) [Preview Abstract] |
Wednesday, March 15, 2006 10:48AM - 11:00AM |
N38.00015: Role of Disorder and Oxygen Reduction on Transport Properties in Pr$_{1.83}$Ce$_{0.17}$CuO$_{4\pm\delta}$ J. S. Higgins, Y. Dagan, M. C. Barr, R. L. Greene, B. D. Weaver We present a study on the effects of changing the oxygen content in the electron-doped superconducting cuprate Pr$_{2-x}$Ce$_{x}$CuO$_{4\pm\delta}$ (PCCO). Epitaxial, c-axis oriented, overdoped (x = 0.17) thin films were grown using a pulsed laser deposition technique, and the oxygen content was adjusted during a post-growth annealing process. In addition to the transition temperature (T$_{c}$), measurements of the Hall effect and resistivity were performed at low temperatures (T $<$ T$_{c} $, H $>$ H$_{C2}$) in several films of different oxygen content. We compare the disorder observed in these oxygenated samples with disorder induced by proton irradiation in an optimally annealed (x = 0.17) film. An analysis of the data demonstrates that a change in the oxygen content of PCCO has two separable effects: 1) a disorder effect, and 2) a doping effect similar to that of cerium. [Preview Abstract] |
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