Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A10: Superconductivity: Electronic and Thermal Transport Properties |
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Sponsoring Units: DMP Chair: Michael Bleiweiss, Naval Academy Preparatory School Room: Colorado Convention Center Korbel 1E |
Monday, March 5, 2007 8:00AM - 8:12AM |
A10.00001: High Field Hall Effect and Resistivity in High-$T_{c}$ La$_{2-x}$Sr$_{x}$CuO$_{4}$ Fedor Balakirev, Jonathan Betts, Albert Migliori, Ichiro Tsukada, Yoichi Ando, Gregory Boebinger Hall effect and resistivity measurements were performed in a set of La$_{2-x}$Sr$_{x}$CuO$_{4}$ thin film samples in magnetic field up to 60T. The Sr doping, x, was varied between 0.08 and 0.22. The resistivity and Hall voltage were measured simultaneously using digital lockin technique developed at NHMFL. We find a pronounced minimum in the doping dependence of the Hall coefficient, suggesting a common phenomenon which is generic for high temperature superconductors. This Hall effect anomaly is most readily associated with a phase transition near optimal doping where superconductivity is most robust. [Preview Abstract] |
Monday, March 5, 2007 8:12AM - 8:24AM |
A10.00002: In-plane resistivity and Hall effect data of La$_{2-x}$Sr$_{x}$CuO$_{4}$ in high magnetic fields Scott Riggs, Fedor Balakirev, Albert Migliori, Jon Betts, Greg Boebinger, Genna Logvenov, Tony Bollinger, Ivan Bozovic The effects of stoichiometry (Sr concentration in La$_{2-x}$Sr$_{x}$CuO$_{4 }$- LSCO) are of extreme interest in exploring the phase space of high-temperature superconductivity (HTS). Carrier doping in LSCO is determined by the Sr concentration rather than non-stoichiometric oxygen, thus controlling the carrier concentration in LSCO tends to be easier than in the other HTS cuprates. Nevertheless, precise control of Sr at a precision greater than 0.005 is extremely difficult using existing single crystal growth techniques. We use our unique MBE growth chamber to grow combinatorial thin films of LSCO with a uniform gradient of Sr concentration across a single substrate. Intense magnetic fields suppress superconductivity, revealing the underlying normal state to temperatures below 0.3K. We report the Hall effect and resistivity with an extremely fine $\sim $x=0.0002 Sr resolution in fields up to 60T. [Preview Abstract] |
Monday, March 5, 2007 8:24AM - 8:36AM |
A10.00003: Low-temperature specific heat and thermal Hall conductivity in a vortex state of d-wave superconductors Ashot Melikyan, Oskar Vafek We analyze the mixed state of $d$-wave lattice superconductors focusing on the quasiparticle contribution to the specific heat and the thermal Hall conductivity at intermediate magnetic fields $H_{c1}\ll H \ll H_{c2}$. In the ultra-low temperature regime $T\ll T_0\approx v_D^2/(v_F l)$ the specific heat follows a general scaling form $C[T, H=hc/el^2] =(T/v_F l) \Phi[ v_F/(Tl), v_F/v_D, k_F l]$. In this regime the specific heat exhibits oscillatory behavior as a $2\pi$-periodic function of $k_F l$: in general it has an activated form $C\propto \exp(-\Delta_m/T)$ except for a discrete set of $k_Fl$ where $\Delta_m=0$ and $C\propto T^2$. At temperatures $T_0\ll T\ll \Delta$, the $k_Fl$-oscillations become unobservable due to thermal broadening and the Simon-Lee scaling is recovered. The results of the analysis of the thermal Hall conductivity are similar: in particular, at the lowest temperatures, $\kappa_{xy}$ is an oscillating $2\pi$-periodic function of $k_Fl$. We calculate the scaling functions numerically and compare our results with the existing experimental data on the specific heat and thermal Hall conductivity. [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 8:48AM |
A10.00004: The Hall Number, Optical Sum Rule and Carrier Density for the $t-t'-J$ Model Sriram Shastry, Jan Haerter Mott Hubbard systems, epitomizing strong correlations and a sensitivity to half filling, display striking departures from band theory for many measurables. E.g. consider two quantities; the Hall constant $R_H$ and the optical conductivity sum rule $\omega_P^2/8$. These are often inverted to give the carrier densities $n_H\equiv 1/ q_e c R_H$ and $n_{Op}=\frac{m}{ 4 \pi q_e^2} \omega_P^2$. There is considerable difficulty in reconciling these with $x$, the ``chemical'' estimate of density in many High $T_c$ systems[1]. We have argued previously[2] that the Hall constant is a manybody object, that need not scale simply with $x$. In this work, we compute the variables $n_H$ and $n_{Op}$ for a $t-t'-J$ model by using exact diagonalization of small clusters and different dopings $x$. We compute the Kubo formulas exactly for small clusters, and also the high frequency Hall constant for even larger systems, and obtain a strong dependence of these variables on the ratio $t'/t$. We also comment on the departure from Luttinger's theorem for the Fermi surface for these clusters, defining the same from the tower of excited states for a given wave vector for an added particle or hole. [1] W. Padilla {\em et.al.}, ~Phys. Rev. {\bf B 72}, 060511(2005). [2] B. S. Shastry, B. I. Shraiman and R. R. P. Singh, Phys. Rev. Lett.{\bf 70}, 2004(1993). [Preview Abstract] |
Monday, March 5, 2007 8:48AM - 9:00AM |
A10.00005: Nernst effect as a probe of superconducting fluctuations in Nb$_{0.15}$Si$_{0.85}$ A. Pourret, H. Aubin, J. Lesueur, K. Behnia, C. Marrache-Kikuchi, L. Berg\'{e}, L. Dumoulin We present a study of the Nernst effect in thin films of the amorphous superconductor Nb$_{0.15}$Si$_{0.85}$ . A finite Nernst signal was resolved at temperatures well above T$_{c}$ and at relatively high magnetic fields (1). In the zero-field limit and close to T$_{c}$, our results are in very good agreement with a simple relation derived from the theory by Ussishkin, Sondhi and Huse (2) for a two-dimensional superconductor. According to the theory, the magnitude of the Nernst signal generated by the fluctuating Cooper pairs depends only on the superconducting coherence length $\xi$. Far above T$_{c}$ and/or in presence of a finite magnetic field, a departure from this relation is observed. Yet, even in this regime, the amplitude of the Nernst coefficient depends on a single length scale set by $\xi$ and by the magnetic length $l_{B}$. This observation allows to establish a phenomenological relation for the Nernst coefficient, for all magnetic fields and temperatures above T$_{c}$, which depends only on the size of the superconducting fluctuations set by $\xi$ and/or $\l_{B}$.\\ (1) A. Pourret \emph{et al.}, Nature Physics \textbf{2}, 683 - 686 (2006)\\ (2) I. Ussishkin, S. L. Sondhi and D. A. Huse, Phys. Rev. Lett. \textbf{89}, 287001 (2002) [Preview Abstract] |
Monday, March 5, 2007 9:00AM - 9:12AM |
A10.00006: On the heat current in the magnetic field: Nernst-Ettingshausen effect above the superconducting transition Andrei Sergeev, Vladimir Mitin, Michael Reizer For maintaining gauge invariance in a magnetic field, the heat current operator should include the magnetic term. Taking this term into account, we revised calculations of the Nernst-Ettingshausen effect above the superconducting transition. We found that the fluctuations of the modulus of the order parameter do not change the particle-hole asymmetry (PHA) of the thermomagnetic effects. As in the normal state, the thermomagnetic effects in the fluctuation region are proportional to the square of PHA and, therefore, small. Magnetization currents in the electric field contribute to the charge and energy transfer, but not to the heat current. Only in this way, one can obtain the Nernst and Ettingshausen coefficients that satisfy to the Onsager relation Large Nernst effect observed in the high-temperature cuprates requires vortex-like excitations due to the phase fluctuations, which are beyond the Gaussian-fluctuation theory. [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A10.00007: Nernst effect and diamagnetism in a vortex liquid Daniel Podolsky, Srinivas Raghu, Ashvin Vishwanath When a superconductor is warmed above its critical temperature $T_c$, superconductivity is destroyed by fluctuations in the order parameter. These fluctuations are seen in a variety of experimental probes, including conductivity, diamagnetism, and the Nernst effect -- the thermoelectric analogue of the Hall effect. In this talk we will discuss a regime in which superconductivity is destroyed by phase fluctuations arising from a dilute liquid of mobile vortices. The local superconducting correlations in this state lead to unusual properties, which are theoretically captured by a thermally fluctuating XY model in which amplitude fluctuations remain effectively frozen. We find that the Nernst effect and diamagnetic response differ dramatically from those arising from Gaussian fluctuations -- in particular, a more rapid decay with temperature is obtained. We predict a rapid onset of Nernst effect at a temperature $T_{\rm onset}$, and show that this scale tracks $T_c$ rather than the pairing temperature. We predict a close quantitative connection with diamagnetism -- the ratio of magnetization to transverse thermoelectric conductivity $\alpha_{xy}$ reaches a universal value at high temperatures. We compare our results to Nernst effect measurements on the underdoped cuprates, and interpret these results in terms of a dilute vortex liquid over a fairly wide temperature range above $T_c$. [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 9:36AM |
A10.00008: Anomalous quasiparticle thermal transport in the superconducting state of ultra pure URu$_2$Si$_2$ single crystals Yuichi Kasahara, T. Iwasawa, T. Shibauchi, Y. Matsuda, K. Behnia, T. D. Matsuda, Y. Haga, Y. Onuki In heavy fermion superconductor URu$_2$Si$_2$, although it is believed that superconductivity in this material ($T_c\sim1.5$ K) is unconventional, details of the superconducting gap structure are still unknown. To investigate the quasiparticle transport in the superconducting state of URu$_2$Si$_2$, the thermal conductivity $\kappa$ is measured in ultra pure single crystals with residual resistivity ratio $\sim600$. In zero field, $\kappa/T$ shows a steep increase below $T_c$, indicating that the quasiparticle mean free path is strongly enhanced in the superconducting state. A finite residual term of $\kappa/T$ as $T\rightarrow0$ is clearly resolved, together with a $T^2$ dependence at very low temperatures. With applying magnetic field, thermal conductivity grows rapidly at low magnetic fields, and exhibits a $\sqrt{H}$-dependence. These results strongly indicate a presence of line nodes in the superconducting gap function. We found that $\kappa/T$ exhibits a sudden drop at upper critical field, which has never been observed in any superconductors. We discuss this unusual behavior of thermal conductivity in the context of anomalously large $\omega_c\tau(>1)$ and giant magnetoresistance observed in this material. [Preview Abstract] |
Monday, March 5, 2007 9:36AM - 9:48AM |
A10.00009: Low Temperature Heat Transport in the superconducting skutterudite PrOs$_4$Sb$_{12}$: Evidence for nodes in the superconducting gap Robert Hill, Brian Maple, Shiyan Li, Nicolas Doiron-Leyraud, Louis Taillefer Thermal conductivity measurements were performed on single crystal samples of the superconducting skutterudite material PrOs$_4$Sb$_{12}$ both as a function of temperature and as a function of magnetic field applied perpendicular to the heat current. In zero magnetic field we find clear evidence for residual electronic conduction as the temperature tends to zero Kelvin which is consistent with the presence of nodes in the superconducting gap. The field dependence of this electronic conductivity shows a rapid rise immediately above H$_{c1}$, increasing by a factor 10 in 100 mT ($\sim$ 0.05 H$_{c2}$). This is consistent with a semi-classical theory based on a Doppler-shift of the quasiparticle spectrum through coupling to the superfluid flow around magnetic vortices. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:00AM |
A10.00010: Probing the nodal metal in LBCO with heat transport Ramzy Daou, Louis Taillefer, Qiang Li, Genda Gu The cuprate superconductor La$_x$Ba$_{2-x}$CuO$_4$ (LBCO) has a near-zero minimum in the superconducting transition temperature at $x=1/8$. This is accompanied by the emergence of static one-dimensional spin and charge ordering in ``stripes'' [1]. Spectroscopic measurements at the same doping in the normal state have shown that a gap with d-wave symmetry is present in the single particle spectrum [2]. One possible origin of this gap is the destruction of the coherence of the superconducting ground state by phase fluctuations, suppressing $T_c$ while leaving gapped but ``uncondensed'' Cooper pairs and nodal quasiparticles. We present measurements of the thermal conductivity of LBCO at very low temperature in both superconducting and field-induced nodal metal states. [1] P. Abbamonte et al., {\em Nature Physics} {\bf 1}, 155 (2005) [2] T. Valla et al., {\em Science.} 10.1126/1134742 (2006) [Preview Abstract] |
Monday, March 5, 2007 10:00AM - 10:12AM |
A10.00011: One Dimensional Superconducting Transition in Quasi-Two-Dimensional Stripes Matthew Bell, Andrei Sergeev, Aleksandr Verevkin We investigate the nature of the superconducting transition in NbN ultrathin nano stripes where the thickness of the stripe (4 nm) is about or less then the coherence length, and the width (100 nm) is significantly larger then the coherence length. It is well known that in micro stripes the resistive state below the Berezinskii-Kosterlitz-Thouless transition is produced by dissociated vortex-antivortex pairs. However, our data clearly demonstrates that in such structures the resistive state is formed due to one-dimensional phase slip centers (PSCs) at low current densities. Our analysis shows that the resistive state is actually a result from the competition between the PSCs and two-dimensional vortices. At low currents, the PSC mechanism prevails over the contribution from vortices over a broad temperature range. At higher currents, current induced unbinding of vortex-antivortex pairs contributes the most to resistivity, however this effect is limited by electron heating. From this analysis we will develop a current-temperature phase diagram for superconducting nano stripes. [Preview Abstract] |
Monday, March 5, 2007 10:12AM - 10:24AM |
A10.00012: Thermal Expansion at the Superconducting Phase Transition in Nb Richard K. Bollinger, John J. Neumeier, Carlos A.M. dos Santos, Hugo R.Z. Sandim Thermal expansion is an important thermodynamic quantity that is difficult to measure with sufficient precision to observe electronic phase transitions such as the normal to superconducting transition. Thermal expansion data will be presented on the superconducting to normal phase transition of Nb ($T_c$ = 9.27 K), obtained with a novel quartz dilatometer cell. Surprisingly, only one prior report of this measurement has been published.$^{*}$ This report does not clearly show the predicted jump at $T_c$. Thermal expansion data can be used to compute the pressure derivative of $T_c$, and this analysis will be presented. $^{*}$ White, G.K., $\textit{Croyogenics}$ $\textbf{2}$, 292 (1962). [Preview Abstract] |
Monday, March 5, 2007 10:24AM - 10:36AM |
A10.00013: On the Nature of the Superconducting Transition in YBCO M. Yethiraj, S.J. Crowe, D. McK. Paul, D.K. Christen, M. Arai, T. Yokoo, L. Porcar, P.D. Butler In the high-Tc superconductor YBCO, a transition was observed from a hexagonal FLL at low magnetic field (parallel to the c-axis) to a square configuration at high fields. Also seen was a rapid decrease in the Bragg intensity at low temperature (T). It has been the general belief that both the symmetry change and the T-dependence behaviour was due to the d- wave nature of high-Tc superconductivity. However, we observed that the fall-off in intensity with increasing temperature depended on the strength of the applied external field and that excellent fits to this T-dependence could be obtained by simply multiplying the temperature dependence of the familiar Ginzburg-Landau two-fluid model, appropriate for high-kappa materials conventional superconductors, by an exponential factor exp(-aT), with the field-dependent variable `a' being the only free parameter. The impact of these observations on the symmetry of the order parameter will be discussed. [Preview Abstract] |
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