### Session Q4: Pairing Fluctuations Near the Superconductor-Insulator Transition

 Wednesday, March 12, 2008 11:15AM - 11:51AM Q4.00001: Survival of superconducting correlations across the two-dimensional superconductor-insulator transition: A finite-frequency study Invited Speaker: N. Peter Armitage The complex AC conductivity of thin amorphous InO films was studied as a function of magnetic field through the nominal 2D superconductor-insulator transition. We have resolved a significant finite frequency superfluid stiffness well into the insulating regime, giving direct evidence for superconducting correlations in this insulating state. As these superconducting fluctuations are temperature independent at low temperatures, this also represents the first observations of quantum superconducting fluctuations around an insulating ground-state. A phase diagram is established that includes the superconducting state, a transition to a Bose'' insulator and an eventual crossover to a Fermi'' insulating state at high fields. We speculate on the consequences of these observations, their impact on our understanding of the insulating state, and its relevance as a prototype for other insulating states of matter that derive from superconductors. \newline \newline [1] R. Crane et al. Phys Rev B 75, 094506 (2007) \newline [2] R. Crane et al. Phys Rev B 75, 184530 (2007) Wednesday, March 12, 2008 11:51AM - 12:27PM Q4.00002: Theory of the Nernst effect near quantum phase transitions in condensed matter, and in dyonic black holes Invited Speaker: Markus Mueller We present a general hydrodynamic theory of transport in the vicinity of superfluid-insulator transitions in two spatial dimensions described by Lorentz''-invariant quantum critical points. We allow for a weak impurity scattering rate, a magnetic field $B$, and a deviation in the density, $\rho$, from that of the insulator. We show that the frequency-dependent thermal and electric linear response functions, including the Nernst coefficient, are fully determined by a single transport coefficient (a universal electrical conductivity), the impurity scattering rate, and a few thermodynamic state variables. With reasonable estimates for the parameters, our results predict a magnetic field and temperature dependence of the Nernst signal which resembles measurements in the cuprates, including the overall magnitude. Our theory predicts a hydrodynamic cyclotron mode'' which could be observable in ultrapure samples. We also discuss exact results for the zero frequency transport co-efficients of a supersymmetric conformal field theory (CFT), which is solvable by the AdS/CFT correspondence, mapping the CFT to a black hole problem in 3+1 dimensional anti-de Sitter space. These exact results are found to be in full agreement with the general predictions of our hydrodynamic analysis. Wednesday, March 12, 2008 12:27PM - 1:03PM Q4.00003: Observation of the Nernst signal generated by fluctuating Cooper pairs Invited Speaker: Kamran Behnia Long-range order is destroyed in a superconductor warmed above its critical temperature (T$_{c})$. However, amplitude fluctuations of the superconducting order parameter survive and lead to a number of well-established phenomena such as paraconductivity: an excess of charge conductivity due to the presence of short-lived Cooper pairs in the normal state. According to a theory by Ussishkin, Sondhi and Huse, these pairs generate a transverse thermoelectric (Nernst) signal. In two dimensions, the magnitude of the expected signal depends only on universal constants and the superconducting coherence length, so the theory can be unambiguously tested. In a dirty superconductor, the lifetime of Cooper pairs exceeds the elastic scattering time of normal electrons in a wide temperature range above T$_{c}$ and, consequently, their Nernst reponse dominates the one generated by the normal electrons. We studied the Nernst effect of amorphous superconducting films of Nb$_{0.15}$Si$_{0.85}$ and resolved a Nernst signal, which persists deep inside the normal state. The amplitude of the observed signal is in excellent agreement with the theoretical prediction [1]. \newline \newline [1] A. Pourret, H. Aubin, J. Lesueur, C. A. Marrache-Kikuchi, L. Berg\'e, L. Dumoulin, K. Behnia, Nature Phys. 2, 683 (2006). Wednesday, March 12, 2008 1:03PM - 1:39PM Q4.00004: Nernst effect and diamagnetism in phase fluctuating superconductors Invited Speaker: Ashvin Vishwanath We investigate the characteristic signatures that arise when superconductivity is destroyed by thermal phase fluctuations induced by a dilute liquid of mobile vortices. We find that the Nernst effect and diamagnetic response differ significantly from Gaussian fluctuations -- in particular, a much sharper decay with temperature is obtained. We predict a rapid onset of Nernst signal at a temperature that tracks the transition temperature Tc, rather than the pairing temperature. We also predict a close quantitative connection with diamagnetism -- the ratio of magnetization to transverse thermoelectric conductivity $\alpha_{xy}$ is proportional to the temperature over a wide range of fields. We interpret Nernst effect measurements on the underdoped cuprates in terms of a dilute vortex liquid over a broad temperature range above Tc [1]. We also introduce a new formalism to study fluctuating superconductivity that deals directly with the vortex variables. This is applied to analyze the effect of vortex properties such as core energy on Nernst effect and diamagnetism. [1] D. Podolsky, S. Raghu and A. Vishwanath, `Nernst Effect and Diamagnetism in Phase Fluctuating Superconductors', Phys. Rev. Lett. 99, 117004 (2007)