Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A3: New Developments in Strontium Ruthenates |
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Sponsoring Units: DCMP Chair: Victor Yakovenko, University of Maryland Room: Oregon Ballroom 203 |
Monday, March 15, 2010 8:00AM - 8:36AM |
A3.00001: Entropy and Fermi surface considerations in the nematic phase of Sr$_{3}$Ru$_{2}$O$_{7}$ Invited Speaker: The layered perovskite metal Sr$_{3}$Ru$_{2}$O$_{7}$ has generated interest because of the discovery of nematic-like electrical transport properties at low temperatures [1]. The unusual properties are seen in the vicinity of a metamagnetic quantum critical point. They appear to be the result of the formation of a new phase, which can be observed only in the highest purity single crystals, with mean free paths of several thousand angstroms. Recently, my group has concentrated on understanding this phase and determining its boundaries using thermodynamic probes. In this talk I will review the physics that we believe underlies our observations, and then report on the recent progress, showing how measurements of the specific heat and magneto-caloric effect enable the determination of a complete ``entropy landscape'' of phase formation in the vicinity of a quantum critical point [2]. I will also discuss the discovery of de Haas-van Alphen oscillations within the putative electronic nematic phase [3]. \\[4pt] [1] R.A. Borzi, S.A. Grigera, J. Farrell, R.S. Perry, S. Lister, S.L. Lee, D.A. Tennant, Y. Maeno {\&} A.P. Mackenzie, Science \textbf{315}, 214 (2007). \\[0pt] [2] A.W. Rost, R.S. Perry, J.F. Mercure, A.P. Mackenzie {\&} S.A. Grigera, Science \textbf{325}, 1360 (2009). \\[0pt] [3] J.-F. Mercure, S. K. Goh, E. C. T. O'Farrell, R. S. Perry, M. L. Sutherland, A. Rost, S. A. Grigera, R. A. Borzi, P. Gegenwart and A. P. Mackenzie, Phys. Rev. Lett. \textbf{103}, 176401 (2009). [Preview Abstract] |
Monday, March 15, 2010 8:36AM - 9:12AM |
A3.00002: Nematic order in Sr$_3$Ru$_2$O$_7$ and triplet superconductivity in Sr$_2$RuO$_4$ Invited Speaker: In an externally applied magnetic field, ultra-pure crystals of the bilayer ruthenate compound Sr$_3$Ru$_2$O$_7$ undergo a metamagnetic transition at a temperature which can be tuned towards zero as a function of the angle between B and the crystalline c-axis. This ``metamagnetic quantum critical point", however, is enveloped by a nematic fluid phase with order one resistive anisotropy in the ab plane. In this talk, I will discuss the microscopic origins of metamagnetism and the accompanying nematic order in this system. I propose that both can be understood within the framework of an orbital-ordering tendency of the material and present a phase diagram which accounts for much of the experimentally observed phenomena. By contrast, the closely related monolayer compound, Sr$_2$RuO$_4$, is a spin-triplet superconductor; it does not exhibit metamagnetism or nematic order. This drastic difference in the physical properties of the two materials - despite their structural similarity - points towards a possible microscopic mechanism of triplet superconductivity in Sr$_2$RuO$_4$. I will conclude the talk with a discussion of our recent progress in understanding the microscopic origins of superconductivity in Sr$_2$RuO$_4$. [Preview Abstract] |
Monday, March 15, 2010 9:12AM - 9:48AM |
A3.00003: Strong Spin-Orbit Coupling Effects on the Fermi Surface of Sr$_{2}$RuO$_{4}$ Invited Speaker: The Fermi surface of Sr$_{2}$RuO$_{4}$ was studied by a wide variety of probes, establishing this material as the first complex oxide for which the de Haas-van Alphen bulk transport technique [1] and surface- sensitive angle-resolved photoemission spectroscopy (ARPES) [2] have arrived at a precise quantitative agreement. This result was obtained by exploiting temperature as an empirical cleaving parameter in suppressing the photoemission intensity associated with the reconstructed surface of the material [2]. On the basis of STM experiments [3], we have been able to show that this is a consequence of a temperature-dependent increase in the surface density of defects at the mesoscopic scale, and might be used as an effective mean to gain bulk-representative information by ARPES on unstable oxide surfaces. By comparing these bulk ARPES results to first-principle calculations, we provide evidence for the importance of spin-orbit coupling effects [4]. Subtle Fermi surface modifications are observed whenever the bands are nearly degenerate; most importantly, however, spin-orbit coupling induces a strong momentum dependence, normal to the RuO$_2$ planes, for both orbital and spin character. These findings have profound implications for the understanding of unconventional superconductivity in Sr$_{2}$RuO$_{4}$.\\[4pt] [1] Bergemann {\it et al.}, PRL {\bf 84}, 2662 (2000).\\[0pt] [2] A. Damascelli {\it et al.}, PRL {\bf 85}, 5194 (2000).\\[0pt] [3] Y. Pennec {\it et al.}, PRL {\bf 101}, 216103 (2008).\\[0pt] [4] M.W. Haverkort {\it et al.}, PRL {\bf 101}, 026406 (2008). [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:24AM |
A3.00004: Spontaneous Supercurrents in a Chiral p-Wave Superconductor Invited Speaker: In a chiral p-wave superconductor, spontaneous equilibrium supercurrents are expected to flow at the sample edges and at domain walls, separating the two chiralities. MuSR experiments on Sr2RuO4 have been interpreted as evidence for such supercurrents in the bulk and these, as well as many other experiments, are taken as evidence for chiral p-wave superconductivity in Sr2RuO4. However, scanning SQUID and scanning Hall bar measurements failed to see evidence for such currents and put stringent limits on the size of these currents. These currents are not topologically protected, although they are connected to the protected zero energy modes and to the macroscopic angular momentum of the chiral p-wave state. I will discuss Ginzburg-Landau and Boglioubov-de Gennes calculations which explore the conditions under which these currents can be substantially suppressed, while maintaining chiral p-wave order. [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 11:00AM |
A3.00005: Recent Developments in the Studies of Sr$_{2}$RuO$_{4}$: Suppression of the Upper Critical Field and the Interference between the Even-Parity Superconductivity and the Superconductivity of Sr$_{2}$RuO$_{4}$ Invited Speaker: Although there are a number of convincing experimental evidence for odd-parity, spin-triplet superconductivity of Sr$_{2}$RuO$_{4}$, there are still a few puzzling phenomena not yet clarified in this framework. To describe recent progress, we mainly present on the accurate and precise dependence of the upper critical field $H_{c2}$ on the field direction and temperature. Compared with the extension of the initial slope of $H_{c2}$ vs.$ T$ near $T_{c}$, the low-temperature $H_{c2}$ is strongly suppressed only when the field direction is within a few degrees form the exact in-plane direction. Interestingly, if the anisotropic ratio in Sr$_{2}$RuO$_{4}$ is assumed to depend on temperature, the observed angular dependence of $H_{c2}$ is reproduced better at lower temperature with an effective-mass model for an anisotropic three-dimensional superconductor. The magnitude of the suppression is rather comparable to that known for UPt$_{3}$, another strong candidate of the odd-parity superconductor. Concerning another development, we will present on the unusual temperature dependence of the critical current between the so-called 3-K phase (a eutectic crystal of Sr$_{2}$RuO$_{4}$ with Ru-metal inclusions) and lead deposited on the surface of the 3-K phase crystal. The critical current of such proximity junctions exhibits extraordinary dependence, dropping sharply below $T_{c}$ = 1.5 K of Sr$_{2}$RuO$_{4}$, but abruptly starting to increase again below about 1.1 K. Such behavior is most naturally ascribable as an interference between the even-parity and odd-parity superconductivity.. [Preview Abstract] |
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