Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session X8: Focus Session: Novel Superconductors VII: Triplet Pairing and Time Reversal Symmetry Breaking |
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Sponsoring Units: DMP Chair: Igor Mazin, Naval Research Laboratory Room: Colorado Convention Center Korbel 1C |
Friday, March 9, 2007 8:00AM - 8:36AM |
X8.00001: High Resolution Polar Kerr Effect Measurements of Sr$_2$RuO$_{4}$: Evidence for Broken Time Reversal Symmetry in the Superconducting State Invited Speaker: Strontium ruthenate ({Sr$_2$RuO$_{4}$}) is an odd-parity superconductor, which has odd orbital angular momentum and symmetric spin-triplet (p-wave) pairing. Some of the possible p- wave states can further break time-reversal symmetry (TRS), since the condensate has an overall magnetic moment because of either the spin or orbital (or both) parts of the pair wave function. However, this TRS-breaking moment will be screened by the Meissner effect. Previously, tests for broken time- reversal symmetry in {Sr$_2$RuO$_{4}$} relied on surfaces and defects where the Meissner screening is not perfect. However, for an unambigous determination of this effect, a bulk measurement, such as measuring magneto-optic like effects, on high quality crystals, is needed. To this end we developed a new technique of measuring Polar Kerr Effect (PKE) at temperatures much below the transition temperature of {Sr$_2 $RuO$_{4}$} of 1.5 K. The technique is based on a fiber Sagnac interferometer with a zero-area Sagnac loop. This new technique allowed us to measure PKE with an accuracy of 10 nano-radian at 400 mK, while rejecting other artifacts like linear birefringence of the sample. The incident optical power was set to be below 2 micro-Watts in order not to heat up the sample locally at such low temperatures. We have observed non-zero Kerr rotations as big as 65 nanorad appearing below $T_c$ in domains comparable in size with the 25-micron-diameter optical beam. Our results imply a broken time reversal symmetry state in the superconducting state of {Sr$_2$RuO$_{4}$}, similar to $^3$He-A. More recent results on other oxide superconductors will also be described. This work was supported by Center for Probing the Nanoscale, NSF NSEC Grant 0425897 and by the Department of Energy grant DEFG03-01ER45925. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X8.00002: Theory of the high-frequency chiral optical response in a $p_x+ip_y$ superconductor Victor Yakovenko The optical Hall conductivity and the polar Kerr angle are calculated as functions of temperature for a two-dimensional chiral $p_x+ip_y$ superconductor, where the time-reversal symmetry is spontaneously broken. The theoretical estimate for the polar Kerr angle agrees by the order of magnitude with the recent experimental measurement in $\rm Sr_2RuO_4$ by Xia et al.,\ Phys.\ Rev.\ Lett.\ {\bf 97}, 167002 (2006). The theory predicts that the Kerr angle is proportional to the square of the superconducting energy gap and is inversely proportional to the cube of frequency, which can be verified experimentally.\\ Reference: cond-mat/0608148. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X8.00003: Instability of Singlet Superconductivity with Respect to the Appearance of a Triplet Component in a Vortex Phase. Andrei Lebed, Omjyoti Dutta We show [1] that a vortex phase in a singlet d(s)-wave superconductor is absolutely unstable with respect to a generation of a triplet component of a superconducting order parameter. The triplet component, which appears for both attractive and repulsive interactions in a triplet channel, is shown to break three important symmetries of an internal superconducting order parameter: spin-rotational, parity [1], and time-reversal [2] ones. As a result, Cooper pairs are characterized by non-zero angular momenta [2] and non-zero spins [1], polarized in a plane, perpendicular to the external magnetic field. The above mentioned effects are expected to be of the order of unity in almost all modern superconductors such as MgB$_{2}$, high-Tc, organic, and some others. [1] A.G. Lebed, Phys. Rev. Lett. \textbf{96}, 037002 (2006). [2] Omjyoti Dutta and A.G. Lebed, Nature, submitted (2006). [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X8.00004: Broken time-reversal symmetry in a vortex phase of a superconductor under perpendicular magnetic field. Omjyoti Dutta, Andrei Lebed A vortex phase in a singlet superconductor is absolutely unstable with respect to a generation of a triplet component of a superconducting order parameter. The triplet component, which appears for both attractive and repulsive interactions in a triplet channel, breaks spin-rotational and parity symmetries[1]. Here we show[2] that in a perpendicular magnetic field, in addition to the above mentioned symmetries, the order parameter also breaks time reversal symmetry. As a result, Cooper pairs carry non-zero angular momenta. The above mentioned effects are expected to be of the order of unity in almost all modern superconductors such as high-Tc and organic ones. We suggest experimental studies to discover triplet-singlet mixing phenomenon which characterizes this novel type-IV superconductivity. [1] A. G. Lebed, Phys. Rev. Lett. 96, 037002. [2]O. Dutta and A. G. Lebed, Nature, submitted (2006) [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X8.00005: Aligning chiral order parameter domains in Sr2RuO4: Josephson interferometry measurements Francoise Kidwingira, J.D. Strand, D.J. Van Harlingen, Y. Maeno There is compelling evidence that the ruthenate superconductor Sr2RuO4 forms chiral order parameters of the form px+ipy and px-ipy. In zero magnetic field, these states are degenerate and result in the formation of a dynamical domain structure that has been detected by Josephson interferometry experiments [1]. However, the degeneracy between the order parameters can be lifted by applying a magnetic field while cooling the system through the superconducting transition. We present Josephson interferometry measurements on field-cooled Josephson junctions that show evidence for domain alignment, manifested by the enhancement of the critical current and qualitative changes of the critical current modulation pattern in applied magnetic field. We also report evidence for memory effects in the domain chirality. [1] Francoise Kidwingira \textit{et al}., \textit{Science}, October 26 2006 (10.1126/science.1133239). [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X8.00006: Probing Bound States in p-wave Superconductors using Shot Noise C.J. Bolech, Eugene Demler The zero-energy bound states at the edges or vortex cores of chiral p-wave superconductors are expected to behave like Majorana fermions. We introduce a model Hamiltonian that describes the tunnelling process when electrons are injected into such states. Using a non-equilibrium green function formalism, we find exact analytic expressions for the tunnelling current and noise and identify experimental signatures of the Majorana nature of the bound states to be found in the shot noise. We discuss the results in the context of different candidate materials that are believed to support triplet superconductivity. [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X8.00007: Skyrmion Flux Lattices in $p\,$-wave Superconductors Qi Li, John Toner, Dietrich Belitz In $p\,$-wave superconductors, topological excitations known as skyrmions are allowed, in addition to the usual vortices. In strongly type-II materials in an external magnetic field, a skyrmion flux lattice is expected to be energetically favored compared to a vortex flux lattice [1]. We analytically calculate the energy, magnetization curves ($B(H)$), and elasticity of skyrmion flux lattices in $p\,$-wave superconductors near the lower critical field $H_{c1}$, and use these results with the Lindemann criterion to predict their melting curve [2]. In striking contrast to vortex flux lattices, which {\it always} melt at an external field $H > H_{c1}$, skyrmion flux lattices {\it never} melt near $H_{c1}$. This provides a simple and unambiguous test for the presence of skyrmions. In addition, the internal magnetic field distributions (which are measurable by muon spin rotation techniques [3]) of skyrmion and vortex lattices are very different. \medskip\par\noindent [1] A. Knigavko, B. Rosenstein, and Y.F. Chen, Phys. Rev. B {\bf 60}, 550 (1999). \par\noindent [2] Qi Li, John Toner, and D. Belitz, cond-mat/0607391 \par\noindent [3] J.E. Sonier, J. Phys. Cond. Matt. {\bf 16}, S4499 (2004) [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X8.00008: Probing the superconducting order parameter of UPt3 by Josephson Interferometry J.D. Strand, F. Kidwingira, D.J. Van Harlingen, J.P. Davis, W.P. Halperin The unconventional superconductor UPt3 exhibits two superconducting transitions which are believed to correspond to two distinct superconducting phases. The symmetry of the order parameter in these phases has yet to be determined and the origin of the double transition remains an open question. We have fabricated Josephson junctions by evaporating copper and lead films onto UPt3 single crystals. Using a SQUID potentiometer, we observe critical currents that onset at the upper transition temperature of the crystal ($\sim $0.55K) and increase as the temperature is lowered. We are studying the magnetic field dependence of the critical current in edge and corner junctions to obtain direct information about the phase anisotropy of the order parameter and hence its pairing symmetry. We are particularly interested in measurements near the lower transition temperature ($\sim $0.50K) at which the order parameter is predicted to become complex and hence break time-reversal symmetry. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X8.00009: High Resolution Polar Kerr Effect Measurements of High-Temperature Superconductors: Evidence for Broken Time Reversal Symmetry Below the Pseudogap temperature Elizabeth Schemm, Jing Xia, Wolter Siemons, Gertjan Koster, Martin M. Fejer, Aharon Kapitulnik High resolution Polar Kerr Effect (PKE) measurements were performed on YBa$_2$Cu$_3$O$_{7-\delta}$ as a function of temperature for variety of doping levels. In order be able to measure effects beyond our old search for anyon superconductivity, we devised a new technique based on a fiber Sagnac interferometer with a zero-area Sagnac loop. With this technique we show a shotnoise-limited sensitivity of 100 nanorad/$\sqrt{Hz}$ with incident photon power of $\sim$ 10 $\mu$-Watt, in a wide temperature range from 0.3 K to room temperature. Our results indicate that a Time Reversal Symmetry Breaking (TRSB) signal appears in all underdoped YBa$_2$Cu$_3$O$_{7-\delta}$~ samples below the doping dependent pseudogap temperature. The effect increases with decreasing temperature and seems to saturate at a lower temperature close to $T_c$. The saturated size of the effect increases with increasing $\delta$. We will discuss possible origins of this effect and its presence in other high-Tc superconductors. [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X8.00010: Scanning magnetic imaging of strontium ruthenate (Sr$_{2}$RuO$_{4})$ Clifford Hicks, Yoshiteru Maeno, Kathryn Moler Strontium ruthenate is a spin-triplet superconductor with, very likely, a time-reversal symmetry breaking $p_{x}\pm $\textit{ip}$_{y}$ orbital order parameter. This is suggested by several experiments, including recent observation of a Kerr effect that develops when Sr2RuO4 becomes superconducting. Such an order parameter should result in spontaneous edge and domain wall currents which would generate a real-space magnetic signal, but this field yet to be observed. Currently it is estimated that, within the sample and near an edge (in the bulk it is Meissner screened), the field should peak at about 6G. We believe the spontaneous edge field may actually be significantly smaller. We discuss its observability through scanning magnetic probe microscopy and describe current experimental efforts to image and measure this field. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X8.00011: Photoemission study of doping in the strontium ruthenate family Tim Kidd, Tonica Valla, John Rameau, Peter Johnson How dopants are incorporated into low dimensional correlated electron systems and their effects on such materials remain open questions despite an intense research effort over the past few decades. We have used photoemission spectroscopy to investigate the effects of dopants in these systems to measure their influence on electronic properties and phase transitions. In systems such as the high-Tc superconductors, disorder in the spatial distribution of dopants leads directly to nanoscale electronic disorder in the system. Despite this seeming randomness, photoemission studies have shown the electronic states become more well-defined with increasing concentration of dopants. In these systems, however, the dopants are usually incorporated outside the copper oxygen planes important for conduction and superconductivity. Here, we present photoemission data from the strontium ruthenate family that incorporate dopants directly into the conducting planes. Our results indicate that even relatively high dopant concentrations do not necessarily cause a large degree of disorder based broadening in the spectra. We have also detailed the influence of dopants like titanium on the associated magnetic and superconducting phase transitions in these materials. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X8.00012: Mixed pairing state in Ru microdomains embedded in bulk single-crystal Sr$_2$RuO$_4$ Zhenyi Long, Kelly McCarthy, Chrysafis Andreou, David Caven, Zhiqiang Mao, Hiroshi Yaguchi, Yoshiteru Maeno, Ying Liu We performed detailed tunneling measurements on Ru microdomains embedded in Sr$_2$RuO$_4$ using In-Ru/Sr$_2$RuO$_4$ junctions prepared by pressing freshly cut pure In wire onto a cleaved $ab$ face of a Ru-containing Sr$_2$RuO$_4$ single crystal. We observed a superconducting transition temperature of 0.45 K and a $T$ = 0 superconducting energy gap of 0.10 meV that are associated with the Ru microdomains. The systematic behavior observed in the tunneling spectra, including the presence of a Josephson coupling between In and Ru, a zero bias conductance peak (ZBCP), an unusually large magnitude and the temperature and magnetic field dependences of the superconducting energy gap, suggests that both $s$- and $p$-wave pairings are present simultaneously in the interior of a Ru microdomain. We propose the existence of a novel superconducting state featuring the mixing of the $s$- and the $p$-wave pairings in this unique superconducting system and discuss the properties of this mixed pairing state. [Preview Abstract] |
Friday, March 9, 2007 10:48AM - 11:00AM |
X8.00013: Influence of Hydrostatic Pressure on Magnetoacoustic Properties of Sr$_{2}$RuO$_{4}$. O. Svitelskiy, S. Headley, S. Tozer, E. Palm, T. Murphy, A. Suslov, D. Shulyatev The interest in the nanolayered Sr$_{2}$RuO$_{4}$ crystal is due to its unconventional spin-triplet superconductivity. We used ultrasonic pulse-echo technique to obtain further insight into its electronic structure. We investigated longitudinal 30-300 MHz sound wave propagating in [100] direction of the single crystal Sr$_{2}$RuO$_{4}$, grown by floating zone technique, at temperatures down to 300 mK, in magnetic fields up to 18 Tesla, and under hydrostatic pressures up to 7.3 kbar. The decent quality of the material was confirmed by the T$_{c}$ value of 1.25 K. Under the hydrostatic pressure, T$_{c}$ and H$_{c2}$ decreased, the superconducting transition width increased, and the sound speed increased. Data analysis allowed us to separate electron and phonon contributions in the sound attenuation. [Preview Abstract] |
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