Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B39: Superconductivity-Magnetic Properties |
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Sponsoring Units: DCMP Chair: Daniel Agterberg, University of Wisconsin Room: Baltimore Convention Center 342 |
Monday, March 13, 2006 11:15AM - 11:27AM |
B39.00001: Low temperature penetration depth measurements as a probe of anisotropic superconductivity in NbSe$_2$ Antony Carrington, Jon Fletcher, Pierre Rodiere Measurements of the field dependent thermal conductivity [1] and angle resolved photoemission spectroscopy (ARPES) [2] have provided evidence for strong gap anisotropy in NbSe$_2$ ($T_c\simeq7.2$~K). A key result of the ARPES study was that one of the sheets of Fermi surface has a much smaller energy gap than the others. Here we investigate the gap structure of NbSe$_2$ using measurements of the temperature dependence of the magnetic penetration depth in single crystal samples at temperature down to less than 0.1 K. Although there is evidence for moderate gap anisotropy (around a factor two) we do not find any evidence for the energy gap being as small as suggested by the ARPES measurements on any sheet of Fermi surface. We will compare these $\lambda(T)$ measurements to specific heat measurements on the same samples. [1] Boaknin {\it et al.}, Phys. Rev. Lett. \textbf{90}, 117003 (2003) [2] Yokoya {\it et al.} Science \textbf{294}, 2518 (2001). [Preview Abstract] |
Monday, March 13, 2006 11:27AM - 11:39AM |
B39.00002: Phase-sensitive measurements of harmonic response in high-Tc superconducting thin films by means of local microwave microscopy Dragos Mircea, Steven Anlage The microscopic origins of Meissner-state nonlinearities in superconductors are still not clear. Traditionally, microwave nonlinear measurements of superconducting thin films employ a spectrum analyzer to measure the power carried by the harmonic signals (P$_{2f }$and P$_{3f})$. Such measurements have provided strong evidence for the Nonlinear Meissner Effect (NLME) at the critical temperature T$_{c}$ in cuprates [PRB 71, 014507 (2005)]. Investigations of the NLME in underdoped YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (YBCO) thin films have revealed the existence of an additional nonlinear mechanism that onsets at T$_{c}$ and leads to the persistence of P$_{3f}$ above T$_{c}$, a feature which has not been observed in optimally-doped samples. A possible nonlinear source active at and slightly above T$_{c}$ is the current-dependent normal conductivity, as proposed earlier by Mishonov and co-workers [PRB 65, 064519 (2002)]. The measurements performed with a spectrum analyzer do not provide phase information about the harmonic signals and therefore the nature of the nonlinear source (inductive vs. resistive) remains undetermined. However, nonlinear phase-sensitive measurements can be carried out with a network analyzer in the frequency offset mode and such data are instrumental in disentangling the effects of different types of nonlinear mechanisms. Work supported by NSF-GOALI , grant no. DMR-0201261 [Preview Abstract] |
Monday, March 13, 2006 11:39AM - 11:51AM |
B39.00003: Doping Dependence of the Magnetic Penetration Depth and Coherence Length in YBa$_{2}$Cu$_{3}$O$_{y}$ Jeff Sonier, Fergal Callaghan, Jess Brewer, Walter Hardy, Doug Bonn, Ruixing Liang While linear scaling of $T_{c}$ with superfluid density has been inferred from early muon depolarization rate ($\sigma )$ measurements on high-temperature superconductors, this interpretation of $\mu $SR data assumes an invalid relation between $\sigma $ and the in-plane magnetic penetration depth $\lambda _{ab}$. From detailed measurements and analysis of the $\mu $SR line shapes in YBa$_{2}$Cu$_{3}$O$_{y}$ single crystals, we have accurately determined the doping dependence of $\lambda _{ab}$ and the superconducting coherence length $\xi _{ab}$. We find that $T_{c}$ exhibits a sublinear dependence on 1/$\lambda _{ab}^{2}$, and that $\xi _{ab}$ increases with decreasing hole doping. In addition, the $\mu $SR line shapes for $y$ = 6.50 are found to be consistent with $y$ = 0 antiferromagnetism in and around the vortex cores, compatible with static stripes. We find that the antiferromagnetic order occurs only where superconductivity is suppressed, indicating that the ordering of copper spins competes with high-temperature superconductivity. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B39.00004: A magnetic analog of the isotope effect in cuprates Rinat Ofer, Galina Bazalitsky, Amit Kanigel, Amit Keren, Assa Auerbach, James Lord, Alex Amato Since the discovery of superconductivity in the cuprates, it has been speculated that their pairing mechanism is due to magnetic interactions. However, this was never proven. Such a proof would require an experiment similar to the isotope effect in metallic superconductors, namely, a measurement of T$_{c}$ versus the in-plane super-exchange J, with no other structural changes. We have done this experiment using the (Ca$_{x}$La$_{1-x})$(Ba$_{1.75-x}$La$_{0.25+x})$Cu$_{3}$O$_{y}$ system with its 4 different families having different T$_{c}^{max}$. For each family, we measured the N\'{e}el Temperature T$_{N}$, the anisotropies of the magnetic interactions, the spin glass temperature T$_{g}$ of underdoped samples, and, of course, T$_{c}$ from under to overdoped compounds. These properties were determined using the zero field muon spin resonance ($\mu $SR) technique, where one injects polarized muons into the sample and measures the time evolution of their spin polarization. Our measurements allow us to demonstrate that T$_{c}^{max}$J$_{f}^{-1}$=const where f is a family index, and to prove experimentally that pairing in the cuprates stems from magnetic interactions. [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B39.00005: Magnetization curves in underdoped cuprates measured at low T in fields up to 45 Tesla Lu Li, Yayu Wang, Mike Naughton, S. Ono, Yoichi Ando, Nai Phuan Ong Torque magnetometry is capable of resolving the weak diamagnetic which extends to temperatures $T\gg T_c$ in hole-doped cuprate crystals. Recently, we reported$^1$ that the magnetization $M$ above $T_c$ scales accurately as the Nernst signal $e_N$ and remains robust to fields of 33 T. The results strongly support the scenario in which thermally created vortices destroy long-range phase coherence at $T_c$. We have extended these studies to explore the fluctuation magnetization to 45 T in underdoped LSCO and Bi 2201 in a series of samples doped near the critical value $x_c\sim$ 0.055. We investigate the loss of phase coherence as we decrease $x$ below $x_c$ keeping $T$ as low as 0.5 K. We use the $M$-$H$ curves to explore how singular phase fluctuations occur as superfluidity is destroyed when $x$ approaches the insulating Mott limit at low $T$.\\ $^*$Supported by NSF grant (DMR 0213706).\\ 1. Yayu Wang et al., Phys. Rev. Lett., {\bf 95}, xxx \emph{in press}, cond-mat/0503190; Lu Li \etal, Eurpophys. Lett. {\bf 72}, 451-457 (2005). [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B39.00006: Superconductivity and Glassiness in Strongly Underdoped Y$_{1-x}$Pr$_{x}$Ba$_2$Cu$_3$O$_{7-\delta}$ C. C. Almasan, V. Sandu, P. Gyawali, B. J. Taylor, M. B. Maple We performed magnetic investigations on single crystals of Y$_{0.47}$Pr$_{0.53}$Ba$_2$Cu$_3$O$_{7-\delta}$ ($T_{c}=7.0$ K) at temperatures $4 |
Monday, March 13, 2006 12:27PM - 12:39PM |
B39.00007: FFLO surface bound state Shi-Hsin Lin, Pawel Redlinski, Boldizsar Janko, Daniel Agterberg Here we study the inhomogeneous superconducting phase of heavy fermion superconductor CeCoIn5. Specifically, we propose a possible existence of FFLO surface bound states. We anticipate that the node of FFLO superconducting state would coincide with the surface to lower the energy. The order parameter vanishes at the interface and increases towards the bulk. This is similar to the case in vortex core of type II superconductor in that the normal region is confined by superconducting wall, except that one side now is vacuum. We thus anticipate bound states to exists at the surface, just as the Caroli-de Gennes-Matricon states exist in the vortex core. We will perform a self-consistent calculation using Bogoliubov-de Gennes equation to obtain the bound surface state spectrum, which can be directly compared to scanning tunnelling microscope (STM) measurements. [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B39.00008: Current driven FFLO state in unconventional superconductors Matthew Song, Hyeonjin Doh, Hae-Young Kee The typical Fulde-Farell-Larkin-Ovchinnikov (FFLO) state is known as the superconducting state with periodic spatial variation of order parameter which becomes stable under a Zeeman magnetic field. Here, we suggest that a kind of FFLO state, Cooper pair with a finite center of mass momentum, can be induced without a magnetic field. We investigate the FFLO state in $d$-wave superconductivity with an external current. In this case, the direction of the induced center of mass momentum of FFLO state is orthogonal to the direction of the external current. We discuss the detail conditions of the gap function for the current driven FFLO state. [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B39.00009: Formation of non-unitary state near the upper-critical field of Sr$_2$RuO$_4$ Masafumi Udagawa, Youichi Yanase, Masao Ogata We have studied the superconducting state of Sr$_2$RuO$_4$ under a magnetic field parallel to the superconducting plane. On the basis of quasiclassical analysis, we show that non-unitary k$_y(\hat{\mathbf{z}}-i\alpha\hat{\mathbf{y}})$ state is stabilized right at H$_{c2}$ as a result of the competition between spin-orbit interaction and Zeeman energy. As a magnetic field is lowered, this state changes to unitary k$_y\hat{\mathbf{z}}$ state. On the basis of this crossover, we address the origin of the observed double peaks of specific heat and the disappearance of the double peaks at low fields. We have also investigated the position of the phase transition line proposed by Agterberg in terms of the quasiclassical theory, and determined the magnetic field-temperature phase diagram of Sr$_2$RuO$_4$. [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B39.00010: Conductance characteristics between a normal metal and a two-dimensional Fulde-Ferrell-Larkin-Ovchinnikov superconductor Qinghong Cui, Chia-Ren Hu, J.Y.T. Wei, Kun Yang The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has received renewed interest recently due to the experimental indication of its presence in CeCoIn$_{5}$. In this work we explore the possibility of detecting the phase structure of the order parameter directly using conductance spectroscopy through micro-constrictions, which probes the phase sensitive surface Andreev bound states of $d$-wave superconductors. We employ the Blonder-Tinkham-Klapwijk formalism to calculate the conductance characteristics between a normal metal and a 2-dimensional $s$- or $d$-wave superconductor in the Fulde-Ferrell state, for all barrier parameter $z$ from the point contact limit ($z$=0) to the tunneling limit (z$\gg $1). We find that the zero-bias conductance peak due to these surface Andreev bound states observed in the uniform $d$-wave superconductor is split and shifted in the Fulde-Ferrell state. This work was supported by NSF grant No. DMR-0225698, NSERC, CFI/OIT, MMO/OCE and CIAR. Reference: cond-mat/0510717. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B39.00011: A novel homogeneous superconducting state to compete with the inhomogeneous FFLO states in conductors with exchange or Zeeman split Fermi surfaces Chia-Ren Hu The Fulde-Ferrell-Larkin-Ovchinikov (FFLO) states have been proposed as the states to replace the usual BCS state when the Fermi surfaces of spin-up and down electrons are sufficiently different due to exchange or Zeeman splitting. The FF state has a space-varying phase and the LO state has an oscillating order-parameter magnitude. Thus both are inhomogeneous states. Here I propose a novel homogeneous state to compete with the FFLO state: Instead of forming Cooper pairing of one spin-up electron of momentum \textbf{k }and one spin-down electron on momentum --\textbf{k}, this new state involves a generalized Cooper pair which is actually a triplet: One spin-up electron of momentum (1+$\delta )$\textbf{k}, one spin-down electron of momentum --(1--$\delta )$\textbf{k}, AND one boson of momentum --2$\delta $\textbf{k}, which can be a phonon, or a magnon, etc. (This state might also form in Fermion-Boson atomic mixtures, and in quark-gluon plasma.) The value of $\delta $ is such that (1+$\delta )$k$_{F}$ is the spin-up Fermi momentum, and (1--$\delta )$k$_{F}$ is the spin-down Fermi momentum. Since the boson involved does not have a fixed momentum \textbf{q}, this state is not a density wave state of any sort coexisting with superconductivity. A mean field theory of this state will be presented. [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B39.00012: The Fulde-Ferrell-Larkin-Ovchinnikov state in two-dimensional $s$-wave and $d$-wave superconductors Qian Wang, Hongyi Chen, Chia-Ren Hu, Chin-Sen Ting We study the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) states for two-dimensional superconductors under a strong magnetic field at low temperature. Using a tight-binding model, the spatial variation of the superconducting order parameter is self-consistently determined by solving Bogliubov-de-Gennes equations iteratively. We then calculate the magnetization and the LDOS of the FFLO state. When the magnetic field is parallel to the superconducting plane, our results show that for 2D s-SC (d-SC), 1D stripe (2D lattice) solutions are more energetically favorable. At certain symmetric sites, we find that the features in the local density of states can be ascribed to two types of bound states. We also investigate the effect of impurities on the FFLO state and the vortex structure when the FFLO state is subjected to an orbital magnetic field. [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 1:51PM |
B39.00013: Quantum fluctuations in LOFF superconductors Kirill Samokhin, Maxim Mar'enko We study the order parameter fluctuations near the phase transition into the Larkin-Ovchinnikov-Fulde-Ferrell superconducting state at zero temperature. In contrast to the usual normal metal-to-uniform superconductor transition, the fluctuation corrections are dominated by the modes with the wave vectors away from the origin. We find that the superconducting fluctuations lead to a divergent spin susceptibility and a breakdown of the Fermi-liquid behavior at the quantum critical point in the clean limit. [Preview Abstract] |
Monday, March 13, 2006 1:51PM - 2:03PM |
B39.00014: Helical phases in broken parity superconductors Raminder Kaur, Daniel Agterberg, Kaori Tanaka, Paolo Frigeri, Manfred Sigrist The recently discovered broken inversion superconductors, CePt$_3$Si, Li$_2$Pt$_3$B, and Li$_2$Pd$_3$B, have provided a unique opportunity to study the role of broken parity in the superconducting state. In these superconductors, a magnetic field leads to a novel inhomogeneous superconducting state, a helical phase. Although the origin of the helical phase is quite different from another well known inhomogeneous superconducting phase, Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, the consequences of both phases are similar, for example; a relative enhancement of transition temperature as a function of magnetic field over that of a uniform state. FFLO states are quite unstable and hard to observe experimentally as that exist only in the low temperature and high field regime. Until now, the FFLO state has only been identified in CeCoIn$_5 $ as an unusual magnetic field enhanced superconducting state near a quantum critical point. We argue that the helical phases is more stable than the FFLO state and will exist in all broken parity superconductors. We present results on the microscopic quasiparticle structure and the stability of the helical phase in broken inversion superconductors. [Preview Abstract] |
Monday, March 13, 2006 2:03PM - 2:15PM |
B39.00015: Superconductivity in PrRu$_{4}$As$_{12}$ single crystals W. M. Yuhasz, P. -C. Ho , T. A. Sayles, T. Yanagisawa, N. A. Frederick, M. B. Maple Single crystals of the filled skutterudite compound PrRu$_{4}$As$_{12}$ were characterized by magnetization, specific heat, and electrical resistivity measurements. These measurements reveal the occurrence of superconductivity below $\sim 2.4$~K. The magnetic susceptibility exhibits behavior consistent with a Pr$^{3+}$ ninefold degenerate $J=4$ Hund's rule ground state multiplet split in a tetrahedral crystalline electric field with either a nonmagnetic singlet or doublet ground state. Fits to the specific heat data indicate an electronic contribution to the specific heat $\gamma$ of $\sim 70$~mJ/mol~K$^{2}$ and a Debye temperature $\Theta_{\mathrm{D}}$ of $\sim 344$ K. The value of $\Delta C/\gamma T_c \approx 1.53$ for PrRu$_{4}$As$_{12}$ is close to the weak-coupling BCS value of $1.43$. Electrical resistivity measurements in field were used to determine $H_{c2} $ as a function of temperature from which the zero temperature value of the orbital critical field $^{*}H_{c2}(0)$ was calculated and used to estimate the coherence length $\xi_{0}$ of PrRu$_{4}$As$_{12}$. In contrast to PrRu$_{4}$As$_{12}$, the compound PrOs$_{4}$As$_{12}$ displays two phase transitions at $2.2$~K and $2.3$~K in zero field, one of which is antiferromagnetic and the nature of the other is yet to be determined. PrOs$_{4}$As$_{12}$ also displays heavy fermion behavior with an enhanced electronic specific heat coefficient $\gamma$ on the order of $200$~mJ/mol~K$^{2}$ and a $\Theta_{\mathrm{D}}$ of only $260$~K. This research was supported by the U.S. DOE (No.~DE-FG02-04ER46105) and NSF (No.~DMR 0335173). [Preview Abstract] |
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