Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Z23: Superconductivity: Thermodynamics, etc. |
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Sponsoring Units: DCMP Chair: S.L. Budko, Ames Laboratory Room: D165 |
Friday, March 25, 2011 11:15AM - 11:27AM |
Z23.00001: Evolution of ground state and upper critical field in R$_{1-x}$Gd$_x$Ni$_2$B$_2$C (R = Lu, Y): Coexistence of superconductivity and spin-glass state S.L. Bud'ko, V.G. Kogan, H. Hodovanets, S. Ran, S.A. Moser, M.J. Lampe, P.C. Canfield We report effects of local magnetic moment, Gd$^{3+}$, doping ($x \leq 0.3$) on superconducting and magnetic properties of the closely related Lu$_{1-x}$Gd$_x$Ni$_2$B$_2$C and Y$_{1- x}$Gd$_x$Ni$_2$B$_2$C series. The superconducting transition temperature decreases and the heat capacity jump associated with it drops rapidly with Gd-doping; qualitative changes with doping are also observed in the temperature-dependent upper critical field behavior, and a region of coexistence of superconductivity and spin-glass state is delineated on the $x - T$ phase diagram. The evolution of superconducting properties can be understood within the Abrikosov-Gor'kov theory of magnetic impurities in superconductors taking into account the paramagnetic effect on upper critical field and the details of the $x - T$ phase diagrams. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z23.00002: Micro-Calorimeter for Heat Capacity Studies of Sub-Microgram Superconducting Crystals Carlos Chaparro, U. Welp, L. Fang, W.K. Kwok, M. Eskildsen A steady-state ac-temperature micro-calorimeter for heat capacity measurement of sub-microgram superconducting samples is presented. It utilizes thermocouple thermometers nano-patterned onto 150 nm thick Si$_{3}$N$_{4}$ membranes. Theoretical models, mathematical relations describing the operation of the calorimeter and calibration procedures are discussed. The system achieves a resolution of 10$^{-4}$; and allows for measurements from 5 K to room temperature. The calorimeter can be continuously rotated in a split-coil magnet generating up to 8 T. To demonstrate the performance of our device we present measurements of the specific heat of single crystals of SmFeAsO$_{0.85}$F$_{0.15}$ (120 $\mu $m wide and 10 $\mu $m thick, crystal w1) and of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ (150 $\mu $m wide and 30 $\mu $m thick). [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z23.00003: Search for quantum critical behavior in the specific heat of HTS cuprates Jeffery Tallon, Felix Barber, John Cooper, John Loram Much evidence has been accumulated suggesting the presence of a quantum critical point in the lightly overdoped regime for high-$T_{c}$ superconductors, around $p$=0.19 holes per Cu. It is however not decisive. In the neighborhood of a quantum critical point the normal-state electronic specific heat should contain a $T$ln($T)$ term. While this would be concealed by the onset of superconductivity it is in principle recoverable using the entropy balance that exists in a second-order phase transition. This paper reviews the data for Y$_{0.8}$Ca$_{0.2}$Ba$_{2}$Cu$_{3}$O$_{7-d}$ and Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$ and places limits on the magnitude and doping evolution of a $T$ln($T)$ term in the specific heat across the phase diagram. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z23.00004: Thermodynamic effects in superconducting hybrid devices Laetitia Pascal, Sukumar Rajauria, Hung Nguyen, Bernard Pannetier, Frank Hekking, Herve Courtois Investigating thermal transport in hybrid superconducting nanostructures can yield a better understanding of such devices and give access to new and useful phenomena. Sub-gap biased superconducting-insulator-normal metal (SIN) junctions exhibit electron cooling, which is useful for achieving electronic temperatures below the cryostat bath temperature. We have designed an experiment to allow independent monitoring of the electron and phonon populations temperatures. An electronic cooler was studied under out-of-equilibrium conditions, in both cooling and heating regimes. The results are interpreted using a thermal model, which takes into account the electron, phonon and photon heat transfer. The Kapitza and electron-phonon couplings, along with a generalized circuit theory approach for the photon heat transfer, will be discussed. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z23.00005: Caloric determination of the anisotropic phase diagram of BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ crystals Wai-Kwong Kwok, Ulrich Welp, Carlos Chaparro, Lei Fang, Alexei Koshelev We report specific heat measurements on a series of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ single crystals with phosphorous contents ranging from optimal doping (x$\sim $0.3, T$_{c}$ = 29.5 K) to highly overdoped (x$\sim $0.6, T$_{c}$ = 11K). We find a sharp superconducting transition at T$_{c}$ for all doping levels, a suppression of the $\Delta $C-step at T$_{c }$with increasing doping and enhanced magnetic field dependence at higher doping. The phase diagrams determined from specific heat data show a decrease of dH$_{c2}$/dT with increasing doping and a nearly constant superconducting anisotropy of $\Gamma \sim $2.5. Our results will be compared with the proposed ``universal'' scaling of $\Delta $C$_{p}$/T$_{c}$ and dH$_{c2}$/dT due to quantum criticality and non Fermi liquid behavior [1] and due to strong pair-breaking and non-magnetic interband scattering [2], respectively. \\[4pt] [1] J. Zaanen, Phys. Rev. B 80, 212502 (2009) \\[0pt] [2] V. G. Kogan, Phys. Rev. B 80, 214532 (2009) [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z23.00006: Growth of thin superconducting films and heterostructures by atomic layer deposition Jeffrey Klug, Thomas Proslier, Nicholas Becker, Jeffrey Elam, James Norem, John Zasadzinski, Michael Pellin We report the use of atomic layer deposition (ALD) to synthesize thin superconducting films and superconductor-insulator (S-I) heterostructures. ALD uses sequential self-saturating surface chemical reactions to produce uniform coatings with atomic scale control on substrates with arbitrary shape. The ALD process therefore offers the possibility of conformally coating complex shapes with precise, layered structures with tightly constrained morphology and chemical properties. Among other applications, such coatings may enable the production of superconducting radio frequency (SRF) structures with significantly better performance and yield than those obtained from bulk niobium. Furthermore, the atomic-scale thickness control afforded by ALD enables the study of superconductivity and associated phenomena in homogeneous layers in the ultra-thin film limit. In this respect, we will present results of ALD-grown Nb$_{1-x}$Ti$_x$N-based films and S-I heterostructures. Our program looks both at the metallurgy and superconducting properties of these coatings, and also their performance in working SRF structures. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z23.00007: Laser Processing on the Surface of Niobium Superconducting Radio-Frequency Accelerator Cavities Senthilraja Singaravelu, Michael Klopf, Geoffrey Krafft, Michael Kelley Superconducting Radio frequency (SRF) niobium cavities are at the heart of an increasing number of particle accelerators.~ Their performance is dominated by a several nm thick layer at the interior surface. ~Maximizing its smoothness is found to be critical and aggressive chemical treatments are employed to this end.~ We describe laser-induced surface melting as an alternative ``greener'' approach.~ Modeling guided selection of parameters for irradiation with a Q-switched Nd:YAG laser.~ The resulting topography was examined by SEM, AFM and Stylus Profilometry. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z23.00008: Charge order phase diagrams of hole-doped cuprates Robert Markiewicz, Jose Lorenzana, Goetz Seibold, Arun Bansil ``Stripe''-like phases in the cuprates can be dominated by either spin or charge fluctuations. We calculate the phase diagram of charge-order phases stabilized by a lattice distortion using Gutzwiller approximation (GA) + RPA, and compare it to the magnetic phase diagrams [1]. The stripe periodicity is determined by Fermi surface [double] nesting, and hence is very similar for charge or magnetic stripes. A detailed analysis of the susceptibility reveals that the leading charge instability in Bi2201 and Bi2212 is an electronically driven phonon soft mode associated with a ``Pomeranchuk wave.'' This instability has the pseudogap doping dependence and shares many properties with the phase seen in scanning tunneling microscopy (STM) Work supported in part by the USDOE and by a Marie Curie Grant. \\[4pt] [1] R.S. Markiewicz {\it et al.}, Phys. Rev. B{\bf 81}, 014509 (2010). [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z23.00009: Using Disorder to Detect Locally Ordered Electron Nematics via Hysteresis Erica Carlson, Karin Dahmen The interplay between charge, orbital, and lattice degrees of freedom in correlated electron systems has resulted in many proposals for new electronic phases of matter. An electron nematic breaks the point group symmetry of the host crystal, often from $C_6$ or $C_4$ rotational symmetry to $C_2$. Electron nematics have been reported in several condensed matter systems including cuprate and iron arsenic based high temperature superconductors, and they have been proposed to exist in many other materials. However, the combination of reduced dimensionality and material disorder typically limits the spatial range over which electron nematic order persists, rendering its experimental detection extremely difficult. Despite the tantalizing possible connection between the phase and high temperature superconductivity, there is surprisingly little guidance in the literature about how to detect the remaining disordered electron nematic. We propose a general method for detecting disordered electron nematics in bulk condensed matter systems using nonequilibrium methods. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z23.00010: Superconducting-to-Normal State Switching Experiments using Graphene-based Josephson Junctions Joseph Lambert, Steven Carabello, Roberto Ramos We report results of ongoing superconductor-to-normal state switching experiments using graphene-based Josephson junctions. These devices consist of a single-layer graphene flake contacted by two superconducting parallel leads separated by a few hundred nanometers. Through the proximity effect, the superconducting state is induced in the graphene region below the leads and the Josephson supercurrent is mediated through the normal graphene region by multiple Andreev reflections. The Josephson effect has been firmly demonstrated in these devices, where supercurrents in the hysteretic current-voltage characteristic, Shapiro steps, the Fraunhofer-like diffraction pattern in the critical current versus external magnetic field, and the current-phase relationship have been observed. We report on work in progress, in measuring I-V characteristics, thermal activation and microwave resonant activation in graphene-based junctions, at various temperatures below 1 Kelvin. We modulate the density of charge carriers using a back-gate voltage, which tunes the critical current. This provides another knob for studying these state switching properties. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z23.00011: Noise spectroscopy: a sensitive probe to explore hot electron effect in highly correlated systems Sudeshna Samanta, Arup K. Raychaudhuri Non-linear electrical conductance in ferromagnetic insulating (FMI) state of manganites can give rise to reversible colossal electro-resistance and current induced resistance change due to heating of the electrons in the system. In FMI state ($<$120K), the temperature of the lattices or phonon ($T_{ph})$ and electrons ($T_{e})$ in the sample can decouple by high input power density giving rise to heating of the electronic bath. We investigated whether white noise like Nyquist noise can be used to measure $T_{e}$ (which is expected to be larger than $T_{ph})$ when the two baths get decoupled. The use of the Nyquist noise to measure $T_{e}$ assumes that the electron bath forms a proper temperature bath in equilibrium. A dc stressing current was used to heat the electron bath while a small ac signal was used to measure the noise. With enhanced power input to the electron system, the white noise enhances and there is a large deviation from the simple estimate of Nyquist relation (4$k_{B}T_{e}R_{sample})$ indicating that the electron system is not in thermal equilibrium and is a non-ergodic system where Fluctuation Dissipation Theorem has broken down. [Preview Abstract] |
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