Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X2: Vortex Dynamics and Josephson Lasers in Superconductors |
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Sponsoring Units: DCMP Chair: Alexander Gurevich, Florida State University Room: Spirit of Pittsburgh Ballroom BC |
Thursday, March 19, 2009 2:30PM - 3:06PM |
X2.00001: Thermodynamics and Flow of the Vortex Matter at the Second-Order Glass Transition in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Invited Speaker: We study the low temperature phase diagram of the vortex matter in the high-T$_c$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_8$. By employing vortex shaking the vortex system is relaxed towards the equilibrium state. We thus reveal a novel second-order glass transition, manifested by a sharp reversible kink in the measured local magnetization \footnote{HB, N.~Avraham, Y.~Myasoedov, H.~Shtrikman, E.~Zeldov, B.~Rosenstein, E.H.~Brandt, T.~Tamegai, \emph{Phys.~Rev.~Lett.}~\textbf{95}, 257004 (2005)}. The glass line bisects the first-order melting line close to its extremum below which disorder is dominant. Consequently, the phase diagram consists of four thermodynamic phases: At high fields, above the melting line, we find amorphous vortex glass and liquid phases; Surprisingly, at low fields the glass transition separates between a low-temperature Bragg glass and a thermally depinned variant of it - possibly a perfect lattice. Studying the oxygen doping dependence of the vortex phase diagram we unexpectedly find that the novel low- temperature glass transition, along which quenched disorder should play a dominant role, has the same anisotropy dependence as that of the high-temperature melting line, where disorder is negligible \footnote{HB, T.~Verdene, Y.~Myasoedov, H.~Shtrikman, E.~Zeldov, B.~Rosenstein, D.~Li, T.~Tamegai, \emph{Phys. Rev. Lett.}, \textbf{98}, 167004 (2007)}. Finally, we utilize an indirect measurement technique to reconstruct the low- temperature I-V characteristics in the region which is inaccessible by transport measurements \footnote{HB, Y.~Myasoedov, H.~Shtrikman, E.~Zeldov, E.H.~Brandt, G.P.~Mikitik, T.~Tamegai, T.~Sasagawa, \emph{unpublished}}. At high temperatures the bulk resistance is of a thermally activated flux flow with linear I-V both in the liquid phase above the melting line as well as below it within the ordered phase. At lower temperatures, on approaching the glass transition, the temperature dependence of the bulk resistance becomes much sharper. This deviation from a simple Arrhenius behavior tracks the glass line, and may signify criticality. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X2.00002: Nanomechanics of Individual, Isolated Vortices in a Cuprate Superconductor Invited Speaker: |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X2.00003: Structure and stability of dynamic coherent states in intrinsic Josephson-junction stacks Invited Speaker: Intrinsic Josephson-junction stacks are realized in mesas fabricated out of high-temperature superconductors. Phase oscillations in different junctions can be synchronized via coupling to the intrinsic cavity mode leading to powerful electromagnetic radiation in terahertz frequency range [1,2]. As homogeneous oscillations do not couple directly to the cavity modes, the mechanism of mode excitations is a nontrivial issue. New inhomogeneous dynamic state providing such coupling has been demonstrated recently [3]. In this state, the stack spontaneously splits into two subsystems with different phase-oscillation patterns. The phase shift between the oscillations in the two subsystems is static and varies from 0 to 2$\pi$ in a narrow region near the stack center (phase kink). The oscillating electric and magnetic fields are almost homogeneous in all the junctions. The formation of this state promotes efficient pumping of the energy into the cavity resonance. We will also discuss (i) stability of coherent states (ii) synchronization in inhomogeneous mesas, and (iii) mechanisms of damping of the resonance mode.\newline [1]L. Ozyuzer \textit{et al.} , Science \textbf{318}, 1291 (2007). \newline [2]A. E. Koshelev and L. N. Bulaevskii, Phys. Rev. B \textbf{77}, 014530 (2008). \newline [3]Sh. Lin and X. Hu Phys.Rev.\ Lett., \textbf{100}, 247006 (2008); A. E. Koshelev, Phys. Rev., B \textbf{78}, 174509 (2008).\newline *\underline{\emph{In collaboration with}} L. Bulaevskii (LANL), U. Welp, C. Kurter, K. Gray (MSD, ANL), L. Ozyuzer (Izmir Institute of Technology, Turkey), K. Kadowaki (Tsukuba University, Japan) [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X2.00004: Josephson LASER Working at THz Frequencies in Intrinsic Josephson Junctions Invited Speaker: Strong, continuous and monochromatic THz electromagnetic waves with power of order of $\mu$W have successfully been generated with the mesa fabricated on the single crystal of high-$T_c$ superconductor $\mathrm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ by either ion milling or FIB (Focused Ion Beam) method\footnote {L. Ozyuzer, \textit{et al.}, Science \textbf{318} (2007) 1291, K. Kadowaki, \textit{et al.}, Physica \textbf{C468} 634.}. The frequency, $f$, of the radiation depends strongly on the shape and the size of the mesa. In the case of rectangular shape it follows the relation, $f$=$c/2nw$, where $c$ is the velocity of lihgt in vaccum, $n$ the refractivity of the superconductor and $w$ the width of the mesa(shorter edge dimension), while it only depends on the radius $a$ in the case of cylindrical mesa. Higher harmonics are also observed. Another stringent requirement for the radiation is the $ac$-Josephson effect, which must be fulfilled in each intrinsic junction with the same frequency determined by the equation: $fh=2eV/N=2ev_n$, where $V$ is the voltage across the whole junction, $N$ the number of junctions involved in the mesa, $v_n$ the voltage appearing betweeen each junctions, $h$ the Planck constant, $e$ the elementary charge. Since the radiation is monochromatic, $v_n$ must be identical and synchronized coherently in all junctions in the mesa. A simple phenomenological interpretation of this synchronization is that it may occur due to the cavity resonance effect inside the mesa. The peculiar temperature dependence and the anisotropic directivity of radiation power observed experimentally may give a hint to understand the mechanism of such synchronized THz radiation from intrinsic Josephson junctions. We think that nonlinearity to be inherent in the Josephson junction as well as thermal nonequilibrium effect plays a crucial role for the synchronized THz oscillation. A more detailed view for the mechanism based on the experimental results will be presented. [Preview Abstract] |
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