Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H12: Vortices in Superconductors III |
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Sponsoring Units: DCMP Chair: M. Tikham, Harvard University Room: LACC 402A |
Tuesday, March 22, 2005 8:00AM - 8:12AM |
H12.00001: Momentum dependence of the half-flux quantum effect in YBCO-Nb rings John Kirtley, Chang Tsuei, A. Ariando, Hans Hilgenkamp We have studied the half-flux quantum effect\footnote{C.C. Tsuei {\it et al.} Phys. Rev. Lett. {\bf 73} 593(1994)} in YBa$_2$Cu$_3$O$_{7-\delta}$(YBCO)-Nb rings with ramp-type junctions,\footnote{H.J.H. Smilde {\it et al.} Appl. Phys. Lett. {\bf 80}, 4579(2002)} in a test for a time reversal symmetry breaking order parameter.\footnote{T.K. Ng and C. Varma, cond-mat/0403379} The angle of one junction normal relative to the YBCO $a$-axis was held fixed, while the other was varied in 5 degree intervals from ring to ring. The epitaxial YBCO thin film rings, with inside and outside diameters of 30$\mu$m and 130$\mu$m, were cooled in various fields and imaged at 4.2K with a SQUID microscope. The spontaneously generated flux $\Phi$ in the rings, when cooled in zero field, alternated systematically with the second junction angle from nearly zero flux ($N=0$) to nearly $\Phi=\Phi_0/2=h/4e$ ($N=1/2$) in a manner consistent with a predominantly $d_{x^2-y^2}$ Cooper pairing symmetry. The transition between the $N=0$ and $N=1/2$ states occured at angles slightly different from multiples of 45$^o$, consistent with a small $a-b$ plane anisotropy in the gap. Deviations from $\Phi_0/2$ flux were small, indicating little, if any, imaginary component to the order parameter, independent of Cooper pair momentum. [Preview Abstract] |
Tuesday, March 22, 2005 8:12AM - 8:24AM |
H12.00002: Measuring the inertial mass of a vortex in the cuprate superconductors Lorenz Bartosch, Subir Sachdev, Leon Balents Scanning tunneling microscopy measurements by Hoffman {\em et al}.~[1] on the vortex lattice of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ exhibit halos in the vicinity of each vortex with modulations in the tunneling conductance at a period close to 4 lattice spacings. We discuss these observations in the light of a novel theory for superfluids in the vicinity of a Mott transition~[2]. Such superfluids have a `quantum order' which is reflected in a degeneracy in the vortex excitation spectrum, and leads to modulations when a vortex is localized. Our interpretation implies that the size of the halos is determined by the zero-point fluctuations of the quantum vortices. We will show how the existing scanning tunneling microscopy measurements, when combined with measurements of the London penetration length, lead to a quantitative estimate of the vortex inertial mass. \begin{thebibliography}{99} \bibitem{1} J.~E.~Hoffman {\em et al.}, Science {\bf 295}, 466 (2002). \bibitem{2} L.~Balents, L.~Bartosch, A.~Burkov, S.~Sachdev, and K.~Sengupta, cond-mat/0408329, cond-mat/0409470. \end{thebibliography} [Preview Abstract] |
Tuesday, March 22, 2005 8:24AM - 8:36AM |
H12.00003: Criticality in the $N$-flavor London model Asle Sudbo, Jo Smiseth, Eivind Smorgrav, Egor Babaev We consider the critical properties of $N$-flavor London model in $d=2+1$ dimensions in the phase-only approximation, with {\it no inter-flavor Josephson coupling}. The model applies to superconducting phases of projected metallic states of light atoms and as effective theories for easy-plane quantum antiferromagnets. MC simulations with $N=2$ and unequal bare phase stiffnesses of the components show two anomalies in the specific heat. From the critical exponents $\alpha$ and $\nu$, the mass of the gauge field, and vortex correlation functions, we conclude that these anomalies correspond to an {\it inverted} 3D$xy$ and a 3D$xy$ fixed point. The $N=2$ model with equal phase stiffnesses exhibits one {\it non}-3D$xy$ critical point due to self-duality. For $N=3$ and unequal bare phase stiffnesses we find two neutral 3D$xy$ fixed points and one charged fixed point. The model with the two lower phase stiffnesses equal exhibits one neutral fixed point and one charged fixed point. We find a {\it non}- 3D$xy$ fixed point with $N=3$ and equal bare phase stiffnesses. For the general $N$-flavor model with unequal phase stiffnesses there are $N$ fixed points, namely one inverted 3D$xy$ fixed point, and $N-1$ fixed points in the (neutral) 3D$xy$ universality class. Hence, we find superfluid modes arising from charged condensates. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H12.00004: Coherent oscillations in a multilayered Josephson junction Serhii Shafranjuk, John Ketterson The quasiclassical model is implemented to describe coherent oscillations between the quantized ABS bands in a multilayered Josephson structure (MJ) with an SIS$^{\prime }$ unit cell. Using the model, we compute the local electron density of states versus the layer thickness ratio, purity, and the interface barrier transparency. Our analysis shows that narrow ABS bands are formed in a ``clean'' low-transparency MJ, when the electron transport across the layers is ballistic. Elastic scattering of electrons by non-magnetic impurity atoms in the MJ structure weakens the ABS bands, particularly when $\Delta \tau _{i}<<1$. When a bias voltage pulse $V\left( t\right) $ is applied across the MJ structure, it induces an a.c. current pulse, which is treated as a weak perturbation. The a.c. pulse induces coherent oscillations between two adjacent ABS bands of the MJ. The oscillations are pronounced in the Josephson supercurrent, and may be observed using an additional SIS Josephson junction connected in series with the MJ structure. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H12.00005: One-dimensional Superconducting NbSe2 Zhili Xiao*, Yew-San Hor, Ulrich Welp, Yasuo Ito*, John F. Mitchell, Russ E. Cook, Wai-Kwong Kwok, George W. Crabtree Niobium diselenide (NbSe$_{2})$ in bulk crystal form has been used in the past as a model system to explore vortex physics due to its extremely weak vortex pinning and the pronounced peak effect. The layered crystalline structure of NbSe$_{2}$ favors the formation of plate-like samples and resists the growth of one-dimensional NbSe$_{2}$ structures. Here we demonstrate a novel method of synthesizing 1D NbSe$_{2}$ structures including wires and ribbons. Our two-step approach includes the synthesis of 1D NbSe$_{3}$ nanostructures and their conversion into NbSe$_{2}$ through the reduction of selenium while maintaining their shape. The converted 1D NbSe$_{2}$ samples are superconducting with transition temperatures of about 7.0$\sim $7.2K, similar to that of bulk NbSe$_{2}$ crystals. The synthesis, structural characterization and physical properties measured with both transport and magnetization will be reported. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H12.00006: Novel type of vortex rectification in plain Pb/Ge superconducting films Farkhad Aliev, Juan Sierra, Vladimir Pryadun, Dusan Golubovic, Victor Moshchalkov Superconducting Pb/Ge films in a perpendicular magnetic field are found to rectify alternating currents. The rectified voltage appears both along and transverse to the alternating current and strongly depends on the magnetic field, temperature and AC drive. In superconductors with periodic pinning centres rectification voltage varies periodically with number of vortices on pinning centre. Depending on applied AC drive, rectification was observed down to below T$_{c}$/2. Interestingly, not far below T$_{c}$ the rectification voltage is opposite on the opposite film sides. The new rectification phenomenon seems to be due to local excess of critical current at sample edges and appearance of corresponding electric field due to relatively slow ( induced by vortex pinning) process of\textbf{ }relaxation of the distribution of superconducting currents which screen the external magnetic field and which are forced to change by the application of the AC current (F.G.Aliev, et al., Cond. Mat.405656). [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H12.00007: Vortex Pinning in a Superconducting Thin Film Pierced by Ferromagnetic Columnar Defects M. Amin Kayali Using London's approximation, I study spontaneous creation of vortices in a superconducting thin film pierced by ferromagnetic columnar defects. The magnetic fields, screening current and pinning force are calculated for the case when the film is pierced by one ferromagnetic columnar defect then I generalize the results for to study the effect of a square array of such defects on superconductor. [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H12.00008: Depinning transitions of flux lines from extended defects in superconductors Anatoli Polkovnikov, David Nelson, Yariv Kafri With magnetic force microscopy experiments in mind we study depinning transitions of individual flux lines from extended defects (columnar pins and twin planes) in superconductors. It is shown that the critical exponents characterizing the unbinding transition of a single flux line from a twin plane are very sensitive to the presence of other flux lines in the plane. In particular, the transition becomes discontinuous if the density of these lines becomes large enough. We argue that measuring the properties of the unbinding transition serves as an extremely sensitive probe of the Luttinger liquid parameter, which characterizes the long-distance physics of vortex lines in two dimensions. In addition, we discuss the effects of point disorder on the critical properties of the depinning transition from both a columnar pin and a twin plane. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H12.00009: Peak Effect at Disorder-driven Dimensional Crossover in the Mixed Phase of Layered Superconductors Jose P. Rodriguez Collective pinning of the vortex lattice in layered superconductors with material defect centers is studied through a duality analysis of the corresponding $XY$ model with uniform frustration[1]. We find first that long-range vortex-glass order across layers emerges from the decoupled vortex-liquid phase as the temperature is lowered. The resulting low-temperature phase shows weak superconductivity across layers. It crosses over to a defective vortex lattice with strong inter-layer coupling when the relative disorder scale between adjacent vortex lattices in isolated layers matches the Josephson penetration depth[2]. In particular, the Larkin length $L_c^c$ extracted from the phase correlations between different layers at low temperature saturates down to the inter-layer spacing at the dimensional crossover into the weak superconductor. Collective pinning theory then implies that a peak in the critical current flowing along layers exists there. We then notably predict that the (second) peak field is diminished by an increase in point disorder before it saturates. Comparison to the peak effect shown by organic superconductors and by high-temperature superconductors is made where possible.\\[4pt] [1] J.P. Rodriguez, Phys. Rev. B {\bf 69}, 100503(R) (2004).\\[0pt] [2] J.P. Rodriguez, Physica C {\bf 404}, 311 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H12.00010: Dissipation due to vortices in a bilayer thin film superconductor Wei Zhang, H.A. Fertig Vortex dynamics in a bilayer thin film superconductor are studied through a Josephson-coupled two layer XY model. A duality transformation and renormalization group analysis show that there are three phases for this system: free vortex phase, logarithmically confined vortex-antivortex pair phase, and a linearly confined phase. The phases may be distinquished by measuring the resistance to counterflow current in a bilayer superconductor. For a geometry in which current is injected and removed from the two layers at the same edge by an ideal (dissipationless) lead, we argue that the three phases yield distinct behaviors: metallic conductivity in the free vortex phase, a power law I-V in the logarithmically confined phase, and true dissipationless superconductivity in the linearly confined phase. Numerical simulations of a resistively shunted junction model reveal size dependences for the conductance of this system that support these expectations. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H12.00011: Dendritic flux avalanches and nonlocal electrodynamics in thin superconducting films A. Gurevich, I. Aranson, V. Vlasko-Vlasov, V. Vinokur, U. Welp, M. Welling, R. Wijngaarden We present numerical and analytical studies of coupled nonlinear Maxwell and thermal diffusion equations, which describe nonisothermal dendritic flux penetration in superconducting films. We show that spontaneous branching of propagating flux filaments occurs due to nonlocal magnetic flux diffusion and positive feedback between flux motion and Joule heating. The branching is triggered by a thermomagnetic edge instability, which causes periodic stratification of the critical state. The resulting distribution of magnetic microavalanches depends on a spatial distribution of defects. Our results are in good agreement with experiments on Nb films. [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H12.00012: Simulations of the intermediate state in type-I superconductors A.D. Hernandez , D. Dominguez We present simulations of the itermediate state of type-I superconducting films. We solve numerically the time dependent Ginzburg-Landau equations taking into account the demagnetizing fields via the Biot-Savart law. We reproduce several features of the intermediate state observed in experiments, particularly droplet and laberynthine striped patterns are obtained depending on the applied field $H_a$ and magnetic history. For small square samples we find an important influence of the surface barriers which leads to a saw-tooth behavior of the magnetization as a function of $H_a$ and very geometric patterns when slowly increasing $H_a$, and to a positive magnetization and symmetry-breaking structures when slowly decreasing $H_a$. As a general feature we find that there is a strong influence on the initial conditions and magnetic history in the structures of the intermediate state patterns observed. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H12.00013: Vortex sub-lattice melting in a two-component superconductor Eivind Smorgrav, Asle Sudbo, Jo Smiseth, Egor Babaev We consider the vortex matter in a three-dimensional two- component superconductor with individually conserved condensates with different bare phase stiffnesses in a finite magnetic field, such as the projected superconducting state of liquid metallic hydrogen. The ground state is an Abrikosov lattice of composite, i.e. co-centered, vortices in both order parameters. We investigate quantitatively two novel phase transitions when temperature is increased at fixed magnetic field. (i) A ``vortex sub-lattice melting" phase transition where vortices in the field with lowest phase stiffness ("light vortices") loose co-centricity with the vortices with large phase stiffness ("heavy vortices"), thus entering a liquid state. Remarkably, the structure factor of the light vortex sub-lattice vanishes continuously. This novel transition, which has no counterpart in one-component superconductors, is shown to be in the 3d XY universality class. Across this transition, the lattice of heavy vortices remains intact. [Preview Abstract] |
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