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supr-con/9507007

Kikuo Harigaya

Kikuo Harigaya

Nonlinear optical response in doped conjugated polymers

Accepted for publication in J. Phys.: Condens. Matter

J. Phys.: Condens. Matter 7 (1995) 7529-7534

10.1088/0953-8984/7/38/011

supr-con chem-ph cond-mat.supr-con mtrl-th

Exciton effects on conjugated polymers are investigated in soliton lattice states. We use the Su-Schrieffer-Heeger model with long-range Coulomb interactions. The Hartree-Fock (HF) approximation and the single-excitation configuration- interaction (single-CI) method are used to obtain optical absorption spectra. The third-harmonic generation (THG) at off-resonant frequencies is calculated as functions of the soliton concentration and the chain length of the polymer. The magnitude of the THG at the 10 percent doping increases by the factor about 10^2 from that of the neutral system. This is owing to the accumulation of the oscillator strengths at the lowest exciton with increasing the soliton concentration. The increase by the order two is common for several choices of Coulomb interaction strengths.

supr-con/9508001

Valerii Vinokur

I. Aranson, B.Ya. Shapiro and V. Vinokur (ANL)

Nucleation and Growth of Normal Phase in Thin Superconducting Strips

4 pages, Revtex, 2 figures avilable from author via valerii_vinokur@qmgate.anl.gov

10.1103/PhysRevLett.76.142

supr-con cond-mat.supr-con

We investigate the kinetics of normal phase nucleation and flux line condensation in the type-II superconductors by numerical study of the time-dependent Ginzburg-Landau equation. We have shown that under the sufficient transport current the normal phase nucleates in the superconducting strips in a form of the macroscopic droplets having the multiple topological charge. We discuss the stability and the dynamics of the droplets. We found that pinning suppresses the droplet formation.

supr-con/9508002

Masanori Ichioka

M. Ichioka(Kyoto Univ.), N. Enomoto, N. Hayashi, and K. Machida(Okayama Univ.)

$s$- and $d_{xy}$-wave components induced around a vortex in $d_{x^2-y^2}$-wave superconductors

RevTex,9 pages, 6 figures in a uuencoded file

Phys. Rev. B53, 2233 (1996).

10.1103/PhysRevB.53.2233

supr-con cond-mat.supr-con

Vortex structure of $d_{x^2-y^2}$-wave superconductors is microscopically analyzed in the framework of the quasi-classical Eilenberger equations. If the pairing interaction contains an $s$-wave ($d_{xy}$-wave) component in addition to a $d_{x^2-y^2}$-wave component, the $s$-wave ($d_{xy}$-wave) component of the order parameter is necessarily induced around a vortex in $d_{x^2-y^2}$-wave superconductors. The spatial distribution of the induced $s$-wave and $d_{xy}$-wave components is calculated. The $s$-wave component has opposite winding number around vortex near the $d_{x^2-y^2}$-vortex core and its amplitude has the shape of a four-lobe clover. The amplitude of $d_{xy}$-component has the shape of an octofoil. These are consistent with results based on the GL theory.

supr-con/9508003

Asai Yoshihiro

Yoshihiro Asai (ETL, Tsukuba, Japan)

Superconducting, magnetic, and charge correlations in the doped two-chain Hubbard model

RevTeX 3.0, 11 pages, 5 figs via FAX, Phys.Rev.B to appear

10.1103/PhysRevB.52.10390

YA#HUB003Y95M01

supr-con cond-mat.supr-con

Superconducting, magnetic, and charge correlation functions and dynamic spin correlation functions of the doped two-chain Hubbard model is studied with the projector Quantum Monte carlo method and Lanczos recursion method. Of the three correlation functions, the interchain singlet superconducting correlation function is the most long range. Our data is not consistent with the Luther-Emery picture.

supr-con/9508004

Steven Spencer

Steven Spencer and Henrik Jeldoft Jensen

The Bean Critical State: Infinitely Unstable

8 pages including 4 figures. Whole paper in uuencoded postscript file bean.uu

10.1016/0921-4534(96)00240-7

supr-con cond-mat.supr-con

The threshold for creep in the Bean critical state is investigated. We perturb the Bean state by an energy $\Delta\epsilon$. We find that no matter how small $\Delta\epsilon$ is it will always be able to induce creep somewhere on the Bean profile. This finding has important consequences for the interpretation of low temperature creep phenomena in terms of quantum creep.

supr-con/9508005

Zbigniew J. Koziol

Z. Kozio{\l} and R.A. Dunlap

Nonlinear Flux Diffusion and ac Susceptibility of Superconductors - Exact Numerical Results

9 LaTeX pages, 4 Postscript figures, WWW version available at http://is.dal.ca/~zkoziol/super.html

10.1063/1.361702

supr-con cond-mat.supr-con

The ac response of a slab of material with electrodynamic characteristics $E\sim j^{\kappa+1}$, $\kappa\geq0$, is studied numerically. From the solutions of the nonlinear diffusion equation, the fundamental and higher-order components of the harmonic susceptibility are obtained. A large portion of the data for every $\kappa$ can be scaled by a single parameter, $\xi$ =$t^{1/(\kappa+2)}\cdot H_0^{\kappa/(\kappa+2)}/D$, where $t$ is the period of the ac field at the surface, $H_0$ is its amplitude and $D$ is the slab thickness. This is, however, only an approximate scaling property: The field penetration into a nonlinear medium is a more complex phenomenon than in the linear case. In particular, the susceptibility values are not uniquely defined by a set of only two parameters, such as $\kappa$ and $\xi$, while one parameter, i.e. $t^{1/2}$/D, is sufficient to describe the electrodynamic response of a linear medium.

supr-con/9508006

Zbigniew J. Koziol

Z. Kozio{\l} and R.A. Dunlap

Magnetostriction of a Superconductor: -Results from the Critical-State Model

9 LaTeX pages, 4 Postscript figures, WWW version available at http://is.dal.ca/~zkoziol/super.html

10.1063/1.361697

supr-con cond-mat.supr-con

In many cases, the critical-state theory can be treated as a suffi ciently accurate approximation for the modelling of the magnetic properties of superconductors. In the present work, the magnetostrictive hysteresis is computed for a quite general case of the modified Kim-Anderson model. The results obtained reproduce many features of the giant magnetostriction (butterfly-shaped curves) reported in the literature for measurements made on single-crystal samples of the high-temperature superconductor $Bi_2Sr_2CaCu_2O_8$. It is shown that addition of a contribution to the magnetostriction in the superconducting state which is of similar origin as in the normal state, offers a broader phenomenological interpretation of the complex magnetostriction hysteresis found in such heavy-fermion compounds as $UPt_3$, $URu_2Si_2$ or $UBe_{13}$.

supr-con/9509001

Kenneth Holmlund and Petter Minnhagen

Langevin Simulations of Two Dimensional Vortex Fluctuations: Anomalous Dynamics and a New $IV$-exponent

Submitted to PRB, RevTeX format, 28 pages and 13 figures, figures in postscript format are available at http://www.tp.umu.se/~holmlund/papers.html

10.1103/PhysRevB.54.523

supr-con cond-mat.supr-con

The dynamics of two dimensional (2D) vortex fluctuations are investigated through simulations of the 2D Coulomb gas model in which vortices are represented by soft disks with logarithmic interactions. The simulations trongly support a recent suggestion that 2D vortex fluctuations obey an intrinsic anomalous dynamics manifested in a long range 1/t-tail in the vortex correlations. A new non-linear IV-exponent a, which is different from the commonly used AHNS exponent, a_AHNS and is given by a = 2a_AHNS - 3, is confirmed by the simulations. The results are discussed in the context of earlier simulations, experiments and a phenomenological description.

supr-con/9510001

Vladimir Dzhunushaliev

V.D.Dzhunushaliev

The string model of the Cooper pair in the anisotropic superconductor

final version, Latex, 9p, to be published in Phys. Rev. B

Phys.Rev.B, 54, 10121(1996)

10.1103/PhysRevB.54.10121

supr-con cond-mat.supr-con hep-th

The analogy between the Cooper pair in high temperature superconductor and the quark-antiquark pair in quantum chromodynamics (QCD) is proposed. In QCD the nonlinear chromodynamical field between a quark and an antiquark is confined to a tube. So we assume that there is the strong interaction between phonons which can confine them to some tube too. This tube is described using the nonlinear Schr\"odinger equation. We show that it has an infinite spectrum of axially symmetric (string) solutions with negative finite linear energy density. The one-dimensional nonlinear Schr\"odinger equation has a finite spectrum (hence, it has a steady-state) which describes the Cooper pair squezeed between anisotropy planes in the superconductor. It is shown that in this model the transition temperature is approximately 45 K.

supr-con/9510002

P. Calvani, M. Capizzi, S. Lupi, P. Maselli, A. Paolone, (Dipartimento di Fisica, Universita di Roma ``La Sapienza'', Roma, Italy) and P. Roy (Laboratoire pour l'Utilization de Rayonnement Electromagnetique, Universite Paris-Sud, Orsay, France)

Polaronic optical absorption in electron-doped and hole-doped cuprates

File latex, 31 p., submitted to Physical Review B. Figures may be faxed upon request

10.1103/PhysRevB.53.2756

supr-con cond-mat.supr-con

Polaronic features similar to those previously observed in the photoinduced spectra of cuprates have been detected in the reflectivity spectra of chemically doped parent compounds of high-critical-temperature superconductors, both $n$-type and $p$-type. In Nd$_2$CuO$_{4-y}$ these features, whose intensities depend both on doping and temperature, include local vibrational modes in the far infrared and a broad band centered at $\sim$ 1000 cm$^{-1}$. The latter band is produced by the overtones of two (or three) local modes and is well described in terms of a small-polaron model, with a binding energy of about 500 cm$^{-1}$. Most of the above infrared features are shown to survive in the metallic phase of Nd$_{2-x}$Ce$_x$Cu0$_{4-y}$, Bi$_2$Sr$_2$CuO$_6$, and YBa$_2$Cu$_3$O$_{7-y}$, where they appear as extra-Drude peaks. The occurrence of polarons is attributed to local modes strongly coupled to carriers, as shown by a comparison with tunneling results.

supr-con/9511001

Dmitry A. Ryndyk (Institute for Physics of Microstructures of Russian Academy of Sciencies, Nizhny Novgorod, Russia)

Synchronization in one-dimensional array of Josephson coupled thin layers

REVTeX, 8 pages, no figures

supr-con cond-mat.supr-con

We obtain self-consistent macroscopic equations describing interlayer Josephson effect and intralayer disequilibrium in one-dimensional array of Josephson coupled layers. We show that ``nonequilibrium coupling'' can lead to effective spatial and time synchronization and formation of coherent dynamic resistive state (collective Josephson effect) in Nb-AlO-Nb stacked junctions and HTSC (intrinsic Josephson effect). We propose it to be the origin of collective switching phenomena observed in PbBiSrCaCuO.

supr-con/9511002

Juan I. Collar

V.Jeudy, J.I.Collar, T.A.Girard, D.Limagne and G.Waysand (G.P.S., Univ. Paris 7)

S-35 Beta Irradiation of a Tin Strip in a State of Superconducting Geometrical Metastability

Compressed PostScript (filename.ps.Z), 9 pages, 2 figures

Nucl.Instrum.Meth. A373 (1996) 65-67

10.1016/0168-9002(95)01451-9

JICUSC-95-8

supr-con cond-mat.supr-con hep-ex nucl-ex

We report the first energy loss spectrum obtained with a geometrically metastable type I superconducting tin strip irradiated by the beta-emission of S-35. (Nucl. Instr. Meth. A, in press)

supr-con/9512001

Masanori Ichioka

M. Ichioka (Kyoto), N. Hayashi, N. Enomoto and K. Machida (Okayama)

Vortex structure in $d$-wave superconductors

22 pages, RevTex, 23 figures available upon request

Phys. Rev. B 53, 15316 (1996).

10.1103/PhysRevB.53.15316

supr-con cond-mat.supr-con

Vortex structure of pure $d_{x^2-y^2}$-wave superconductors is microscopically analyzed in the framework of the quasi-classical Eilenberger equations. Selfconsistent solution for the $d$-wave pair potential is obtained for the first time in the case of an isolated vortex. The vortex core structure, i.e., the pair potential, the supercurrent and the magnetic field, is found to be fourfold symmetric even in the case that the mixing of $s$-wave component is absent. The detailed temperature dependences of these quantities are calculated. The fourfold symmetry becomes clear when temperature is decreased. The local density of states is calculated for the selfconsistently obtained pair potential. From the results, we discuss the flow trajectory of the quasiparticles around a vortex, which is characteristic in the $d_{x^2-y^2}$-wave superconductors. The experimental relevance of our results to high temperature superconductors is also given.

supr-con/9512002

Juan I. Collar

V.Jeudy, D.Limagne, G.Waysand, J.I.Collar and T.A.Girard (G.P.S., Univ. Paris 7)

Beta Irradiation of a Geometrically Metastable Superconducting Strip Detector with a Magnetic Flux Penetration Read-Out

Compressed PostScript (filename.ps.Z), 8 pages, 2 figures

10.1016/0168-9002(95)01063-7

JICUSC-95-9

supr-con cond-mat.supr-con

Geometrical metastability, observed in superconducting type I tin flat strips, has been previously proposed as a principle for particle detection. The energy deposition of an incoming beta-particle induces the rupture of the metastability and consequently the penetration of multiquantum flux tubes into a superconducting tin strip. We present here the first absorption spectra from two beta sources, which demonstrate the linearity and energy-resolution of these detectors (presented at the 6th International Workshop on Low Temperature Detectors for Dark Matter and Neutrinos (LTD-6), Interlaken, Switzerland, Sept. 1995)

supr-con/9512003

Eduardo Jagla

E.A.Jagla and C. A. Balseiro

Vortex structure and resistive transitions in high-Tc superconductors

8 pages + 6 figures

10.1103/PhysRevB.53.15305

supr-con cond-mat.supr-con

The nature of the resistive transition for a current applied parallel to the magnetic field in high-Tc materials is investigated by numerical simulation on the three dimensional Josephson junction array model. It is shown by using finite size scaling that for samples with disorder the critical temperature Tp for the c axis resistivity corresponds to a percolation phase transition of vortex lines perpendicularly to the applied field. The value of Tp is higher than the critical temperature for j perpendicular to H, but decreases with the thickness of the sample and with anisotropy. We predict that critical behavior around Tp should reflect in experimentally accessible quantities, as the I-V curves.

supr-con/9601001

Modanese Giovanni

G. Modanese (INFN, Trento)

Updating the theoretical analysis of the weak gravitational shielding experiment

LaTex, 6 pages. Only formal changes in title and abstract

Proceedings of the 1997 IAF (International Astronautical Federation) Congress, nr. IAA-97-4.107

UTF-367/96

supr-con cond-mat.supr-con gr-qc hep-ph

The most recent data about the weak gravitational shielding produced recently through a levitating and rotating HTC superconducting disk show a very weak dependence of the shielding value ($\sim 1 \%$) on the height above the disk. We show that whilst this behaviour is incompatible with an intuitive vectorial picture of the shielding, it is consistently explained by our theoretical model. The expulsive force observed at the border of the shielded zone is due to energy conservation.

supr-con/9602001

Franco Nori

C. Reichhardt, C. J. Olson, J. Groth, Stuart Field, and Franco Nori

Vortex Plastic Flow, $B(x,y,H(t)), M(H(t)), J_c(B(t))$, Deep in the Bose Glass and Mott-Insulator Regimes

4 pages, 4 PostScript Figures

Phys. Rev. B 53 (1996) R8898-R8901

10.1103/PhysRevB.53.R8898

supr-con cond-mat.mtrl-sci cond-mat.supr-con

We present simulations of flux-gradient-driven superconducting vortices interacting with strong columnar pinning defects as an external field $H(t)$ is quasi-statically swept from zero through a matching field $B_{\phi}$. We analyze several measurable quantities, including the local flux density $ B(x,y,H(t))$, magnetization $M(H(t))$, critical current $J_{c}(B(t))$, and the individual vortex flow paths. We find a significant change in the behavior of these quantities as the local flux density crosses $B_{\phi}$, and quantify it for many microscopic pinning parameters. Further, we find that for a given pin density $J_c(B)$ can be enhanced by maximizing the distance between the pins for $ B < B_{\phi} $.

supr-con/9602002

Santi Gilles

G. Santi, T. Jarlborg, M. Peter and M. Weger

s and d-wave symmetries of the solutions of the Eliashberg equations

Uuencoded LaTeX file anf 6 Postscript figures (14 pages). Accepted for publication in Physica C

10.1016/0921-4534(96)00052-4

supr-con cond-mat.supr-con

We examine the different possible symmetries of the superconducting gap obtained by solving the Eliashberg equations. We consider an electron-phonon interaction in a strong coupling scenario. The Coulomb pseudopotential plays the crucial role of providing the repulsion needed to favour the d-wave symmetry. But the key parameter that allows very anisotropic solutions even with very strong coupling is the small angular range of the interaction due to predominantly electron-phonon forward scattering that is found in the high-Tc superconductors. We find both s and d-wave solutions whose stability depends mainly on the angular range of the interaction.

supr-con/9602003

Durga Prasad Choudhury

S. Oxx, D. P. Choudhury, Balam A. Willemsen, H. Srikanth, S. Sridhar (Northeastern Univ), B. K. Cho, P. C. Canfield (Ames Lab)

Vortex Response and Critical Fields observed via rf penetration depth measurements on the superconductor YNi_2B_2C

6 LaTeX pages with 6 EPS figures, uses RevTeX, multicol and epsf. One figure bitmapped. Better PS file available at http://sagar.physics.neu.edu/preprints.html Submitted to Physica C

10.1016/0921-4534(96)00230-4

supr-con cond-mat.supr-con

Measurements of the rf penetration depth \lambda(T,H,\theta ) are used to study the superconducting order parameter, vortex dynamics in the mixed state and delineate critical fields in the borocarbide superconductor YNi_2B_2C. The lower critical field has an anomalous T dependence, H_{c1}(T)=1.12[1-(T/T_c)] kOe, which is however consistent with independent superfluid density measurements at microwave frequencies. The vortex response is dominated by viscous flux flow, indicative of extremely weak pinning, and is parametrized by a field scale H_{c2,eff}. The angular dependence of the vortex contribution \lambda(\theta) is in good agreement with the Coffey-Clem model. Structure is seen in the depairing transition in the vicinity of the upper critical field, with the existence of well-defined critical fields H_{c2a}, H_{c2b} and H_{c2c}, with the vortex field scale H_{c2,eff} closest to H_{c2b}. Overall the measurements indicate that YNi_2B_2C has a rich and unusual field dependence of its transport parameters.

supr-con/9604001

Alexey V. Samokhvalov (Institute for physics of microstructures, Nizhny Novgorod, Russia)

Vortex loops entry into type--II superconductors

LaTeX file ( 22 pages & 7 figures ); the required TeX style file "bezier.sty" is attached; accepted for publication in Physica C

Physica C 259 (1996) 337

10.1016/0921-4534(96)00003-2

supr-con cond-mat.supr-con

The magnetic field distribution, the magnetic flux, and the free energy of an Abrikosov vortex loop near a flat surface of type--II superconductors are calculated in the London approximation. The shape of such a vortex line is a semicircle of arbitrary radius. The interaction of the vortex half--ring and an external homogeneous magnetic field applied along the surface is studied. The magnitude of the energy barrier against the vortex expansion into superconductor is found. The possibilities of formation of an equilibrium vortex line determined by the structure of the applied magnetic field by creating the expanding vortex loops near the surface of type--II superconductor are discussed.

supr-con/9604002

Kamran Behnia

K. Behnia(1), H. Aubin(1), L. Taillefer(2), R. Gagnon(2) ((1) Universit\'e Paris-Sud (2) McGill University)

Low temperature thermal conductivity of Zn-doped YBCO: evidence for impurity-induced electronic bound states

8 pages, 3 Encapsulated Postscript figures

KB-96-1

supr-con cond-mat.supr-con

The thermal conductivity of Zn-doped YBCO crystals was studied at low temperature (0.15 < T < 0.8 K) for different concentrations of Zn impurities. A small amount of Zn induces a dramatic decrease in the non-linear component of the low-temperature thermal conductivity. Moreover, the magnitude of the linear component (obtained by extrapolating the data to T=0) is found to depend on Zn concentration. After an initial decrease, this linear term, associated with the electronic contribution to the conductivity, increases with increasing Zn dopage. Such an increase is consistent with the introduction of low-energy excitations by Zn impurities as expected for a $d_{x^2-y^2}$ superconducting state in contrast to an anisotropic s-wave gap. The results are compared to quantitative predictions of available theoretical models.

supr-con/9605001

Prozorov Ruslan

R. Prozorov, A. Poddar, E. Sheriff, A. Shaulov, Y. Yeshurun (Institute for Superconductivity, Physics Department, Bar-Ilan University, Ramat-Gan, Israel)

Irreversible magnetization in thin YBCO films rotated in external magnetic field

14 pages, 7 PostScript figures

10.1016/0921-4534(96)00266-3

supr-con cond-mat.supr-con

The magnetization M of a thin YBaCuO film is measured as a function of the angle $\theta $ between the applied field H and the c-axis. For fields above the first critical field, but below the Bean's field for first penetration H*, M is symmetric with respect to $\theta =\pi $ and the magnetization curves for forward and backward rotation coincide. For H>H* the curves are asymmetric and they do not coincide. These phenomena have a simple explanation in the framework of the Bean critical state model.

supr-con/9605002

R. D. Parmentier

C. Soriano, G. Costabile, R. D. Parmentier (INFM & Dept. Physics, Univ. Salerno)

Coupling of Josephson flux-flow oscillators to an external RC load

uuencoded, gzipped tar archive containing 11 pages of REVTeX text + 4 PostScript figures. To appear in Supercond. Sci. Technol

10.1088/0953-2048/9/7/011

supr-con cond-mat.supr-con

We investigate by numerical simulations the behavior of the power dissipated in a resistive load capacitively coupled to a Josephson flux flow oscillator and compare the results to those obtained for a d.c. coupled purely resistive load. Assuming realistic values for the parameters R and C, both in the high- and in the low-Tc case the power is large enough to allow the operation of such a device in applications.

supr-con/9606001

Hagen Kleinert

Hagen Kleinert and Adriaan Schakel

New Universality Class in the Superconductive Phase Transition

Invited talk presented by A. Schakel at the 10'th International Conference on Problems of Quantum Field Theory, Alushta, Ukraine, May 13-17, 1996. Author Information under http://www.physik.fu-berlin.de/~kleinert/institution.html; HTML version and PostScript version of paper available at http://www.physik.fu-berlin.de/~kleinert/kleiner_re247/preprint.html

supr-con cond-mat.supr-con

The superconductive phase transition in the Ginzburg-Landau theory (or Coulomb-Higgs phase transition of scalar QED in 3D) is discussed in a dual formulation which focuses on the magnetic rather than the electric excitations of the system. Renormalization group analysis of the dual formulation reveals the transition to be of second order and of a new universality class. Whereas coherence length and specific heat have XY-model exponents, the magnetic penetration depth shows mean-field behavior. Experimental evidence for these predictions is discussed.

supr-con/9606002

Nobuhiko Hayashi

N. Hayashi, M. Ichioka, K. Machida (Okayama Univ.)

Star-shaped Local Density of States around Vortices in a Type II Superconductor

4 pages, REVTeX, 3 figures available upon request

Phys. Rev. Lett. 77, 4074 (1996).

10.1103/PhysRevLett.77.4074

supr-con cond-mat.supr-con

The electronic structure of vortices in a type II superconductor is analyzed within the quasi-classical Eilenberger framework. The possible origin of a sixfold ``star'' shape of the local density of states, observed by scanning tunneling microscope experiments on NbSe$_2$, is examined in the light of the three effects; the anisotropic pairing, the vortex lattice, and the anisotropic density of states at the Fermi surface. Outstanding features of split parallel rays of this star are well explained in terms of an anisotropic $s$-wave pairing. This reveals a rich internal electronic structure associated with a vortex core.

supr-con/9606003

Prozorov Ruslan

Y. Abulafia, A. Shaulov, Y. Wolfus, R. Prozorov, L. Burlachkov, and Y. Yeshurun, D. Majer E. Zeldov, H. W\"uhl, V.B. Geshkenbein, V.M. Vinokur

Plastic vortex-creep in YBa$_2$Cu$_3$O$_{7-x}$ crystals

10 pages, RevTeX type, 4 PS figures

10.1103/PhysRevLett.77.1596

supr-con cond-mat.supr-con

Local magnetic relaxation measurements in YBa$_2$Cu$_3$O$_{7-x}$ crystals show evidence for plastic vortex-creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the ''fishtail'' magnetization. In this range the activation energy $U_{pl}$, which decreases with field, drops below the elastic (collective) creep activation energy, $U_{el}$, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa$_2$Cu$_3$O$_{7-x}$.

supr-con/9607001

Michael Simkin

M.V.Simkin (Brown University)

On Mean-Field Theory of Quantum Phase Transition in Granular Superconductors

Physica C 267 (1996) 161

10.1016/0921-4534(96)00376-0

supr-con cond-mat.supr-con

In previous work on quantum phase transition in granular superconductors, where mean-field theory was used, an assumption was made that the order parameter as a function of the mean field is a convex up function. Though this is not always the case in phase transitions, this assumption must be verified, what is done in this article.

supr-con/9607002

J.J. Betouras, R. Joynt, Z.-W. Dong, T. Venkatesan, P. Hadley

Ginzburg-Landau Theory of Josephson Field Effect Transistors

REVTEX, 4 figures upon request, submitted to Appl. Phys. Lett

10.1063/1.117661

supr-con cond-mat.supr-con

A theoretical model of high-T_c Josephson Field Effect Transistors (JoFETs) based on a Ginzburg-Landau free energy expression whose parameters are field- and spatially- dependent is developed. This model is used to explain experimental data on JoFETs made by the hole-overdoped Ca-SBCO bicrystal junctions (three terminal devices). The measurements showed a large modulation of the critical current as a function of the applied voltage due to charge modulation in the bicrystal junction. The experimental data agree with the solutions of the theoretical model. This provides an explanation of the large field effect, based on the strong suppresion of the carrier density near the grain boundary junction in the absence of applied field and the subsequent modulation of the density by the field.

supr-con/9607003

J. I. Martin, M. Velez, F. Guinea and J. L. Vicent

Flux flow resistivity and vortex viscosity of high-Tc films

3 pages. 4 figures upon request

10.1103/PhysRevB.55.5659

supr-con cond-mat.supr-con

The flux flow regime of high-T$_{\rm c}$ samples of different normal state resistivities is studied in the temperature range where the sign of the Hall effect is reversed. The scaling of the vortex viscosity with normal state resistivity is consistent with the Bardeen-Stephen theory. Estimates of the influence of possible mechanisms suggested for the sign reversal of the Hall effect are also given.

supr-con/9607004

Durga P. Choudhury

S. Sridhar, S. Oxx, B.A. Willemsen, H. Srikanth, and D.P. Choudhury (Physics Department, Northeastern University)

Comments on "Vortex Glass and Lattice Melting Transitions in a YNi_2B_2C Single Crystal"

1 page, LaTeX type, 1 PostScript figure, Uses PRABIB.STY file, 600 dpi PS file available at http://sagar.physics.neu.edu/preprints.html To appear in Physical Review Letters

10.1103/PhysRevLett.77.2145

supr-con cond-mat.supr-con

Recently, Mun et.al. (Phys. Rev. Lett., 76, 2790 (1996)) have published their results on single crystal YNi_2B_2C, claiming that their experimental observations can be explained in terms of formation of Vortex Glass and Lattice melting. Our experiments, carried out on samples obtained from the SAME source, reveal a much richer phase diagram and span wider regions of experimental parameter space than Mun et. al. that encompasses most of their observations. We speculate that this material has anomalous intrinsic properties and the results cannot be explained by simple models about the flux lattice.

supr-con/9607005

Franco Nori

C. Reichhardt, J. Groth, C. J. Olson, Stuart Field, Franco Nori

Spatio-temporal dynamics and plastic flow of vortices in superconductors with periodic arrays of pinning sites

Phys. Rev. B 54 (1996) 16108-16115

10.1103/PhysRevB.54.16108

supr-con cond-mat.mtrl-sci cond-mat.supr-con nlin.PS patt-sol

We present simulations of flux-gradient-driven superconducting rigid vortices interacting with square and triangular arrays of columnar pinning sites in an increasing external magnetic field. These simulations allow us to quantitatively relate spatio-temporal microscopic information of the vortex lattice with typically measured macroscopic quantities, such as the magnetization $M(H)$. The flux lattice does not become completely commensurate with the pinning sites throughout the sample at the magnetization matching peaks, but forms a commensurate lattice in a region close to the edge of the sample. Matching fields related to unstable vortex configurations do not produce peaks in $M(H)$. We observe a variety of evolving complex flux profiles, including flat terraces or plateaus separated by winding current-carrying strings and, near the peaks in $M(H)$, plateaus only in certain regions, which move through the sample as the field increases.

supr-con/9607006

Andreas Schoenenberger

A. Sch\"onenberger, A. Larkin, E. Heeb, V. Geshkenbein, G. Blatter

Strong Pinning and Plastic Deformations of the Vortex Lattice

12 pages, TeX type, Postscript figures

10.1103/PhysRevLett.77.4636

supr-con cond-mat.supr-con

We investigate numerically the dynamically generated plastic deformations of a 3D vortex lattice (VL) driven through a disorder potential with isolated, strong pinning centers (point-like or extended along the field direction). We find that the VL exhibits a very peculiar dynamical behavior in the plastic flow regime, in particular, topological excitations consisting of three or four entangled vortices are formed. We determine the critical current density $j_c$ and the activation energy for depinning $U_c$ in the presence of a finite density of strong pinning centers.

supr-con/9607007

Frithjof Anders

F. B. Anders and K. Gloos

Towards a Microscopic Theory for Metallic Heavy-Fermion Point Contacts

3 pages, TeX type, 1 eps figure include, SCES96 Z\"urich

Physica B, 230-232, 437 (1997)

10.1016/S0921-4526(96)00604-7

supr-con cond-mat.supr-con

The bias-dependent resistance R(V) of NS-junctions is calculated using the Keldysh formalism in all orders of the transfer matrix element. We present a compact and simple formula for the Andreev current, that results from the coupling of electrons and holes on the normal side via the anomalous Green's function on the superconducting side. Using simple BCS Nambu-Green's functions the well known Blonder-Tinkam-Klapwijk theory can be recovered. Incorporating the energy-dependent quasi-particle lifetime of the heavy fermions strongly reduces the Andreev-reflection signal.

supr-con/9608001

Jorge Berger and Jacob Rubinstein (Technion)

Topology and Phase Transitions in the Little-Parks Experiment

Latex, 26 pages, 5 figs upon request

SIAM J Appl Math {\bf 58} (1998) 103--121

supr-con cond-mat.supr-con quant-ph

This is an analytic study of the problem of transitions between normal and superconducting phases for a sample which encloses a magnetic flux. A preliminary study of this problem, based on numerical minimization of the free energy for a particular form of the thickness of the sample, was published in Phys. Rev. Lett. {\bf 75}, 320 (1995). For a sample of uniform thickness the order parameter is uniform, but even infinitesimal deviations from uniform thickness give rise to a singly connected state in which the order parameter vanishes at a suitable layer, so that the superconducting part does not enclose the magnetic field. The stability domain of this singly connected state is a line segment in the magnetic field-temperature plane, delimited by two critical points. The phase diagram contains several bifurcation lines, which are systematically analyzed.

supr-con/9608002

Sean M. Quinlan

P. J. Hirschfeld, S. M. Quinlan, and D. J. Scalapino

c-Axis Infrared Conductivity of a d_{x^2-y^2}-Wave Superconductor with Impurity and Spin-Fluctuation Scattering

18 pages, 8 Postscript figures, REVTeX 3.0 with psfig macros

10.1103/PhysRevB.55.12742

supr-con cond-mat.supr-con

Results of a calculation of the c-axis infrared conductivity sigma_c for a d_{x^2-y^2}-wave superconductor which include both elastic impurity and inelastic spin-fluctuation scattering are presented and compared with the ab-plane conductivity sigma_{ab} in the same model. In this model,the interlayer c-axis coupling is taken to be weak and diffusive. While in clean systems, inelastic scattering leads to a peak at omega = 4*Delta_0 in sigma_{ab} for T < T_c, it has little effect on the corresponding sigma_c, which exhibits structure only at omega = 2*Delta_0 and is directly related to the single-particle density of states N(omega). The c-axis penetration depth lambda_c in the same model is predicted to vary as T^3 at low temperatures in clean samples. We discuss recent optical experiments on the cuprates and compare with these predictions.

supr-con/9608003

Oleg Tchernyshyov

A. S. Blaer (1), H. C. Ren (2), and O. Tchernyshyov (1) ((1) Columbia University, (2) Rockefeller University)

Extended bound states and resonances of two fermions on a periodic lattice

21 pages, RevTeX, 4 Postscript figures, arithmetic errors corrected. An abbreviated version (no appendix) appeared in PRB on March 1, 1997

Phys. Rev. B 55, 6035 (1997)

10.1103/PhysRevB.55.6035

CU-TP-771, RU-96-6B

supr-con cond-mat.supr-con

The high-$T_c$ cuprates are possible candidates for d-wave superconductivity, with the Cooper pair wave function belonging to a non-trivial irreducible representation of the lattice point group. We argue that this d-wave symmetry is related to a special form of the fermionic kinetic energy and does not require any novel pairing mechanism. In this context, we present a detailed study of the bound states and resonances formed by two lattice fermions interacting via a non-retarded potential that is attractive for nearest neighbors but repulsive for other relative positions. In the case of strong binding, a pair formed by fermions on adjacent lattice sites can have a small effective mass, thereby implying a high condensation temperature. For a weakly bound state, a pair with non-trivial symmetry tends to be smaller in size than an s-wave pair. These and other findings are discussed in connection with the properties of high-$T_c$ cuprate superconductors.

supr-con/9608004

Georg Bruun

G. M. Bruun, V. N. Nicopoulos, N. F. Johnson

Ginzburg-Landau-Gor'kov Theory of Magnetic oscillations in a type-II 2-dimensional Superconductor

39 pages, 8 figures. This is a replacement of supr-con/9608004. Several sections changed or added, including a section on the effect of spin and the effect of a conserved number of particles. To be published in Phys. Rev. B

PHYS REV B 56: (2) 809-825 JUL 1 1997

10.1103/PhysRevB.56.809

supr-con cond-mat.supr-con

We investigate de Haas-van Alphen (dHvA) oscillations in the mixed state of a type-II two-dimensional superconductor within a self-consistent Gor'kov perturbation scheme. Assuming that the order parameter forms a vortex lattice we can calculate the expansion coefficients exactly to any order. We have tested the results of the perturbation theory to fourth and eight order against an exact numerical solution of the corresponding Bogoliubov-de Gennes equations. The perturbation theory is found to describe the onset of superconductivity well close to the transition point $H_{c2}$. Contrary to earlier calculations by other authors we do not find that the perturbative scheme predicts any maximum of the dHvA-oscillations below $H_{c2}$. Instead we obtain a substantial damping of the magnetic oscillations in the mixed state as compared to the normal state. We have examined the effect of an oscillatory chemical potential due to particle conservation and the effect of a finite Zeeman splitting. Furthermore we have investigated the recently debated issue of a possibility of a sign change of the fundamental harmonic of the magnetic oscillations. Our theory is compared with experiment and we have found good agreement.

supr-con/9608005

Robert B. Laughlin

R. B. Laughlin (Stanford)

Evidence for Quasiparticle Decay in Photoemission from Underdoped Cuprates

4 pages of ReVTeX, 3 eps figures

Phys. Rev. Lett. 79, 1726 (1997)

10.1103/PhysRevLett.79.1726

p01jul96

supr-con cond-mat.supr-con

I argue that the ``gap'' recently observed at the Brillouin zone face of cuprate superconductors in photoemission by Marshall et al [Phys. Rev. Lett. 76, 4841 (1996)] and Ding et al [Nature 382, 54 (1996)] is evidence for the decay of the injected hole into a spinon-holon pair.

supr-con/9608006

Bozidar Mitrovic

B. Mitrovic and M. Castle (Brock University, Canada)

Strong coupling theory of the interlayer tunneling model for high temperature superconductors

23 pages, 7 Postscript figures, REVTeX 3.0 with epsf macros

10.1088/0953-8984/9/42/015

supr-con cond-mat.supr-con

The interlayer pair tunneling model of Anderson et al. is generalized to include the strong coupling effects associated with in-plane interactions. The equations for the superconducting transition temperature T_{c} are solved numerically for several models of electron-optical phonon coupling. The nonmagnetic in-plane impurity scattering suppresses T_{c} in all cases considered, and it is possible to obtain a fair agreement with experiments for a reasonable choice of parameters. For the anisotropic electron-phonon coupling proposed by Song and Annett we find that the interlayer pair tunneling can stabilize the d_{x^2-y^2}-wave superconducting state with a high T_{c}. Moreover, in this case there is a possibility of an impurity induced crossover from the d$_{x^2-y^2}$-wave state stabilized by the interlayer tunneling to the s-wave state at a low impurity concentration. We also calculate the isotope effect associated with the in-plane oxygen optic mode and its dependence on the strength of the interlayer pair tunneling. Small positive values of the isotope exponent are obtained for strengths of pair tunneling that give high transition temperatures.

supr-con/9608007

Francesca Federici, Andrei A. Varlamov

The Fluctuation Induced Pseudogap in the Infrared Optical Conductivity of High Temperature Superconductors

8 pages, 4 eps figures, Submitted to Phys. Rev. B

10.1103/PhysRevB.55.6070

supr-con cond-mat.supr-con

We study the effect of fluctuations on the {\bf ac} conductivity of a layered superconductor both for $c$-axis and $ab$-plane electromagnetic wave polarizations. The fluctuation contributions of different physical nature and signs (paraconductivity, Maki-Thompson anomalous contribution, one-electron density of states renormalization) are found to be suppressed by the external field at different characterisitic frequencies ($ \omega_{\rm AL}\sim T-T_{\rm c}$, $\omega_{\rm MT} \sim \max\{ T-T_{\rm c}, \tau_{\varphi}^{-1}\}$, $ \omega_{\rm DOS} \sim \min\{T,\tau ^{-1}\}$ for the $2D$ case). As a result the appearance of the nonmonotonic frequency dependence (pseudogap) in the infrared optical conductivity of HTS film is predicted. The effect has to be especially pronounced in the case of the electromagnetic field polarization along $c$-axis.

supr-con/9608008

Ruslan Prozorov

R. Prozorov, M. Konczykowski, B. Schmidt, Y. Yeshurun, A. Shaulov, C. Villard, G. Koren

On the origin of the irreversibility line in thin YBaCuO7 films with and without columnar defects

19 pages, LaTex, 6 PostScript figures; Author's Homepage: http://www.biu.ac.il:80/~prozorr

10.1103/PhysRevB.54.15530

supr-con cond-mat.supr-con

We report on measurements of the angular dependence of the irreversibility temperature $T_{irr}(\theta) $ in $YBa_2Cu_3O_{7-\delta }$ thin films, defined by the onset of a third harmonic signal and measured by a miniature Hall probe. From the functional form of $T_{irr}(\theta)$ we conclude that the origin of the irreversibility line in unirradiated films is a dynamic crossover from an unpinned to a pinned vortex liquid. In irradiated films the irreversibility temperature is determined by the trapping angle.

supr-con/9609001

Durga P. Choudhury

Durga P. Choudhury, Balam A. Willemsen, John S. Derov and S. Sridhar (Physics Department, Northeastern University and Rome Laboratory, Hanscom AFB.)

Nonlinear Response of HTSC Thin Film Microwave Resonators in an Applied DC Magnetic Field

4 pages, LaTeX type, Uses IEEE style files, 600 dpi PostScript file with color figures available at http://sagar.physics.neu.edu/preprints.html Submitted to IEEE Transactions on Applied Superconductivity

10.1109/77.620744

supr-con cond-mat.supr-con

The non-linear microwave surface impedance of patterned YBCO thin films, was measured using a suspended line resonator in the presence of a perpendicular DC magnetic field of magnitude comparable to that of the microwave field. Signature of the virgin state was found to be absent even for relatively low microwave power levels. The microwave loss was initially found to decrease for small applied DC field before increasing again. Also, non-linearities inherent in the sample were found to be substantially suppressed at low powers at these applied fields. These two features together can lead to significant improvement in device performance.

supr-con/9609002

Durga P. Choudhury

Balam A. Willemsen, J. S. Derov and S.Sridhar (Physics Department, Northeastern University and Rome Laboratory, Hanscom AFB)

Critical State Flux Penetration and Linear Microwave Vortex Response in YBa_2Cu_3O_{7-x} Films

20 pages, LaTeX type, Uses REVTeX style files, Submitted to Physical Review B, 600 dpi PostScript file with high resolution figures available at http://sagar.physics.neu.edu/preprints.html

10.1103/PhysRevB.56.11989

supr-con cond-mat.supr-con

The vortex contribution to the dc field (H) dependent microwave surface impedance Z_s = R_s+iX_s of YBa_2Cu_3O_{7-x} thin films was measured using suspended patterned resonators. Z_s(H) is shown to be a direct measure of the flux density B(H) enabling a very precise test of models of flux penetration. Three regimes of field-dependent behavior were observed: (1) Initial flux penetration occurs on very low field scales H_i(4.2K) 100Oe, (2) At moderate fields the flux penetration into the virgin state is in excellent agreement with calculations based upon the field-induced Bean critical state for thin film geometry, parametrized by a field scale H_s(4.2K) J_c*d 0.5T, (3) for very high fields H >>H_s, the flux density is uniform and the measurements enable direct determination of vortex parameters such as pinning force constants \alpha_p and vortex viscosity \eta. However hysteresis loops are in disagreement with the thin film Bean model, and instead are governed by the low field scale H_i, rather than by H_s. Geometric barriers are insufficient to account for the observed results.

supr-con/9609003

Hasegawa Yasumasa

Yasumasa Hasegawa (Himeji Institute of Technology)

Density of States and NMR Relaxation Rate in Anisotropic Superconductivity with Intersecting Line Nodes

7 pages, 4 PostScript Figures, LaTeX, to appear in J. Phys. Soc. Jpn

10.1143/JPSJ.65.3131

supr-con cond-mat.supr-con

We show that the density of states in an anisotropic superconductor with intersecting line nodes in the gap function is proportional to $E log (\alpha \Delta_0 /E)$ for $|E| << \Delta_0$, where $\Delta_0$ is the maximum value of the gap function and $\alpha$ is constant, while it is proportional to $E$ if the line nodes do not intersect. As a result, a logarithmic correction appears in the temperature dependence of the NMR relaxation rate and the specific heat, which can be observed experimentally. By comparing with those for the heavy fermion superconductors, we can obtain information about the symmetry of the gap function.

supr-con/9609004

Masanori Ichioka

Naoki Enomoto, Masanori Ichioka and Kazushige Machida (Okayama Univ.)

Ginzburg Landau theory for d-wave pairing and fourfold symmetric vortex core structure

12 pages including 8 eps figs, LaTeX with jpsj.sty & epsfig

J. Phys. Soc. Jpn. 66, 204 (1997).

10.1143/JPSJ.66.204

supr-con cond-mat.supr-con

The Ginzburg Landau theory for d_{x^2-y^2}-wave superconductors is constructed, by starting from the Gor'kov equation with including correction terms up to the next order of ln(T_c/T). Some of the non-local correction terms are found to break the cylindrical symmetry and lead to the fourfold symmetric core structure, reflecting the internal degree of freedom in the pair potential. Using this extended Ginzburg Landau theory, we investigate the fourfold symmetric structure of the pair potential, current and magnetic field around an isolated single vortex, and clarify concretely how the vortex core structure deviates from the cylindrical symmetry in the d_{x^2-y^2}-wave superconductors.