arXiv subjects

D. Stroud

Publications and source records attributed to D. Stroud.

At least 55 records · Page 3Linked to original sources

arXiv2001

Negative Magnetoresistance Produced by Hall Fluctuations in a Ferromagnetic Domain Structure

We present a model for a negative magnetoresistance (MR) that would develop in a material with many ferromagnetic domains even if the individual domains have no magnetoresistance and even if there is no boundary resistance. The negative MR is due to a classical current-distortion effect arising from spatial variations in the Hall conductivity, combined with a change in domain structure due to an applied magnetic field. The negative MR can exceed 1000% if the product of the carrier relaxation time and the internal magnetic field due to spontaneous magnetization is sufficiently large.

cond-mat.mtrl-sci↗

arXiv2000

Resonant-Cavity-Induced Phase Locking and Voltage Steps in a Josephson Array

We describe a simple dynamical model for an underdamped Josephson junction array coupled to a resonant cavity. From numerical solutions of the model in one dimension, we find that (i) current-voltage characteristics of the array have self-induced resonant steps (SIRS), (ii) at fixed disorder and coupling strength, the array locks into a coherent, periodic state above a critical number of active Josephson junctions, and (iii) when $N_a$ active junctions are synchronized on an SIRS, the energy emitted into the resonant cavity is quadratic with $N_a$. All three features are in agreement with a recent experiment [Barbara {\it et al}, Phys. Rev. Lett. {\bf 82}, 1963 (1999)]}.

cond-mat.supr-con↗

arXiv2000

Fractional vortices in the XY model with $π$ bonds

We define a new set of excitations in the XY model which we call ``fractional vortices''. In the frustrated XY model containing $π$ bonds, we make the ansatz that the ground state configurations can be characterized by pairs of oppositely charged fractional vortices. For a chain of $π$ bonds, the ground state energy and the phase configurations calculated on the basis of this ansatz agree well with the results from direct numerical simulations. Finally, we discuss the possible connection of these results to some recent experiments by Kirtley {\it et al} [Phys. Rev. B {\bf 51}, R12057 (1995)] on high-T$_c$ superconductors where fractional flux trapping was observed along certain grain boundaries.

cond-mat↗

arXiv2000

Tilt Modulus and Angle-Dependent Flux Lattice Melting in the Lowest Landau Level Approximation

For a clean high-T$_c$ superconductor, we analyze the Lawrence-Doniach free energy in a tilted magnetic field within the lowest Landau level (LLL) approximation. The free energy maps onto that of a strictly $c$-axis field, but with a reduced interlayer coupling. We use this result to calculate the tilt modulus $C_{44}$ of a vortex lattice and vortex liquid. The vortex contribution to $C_{44}$ can be expressed in terms of the squared $c$-axis Josephson plasmon frequency $ω_{pl}^2$. The transverse component of the field has very little effect on the position of the melting curve.

cond-mat.supr-con↗

arXiv2000

Ab Initio Molecular Dynamics Simulation of Liquid Ga_xAs_{1-x} Alloys

We report the results of ab initio molecular dynamics simulations of liquid Ga_xAs_{1-x} alloys at five different concentrations, at a temperature of 1600 K, just above the melting point of GaAs. The liquid is predicted to be metallic at all concentrations between x = 0.2 and x = 0.8, with a weak resistivity maximum near x = 0.5, consistent with the Faber-Ziman expression. The electronic density of states is finite at the Fermi energy for all concentrations; there is, however, a significant pseudogap especially in the As-rich samples. The Ga-rich density of states more closely resembles that of a free-electron metal. The partial structure factors show only a weak indication of chemical short-range order. There is also some residue of the covalent bonding found in the solid, which shows up in the bond-angle distribution functions of the liquid state. Finally, the atomic diffusion coefficients at 1600K are calculated to be 2.1 \times 10^{-4} cm^2/sec for Ga ions in Ga_{0.8}As_{0.2} and 1.7 \times 10^{-4} cm^2/sec for As ions in Ga_{0.2}As_{0.8}.

cond-mat↗

arXiv2000

Conductivity Due to Classical Phase Fluctuations in a Model For High-T_c Superconductors

We consider the real part of the conductivity, σ_1(ω), arising from classical phase fluctuations in a model for high-T_c superconductors. We show that the frequency integral of that conductivity, \int_0^\infty σ_1 dω, is non-zero below the superconducting transition temperature $T_c$, provided there is some quenched disorder in the system. Furthermore, for a fixed amount of quenched disorder, this integral at low temperatures is proportional to the zero-temperature superfluid density, in agreement with experiment. We calculate σ_1(ω) explicitly for a model of overdamped phase fluctuations.

cond-mat.supr-con↗

arXiv2000

Model for a Josephson junction array coupled to a resonant cavity

We describe a simple Hamiltonian for an underdamped Josephson array coupled to a single photon mode in a resonant cavity. Using a Hartree-like mean-field theory, we show that, for any given strength of coupling between the photon field and the Josephson junctions, there is a transition from incoherence to coherence as a function of N, the number of Josephson junctions in the array. Above that value of N, the energy in the photon field is proportional to N^2, indicative of coherent emission. Both of these features agree with recent experiments by Barbara et al. The experimentally observed voltage steps may require an explicit solution of the dynamical equations.

cond-mat.supr-con↗

arXiv1999

Exact results and scaling properties of small-world networks

We study the distribution function for minimal paths in small-world networks. Using properties of this distribution function, we derive analytic results which greatly simplify the numerical calculation of the average minimal distance, $\bar{\ell}$, and its variance, $σ^2$. We also discuss the scaling properties of the distribution function. Finally, we study the limit of large system sizes and obtain some analytic results.

cond-mat.stat-mech↗

arXiv1999

Dynamical Phase Transition in a Fully Frustrated Square Josephson Array

We study dynamical phase transitions at temperature T = 0 in a fully frustrated square Josephson junction array subject to a driving current density which has nonzero components parallel to both axes of the lattice. Our numerical results show clear evidence for three dynamical phases: a pinned vortex lattice characterized by zero time-averaged voltages, a ``plastic'' phase in which both voltages are nonzero, and a moving lattice phase in which only one of the time-average voltage components is nonzero. The last of these has a finite transverse critical current.

cond-mat.supr-con↗

arXiv1999

Study of Vortex Avalanches using the Bassler-Paczuski Model

We have carried out a model calculation of the flux noise produced by vortex avalanches in a Type-II superconductor, using a simple kinetic model proposed by Bassler and Paczuski. Over a broad range of frequencies, we find that the flux noise $S_Φ(ω)$ has a power-law dependence on frequency, $S_Φ(ω) \sim ω^{-s}$, with $s \sim 1.4$ in reasonable agreement with experiment. In addition, for small lattices, the calculated $S_Φ(ω)$ has a high-frequency knee, which is seen in some experiments, and which is due to the finite lattice size. Deviations between calculation and experiment are attributed mostly to uncertainties in the measured critical current densities and pinning strengths of the experimental samples.

cond-mat.supr-con↗

arXiv1999

Evolutionary dynamics in the Bak-Sneppen model on small-world networks

We study the dynamics of the Bak-Sneppen model on small-world networks. For each site in the network, we define a ``connectance,'' which measures the distance to all other sites. We find radically different patterns of activity for different sites, depending on their connectance and also on the topology of the network. For a given network, the site with the minimal connectance shows long periods of stasis interrupted by much smaller periods of activity. In contrast, the activity pattern for the maximally connected site appears uniform on the same time scale. We discuss the significance of these results for speciation events.

cond-mat↗

arXiv1998

Critical Currents of Josephson-Coupled Wire Arrays

We calculate the current-voltage characteristics and critical current I_c^{array} of an array of Josephson-coupled superconducting wires. The array has two layers, each consisting of a set of parallel wires, arranged at right angles, such that an overdamped resistively-shunted junction forms wherever two wires cross. A uniform magnetic field equal to f flux quanta per plaquette is applied perpendicular to the layers. If f = p/q, where p and q are mutually prime integers, I_c^{array}(f) is found to have sharp peaks when q is a small integer. To an excellent approximation, it is found in a square array of n^2 plaquettes, that I_c^{array}(f) \propto (n/q)^{1/2} for sufficiently large n. This result is interpreted in terms of the commensurability between the array and the assumed q \times q unit cell of the ground state vortex lattice.

cond-mat.supr-con↗

arXiv1997

Anomalous Relaxation in the XY Gauge Glass

To study relaxation dynamics of the two-dimensional XY gauge glass, we integrate directly the equations of motion and investigate the energy function. As usual, it decays exponentially at high temperatures; at low but non-zero temperatures, it is found to exhibit an algebraic relaxation. We compute the relaxation time $τ$ as a function of the temperature $T$ and find that the rapid increase of $τ$ at low temperatures is well described by $τ\sim (T-T_g)^{-b}$ with $T_g = 0.22 \pm 0.02$ and $b = 0.76 \pm 0.05$, which strongly suggests a finite-temperature glass transition. The decay of vorticity is also examined and explained in terms of a simple heuristic model, which attributes the fast relaxation at high temperatures to annihilation of unpinned vortices.

cond-mat.supr-con↗

arXiv1997

Magnetization Jump in a Model for Flux Lattice Melting at Low Magnetic Fields

Using a frustrated XY model on a lattice with open boundary conditions, we numerically study the magnetization change near a flux lattice melting transition at low fields. In both two and three dimensions, we find that the melting transition is followed at a higher temperature by the onset of large dissipation associated with the zero-field XY transition. It is characterized by the proliferation of vortex-antivortex pairs (in 2D) or vortex loops (in 3D). At the upper transition, there is a sharp increase in magnetization, in qualitative agreement with recent local Hall probe experiments.

cond-mat↗

arXiv1997

Flux noise and Fluctuation conductivity in Unfrustrated Josephson Junction Arrays

We study the flux noise $S_Φ(ω)$ and finite frequency conductivity $σ_1(ω)$ in two dimensional unfrustrated Josephson junction arrays (JJA's), by numerically solving the equations of the coupled overdamped resistively-shunted-junction model with Langevin noise. We find that $S_Φ(ω)\propto ω^{-3/2}$ at high frequencies $ω$ and flattens at low $ω$, indicative of vortex diffusion, while $σ_1 \propto ω^{-2}$ at sufficiently high $ω$. Both quantities show clear evidence of critical slowing down and possibly scaling behavior near the Kosterlitz-Thouless-Berezinskii (KTB) transition. The critical slowing down of $S_Φ$, but not its frequency dependence, is in agreement with recent experiments on Josephson junction arrays.

cond-mat.supr-con↗

arXiv1996

Quantum Melting of a Two-Dimensional Vortex Lattice at Zero Temperature

We consider the quantum melting of a two-dimensional flux lattice at temperature T = 0 in the ``superclean limit.'' In this regime, we find that vortex motion is dominated by the Magnus force. A Lindemann criterion predicts melting when $n_v/n_p \geq β$, where $n_v$ and $n_p$ are the areal number densities of vortex pancakes and Cooper pairs, and $β\approx 0.1$. A second criterion is derived by using Wigner crystal and Laughlin wave functions for the solid and liquid phases respectively, and setting the two energies equal. This gives a melting value similar to the Lindemann result. We discuss the numerical value of the melting field at $T = 0$ for thin layers of low-T$_c$ superconductor, such as $a-MoGe$, and single layers of high-T$_c$ materials.

cond-mat↗

arXiv1996

Dynamical Phase Transition in a Driven Disordered Vortex Lattice

Using Langevin dynamics, we have investigated the dynamics of vortices in a disordered two dimensional superconductor subjected to a uniform driving current. The results provide direct numerical evidence for a dynamical phase transition between a plastic flow regime and a moving ``hexatic glass." The simulated current-voltage characteristics are in excellent agreement with recent transport measurements on amorphous ${\rm Mo_{77}Ge_{23}}$ thin film superconductors.

cond-mat↗

arXiv1996

First-Order Melting and Dynamics of Flux Lines in a Model for YBa$_2$Cu$_3$O$_{7-δ}$

We have studied the statics and dynamics of flux lines in a model for YBCO, using both Monte Carlo simulations and Langevin dynamics. For a clean system, both approaches yield the same melting curve, which is found to be weakly first order with a heat of fusion of about $0.02 k_BT_m$ per vortex pancake at a field of $50 {\rm kG}.$ The time averaged magnetic field distribution experienced by a fixed spin is found to undergo a qualitative change at freezing, in agreement with NMR and $μ{\rm SR}$ experiments. Melting in the clean system is accompanied by a proliferation of free disclinations which show a clear B-dependent 3D-2D crossover from long disclination lines parallel to the c-axis at low fields, to 2D ``pancake'' disclinations at higher fields. Strong point pins produce a logarithmical $\ln t$ relaxation which results from slow annealing out of disclinations in disordered samples.

cond-mat↗