arXiv subjects

David Stroud

Publications and source records attributed to David Stroud.

28 records · Page 2Linked to original sources

arXiv2005

Long Josephson Junction in a Resonant Cavity

We present a model to describe an underdamped long Josephson junction coupled to a singlemode electromagnetic cavity, and carry out numerical calculations using this model in various regimes. The coupling may occur through either the electric or the magnetic eld of the cavity mode. When a current is injected into the junction, we nd that the time-averaged voltage exhibits self-induced resonant steps due to coupling between the current in the junction and the electric eld of the cavity mode. These steps are similar to those observed and calculated in small Josephson junctions. When a soliton is present in the junction (corresponding to a quantum of magnetic ux parallel to the junction plates), the SIRS's disappear if the electric eld in the cavity is spatially uniform. If the cavity mode has a spatially varying electric eld, there is a strong coupling between the soliton and the cavity mode. This coupling causes the soliton to become phase-locked to the cavity mode, and produces step-like anomalies on the soliton branch of the IV characteristics. If the coupling is strong enough, the frequency of the cavity mode is greatly red-shifted from its uncoupled value. We present simple geometrical arguments which account for this behavior.

cond-mat.supr-con↗

arXiv2004

Surface Plasmon Dispersion Relations in Chains of Metallic Nanoparticles: Exact Quasistatic Calculation

We calculate the surface plasmon dispersion relations for a periodic chain of spherical metallic nanoparticles in an isotropic host, including all multipole modes in a generalized tight-binding approach. For sufficiently small particles ($kd \ll 1$, where $k$ is the wave vector and $d$ is the interparticle separation), the calculation is exact. The lowest bands differ only slightly from previous point-dipole calculations provided the particle radius $a \lesssim d/3$, but differ substantially at smaller separation. We also calculate the dispersion relations for many higher bands, and estimate the group velocity $v_g$ and the exponential decay length $ξ_D$ for energy propagation for the lowest two bands due to single-grain damping. For $a/d=0.33$, the result for $ξ_D$ is in qualitative agreement with experiments on gold nanoparticle chains, while for larger $a/d$, such as $a/d=0.45$, $v_g$ and $ξ_D$ are expected to be strongly $k$-dependent because of the multipole corrections. When $a/d \sim 1/2$, we predict novel percolation effects in the spectrum, and find surprising symmetry in the plasmon band structure. Finally, we reformulate the band structure equations for a Drude metal in the time domain, and suggest how to include localized driving electric fields in the equations of motion.

cond-mat.mtrl-sci↗

arXiv2003

Theory of the Optical Properties of a DNA-Modified Gold Nanoparticle System

We describe a simple model for the melting and optical properties of a DNA/gold nanoparticle aggregate. The aggregate is modeled as a cluster of gold nanoparticles on a periodic lattice connected by DNA bonds, and the extinction coefficient is computed using the discrete dipole approximation. The optical properties at fixed wavelength change dramatically at the melting transition, which is found to be higher and narrower in temperature for larger particles, and much sharper than that of an isolated DNA link. All these features are in agreement with available experiments.

cond-mat↗

arXiv2002

Models for Enhanced Absorption in Inhomogeneous Superconductors

We discuss the low-frequency absorption arising from quenched inhomogeneity in the superfluid density rho_s of a model superconductor. Such inhomogeneities may arise in a high-T_c superconductor from a wide variety of sources, including quenched random disorder and static charge density waves such as stripes. Using standard classical methods for treating randomly inhomogeneous media, we show that both mechanisms produce additional absorption at finite frequencies. For a two-fluid model with weak mean-square fluctuations <(d rho_s)^2 > in rho_s and a frequency-independent quasiparticle conductivity, the extra absorption has oscillator strength proportional to the quantity <(d rho_s)^2>/rho_s, as observed in some experiments. Similar behavior is found in a two-fluid model with anticorrelated fluctuations in the superfluid and normal fluid densities. The extra absorption typically occurs as a Lorentzian centered at zero frequency. We present simple model calculations for this extra absorption under conditions of both weak and strong fluctuations. The relation between our results and other model calculations is briefly discussed.

cond-mat.supr-con↗

arXiv2002

Transition Spectra for a BCS Superconductor with Multiple Gaps: Model Calculations for MgB_2

We analyze the qualitative features in the transition spectra of a model superconductor with multiple energy gaps, using a simple extension of the Mattis-Bardeen expression for probes with case I and case II coherence factors. At temperature T = 0, the far infrared absorption edge is, as expected, determined by the smallest gap. However, the large thermal background may mask this edge at finite temperatures and instead the secondary absorption edges found at Delta_i+Delta_j may become most prominent. At finite T, if certain interband matrix elements are large, there may also be absorption peaks at the gap difference frequencies | Delta_i-Delta_j | . We discuss the effect of sample quality on the measured spectra and the possible relation of these predictions to the recent infrared absorption measurement on MgB_2.

cond-mat.supr-con↗

arXiv2002

Superfluid Inhomogeneity and Microwave Absorption in Model High-T_c Superconductors

We investigate the microwave absorption arising from inhomogeneity in the superfluid density of a model high-T_c superconductor. Such inhomogeneities may arise from a wide variety of sources, including quenched random disorder and static charge density waves such as stripes. We show that both mechanisms will inevitably produce additional absorption at finite frequencies. We present simple model calculations for this extra absorption, and discuss applications to other transport properties in high-T_c materials. Finally, we discuss the connection of these predictions to recent measurements by Corson et al. of absorption by the high-temperature superconductor BSCCO in the THz frequency regime.

cond-mat.supr-con↗

arXiv2002

Effective Macroscopic Response of a Composite with Small Deviations from Periodicity: Application to Colloidal Crystals

Using the spectral approach, we analyze the effective properties of a composite which deviates slightly from periodicity. We find that, when the inclusions are randomly displaced from their equilibrium positions, the sharp resonances seen in the periodic case are broadened, and an additional branch cut appears. We use these results to analyze the effective dielectric constant of a colloidal crystal.

cond-mat.mtrl-sci↗

arXiv2002

Structural and Superconducting Transitions in Mg_{1-x}Al_{x}B_2

From systematic ab initio calculations of the alloy system Mg_{1-x}Al_{x}B_2, we find a strong tendency for the formation of a superstructure characterized by Al-rich layers. We also present a simple model, based on calculated energies and an estimate of the configurational entropy, which suggests that the alloy has two separate concentration regimes of phase separation, with critical points near x = 0.25 and x = 0.75. These results, together with calculations of electronic densities of states in several ionic arrangements, give a qualitative explanation for the observed structural instabilities, as well as the x-dependence of the superconducting T_c for x<0.6.

cond-mat.supr-con↗

arXiv1998

Nature of the Low Field Transition in the Mixed State of High Temperature Superconductors

We have numerically studied the statics and dynamics of a model three-dimensional vortex lattice at low magnetic fields. For the statics we use a frustrated 3D XY model on a stacked triangular lattice. We model the dynamics as a coupled network of overdamped resistively-shunted Josephson junctions with Langevin noise. At low fields, there is a weakly first-order phase transition, at which the vortex lattice melts into a line liquid. Phase coherence parallel to the field persists until a sharp crossover, conceivably a phase transition, near $T_{\ell} > T_m$ which develops at the same temperature as an infinite vortex tangle. The calculated flux flow resistivity in various geometries near $T=T_{\ell}$ closely resembles experiment. The local density of field induced vortices increases sharply near $T_\ell$, corresponding to the experimentally observed magnetization jump. We discuss the nature of a possible transition or crossover at $T_\ell$(B) which is distinct from flux lattice melting.

cond-mat↗

arXiv1993

Mean Field Theory for Underdamped Josephson Junction Arrays with an Offset Voltage

We study a model Hamiltonian for superconductivity in underdamped Josephson junction arrays in the presence of an offset voltage between the array and the substrate. We develop an approximate zero-temperature (T = 0) phase diagram as a function of Josephson coupling, charging energy, and offset voltage, using a simple Hartree-type mean-field approximation. With diagonal charging energy, the calculated phase diagram is periodic in offset voltage, in agreement with previous results. At a special value of this voltage such that states with n and n+1 Cooper pairs per grain are degenerate, only an infinitesimal Josephson coupling is needed to establish long-range phase coherence in this approximation. With both diagonal and nearest-neighbor charging energies, the T = 0 phase diagram has two types of insulating lobes with different kinds of charge order, and two types of superconducting regions. One of these is a ``supersolid'' in which long-range phase coherence coexists with a frozen charge density wave. We briefly discuss connections to previous calculations, and possible relevance to experiments.

cond-mat↗