arXiv subjects

D. Stroud

Publications and source records attributed to D. Stroud.

At least 19 recordsLinked to original sources

arXiv2015

Spectral Representation for the Effective Macroscopic Response of a Polycrystal: Application to Third-Order Nonlinear Susceptibility

Erratum: In our paper, we show that the spectral representation for isotropic two-component composites also applies to uniaxial polycrystals. We have learned that this result was, in fact, first conjectured by G.W. Milton. While our derivation is more detailed, our result for the spectral function is the same as Milton's. We very much regret not having been aware of this work at the time of writing our paper. Original abstract: We extend the spectral theory used for the calculation of the effective linear response functions of composites to the case of a polycrystalline material with uniaxially anisotropic microscopic symmetry. As an application, we combine these results with a nonlinear decoupling approximation as modified by Ma et al., to calculate the third-order nonlinear optical susceptibility of a uniaxial polycrystal, assuming that the effective dielectric function of the polycrystal can be calculated within the effective-medium approximation.

cond-mat↗

arXiv2015

Faraday Rotation and One-Way Propagation of Plasmon Waves on a Nanoparticle Chain

We calculate the dispersion relations of plasmonic waves propagating along a chain of metallic nanoparticles in the presence of both a static magnetic field ${\bf B}$ and a liquid crystalline host. The dispersion relations are obtained using the quasistatic approximation and a dipole-dipole approximation to treat the interaction between surface plasmons on different nanoparticles. For a plasmons propagating along a particle chain in a nematic liquid crystalline host and a field parallel to the chain, we find a small, but finite, Faraday rotation angle. In a cholesteric liquid crystal host and an applied magnetic field parallel to the chain, the dispersion relations for left- and right-moving waves are found to be different. For some frequencies, the plasmonic wave may propagate only in one of the two directions.

cond-mat.mes-hall↗

arXiv2014

Model of the Longitudinal Spin Seebeck Coefficient of InSb in a Magnetic Field

We develop a simple theory for the longitudinal spin Seebeck effect in n-doped InSb in an external magnetic field. We consider spin-$1/2$ electrons in the conduction band of InSb with a temperature gradient parallel to the applied magnetic field. In the absence of spin-orbit interactions, a Boltzmann equation approach leads to a spin current parallel to the field and proportional to the temperature gradient. The calculated longitudinal spin Seebeck coefficients oscillates as a function of magnetic field B; the peak positions are approximately periodic in 1/B. The oscillations arise when the Fermi energy crosses the bottom of a Landau band.

cond-mat.mes-hall↗

arXiv2014

Graphene with adatoms: tuning the magnetic moment with an applied voltage

We show that, in graphene with a small concentration of adatoms, the total magnetic moment $μ_T$ can be switched on and off by varying the Fermi energy $E_F$, either by applying a gate voltage or by suitable chemical doping. Our calculation is carried out using a simple tight-binding model described previously, combined with a mean-field treatment of the electron-electron interaction on the adatom. The values of $E_F$ at which the moment is turned on or off are controlled by the strength of the hopping between the graphene sheet and the adatom, the on-site energy of the adatom, and the strength of the electron-electron correlation energy U. Our result is in qualitatively consistent with recent experiments by Nair {\it et al.} [Nat.\ Commun.\ {\bf 4}, 2010 (2013)].

cond-mat.mes-hall↗

arXiv2013

Theory of plasmonic waves on a chain of metallic nanoparticles in a liquid crystalline host

A chain of metallic particles, of sufficiently small diameter and spacing, allows linearly polarized plasmonic waves to propagate along the chain. In this paper, we describes how these waves are altered when the liquid crystal host is a nematic or a cholesteric liquid crystal (NLC or CLC) with or without an applied magnetic field. We find that, in general, the liquid crystal host, either NLC or CLC, alters the dispersion relations of the transverse ($T$) and longitudinal ($L$) waves significantly from the dispersion relations for an isotropic host. We show that by altering the director axis of the liquid crystal relative to the long axis of the metallic chain, that the $T$ branch can be split into two non-degenerate linearly polarized branches (NLC host) or two non-degenerate elliptically polarized branches (CLC host). When an external magnetic field is applied parallel to both the long axis of the metallic particles and the director of the CLC host, we find that the dispersion relations are odd in an exchange in sign for $ω$ for the non-degenerate elliptically polarized $T$ branches. That is, the application of an external magnetic field leads to the realization of a one-way waveguide.

cond-mat.mes-hall↗

arXiv2012

Experimental Demonstration Of Scanned Spin-Precession Microscopy

Progress in spintronics has been aided by characterization tools tailored to certain archetypical materials. New device structures and materials will require characterization tools that are material independent, provide sufficient resolution to image locally-varying spin properties and enable subsurface imaging. Here we report the demonstration of a novel spin-microscopy tool based on the variation of a global spin-precession signal in response to the localized magnetic field of a scanned probe. We map the local spin density in optically pumped GaAs from this spatially-averaged signal with a resolution of 5.5 microns. This methodology is also applicable to other spin properties and its resolution can be improved. It can extend spin microscopy to device structures not accessible by other techniques, such as buried interfaces and non-optically active materials, due to the universal nature of magnetic interactions between the spins and the probe.

cond-mat.mes-hall↗

arXiv2012

Plasmonic Waves on a Chain of Metallic Nanoparticles: Effects of a Liquid Crystalline Host or an Applied Magnetic Field

A chain of metallic particles, of sufficiently small diameter and spacing, allows linearly polarized plasmonic waves to propagate along the chain. In this paper, we consider how these waves are altered by an anisotropic host (such as a nematic liquid crystal) or an applied magnetic field. In a liquid crystalline host, with principal axis (director) oriented either parallel or perpendicular to the chain, we find that the dispersion relations of both the longitudinal ($L$) and transverse ($T$) modes are significantly altered relative to those of an isotropic host. Furthermore, when the director is perpendicular to the chain, the doubly degenerate $T$ branch is split by the anisotropy of the host material. With an applied magnetic field ${\bf B}$ parallel to the chain, the propagating transverse modes are circularly polarized, and the left and right circularly polarized branches have slightly different dispersion relations. As a result, if a linearly polarized transverse wave is launched along the chain, it undergoes Faraday rotation. For parameters approximating that of a typical metal and for a field of 2T, the Faraday rotation is of order 1$^o$ per ten interparticle spacings, even taking into account single-particle damping. If ${\bf B}$ is perpendicular to the chain, one of the $T$ branches mixes with the $L$ branch to form two elliptically polarized branches. Our calculations include single-particle damping and can, in principle, be generalized to include radiation damping. The present work suggests that the dispersion relations of plasmonic waves on chain of nanoparticles can be controlled by immersing the chain in a nematic liquid crystal and varying the director axis, or by applying a magnetic field.

cond-mat.mes-hall↗

arXiv2011

Photonic band structures of periodic arrays of pores in a metallic host: tight-binding beyond the quasistatic approximation

We have calculated the photonic band structures of metallic inverse opals and of periodic linear chains of spherical pores in a metallic host, below a plasma frequency $ω_{\text{p}}$. In both cases, we use a tight-binding approximation, assuming a Drude dielectric function for the metallic component, but without making the quasistatic approximation. The tight-binding modes are linear combinations of the single-cavity transverse magnetic (TM) modes. For the inverse-opal structures, the lowest modes are analogous to those constructed from the three degenerate atomic p-states in fcc crystals. For the linear chains, in the limit of small spheres compared to a wavelength, the results are the "inverse" of the dispersion relation for metal spheres in an insulating host, as calculated by Brongersma \textit{et al.} [Phys.\ Rev.\ B \textbf{62}, R16356 (2000)]. Because the electromagnetic fields of these modes decay exponentially in the metal, there are no radiative losses, in contrast to the case of arrays of metallic spheres in air. We suggest that this tight-binding approach to photonic band structures of such metallic inverse materials may be a useful approach for studying photonic crystals containing metallic components, even beyond the quasistatic approximation.

cond-mat.mes-hall↗

arXiv2011

Imaging spin properties using spatially varying magnetic fields

We present a novel method to image spin properties of spintronic systems using the spatially confined field of a scanned micromagnetic probe, in conjunction with existing electrical or optical global spin detection schemes. It is thus applicable to all materials systems susceptible to either of those approaches. The proposed technique relies on numerical solutions to the spin diffusion equation in the presence of spatially varying fields to obtain the local spin response to the micromagnetic probe field. These solutions also provide insight into the effects of inhomogeneities on Hanle measurements.

cond-mat.mes-hall↗

arXiv2010

Tunable Band Gap in Graphene with a Non-Centrosymmetric Superlattice Potential

We show that, if graphene is subjected to the potential from an external superlattice, a band gap develops at the Dirac point provided the superlattice potential has broken inversion symmetry. As a numerical example, we calculate the band structure of graphene in the presence of an external potential due to periodically patterned gates arranged in a triangular graphene superlattice (TGS) with broken inversion symmetry, and find that a band gap is created at both the original and "second generation" Dirac point. The gap can be controlled, in principle, by changing the external potential and the lattice constant of the TGS.

cond-mat.mes-hall↗

arXiv2010

Magnonic Crystal with Two-Dimensional Periodicity as a Waveguide for Spin Waves

We describe a simple method of including dissipation in the spin wave band structure of a periodic ferromagnetic composite, by solving the Landau-Lifshitz equation for the magnetization with the Gilbert damping term. We use this approach to calculate the band structure of square and triangular arrays of Ni nanocylinders embedded in an Fe host. The results show that there are certain bands and special directions in the Brillouin zone where the spin wave lifetime is increased by more than an order of magnitude above its average value. Thus, it may be possible to generate spin waves in such composites decay especially slowly, and propagate especially large distances, for certain frequencies and directions in ${\bf k}$-space.

cond-mat.mes-hall↗

arXiv2008

Model for the magnetoresistance and Hall coefficient of inhomogeneous graphene

We show that when bulk graphene breaks into n-type and p-type puddles, the in-plane resistivity becomes strongly field dependent in the presence of a perpendicular magnetic field, even if homoge- neous graphene has a field-independent resistivity. We calculate the longitudinal resistivity ρ_{xx} and Hall resistivity ρ_{xy} as a function of field for this system, using the effective-medium approximation. The conductivity tensors of the individual puddles are calculated using a Boltzmann approach suit- able for the band structure of graphene near the Dirac points. The resulting resistivity agrees well with experiment, provided that the relaxation time is weakly field-dependent. The calculated Hall resistivity has the sign of the majority carrier and vanishes when there are equal number of n and p type puddles.

cond-mat.mes-hall↗

arXiv2008

Model of multiphoton transitions in a current-biased Josephson junction

We present a simple model for multiphoton transitions between the quasi-bound states of a current-driven Josephson junction. The transitions are induced by applying an ac voltage with controllable frequency and amplitude across the junction. The voltage induces transitions when the ac frequency equals n times the splitting between the ground and first excited quasi-bound state of the junction. We calculate the transition matrix elements as functions of the dc bias current I, and the frequency and amplitude of the ac voltage, for representative junction parameters. We also calculate the frequency-dependent absorption coefficient by solving the relevant Bloch equations when the ac amplitude is sufficiently small. In this regime, the absorption coefficient is a sum of Lorentzian lines centered at the n-photon absorption frequency, of strength proportional to the squared matrix elements. For fixed ac voltage amplitude, the n-photon transition rate usually decreases with increasing n. We also find a characteristic even-odd effect: The absorption coefficient typically increases with I for n even but decreases for n odd. Our results agree qualitatively with recent experiments.

cond-mat.supr-con↗

arXiv2008

Numerical Study of Energy Loss by a Nanomechanical Oscillator Coupled to a Cooper Pair Box

We calculate the dynamics of a nanomechanical oscillator (NMO) coupled capacitively to a Cooper pair box (CPB), by solving a stochastic Schrodinger equation with two Lindblad operators. Both the NMO and the CPB are assumed dissipative, and the coupling is treated within the rotating wave approximation. We show numerically that, if the CPB decay time is smaller than the NMO decay time, the coupled NMO will lose energy faster, and the coupled CPB more slowly, than do the uncoupled NMO and CPB. The results show that the efficiency of energy loss by an NMO can be substantially increased if the NMO is coupled to a CPB.

cond-mat.supr-con↗

arXiv2007

Suppression of Tunneling in a Superconducting Persistent-Current Qubit

We consider a superconducting persistent-current qubit consisting of a three-junction superconducting loop in an applied magnetic field. We show that by choosing the field, Josephson couplings, and offset charges suitably, we can perfectly suppress the tunneling between two oppositely directed states of circulating current, leading to a vanishing of the splitting between the two qubit states. The suppression arises for interference between tunneling along different paths, and is analogous to that predicted previously for magnetic particles with half-integer spin.

cond-mat.supr-con↗

arXiv2005

Surface-Enhanced Plasmon Splitting in a Liquid-Crystal-Coated Gold Nanoparticle

We show that, when a gold nanoparticle is coated by a thin layer of nematic liquid crystal, the deformation produced by the nanoparticle surface can enhance the splitting of the nanoparticle surface plasmon. We consider three plausible liquid crystal director configurations in zero electric field: boojum pair (north-south pole configuration), baseball (tetrahedral), and homogeneous. From a calculation using the Discrete Dipole Approximation, we find that the surface plasmon splitting is largest for the boojum pair, intermediate for the homogeneous, and smallest for the baseball configuration. The boojum pair results are in good agreement with experiment. We conclude that the nanoparticle surface has a strong effect on the director orientation, but, surprisingly, that this deformation can actually enhance the surface plasmon splitting.

cond-mat.mtrl-sci↗

arXiv2004

Splitting of Surface Plasmon Frequencies of Metal Particles in a Nematic Liquid Crystal

We calculate the effective dielectric function for a suspension of small metallic particles immersed in a nematic liquid crystal (NLC) host. For a random suspension of such particles in the dilute limit, we calculate the effective dielectric tensor exactly and show that the surface plasmon (SP)resonance of such particles splits into two resonances, polarized parallel and perpendicular to the NLC director. At higher concentrations, we calculate this splitting using a generalized Maxwell-Garnett approximation, which can also be applied to a small metal particle coated with NLC. To confirm the accuracy of the MGA for NLC-coated spheres, we also use the Discrete Dipole Approximation. The calculated splitting is comparable to that observed in recent experiments on NLC-coated small metal particles

cond-mat.mtrl-sci↗

arXiv2004

Ab Initio Studies of Liquid and Amorphous Ge

We review our previous work on the dynamic structure factor S(k,omega) of liquid Ge (l-Ge) at temperature T = 1250 K, and of amorphous Ge (a-Ge) at T = 300 K, using ab initio molecular dynamics [Phys. Rev. B67, 104205 (2003)]. The electronic energy is computed using density-functional theory, primarily in the generalized gradient approximation, together with a plane wave representation of the wave functions and ultra-soft pseudopotentials. We use a 64-atom cell with periodic boundary conditions, and calculate averages over runs of up to 16 ps. The calculated liquid S(k,omega) agrees qualitatively with that obtained by Hosokawa et al, using inelastic X-ray scattering. In a-Ge, we find that the calculated S(k,omega) is in qualitative agreement with that obtained experimentally by Maley et al. Our results suggest that the ab initio approach is sufficient to allow approximate calculations of S(k,omega) in both liquid and amorphous materials.

cond-mat.mtrl-sci↗