David Stroud - Academia.edu (original) (raw)

Papers by David Stroud

ABSTRACT One-way electromagnetic waveguides are of special interest because of complete suppressi... more ABSTRACT One-way electromagnetic waveguides are of special interest because of complete suppression of back-scattering by disorder. Such waveguides support a unique class of photonic modes that completely forbid propagation in the opposite direction. We show that a one-way electromagnetic waveguide can be realized at the interface of two dissimilar lossless metals in an external magnetic field parallel to the interface. Electromagnetic surface plasmon modes bound to the interface of the two metals and propagating parallel to it and normal to the direction of the external magnetic field, with the electric field polarized normal to the plane of the interface, support one-way electromagnetic propagation in a range of frequencies. Increasing the magnetic field increases the window of frequencies for one-way propagation. Adding damping reduces the range of frequencies. Details of the calculation and plots showing the dispersion relation will be presented.

ABSTRACT Spin transport through graphene is strongly influenced by the presence of adatoms with u... more ABSTRACT Spin transport through graphene is strongly influenced by the presence of adatoms with unpaired spins, such as hydrogen adatoms. In this work, we calculate the local density of states (LDOS) for a simple model of hydrogen on graphene using a tight binding model. The model includes nearest neighbor hopping between carbon atoms, the value of the hydrogen energy level, hopping between the carbon and hydrogen atoms, and a Hubbard U-term to account for the on-site Coulomb interaction. When U = 0, we develop an exact analytic equation for the LDOS on the adatom site, and for the total density of states (DOS). When U !=0, we carry out the same calculation but treat the Hubbard term using mean-field theory. We find that the hydrogen adatom has a net non-integer spin polarization, and that some of the electronic density is transferred from the hydrogen adatom to the graphene host. Possible implications of these results for spin transport through graphene will be discussed.

Physical review. B, Condensed matter, 1994

... TC; (I h) t ci - c (I h) Tj (2 h) T.. *--Tj(2 h) 10 083 50 RF JARDIM, L. BEN-DOR, D. STROUD, ... more ... TC; (I h) t ci - c (I h) Tj (2 h) T.. *--Tj(2 h) 10 083 50 RF JARDIM, L. BEN-DOR, D. STROUD, AND MB MAPLE The ... as low as 5 K.' 5 However, even if the superconducting grain volume is small but there were superconducting regions as large as 5 tim, appreciable diamagnetism ...

Physical Review B Condensed Matter, 1994

We describe a method for treating fluctuations in two-dimensional superconducting films in zero m... more We describe a method for treating fluctuations in two-dimensional superconducting films in zero magnetic field. The method involves expanding the order parameter ψ(x,y) in empty-lattice Wannier functions of a fictitious square lattice. Despite the discrete basis, the order parameter is continuous and has no unphysical pinning. The thermodynamics of the model is a function of a single variable analogous to the Josephson coupling in granular superconductors. We estimate the Kosterlitz-Thouless (KT) transition temperature Tc of the model by Monte Carlo techniques. If amplitude fluctuations are neglected, the model reduces to a partially frustrated XY Hamiltonian, even in zero magnetic field. With amplitude fluctuations, Tc is further reduced, the Coulomb-gas scaling hypothesis of Minnhagen is automatically satisfied, and the jump in superfluid density at the transition may possibly be nonuniversal. Snapshots of ψ(x,y) near Tc reveal the rapid development of pairs of oppositely charged vortices, accompanied by zeros of the order parameter, and, above Tc, by unpaired vortices, in agreement with the original KT picture. The extension of this approach to layered three-dimensional superconductors is briefly discussed.

Eprint Arxiv Supr Con 9506002, Jun 20, 1995

We present the first non-mean-field calculation of the magnetization M\(T\) of YBa2Cu3O7-δ both a... more We present the first non-mean-field calculation of the magnetization M\(T\) of YBa2Cu3O7-δ both above and below the flux-lattice melting temperature Tm\(H\), in good agreement with experiment. Fluctuations in both order parameter ψ\(r\) and magnetic induction B are included in the Ginzburg-Landau free energy. The second derivative \(∂2M/∂T2\)H is predicted to be negative throughout the vortex liquid state and positive in the solid state. The discontinuity in entropy at melting is calculated to be ~0.034kB per flux line per layer at 50 kOe.

Phys Rev B, 1985

We present a density-functional theory of the surface tension of liquid metal alloys. The theory ... more We present a density-functional theory of the surface tension of liquid metal alloys. The theory is applied to miscible alloy NaxK1-x and the results are similar to typical miscible alloys. The phenomenon of segregation of the low-surface-tension component to the surface is obtained with use of a simple hyperbolic tangent concentration profile. An estimation of the surface concentration is made within the monolayer surface model.

Aps Meeting Abstracts, Mar 1, 2005

We present a model for an underdamped long Josephson junction coupled to a single-mode electromag... more We present a model for an underdamped long Josephson junction coupled to a single-mode electromagnetic cavity, and carry out numerical calculations using this model in various regimes. The coupling may occur through either the electric or the magnetic field of the cavity mode. When a current is injected into the junction, we find that the time-averaged voltage exhibits self-induced resonant steps due to coupling between the current in the junction and the electric field 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 flux parallel to the junction plates), the SIRS's disappear if the electric field in the cavity is spatially uniform. If the cavity mode has a spatially varying electric field, 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 and a simple analytical model which account for this behavior. This work was supported by NSF grant DMR04-13395.

We numerically study the superconductor-insulator phase transition in a model disordered 2D super... more We numerically study the superconductor-insulator phase transition in a model disordered 2D superconductor as a function of applied magnetic field. The calculation involves quantum Monte Carlo calculations of the (2+1)D XY model in the presence of both disorder and magnetic field. The XY coupling is assumed to have the form -J\cos(\theta_i-\theta_j-A_{ij}), where A_{ij} has a mean of zero and a standard deviation \Delta A_{ij}. In a real system, such a model would be approximately realized by a 2D array of small Josephson-coupled grains with slight spatial disorder and a uniform applied magnetic field. The different values \Delta A_{ij} then corresponds to an applied field such that the average number of flux quanta per plaquette has various integer values N: larger N corresponds to larger \Delta A_{ij}. For any value of \Delta A_{ij}, there appears to be a critical coupling constant K_c(\Delta A_{ij})=\sqrt{[J/(2U)]_c}, where U is the charging energy, above which the system is a Mott insulator; there is also a corresponding critical conductivity \sigma^*(\Delta A_{ij}) at the transition. For \Delta A_{ij}=\infty, the order parameter of the transition is a renormalized coupling constant g. Using a numerical technique appropriate for disordered systems, we show that the transition at this value of \Delta A_{ij} takes place from an insulating (I) phase to a Bose glass (BG) phase, and that the dynamical critical exponent characterizing this transition is z \sim 1.3. By contrast, z=1 for this model at \Delta A_{ij}=0. We suggest that the superconductor to insulator transition is actually of this I to BG class at all nonzero \Delta A_{ij}'s, and we support this interpretation by both numerical evidence and an analytical argument based on the Harris criterion.

Phys Rev B, 1979

The optical properties of binary composites may be affected by the presence or absence of infinit... more The optical properties of binary composites may be affected by the presence or absence of infinite connected paths of either component. In a model composite of Drude metal and insulator, we find not only that the Drude peak in the real conductivity, Re σeff(ω), disappears below the metal percolation threshold, but also that the metal plasmon peak in the energy-loss function -Im ɛ-1eff(ω) vanishes at the insulator threshold. The integrated strength of the percolation modes is found to vary near the percolation threshold in the limit of small damping, according to the conductivity exponents t and s as defined by Straley. These effects are illustrated by elementary calculations based on the effective medium approximation. Similar phenomena are found in other kinds of composites, and the possibility that these effects may have been observed in polarized transmission experiments is discussed. New sum rules, analogous to those of Bergman, are derived within the quasistatic approximation for Re σeff(ω) and -Im ɛ-1eff(ω). These are used to make statements about the center of gravity of the impurity band in these quantities, and the way in which this is affected by percolation phenomena.

Physical Review B, 2014

We develop a simple theory for the spin-dependent Seebeck effect in n-doped InSb in an external m... more We develop a simple theory for the spin-dependent 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 spin-dependent Seebeck coefficient 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.

Physical Review B Condensed Matter, 1994

Materials Science Forum, 1985

ABSTRACT

We describe a simple method of including dissipation in the spin wave band structure of a periodi... more 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 bfk{\bf k}bfk-space.

The self consistent effective medium approximation (SEMA) is used to study three-dimensional rand... more The self consistent effective medium approximation (SEMA) is used to study three-dimensional random conducting composites under the influence of a strong magnetic field {\bf B}, in the case where all constituents exhibit isotropic response. Asymptotic analysis is used to obtain almost closed form results for the strong field magnetoresistance and Hall resistance in various types of two- and three-constituent isotropic mixtures for the entire range of compositions. Numerical solutions of the SEMA equations are also obtained, in some cases, and compared with those results. In two-constituent free-electron-metal/perfect-insulator mixtures, the magnetoresistance is asymptotically proportional to ∣bfB∣|{\bf B}|∣bfB∣ at {\em all concentrations above the percolation threshold}. In three-constituent metal/insulator/superconductor mixtures a line of critical points is found, where the strong field magnetoresistance switches abruptly from saturating to non-saturating dependence on ∣bfB∣|{\bf B}|∣bfB∣, at a certain value of the insulator-to-superconductor concentration ratio. This transition appears to be related to the phenomenon of anisotropic percolation.

Phys Rev B, 2001

Scaling theory, duality symmetry, and numerical simulations of a random network model are used to... more Scaling theory, duality symmetry, and numerical simulations of a random network model are used to study the magnetoresistance of a metal/insulator/perfect conductor composite with a disordered columnar microstructure. The phase diagram is found to have a critical line which separates regions of saturating and nonsaturating magnetoresistance. The percolation problem which describes this line is a generalization of anisotropic percolation. We locate the percolation threshold and determine the values of the critical exponents t||=t⊥=s||=s⊥=1.30+/-0.02, ν=4/3+/-0.02, which are the same as in two-constituent two-dimensional isotropic percolation. We also determine the exponents which characterize the critical dependence on magnetic field, and confirm numerically that ν is independent of anisotropy. We propose and test a complete scaling description of the magnetoresistance in the vicinity of the critical line.

ABSTRACT One-way electromagnetic waveguides are of special interest because of complete suppressi... more ABSTRACT One-way electromagnetic waveguides are of special interest because of complete suppression of back-scattering by disorder. Such waveguides support a unique class of photonic modes that completely forbid propagation in the opposite direction. We show that a one-way electromagnetic waveguide can be realized at the interface of two dissimilar lossless metals in an external magnetic field parallel to the interface. Electromagnetic surface plasmon modes bound to the interface of the two metals and propagating parallel to it and normal to the direction of the external magnetic field, with the electric field polarized normal to the plane of the interface, support one-way electromagnetic propagation in a range of frequencies. Increasing the magnetic field increases the window of frequencies for one-way propagation. Adding damping reduces the range of frequencies. Details of the calculation and plots showing the dispersion relation will be presented.

ABSTRACT Spin transport through graphene is strongly influenced by the presence of adatoms with u... more ABSTRACT Spin transport through graphene is strongly influenced by the presence of adatoms with unpaired spins, such as hydrogen adatoms. In this work, we calculate the local density of states (LDOS) for a simple model of hydrogen on graphene using a tight binding model. The model includes nearest neighbor hopping between carbon atoms, the value of the hydrogen energy level, hopping between the carbon and hydrogen atoms, and a Hubbard U-term to account for the on-site Coulomb interaction. When U = 0, we develop an exact analytic equation for the LDOS on the adatom site, and for the total density of states (DOS). When U !=0, we carry out the same calculation but treat the Hubbard term using mean-field theory. We find that the hydrogen adatom has a net non-integer spin polarization, and that some of the electronic density is transferred from the hydrogen adatom to the graphene host. Possible implications of these results for spin transport through graphene will be discussed.

Physical review. B, Condensed matter, 1994

... TC; (I h) t ci - c (I h) Tj (2 h) T.. *--Tj(2 h) 10 083 50 RF JARDIM, L. BEN-DOR, D. STROUD, ... more ... TC; (I h) t ci - c (I h) Tj (2 h) T.. *--Tj(2 h) 10 083 50 RF JARDIM, L. BEN-DOR, D. STROUD, AND MB MAPLE The ... as low as 5 K.' 5 However, even if the superconducting grain volume is small but there were superconducting regions as large as 5 tim, appreciable diamagnetism ...

Physical Review B Condensed Matter, 1994

We describe a method for treating fluctuations in two-dimensional superconducting films in zero m... more We describe a method for treating fluctuations in two-dimensional superconducting films in zero magnetic field. The method involves expanding the order parameter ψ(x,y) in empty-lattice Wannier functions of a fictitious square lattice. Despite the discrete basis, the order parameter is continuous and has no unphysical pinning. The thermodynamics of the model is a function of a single variable analogous to the Josephson coupling in granular superconductors. We estimate the Kosterlitz-Thouless (KT) transition temperature Tc of the model by Monte Carlo techniques. If amplitude fluctuations are neglected, the model reduces to a partially frustrated XY Hamiltonian, even in zero magnetic field. With amplitude fluctuations, Tc is further reduced, the Coulomb-gas scaling hypothesis of Minnhagen is automatically satisfied, and the jump in superfluid density at the transition may possibly be nonuniversal. Snapshots of ψ(x,y) near Tc reveal the rapid development of pairs of oppositely charged vortices, accompanied by zeros of the order parameter, and, above Tc, by unpaired vortices, in agreement with the original KT picture. The extension of this approach to layered three-dimensional superconductors is briefly discussed.

Eprint Arxiv Supr Con 9506002, Jun 20, 1995

We present the first non-mean-field calculation of the magnetization M\(T\) of YBa2Cu3O7-δ both a... more We present the first non-mean-field calculation of the magnetization M\(T\) of YBa2Cu3O7-δ both above and below the flux-lattice melting temperature Tm\(H\), in good agreement with experiment. Fluctuations in both order parameter ψ\(r\) and magnetic induction B are included in the Ginzburg-Landau free energy. The second derivative \(∂2M/∂T2\)H is predicted to be negative throughout the vortex liquid state and positive in the solid state. The discontinuity in entropy at melting is calculated to be ~0.034kB per flux line per layer at 50 kOe.

Phys Rev B, 1985

We present a density-functional theory of the surface tension of liquid metal alloys. The theory ... more We present a density-functional theory of the surface tension of liquid metal alloys. The theory is applied to miscible alloy NaxK1-x and the results are similar to typical miscible alloys. The phenomenon of segregation of the low-surface-tension component to the surface is obtained with use of a simple hyperbolic tangent concentration profile. An estimation of the surface concentration is made within the monolayer surface model.

Aps Meeting Abstracts, Mar 1, 2005

We present a model for an underdamped long Josephson junction coupled to a single-mode electromag... more We present a model for an underdamped long Josephson junction coupled to a single-mode electromagnetic cavity, and carry out numerical calculations using this model in various regimes. The coupling may occur through either the electric or the magnetic field of the cavity mode. When a current is injected into the junction, we find that the time-averaged voltage exhibits self-induced resonant steps due to coupling between the current in the junction and the electric field 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 flux parallel to the junction plates), the SIRS's disappear if the electric field in the cavity is spatially uniform. If the cavity mode has a spatially varying electric field, 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 and a simple analytical model which account for this behavior. This work was supported by NSF grant DMR04-13395.

We numerically study the superconductor-insulator phase transition in a model disordered 2D super... more We numerically study the superconductor-insulator phase transition in a model disordered 2D superconductor as a function of applied magnetic field. The calculation involves quantum Monte Carlo calculations of the (2+1)D XY model in the presence of both disorder and magnetic field. The XY coupling is assumed to have the form -J\cos(\theta_i-\theta_j-A_{ij}), where A_{ij} has a mean of zero and a standard deviation \Delta A_{ij}. In a real system, such a model would be approximately realized by a 2D array of small Josephson-coupled grains with slight spatial disorder and a uniform applied magnetic field. The different values \Delta A_{ij} then corresponds to an applied field such that the average number of flux quanta per plaquette has various integer values N: larger N corresponds to larger \Delta A_{ij}. For any value of \Delta A_{ij}, there appears to be a critical coupling constant K_c(\Delta A_{ij})=\sqrt{[J/(2U)]_c}, where U is the charging energy, above which the system is a Mott insulator; there is also a corresponding critical conductivity \sigma^*(\Delta A_{ij}) at the transition. For \Delta A_{ij}=\infty, the order parameter of the transition is a renormalized coupling constant g. Using a numerical technique appropriate for disordered systems, we show that the transition at this value of \Delta A_{ij} takes place from an insulating (I) phase to a Bose glass (BG) phase, and that the dynamical critical exponent characterizing this transition is z \sim 1.3. By contrast, z=1 for this model at \Delta A_{ij}=0. We suggest that the superconductor to insulator transition is actually of this I to BG class at all nonzero \Delta A_{ij}'s, and we support this interpretation by both numerical evidence and an analytical argument based on the Harris criterion.

Phys Rev B, 1979

The optical properties of binary composites may be affected by the presence or absence of infinit... more The optical properties of binary composites may be affected by the presence or absence of infinite connected paths of either component. In a model composite of Drude metal and insulator, we find not only that the Drude peak in the real conductivity, Re σeff(ω), disappears below the metal percolation threshold, but also that the metal plasmon peak in the energy-loss function -Im ɛ-1eff(ω) vanishes at the insulator threshold. The integrated strength of the percolation modes is found to vary near the percolation threshold in the limit of small damping, according to the conductivity exponents t and s as defined by Straley. These effects are illustrated by elementary calculations based on the effective medium approximation. Similar phenomena are found in other kinds of composites, and the possibility that these effects may have been observed in polarized transmission experiments is discussed. New sum rules, analogous to those of Bergman, are derived within the quasistatic approximation for Re σeff(ω) and -Im ɛ-1eff(ω). These are used to make statements about the center of gravity of the impurity band in these quantities, and the way in which this is affected by percolation phenomena.

Physical Review B, 2014

We develop a simple theory for the spin-dependent Seebeck effect in n-doped InSb in an external m... more We develop a simple theory for the spin-dependent 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 spin-dependent Seebeck coefficient 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.

Physical Review B Condensed Matter, 1994

Materials Science Forum, 1985

ABSTRACT

We describe a simple method of including dissipation in the spin wave band structure of a periodi... more 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 bfk{\bf k}bfk-space.

The self consistent effective medium approximation (SEMA) is used to study three-dimensional rand... more The self consistent effective medium approximation (SEMA) is used to study three-dimensional random conducting composites under the influence of a strong magnetic field {\bf B}, in the case where all constituents exhibit isotropic response. Asymptotic analysis is used to obtain almost closed form results for the strong field magnetoresistance and Hall resistance in various types of two- and three-constituent isotropic mixtures for the entire range of compositions. Numerical solutions of the SEMA equations are also obtained, in some cases, and compared with those results. In two-constituent free-electron-metal/perfect-insulator mixtures, the magnetoresistance is asymptotically proportional to ∣bfB∣|{\bf B}|∣bfB∣ at {\em all concentrations above the percolation threshold}. In three-constituent metal/insulator/superconductor mixtures a line of critical points is found, where the strong field magnetoresistance switches abruptly from saturating to non-saturating dependence on ∣bfB∣|{\bf B}|∣bfB∣, at a certain value of the insulator-to-superconductor concentration ratio. This transition appears to be related to the phenomenon of anisotropic percolation.

Phys Rev B, 2001

Scaling theory, duality symmetry, and numerical simulations of a random network model are used to... more Scaling theory, duality symmetry, and numerical simulations of a random network model are used to study the magnetoresistance of a metal/insulator/perfect conductor composite with a disordered columnar microstructure. The phase diagram is found to have a critical line which separates regions of saturating and nonsaturating magnetoresistance. The percolation problem which describes this line is a generalization of anisotropic percolation. We locate the percolation threshold and determine the values of the critical exponents t||=t⊥=s||=s⊥=1.30+/-0.02, ν=4/3+/-0.02, which are the same as in two-constituent two-dimensional isotropic percolation. We also determine the exponents which characterize the critical dependence on magnetic field, and confirm numerically that ν is independent of anisotropy. We propose and test a complete scaling description of the magnetoresistance in the vicinity of the critical line.