Publications (79)202.98 Total impact
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ABSTRACT: Undoped bilayer graphene is a twodimensional semimetal with a lowenergy excitation spectrum that is parabolic in the momentum. As a result, the screening of an arbitrary external charge $Ze$ is accompanied by a reconstruction of the ground state: valence band electrons (for $Z>0$) are promoted to form a space charge around the charge while the holes leave the physical picture. The outcome is flat neutral object resembling the regular atom except that for $Z \gg 1$ it is described by a strictly linear ThomasFermi theory. This theory also predicts that the bilayer's static dielectric constant is the same as that of a twodimensional electron gas in the longwavelength limit.  [Show abstract] [Hide abstract]
ABSTRACT: We present a systematic approach based on the multiple scattering formalism due to Balian and Duplantier [Balian and Duplantier, Ann. Phys. (NY) \textbf{104}, 300 (1977); \textbf{112}, 165 (1978)] for the calculation of the Casimir interaction between arbitrarily shaped smooth conductors. The leading twopoint scattering term of the expansion has a simple compact form and for many geometries captures the bulk of the interaction effect; it is an improvement on the uncontrolled proximity force approximation which can be extended to finite temperatures. The inclusion of terms beyond the twopoint approximation provides an accuracy check and reveals cases where the twopoint approximation is insufficient. We also analyze the anomalous situations involving long cylindrical conductors where the twopoint scattering approximation fails. In such cases summation of the entire scattering series is carried out and a topological argument is put forward as an explanation of the result.  [Show abstract] [Hide abstract]
ABSTRACT: Surface electrons of strong topological insulator wires acquire a Berry phase difference of $\pi$ on orbiting the wire. This can be detected in response of clean wires (whose Fermi level is tuned to the Dirac point) to the presence of the AharonovBohm flux. Specifically, at halfodd integer applied flux (in units of $hc/e$), long wires undergo semimetalsemiconductor transitions characterized by logarithmically divergent susceptibility. Associated with these are oscillations of magnetization (persistent current) that vanish both at integer and halfodd integer flux. Additionally wires of arbitrary aspect ratio exhibit conductance maxima at halfodd integer applied flux and minima at integer flux. For long wires the maxima are sharp with their height approaching $e^{2}/h$. Short wires are characterized by a universal conductivity $e^{2}/\pi h$ attained in the disc limit.  [Show abstract] [Hide abstract]
ABSTRACT: We discuss the formalism of Balian and Duplantier [Balian and Duplantier, Ann. Phys. (NY) 104, 300 (1977); 112, 165 (1978)] for the calculation of the Casimir energy for an arbitrary smooth compact surface and use it to give some examples: a finite cylinder with hemispherical caps, a torus, an ellipsoid of revolution, a cube with rounded corners and edges, and a drum made of disks and part of a torus. We propose a model function that approximately captures the shape dependence of the Casimir energy.  [Show abstract] [Hide abstract]
ABSTRACT: The field of charged impurities in narrowband gap semiconductors and Weyl semimetals can create electronhole pairs when the total charge $Ze$ of the impurity exceeds a value $Z_{c}e$. The particles of one charge escape to infinity, leaving a screening space charge. The result is that the observable dimensionless impurity charge $Q_{\infty}$ is less than $Z$ but greater than $Z_{c}$. There is a corresponding effect for nuclei with $Z >Z_{c} \approx 170$, however in the condensed matter setting we find $Z_{c} \simeq 10$. ThomasFermi theory indicates that $Q_{\infty} = 0$ for the Weyl semimetal, but we argue that this is a defect of the theory. For the case of a highlycharged recombination center in a narrow bandgap semiconductor (or of a supercharged nucleus), the observable charge takes on a nearly universal value. In Weyl semimetals the observable charge takes on the universal value $Q_{\infty} = Z_{c}$ set by the reciprocal of material's fine structure constant.  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the increase in the number of modes (the Kac number) per unit length and the change in the zeropoint energy (the Casimir energy) of the electromagnetic field resulting from the introduction of a thin, perfectly conducting cylindrical shell of elliptical cross section. Along the way we give a route to the calculation of these physical quantities. The Casimir energy is found to be attractive with the circular case corresponding to the energy maximum and the large eccentricity limit being the divergent energy minimum. As a result, with only Casimir stresses present, a fixedarea shell is unstable and might collapse onto itself. This instability is argued to persist at arbitrary temperature. 
Article: Dynamic Infrared ElectroOptic Response of Soluble Organic Semiconductors in Thin Film Transistors
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ABSTRACT: We use a frequencydependent electrooptic technique to measure the hole mobility in small molecule organic semiconductors, such as 6,13 bis(triisopropylsilylethynyl)pentacene. Measurements are made on semiconductor films in bottom gate, bottom contact fieldeffect transistors (FETs.) Because of the buried metal layer effect the maximum response, due to absorption in the charge layer, will be for a dielectric film ~ 1/4 of a wavelength (in the dielectric) (e.g. ~ 1 micron thick in the infrared.) Results are presented for FETs prepared with both spincast polymer and alumina dielectrics prepared by atomic layer deposition. At low frequencies the results are fit to solutions to a nonlinear differential equation describing the spatial dependence of flowing charge in the FET channel, which allows us to study multiple crystals forming across one set of drainsource contacts. FETs prepared on alumina dielectrics show interesting deviations from the model at high frequencies, possibly due to increased contact impedance.  [Show abstract] [Hide abstract]
ABSTRACT: A Coulomb impurity placed in an undoped Weyl semimetal spontaneously surrounds itself with a cloud of condensed Weyl fermions. We find that the groundstate of this system exhibits an experimentally accessible Landau zerocharge effect: the fermion condensate completely screens out the impurity charge. In a narrow band gap semiconducor this effect manifests itself in the near universality of observable charge of a highlycharged recombination center.  [Show abstract] [Hide abstract]
ABSTRACT: A Coulomb impurity placed in an undoped Weyl semimetal spontaneously surrounds itself with a cloud of condensed Weyl fermions. We study this system within the ThomasFermi approximation. We find that the groundstate of the system is electrically neutral and exhibits an experimentally accessible Landau zerocharge effect: the impurity charge is screened out at any finite distance in the limit of vanishing impurity size. Specifically, we show how in this limit the ThomasFermi equation for the electrostatic potential transforms into the GellMannLow equation for the charge.  [Show abstract] [Hide abstract]
ABSTRACT: The local conductivity in the channel of a thinfilm fieldeffect transistor is proportional to the charge density induced by the local gate voltage. We show how this determines the frequency and positiondependence of the charge induced in the channel for the case of "zero applied current": zero drainsource voltage with charge induced by a squarewave voltage applied to the gate, assuming constant mobility and negligible contact impedances. An approximate expression for the frequency dependence of the induced charge in the center of the channel can be conveniently used to determine the charge mobility. Fits of electrooptic measurements of the induced charge in organic transistors are used as examples.  [Show abstract] [Hide abstract]
ABSTRACT: We compute the magnetic response of hollow semimetal cylinders and rings to the presence of an axial AharonovBohm magnetic flux, in the absence of interactions. We predict nullification of the AharonovBohm effect for a class of dispersion laws that includes "nonrelativistic" dispersion and demonstrate that at zero flux the groundstate of a very short "armchair" graphene tube will exhibit a ferromagnetic broken symmetry. We also compute the diamagnetic response of bulk semimetals to the presence of a uniform magnetic field, specifically predicting that the susceptibility has a logarithmic dependence on the size of the sample.  [Show abstract] [Hide abstract]
ABSTRACT: A magnetic flux applied along the axis of a nanotube can counteract the effect of the tube chirality and dramatically affect its conductance, leading to a way to determine the chirality of a nanotube. The effect of the applied flux is strongest in the long tube limit where the conductance is (i) either a sequence of sharp $4e^{2}/h$ height peaks located at integer (in units of the flux quantum) values of the flux (for an armchair tube) or (ii) a periodic sequence of pairs of $2e^{2}/h$ height peaks for a chiral tube, with the spacing determined by the chirality. In the short tube limit the conductance takes on the value that gives the universal conductivity of an undoped graphene sheet, with a small amplitude modulation periodic in the flux.  [Show abstract] [Hide abstract]
ABSTRACT: We compute the generic mode sum that quantifies the effect on the spectrum of a harmonic field when a spherical shell is inserted into vacuum. This encompasses a variety of problems including the Weyl spectral problem and the Casimir effect of quantum electrodynamics. This allows us to resolve several longstanding controversies regarding the question of universality of the Casimir selfenergy; the resolution comes naturally through the connection to the Weyl problem. Specifically we demonstrate that in the case of a scalar field obeying Dirichlet or Neumann boundary conditions on the shell surface the Casimir selfenergy is cutoffdependent while in the case of the electromagnetic field perturbed by a conductive shell the Casimir selfenergy is universal. We additionally show that an analog nonrelativistic Casimir effect due to zeropoint magnons takes place when a nonmagnetic spherical shell is inserted inside a bulk ferromagnet.  [Show abstract] [Hide abstract]
ABSTRACT: We study the energy spectrum for an aperiodic Josephson junction ladder, as a function of frustration. Frustration is brought about by application of a transverse magnetic field, and aperiodicity is imposed by the arrangement of plaquettes with two incommensurate areas. We study the effect of the incommensurate plaquette areas in conjunction with that of the aperiodicity. The structure of the energy spectrum at deep minima is shown to be described by a model that treats the plaquettes independently. The energy spectrum is a quasiperiodic function of frustration; short range correlations in the arrangement of plaquettes have a small effect on the energy power spectrum.  [Show abstract] [Hide abstract]
ABSTRACT: We study an array of inductively coupled Josephson junctions containing a single phaseslip site in the presence of an alternating current. A phaseslip site with reduced critical current can serve as a permanent source of solitary pulses which can synchronize the dynamics of an infinitely large array of junctions. We find that for definite values of parameters, increasing the coupling constant may prevent this synchronization in a system with periodic boundary conditions. This dependence on the boundary condition can be used as a coding mechanism and the information can be retrieved elsewhere in the array.  [Show abstract] [Hide abstract]
ABSTRACT: The Casimir selfenergy of a boundary is ultravioletdivergent. In many cases the divergences can be eliminated by methods such as zetafunction regularization or through physical arguments (ultraviolet transparency of the boundary would provide a cutoff). Using the example of a massless scalar field theory with a single Dirichlet boundary we explore the relationship between such approaches, with the goal of better understanding the origin of the divergences. We are guided by the insight due to Dowker and Kennedy (1978) and Deutsch and Candelas (1979), that the divergences represent measurable effects that can be interpreted with the aid of the theory of the asymptotic distribution of eigenvalues of the Laplacian discussed by Weyl. In many cases the Casimir selfenergy is the sum of cutoffdependent (Weyl) terms having geometrical origin, and an "intrinsic" term that is independent of the cutoff. The Weyl terms make a measurable contribution to the physical situation even when regularization methods succeed in isolating the intrinsic part. Regularization methods fail when the Weyl terms and intrinsic parts of the Casimir effect cannot be clearly separated. Specifically, we demonstrate that the Casimir selfenergy of a smooth boundary in two dimensions is a sum of two Weyl terms (exhibiting quadratic and logarithmic cutoff dependence), a geometrical term that is independent of cutoff, and a nongeometrical intrinsic term. As byproducts we resolve the puzzle of the divergent Casimir force on a ring and correct the sign of the coefficient of linear tension of the Dirichlet line predicted in earlier treatments. Comment: 13 pages, 1 figure, minor changes to the text, extra references added, version to be published in J. Phys. A  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the origin of fractional Shapiro steps in arrays consisting of a few overdamped Josephson junctions. We show that when the symmetry reduces the equations to that of a single junction equation, only integer steps appear. Otherwise, fractional steps will appear when the evolution equations contain second (or higher) order derivatives or nonsinusoidal terms. We make a point of distinguishing the last two possibilities in the generation of the fractional steps.  [Show abstract] [Hide abstract]
ABSTRACT: Casimir forces are a manifestation of the change in the zeropoint energy of the vacuum caused by the insertion of boundaries. We show how the Casimir force can be efficiently computed by consideration of the vacuum fluctuations that are suppressed by the boundaries, and rederive the scalar Casimir effects for a series of the Dirichlet geometries. For the planar case a finite universal force is automatically found. Consistent with other calculations of the effect, for curved geometries divergent (nonuniversal) expressions are encountered. They are interpreted geometrically following Candelas and Deutsch (1979) as largely due to the divergent selfenergy of the boundary contributing to the force. This viewpoint is supported by explicit calculations for a wedgecircular arc geometry in two dimensions where nonuniversal and universal contributions into the effect can be unambiguously separated. We also give a heuristic derivation of the purely geometrical expression (Sen, 1981) for the nonuniversal piece of the Casimir energy due to an arbitrary smooth twodimensional Dirichlet boundary of a compact region.  [Show abstract] [Hide abstract]
ABSTRACT: Casimir forces are a manifestation of the change in the zeropoint energy of the vacuum caused by the insertion of boundaries. We show how the Casimir force can be computed by consideration of the vacuum fluctuations that are suppressed by the boundaries, and rederive the scalar Casimir effects for a series of geometries. For the planar case a finite universal force is automatically found. For curved geometries formally divergent expressions are encountered which we argue are largely due to the divergent selfenergy of the boundary contributing to the force. This idea is supported by computing the effect for a fixed perimeter wedgearc geometry in two dimensions.  [Show abstract] [Hide abstract]
ABSTRACT: We study the dynamics of a triangular singleplaquette Josephsonjunction array in the development of the fractional Shapiro steps. We show that synchronization on fractional steps can happen due to an intricate interplay of the three junctions as the plaquette is made dynamically unsymmetric, either by applying an external magnetic field or by changing the configuration of external currents. We propose a mechanism for synchronization when the asymmetry is only due to the frustration induced by the magnetic field.
Publication Stats
1k  Citations  
202.98  Total Impact Points  
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Institutions

19902014

University of Kentucky
 Department of Physics & Astronomy
Lexington, Kentucky, United States


2001

University of Virginia
 Department of Physics
Charlottesville, Virginia, United States


2000

Michigan State University
East Lansing, Michigan, United States


19982000

University of Alabama
 Department of Physics and Astronomy
Tuscaloosa, AL, United States
