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Articles 391 - 420 of 490
Full-Text Articles in Physics
Comment On "Density And Physical Current Density Functional Theory", Xiao-Yin Pan, Viraht Sahni
Comment On "Density And Physical Current Density Functional Theory", Xiao-Yin Pan, Viraht Sahni
Publications and Research
In this letter to the editor, the authors comment on an earlier article they had published, "Density and Physical Current Density Functional Theory" (Pan, X.-Y. and Sahni, V. (2010), Density and physical current density functional theory. Int. J. Quantum Chem., 110: 2833–2843. doi: 10.1002/qua.22862).
Dipolar Interactions, Long Range Order And Random Fields In A Single Molecule Magnet, Mn12-Acetate, Bo Wen
Dipolar Interactions, Long Range Order And Random Fields In A Single Molecule Magnet, Mn12-Acetate, Bo Wen
Dissertations, Theses, and Capstone Projects
In this thesis, I will present an experimental study of two single molecule magnets, Mn12-ac and Mn12-ac-MeOH. I will show that in both systems, the temperature dependence of the inverse susceptibility yields a positive intercept on the temperature axis (a positive Weiss temperature), implying the existence of a ferromagnetic phase at low temperature.
Applying a magnetic field in the transverse direction moves the Weiss temperature downward towards zero. This implies that the transverse field triggers mechanisms in the system that compete with the dipolar interaction and suppress the long-range ordering.
I will then show that the …
Quantum Dynamics Of Vortices In Mesoscopic Magnetic Disks, Ricardo Zarzuela, Eugene M. Chudnovsky, J M. Hernandez, J Tajeda
Quantum Dynamics Of Vortices In Mesoscopic Magnetic Disks, Ricardo Zarzuela, Eugene M. Chudnovsky, J M. Hernandez, J Tajeda
Publications and Research
Model of quantum depinning of magnetic vortex cores from line defects in a disk geometry and under the application of an in-plane magnetic field has been developed within the framework of the Caldeira-Leggett theory. The corresponding instanton solutions are computed for several values of the magnetic field. Expressions for the crossover temperature Tc and for the depinning rate \Gamma(T) are obtained. Fitting of the theory parameters to experimental data is also presented.
Turbulent Fronts Of Quantum Detonation In Molecular Magnets, Dmitry A. Garanin
Turbulent Fronts Of Quantum Detonation In Molecular Magnets, Dmitry A. Garanin
Publications and Research
Dipolar-controlled quantum deflagration going over into quantum detonation in the elongated Mn_12 Ac molecular magnet in a strong transverse field has been considered within the full 3d model. It is shown that within the dipolar window around tunneling resonances the deflagration front is non-flat. With increasing bias, dipolar instability makes the front turbulent, while its speed reaches sonic values, that is a signature of detonation.
Black Hole Formation In Fuzzy Sphere Collapse, Norihiro Izuka, Daniel N. Kabat, Roy Shubho, Debajyoti Sarkar
Black Hole Formation In Fuzzy Sphere Collapse, Norihiro Izuka, Daniel N. Kabat, Roy Shubho, Debajyoti Sarkar
Publications and Research
We study the collapse of a fuzzy sphere, that is a spherical membrane built out of D0-branes, in the BFSS model. At weak coupling, as the sphere shrinks, open strings are produced. If the initial radius is large then open string production is not important and the sphere behaves classically. At intermediate initial radius the back-reaction from open string production is important but the fuzzy sphere retains its identity. At small initial radius the sphere collapses to form a black hole. The crossover between the later two regimes is smooth and occurs at the correspondence point of Horowitz and Polchinski.
On Three Dimensions As The Preferred Dimensionality Of Space Via The Brandenberger-Vafa Mechanism, Brian Greene, Daniel N. Kabat, Stefanos Marnerides
On Three Dimensions As The Preferred Dimensionality Of Space Via The Brandenberger-Vafa Mechanism, Brian Greene, Daniel N. Kabat, Stefanos Marnerides
Publications and Research
In previous work it was shown that, in accord with the Brandenberger-Vafa mechanism, three is the maximum number of spatial dimensions that can grow large cosmologically from an initial thermal fluctuation. Here we complement that work by considering the possibility of successive fluctuations. Suppose an initial fluctuation causes at least one dimension to grow, and suppose successive fluctuations occur on timescales of order alpha'^{1/2}. If the string coupling is sufficiently large, we show that such fluctuations are likely to push a three-dimensional subspace to large volume where winding modes annihilate. In this setting three is the preferred number of large …
Landau-Zener Dynamics Of A Nanoresonator Containing A Tunneling Spin, Michael F. O'Keeffe, Eugene M. Chudnovsky, Dmitry A. Garanin
Landau-Zener Dynamics Of A Nanoresonator Containing A Tunneling Spin, Michael F. O'Keeffe, Eugene M. Chudnovsky, Dmitry A. Garanin
Publications and Research
We study the Landau-Zener dynamics of a tunneling spin coupled to a torsional resonator. For strong spin-phonon coupling, when the oscillator frequency is large compared to the tunnel splitting, the system exhibits multiple Landau-Zener transitions. Entanglement of spin and mechanical angular momentum results in abrupt changes of oscillator dynamics which coincide in time with spin transitions. We show that a large number of spins on a single oscillator coupled only through the in-phase phonon field behaves as a single large spin, greatly enhancing the spin-phonon coupling. We compare purely quantum and semiclassical dynamics of the system and discuss their experimental …
Mutual Information In Hawking Radiation, Norihiro Izuka, Daniel N. Kabat
Mutual Information In Hawking Radiation, Norihiro Izuka, Daniel N. Kabat
Publications and Research
We compute the mutual information of two Hawking particles emitted consecutively by an evaporating black hole. Following Page, we find that the mutual information is of order exp(-S) where S is the entropy of the black hole. We speculate on implications for black hole unitarity, in particular on a possible failure of locality at large distances.
Black Hole Formation At The Correspondence Point, Norihiro Izuka, Daniel N. Kabat, Roy Shubho, Debajyoti Sarkar
Black Hole Formation At The Correspondence Point, Norihiro Izuka, Daniel N. Kabat, Roy Shubho, Debajyoti Sarkar
Publications and Research
We study the process of bound state formation in a D-brane collision. We consider two mechanisms for bound state formation. The first, operative at weak coupling in the worldvolume gauge theory, is pair creation of W-bosons. The second, operative at strong coupling, corresponds to formation of a large black hole in the dual supergravity. These two processes agree qualitatively at intermediate coupling, in accord with the correspondence principle of Horowitz and Polchinski. We show that the size of the bound state and timescale for formation of a bound state agree at the correspondence point. The timescale involves matching a parametric …
Cft Representation Of Interacting Bulk Gauge Fields In Ads, Daniel N. Kabat, Gilad Lifschytz
Cft Representation Of Interacting Bulk Gauge Fields In Ads, Daniel N. Kabat, Gilad Lifschytz
Publications and Research
We develop the representation of interacting bulk gauge fields and charged scalar matter in AdS in terms of non-local observables in the dual CFT. We work in holographic gauge in the bulk, A_z = 0. The correct statement of micro-causality in holographic gauge is somewhat subtle, so we first discuss it from the bulk point of view. We then show that in the 1/N expansion CFT correlators can be lifted to obtain bulk correlation functions which satisfy micro-causality. This requires adding an infinite tower of higher-dimension multi-trace operators to the CFT definition of a bulk observable. For conserved currents the …
Reversal Of Magnetization Of A Single-Domain Magnetic Particle By The Ac Field Of Time-Dependent Frequency, Liufei Cai, Dmitry A. Garanin, Eugene M. Chudnovsky
Reversal Of Magnetization Of A Single-Domain Magnetic Particle By The Ac Field Of Time-Dependent Frequency, Liufei Cai, Dmitry A. Garanin, Eugene M. Chudnovsky
Publications and Research
We report numerical and analytical studies of the reversal of the magnetic moment of a single- domain magnetic particle by a circularly polarized ac field of time-dependent frequency. For the time-linear frequency sweep, the phase diagrams are computed that illustrate the dependence of the reversal on the frequency sweep rate v , the amplitude of the ac field h , the magnetic anisotropy field d , and the damping parameter . It is shown that the most efficient magnetization reversal requires a non-linear time dependence of the frequency, ! ( t ) , for which an exact analytical formula is …
Excitation Modes Of Vortices In Submicron Magnetic Disks, Ricardo Zarzuela, Eugene M. Chudnovsky, Javier Tejada
Excitation Modes Of Vortices In Submicron Magnetic Disks, Ricardo Zarzuela, Eugene M. Chudnovsky, Javier Tejada
Publications and Research
Classical and quantum theory of spin waves in the vortex state of a mesoscopic sub-micron magnetic disk has been developed with account of the finite mass density of the vortex. Oscillations of the vortex core resemble oscillations of a charged string in a potential well in the presence of the magnetic field. Conventional gyroscopic frequency appears as a gap in the spectrum of spin waves of the vortex. The mass of the vortex has been computed that agrees with experimental findings. Finite vortex mass generates a high-frequency branch of spin waves. Effects of the external magnetic field and dissipation have …
Diffractive Effects And General Boundary Conditions In Casimir Energy, Dimitra Karabali, V P. Nair
Diffractive Effects And General Boundary Conditions In Casimir Energy, Dimitra Karabali, V P. Nair
Publications and Research
The effect of edges and apertures on the Casimir energy of an arrangement of plates and boundaries can be calculated in terms of an effective nonlocal lower-dimensional field theory that lives on the boundary. This formalism has been developed in a number of previous papers and applied to specific examples with Dirichlet boundary conditions. Here we generalize the formalism to arbitrary boundary conditions. As a specific example, the geometry of a flat plate and a half-plate placed parallel to it is considered for a number of different boundary conditions and the area-dependent and edge dependent contributions to the Casimir energy …
Hohenberg-Kohn Theorem Including Electron Spin, Xiao-Yin Pan, Viraht Sahni
Hohenberg-Kohn Theorem Including Electron Spin, Xiao-Yin Pan, Viraht Sahni
Publications and Research
The Hohenberg-Kohn theorem is generalized to the case of a finite system of N electrons in external electrostatic epsilon(r) = -del nu(r) and magnetostatic B(r) = del x A(r) fields in which the interaction of the latter with both the orbital and spin angular momentum is considered. For a nondegenerate ground state a bijective relationship is proved between the gauge invariant density rho(r) and physical current density j(r) and the potentials {nu(r), A(r)}. The possible many-to-one relationship between the potentials {v(r), A(r)} and the wave function is explicitly accounted for in the proof. With the knowledge that the basic variables …
The Quantum Effective Action, Wave Functions And Yang-Mills(2+1), V. Parameswaran Nair
The Quantum Effective Action, Wave Functions And Yang-Mills(2+1), V. Parameswaran Nair
Publications and Research
No abstract provided.
Hohenberg-Kohn And Percus-Levy-Lieb Proofs Of Density-Functional Theory, Viraht Sahni, Xiao-Yin Pan
Hohenberg-Kohn And Percus-Levy-Lieb Proofs Of Density-Functional Theory, Viraht Sahni, Xiao-Yin Pan
Publications and Research
The premise of density-functional theory is that knowledge of the ground-state density uniquely determines the Hamiltonian, and thereby, via solution of the corresponding time-independent Schrodinger equation, all the properties of the system. The density therefore constitutes a basic variable of quantum mechanics. There are at present two paths from the density to the Hamiltonian: the Hohenberg and Kohn proof of the bijectivity between the external potential and the basic variable, and the Percus, Levy, and Lieb constrained-search proof. We argue the Hohenberg- and Kohn-type proof to be the more fundamental, and that this is the case in general when both …
Demonstration Of The Gunnarsson-Lundqvist Theorem And The Multiplicity Of Potentials For Excited States, Yu-Qi Li, Xiao-Yin Pan, Biao Li, Viraht Sahni
Demonstration Of The Gunnarsson-Lundqvist Theorem And The Multiplicity Of Potentials For Excited States, Yu-Qi Li, Xiao-Yin Pan, Biao Li, Viraht Sahni
Publications and Research
The Gunnarsson-Lundqvist (GL) theorem of density functional theory states that there is a one-to-one relationship between the density of the lowest nondegenerate excited state of a given symmetry and the external potential. As a consequence, knowledge of this excited state density determines the external potential uniquely. [The GL theorem is the equivalent for such excited states of theHohenberg-Kohn (HK) theorem for nondegenerate ground states.] For other excited states, there is no equivalent of the GL or HK theorem. For these states, there thus exist multiple potentials that generate the excited-state density. We show, by example, the satisfaction that the GL …
Advances In Ultrafast Time Resolved Fluorescence Physics For Cancer Detection In Optical Biopsy, R. R. Alfano
Advances In Ultrafast Time Resolved Fluorescence Physics For Cancer Detection In Optical Biopsy, R. R. Alfano
Publications and Research
We discuss the use of time resolved fluorescence spectroscopy to extract fundamental kinetic information on molecular species in tissues. The temporal profiles reveal the lifetime and amplitudes associated with key active molecules distinguishing the local spectral environment of tissues. The femtosecond laser pulses at 310 nm excite the tissue. The emission profile at 340 nm from tryptophan is non-exponential due to the micro-environment. The slow and fast amplitudes and lifetimes of emission profiles reveal that cancer and normal states can be distinguished. Time resolved optical methods offer a new cancer diagnostic modality for the medical community.
Aligned Layers Of Silver Nano-Fibers, Andrii B. Golovin, Jeremy Stromer, Liubov Kreminska
Aligned Layers Of Silver Nano-Fibers, Andrii B. Golovin, Jeremy Stromer, Liubov Kreminska
Publications and Research
We describe a new dichroic polarizers made by ordering silver nano-fibers to aligned layers. The aligned layers consist of nano-fibers and self-assembled molecular aggregates of lyotropic liquid crystals. Unidirectional alignment of the layers is achieved by means of mechanical shearing. Aligned layers of silver nano-fibers are partially transparent to a linearly polarized electromagnetic radiation. The unidirectional alignment and density of the silver nano-fibers determine degree of polarization of transmitted light. The aligned layers of silver nano-fibers might be used in optics, microwave applications, and organic electronics.
Yousef Studies The Physics Of Life And Health, Aldemaro Romero Jr.
Yousef Studies The Physics Of Life And Health, Aldemaro Romero Jr.
Publications and Research
No abstract provided.
Hamad Studies, Teaches Properties Of Light, Aldemaro Romero Jr.
Hamad Studies, Teaches Properties Of Light, Aldemaro Romero Jr.
Publications and Research
No abstract provided.
Casimir Effect: Edges And Diffraction, Dimitra Karabali
Casimir Effect: Edges And Diffraction, Dimitra Karabali
Publications and Research
The Casimir effect refers to the existence of a macroscopic force between conducting plates in vacuum due to quantum fluctuations of fields. These forces play an important role, among other things, in the design of nano-scale mechanical devices. Accurate experimental observations of this phenomenon have motivated the development of new theoretical approaches in dealing with the effects of different geometries, temperature etc. In this talk, I will focus on a new method we have developed in calculating the contribution to the Casimir effect due to diffraction from edges and holes in different geometries, at zero and at finite temperature.
Optomechanics Of Cavity Driven Nanoparticles, Joel T. Rubin
Optomechanics Of Cavity Driven Nanoparticles, Joel T. Rubin
Dissertations, Theses, and Capstone Projects
The subject of this thesis is the opto-mechanical interaction of a spherical high-Q microresonator and a subwavelength particle, which, at optical wavelengths, corresponds to a size on the order of nanometers. After a review of the basic theory of spherical resonators and multi-sphere scattering, the full self-consistent electromagnetic field of the coupled resonator-particle system is derived. The particle-induced frequency shift and broadening is calculated by examining the poles of the scattering coefficients of the resonator. The force exerted on the particle by the field is determined via the Maxwell stress tensor, and is found to be in general non-conservative. From …
Aperture Array Photonic Metamaterials: Theoretical Approaches, Numerical Techniques And A Novel Application, Eli Lansey
Aperture Array Photonic Metamaterials: Theoretical Approaches, Numerical Techniques And A Novel Application, Eli Lansey
Dissertations, Theses, and Capstone Projects
Optical or photonic metamaterials that operate in the infrared and visible frequency regimes show tremendous promise for solving problems in renewable energy, infrared imaging, and telecommunications. However, many of the theoretical and simulation techniques used at lower frequencies are not applicable to this higher-frequency regime. Furthermore, technological and financial limitations of photonic metamaterial fabrication increases the importance of reliable theoretical models and computational techniques for predicting the optical response of photonic metamaterials.
This thesis focuses on aperture array metamaterials. That is, a rectangular, circular, or other shaped cavity or hole embedded in, or penetrating through a metal film. The research …
Quantum Tunneling Of The Magnetic Moment In A Free Nanoparticle, Michael F. O'Keeffe, Eugene M. Chudnovsky, Dmitry A. Garanin
Quantum Tunneling Of The Magnetic Moment In A Free Nanoparticle, Michael F. O'Keeffe, Eugene M. Chudnovsky, Dmitry A. Garanin
Publications and Research
We study tunneling of the magnetic moment in a particle that has full rotational freedom. Exact energy levels are obtained and the ground-state magnetic moment is computed for a symmetric rotor. The effect of the mechanical freedom on spin tunneling manifests itself in a strong dependence of the magnetic moment on the moments of inertia of the rotor. Energy of the particle exhibits quantum phase transitions between states with different values of the magnetic moment. Particles of various shapes are investigated and quantum phase diagram is obtained.
Diffuse Optical Imaging Using Decomposition Methods, Binlin Wu, M. Alrubaiee, W. Cai, M. Xu, S. K. Gayen
Diffuse Optical Imaging Using Decomposition Methods, Binlin Wu, M. Alrubaiee, W. Cai, M. Xu, S. K. Gayen
Publications and Research
Diffuse optical imaging (DOI) for detecting and locating targets in a highly scattering turbid medium is treated as a blind source separation (BSS) problem. Three matrix decomposition methods, independent component analysis (ICA), principal component analysis (PCA), and nonnegative matrix factorization (NMF) were used to study the DOI problem. The efficacy of resulting approaches was evaluated and compared using simulated and experimental data. Samples used in the experiments included Intralipid-10% or Intralipid-20% suspension in water as the medium with absorptive or scattering targets embedded.
Fluids, Anomalies And The Chiral Magnetic Effect: A Group-Theoretic Formulation, V. Parameswaran Nair, Rashmi Ray, Shubho Roy
Fluids, Anomalies And The Chiral Magnetic Effect: A Group-Theoretic Formulation, V. Parameswaran Nair, Rashmi Ray, Shubho Roy
Publications and Research
It is possible to formulate fluid dynamics in terms of group-valued variables. This is particularly suited to the cases where the fluid has nonabelian charges and is coupled to nonabelian gauge fields. We explore this formulation further in this paper. An action for a fluid of relativistic particles (with and without spin) is given in terms of the Lorentz and Poincare (or de Sitter) groups. Considering the case of particles with flavor symmetries, a general fluid action which also incorporates all flavor anomalies is given. The chiral magnetic and chiral vorticity effects as well as the consequences of the mixed …
Conservation Of Angular Momentum In A Flux Qubit, Eugene M. Chudnovsky, Dmitry A. Garanin, Michael F. O'Keeffe
Conservation Of Angular Momentum In A Flux Qubit, Eugene M. Chudnovsky, Dmitry A. Garanin, Michael F. O'Keeffe
Publications and Research
Oscillations of superconducting current between clockwis e and counterclockwise directions in a flux qubit do not conserve the angular momentum of the qubit. T o compensate for this effect the solid containing the qubit must oscillate in unison with the current. This requires entanglement of quantum states of the qubit with quantum states of a macrosco pic body. The question then arises whether slow decoherence of quantum oscillations of the cur rent is consistent with fast decoherence of quantum states of a macroscopic solid. This problem is ana lyzed within an exactly solvable quantum model of a qubit embedded in …
Collapse Of Skyrmions In Two-Dimensional Ferromagnets And Antiferromagnets, Luifei Cai, Eugene M. Chudnovsky, Dmitry A. Garanin
Collapse Of Skyrmions In Two-Dimensional Ferromagnets And Antiferromagnets, Luifei Cai, Eugene M. Chudnovsky, Dmitry A. Garanin
Publications and Research
Collapse of a skyrmion due to the discreteness of a crystal lattice in isotropic two-dimensional ferro- and antiferromagnets has been studied analytically and by numerical solution of equations of motion for up to 2000 x 2000 classical spins on a square lattice coupled via Heisenberg exchange interaction. Excellent agreement between analytical and numerical results has been achieved. The lifetime of the skyrmion scales with its initial size, lambda_0, as(lambda_0/a)^5 in ferromagnets and as (lambda_0/a)^2.15 in antiferromagnets, with a being the lattice parameter. This makes antiferromagnetic skyrmions significantly shorter lived than ferromagnetic skyrmions.
Dipolar-Controlled Spin Tunneling And Relaxation In Molecular Magnets, Dmitry A. Garanin
Dipolar-Controlled Spin Tunneling And Relaxation In Molecular Magnets, Dmitry A. Garanin
Publications and Research
Spin tunneling in molecular magnets controlled by dipole-dipole interactions (DDI) in the disordered state has been considered numerically on the basis of the microscopic model using the quantum mean-field approximation. In the actual case of a strong DDI spin coherence is completely lost and there is a slow relaxation of magnetization, described by t^{3/4} at short times. Fast precessing nuclear spins, included in the model microscopically, only moderately speed up the relaxation.