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Articles 61 - 87 of 87
Full-Text Articles in Physical Sciences and Mathematics
Modeling An Electrosensory Landscape: Behavioral And Morphological Optimization In Elasmobranch Prey Capture, Brandon R. Brown
Modeling An Electrosensory Landscape: Behavioral And Morphological Optimization In Elasmobranch Prey Capture, Brandon R. Brown
Physics and Astronomy
Most biological sensory systems benefit from multiple sensors. Elasmobranchs (sharks, skates and rays) possess an array of electroreceptive organs that facilitate prey location, mate location and navigation. Here, the perceived electrosensory landscape for an elasmobranch approaching prey is mathematically modeled. The voltages that develop simultaneously in dozens of separate sensing organs are calculated using electrodynamics. These voltages lead directly to firing rate modifications in the primary afferent nerves. The canals connecting the sense organs to an elasmobranch's surface exhibit great variation of location and orientation. Here, the voltages arising in the sense organs are found to depend strongly on the …
Quantum Anomaly In Molecular Physics, Horacio E. Camblong
Quantum Anomaly In Molecular Physics, Horacio E. Camblong
Physics and Astronomy
The interaction of an electron with a polar molecule is shown to be the simplest realization of a quantum anomaly in a physical system. The existence of a critical dipole moment for electron capture and formation of anions, which has been confirmed experimentally and numerically, is derived. This phenomenon is a manifestation of the anomaly associated with quantum symmetry breaking of the classical scale invariance exhibited by the point-dipole interaction. Finally, analysis of symmetry breaking for this system is implemented within two different models: point dipole subject to an anomaly and finite dipole subject to explicit symmetry breaking.
Renormalization Of The Inverse Square Potential, Horacio E. Camblong
Renormalization Of The Inverse Square Potential, Horacio E. Camblong
Physics and Astronomy
The quantum-mechanical D-dimensional inverse square potential is analyzed using field-theoretic renormalization techniques. A solution is presented for both the bound-state and scattering sectors of the theory using cutoff and dimensional regularization. In the renormalized version of the theory, there is a strong-coupling regime where quantum-mechanical breaking of scale symmetry takes place through dimensional transmutation, with the creation of a single bound state and of an energy-dependent s-wave scattering matrix element.
Heating And Ionization Of The Intergalactic Medium By An Early X-Ray Background, Aparna Venkatesan, Mark L. Giroux, J Shull
Heating And Ionization Of The Intergalactic Medium By An Early X-Ray Background, Aparna Venkatesan, Mark L. Giroux, J Shull
Physics and Astronomy
Observational studies indicate that the intergalactic medium (IGM) is highly ionized up to redshifts just over 6. A number of models have been developed to describe the process of reionization and the effects of the ionizing photons from the first luminous objects. In this paper we study the impact of an X-ray background, such as high-energy photons from early quasars, on the temperature and ionization of the IGM prior to reionization, before the fully ionized bubbles associated with individual sources have overlapped. X-rays have large mean free paths relative to EUV photons, and their photoelectrons can have significant effects on …
Laser Stabilization At 1536 Nm Using Regenerative Spectral Hole Burning, P B. Sellin, N M. Strickland, Thomas Böttger, J L. Carlsten, R L. Cone
Laser Stabilization At 1536 Nm Using Regenerative Spectral Hole Burning, P B. Sellin, N M. Strickland, Thomas Böttger, J L. Carlsten, R L. Cone
Physics and Astronomy
Laser frequency stabilization giving a 500-Hz Allan deviation for a 2-ms integration time with drift reduced to 7 kHz/min over several minutes was achieved at 1536 nm in the optical communication band. A continuously regenerated spectral hole in the inhomogeneously broadened 4I15/2(1)!4I13/2(1) optical absorption of an Er31:Y2SiO5 crystal was used as the short-term frequency reference, while a variation on the locking technique allowed simultaneous use of the inhomogeneously broadened absorption line as a long-term reference. The reported frequency stability was achieved without vibration isolation. Spectral hole burning frequency stabilization provides ideal laser sources for high-resolution spectroscopy, real-time optical signal processing, …
A Theory For Stretchiness, Brandon R. Brown
A Theory For Stretchiness, Brandon R. Brown
Physics and Astronomy
No abstract available.
Focus: New Info From Slow Muons, Brandon R. Brown
Focus: New Info From Slow Muons, Brandon R. Brown
Physics and Astronomy
No abstract available.
A Spectroscopic Redshift For The Cl 0024+16 Multiple Arc System: Implications For The Central Mass Distribution, Tom Broadhurst, Xiaosheng Huang, Brenda Frye, Richard Ellis
A Spectroscopic Redshift For The Cl 0024+16 Multiple Arc System: Implications For The Central Mass Distribution, Tom Broadhurst, Xiaosheng Huang, Brenda Frye, Richard Ellis
Physics and Astronomy
We present a spectroscopic redshift of z = 1.675 for the well-known multiply lensed system of arcs seen in the z = 0.39 cluster Cl 0024+16. In contrast to earlier work, we find that the lensed images are accurately reproduced by a projected mass distribution which traces the locations of the brightest cluster elliptical galaxies, suggesting that the most significant minima of the cluster potential are not dynamically erased. The averaged mass profile is shallow and consistent with predictions of recent numerical simulations. The source redshift enables us to determine an enclosed cluster mass of M(<100 kpc h-1) = …
Herding Molecules, Brandon R. Brown
Simulation Of Dc Resistivity Data: Questioning Critical Scaling For The High-Tc Copper Oxides, Brandon R. Brown
Simulation Of Dc Resistivity Data: Questioning Critical Scaling For The High-Tc Copper Oxides, Brandon R. Brown
Physics and Astronomy
We simulate dc V-I data using Gaussian distributions of depinning currents for the mixed state of a copper-oxide superconductor and subsequently submit the data to a critical scaling analysis. Though the simple model for simulation assumes no phase transition for the vortex matter, the generated data exhibit a collapse matching those in the experimental literature. The resulting would-be critical parameters depend on the simulated sensitivity level, reinforcing doubts concerning the utility of the scaling analysis for these systems.
The Optical Depth To Reionization As A Probe Of Cosmological And Astrophysical Parameters, Aparna Venkatesan
The Optical Depth To Reionization As A Probe Of Cosmological And Astrophysical Parameters, Aparna Venkatesan
Physics and Astronomy
Current data of high-redshift absorption-line systems imply that hydrogen reionization occurred before redshifts of about 5. Previous works on reionization by the first stars or quasars have shown that such scenarios are described by a large number of cosmological and astrophysical parameters. Here we adopt a semianalytic model of stellar reionization in order to quantify how the optical depth to reionization depends on such parameters, and combine this with constraints from the cosmic microwave background (CMB). We find this approach to be particularly useful in alleviating the well-known degeneracy in CMB parameter extraction between the optical depth to reionization and …
Electrical Resistivity Of A Thin Metallic Film, Horacio E. Camblong
Electrical Resistivity Of A Thin Metallic Film, Horacio E. Camblong
Physics and Astronomy
The electrical resistivity of a pure sample of a thin metallic film is found to depend on the boundary conditions. This conclusion is supported by a free-electron model calculation and confirmed by an ab initio relativistic Korringa-Kohn-Rostoker computation. The low-temperature resistivity is found to be zero for a free-standing film (reflecting boundary conditions) but nonzero when the film is sandwiched between two semi-infinite samples of the same material (outgoing boundary conditions). In the latter case, this resistivity scales inversely with the number of monolayers and is due to the background diffusive scattering by a finite lattice.
Neutron Stars And Black Holes As Machos, Aparna Venkatesan, Angela V. Olinto, James W. Truran
Neutron Stars And Black Holes As Machos, Aparna Venkatesan, Angela V. Olinto, James W. Truran
Physics and Astronomy
We consider the contribution of neutron stars and black holes to the dynamical mass of galactic halos. In particular, we show that if these compact objects were produced by an early generation of stars with initial metallicity 10-4 Z☉, they can contribute at most 30%-40% of the Galactic halo mass without creating supersolar levels of enrichment. We show that the case for halo neutron stars and black holes cannot be rejected on metal overproduction arguments alone because of the critical factor of the choice of progenitor metallicity in determining the yields. We show that this scenario satisfies …
Neutron-Irradiation Effects On The V-I Characteristics Of Yba2cu3o7-Δ Twinned Crystals:Linking Transport Results In A Variety Of Copper Oxide Superconductors, Brandon R. Brown
Neutron-Irradiation Effects On The V-I Characteristics Of Yba2cu3o7-Δ Twinned Crystals:Linking Transport Results In A Variety Of Copper Oxide Superconductors, Brandon R. Brown
Physics and Astronomy
We have investigated the depinning process of magnetic flux vortices in twinned crystals of YBa2Cu3O7-δ in intermediate applied magnetic fields parallel to the crystallographic c axis through precise resistivity measurements before and after fast-neutron irradiation. Of particular interest for each field is the power-law dependence of the electric field on current density, E∝Jn, at a well defined temperature which separates a nonlinear region below from a more linear, dissipative region above. We find that n shows a striking dependence on the estimated value of the Larkin-Ovchinnikov length. The relation is not only manifest …
Constraints On The Production Of Ultra–High-Energy Cosmic Rays By Isolated Neutron Stars, Aparna Venkatesan, M Coleman Miller, Angela V. Olinto
Constraints On The Production Of Ultra–High-Energy Cosmic Rays By Isolated Neutron Stars, Aparna Venkatesan, M Coleman Miller, Angela V. Olinto
Physics and Astronomy
The energetics, spectrum, and composition of cosmic rays with energies below ~1015 eV are fairly well explained by models involving supernova shocks. In contrast, no widely accepted theory exists for the origin of ultra-high-energy cosmic rays (UHECRs), which have energies above 1015 eV. Instead of proposing a specific model, here we place strong constraints on any model of UHECRs involving isolated neutron stars (no companions). We consider the total power requirements and show that the only viable power source associated with isolated neutron stars is rotation. Mechanisms based on accretion from the interstellar medium fall short of the …
Oblique Electron Transport In The Presence Of Collinear And Noncollinear Magnetizations, Horacio E. Camblong
Oblique Electron Transport In The Presence Of Collinear And Noncollinear Magnetizations, Horacio E. Camblong
Physics and Astronomy
The nonlocal real‐space Kubo approach to electron transport in magnetic multilayers is applied to a new geometry, ‘‘oblique transport,’’ whose complexity is traced back to a combination of the layering, of the nonlocal character of the linear response, and of the oblique direction transport in this new geometry. The problem is dealt with by applying a condition on the average current density vector or on the average electric‐field vector, depending upon the external driving conditions. Its solution exhibits a characteristic anisotropy and it yields the global oblique conductance and magnetoresistance as simple trigonometric expressions in terms of the in‐plane and …
Type-Ii Interface Exciton In Znse/(Zn,Mn)Se Heterostructures, V V. Rossin, Thomas Böttger, F Henneberger
Type-Ii Interface Exciton In Znse/(Zn,Mn)Se Heterostructures, V V. Rossin, Thomas Böttger, F Henneberger
Physics and Astronomy
Two emission bands are observed in the photoluminescence spectrum of ZnSe/~Zn,Mn!Se heterostructures. These bands emerge in a magnetic field and are associated with interface excitons formed as a result of a magnetic-field-induced type-I–type-II transition of the band alignment. Time-resolved measurements yield lifetimes in the ns range signifying a relatively large spatial separation of electrons and holes and hence low optical oscillator strength. These features are confirmed by a theoretical analysis of the interface exciton state revealing a reduced importance of the electron-hole Coulomb interaction in comparison with type-II excitons in quantum wells.
Scaling Of Thin-Film Nd1.85ce0.15cuo4-Y Resistivity-Current Isotherms At Low Fields: Implications For Vortex Phase Transitions And Universality, Brandon R. Brown
Scaling Of Thin-Film Nd1.85ce0.15cuo4-Y Resistivity-Current Isotherms At Low Fields: Implications For Vortex Phase Transitions And Universality, Brandon R. Brown
Physics and Astronomy
We have investigated a second-order phase transition of the vortex lattice in thin-film Nd1.85Ce0.15CuO4-y from 1 mT to 1 T applied magnetic fields. The dc resistivity-current density data for each field exhibit critical scaling consistent with a second-order vortex-liquid to vortex-solid phase transition. The high-field data support the vortex-glass model and have the same critical exponents as thin-film YBa2Cu3O7-δ. At low fields we observe a change in the critical exponents and universal functions. The critical regime diminishes as the field is decreased but remains nonzero even at 1 mT. …
Electron Transport In Magnetic Inhomogeneous Media, Horacio E. Camblong
Electron Transport In Magnetic Inhomogeneous Media, Horacio E. Camblong
Physics and Astronomy
Giant magnetoresistance has been observed in both magnetic multilayers and magnetic granular solids. We develop a framework for unifying these particular realizations of inhomogeneous magnetic media, based on the real-space Kubo formula. It constitutes a spin-dependent form of linear response theory, associated with internal spin-dependent fields arising from spin accumulation; moreover, we discuss the physical meaning of these spin dependences. For magnetic multilayers we discuss the particular cases of collinear and noncollinear configurations, and we consider limiting cases of the elastic mean-free path to inhomogeneity-scale ratio for granular solids. Furthermore, we introduce the concept of magnetically self-averaging systems, which include …
Linear Transport Theory Of Magnetoconductance In Metallic Multilayers: A Real-Space Approach, Horacio E. Camblong
Linear Transport Theory Of Magnetoconductance In Metallic Multilayers: A Real-Space Approach, Horacio E. Camblong
Physics and Astronomy
The linear transport behavior of metallic multilayers is studied by using the real-space Kubo formula, for a model Hamiltonian consisting of zero-range spin-dependent impurity-scattering potentials. The resulting theory allows a direct comparison with the Boltzmann equation approach in the quasiclassical limit, which is expected to be a good approximation for most magnetic multilayers. Furthermore, the regimes for which quantum corrections might be needed are explicitly indicated. It is explicitly shown that: (i) periodicity is not required as a mechanism for giant magnetoresistance, and (ii) the two main geometries, current in the plane of the layers and perpendicular to the plane …
Scaling Of Voltage-Current Characteristics Of Thin-Film Y-Ba-Cu-0 At Low Magnetic Fields, Brandon R. Brown
Scaling Of Voltage-Current Characteristics Of Thin-Film Y-Ba-Cu-0 At Low Magnetic Fields, Brandon R. Brown
Physics and Astronomy
We have measured voltage-current characteristics for YBa2Cu3O7 thin films in magnetic fields from 5 T to ambient, including some in the millitesla range. In all cases, the resistivity-current density isotherms can be separated into two classes: those which exhibit upward curbature, and have constant resistivity at low currents, and those which exhibit downward curvature at all currents. These two classes of isotherms are separated by a field-dependent temperture Tg. For each field, the isotherms scale in a manner consistent with a three dimensional vortex-liquid to vortex-glass phase transition. The region that can be scaled is several kelving at 5 T …
Effective Internal Fields And Magnetization Buildup For Magnetotransport In Magnetic Multilayered Structures, Horacio E. Camblong
Effective Internal Fields And Magnetization Buildup For Magnetotransport In Magnetic Multilayered Structures, Horacio E. Camblong
Physics and Astronomy
Starting with the Kubo formula for electric conductivity we derive a set of equations that define the spin diffusion that is present when current is driven through inhomogeneous magnetic media. We show the spin accumulation, or nonequilibrium magnetization, attendant to charge transport through regions of inhomogeneous magnetization is governed by the same equations found in a thermodynamic approach to magnetoelectric transport.
Theory Of Magnetotransport In Inhomogeneous Magnetic Structures, Horacio E. Camblong
Theory Of Magnetotransport In Inhomogeneous Magnetic Structures, Horacio E. Camblong
Physics and Astronomy
The origin of the giant magnetoresistance of magnetic multilayers and magnetic granular solids is investigated through a unified spin‐dependent linear transport theory, in which the primary source of electrical resistivity is short‐range scattering by impurities in the different magnetic or nonmagnetic regions and at the interfaces. Our theory predicts that magnetotransport in granular solids is similar to that for currents perpendicular to the plane of the layers in multilayers in that their magnetoresistance is independent of the average distance between adjacent magnetic regions.
Magnetoresistance Of Multilayered Structures For Currents Perpendicular To The Plane Of The Layers, Horacio E. Camblong
Magnetoresistance Of Multilayered Structures For Currents Perpendicular To The Plane Of The Layers, Horacio E. Camblong
Physics and Astronomy
While most transport measurements on multilayered structures have been done for currents in the plane of the layers, there is an emerging interest in the geometry where the currents are perpendicular to the plane of the layers. We discuss the field and current patterns in these two cases. For the latter, the elastic mean free path is not a relevant length scale; rather, it is the spin-flip mean free path that is important. In the case of currents perpendicular to the plane of the layers, one must distinguish between models which allow spin mixing of the currents and those in …
Magnetic Multilayers: Quasiclassical Transport Via The Kubo Formula, Horacio E. Camblong
Magnetic Multilayers: Quasiclassical Transport Via The Kubo Formula, Horacio E. Camblong
Physics and Astronomy
A real‐space quantum approach, based on the Kubo formula, is used to describe the quasiclassical transport behavior of metallic multilayers. We emphasize the differences between the cases of current in the plane of the layers, for which size effects play a dominant role and current perpendicular to the planes, for which we provide several proofs that the so‐called series resistor model holds for all length scales.
Novel Results For Quasiclassical Linear Transport In Metallic Multilayers, Horacio E. Camblong
Novel Results For Quasiclassical Linear Transport In Metallic Multilayers, Horacio E. Camblong
Physics and Astronomy
We analyze the linear transport behavior of metallic multilayers via the Kubo formula for a Hamiltonian with zero-range spin-dependent potentials. We find a direct connection between the Boltzmann and the Kubo approaches. Our two-point transport theory validates the quasiclassical approach for multilayered structures and introduces a new treatment of interfaces via angle-dependent coherent transmission coefficients.
Inverse Ac Josephson Effect At Terahertz Frequencies, W C. Danchi, William Golightly, E C. Sutton
Inverse Ac Josephson Effect At Terahertz Frequencies, W C. Danchi, William Golightly, E C. Sutton
Physics and Astronomy
The inverse ac Josephson effect occurs when a Josephson junction driven by a microwave source of frequency f produces constant‐voltage steps at integer multiples of h f/2e. For low‐leakage current hysteretic junctions driven at microwave frequencies below about 100 GHz, some of these steps can cross the zero dc bias current axis. These zero‐crossing steps allow modern series array voltage standards to operate without individually biasing the junctions in the array. We reexamine the theory behind these steps and show that they can exist at frequencies much higher than thought previously. The Riedel singularity in the supercurrent …