Showing papers by "John B Ketterson published in 2006"
TL;DR: In this paper, the authors showed numerically that switching is possible with simple sinusoidal pulses; however, the optimum approach is to use a frequency-swept (chirped) rf magnetic pulse, the shape of which can be derived analytically.
Abstract: The authors study magnetization reversal using various rf magnetic pulses The authors show numerically that switching is possible with simple sinusoidal pulses; however, the optimum approach is to use a frequency-swept (chirped) rf magnetic pulse, the shape of which can be derived analytically Switching times of the order of nanoseconds can be achieved with relatively small rf fields, independent of the anisotropy’s strength
65 citations
TL;DR: In this paper, the authors used the full Sommerfeld integral formalism as well as an asymptotic formalism to study the near and far-field radiation patterns of an electric dipole in the vicinity of a planar dielectric half space.
Abstract: We have used the full Sommerfeld integral formalism as well as an asymptotic formalism to study the near- and far-field radiation patterns of an electric dipole in the vicinity of a planar dielectric half space. We present systematic results for the polarization dependence of the radiation patterns in both half spaces and the ratio of the integrated power radiated into the two half spaces as a function of the relative refractive index as well as the dipole position. We find that the radiation patterns are highly structured and directed. Furthermore, the ratio of the integrated power increases significantly on increasing the relative refractive index, which can be exploited to enhance the sensitivity of spectroscopic studies of surface-bound molecules; however this ratio drops quickly for a dipole more than 0.2 wavelength from the interface.
24 citations
TL;DR: In this paper, a method for numerically simulating magnetic spin waves is presented, which can calculate the energy absorbed by the various modes excited by a position and time-dependent H1 field in a ferromagnetic body of arbitrary shape in the presence of a (uniform or non-uniform) static H0 field as well as the internal exchange and anisotropy fields.
21 citations
TL;DR: In this paper, the spectroscopic observation of bound excitons in natural Cu 2 O 2 O O O 2 samples is reported, which are generated by impurity capture of free exciton.
Abstract: We report the spectroscopic observation of bound excitons in natural ${\mathrm{Cu}}_{2}\mathrm{O}$ samples, which are generated by impurity capture of free excitons. Due to the broken symmetry of a bound exciton, its radiative recombination rate increases, causing a greatly enhanced luminescence intensity compared with that for a free exciton. Moreover, the bound exciton luminescence serves as an internal monitor of the free ortho- and free para-exciton densities, whereas the free para-exciton luminescence is allowed only via weak ${\ensuremath{\Gamma}}_{25}^{\ensuremath{-}}$ phonon emission. At high excitation levels, the bound exciton luminescence shows a density-dependent loss of excitons that is well explained by a recently proposed biexciton formation model. The temperature dependence of the bound ortho- and bound para-exciton luminescence indicates a ${T}^{\ensuremath{-}1}$-dependence for the exciton-impurity capture rate.
21 citations
TL;DR: In this paper, the authors show that magnetization reversal in spin-injection devices can be significantly faster when using a chirped rf current, rather than a dc, pulse.
Abstract: We show that magnetization reversal in spin-injection devices can be significantly faster when using a chirped rf current, rather than a dc, pulse. Although one can use a simple sinusoidal rf pulse, an optimized series of alternating, equal-amplitude, pulses of varying width (a digitized approximation to a chirped rf pulse) produces more efficient switching.
21 citations
TL;DR: In this paper, the angular radiation pattern from an array of dye molecules embedded in a polymethylmethacrylate film deposited on a dielectric hemispherical lens was investigated.
Abstract: We report measurements on the angular radiation pattern from an array of dye molecules embedded in a polymethylmethacrylate film deposited on a dielectric hemispherical lens. The radiation pattern is both highly structured and directed, with most of the power being radiated into the media having the higher refractive index. We also present a simulation of the far-field radiation pattern of a dipole embedded in a thin dielectric layer, which apparently has not been investigated before. The simulation matches the experimental results rather well.
16 citations
TL;DR: In this paper, the authors demonstrate controlled loading of a closely packed array of optical traps, which is formed from the interference of two pairs of coherent laser beams via an optical setup that allows for simple, continuous variation of lattice parameters over a very wide range.
Abstract: In this paper we demonstrate controlled loading of a closely packed array of optical traps. We also describe the technical advantages of our method of filling the trap array (which makes use of an independent, steerable trap created by a separate objective lens), as well of our specific implementation of array generation by multi-beam interference. Microscopic polystyrene spheres are trapped and subsequently assembled into sites on a two-dimensional optical lattice, which is formed from the interference of two pairs of coherent laser beams via an optical setup that allows for simple, continuous variation of lattice parameters over a very wide range. Individual particles in the initial assembly are dynamically manipulated with the independent laser beam, which offers the freedom to generate either defect-free lattices or a lattice with designer defects. As examples we demonstrate the assembly of a defect-free square lattice and a lattice with a single vacancy.
14 citations
TL;DR: In this article, the authors describe two state-of-the-art ultrasonic interferometers designed for pulse-echo ultrasound measurements in the short-pulse (40 ms) and intermediate pulse (400 ms) 60 T magnets of the National High Magnetic Field Laboratory facilities at Los Alamos, NM.
Abstract: We describe two state-of-the-art ultrasonic interferometers designed for pulse-echo ultrasound measurements in the short-pulse (40 ms) and intermediate-pulse (400 ms) 60 T magnets of the National High Magnetic Field Laboratory facilities at Los Alamos, NM. One interferometer utilizes a more commonly used scheme which has been adapted for a pulsed field environment. The second interferometer exploits digital processing of the undetected signal: the ultrasonic signal is digitized, stored in a computer, and later, analyzed.
6 citations
TL;DR: In this article, the authors describe a basic ultrasonic system intended for measurements in quasicontinuous fields, which operates in the frequency range from 10 to 1000 MHz and enables precise measurements of changes in the ultrasonic velocity and attenuation in solids using the pulse echo technique.
Abstract: Portable, superheterodyne, ultrasonic interferometers have been constructed for use with the continuous and pulsed magnetic field facilities of the National High Magnetic Field Laboratory sites at Tallahassee, FL and Los Alamos, NM. The instrumentation operates in the frequency range from 10 to 1000 MHz and enables precise measurements of changes in the ultrasonic velocity and attenuation in solids using the pulse-echo technique. In this first of a two part article we describe a basic ultrasonic system intended for measurements in quasicontinuous fields.
6 citations
5 citations
TL;DR: In this paper, the lateral conductivity of the middle Al layer of a SINIS multiterminal device is studied as a function of the current injected perpendicular to the layers in a regime where a gap-difference-like feature is observed in the current-voltage characteristic.
Abstract: The lateral conductivity of the middle Al layer of a $\mathrm{Nb}∕\mathrm{Al}∕\mathrm{Al}{\mathrm{O}}_{x}∕\mathrm{Al}∕\mathrm{Al}{\mathrm{O}}_{x}∕\mathrm{Al}∕\mathrm{Nb}$ (SINIS) multiterminal device is studied as a function of the current injected perpendicular to the layers in a regime where a gap-difference-like feature is observed in the current-voltage characteristic. The response of the Al layer does not confirm an earlier reported observation of superconductivity in the Al far above its transition temperature [Blamire et al., Phys. Rev. Lett. 66, 220 (1991)].
TL;DR: In this article, the authors demonstrate the sequential spatial separation of a solution consisting of a mixture of two microspheres with different diameters using a dynamic optical interferometery scheme.
Abstract: We demonstrate the sequential spatial separation of a solution consisting of a mixture of two microspheres with different diameters using a dynamic optical interferometery scheme. Two coherent lasers beams are focused together through an objective lens to form an in-plane standing wave. By linearly increasing the phase of one of incoming beams relative to the other, the optical lattice is translated. The optical forces on particles with different sizes depends on the spacing of the standing wave relative to the particle diameter; therefore, by adjusting the spacing of the standing wave so as to minimize the interaction of particles of one size with the optical lattice, all other particles can be swept out by the translating potential wells that are associated with the intensity maxima of the standing wave, while the selected particles remain trapped in the overall center of the Gaussian beam envelope of the optical lattice. Here, we demonstrate the selectivity of this optical conveyor belt by dragging smaller particles out to one side of an ensemble while simultaneously keeping the larger ones trapped. The Brownian dynamics of particles translated in an optical lattice and measurements of the associated optical force are also presented.
TL;DR: In this paper, postgrowth annealing studies on heteroepitaxial MnAs thin films deposited on GaAs and Si by molecular-beam epitaxy were carried out and the crystal orientation of the MnAs∕Si(001) film changes, and is accompanied by a shift from paramagnetic to ferromagnetic behavior.
Abstract: We have carried out postgrowth annealing studies on heteroepitaxial MnAs thin films deposited on GaAs(001) and Si(001) by molecular-beam epitaxy. Before annealing, a paramagnetic behavior is exhibited for MnAs∕Si(001), whereas a ferromagnetic behavior is observed for MnAs∕GaAs(001). The paramagnetic β-MnAs phase domains of the as-grown MnAs∕GaAs(001) sample disappear completely after postgrowth annealing. Surprisingly, after postgrowth annealing, the crystal orientation of the MnAs∕Si(001) film changes, and is accompanied by a shift from paramagnetic to ferromagnetic behavior. We attribute these observations to relaxation of the biaxial strain associated with the substrates.
TL;DR: Using Zr oxide for the tunnel barrier formation, it is shown experimentally that the effect of pinholes is suppressed in the double-barrier versus single-branched junction configuration as discussed by the authors.
Abstract: Using Zr oxide for the tunnel barrier formation, it is shown experimentally that the effect of pinholes is suppressed in the double-barrier versus single-barrier junction configuration. It is found that Josephson current in Nb∕Zr double-barrier junctions is much lower than in Nb∕Al junctions with comparable specific tunneling resistance and middle layer thickness. Multiple oxidation process is used to improve the quality of superconductor-insulator-superconductor Zr-based junctions.
TL;DR: In this article, the authors report on novel ferromagnetic Mn/Ge multilayers for spintronics applications investigated both experimentally and theoretically, which are grown on GaAs (001) substrate by molecular beam epitaxy.
Abstract: We report on novel ferromagnetic Mn/Ge multilayers for spintronics applications investigated both experimentally and theoretically. Two Mn/Ge multilayers are grown on GaAs (001) substrates by molecular beam epitaxy. The period of each multilayer consists of an Mn layer of varying thickness (0.6 and 5 A) and a 10 A thick Ge spacer layer. From temperature-dependent magnetization and hysteresis loop measurements, the Mn (0.6 A)/Ge (10 A) multilayer showed very weak ferromagnetic ordering, which is persistent up to 260 K, whereas the Mn (5 A)/Ge (10 A) multilayer exhibited strong ferromagnetism up to 305 K. The coercive field of the Mn (5 A)/Ge (10 A) multilayer was 277 Oe at 200 K. Density functional electronic band structure calculations on a number of Mn/Ge (001) multilayers determined them to be ferromagnetic, and estimates of their critical temperatures are reported.
TL;DR: In this article, a simple classical analogy is a pendulum with a mass m = C (h/2e) 2 and a rigidity k = hJc/ 2e; here C is the junction capacitance and Jc is the critical current.
Abstract: Renewed interest in collective oscillations in Josephson junctions has been generated by recent progress in quantum information technology[1–6]. Quantization of the Josephson plasmon in SIS junctions (where S is a superconductor and I is a dielectric barrier) has been successfuly used in single and double quantum logic gates[1–6]. Several types of collective oscillations have been studied to date. The Josephson plasmon (JP) has attracted the most attention due to its utilization in various superconducting devices. The JP involves oscillations of the Josephson supercurrent about its equilibrium value. A simple classical analogy is a pendulum with a mass m = C (h/2e) 2 and a rigidity k = hJc/2e; here C is the junction capacitance and Jc is the critical current. In such a pendulum, the phase difference φ across the junction serves as a coordinate, while the velocity φ is related to the charge Q = Chφ/2e accumulated on the junction electrodes; here we used the Josephson relationship V = hφ/2e relating the voltage V and φ. The JP pendulum frequency is then ωJP = √ 2eJc/hC.
TL;DR: In this paper, the authors measured photoluminescence of the quadrupolar transitions of the yellow series excitons of cuprous oxide at 2 K by tuning the excitation energy.
Abstract: We present our measured photoluminescence of the quadrupolar transitions of the yellow series excitons of cuprous oxide at 2 K By tuning the excitation energy in the two-photon quadrupolar absorption scheme of Cu2O, it is found that the band-gap value associated with this optical process can be characterized by a sharp resonance at 2036 eV at cryogenic temperature This energy value is helpful to guide a proper excitation for creating cold excitonic gas in this crystal
TL;DR: The velocity of longitudinal ultrasound propagated along the a axis in URu2Si2 has been studied at temperatures down to 40 mK and in magnetic fields of up to 45 T applied along the c axis.
Abstract: The velocity of longitudinal ultrasound propagated along the a axis in URu2Si2 has been studied at temperatures down to 40 mK and in magnetic fields of up to 45 T applied along the c axis. The frequencies of the observed acoustic quantum oscillations (560 T and 1070 T), as well as calculated effective masses (14.5me and 22me), are in reasonable agreement with the known dHvA extremal cross sections of the Fermi surface in URu2Si2. It is shown that features observed previously at temperatures of about 0.5 K in magnetic fields between about 29 and 30 T in the magnetoresistance. Hall coefficient and ultrasound velocity are associated with quantum oscillations, which are clearly seen at lower temperatures. A hysteresis observed at 40 mK for all three steps of the metamagnetic transition confirms that they all are first order transitions.