arXiv subjects

S. J. Allen

Publications and source records attributed to S. J. Allen.

7 recordsLinked to original sources

arXiv2010

Micro-Structured Ferromagnetic Tubes for Spin Wave Excitation

Micron scale ferromagnetic tubes placed on the ends of ferromagnetic CoTaZr spin waveguides are explored in order to enhance the excitation of Backward Volume Magnetostatic Spin Waves. The tubes produce a closed magnetic circuit about the signal line of the coplanar waveguide and are, at the same time, magnetically contiguous with the spin waveguide. This results in a 10 fold increase in spin wave amplitude. However, the tube geometry distorts the magnetic field near the spin waveguide and relatively high biasing magnetic fields are required to establish well defined spin waves. Only the lowest (uniform) spin wave mode is excited.

cond-mat.other↗

arXiv2009

Resonant coupling of coplanar waveguides with ferromagnetic tubes

Resonant coupling of coplanar waveguides is explored by wrapping proximate shorted ends of the waveguides with micron size ferromagnetic Co90Ta5Zr5 tubes. Ferromagnetic resonance and up to 7 outer surface modes are identified. Experimental results for these contorted rectangular tubes are in good agreement with micromagnetic simulations and model calculations of magnetostatic modes for an elliptical ferromagnetic tube. These results indicate that the modes are largely determined by tube topology and dimensions but less so by the detailed shape.

cond-mat.mtrl-sci↗

arXiv2008

Dispersion in magnetostatic CoTaZr spin wave-guides

Magnetostatic spin wave dispersion and loss are measured in micron scale spin wave-guides in ferromagnetic, metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion and up to three different modes are identified. Attenuation lengths of the order of 3 microns are several of orders of magnitude shorter than that predicted from eddy currents in these thin wires.

cond-mat.mtrl-sci↗

arXiv2008

Dispersion and spin wave "tunneling" in nano-structured magnetostatic spin waveguides

Magnetostatic spin wave dispersion and loss are measured in micron scale spin wave-guides in ferromagnetic, metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion. The measured attenuation lengths, of the order of 3um, are several of orders of magnitude shorter than that predicted from eddy currents in these thin wires. Spin waves effectively "tunnel" through air gaps, produced by focused ion beam etching, as large as 1.5 um.

cond-mat.mtrl-sci↗

arXiv1999

Nonequilibrium ac Josephson effect in mesoscopic Nb-InAs-Nb junctions

Microwave irradiation of Nb-InAs-Nb junctions reveals frequency-doubled Josephson currents which persist to high temperatures, in the absence of a critical current. A nonequilibrium dynamical model, based on time-dependent Andreev bound states, successfully accounts for the resulting half-integer Shapiro step and an enhancement in the conductance near zero bias.

cond-mat.supr-con↗

arXiv1998

Excitonic Dynamical Franz-Keldysh Effect

The Dynamical Franz-Keldysh Effect is exposed by exploring near-bandgap absorption in the presence of intense THz electric fields. It bridges the gap between the DC Franz- Keldysh effect and multi-photon absorption and competes with the THz AC Stark Effect in shifting the energy of the excitonic resonance. A theoretical model which includes the strong THz field non-perturbatively via a non-equilibrium Green Functions technique is able to describe the Dynamical Franz-Keldysh Effect in the presence of excitonic absorption.

cond-mat.mes-hall↗

arXiv1997

On the absence of Shapiro-like steps in certain mesoscopic S-N-S junctions

In DC transport through mesoscopic S-N-S junctions, it is known that the Josephson coupling decreases exponentially with increasing temperature, but the phase dependence of the conductance persists to much higher temperatures and decreases only as 1/T. It is pointed out here that, despite the fact that such a phase-dependent conductance does bring about an AC current for a pure DC voltage, it cannot, by itself, lead to the formation of Shapiro steps.

cond-mat.supr-con↗