Showing papers by "John B Ketterson published in 1986"

Superconducting properties of V/Fe superlattices

Abstract: Measurements are reported on the superconducting properties of V/Fe superlatitices showing the interplay between ferromagnetism and superconductivity. When the V layer thickness is on the order of the BCS coherence length and the Fe layer is only a few atomic planes thick, a 2D–3D crossover is observed in the temperature dependence of the parallel upper critical field Hc2‖. This implies the coexistence of superconductivity and ferromagnetism in the Fe layers. Three-dimensional behavior for thinner Fe layers is observed (∼1 atomic plane) and 2D behavior for thicker Fe layers (>10 atomic planes).

Possibility of obtaining coherent radiation from a solid state undulator

Abstract: The idea of using a crystal lattice or a superlattice as an undulator for a free electron laser is explored. A purely classical treatment of relativistic positrons channeling through the proposed structure is performed in a self‐consistent fashion involving the wave equation for the radiating electromagnetic field and the kinetic equation for the positron distribution function. The (positive) gain coefficient for a forward radiating field is obtained in closed form. Matching the Kumakhov channeling resonance to the undulator frequency further enhances the gain. This result, combined with a feedback mechanism arising from Bragg diffraction within the basic crystal lattice, leads to an instability of the radiation inside the crystal. A simple threshold condition involving the atomic scattering factor and the net gain is formulated. Finally, a numerical estimate of the Kumakhov‐enhanced gain coefficient is made for the (110) planar channeling in a strain‐modulated Si superlattice.

Superconducting properties of Fe/V/Fe sandwiches

Abstract: A study has been made of the superconducting properties of Fe/V/Fe sandwiches with V thickness ranging from 153 AP (atomic planes) to 5048 AP. All 20 samples studied were prepared in a single run. At zero magnetic field, the suppression of the transition temperature follows an inverse square dependence on the thickness, implying a strong pair-breaking effect in the Fe layers. The upper critical field data cross over smoothly from the 2D regime to the 3D regime with increasing V thickness. A comparison with some thin-film theories is given.

Collective modes and sound propagation in a magnetic field in superfluid3He-B

Abstract: A high-resolution, ultrasonic (12–89 MHz) acoustic impedance technique has been used to investigate the order parameter collective modes in superfluid3He-B over a pressure range of 0–15 bar and in magnetic fields up to 180 mT. In agreement with earlier experiments, theJ=2 real squashing mode has been observed to split into five components in small magnetic fields. However, contrary to earlier theoretical estimates, the Zeeman shifts have been found to become extremely nonlinear as the magnetic field is increased. The extent of this nonlinearity is largest at low pressures and at temperatures close toTc. In comparison with recent theoretical work, the nonlinear Zeeman shifts may be explained as a result of two effects. First, there is a significant distortion of the B-phase energy gap in large magnetic fields. Second, there is an important coupling between the sameJzsubstates of the differentJ modes. In this sense the nonlinear evolution of the real squashing mode constitutes the observation of the Paschen-Back effect in3He-B. A comparison of the observed Zeeman shifts with theoretical expressions has yielded information about particle-particle and particle-hole interaction effects in the superfluid. In the limitT → 0 and above a threshold field, the real squashing mode has been found to possess additional structure. TheJz=0 substate has been observed to split into a doublet. The separation between the two components of the doublet is of the order of 100–200 kHz and remains independent of the magnetic field. The origin of the doublet may be understood in terms of a recent theory which postulates a texture-dependent collective mode frequency. Further, at extremely small fields the effects due to dispersion of the real squashing modes have been found to be important. The magnitude of the dispersion-induced mode splitting in zero field is found to be consistent with theoretical predictions. TheJ=2 squashing mode has also been studied in the presence of a magnetic field. TheJz=0 state of the squashing mode is observed to shift to lower temperatures in a magnetic field. An additional field dependence of the observed acoustic impedance is interpreted as the evolution of theJz=−1, −2 states, but appears to be inconsistent with theoretical predictions.

Charged particle high frequency laser

Abstract: A high frequency laser for generating a coherent collimated beam in the ultraviolet to x-ray range using a high energy charged particle beam is provided. The laser utilizes an undulator which in one embodiment is a solid state superlattice composed of alternating layers of two materials have slightly different lattice spacing to provide strain modulation. A beam of charged particles is injected into the undulator to stimulate emission at short wavelengths in ultraviolet to hard x-ray range. The feedback required for lasing action is provided by Bragg diffraction involving the basic crystal lattice or other mirror device such as a superlattice mirror. In another embodiment, a dynamic undulator is used comprising infrared electromagnetic radiation. High energy Lithium ions are injected into the undulator resulting in stimulation of high frequency radiation. The feedback is provided by a superlattice mirror.

NMR in normal 3He using a meander-line coil.

Abstract: A simple technique is proposed for probing the single-particle and collective excitations in a Fermi liquid via a standing magnetic surface wave of arbitrary $\ensuremath{\omega}$ and fixed $k$ generated by a meanderline coil. The calculated power absorption spectrum for $^{3}\mathrm{He}$ displays singularities associated with the $l=0$ spin-wave mode and a Doppler-shifted spin resonance of the single-particle excitations.

Coherent short wave radiation from a solid state free electron laser

Abstract: The idea of using a crystal lattice or a superlattice as an undulator for a free electron laser is explored. A purely classical treatment of relativistic positrons channeling through the proposed structure is performed in a self-consistent fashion involving the wave equation for the radiating electromagnetic field and the kinetic equation for the positron distribution function. The gain coefficient for a forward radiating field is obtained in closed analytical form and is studied numerically for a (110) planar channeling in Si. This result, combined with a feedback mechanism arising from Bragg diffraction within the basic crystal lattice, may lead to an instability of the radiation inside the crystal.

Magnetic Transitions in Fe/Cr Superlattices

Abstract: Several series of multilayered Fe/Cr films have been studied, each with a fixed thickness of Fe and several thicknesses of Cr. The coer-civity and in plane anisotropy field were found to increase abruptly for larger Cr thicknesses. The origin of this change in behavior may involve a paramagnetic/antiferromagnetic transition in the Cr.

Superconducting Properties of v/cr Artificial Metallic Superlattices

Abstract: We have studied the superconducting properties of V/Cr superlattices. Superlattices of V100Crx, V150Crx, and V200Crx, with x varying between 3 and 80 atomic planes have been prepared. The films have a strong (110) texture and were grown on sapphire substrates at 250°C in an ultra-high vacuum system. The zero field transition temperature of the V150Crx and V200Crx, but not the V100Crx films have been adequately modeled using Werthamer’s theory for proximity effect coupled films. The films have a dimensional crossover in the parallel upper critical field when the thickness of the Cr layers is about 60 atomic planes.

Angular Dependent Critical Fields in NbTi-Ge Superlattices in the Weakly Localized Regime

Abstract: The angular dependence of the upper critical fields, H c (θ) for a set of NbTi-Ge superlattices were studied at various temperatures. The behavior of H c (θ) at lower temperatures deviates from the Tinkham expression which is expected to be valid only in the Ginzberg-Landau regime close to T c . We examine a model for calculating H c (θ) involving the lowest eigenvalue of the gauge invariant diffusion equation (subject to boundary conditions appropriate to a slab) in the de Gennes expression for the upper critical field of a dirty superconductor at all temperatures. The disorder related localization and interaction effects as well, as the paramagnetic limiting effect, are also considered.

Technique to produce coherent X-ray radiation via laser pumping of a relativistic ion beam

Abstract: The level population of a beam of relativistic positive ions with Z≥2 having a single bound electron may be inverted by the application of a ‘‘π pulse’’ of laser radiation tuned to the Doppler‐shifted 1s–2p transition. When the laser beam and ion beam move in opposite directions, the required laser frequency is reduced by a factor 2γ. Subsequently applied short wavelength resonant radiation moving in the same direction as the ion beam (with an inverted population) will be amplified via stimulated emission, the wavelength in the lab frame now being shorter than the original laser wavelength by a factor (2γ)2.

Technique to produce coherent x‐ray radiation via laser pumping of a relativistic ion beam

Abstract: The level population of a beam of relativistic positive ions with Z≥2 having a single bound electron may be inverted by the application of a ‘‘π pulse’’ of laser radiation tuned to the Doppler‐shifted 1s–2p transition. When the laser beam and ion beam move in opposite directions, the required laser frequency is reduced by a factor 2γ. Subsequently applied short wavelength resonant radiation moving in the same direction as the ion beam (with an inverted population) will be amplified via stimulated emission, the wavelength in the lab frame now being shorter than the original laser wavelength by a factor (2γ)2.