690 research outputs found
Echo spectroscopy of bulk Bogoliubov excitations in trapped Bose-Einstein condensates
We propose and demonstrate an echo method to reduce the inhomogeneous
linewidth of Bogoliubov excitations, in a harmonically-trapped Bose-Einstein
condensate. Our proposal includes the transfer of excitations with momentum +q
to -q using a double two photon Bragg process, in which a substantial reduction
of the inhomogeneous broadening is calculated. Furthermore, we predict an
enhancement in the method's efficiency for low momentum due to many-body
effects. The echo can also be implemented by using a four photon process, as is
demonstrated experimentally.Comment: 4 pages, 5 figure
Direct observation of the phonon energy in a Bose-Einstein condensate by tomographic imaging
The momentum and energy of phonons in a Bose-Einstein condensate are measured
directly from a time-of-flight image by computerized tomography. We find that
the same atoms that carry the momentum of the excitation also carry the
excitation energy. The measured energy is in agreement with the Bogoliubov
spectrum. Hydrodynamic simulations are performed which confirm our observation.Comment: Letter, 5 figure
Near-Field Scanning Microwave Microscopy: Measuring Local Microwave Properties and Electric Field Distributions
We describe the near-field microwave microscopy of microwave devices on a
length scale much smaller than the wavelength used for imaging. Our microscope
can be operated in two possible configurations, allowing a quantitative study
of either material properties or local electric fields.Comment: 4 pages, 8 figures, minor corrections to text and 2 figure
Low Power Superconducting Microwave Applications and Microwave Microscopy
We briefly review some non-accelerator high-frequency applications of
superconductors. These include the use of high-Tc superconductors in front-end
band-pass filters in cellular telephone base stations, the High Temperature
Superconductor Space Experiment, and high-speed digital electronics. We also
present an overview of our work on a novel form of near-field scanning
microscopy at microwave frequencies. This form of microscopy can be used to
investigate the microwave properties of metals and dielectrics on length scales
as small as 1 mm. With this microscope we have demonstrated quantitative
imaging of sheet resistance and topography at microwave frequencies. An
examination of the local microwave response of the surface of a heat-treated
bulk Nb sample is also presented.Comment: 11 pages, including 6 figures. Presented at the Eight Workshop on RF
Superconductivity. To appear in Particle Accelerator
Near-Field Microwave Microscopy of Materials Properties
Near-field microwave microscopy has created the opportunity for a new class
of electrodynamics experiments of materials. Freed from the constraints of
traditional microwave optics, experiments can be carried out at high spatial
resolution over a broad frequency range. In addition, the measurements can be
done quantitatively so that images of microwave materials properties can be
created. We review the five major types of near-field microwave microscopes and
discuss our own form of microscopy in detail. Quantitative images of microwave
sheet resistance, dielectric constant, and dielectric tunability are presented
and discussed. Future prospects for near-field measurements of microwave
electrodynamic properties are also presented.Comment: 31 pages, 9 figures, lecture given at the 1999 NATO ASI on Microwave
Superconductivity Changes suggested by editor, including full reference
Frequency Following Imaging of Electric Fields from Resonant Superconducting Devices using a Scanning Near-Field Microwave Microscope
We have developed a scanning near-field microwave microscope that operates at
cryogenic temperatures. Our system uses an open-ended coaxial probe with a 200
mm inner conductor diameter and operates from 77 to 300 K in the 0.01-20 GHz
frequency range. In this paper, we present microwave images of the electric
field distribution above a Tl2Ba2CaCu2O8 microstrip resonator at 77 K, measured
at several heights. In addition, we describe the use of a frequency-following
circuit to study the influence of the probe on the resonant frequency of the
device.Comment: 4 pages, postscript file with 6 figures conference proceeding for the
Applied Superconductivity Conference 199
Superconducting Material Diagnostics using a Scanning Near-Field Microwave Microscope
We have developed scanning near-field microwave microscopes which can image
electrodynamic properties of superconducting materials on length scales down to
about 2 m. The microscopes are capable of quantitative imaging of sheet
resistance of thin films, and surface topography. We demonstrate the utility of
the microscopes through images of the sheet resistance of a YBa2Cu3O7-d thin
film wafer, images of bulk Nb surfaces, and spatially resolved measurements of
Tc of a YBa2Cu3O7-d thin film. We also discuss some of the limitations of the
microscope and conclude with a summary of its present capabilities.Comment: 6 pages with 9 figures, Proceedings of the Applied Superconductivity
Conference 199
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