117 research outputs found
Imaging Local Sources of Intermodulation in Superconducting Microwave Devices
This work presents new experimental results on low-temperature (LT)
characterization of local rf properties of passive superconducting (SC)
microwave devices using a novel Laser Scanning Microscope (LSM). In this
technique, a modulated laser beam is focused onto and scanned over the surface
of a resonant SC device to probe the spatial distribution of rf current. The
highly localized photo-induced change of the kinetic inductance of the SC
device produces both a shift of the resonant frequency f_0 and change of the
quality factor Q. An image of these changes is recorded as the laser spot is
scanned over the device. We present the first measurements of spatially
resolved intermodulation response in a High Temperature Superconducting (HTS)
co-planar waveguide resonator, opening up a new window into the local origins
of nonlinearity in the HTS materials.Comment: 4 Pages, to be published in IEEE Trans. Appl. Supercond., June 200
Effect of LaAlO Surface Topography on RF Current Distribution in Superconducting Microwave Devices
A laser scanning microscope with a thermal spot size of about 4 m is
used to measure a quantity proportional to the rf current density in an
operating superconducting co-planar waveguide microwave resonator. The twinning
of the LaAlO substrate produces a meandering of the current at the edges
due to irregularities in the wet etching of the YBaCuO
film associated with substrate twin domain blocks, and a ( 20%)
enhancement of the rf photoresponse at these locations. These irregularities
are candidates for enhanced nonlinear response from the device. The effects of
substrate twinning and the resulting edge features on the superconducting film
are discussed and analyzed.Comment: 13 pages, 4 figures, submitted to Applied Physics Letter
Superconducting RF Metamaterials Made with Magnetically Active Planar Spirals
Superconducting metamaterials combine the advantages of low-loss, large
inductance (with the addition of kinetic inductance), and extreme tunability
compared to their normal metal counterparts. Therefore, they allow realization
of compact designs operating at low frequencies. We have recently developed
radio frequency (RF) metamaterials with a high loaded quality factor and an
electrical size as small as 658, ( is the free space
wavelength) by using Nb thin films. The RF metamaterial is composed of truly
planar spirals patterned with lithographic techniques. Linear transmission
characteristics of these metamaterials show robust Lorentzian resonant peaks in
the sub- 100 MHz frequency range below the of Nb. Though Nb is a
non-magnetic material, the circulating currents in the spirals generated by RF
signals produce a strong magnetic response, which can be tuned sensitively
either by temperature or magnetic field thanks to the superconducting nature of
the design. We have also observed strong nonlinearity and meta-stable jumps in
the transmission data with increasing RF input power until the Nb is driven
into the normal state. We discuss the factors modifying the induced magnetic
response from single and 1-D arrays of spirals in the light of numerical
simulations.Comment: 4 pages, 7 figure
Microscopic examination of hot spots giving rise to nonlinearity in superconducting resonators
We investigate the microscopic origins of nonlinear rf response in
superconducting electromagnetic resonators. Strong nonlinearity appearing in
the transmission spectra at high input powers manifests itself through the
emergence of jumplike features near the resonant frequency that evolve toward
lower quality factor with higher insertion loss as the rf input power is
increased. We directly relate these characteristics to the dynamics of
localized normal regions (hot spots) caused by microscopic features in the
superconducting material making up the resonator. A clear observation of
hot-spot formation inside a Nb thin film self-resonant structure is presented
by employing the microwave laser scanning microscope, and a direct link between
microscopic and macroscopic manifestations of nonlinearity is established.Comment: 5 pages, 4 figure
Effect of LaAlO3 twin-domain topology on local dc and microwave properties of cuprate films
Different imaging modes of low temperature laser scanning microscopy (LTLSM)
have been applied to probe local optical and superconducting properties, as
well as the spatial variations in thermoelectric and electronic (both dc and
rf) transport, in a YBa_2Cu_3O_6.95 /LaAlO_3 (YBCO/LAO) superconducting
microstrip resonator with micron-range resolution. Additionally, the local
sources of microwave nonlinearity (NL) were mapped in two-dimensions
simultaneously by using the LTLSM in two-tone rf intermodulation distortion
contrast mode as a function of (x,y) position of the laser beam perturbation on
the sample. The influence of the direction of individual twin-domain YBCO
blocks on its NL properties was analyzed in detail. The result shows the direct
spatial correlation between NL microwave and dc electronic transport properties
of the YBCO film that are imposed by the underlying twin-domain topology of the
LAO substrate. In these circumstances, the scale of local NL current densities
J_IM(x,y) in different areas of the YBCO microstrip quantitatively coincide
with the scale of local critical current densities J_c(x,y) measured at the
same positions.Comment: 10 pages, 11 figure
Microwave Current Imaging in Passive HTS Components by Low-Temperature Laser Scanning Microscopy (LTLSM)
We have used the LTLSM technique for a spatially resolved investigation of
the microwave transport properties, nonlinearities and material inhomogeneities
in an operating coplanar waveguide YBa_2Cu_3O_{7-\delta} (YBCO) microwave
resonator on an LaAlO_3 (LAO) substrate. The influence of twin-domain blocks,
in-plane rotated grains, and micro-cracks in the YBCO film on the nonuniform rf
current distribution were measured with a micrometer-scale spatial resolution.
The impact of the peaked edge currents and rf field penetration into weak links
on the linear device performance were studied as well. The LTLSM capabilities
and its future potential for non-destructive characterization of the microwave
properties of superconducting circuits are discussed.Comment: 8 pages, 9 figures, 2-column format, presented at High Temperature
Superconductors in High Frequency Fields 2004, Journal of Superconductivity
(in press
Laser Scanning Microscopy of HTS Films and Devices
The work describes the capabilities of Laser Scanning Microscopy (LSM) as a
spatially resolved method of testing high_Tc materials and devices. The earlier
results obtained by the authors are briefly reviewed. Some novel applications
of the LSM are illustrated, including imaging the HTS responses in rf mode,
probing the superconducting properties of HTS single crystals, development of
twobeam laser scanning microscopy. The existence of the phase slip lines
mechanism of resistivity in HTS materials is proven by LSM imaging.Comment: 17 pages, 21 figures, Submitted to Fizika Nizkikh Temperatur (Low
Temperature Physics
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