262 research outputs found
Single Superconducting Split-Ring Resonator Electrodynamics
We investigate the microwave electrodynamic properties of a single
superconducting thin film split-ring resonator (SRR). The experiments were
performed in an all-Nb waveguide, with Nb wires and Nb SRRs. Transmission data
showed a high-Q stopband for a single Nb SRR ( at 4.2 K)
below , and no such feature for a Cu SRR, or closed Nb loops, of similar
dimensions. Adding SRRs increased the bandwidth, but decreased the insertion
loss of the features. Placing the Nb SRR into an array of wires produced a
single, elementary negative-index passband ( at 4.2 K).
Changes in the features due to the superconducting kinetic inductance were
observed. Models for the SRR permeability, and the wire dielectric response,
were used to fit the data.Comment: 4 pages, 3 figures, RevTex, submitted to Applied Physics Letters.
Updated version includes mention of bianisotropy, better looking figures, and
different temperature dat
Extremely strong-coupling superconductivity and anomalous lattice properties in the beta-pyrochlore oxide KOs2O6
Superconducting and normal-state properties of the beta-pyrochlore oxide
KOs2O6 are studied by means of thermodynamic and transport measurements. It is
shown that the superconductivity is of conventional s-wave type and lies in the
extremely strong-coupling regime. Specific heat and resistivity measurements
reveal that there are characteristic low-energy phonons that give rise to
unusual scattering of carriers due to strong electron-phonon interactions. The
entity of the low-energy phonons is ascribed to the heavy rattling of the K ion
confined in an oversized cage made of OsO6 octahedra. It is suggested that this
electron-rattler coupling mediates the Cooper pairing, resulting in the
extremely strong-coupling superconductivity.Comment: 17 pages (only 4 pages included here. go to
http://hiroi.issp.u-tokyo.ac.jp/Published%20papers/K-SC6.pdf for full pages),
to be published in PR
Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities
A model for the onset of the reduction in SRF cavity quality factor, the
so-called Q-drop, at high accelerating electric fields is presented. Breakdown
of the surface barrier against magnetic flux penetration at the cavity equator
is considered to be the critical event that determines the onset of Q-drop. The
worst case of triangular grooves with low field of first flux penetration Hp,
as analyzed previously by Buzdin and Daumens, [1998 Physica C 294: 257], was
adapted. This approach incorporates both the geometry of the groove and local
contamination via the Ginzburg-Landau parameter kappa, so the proposed model
allows new comparisons of one effect in relation to the other. The model
predicts equivalent reduction of Hp when either roughness or contamination were
varied alone, so smooth but dirty surfaces limit cavity performance about as
much as rough but clean surfaces do. When in combination, contamination
exacerbates the negative effects of roughness and vice-versa. To test the model
with actual data, coupons were prepared by buffered chemical polishing and
electropolishing, and stylus profilometry was used to obtain distributions of
angles. From these data, curves for surface resistance generated by simple flux
flow as a function of magnetic field were generated by integrating over the
distribution of angles for reasonable values of kappa. This showed that
combined effects of roughness and contamination indeed reduce the Q-drop onset
field by ~30%, and that that contamination contributes to Q-drop as much as
roughness. The latter point may be overlooked by SRF cavity research, since
access to the cavity interior by spectroscopy tools is very difficult, whereas
optical images have become commonplace. The model was extended to fit cavity
test data, which indicated that reduction of the superconducting gap by
contaminants may also play a role in Q-drop.Comment: 15 pages with 7 figure
Low temperature laser scanning microscopy of a superconducting radio-frequency cavity
An apparatus was developed to obtain, for the first time, 2D maps of the
surface resistance of the inner surface of an operating superconducting
radio-frequency niobium cavity by a low-temperature laser scanning microscopy
technique. This allows identifying non-uniformities of the surface resistance
with a spatial resolution of about one order of magnitude better than with
earlier methods and surface resistance resolution of ~ 1 micro-Ohm at 3.3 GHz.
A signal-to-noise ratio of about 10 dB was obtained with 240 mW laser power and
1 Hz modulation frequency. The various components of the apparatus, the
experimental procedure and results are discussed in detail in this
contribution.Comment: 19 pages, 17 figures, submitted to Rev. Sci. Instru
On the Response of an OST to a Point-like Heat Source
A new technique of superconducting cavity diagnostics has been introduced by
D. Hartrill at Cornell University, Ithaca, USA. Oscillating Superleak
Transducers (OST) detect the heat transferred from a cavity's quench point via
"Second Sound" through the superfluid He bath, needed to cool the
superconducting cavity. The observed response of an OST is a complex, but
reproducible pattern of oscillations. A small helium evaporation cryostat was
built which allows the investigation of the response of an OST in greater
detail. The distance between a point-like electrical heater and the OST can be
varied. The OST can be mounted either parallel or perpendicular to the plate,
housing the heat source. If the artificial quench-point releases an amount of
energy compatible to a real quench spot on a cavity's surface, the OST signal
starts with a negative pulse, which is usually strong enough to allow automatic
detection. Furthermore, the reflection of the Second Sound on the wall is
observed. A reflection coefficient R = 0.39 +- 0.05 of the glass wall is
measured. This excludes a strong influence of multiple reflections in the
complex OST response. Fourier analyses show three main frequencies, found in
all OST spectra. They can be interpreted as modes of an oscillating circular
membrane.Comment: 10 pages, 16 figure
Superconducting Microwave Cavity Made of Bulk MgB2
We report the successful manufacture and characterization of a microwave
resonant cylindrical cavity made of bulk MgB2 superconductor (Tc = 38.5 K),
which has been produced by the Reactive Liquid Mg Infiltration technique. The
quality factor of the cavity for the TE011 mode, resonating at 9.79 GHz, has
been measured as a function of the temperature. At T = 4.2 K, the unloaded
quality factor is 2.2x10^5; it remains of the order of 10^5 up to T ~ 30 K. We
discuss the potential performance improvements of microwave cavities built from
bulk MgB2 materials produced by reactive liquid Mg infiltration.Comment: 7 pages, 2 embedded figures, accepted for publication in Supercond.
Sci. Techno
High Gradients In Linear Collider Superconducting Accelerator cavities By high Pulsed Power To Suppress Field Emission
Observation of a Chiral State in a Microwave Cavity
A microwave experiment has been realized to measure the phase difference of
the oscillating electric field at two points inside the cavity. The technique
has been applied to a dissipative resonator which exhibits a singularity --
called exceptional point -- in its eigenvalue and eigenvector spectrum. At the
singularity, two modes coalesce with a phase difference of We
conclude that the state excited at the singularity has a definitiv chirality.Comment: RevTex 4, 5 figure
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