30 research outputs found
Angular Magnetoresistance Oscillations in the Molecular Organic Conductor (DMET)<sub>2</sub>I<sub>3</sub>: Experiment and Calculation
Thesis advisor: Michael J. NaughtonQuasi-one dimensional (Q1D) molecular organic conductors are among the most exciting materials in condensed matter physics, exhibiting nearly every known ground state. They are highly anisotropic, structurally and electronically, and show large oscillatory phenomena in conductivity for magnetic field rotated in different crystalline planes. Several theoretical works have been published to explain these angular magnetoresistance oscillation (AMRO) effects, but the underlying physics remains illunderstood. Here, we present measurements and calculations of magnetotransport in the molecular organic (super)conductor (DMET)2I3 which detect and simulate all known AMRO phenomena for Q1D systems. Employing, for the first time, the true triclinic crystal structure in the calculations, these results address the mystery of the putative vanishing of the primary AMRO phenomenon, the Lebed magic angle effect, for orientations in which it is expected to be strongest. They also show a common origin for Lebed and so-called "Lee-Naughton" oscillations, and confirm the generalized nature of AMRO in Q1D systems. Furthermore, we report the temperature dependence of the upper critical magnetic field in (DMET)2I3, for magnetic field applied along the intrachain, interchain, and interplane directions. The upper critical field exhibits orbital saturation at low temperature for field in all directions, implying that superconductivity in (DMET)2I3 is conventional spin singlet.Thesis (PhD) — Boston College, 2010.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics
Flux expulsion in niobium superconducting radio-frequency cavities of different purity and essential contributions to the flux sensitivity
Magnetic flux trapped during the cooldown of superconducting radio-frequency
cavities through the transition temperature due to incomplete Meissner state is
known to be a significant source of radio-frequency losses. The sensitivity of
flux trapping depends on the distribution and the type of defects and
impurities which pin vortices, as well as the cooldown dynamics when the cavity
transitions from a normal to superconducting state. Here we present the results
of measurements of the flux trapping sensitivity on 1.3 GHz elliptical cavities
made from large-grain niobium with different purity for different cooldown
dynamics and surface treatments. The results show that lower purity material
results in a higher fraction of trapped flux and that the trapped flux
sensitivity parameter is significantly affected by surface treatments but
without much change in the mean free path . We discuss our results within an
overview of published data on the dependencies of on and frequency
using theoretical models of rf losses of elastic vortex lines driven by
weak rf currents in the cases of sparse strong pinning defects and collective
pinning by many weak defects. Our analysis shows how multiscale pinning
mechanisms in cavities can result in a maximum in similar to that
observed by the FNAL and Cornell groups and how pinning characteristics can be
extracted from the experimental data. Here the main contribution to come
from weak pinning regions at the cavity surface, where dissipative oscillations
along trapped vortices perpendicular to the surface propagate into the bulk
well beyond the layer of rf screening current
Role of Thermal Resistance on the Performance of Superconducting Radio Frequency Cavities
Thermal stability is an important parameter for the operation of the
superconducting radio frequency (SRF) cavities used in particle accelerators.
The rf power dissipated on the inner surface of the cavities is conducted to
the helium bath cooling the outer cavity surface and the equilibrium
temperature of the inner surface depends on the thermal resistance. In this
manuscript, we present the results of direct measurements of thermal resistance
on 1.3 GHz single cell SRF cavities made from high purity large grain and fine
grain niobium as well as their rf performance for different treatments applied
to outer cavity surface in order to investigate the role of the Kapitza
resistance to the overall thermal resistance and to the SRF cavity performance.
The results show no significant impact of the thermal resistance to the SRF
cavity performance after chemical polishing, mechanical polishing or
anodization of the outer cavity surface. Temperature maps taken during the rf
test show non-uniform heating of the surface at medium rf fields. Calculations
of Q0(Bp) curves using the thermal feedback model show good agreement with
experimental data at 2 K and 1.8 K when a pair-braking term is included in the
calculation of the BCS surface resistance. These results indicate local
intrinsic non-linearities of the surface resistance, rather than purely thermal
effects, to be the main cause for the observed field dependence of Q0(Bp)
Analysis of post wet chemistry heat treatment effects on Nb SRF surface resistance
Most of the current research in superconducting radio frequency (SRF)
cavities is focused on ways to reduce the construction and operating cost of
SRF based accelerators as well as on the development of new or improved cavity
processing techniques. The increase in quality factors is the result of the
reduction of the surface resistance of the materials. A recent test on a 1.5
GHz single cell cavity made from ingot niobium of medium purity and heat
treated at 1400 C in a ultra-high vacuum induction furnace resulted in a
residual resistance of about 1nanoohm and a quality factor at 2.0 K increasing
with field up to 5x10^10 at a peak magnetic field of 90 mT. In this
contribution, we present some results on the investigation of the origin of the
extended Q0-increase, obtained by multiple HF rinses, oxypolishing and heat
treatment of all Nb cavities.Comment: To be appear in proceeding of SRF 201
Direct-write, focused ion beam-deposited,7 K superconducting C-Ga-O nanowire
We have fabricated C-Ga-O nanowires by gallium focused ion beam-induced
deposition from the carbon-based precursor phenanthrene. The electrical
conductivity of the nanowires is weakly temperature dependent below 300 K, and
indicates a transition to a superconducting state below Tc = 7 K. We have
measured the temperature dependence of the upper critical field Hc2(T), and
estimate a zero temperature critical field of 8.8 T. The Tc of this material is
approximately 40% higher than that of any other direct write nanowire, such as
those based on C-W-Ga, expanding the possibility of fabricating direct-write
nanostructures that superconduct above liquid helium temperaturesComment: Accepted for AP
Participatory Ranking of Fodders in the Western Hills of Nepal
Fodder is an important source of feed of the ruminants in Nepal. In the mid hills of Nepal, farmers generally practice integrated farming system that combines crop cultivation with livestock husbandry and agroforestry. Tree fodders are good sources of protein during the forage and green grass scarcity periods especially in dry season. Local communities possess indigenous knowledge for the selection of grasses and tree fodders at different seasons in mid hills of western Nepal. A study was conducted on the perception of farmers with respect to selection of fodder species in eight clusters in Kaski and Lumjung districts that range 900-2000 meter above sea level and receive average precipitation of 2000- 4500mm per annum. During the fodder preference ranking, farmers prepared the inventory of fodders found around the villages and nearby forests and selected top ten most important fodders in terms of their availability, palatability, fodder yield, milk yield and milk fat yield. In total, 23 top ranking fodders species were selected from the eight clusters. These fodder species were also ranked using pairwise ranking and weighted scoring methods and ranking was done on the basis of merit numbers obtained from weighted scores. The analysis revealed Artocarpus lakoocha as best tree fodder followed by Ficus semicordata, Thysanolena maxima and Ficus calvata. Similarly, the calendar of fodders trees for lopping season and the best feeding time was prepared on the basis of farmers\u27 local knowledge. This study suggests strategies for promotion of locally preferred tree fodder species and supplementing tree fodder with feed in different seasons depending on their availability and local preferences
Effect of Low Temperature Baking in Nitrogen on the Performance of a Niobium Superconducting Radio Frequency Cavity
We report the rf performance of a single-cell superconducting radiofrequency
cavity after low temperature baking in a nitrogen environment. A significant
increase in quality factor has been observed when the cavity was heat treated
in the temperature range of 120-160 {\deg}C with a nitrogen partial pressure of
~25 mTorr. This increase in quality factor as well as the Q-rise phenomenon
(anti-Q-slope) is similar to those previously obtained with high temperature
nitrogen doping as well as titanium doping. In this study, a cavity N2-treated
at 120 {\deg}C and at140 {\deg}C, showed no degradation in accelerating
gradient, however the accelerating gradient was degraded by 25 with a 160
{\deg}C N2 treatment. Sample coupons treated in the same conditions as the
cavity were analyzed by scanning electron microscope, x-ray photoelectron
spectroscopy and secondary ion mass spectroscopy revealed a complex surface
composition of Nb_2O5, NbO and NbN(1-x)Ox within the rf penetration depth.
Furthermore, magnetization measurements showed no significant change on bulk
superconducting properties