2,189 research outputs found
Magnetic field induced lattice anomaly inside the superconducting state of CeCoIn: evidence of the proposed Fulde-Ferrell-Larkin-Ovchinnikov state
We report high magnetic field linear magnetostriction experiments on
CeCoIn single crystals. Two features are remarkable: (i) a sharp
discontinuity in all the crystallographic axes associated with the upper
superconducting critical field that becomes less pronounced as the
temperature increases; (ii) a distinctive second order-like feature observed
only along the c-axis in the high field (10 T ) low
temperature ( 0.35 K) region. This second order transition is
observed only when the magnetic field lies within 20 of the ab-planes and
there is no signature of it above , which raises questions regarding
its interpretation as a field induced magnetically ordered phase. Good
agreement with previous results suggests that this anomaly is related to the
transition to the Fulde-Ferrel-Larkin-Ovchinnikov superconducting state.Comment: 3 figures, 5 page
An algorithm for LET-analysis
An algorithm for the derivation of LET-distributions from pulse- height spectra obtained with proportional counters is described. The method is based on Fourier transformation: it is applicable to spherical as well as non-spherical proportional counters. The relation between the energy mean, LD, of LET and the energy mean, yD, of the lineal energy density is given
Corrosion Resistant Coatings for High Temperature Applications
Efforts to increase efficiency of energy conversion devices have required their operation at ever higher temperatures. This will force the substitution of higher-temperature structural ceramics for lower temperature materials, largely metals. Yet, many of these ceramics will require protection from high temperature corrosion caused by combustion gases, atmospheric contaminants, or the operating medium. This paper discusses examples of the initial development of such coatings and materials for potential application in combustion, aluminum smelting, and other harsh environments
High temperature superconductors for magnetic suspension applications
High temperature superconductors (HTS) hold the promise for applications in magnetic levitation bearings, vibration damping, and torque coupling. Traditional magnetic suspension systems require active feedback and vibration controls in which power consumption and low frequency vibration are among the major engineering concerns. HTS materials have been demonstrated to be an enabling approach towards such problems due to their flux trapping properties. In our laboratory at TCSUH, we have been conducting a series of experiments to explore various mechanical applications using HTS. We have constructed a 30 lb. model flywheel levitated by a hybrid superconducting magnetic bearing (HSMB). We are also developing a levitated and vibration-dampled platform for high precision instrumentation. These applications would be ideal for space usages where ambient temperature is adequate for HTS to operate properly under greatly reduced cryogenic requirements. We will give a general overview of these potential applications and discuss the operating principles of the HTS devices we have developed
Fermi Surface of Alpha-Uranium at Ambient Pressure
We have performed de Haas-van Alphen measurements of the Fermi surface of
alpha-uranium single crystals at ambient pressure within the alpha-3 charge
density wave (CDW) state from 0.020 K - 10 K and magnetic fields to 35 T using
torque magnetometry. The angular dependence of the resulting frequencies is
described. Effective masses were measured and the Dingle temperature was
determined to be 0.74 K +/- 0.04 K. The observation of quantum oscillations
within the alpha-3 CDW state gives new insight into the effect of the charge
density waves on the Fermi surface. In addition we observed no signature of
superconductivity in either transport or magnetization down to 0.020 K
indicating the possibility of a pressure-induced quantum critical point that
separates the superconducting dome from the normal CDW phase.Comment: 11 pages, 4 figures, 3 table
Pressure Evolution of a Field Induced Fermi Surface Reconstruction and of the Neel Critical Field in CeIn3
We report high-pressure skin depth measurements on the heavy fermion material
CeIn3 in magnetic fields up to 64 T using a self-resonant tank circuit based on
a tunnel diode oscillator. At ambient pressure, an anomaly in the skin depth is
seen at 45 T. The field where this anomaly occurs decreases with applied
pressure until approximately 1.0 GPa, where it begins to increase before
merging with the antiferromagnetic phase boundary. Possible origins for this
transport anomaly are explored in terms of a Fermi surface reconstruction. The
critical magnetic field at which the Neel ordered phase is suppressed is also
mapped as a function of pressure and extrapolates to the previous ambient
pressure measurements at high magnetic fields and high pressure measurements at
zero magnetic field.Comment: 15 pages, 5 figure
The energy gap of intermediate-valent SmB6 studied by point-contact spectroscopy
We have investigated the intermediate valence narrow-gap semiconductor SmB6
at low temperatures using both conventional spear-anvil type point contacts as
well as mechanically controllable break junctions. The zero-bias conductance
varied between less than 0.01 mikrosiemens and up to 1 mS. The position of the
spectral anomalies, which are related to the different activation energies and
band gaps of SmB6, did not depend on the the contact size. Two different
regimes of charge transport could be distinguished: Contacts with large zero -
bias conductance are in the diffusive Maxwell regime. They had spectra with
only small non-linearities. Contacts with small zero - bias conductance are in
the tunnelling regime. They had larger anomalies, but still indicating a finite
45 % residual quasiparticle density of states at the Fermi level at low
temperatures of T = 0.1 K. The density of states derived from the tunelling
spectra can be decomposed into two energy-dependent parts with Eg = 21 meV and
Ed = 4.5 meV wide gaps, respectively.Comment: 9 pages incl. 13 figure
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
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