3,369 research outputs found
Uncovering the Hidden Order in URu2Si2 by Impurity Doping
We report the use of impurities to probe the hidden order parameter of the
strongly correlated metal URu_2Si_2 below the transition temperature T_0 ~ 17.5
K. The nature of this order parameter has eluded researchers for more than two
decades, but is accompanied by the development of a partial gap in the single
particle density of states that can be detected through measurements of the
electronic specific heat and nuclear spin-lattice relaxation rate. We find that
impurities in the hidden order phase give rise to local patches of
antiferromagnetism. An analysis of the coupling between the antiferromagnetism
and the hidden order reveals that the former is not a competing order parameter
but rather a parasitic effect of the latter.Comment: 4 pages, 4 figure
A robust but disordered collapsed-volume phase in a cerium alloy under the application of pulsed magnetic fields
We report synchrotron x-ray powder diffraction measurements of
Ce0.8La0.1Th0.1 subject to pulsed magnetic fields as high as 28 Tesla. This
alloy is known to exhibit a continuous volume collapse on cooling at ambient
pressure, which is a modification of the gamma -> alpha transition in elemental
cerium. Recently, it has been suggested on the basis of field-cooled
resistivity and pulsed field magnetization measurements that the volume
collapse in this alloy can be suppressed by the application of magnetic fields.
Conversely, our direct diffraction measurements show a robust collapsed phase,
which persists in magnetic fields as high as 28 Tesla. We also observe
nanoscale disorder in the collapsed phase, which increasingly contaminates the
high temperature phase on thermal cycling.Comment: 6 pages, 6 figures, submitted to PR
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
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
Simulations of the effects of tin composition gradients on the superconducting properties of Nb3Sn conductors
In powder-in-tube (PIT) Nb3Sn composites, the A15 phase forms between a
central tin-rich core and a coaxial Nb tube, thus causing the tin content and
superconducting properties to vary with radius across the A15 layer. Since this
geometry is also ideal for magnetic characterization of the superconducting
properties with the field parallel to the tube axis, a system of concentric
shells with varying tin content was used to simulate the superconducting
properties, the overall severity of the Sn composition gradient being defined
by an index N. Using well-known scaling relationships and property trends
developed in an earlier experimental study, the critical current density for
each shell was calculated, and from this the magnetic moment of each shell was
found. By summing these moments, experimentally measured properties such as
pinning-force curves and Kramer plots could be simulated. We found that
different tin profiles have only a minor effect on the shape of Kramer plots,
but a pronounced effect on the irreversibility fields defined by the
extrapolation of Kramer plots. In fact, these extrapolated values H_K are very
close to a weighted average of the superconducting properties across the layer
for all N. The difference between H_K and the upper critical field commonly
seen in experiments is a direct consequence of the different ways measurements
probe the simulated Sn gradients. Sn gradients were found to be significantly
deleterious to the critical current density Jc, since reductions to both the
elementary pinning force and the flux pinning scaling field H_K compound the
reduction in Jc. The simulations show that significant gains in Jc of Nb3Sn
strands might be realized by circumventing strong compositional gradients of
tin.Comment: 10 pages, 8 figures, 2 tables, submitted to J. Appl. Phy
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
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