5,791 research outputs found
Carbon doping of superconducting magnesium diboride
We present details of synthesis optimization and physical properties of
nearly single phase carbon doped MgB2 with a nominal stoichiometry of
Mg(B{0.8}C{0.2})2 synthesized from magnesium and boron carbide (B4C) as
starting materials. The superconducting transition temperature is ~ 22 K (~ 17
K lower than in pure MgB2). The temperature dependence of the upper critical
field is steeper than in pure MgB2 with Hc2(10K) ~ 9 T. Temperature dependent
specific heat data taken in different applied magnetic fields suggest that the
two-gap nature of superconductivity is still preserved for carbon doped MgB2
even with such a heavily suppressed transition temperature. In addition, the
anisotropy ratio of the upper critical field for T/Tc ~ 2/3 is gamma ~ 2. This
value is distinct from 1 (isotropic) and also distinct from 6 (the value found
for pure MgB2).Comment: 11 pages, 13 figures, submitted to Physica
Supersymmetry and Dark Matter post LHC8: why we may expect both axion and WIMP detection
In the post-LHC8 era, it is perceived that what is left of SUSY model
parameter space is highly finetuned in the EW sector (EWFT). We discuss how
conventional measures overestimate EWFT in SUSY theory. Radiatively-driven
natural SUSY (RNS) models maintain the SUSY GUT paradigm with low EWFT at 10%
level, but are characterized by light higgsinos ~100-300 GeV and a thermal
underabundance of WIMP dark matter. Implementing the SUSY DFSZ solution to the
strong CP problem explains the small \mu parameter but indicates dark matter
should be comprised mainly of axions with a small admixture of higgsino-like
WIMPs. While RNS might escape LHC14 searches, we would expect ultimately direct
detection of both WIMPs and axions. An e^+e^- collider with \sqrt{s} ~ 500-600
GeV should provide a thorough search for the predicted light higgsinos.Comment: 11 pages; 5 figures: Talk given at Particle Physics and Cosmology
meeting (PPC2013), July 8, 2013, Deadwood, South Dakot
Effects of Boron Purity, Mg Stoichiometry and Carbon Substitution on Properties of Polycrystalline MgB
By synthesizing MgB using boron of different nominal purity we found
values of the residual resistivity ratio () from 4 to
20, which covers almost all values found in literature. To obtain high values
of , high purity reagents are necessary. With the isotopically pure boron
we obtained the highest 20 for the stoichiometric compound. We also
investigated MgB samples with 0.8 1.2. For the range
MgB up to MgB we found average values
of between 14 and 24. For smaller variations in stoichiometry () . All of our data point to the conclusion that high
() and low () are intrinsic
material properties associated with high purity MgB. In addition we have
performed initial work on optimizing the formation of carbon doped MgB
via the use of BC. Nearly single phase material can be formed by reaction
of nominal Mg(BC) for 24 hours at . The
for this composition is between and (depending on
criterion).Comment: accepted to Physica C, special MgB2 issu
Is natural higgsino-only dark matter excluded?
The requirement of electroweak naturalness in supersymmetric (SUSY) models of
particle physics necessitates light higgsinos not too far from the weak scale
characterized by m(weak)~ m(W,Z,h)~100 GeV. On the other hand, LHC Higgs mass
measurements and sparticle mass limits point to a SUSY breaking scale in the
multi-TeV regime. Under such conditions, the lightest SUSY particle is expected
to be a mainly higgsino-like neutralino with non-negligible gaugino components
(required by naturalness). The computed thermal WIMP abundance in natural SUSY
models is then found to be typically a factor 5-20 below its measured value. To
gain concordance with observations, either an additional DM particle (the axion
is a well-motivated possibility) must be present or additional non-thermal
mechanisms must augment the neutralino abundance. We compare present direct and
indirect WIMP detection limits to three natural SUSY models based on gravity-,
anomaly- and mirage-mediation. We show that the case of natural higgsino-only
dark matter where non-thermal production mechanisms augment its relic density,
is essentially excluded by a combination of direct detection constraints from
PandaX-II, LUX and Xenon-1t experiments, and by bounds from Fermi-LAT/MAGIC
observations of gamma rays from dwarf spheroidal galaxies.Comment: 16 pages with 6 .png figures; some added references for version
Supersymmetry with radiatively-driven naturalness: implications for WIMP and axion searches
By insisting on naturalness in both the electroweak and QCD sectors of the
MSSM, the portrait for dark matter production is seriously modified from the
usual WIMP miracle picture. In SUSY models with radiatively-driven naturalness
(radiative natural SUSY or RNS) which include a DFSZ-like solution to the
strong CP and SUSY mu problems, dark matter is expected to be an admixture of
both axions and higgsino-like WIMPs. The WIMP/axion abundance calculation
requires simultaneous solution of a set of coupled Boltzmann equations which
describe quasi-stable axinos and saxions. In most of parameter space, axions
make up the dominant contribution of dark matter although regions of WIMP
dominance also occur. We show the allowed range of PQ scale f_a and compare to
the values expected to be probed by the ADMX axion detector in the near future.
We also show WIMP detection rates which are suppressed from usual expectations
because now WIMPs comprise only a fraction of the total dark matter.
Nonetheless, ton-scale noble liquid detectors should be able to probe the
entirety of RNS parameter space. Indirect WIMP detection rates are less
propitious since they are reduced by the square of the depleted WIMP abundance.Comment: 25 pages with 14 .png figures; submitted to special issue
"Supersymmetry and Dark Matter" in Symmetry, edited by D. Clin
Superparticle phenomenology from the natural mini-landscape
The methodology of the heterotic mini-landscape attempts to zero in on
phenomenologically viable corners of the string landscape where the effective
low energy theory is the Minimal Supersymmetric Standard Model with localized
grand unification. The gaugino mass pattern is that of mirage-mediation. The
magnitudes of various SM Yukawa couplings point to a picture where scalar soft
SUSY breaking terms are related to the geography of fields in the compactified
dimensions. Higgs fields and third generation scalars extend to the bulk and
occur in split multiplets with TeV scale soft masses. First and second
generation scalars, localized at orbifold fixed points or tori with enhanced
symmetry, occur in complete GUT multiplets and have much larger masses. This
picture can be matched onto the parameter space of generalized mirage
mediation. Naturalness considerations, the requirement of the observed
electroweak symmetry breaking pattern, and LHC bounds on m(gluino) together
limit the gravitino mass to the m_{3/2}~ 5-60 TeV range. The mirage unification
scale is bounded from below with the limit depending on the ratio of squark to
gravitino masses. We show that while natural SUSY in this realization may
escape detection even at the high luminosity LHC, the high energy LHC with
\sqrt{s}=33 TeV could unequivocally confirm or exclude this scenario. It should
be possible to detect the expected light higgsinos at the ILC if these are
kinematically accessible, and possibly also discriminate the expected
compression of gaugino masses in the natural mini-landscape picture from the
mass pattern expected in models with gaugino mass unification. The thermal WIMP
signal should be accessible via direct detection searches at the multi-ton
noble liquid detectors such as Xenon-nT or LZ.Comment: 33 pages with 32 .png figure
Defining Petrophysical Units of the Palmer Deep Sites from Leg 178
Palmer Deep, on the inner continental shelf southwest of Anvers
Island off the Antarctic Peninsula, is a glacially overdeepened basin
consisting of three subbasins. Two sites, 1098 and 1099, were drilled in
the Palmer Deep area.
A high-resolution porosity curve has been calculated from density
data and subsequently plotted against the shipboard lithologic logs.
These new data correspond accurately to the lithologic logs, magnetic
susceptibility, and gamma ray attenuation (GRA) density data and offer
information on the heterogeneity of the sediments.
Petrophysical groups have been generated to investigate interrelationships
between different physical attributes. To develop these petrophysical
groups, crossplots of the available physical properties data
were performed. The results for the GRA density and magnetic susceptibility
crossplots demonstrate distinct clusters. Plotting the magnetic
susceptibility and GRA density data logs (divided into these new petrophysical
groups) against lithology provided information to subdivide
the lithologic unit(s) into a series of petrophysical units
Enhanced Critical parameters of nano-Carbon doped MgB2 Superconductor
The high field magnetization and magneto transport measurements are carried
out to determine the critical superconducting parameters of MgB2-xCx system.
The synthesized samples are pure phase and the lattice parameters evaluation is
carried out using the Rietveld refinement. The R-T(H) measurements are done up
to a field of 140 kOe. The upper critical field values, Hc2 are obtained from
this data based upon the criterion of 90% of normal resistivity i.e. Hc2=H at
which Rho=90%Rho; where RhoN is the normal resistivity i.e., resistivity at
about 40 K in our case. The Werthamer-Helfand-Hohenberg (WHH) prediction of
Hc(0) underestimates the critical field value even below than the field up to
which measurement is carried out. After this the model, the Ginzburg Landau
theory (GL equation) is applied to the R-T(H) data which not only calculates
the Hc2(0) value but also determines the dependence of Hc2 on temperature in
the low temperature high field region. The estimated Hc(0)=157.2 kOe for pure
MgB2 is profoundly enhanced to 297.5 kOe for the x=0.15 sample in MgB2-xCx
series. Magnetization measurements are done up to 120 kOe at different
temperatures and the other parameters like irreversibility field, Hirr and
critical current density Jc(H) are also calculated. The nano carbon doping
results in substantial enhancement of critical parameters like Hc2, Hirr and
Jc(H) in comparison to the pure MgB2 sample.Comment: 25 pages with 9 Figs:
comments/suggestions([email protected]
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