99,466 research outputs found
Severe Fermi Surface Reconstruction at a Metamagnetic-Transition in CaSrRuO (for )
We report an electrical transport study in CaSrRuO single
crystals at high magnetic fields (). For , the Hall constant
decreases sharply at an anisotropic metamagnetic (MM) transition
reaching its value for SrRuO at high fields. A sharp decrease in the
coefficient of the resistivity -term and a change in the structure of
the angular magnetoresistance oscillations (AMRO) for rotating in the
planes, confirms the reconstruction of the Fermi surface (FS). Our observations
and LDA calculations indicate a strong dependence of the FS on the Ca
concentration and suggest the coexistence of itinerant and localized electronic
states in single layered ruthenates.Comment: 5 pages, 4 fig
Density functional approach to finite temperature nuclear properties and the role of a momentum dependent isovector interaction
Using a density functional approach based on a Skyrme interaction,
thermodynamic properties of finite nuclei are investigated at non-zero
temperture. The role of a momentum dependent isovector term is now studied
besides volume, symmetry, surface and Coulomb effects. Various features
associated with both mechanical and chemical instability and the liquid-gas
coexistence curve are sensitive to the Skyrme interaction. The separated
effects of the isoscalar term and the isovector term of momentum dependent
interaction are studied for a modified SKM() interaction. The frequently
used Skyrme interaction SLy4 is one of the cases considered and is shown to
have better features for neutron star studies due to a larger symmetry energy.Comment: 14 pages, 6 figures, added more discussio
Pressure effects on the heavy-fermion antiferromagnet CeAuSb2
The f-electron compound CeAuSb2, which crystallizes in the ZrCuSi2-type
tetragonal structure, orders antiferromagnetically between 5 and 6.8 K, where
the antiferromagnetic transition temperature T_N depends on the occupancy of
the Au site. Here we report the electrical resistivity and heat capacity of a
high-quality crystal CeAuSb2 with T_N of 6.8 K, the highest for this compound.
The magnetic transition temperature is initially suppressed with pressure, but
is intercepted by a new magnetic state above 2.1 GPa. The new phase shows a
dome shape with pressure and coexists with another phase at pressures higher
than 4.7 GPa. The electrical resistivity shows a T^2 Fermi liquids behavior in
the complex magnetic state, and the residual resistivity and the T^2
resistivity coefficient increases with pressure, suggesting the possibility of
a magnetic quantum critical point at a higher pressure.Comment: 5 pages, 5 firure
Conductance Correlations Near Integer Quantum Hall Transitions
In a disordered mesoscopic system, the typical spacing between the peaks and
the valleys of the conductance as a function of Fermi energy is called
the conductance energy correlation range . Under the ergodic hypothesis,
the latter is determined by the half-width of the ensemble averaged conductance
correlation function: . In
ordinary diffusive metals, , where is the diffusion constant
and is the linear dimension of the phase-coherent sample. However, near a
quantum phase transition driven by the location of the Fermi energy , the
above picture breaks down. As an example of the latter, we study, for the first
time, the conductance correlations near the integer quantum Hall transitions of
which is a critical coupling constant. We point out that the behavior of
is determined by the interplay between the static and the dynamic
properties of the critical phenomena.Comment: 4 pages, 4 figures, minor corrections, to appear in Phys. Rev. Let
Prompt Iron Enrichment, Two r-Process Components, and Abundances in Very Metal-Poor Stars
We present a model to explain the wide range of abundances for heavy
r-process elements (mass number A > 130) at low [Fe/H]. This model requires
rapid star formation and/or an initial population of supermassive stars in the
earliest condensed clots of matter to provide a prompt or initial Fe inventory.
Subsequent Fe and r-process enrichment was provided by two types of supernovae:
one producing heavy r-elements with no Fe on a rather short timescale and the
other producing light r-elements (A < or = 130) with Fe on a much longer
timescale.Comment: 5 pages, 2 postscript figures, to appear in ApJ
How Algorithmic Confounding in Recommendation Systems Increases Homogeneity and Decreases Utility
Recommendation systems are ubiquitous and impact many domains; they have the
potential to influence product consumption, individuals' perceptions of the
world, and life-altering decisions. These systems are often evaluated or
trained with data from users already exposed to algorithmic recommendations;
this creates a pernicious feedback loop. Using simulations, we demonstrate how
using data confounded in this way homogenizes user behavior without increasing
utility
Coupling of spin and orbital excitations in the iron-based superconductor FeSe(0.5)Te(0.5)
We present a combined analysis of neutron scattering and photoemission
measurements on superconducting FeSe(0.5)Te(0.5). The low-energy magnetic
excitations disperse only in the direction transverse to the characteristic
wave vector (1/2,0,0), whereas the electronic Fermi surface near (1/2,0,0)
appears to consist of four incommensurate pockets. While the spin resonance
occurs at an incommensurate wave vector compatible with nesting, neither
spin-wave nor Fermi-surface-nesting models can describe the magnetic
dispersion. We propose that a coupling of spin and orbital correlations is key
to explaining this behavior. If correct, it follows that these nematic
fluctuations are involved in the resonance and could be relevant to the pairing
mechanism.Comment: 4 pages, 4 figures; accepted versio
Supernovae versus Neutron Star Mergers as the Major r-Process Sources
I show that recent observations of r-process abundances in metal-poor stars
are difficult to explain if neutron star mergers (NSMs) are the major r-process
sources. In contrast, such observations and meteoritic data on Hf182 and I129
in the early solar system support a self-consistent picture of r-process
enrichment by supernovae (SNe). While further theoretical studies of r-process
production and enrichment are needed for both SNe and NSMs, I emphasize two
possible direct observational tests of the SN r-process model: gamma rays from
decay of r-process nuclei in SN remnants and surface contamination of the
companion by SN r-process ejecta in binaries.Comment: 5 pages, to appear in ApJ
Superconducting energy gap in MgCNi3 single crystals: Point-contact spectroscopy and specific-heat measurements
Specific heat has been measured down to 600 mK and up to 8 Tesla by the
highly sensitive AC microcalorimetry on the MgCNi3 single crystals with Tc ~ 7
K. Exponential decay of the electronic specific heat at low temperatures proved
that a superconducting energy gap is fully open on the whole Fermi surface, in
agreement with our previous magnetic penetration depth measurements on the same
crystals. The specific-heat data analysis shows consistently the strong
coupling strength 2D/kTc ~ 4. This scenario is supported by the direct gap
measurements via the point-contact spectroscopy. Moreover, the spectroscopy
measurements show a decrease in the critical temperature at the sample surface
accounting for the observed differences of the superfluid density deduced from
the measurements by different techniques
Distinct Fermi-Momentum Dependent Energy Gaps in Deeply Underdoped Bi2212
We use angle-resolved photoemission spectroscopy applied to deeply underdoped
cuprate superconductors Bi2Sr2(Ca,Y)Cu2O8 (Bi2212) to reveal the presence of
two distinct energy gaps exhibiting different doping dependence. One gap,
associated with the antinodal region where no coherent peak is observed,
increases with underdoping - a behavior known for more than a decade and
considered as the general gap behavior in the underdoped regime. The other gap,
associated with the near nodal regime where a coherent peak in the spectrum can
be observed, does not increase with less doping - a behavior not observed in
the single particle spectra before. We propose a two-gap scenario in momentum
space that is consistent with other experiments and may contain important
information on the mechanism of high-Tc superconductivity.Comment: 12 pages, 3 figures, submitted to Scienc
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