1,318 research outputs found
Probing early-universe phase transitions with CMB spectral distortions
Global, symmetry-breaking phase transitions in the early universe can
generate scaling seed networks which lead to metric perturbations. The acoustic
waves in the photon-baryon plasma sourced by these metric perturbations, when
Silk damped, generate spectral distortions of the cosmic microwave background
(CMB). In this work, the chemical potential distortion () due to scaling
seed networks is computed and the accompanying Compton -type distortion is
estimated. The specific model of choice is the nonlinear -model
for , but the results remain the same order of magnitude for other
scaling seeds. If CMB anisotropy constraints to the model are saturated,
the resulting chemical potential distortion .Comment: 17 pages, 6 figures, v2: References added, submitted to Phys. Rev.
Spectral distortions from the dissipation of tensor perturbations
Spectral distortions of the cosmic microwave background (CMB) may become a
powerful probe of primordial perturbations at small scales. Existing studies of
spectral distortions focus almost exclusively on primordial scalar metric
perturbations. Similarly, vector and tensor perturbations should source CMB
spectral distortions. In this paper, we give general expressions for the
effective heating rate caused by these types of perturbations, including
previously neglected contributions from polarization states and higher
multipoles. We then focus our discussion on the dissipation of tensors, showing
that for nearly scale invariant tensor power spectra, the overall distortion is
some six orders of magnitudes smaller than from the damping of adiabatic scalar
modes. We find simple analytic expressions describing the effective heating
rate from tensors using a quasi-tight coupling approximation. In contrast to
adiabatic modes, tensors cause heating without additional photon diffusion and
thus over a wider range of scales, as recently pointed out by Ota et. al 2014.
Our results are in broad agreement with their conclusions, but we find that
small-scale modes beyond k< 2x10^4 Mpc^{-1} cannot be neglected, leading to a
larger distortion, especially for very blue tensor power spectra. At small
scales, also the effect of neutrino damping on the tensor amplitude needs to be
included.Comment: 14 pages, 7 figures, accepted version (MNRAS
Crystal-field effects in the mixed-valence compounds Yb2M3Ga9 (M= Rh, Ir)
Magnetic susceptibility, heat capacity, and electrical resistivity
measurements have been carried out on single crystals of the intermediate
valence compounds Yb2Rh3Ga9 and Yb2Ir3Ga9. These measurements reveal a large
anisotropy due apparently to an interplay between crystalline electric field
(CEF) and Kondo effects. The temperature dependence of magnetic susceptibility
can be modelled using the Anderson impurity model including CEF within an
approach based on the Non-Crossing Approximation.Comment: Accepted to Phys. Rev.
Pair-Breaking in Rotating Fermi Gases
We study the pair-breaking effect of rotation on a cold Fermi gas in the
BCS-BEC crossover region. In the framework of BCS theory, which is supposed to
be qualitatively correct at zero temperature, we find that in a trap rotating
around a symmetry axis, three regions have to be distinguished: (A) a region
near the rotational axis where the superfluid stays at rest and where no pairs
are broken, (B) a region where the pairs are progressively broken with
increasing distance from the rotational axis, resulting in an increasing
rotational current, and (C) a normal-fluid region where all pairs are broken
and which rotates like a rigid body. Due to region B, density and current do
not exhibit any discontinuities.Comment: 4 pages, 2 figures; v2: discussion clarified, typos corrected, one
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On the approximation of the limit cycles function
We consider planar vector fields depending on a real parameter. It is assumed that this vector field has a family of limit cycles which can be described by means of the limit cycles function . We prove a relationship between the multiplicity of a limit cycle of this family and the order of a zero of the limit cycles function. Moreover, we present a procedure to approximate , which is based on the Newton scheme applied to the Poincar'e function and represents a continuation method. Finally, we demonstrate the effectiveness of the proposed procedure by means of a Li'enard system. The obtained result supports a conjecture by Lins, de Melo and Pugh
Unconventional magnetism in multivalent charge-ordered YbPtGe probed by Pt- and Yb-NMR
Detailed Pt- and Yb nuclear magnetic resonance (NMR) studies
on the heterogeneous mixed valence system YbPtGe are reported. The
temperature dependence of the Pt-NMR shift indicates the
opening of an unusual magnetic gap below 200\,K. was analyzed by a
thermal activation model which yields an isotropic gap \,K. In contrast, the spin-lattice relaxation rate () does
not provide evidence for the gap. Therefore, an intermediate-valence picture is
proposed while a Kondo-insulator scenario can be excluded. Moreover,
() follows a simple metallic behavior, similar to the reference
compound YPtGe. A well resolved NMR line with small shift is assigned to
divalent Yb. This finding supports the proposed model with two sub-sets
of Yb species (di- and trivalent) located on the Yb2 and Yb1 site of the
YbPtGe lattice.Comment: Submitted in Physical Review B (Rapid Communication
Dulac-Cherkas functions for generalized Liénard systems
Dulac-Cherkas functions can be used to derive an upper bound for the number of limit cycles of planar autonomous differential systems including criteria for the non-existence of limit cycles, at the same time they provide information about their stability and hyperbolicity. In this paper, we present a method to construct  a special class of Dulac-Cherkas functions for  generalized Liénard systems of the type with . In case ,  linear differential equations play a key role in this process, for , we have to solve a system of linear differential and algebraic equations, where the number of equations is larger than the number of unknowns. Finally,  we show that Dulac-Cherkas functions can be used to construct generalized Liénard systems with any possessing limit cycles
Mars: Noachian hydrology by its statistics and topology
Discrimination between fluvial features generated by surface drainage and subsurface aquifer discharges will provide clues to the understanding of early Mars' climatic history. Our approach is to define the process of formation of the oldest fluvial valleys by statistical and topological analyses. Formation of fluvial valley systems reached its highest statistical concentration during the Noachian Period. Nevertheless, they are a scarce phenomenom in Martian history, localized on the craterized upland, and subject to latitudinal distribution. They occur sparsely on Noachian geological units with a weak distribution density, and appear in reduced isolated surface (around 5 x 10(exp 3)(sq km)), filled by short streams (100-300 km length). Topological analysis of the internal organization of 71 surveyed Noachian fluvial valley networks also provides information on the mechanisms of formation
Superconductivity in the New Platinum Germanides MPt4Ge12 (M = Rare-earth and Alkaline-earth Metals) with Filled Skutterudite Structure
New germanium-platinum compounds with the filled-skutterudite crystal
structure were synthesized. The structure and composition were investigated by
X-ray diffraction and microprobe analysis. Magnetic susceptibility, specific
heat, and electrical resistivity measurements evidence superconductivity in
LaPt4Ge12 and PrPt4Ge12 below 8.3K. The parameters of the normal and
superconducting states were established. Strong coupling and a crystal electric
field singlet groundstate is found for the Pr compound. Electronic structure
calculations show a large density of states at the Fermi level. Similar
behavior with lower T_c was observed for SrPt4Ge12 and BaPt4Ge12.Comment: RevTeX, 4 figures, submitted to Physical Review Letters July 12, 200
On the origin of heavy quasiparticles in LiV_2O_4
An explanation is provided for the heavy quasiparticle excitations in
LiV_2O_4. It differs considerably from that of other known heavy-fermion
systems. Main ingredients of our theory are the cubic spinel structure of the
material and strong short-range correlations of the d electrons. The large
gamma-coefficient is shown to result from excitations of Heisenberg spin 1/2
rings and chains. The required coupling constant is calculated from LDA+U
calculations and is found to be of the right size. Also the calculated
Sommerfeld-Wilson ratio is reasonably close to the observed one.Comment: REVTEX, 5 pages, 2 figure
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