1,437 research outputs found
Cosmological Constant from Decoherence
We address the issue why a cosmological constant (dark energy) possesses a
small positive value instead of being zero. Motivated by the cosmic landscape
picture, we mimic the dark energy by a scalar field with potential wells and
show that other degrees of freedom interacting with it can localize this field
by decoherence in one of the wells. Dark energy can then acquire a small
positive value. We also show that the additional degrees of freedom enhance the
tunneling rate between the wells. The consideration is performed in detail for
the case of two wells and then extended to a large number of wells.Comment: 39 pages, 2 figures, final versio
Incoherent multi-gap optical solitons in nonlinear photonic lattices
We demonstrate numerically that partially incoherent light can be trapped in
the spectral band gaps of a photonic lattice, creating partially incoherent
multi-component spatial optical solitons in a self-defocusing nonlinear
periodic medium. We find numerically such incoherent multi-gap optical solitons
and discuss how to generate them in experiment by interfering incoherent light
beams at the input of a nonlinear periodic medium.Comment: 9 pages, 5 figure
Break up of heavy fermions at an antiferromagnetic instability
We present results of high-resolution, low-temperature measurements of the
Hall coefficient, thermopower, and specific heat on stoichiometric YbRh2Si2.
They support earlier conclusions of an electronic (Kondo-breakdown) quantum
critical point concurring with a field induced antiferromagnetic one. We also
discuss the detachment of the two instabilities under chemical pressure. Volume
compression/expansion (via substituting Rh by Co/Ir) results in a
stabilization/weakening of magnetic order. Moderate Ir substitution leads to a
non-Fermi-liquid phase, in which the magnetic moments are neither ordered nor
screened by the Kondo effect. The so-derived zero-temperature global phase
diagram promises future studies to explore the nature of the Kondo breakdown
quantum critical point without any interfering magnetism.Comment: minor changes, accepted for publication in JPS
Hall effect in heavy-fermion metals
The heavy fermion systems present a unique platform in which strong
electronic correlations give rise to a host of novel, and often competing,
electronic and magnetic ground states. Amongst a number of potential
experimental tools at our disposal, measurements of the Hall effect have
emerged as a particularly important one in discerning the nature and evolution
of the Fermi surfaces of these enigmatic metals. In this article, we present a
comprehensive review of Hall effect measurements in the heavy-fermion
materials, and examine the success it has had in contributing to our current
understanding of strongly correlated matter. Particular emphasis is placed on
its utility in the investigation of quantum critical phenomena which are
thought to drive many of the exotic electronic ground states in these systems.
This is achieved by the description of measurements of the Hall effect across
the putative zero-temperature instability in the archetypal heavy-fermion metal
YbRhSi. Using the CeIn (with Co, Ir) family of systems as
a paradigm, the influence of (antiferro-)magnetic fluctuations on the Hall
effect is also illustrated. This is compared to prior Hall effect measurements
in the cuprates and other strongly correlated systems to emphasize on the
generality of the unusual magnetotransport in materials with non-Fermi liquid
behavior.Comment: manuscript accepted in Adv. Phy
Moving Focus from Weight to Health. What Are the Components Used in Interventions to Improve Cardiovascular Health in Children?
Obesity in childhood impacts on many areas of the child's current and future health, including their cardiovascular health. To date many attempts have been made to design interventions to tackle excess childhood weight but with limited success. We aimed to establish the components common to interventions in children that improve cardiovascular health parameters
Comment on "Zeeman-Driven Lifshitz Transition: A Model for the Experimentally Observed Fermi-Surface Reconstruction in YbRh2Si2"
In Phys. Rev. Lett. 106, 137002 (2011), A. Hackl and M. Vojta have proposed
to explain the quantum critical behavior of YbRh2Si2 in terms of a
Zeeman-induced Lifshitz transition of an electronic band whose width is about 6
orders of magnitude smaller than that of conventional metals. Here, we note
that the ultra-narrowness of the proposed band, as well as the proposed
scenario per se, lead to properties which are qualitatively inconsistent with
the salient features observed in YbRh2Si2 near its quantum critical point.Comment: 3 page
H - T phase diagram of YbCo2Si2 with H // [100]
We report on the first high-resolution dc-magnetisation () measurements on
a single crystal of \ycs. was measured down to 0.05 K and in fields up to
12 T, with the magnetic field parallel to the crystallographic direction
[100]. Two antiferromagnetic (AFM) phase transitions have been detected in a
field T at K and K, in form of a
sharp cusp and a sudden drop in , respectively. These signatures
suggest that the phase transitions are order at and
order at . The upper transition is suppressed by a critical field
T. The field-dependent magnetisation shows two hysteretic
metamagnetic-like steps at the lowest temperature, followed by a sharp kink,
which separates the AFM region from the paramagnetic one. The magnetic
phase diagram of \ycs has been deduced from the isothermal and isofield curves.
Four AFM regions were identified which are separated by and
order phase-transition lines.Comment: 5 Pages, 3 figure
On Silicon Carbide Grains as the Carrier of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars
The mysterious 21mu emission feature seen in 12 proto-planetary nebulae
(PPNe) remains unidentified since its first detection in 1989. Over a dozen of
candidate materials have been proposed within the past decade, but none of them
has received general acceptance. Very recently, silicon carbide (SiC) grains
with impurities were suggested to be the carrier of this enigmatic feature,
based on recent laboratory data that doped SiC grains exhibit a resonance at
\~21mu. This proposal gains strength from the fact that SiC is a common dust
species in carbon-rich circumstellar envelopes. However, SiC dust has a strong
vibrational band at ~11.3mu. We show in this Letter that in order to be
consistent with the observed flux ratios of the 11.3mu feature to the 21mu
feature, the band strength of the 21mu resonance has to be very strong, too
strong to be consistent with current laboratory measurements. But this does not
yet readily rule out the SiC hypothesis since recent experimental results have
demonstrated that the 21mu resonance of doped SiC becomes stronger as the C
impurity increases. Further laboratory measurements of SiC dust with high
fractions of C impurity are urgently needed to test the hypothesis of SiC as
the carrier of the 21mu feature.Comment: 14 pages, 3 figures, accepted for publication in ApJ
The SiC problem: astronomical and meteoritic evidence
Pre-solar grains of silicon carbide found in meteorites and interpreted as
having had an origin around carbon stars from their isotopic composition, have
all been found to be of the beta-SiC polytype. Yet to date fits to the 11.3
microns SiC emission band of carbon stars had been obtained only for alpha-SiC
grains. We present thin film infrared (IR) absorption spectra measured in a
diamond anvil cell for both the alpha- and beta- polymorphs of synthetic SiC
and compare the results with previously published spectra taken using the KBr
matrix method. We find that our thin film spectra have positions nearly
identical to those obtained previously from finely ground samples in KBr.
Hence, we show that this discrepancy has arisen from inappropriate `KBr
corrections' having been made to laboratory spectra of SiC particles dispersed
in KBr matrices. We re-fit a sample of carbon star mid-IR spectra, using
laboratory data with no KBr correction applied, and show that beta-SiC grains
fit the observations, while alpha-SiC grains do not. The discrepancy between
meteoritic and astronomical identifications of the SiC-type is therefore
removed. This work shows that the diamond anvil cell thin film method can be
used to produce mineral spectra applicable to cosmic environments without
further manipulation.Comment: to be published in Astrophysical Journal Letter 4 pages, 3 figure
Macroscopic constitutive model for ergodic and non-ergodic lead-free relaxors
A fully electromechanically coupled, three dimensional phenomenological constitutive model for relaxor ferroelectric materials was developed for the use in a finite-element-method (FEM) solution procedure. This macroscopic model was used to simulate the macroscopic electromechanical response of lead-free ergodic 0.94Na1/2Bi1/2TiO3â0.06BaTiO3 and non-ergodic 0.90Na1/2Bi1/2TiO3â0.06BaTiO3â0.04K0.5Na0.5NbO3 relaxor materials. The presented constitutive model is capable of accounting for the observed pinched hysteretic response as well as non-deviatoric polarization induced strain and internal order transitions. Time integration of the history dependent internal variables is done with a predictor-corrector integration scheme. The adaptability of the constitutive model regarding the pinching of the hystereses is shown. Simulations are compared to experimental observations
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