11,433 research outputs found
The double life of electrons in magnetic iron pnictides, as revealed by NMR
We present a phenomenological, two-fluid approach to understanding the
magnetic excitations in Fe pnictides, in which a paramagnetic fluid with
gapless, incoherent particle-hole excitations coexists with an
antiferromagnetic fluid with gapped, coherent spin wave excitations. We show
that this two-fluid phenomenology provides an excellent quantitative
description of NMR data for magnetic "122" pnictides, and argue that it finds a
natural justification in LSDA and spin density wave calculations. We further
use this phenomenology to estimate the maximum renormalisation of the ordered
moment that can follow from low-energy spin fluctuations in Fe pnictides. We
find that this is too small to account for the discrepancy between ab intio
calculations and neutron scattering measurements.Comment: Accepted for publication in Europhys. Lett. 6 pages, 4 figure
Magnetic and Structural Studies of the Quasi-Two-Dimensional Spin-Gap System (CuCl)LaNb2O7
We report magnetization, nuclear magnetic resonance (NMR), nuclear quadrupole
resonance (NQR), and transmission electron microscopy (TEM) studies on the
quasi-two-dimensional spin-gap system (CuCl)LaNb2O7, a possible candidate for
the J1-J2 model on a square lattice. A sharp single NQR line is observed at the
Cu and Cl sites, indicating that both Cu and Cl atoms occupy a unique site.
However, the electric field gradient tensors at the Cu, Cl, and La sites do not
have axial symmetry. This is incompatible with the reported crystal structure.
Thus the J1-J2 model has to be modified. We propose alternative two-dimensional
dimer models based on the NMR, NQR, and TEM results. The value of the hyperfine
coupling constant at the Cu sites indicates that the spin density is mainly on
the d(3z2-r2) orbital (z parallel c). At 1.5 K, Cu- and Nb-NMR signals
disappear above the critical field Bc1 = 10.3 T determined from the onset of
the magnetization, indicating a field-induced magnetic phase transition at Bc1.Comment: 9 pages, 16 figure
Understanding past population dynamics: Bayesian coalescent-based modeling with covariates
Effective population size characterizes the genetic variability in a population and is a parameter of paramount importance in population genetics. Kingman's coalescent process enables inference of past population dynamics directly from molecular sequence data, and researchers have developed a number of flexible coalescent-based models for Bayesian nonparametric estimation of the effective population size as a function of time. A major goal of demographic reconstruction is understanding the association between the effective population size and potential explanatory factors. Building upon Bayesian nonparametric coalescent-based approaches, we introduce a flexible framework that incorporates time-varying covariates through Gaussian Markov random fields. To approximate the posterior distribution, we adapt efficient Markov chain Monte Carlo algorithms designed for highly structured Gaussian models. Incorporating covariates into the demographic inference framework enables the modeling of associations between the effective population size and covariates while accounting for uncertainty in population histories. Furthermore, it can lead to more precise estimates of population dynamics. We apply our model to four examples. We reconstruct the demographic history of raccoon rabies in North America and find a significant association with the spatiotemporal spread of the outbreak. Next, we examine the effective population size trajectory of the DENV-4 virus in Puerto Rico along with viral isolate count data and find similar cyclic patterns. We compare the population history of the HIV-1 CRF02_AG clade in Cameroon with HIV incidence and prevalence data and find that the effective population size is more reflective of incidence rate. Finally, we explore the hypothesis that the population dynamics of musk ox during the Late Quaternary period were related to climate change
Half-magnetization plateaux in Cr spinels
Magnetization plateaux, visible as anomalies in magnetic susceptibility at
low temperatures, are one of the hallmarks of frustrated magnetism. An
extremely robust half-magnetization plateau is observed in the spinel oxides
CdCr2O4 and HgCr2O4, where it is accompanied by a substantial lattice
distortion. We give an overview of the present state experiment for CdCr2O4 and
HgCr2O4, and show how such a half-magnetization plateau arises quite naturally
in a simple model of these systems, once coupling to the lattice is taken into
account.Comment: 8 pages latex using IOP macros, from review talk given at RHMF 2006
(Sendai
Information capacity of genetic regulatory elements
Changes in a cell's external or internal conditions are usually reflected in
the concentrations of the relevant transcription factors. These proteins in
turn modulate the expression levels of the genes under their control and
sometimes need to perform non-trivial computations that integrate several
inputs and affect multiple genes. At the same time, the activities of the
regulated genes would fluctuate even if the inputs were held fixed, as a
consequence of the intrinsic noise in the system, and such noise must
fundamentally limit the reliability of any genetic computation. Here we use
information theory to formalize the notion of information transmission in
simple genetic regulatory elements in the presence of physically realistic
noise sources. The dependence of this "channel capacity" on noise parameters,
cooperativity and cost of making signaling molecules is explored
systematically. We find that, at least in principle, capacities higher than one
bit should be achievable and that consequently genetic regulation is not
limited the use of binary, or "on-off", components.Comment: 17 pages, 9 figure
Information Flow through a Chaotic Channel: Prediction and Postdiction at Finite Resolution
We reconsider the persistence of information under the dynamics of the
logistic map in order to discuss communication through a nonlinear channel
where the sender can set the initial state of the system with finite
resolution, and the recipient measures it with the same accuracy. We separate
out the contributions of global phase space shrinkage and local phase space
contraction and expansion to the uncertainty in predicting and postdicting the
state of the system. Thus, we determine how the amplification parameter, the
time lag, and the resolution influence the possibility for communication. A
novel representation for real numbers is introduced that allows for a
visualization of the flow of information between scales.Comment: 14 pages, 13 figure
Quantum conditional operator and a criterion for separability
We analyze the properties of the conditional amplitude operator, the quantum
analog of the conditional probability which has been introduced in
[quant-ph/9512022]. The spectrum of the conditional operator characterizing a
quantum bipartite system is invariant under local unitary transformations and
reflects its inseparability. More specifically, it is shown that the
conditional amplitude operator of a separable state cannot have an eigenvalue
exceeding 1, which results in a necessary condition for separability. This
leads us to consider a related separability criterion based on the positive map
, where is an Hermitian operator. Any
separable state is mapped by the tensor product of this map and the identity
into a non-negative operator, which provides a simple necessary condition for
separability. In the special case where one subsystem is a quantum bit,
reduces to time-reversal, so that this separability condition is
equivalent to partial transposition. It is therefore also sufficient for
and systems. Finally, a simple connection between this
map and complex conjugation in the "magic" basis is displayed.Comment: 19 pages, RevTe
Methods of Nature: Landscapes from the Gettysburg College Collection
Methods of Nature: Landscapes from the Gettysburg College Collection is the third annual exhibition curated by students enrolled in the Art History Methods course. The exhibition is an exciting academic endeavor and incredible opportunity for engaged learning, research, and curatorial experience. The five student curators are Molly Chason ’17, Leah Falk ’18, Shannon Gross ’17, Bailey Harper ’19 and Laura Waters ’19. The selection of artworks in this exhibition includes the depiction of landscape in the nineteenth- and twentieth-century French, American and East Asian cultural traditions in various art forms from traditional media of paintings and prints to utilitarian artifacts of porcelain and a paper folding fan. Landscape paintings in this exhibition are inspired by nature, specific locales and literature. Each object carries a distinctive characteristic, a mood, and an ambience. Collectively, they present a multifaceted view of the landscape in the heart and mind of the artists and intended viewers. [excerpt]https://cupola.gettysburg.edu/artcatalogs/1020/thumbnail.jp
Exact Results for the Crossover from Gaussian to Non-Gaussian Order Parameter Fluctuations in Quasi One-Dimensional Electronic Systems
The physics of quasi one-dimensional Peierls systems is dominated by order
parameter fluctuations. We present an algorithm which allows for the first time
to exactly calculate physical properties of the electrons gas coupled to
classical order parameter fluctuations. The whole range from the Gaussian
regime dominated by amplitude fluctuations to the non-Gaussian regime dominated
by phase fluctuations is accessible. Our results provide insight into the
'pseudogap' phenomenon occurring in underdoped high-temperature
superconductors, quasi one-dimensional organic conductors and liquid metals.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
Negative thermal expansion in the plateau state of a magnetically-frustrated spinel
We report on negative thermal expansion (NTE) in the high-field,
half-magnetization plateau phase of the frustrated magnetic insulator CdCr2O4.
Using dilatometry, we precisely map the phase diagram at fields of up to 30T,
and identify a strong NTE associated with the collinear half-magnetization
plateau for B > 27T. The resulting phase diagram is compared with a microscopic
theory for spin-lattice coupling, and the origin of the NTE is identified as a
large negative change in magnetization with temperature, coming from a
nearly-localised band of spin excitations in the plateau phase. These results
provide useful guidelines for the discovery of new NTE materials.Comment: 6 pages, 2 figure
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