2,561 research outputs found
Scaling and data collapse from local moments in frustrated disordered quantum spin systems
Recently measurements on various spin-1/2 quantum magnets such as
HLiIrO, LiZnMoO, ZnCu(OH)Cl and 1T-TaS
-- all described by magnetic frustration and quenched disorder but with no
other common relation -- nevertheless showed apparently universal scaling
features at low temperature. In particular the heat capacity C[H,T] in
temperature T and magnetic field H exhibits T/H data collapse reminiscent of
scaling near a critical point. Here we propose a theory for this scaling
collapse based on an emergent random-singlet regime extended to include
spin-orbit coupling and antisymmetric Dzyaloshinskii-Moriya (DM) interactions.
We derive the scaling with at small , with (0,1,2) an integer exponent whose value
depends on spatial symmetries. The agreement with experiments indicates that a
fraction of spins form random valence bonds and that these are surrounded by a
quantum paramagnetic phase. We also discuss distinct scaling for magnetization
with a -dependent subdominant term enforced by Maxwell's relations.Comment: v2. Expanded argument in Appendix 2 and revised for clarity. v3.
Fixed typo in Fig 3 caption. Main text 4 pages 4 figures, Appendix 6 pages 1
figur
Dosage compensation in birds
AbstractThe Z and W sex chromosomes of birds have evolved independently from the mammalian X and Y chromosomes [1]. Unlike mammals, female birds are heterogametic (ZW), while males are homogametic (ZZ). Therefore male birds, like female mammals, carry a double dose of sex-linked genes relative to the other sex. Other animals with nonhomologous sex chromosomes possess “dosage compensation” systems to equalize the expression of sex-linked genes. Dosage compensation occurs in animals as diverse as mammals, insects, and nematodes, although the mechanisms involved differ profoundly [2]. In birds, however, it is widely accepted that dosage compensation does not occur [3–5], and the differential expression of Z-linked genes has been suggested to underlie the avian sex-determination mechanism [6]. Here we show equivalent expression of at least six of nine Z chromosome genes in male and female chick embryos by using real-time quantitative PCR [7]. Only the Z-linked ScII gene, whose ortholog in Caenorhabditis elegans plays a crucial role in dosage compensation [8], escapes compensation by this assay. Our results imply that the majority of Z-linked genes in the chicken are dosage compensated
The EULAR–OMERACT rheumatoid arthritis MRI reference image atlas: the metacarpophalangeal joints
This paper presents the metacarpophalangeal (MCP) joint magnetic resonance images of the EULAR–OMERACT rheumatoid arthritis MRI reference image atlas. The illustrations include synovitis in the MCP joints (OMERACT RA magnetic resonance imaging scoring system (RAMRIS), grades 0–3), bone oedema in the metacarpal head and the phalangeal base (grades 0–3), and bone erosion in the metacarpal head and the phalangeal base (grades 0–3, and examples of higher grades). The presented reference images can be used to guide scoring of MCP joints according to the OMERACT RA MRI scoring system
An introduction to the EULAR–OMERACT rheumatoid arthritis MRI reference image atlas
This article gives a short overview of the development and characteristics of the OMERACT rheumatoid arthritis MRI scoring system (RAMRIS), followed by an introduction to the use of the EULAR–OMERACT rheumatoid arthritis MRI reference image atlas. With this atlas, MRIs of wrist and metacarpophalangeal joints of patients with rheumatoid arthritis can be scored for synovitis, bone oedema, and bone erosion, guided by standard reference images
Heat capacity anomaly at the quantum critical point of the Transverse Ising Magnet CoNb_2O_6
The transverse Ising magnet Hamiltonian describing the Ising chain in a
transverse magnetic field is the archetypal example of a system that undergoes
a transition at a quantum critical point (QCP). The columbite CoNbO is
the closest realization of the transverse Ising magnet found to date. At low
temperatures, neutron diffraction has observed a set of discrete collective
spin modes near the QCP. We ask if there are low-lying spin excitations
distinct from these relatively high energy modes. Using the heat capacity, we
show that a significant band of gapless spin excitations exists. At the QCP,
their spin entropy rises to a prominent peak that accounts for 30 of the
total spin degrees of freedom. In a narrow field interval below the QCP, the
gapless excitations display a fermion-like, temperature-linear heat capacity
below 1 K. These novel gapless modes are the main spin excitations
participating in, and affected, by the quantum transition.Comment: 14 pages total, 8 figure
The development of the EULAR–OMERACT rheumatoid arthritis MRI reference image atlas
Based on a previously developed rheumatoid arthritis MRI scoring system (OMERACT 2002 RAMRIS), the development team agreed which joints, MRI features, MRI sequences, and image planes would best illustrate the scoring system in an atlas. After collecting representative examples for all grades for each abnormality (synovitis, bone oedema, and bone erosion), the team met for a three day period to review the images and choose by consensus the most illustrative set for each feature, site, and grade. A predefined subset of images (for example, for erosion—all coronal slices through the bone) was extracted. These images were then re-read by the group at a different time point to confirm the scores originally assigned. Finally, all selected images were photographed and formatted by one centre and distributed to all readers for final approval
Building a quantum superposition of conscious states with integrated information theory
Could there be a quantum superposition of consciousness, as in the Wigner's
friend thought experiment? The integrated information theory (IIT) of
consciousness has turned this into a well-defined question. According to IIT,
consciousness is a measurable physical quantity given by integrated information
(), such that the amount of consciousness in a system corresponds to its
amount of . We use the most recent IIT formalism (IIT4.0) to analyze the
simplest non-zero system known as a feedback dyad. We then propose a
circuit that puts the dyad into a superposition of states which, according to
IIT, would correspond to a superposition of conscious states. We refer to this
as "Schr\"odinger's dyad". We therefore show that either IIT is false or the
simple dyad is conscious and can easily be put into a superposition of
conscious states. We then identify the simplest possible consciousness-collapse
model, which predicts that this superposition is unstable and collapses at a
rate determined by a measure of difference between the superposed conscious
states. Our analysis will enable us to make a number of key observations about
the general structure of integrated information theory (IIT2.0, IIT3.0, IIT4.0,
and QIIT) and the general structure of consciousness-collapse models
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