264 research outputs found
R\'enyi entanglement entropy of critical SU() spin chains
We present a study of the scaling behavior of the R\'{e}nyi entanglement
entropy (REE) in SU() spin chain Hamiltonians, in which all the spins
transform under the fundamental representation. These SU() spin chains are
known to be quantum critical and described by a well known Wess-Zumino-Witten
(WZW) non-linear sigma model in the continuum limit. Numerical results from our
lattice Hamiltonian are obtained using stochastic series expansion (SSE)
quantum Monte Carlo for both closed and open boundary conditions. As expected
for this 1D critical system, the REE shows a logarithmic dependence on the
subsystem size with a prefector given by the central charge of the SU() WZW
model. We study in detail the sub-leading oscillatory terms in the REE under
both periodic and open boundaries. Each oscillatory term is associated with a
WZW field and decays as a power law with an exponent proportional to the
scaling dimension of the corresponding field. We find that the use of periodic
boundaries (where oscillations are less prominent) allows for a better estimate
of the central charge, while using open boundaries allows for a better estimate
of the scaling dimensions. For completeness we also present numerical data on
the thermal R\'{e}nyi entropy which equally allows for extraction of the
central charge.Comment: 8 pages, 13 figure
Spin Bose-Metal and Valence Bond Solid phases in a spin-1/2 model with ring exchanges on a four-leg triangular ladder
We study a spin-1/2 system with Heisenberg plus ring exchanges on a four-leg
triangular ladder using the density matrix renormalization group and Gutzwiller
variational wave functions. Near an isotropic lattice regime, for moderate to
large ring exchanges we find a spin Bose-metal phase with a spinon Fermi sea
consisting of three partially filled bands. Going away from the triangular
towards the square lattice regime, we find a staggered dimer phase with dimers
in the transverse direction, while for small ring exchanges the system is in a
featureless rung phase. We also discuss parent states and a possible phase
diagram in two dimensions.Comment: 4 pages, 5 figures, v3 is the print versio
Exotic Gapless Mott Insulators of Bosons on Multi-Leg Ladders
We present evidence for an exotic gapless insulating phase of hard-core
bosons on multi-leg ladders with a density commensurate with the number of
legs. In particular, we study in detail a model of bosons moving with direct
hopping and frustrating ring exchange on a 3-leg ladder at filling.
For sufficiently large ring exchange, the system is insulating along the ladder
but has two gapless modes and power law transverse density correlations at
incommensurate wave vectors. We propose a determinantal wave function for this
phase and find excellent comparison between variational Monte Carlo and density
matrix renormalization group calculations on the model Hamiltonian, thus
providing strong evidence for the existence of this exotic phase. Finally, we
discuss extensions of our results to other -leg systems and to -layer
two-dimensional structures.Comment: 5 pages, 4 figures; v3 is the print version; supplemental material
attache
Bose Metals and Insulators on Multi-Leg Ladders with Ring Exchange
We establish compelling evidence for the existence of new
quasi-one-dimensional descendants of the d-wave Bose liquid (DBL), an exotic
two-dimensional quantum phase of uncondensed itinerant bosons characterized by
surfaces of gapless excitations in momentum space [O. I. Motrunich and M. P. A.
Fisher, Phys. Rev. B {\bf 75}, 235116 (2007)]. In particular, motivated by a
strong-coupling analysis of the gauge theory for the DBL, we study a model of
hard-core bosons moving on the -leg square ladder with frustrating four-site
ring exchange. Here, we focus on four- and three-leg systems where we have
identified two novel phases: a compressible gapless Bose metal on the four-leg
ladder and an incompressible gapless Mott insulator on the three-leg ladder.
The former is conducting along the ladder and has five gapless modes, one more
than the number of legs. This represents a significant step forward in
establishing the potential stability of the DBL in two dimensions. The latter,
on the other hand, is a fundamentally quasi-one-dimensional phase that is
insulating along the ladder but has two gapless modes and incommensurate power
law transverse density-density correlations. In both cases, we can understand
the nature of the phase using slave-particle-inspired variational wave
functions consisting of a product of two distinct Slater determinants, the
properties of which compare impressively well to a density matrix
renormalization group solution of the model Hamiltonian. Stability arguments
are made in favor of both quantum phases by accessing the universal low-energy
physics with a bosonization analysis of the appropriate quasi-1D gauge theory.
We will briefly discuss the potential relevance of these findings to
high-temperature superconductors, cold atomic gases, and frustrated quantum
magnets.Comment: 33 pages, 16 figures; this is the print version, only very minor
changes from v
The N\'{e}el-VBS transition in three-dimensional SU() antiferromagnets
We present results for the phase diagram of an SU() generalization of the
Heisenberg antiferromagnet on a bipartite three-dimensional anisotropic cubic
(tetragonal) lattice as a function of and the lattice anisotropy .
In the "isotropic" cubic limit, we find a transition from N\'{e}el
to valence bond solid (VBS) between N=9 and N=10. We follow the N\'{e}el-VBS
transition to the limiting cases of (weakly coupled layers) and
(weakly coupled chains). Throughout the phase diagram we find a
direct first-order transition from N\'{e}el at small- to VBS at large-.
In the three-dimensional models studied here, we find no evidence for either an
intervening spin-liquid "photon" phase or a continuous transition, even close
to the limit where the isolated layers undergo continuous
N\'{e}el-VBS transitions.Comment: 6 pages, 5 figure
Non-Fermi-liquid d-wave metal phase of strongly interacting electrons
Developing a theoretical framework for conducting electronic fluids
qualitatively distinct from those described by Landau's Fermi-liquid theory is
of central importance to many outstanding problems in condensed matter physics.
One such problem is that, above the transition temperature and near optimal
doping, high-transition-temperature copper-oxide superconductors exhibit
`strange metal' behaviour that is inconsistent with being a traditional Landau
Fermi liquid. Indeed, a microscopic theory of a strange-metal quantum phase
could shed new light on the interesting low-temperature behaviour in the
pseudogap regime and on the d-wave superconductor itself. Here we present a
theory for a specific example of a strange metal---the 'd-wave metal'. Using
variational wavefunctions, gauge theoretic arguments, and ultimately
large-scale density matrix renormalization group calculations, we show that
this remarkable quantum phase is the ground state of a reasonable microscopic
Hamiltonian---the usual t-J model with electron kinetic energy and two-spin
exchange supplemented with a frustrated electron `ring-exchange' term,
which we here examine extensively on the square lattice two-leg ladder. These
findings constitute an explicit theoretical example of a genuine
non-Fermi-liquid metal existing as the ground state of a realistic model.Comment: 22 pages, 12 figures: 6 pages, 7 figures of main text + 16 pages, 5
figures of Supplementary Information; this is approximately the version
published in Nature, minus various subedits in the main tex
PCORnet Antibiotics and Childhood Growth Study: Process for Cohort Creation and Cohort Description
OBJECTIVES: The National Patient-Centered Clinical Research Network (PCORnet) supports observational and clinical research using health care data. The PCORnet Antibiotics and Childhood Growth Study is one of PCORnet’s inaugural observational studies. We sought to describe the processes used to integrate and analyze data from children across 35 participating institutions, the cohort characteristics, and prevalence of antibiotic use.
METHODS:We included children in the cohort if they had at least one same-day height and weight measured in each of 3 age periods: 1) before 12 months, 2) 12 to 30 months, and 3) after 24 months. We distributed statistical queries that each institution ran on its local version of the PCORnet Common Data Model, with aggregate data returned for analysis. We defined overweight or obesity as age- and sex-specific body mass index ≥85th percentile, obesity ≥95th percentile, and severe obesity ≥120% of the 95th percentile.
RESULTS: A total of 681,739 children met the cohort inclusion criteria, and participants were racially/ethnically diverse (24.9% black, 17.5% Hispanic). Before 24 months of age, 55.2% of children received at least one antibiotic prescription; 21.3% received a single antibiotic prescription; 14.3% received 4 or more; and 33.3% received a broad-spectrum antibiotic. Overweight and obesity prevalence was 27.6% at age 4 to(n = 362,044) and 36.2% at 9 to(n = 58,344).
CONCLUSIONS: The PCORnet Antibiotics and Childhood Growth Study is a large national longitudinal observational study in a diverse population that will examine the relationship between early antibiotic use and subsequent growth patterns in children
Formation of a cytoplasmic salt bridge network in the matrix state is a fundamental step in the transport mechanism of the mitochondrial ADP/ATP carrier
Mitochondrial ADP/ATP carriers catalyze the equimolar exchange of ADP and ATP across the mitochondrial inner membrane. Structurally, they consist of three homologous domains with a single substrate binding site. They alternate between a cytoplasmic and matrix state in which the binding site is accessible to these compartments for binding of ADP or ATP. It has been proposed that cycling between states occurs by disruption and formation of a matrix and cytoplasmic salt bridge network in an alternating way, but formation of the latter has not been shown experimentally. Here, we show that state-dependent formation of the cytoplasmic salt bridge network can be demonstrated by measuring the effect of mutations on the thermal stability of detergent-solubilized carriers locked in a specific state. For this purpose, mutations were made to increase or decrease the overall interaction energy of the cytoplasmic network. When locked in the cytoplasmic state by the inhibitor carboxyatractyloside, the thermostabilities of the mutant and wild-type carriers were similar, but when locked in the matrix state by the inhibitor bongkrekic acid, they correlated with the predicted interaction energy of the cytoplasmic network, demonstrating its formation. Changing the interaction energy of the cytoplasmic network also had a profound effect on the kinetics of transport, indicating that formation of the network is a key step in the transport cycle. These results are consistent with a unique alternating access mechanism that involves the simultaneous rotation of the three domains around a central translocation pathway
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