We present quantum Monte Carlo simulations for the chiral Heisenberg Gross-Neveu-Yukawa quantum phase transition of relativistic fermions with N=4 Dirac spinor components subject to a repulsive, ...local four fermion interaction in (2+1)D. Here we employ a two-dimensional lattice Hamiltonian with a single, spin-degenerate Dirac cone, which exactly reproduces a linear energy-momentum relation for all finite size lattice momenta in the absence of interactions. This allows us to significantly reduce finite size corrections compared to the widely studied honeycomb and π-flux lattices. A Hubbard term dynamically generates a mass beyond a critical coupling of Uc=6.76(1) as the system acquires antiferromagnetic order and SU(2) spin rotational symmetry is spontaneously broken. At the quantum phase transition, we extract a self-consistent set of critical exponents ν=0.98(1), ηϕ=0.53(1), ηψ=0.18(1), and β=0.75(1). We provide evidence for the continuous degradation of the quasiparticle weight of the fermionic excitations as the critical point is approached from the semimetallic phase. Finally, we study the effective "speed of light" of the low-energy relativistic description, which depends on the interaction U, but is expected to be regular across the quantum phase transition. We illustrate that the strongly coupled bosonic and fermionic excitations share a common velocity at the critical point.
Quantitative computed tomography (QCT) is currently undergoing a renaissance, with an increasing number of studies being published and the definition of both QCT-specific osteoporosis thresholds and ...treatment criteria. Compared with dual-energy X-ray absorptiometry, the current standard bone mineral density technique, QCT has a number of pertinent advantages, including volumetric measurements, less susceptibility to degenerative spine changes, and higher sensitivity to changes in bone mass. Disadvantages include the higher radiation doses and less experience with fracture prediction and therapy monitoring. Over the last 10 yr, a number of novel applications have been described allowing assessment of bone mineral density and bone quality in larger patient populations, developments that may substantially improve patient care.
Quantum simulation using synthetic systems is a promising route to solve outstanding quantum many-body problems in regimes where other approaches, including numerical ones, fail
. Many platforms are ...being developed towards this goal, in particular based on trapped ions
, superconducting circuits
, neutral atoms
or molecules
. All of these platforms face two key challenges: scaling up the ensemble size while retaining high-quality control over the parameters, and validating the outputs for these large systems. Here we use programmable arrays of individual atoms trapped in optical tweezers, with interactions controlled by laser excitation to Rydberg states
, to implement an iconic many-body problem-the antiferromagnetic two-dimensional transverse-field Ising model. We push this platform to a regime with up to 196 atoms manipulated with high fidelity and probe the antiferromagnetic order by dynamically tuning the parameters of the Hamiltonian. We illustrate the versatility of our platform by exploring various system sizes on two qualitatively different geometries-square and triangular arrays. We obtain good agreement with numerical calculations up to a computationally feasible size (approximately 100 particles). This work demonstrates that our platform can be readily used to address open questions in many-body physics.
We revisit the effect of local interactions on the quadratic band touching (QBT) of the Bernal honeycomb bilayer model using renormalization group (RG) arguments and quantum Monte Carlo (QMC) ...simulations. We present a RG argument which predicts, contrary to previous studies, that weak interactions do not flow to strong coupling even if the free dispersion has a QBT. Instead, they generate a linear term in the dispersion, which causes the interactions to flow back to weak coupling. Consistent with this RG scenario, in unbiased QMC simulations of the Hubbard model we find compelling evidence that antiferromagnetism turns on at a finite U/t despite the U=0 hopping problem having a QBT. The onset of antiferromagnetism takes place at a continuous transition which is consistent with (2+1)D Gross-Neveu criticality. We conclude that generically in models of bilayer graphene, even if the free dispersion has a QBT, small local interactions generate a Dirac phase with no symmetry breaking and that there is a finite-coupling transition out of this phase to a symmetry-broken state.
The implementation of artificial molecular machines in polymer science is an important objective that challenges chemists and physicists in order to access an entirely new class of smart materials. ...To design such systems, the amplification of a mechanical actuation from the nanoscale up to a macroscopic response in the bulk material is a central issue. In this article we show that bistable c2daisy chain rotaxanes (i.e., molecular muscles) can be linked into main-chain Upy-based supramolecular polymers. We then reveal by an in depth quantitative study that the pH actuation of the mechanically active rotaxane at the nanoscale influences the physical reticulation of the polymer chains by changing the supramolecular behavior of the Upy units. This nanoactuation within the local structure of the main chain polymer results in a mechanically controlled sol–gel transition at the macroscopic level.
We perform an extensive exact diagonalization study of interaction-driven insulators in spin- and valley-polarized moiré flat bands of twisted bilayer graphene aligned with its hexagonal boron ...nitride substrate. In addition to previously reported fractional Chern insulator phases, we provide compelling evidence for competing charge-density-wave phases at multiple fractional fillings of a realistic single-band model. A thorough analysis at different interlayer hopping parameters, motivated by experimental variability, and the role of kinetic energy at various Coulomb interaction strengths highlight the competition between these phases. The interplay of the single-particle and the interaction-induced hole dispersion with the inherent Berry curvature of the Chern bands is intuitively understood to be the driving mechanism for the ground-state selection. The resulting phase diagram features remarkable agreement with experimental findings in a related moiré heterostructure and affirms the relevance of our results beyond the scope of graphene-based materials.
Ample evidence confirms that college graduates in general are not good writers, and there is no reason to believe that graduates of public health programs are any different. The poor quality of ...writing among college graduates is also a concern among employers. This article provides an overview of the concepts and techniques that public health faculty with little background in writing instruction can use to help their students improve their writing.
The scientific literature makes science possible. But to be useful, this literature must be organized and retrievable; activities that in turn are made possible by assigning reference information to ...a source. The source can then be cited in a publication, allowing readers to verify the cited information against its source document. In fact, accurate citation and reference information is the glue that holds science together. Although thousands of style manuals provide extensive details on
how
to format references, researchers are not always taught
when
to cite the literature or to appreciate the critical importance of verifying the accuracy of the cited text and its reference information. Here, I consider the
when, what,
and
why
of citing sources and review some common problems with citations and their references. In doing so, I also raise some questions that can, should, or must be raised at the “sight of a cite and its site.”
Repetitive negative thought plays an important role in the maintenance of mental health problems following bereavement. To date, bereavement researchers have primarily focused on rumination (i.e., ...repetitive thought about negative events and/or negative emotions), yet the interest in worry (i.e., repetitive thought about uncertain future events) is increasing. Both cognitive processes potentially lead to poorer adaptation to bereavement by contributing to loss‐related avoidance and behavioural avoidance of activities. The current study aims to establish the differential associations of rumination and worry with symptoms of depression and prolonged grief and clarify if avoidance processes mediate the associations of rumination and worry with symptom levels. Four hundred seventy‐four recently bereaved adults (82% female) filled out questionnaires assessing rumination, worry, loss‐related and behavioural avoidance, and depression and prolonged grief symptoms. Rumination and worry were both uniquely associated with depression and prolonged grief symptoms. Compared with worry, rumination related more strongly to prolonged grief symptoms, whereas correlations of both cognitive styles with depression symptoms did not differ. Loss‐related avoidance and behavioural avoidance partially mediated the associations of rumination and worry with prolonged grief symptoms. Behavioural avoidance partially mediated the associations of rumination and worry with depression symptoms. Findings suggest that exposure and behavioural activation may be effective interventions to reduce repetitive thinking and psychopathology after bereavement.
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