26,387 research outputs found
Tensor network trial states for chiral topological phases in two dimensions and a no-go theorem in any dimension
Trial wavefunctions that can be represented by summing over locally-coupled
degrees of freedom are called tensor network states (TNSs); they have seemed
difficult to construct for two-dimensional topological phases that possess
protected gapless edge excitations. We show it can be done for chiral states of
free fermions, using a Gaussian Grassmann integral, yielding
and Chern insulator states, in the sense that the fermionic excitations live in
a topologically non-trivial bundle of the required type. We prove that any
strictly short-range quadratic parent Hamiltonian for these states is gapless;
the proof holds for a class of systems in any dimension of space. The proof
also shows, quite generally, that sets of compactly-supported Wannier-type
functions do not exist for band structures in this class. We construct further
examples of TNSs that are analogs of fractional (including non-Abelian) quantum
Hall phases; it is not known whether parent Hamiltonians for these are also
gapless.Comment: 5 pages plus 4 pages supplementary material, inc 3 figures. v2:
improved no-go theorem, additional references. v3: changed to regular article
format; 16 pages, 3 figures, no supplemental material; main change is much
extended proof of no-go theorem. v4: minor changes; as-published versio
How to break the density-anisotropy degeneracy in spherical stellar systems
We present a new non-parametric Jeans code, GravSphere, that recovers the
density and velocity anisotropy of spherical stellar
systems, assuming only that they are in a steady-state. Using a large suite of
mock data, we confirm that with only line-of-sight velocity data, GravSphere
provides a good estimate of the density at the projected stellar half mass
radius, , but is not able to measure or ,
even with 10,000 tracer stars. We then test three popular methods for breaking
this degeneracy: using multiple populations with different
; using higher order `Virial Shape Parameters' (VSPs); and including
proper motion data.
We find that two populations provide an excellent recovery of
in-between their respective . However, even with a total of tracers, we are not able to well-constrain for either
population. By contrast, using 1000 tracers with higher order VSPs we are able
to measure over the range and broadly constrain
. Including proper motion data for all stars gives an even better
performance, with and well-measured over the range .
Finally, we test GravSphere on a triaxial mock galaxy that has axis ratios
typical of a merger remnant, . In this case, GravSphere can become
slightly biased. However, we find that when this occurs the data are poorly
fit, allowing us to detect when such departures from spherical symmetry become
problematic.Comment: 19 pages; 1 table; 11 Figures. Version accepted for publication in
MNRAS. (Minor changes from previously. Appendix B added showing decreasing
bias of VSP estimators with increasing sampling.
The X-ray Evolution of Merging Galaxies
We present here the first study of the X-ray properties of an evolutionary
sample of merging galaxies. Both ROSAT PSPC and HRI data are presented for a
sample of eight interacting galaxy systems, each believed to involve a similar
encounter between two spiral discs of approximately equal size. The mergers
span a large range in age, from completely detached to fully merged systems. A
great deal of interesting X-ray structure is seen, and the X-ray properties of
each individual system are discussed in detail. Along the merging sequence,
several trends are evident: in the case of several of the infrared bright
systems, the diffuse emission is very extended, and appears to arise from
material ejected from the galaxies. The onset of this process seems to occur
very soon after the galaxies first encounter one another, and these ejections
soon evolve into distorted flows. More massive extensions (perhaps involving up
to 1e10 solar masses of hot gas) are seen at the `ultraluminous' peak of the
interaction, as the galactic nuclei coalesce. The amplitude of the evolution of
the X-ray emission through a merger is markedly different from that of the
infrared and radio emission however, and this, we believe, may well be linked
with the large extensions of hot gas observed. The late, relaxed remnants,
appear relatively devoid of gas, and possess an X-ray halo very different from
that of typical ellipticals, a problem for the `merger hypothesis', whereby the
merger of two disc galaxies results in an elliptical galaxy. However, these
systems are still relatively young in terms of total merger lifetime, and they
may still have a few Gyr of evolution to go through, before they resemble
typical elliptical galaxies.Comment: 30 pages, 15 figures, accepted by MNRA
Exponents and bounds for uniform spanning trees in d dimensions
Uniform spanning trees are a statistical model obtained by taking the set of
all spanning trees on a given graph (such as a portion of a cubic lattice in d
dimensions), with equal probability for each distinct tree. Some properties of
such trees can be obtained in terms of the Laplacian matrix on the graph, by
using Grassmann integrals. We use this to obtain exact exponents that bound
those for the power-law decay of the probability that k distinct branches of
the tree pass close to each of two distinct points, as the size of the lattice
tends to infinity.Comment: 5 pages. v2: references added. v3: closed form results can be
extended slightly (thanks to C. Tanguy). v4: revisions, and a figure adde
Animating the Real: Illusions, Musicality and the Live Dancing Body
Animation film frequently uses dance and choreography as part of explicit scenes to help assist with and compliment the narrative. Although animators frequently acknowledge relationships between dance and animation, scarcely any scholarly work considers how animated film principles are used and applied within live dance performances. Additionally, although many scholars discuss the relationship between live dance and music and similarly animation and music, rarely has the collision of all three been scrutinized. In this article, I draw upon principles from animation film and choreomusical scholarship to show a complex relationship between the real and the pretend in a solo street dance performance. Based on detailed movement analysis of a short solo dance performance by Isaac "Turbo" Baptiste, I discuss the ways that moving image can influence live dancing bodies and create endless possibilities for choreomusical play
Bad things happen and can drive you crazy: The causal beliefs of 701 people taking antipsychotics
In almost all countries the public prefers psycho-social explanations of ‘schizophrenia’ to bio-genetic ones. The causal explanations of people who experience psychosis have been under researched, and, if they diverge from the dominant bio-genetic paradigm, can be dismissed as ‘lack of insight’. Of the 832 people completing a survey about their experiences on antipsychotic, 701, from 30 countries, answered an open question about what had caused the problems for which they had been prescribed the drugs. On a Bio-Social likert scale, from 1 = ‘Purely Biological’ to 5 = ‘Purely Social’, the mean score was 4.24. Thematic analysis produced seven themes: Social (75.6%), Psychological (18.4%), Bio-genetic (17.5%), Iatrogenic (17.1%), Drug and Alcohol (10.1%), Medical Condition (6.8%) and Insomnia (6.0%). Those with a psychosis diagnosis were even more likely than others to report a Social cause. The causal beliefs of this sample are consistent with previous studies of people diagnosed with ‘schizophrenia’ and the beliefs of the public. They are also aligned with recent research into the social causes of psychosis. It is argued that rather than dismiss the beliefs as ‘lack of insight’ it is more respectful and productive to listen carefully and adjust our understandings and services accordingly
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