39 research outputs found
Quantum dimer models and exotic orders
We discuss how quantum dimer models may be used to provide "proofs of
principle" for the existence of exotic magnetic phases in quantum spin systems.Comment: 12 pages, 6 figures. Contributed talk at the PITP-Les Houches Summer
School on "Quantum Magnetism", June 200
Deformations of Lifshitz holography
The simplest gravity duals for quantum critical theories with z=2 `Lifshitz'
scale invariance admit a marginally relevant deformation. Generic black holes
in the bulk describe the field theory with a dynamically generated momentum
scale Lambda as well as finite temperature T. We describe the thermodynamics of
these black holes in the quantum critical regime where T >> Lambda^2. The
deformation changes the asymptotics of the spacetime mildly and leads to
intricate UV sensitivities of the theory which we control perturbatively in
Lambda^2/T.Comment: 1+27 pages, 12 figure
Gauss-Bonnet Black Holes and Heavy Fermion Metals
We consider charged black holes in Einstein-Gauss-Bonnet Gravity with
Lifshitz boundary conditions. We find that this class of models can reproduce
the anomalous specific heat of condensed matter systems exhibiting
non-Fermi-liquid behaviour at low temperatures. We find that the temperature
dependence of the Sommerfeld ratio is sensitive to the choice of Gauss-Bonnet
coupling parameter for a given value of the Lifshitz scaling parameter. We
propose that this class of models is dual to a class of models of
non-Fermi-liquid systems proposed by Castro-Neto et.al.Comment: 17 pages, 6 figures, pdfLatex; small corrections to figure 10 in this
versio
Quantum phase transitions of light
Recently, condensed matter and atomic experiments have reached a length-scale
and temperature regime where new quantum collective phenomena emerge. Finding
such physics in systems of photons, however, is problematic, as photons
typically do not interact with each other and can be created or destroyed at
will. Here, we introduce a physical system of photons that exhibits strongly
correlated dynamics on a meso-scale. By adding photons to a two-dimensional
array of coupled optical cavities each containing a single two-level atom in
the photon-blockade regime, we form dressed states, or polaritons, that are
both long-lived and strongly interacting. Our zero temperature results predict
that this photonic system will undergo a characteristic Mott insulator
(excitations localised on each site) to superfluid (excitations delocalised
across the lattice) quantum phase transition. Each cavity's impressive photon
out-coupling potential may lead to actual devices based on these quantum
many-body effects, as well as observable, tunable quantum simulators. We
explicitly show that such phenomena may be observable in micro-machined diamond
containing nitrogen-vacancy colour centres and superconducting microwave
strip-line resonators.Comment: 11 pages, 5 figures (2 in colour
Vortices in polariton OPO superfluids
This chapter reviews the occurrence of quantised vortices in polariton
fluids, primarily when polaritons are driven in the optical parametric
oscillator (OPO) regime. We first review the OPO physics, together with both
its analytical and numerical modelling, the latter being necessary for the
description of finite size systems. Pattern formation is typical in systems
driven away from equilibrium. Similarly, we find that uniform OPO solutions can
be unstable to the spontaneous formation of quantised vortices. However,
metastable vortices can only be injected externally into an otherwise stable
symmetric state, and their persistence is due to the OPO superfluid properties.
We discuss how the currents charactering an OPO play a crucial role in the
occurrence and dynamics of both metastable and spontaneous vortices.Comment: 40 pages, 16 figure
The little skate genome and the evolutionary emergence of wing-like fins
Skates are cartilaginous fish whose body plan features enlarged wing-like pectoral fins, enabling them to thrive in benthic environments1,2. However, the molecular underpinnings of this unique trait remain unclear. Here we investigate the origin of this phenotypic innovation by developing the little skate Leucoraja erinacea as a genomically enabled model. Analysis of a high-quality chromosome-scale genome sequence for the little skate shows that it preserves many ancestral jawed vertebrate features compared with other sequenced genomes, including numerous ancient microchromosomes. Combining genome comparisons with extensive regulatory datasets in developing finsâincluding gene expression, chromatin occupancy and three-dimensional conformationâwe find skate-specific genomic rearrangements that alter the three-dimensional regulatory landscape of genes that are involved in the planar cell polarity pathway. Functional inhibition of planar cell polarity signalling resulted in a reduction in anterior fin size, confirming that this pathway is a major contributor to batoid fin morphology. We also identified a fin-specific enhancer that interacts with several hoxa genes, consistent with the redeployment of hox gene expression in anterior pectoral fins, and confirmed its potential to activate transcription in the anterior fin using zebrafish reporter assays. Our findings underscore the central role of genome reorganization and regulatory variation in the evolution of phenotypes, shedding light on the molecular origin of an enigmatic trait
Holographic Renormalization for Asymptotically Lifshitz Spacetimes
A variational formulation is given for a theory of gravity coupled to a
massive vector in four dimensions, with Asymptotically Lifshitz boundary
conditions on the fields. For theories with critical exponent z=2 we obtain a
well-defined variational principle by explicitly constructing two actions with
local boundary counterterms. As part of our analysis we obtain solutions of
these theories on a neighborhood of spatial infinity, study the asymptotic
symmetries, and consider different definitions of the boundary stress tensor
and associated charges. A constraint on the boundary data for the fields
figures prominently in one of our formulations, and in that case the only
suitable definition of the boundary stress tensor is due to Hollands,
Ishibashi, and Marolf. Their definition naturally emerges from our requirement
of finiteness of the action under Hamilton-Jacobi variations of the fields. A
second, more general variational principle also allows the Brown-York
definition of a boundary stress tensor.Comment: 34 pages, Added Reference
Hemichordate genomes and deuterostome origins
Acorn worms, also known as enteropneust (literally, âgut-breathingâ) hemichordates, are marine invertebrates that share features with echinoderms and chordates. Together, these three phyla comprise the deuterostomes. Here we report the draft genome sequences of two acorn worms, Saccoglossus kowalevskii and Ptychodera flava. By comparing them with diverse bilaterian genomes, we identify shared traits that were probably inherited from the last common deuterostome ancestor, and then explore evolutionary trajectories leading from this ancestor to hemichordates, echinoderms and chordates. The hemichordate genomes exhibit extensive conserved synteny with amphioxus and other bilaterians, and deeply conserved non-coding sequences that are candidates for conserved gene-regulatory elements. Notably, hemichordates possess a deuterostome-specific genomic cluster of four ordered transcription factor genes, the expression of which is associated with the development of pharyngeal âgillâ slits, the foremost morphological innovation of early deuterostomes, and is probably central to their filter-feeding lifestyle. Comparative analysis reveals numerous deuterostome-specific gene novelties, including genes found in deuterostomes and marine microbes, but not other animals. The putative functions of these genes can be linked to physiological, metabolic and developmental specializations of the filter-feeding ancestor