11,961 research outputs found
Liquid crystal phases of ultracold dipolar fermions on a lattice
Motivated by the search for quantum liquid crystal phases in a gas of
ultracold atoms and molecules, we study the density wave and nematic
instabilities of dipolar fermions on the two-dimensional square lattice (in the
plane) with dipoles pointing to the direction. We determine the phase
diagram using two complimentary methods, the Hatree-Fock mean field theory and
the linear response analysis of compressibility. Both give consistent results.
In addition to the staggered (, ) density wave, over a finite range
of densities and hopping parameters, the ground state of the system first
becomes nematic and then smectic, when the dipolar interaction strength is
increased. Both phases are characterized by the same broken four-fold (C)
rotational symmetry. The difference is that the nematic phase has a closed
Fermi surface but the smectic does not. The transition from the nematic to the
smectic phase is associated with a jump in the nematic order parameter. This
jump is closely related to the van Hove singularities. We derive the kinetic
equation for collective excitations in the normal isotropic phase and find that
the zero sound mode is strongly Landau damped and thus is not a well defined
excitation. Experimental implications of our results are discussed.Comment: 8 pages, 4 figures; Erratum added in the appendi
Multi-scale coarse-graining of diblock copolymer self-assembly: from monomers to ordered micelles
Starting from a microscopic lattice model, we investigate clustering,
micellization and micelle ordering in semi-dilute solutions of AB diblock
copolymers in a selective solvent. To bridge the gap in length scales, from
monomers to ordered micellar structures, we implement a two-step coarse
graining strategy, whereby the AB copolymers are mapped onto ``ultrasoft''
dumbells with monomer-averaged effective interactions between the centres of
mass of the blocks. Monte Carlo simulations of this coarse-grained model yield
clear-cut evidence for self-assembly into micelles with a mean aggregation
number n of roughly 100 beyond a critical concentration. At a slightly higher
concentration the micelles spontaneously undergo a disorder-order transition to
a cubic phase. We determine the effective potential between these micelles from
first principles.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett
Ultrafast (but Many-Body) Relaxation in a Low-Density Electron Glass
We present a study of the relaxation dynamics of the photoexcited
conductivity of the impurity states in the low-density electronic glass,
phosphorous-doped silicon Si:P. Using optical pump-terahertz probe spectroscopy
we find strongly temperature and fluence dependent glassy power-law relaxation
occurring over sub-ns time scales. Such behavior is in contrast to the much
longer time scales found in higher electron density glassy systems. We also
find evidence for both multi-particle relaxation mechanisms and/or coupling to
electronic collective modes and a low temperature quantum relaxational regime.Comment: 4 pages, 4 figures, Appeared in Phys. Rev. Let
Energy-based temporal neural networks for imputing missing values
Imputing missing values in high dimensional time series is a difficult problem. There have been some approaches to the problem [11,8] where neural architectures were trained as probabilistic models of the data. However, we argue that this approach is not optimal. We propose to view temporal neural networks with latent variables as energy-based models and train them for missing value recovery directly. In this paper we introduce two energy-based models. The first model is based on a one dimensional convolution and the second model utilizes a recurrent neural network. We demonstrate how ideas from the energy-based learning framework can be used to train these models to recover missing values. The models are evaluated on a motion capture dataset
Global Value Chains During the Great Trade Collapse: A Bullwhip Effect?
This paper analyzes the performance of global value chains during the trade collapse. To do so, it exploits a unique transaction-level dataset on French firms containing information on cross-border monthly transactions matched with data on worldwide intra-.rm linkages as defined by property rights (multinational business groups, hierarchies of firms). This newly assembled dataset allows us to distinguish firm-level transactions among two alternative organizational modes of global value chains: internalization of activities (intra- group trade/trade among related parties) or establishment of supply contracts (arm's length trade/trade among unrelated parties). After an overall assessment of the role of global value chains during the trade collapse, we document that intra-group trade in intermediates was characterized by a faster drop followed by a faster recovery than arm's length trade. Amplified fluctuations in terms of trade elasticities by value chains have been referred to as the "bullwhip effect" and have been attributed to the adjustment of inventories within supply chains. In this paper we first con.rm the existence of such an effect due to trade in inter- mediates, and we underline the role that different organizational modes can play in driving this adjustment.trade collapse, multinational firms, global value chains, hierarchies of firms, vertical integration
Studying the Pc(4450) resonance in J/psi photoproduction off protons
The LHCb has reported the observation of a resonancelike structure, the
Pc(4450), in the J/psi p invariant masses. In our work, we discuss the
feasibility of detecting this structure in J/psi photoproduction, e.g. in the
measurements that have been approved for the experiments in Hall A/C and in
Hall B with CLAS12 at JLab. Also the GlueX Collaboration has already reported
preliminary results. We take into account the experimental resolution effects,
and perform a global fit to world J/psi photoproduction data in order to study
the possibility of observing the Pc(4450) signal in future JLab data. We
present a first estimate of the upper limit for the branching ratio of the
Pc(4450) into the J/psi p channel, and we study the angular distributions of
the differential cross sections. This will shed light on the nature and
couplings of the Pc(4450) structure in the future photoproduction experiments.Comment: NSTAR 2017 conference proceeding
Dynamical ultrametricity in the critical trap model
We show that the trap model at its critical temperature presents dynamical
ultrametricity in the sense of Cugliandolo and Kurchan [CuKu94]. We use the
explicit analytic solution of this model to discuss several issues that arise
in the context of mean-field glassy dynamics, such as the scaling form of the
correlation function, and the finite time (or finite forcing) corrections to
ultrametricity, that are found to decay only logarithmically with the
associated time scale, as well as the fluctuation dissipation ratio. We also
argue that in the multilevel trap model, the short time dynamics is dominated
by the level which is at its critical temperature, so that dynamical
ultrametricity should hold in the whole glassy temperature range. We revisit
some experimental data on spin-glasses in light of these results.Comment: 7 pages, 4 .eps figures. submitted to J. Phys.
The relative influences of disorder and of frustration on the glassy dynamics in magnetic systems
The magnetisation relaxations of three different types of geometrically
frustrated magnetic systems have been studied with the same experimental
procedures as previously used in spin glasses. The materials investigated are
YMoO (pyrochlore system), SrCrGaO (piled
pairs of Kagom\'e layers) and (HO)Fe(SO)(OH) (jarosite
compound). Despite a very small amount of disorder, all the samples exhibit
many characteristic features of spin glass dynamics below a freezing
temperature , much smaller than their Curie-Weiss temperature .
The ageing properties of their thermoremanent magnetization can be well
accounted for by the same scaling law as in spin glasses, and the values of the
scaling exponents are very close. The effects of temperature variations during
ageing have been specifically investigated. In the pyrochlore and the
bi-Kagom\'e compounds, a decrease of temperature after some waiting period at a
certain temperature re-initializes ageing and the evolution at the new
temperature is the same as if the system were just quenched from above .
However, as the temperature is raised back to , the sample recovers the
state it had previously reached at that temperature. These features are known
in spin glasses as rejuvenation and memory effects. They are clear signatures
of the spin glass dynamics. In the Kagom\'e compound, there is also some
rejuvenation and memory, but much larger temperature changes are needed to
observe the effects. In that sense, the behaviour of this compound is
quantitatively different from that of spin glasses.Comment: latex VersionCorrigee4.tex, 4 files, 3 figures, 5 pages (Proceedings
of the International Conference on Highly Frustrated Magnetism (HFM2003),
August 26-30, 2003, Institut Laue Langevin (ILL), Grenoble, France
Breached pairing superfluidity: Possible realization in QCD
We propose a wide universality class of gapless superfluids, and analyze a
limit that might be realized in quark matter at intermediate densities. In the
breached pairing color superconducting phase heavy -quarks, with a small
Fermi surface, pair with light or quarks. The groundstate has a
superfluid and a normal Fermi component simultaneously. We expect a second
order phase transition, as a function of increasing density, from the breached
pairing phase to the conventional color-flavor locked (CFL) phase.Comment: 5 pages, latex, 1 figure; added references; Comment on Ref. [10]
change
Interior Gap Superfluidity
We propose a new state of matter in which the pairing interactions carve out
a gap within the interior of a large Fermi ball, while the exterior surface
remains gapless. This defines a system which contains both a superfluid and a
normal Fermi liquid simultaneously, with both gapped and gapless quasiparticle
excitations. This state can be realized at weak coupling. We predict that a
cold mixture of two species of fermionic atoms with different mass will exhibit
this state. For electrons in appropriate solids, it would define a material
that is simultaneously superconducting and metallic.Comment: 5 page
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