269 research outputs found
Dynamical obstruction in a constrained system and its realization in lattices of superconducting devices
Hard constraints imposed in statistical mechanics models can lead to
interesting thermodynamical behaviors, but may at the same time raise
obstructions in the thoroughfare to thermal equilibration. Here we study a
variant of Baxter's 3-color model in which local interactions and defects are
included, and discuss its connection to triangular arrays of Josephson
junctions of superconductors and \textit{kagom\'e} networks of superconducting
wires. The model is equivalent to an Ising model in a hexagonal lattice with
the constraint that the magnetization of each hexagon is or 0. For
ferromagnetic interactions, we find that the system is critical for a range of
temperatures (critical line) that terminates when it undergoes an exotic first
order phase transition with a jump from a zero magnetization state into the
fully magnetized state at finite temperature. Dynamically, however, we find
that the system becomes frozen into domains. The domain walls are made of
perfectly straight segments, and domain growth appears frozen within the time
scales studied with Monte Carlo simulations. This dynamical obstruction has its
origin in the topology of the allowed reconfigurations in phase space, which
consist of updates of closed loops of spins. As a consequence of the dynamical
obstruction, there exists a dynamical temperature, lower than the (avoided)
static critical temperature, at which the system is seen to jump from a
``supercooled liquid'' to a ``polycrystalline'' phase. In contrast, for
antiferromagnetic interactions, we argue that the system orders for
infinitesimal coupling because of the constraint, and we observe no interesting
dynamical effects
Regional disparities and industrial structure: territorial capital and productivity in Italian firms
We investigate the role of Territorial Capital (TC) on the productivity of Italian firms, constructing indicators for eight dimensions of TC in a first attempt to capture a wide variety of regional resources. When imposing homogeneous TC effects on all firms, we find that technological, social, institutional, financial and infrastructure capital drive productivity. However, only technological and artistic capital contribute to reduce regional disparities. Across industries, financial capital and infrastructure increase productivity in companies operating in a wide range of sectors. Industrial policies should consider sectoral heterogeneity and North-South differences to effectively boost productivity performance
High temperature onset of field-induced transitions in the spin-ice compound Dy2Ti2O7
We have studied the field-dependent ac magnetic susceptibility of single
crystals of Dy2Ti2O7 spin ice along the [111] direction in the temperature
range 1.8 K - 7 K. Our data reflect the onset of local spin ice order in the
appearance of different field regimes. In particular, we observe a prominent
feature at approximately 1.0 T that is a precursor of the low-temperature
metamagnetic transition out of field-induced kagome ice, below which the
kinetic constraints imposed by the ice rules manifest themselves in a
substantial frequency-dependence of the susceptibility. Despite the relatively
high temperatures, our results are consistent with a monopole picture, and they
demonstrate that such a picture can give physical insight to the spin ice
systems even outside the low-temperature, low-density limit where monopole
excitations are well-defined quasiparticles
Molecular random tilings as glasses
We have recently shown [Blunt et al., Science 322, 1077 (2008)] that
p-terphenyl-3,5,3',5'-tetracarboxylic acid adsorbed on graphite self-assembles
into a two-dimensional rhombus random tiling. This tiling is close to ideal,
displaying long range correlations punctuated by sparse localised tiling
defects. In this paper we explore the analogy between dynamic arrest in this
type of random tilings and that of structural glasses. We show that the
structural relaxation of these systems is via the propagation--reaction of
tiling defects, giving rise to dynamic heterogeneity. We study the scaling
properties of the dynamics, and discuss connections with kinetically
constrained models of glasses.Comment: 5 pages, 5 figure
Quality of institutions and productivity of State-Invested Enterprises: International evidence from major telecom companies
The quality of institutions is at the core of the differences in the growth of income and productivity of nations. A growing body of evidence shows how this is also true at the firm level. After taking stock of earlier theoretical and empirical literature on the efficiency of state-owned versus private enterprises, while we consider ownership as the core internal governance mechanism of firms, we add quality of government as a determinant of the external institutional environment. To disentangle the effect of internal and external institutions on firms' productivity, we use different sets of ownership and institutional environment indicators. After having identified the top 350 private, state-invested (i.e. partially state-owned)and state-owned enterprises in the telecommunications industry in EU28 and in more than 60 other countries between 2007 and 2015, we empirically investigate models of firms' productivity augmented with ownership and quality of government. Our findings suggest that, after controlling for the regulatory and competitive conditions at the country level, on average, public ownership has a negative impact on firm-level TFP. This effect is however mitigated by high external institutional quality and even reversed in some countries with a particularly favourable institutional environment
Dynamics of single polymers under extreme confinement
We study the dynamics of a single chain polymer confined to a two dimensional
cell. We introduce a kinetically constrained lattice gas model that preserves
the connectivity of the chain, and we use this kinetically constrained model to
study the dynamics of the polymer at varying densities through Monte Carlo
simulations. Even at densities close to the fully-packed configuration, we find
that the monomers comprising the chain manage to diffuse around the box with a
root mean square displacement of the order of the box dimensions over time
scales for which the overall geometry of the polymer is, nevertheless, largely
preserved. To capture this shape persistence, we define the local tangent field
and study the two-time tangent-tangent correlation function, which exhibits a
glass-like behavior. In both closed and open chains, we observe reptational
motion and reshaping through local fingering events which entail global monomer
displacement.Comment: 22 pages, 18 figures, slightly extended version to appear in JSTA
Quantum dynamics of a domain wall in the presence of dephasing
We compare quantum dynamics in the presence of Markovian dephasing for a particle hopping on a chain and for an Ising domain wall whose motion leaves behind a string of flipped spins. Exact solutions show that on an infinite chain, the transport responses of the models are nearly identical. However, on finite-length chains, the broadening of discrete spectral lines is much more noticeable in the case of a domain wall.This work was supported in part by the ARO grant W911NF-14-1-0272, the NSF grant PHY-1416578, and EPSRC grants EP/K028960/1 and EP/M007065/1
Unconventional magnets in external magnetic fields
This short review surveys phenomena observed when a magnetic field is applied
to a system of localised spins on a lattice. Its focus is on frustrated magnets
in dimension . The interplay of field and entropy is illustrated in
the context of their unusual magnetocaloric properties, where field-tuned
degeneracies assert themselves. Magnetisation plateaux can reveal the physics
of fluctuations, with unusual excitations (such as local modes, extended string
defects or monopoles) involved in plateau termination. Field-tuning lattice
geometry is the final topic, where mechanisms for dimensional reduction and
conversion between different lattice types are discussed.Comment: Plenary Talk at HFM 2008 Conferenc
Spin Dynamics at Very Low Temperature in Spin Ice DyTiO
We have performed AC susceptibility and DC magnetic relaxation measurements
on the spin ice system DyTiO down to 0.08 K. The relaxation time of
the magnetization has been estimated below 2 K down to 0.08 K. The spin
dynamics of DyTiO is well described by using two relaxation times
( (short time) and (long time)). Both and increase on cooling. Assuming the Arrhenius law in the
temperature range 0.5-1 K, we obtained an energy barrier of 9 K. Below 0.5 K,
both and show a clear deviation from the thermal
activated dynamics toward temperature independent relaxation, suggesting a
quantum dynamics.Comment: 4 page
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