412 research outputs found
Transient response of a sandwich structure damped with a fibrous core material
International audienceThis paper investigates the effect of a fibrous core material on the transient response of a sandwich structure. The core material studied is made of entangled carbon fibers cross-linked with epoxy resin. In order to understand its complex behavior, the material is characterized in shear oscillation, and the obtained shear-stress loops are described using Dahl’s dynamic hysteresis model. Then, the transient response of a single-degree-of-freedom system including this material is simulated and the amplitude dependency of the response is analyzed. Sandwich beams with the fibrous core material are tested experimentally with a impact hammer and compared with classical honeycomb and foam cored sandwich beams. The entangled cross-linked fibers are shown to provide high damping resulting in a fast return to the equilibrium position. Both theoretical and experimental studies showed nonlinear damping. In the amplitude range studied, the material is more interesting at high impact amplitude
Finding long cycles in graphs
We analyze the problem of discovering long cycles inside a graph. We propose
and test two algorithms for this task. The first one is based on recent
advances in statistical mechanics and relies on a message passing procedure.
The second follows a more standard Monte Carlo Markov Chain strategy. Special
attention is devoted to Hamiltonian cycles of (non-regular) random graphs of
minimal connectivity equal to three
Uclacyanin Proteins Are Required for Lignified Nanodomain Formation within Casparian Strips
© 2020 The Author(s) Casparian strips (CSs) are cell wall modifications of vascular plants restricting extracellular free diffusion into and out of the vascular system [1]. This barrier plays a critical role in controlling the acquisition of nutrients and water necessary for normal plant development [2–5]. CSs are formed by the precise deposition of a band of lignin approximately 2 μm wide and 150 nm thick spanning the apoplastic space between adjacent endodermal cells [6, 7]. Here, we identified a copper-containing protein, Uclacyanin1 (UCC1), that is sub-compartmentalized within the CS. UCC1 forms a central CS nanodomain in comparison with other CS-located proteins that are found to be mainly accumulated at the periphery of the CS. We found that loss-of-function of two uclacyanins (UCC1 and UCC2) reduces lignification specifically in this central CS nanodomain, revealing a nano-compartmentalized machinery for lignin polymerization. This loss of lignification leads to increased endodermal permeability and, consequently, to a loss of mineral nutrient homeostasis
Quantifying the Expressive Capacity of Quantum Systems: Fundamental Limits and Eigentasks
The expressive capacity of quantum systems for machine learning is limited by
quantum sampling noise incurred during measurement. Although it is generally
believed that noise limits the resolvable capacity of quantum systems, the
precise impact of noise on learning is not yet fully understood. We present a
mathematical framework for evaluating the available expressive capacity of
general quantum systems from a finite number of measurements, and provide a
methodology for extracting the extrema of this capacity, its eigentasks.
Eigentasks are a native set of functions that a given quantum system can
approximate with minimal error. We show that extracting low-noise eigentasks
leads to improved performance for machine learning tasks such as
classification, displaying robustness to overfitting. We obtain a tight bound
on the expressive capacity, and present analyses suggesting that correlations
in the measured quantum system enhance learning capacity by reducing noise in
eigentasks. These results are supported by experiments on superconducting
quantum processors. Our findings have broad implications for quantum machine
learning and sensing applications.Comment: 7 + 21 pages, 4 + 12 figures, 1 tabl
On quantum mean-field models and their quantum annealing
This paper deals with fully-connected mean-field models of quantum spins with
p-body ferromagnetic interactions and a transverse field. For p=2 this
corresponds to the quantum Curie-Weiss model (a special case of the
Lipkin-Meshkov-Glick model) which exhibits a second-order phase transition,
while for p>2 the transition is first order. We provide a refined analytical
description both of the static and of the dynamic properties of these models.
In particular we obtain analytically the exponential rate of decay of the gap
at the first-order transition. We also study the slow annealing from the pure
transverse field to the pure ferromagnet (and vice versa) and discuss the
effect of the first-order transition and of the spinodal limit of metastability
on the residual excitation energy, both for finite and exponentially divergent
annealing times. In the quantum computation perspective this quantity would
assess the efficiency of the quantum adiabatic procedure as an approximation
algorithm.Comment: 44 pages, 23 figure
Rigorous Inequalities between Length and Time Scales in Glassy Systems
Glassy systems are characterized by an extremely sluggish dynamics without
any simple sign of long range order. It is a debated question whether a correct
description of such phenomenon requires the emergence of a large correlation
length. We prove rigorous bounds between length and time scales implying the
growth of a properly defined length when the relaxation time increases. Our
results are valid in a rather general setting, which covers finite-dimensional
and mean field systems.
As an illustration, we discuss the Glauber (heat bath) dynamics of p-spin
glass models on random regular graphs. We present the first proof that a model
of this type undergoes a purely dynamical phase transition not accompanied by
any thermodynamic singularity.Comment: 24 pages, 3 figures; published versio
Relaxation and Metastability in the RandomWalkSAT search procedure
An analysis of the average properties of a local search resolution procedure
for the satisfaction of random Boolean constraints is presented. Depending on
the ratio alpha of constraints per variable, resolution takes a time T_res
growing linearly (T_res \sim tau(alpha) N, alpha < alpha_d) or exponentially
(T_res \sim exp(N zeta(alpha)), alpha > alpha_d) with the size N of the
instance. The relaxation time tau(alpha) in the linear phase is calculated
through a systematic expansion scheme based on a quantum formulation of the
evolution operator. For alpha > alpha_d, the system is trapped in some
metastable state, and resolution occurs from escape from this state through
crossing of a large barrier. An annealed calculation of the height zeta(alpha)
of this barrier is proposed. The polynomial/exponentiel cross-over alpha_d is
not related to the onset of clustering among solutions.Comment: 23 pages, 11 figures. A mistake in sec. IV.B has been correcte
On the freezing of variables in random constraint satisfaction problems
The set of solutions of random constraint satisfaction problems (zero energy
groundstates of mean-field diluted spin glasses) undergoes several structural
phase transitions as the amount of constraints is increased. This set first
breaks down into a large number of well separated clusters. At the freezing
transition, which is in general distinct from the clustering one, some
variables (spins) take the same value in all solutions of a given cluster. In
this paper we study the critical behavior around the freezing transition, which
appears in the unfrozen phase as the divergence of the sizes of the
rearrangements induced in response to the modification of a variable. The
formalism is developed on generic constraint satisfaction problems and applied
in particular to the random satisfiability of boolean formulas and to the
coloring of random graphs. The computation is first performed in random tree
ensembles, for which we underline a connection with percolation models and with
the reconstruction problem of information theory. The validity of these results
for the original random ensembles is then discussed in the framework of the
cavity method.Comment: 32 pages, 7 figure
- …