arXiv subjects

Surachate Limkumnerd

Publications and source records attributed to Surachate Limkumnerd.

13 recordsLinked to original sources

arXiv2026

Pre-failure response spectra predict finite-amplitude fragility

Failure theories often identify a single leading route to failure: the most unstable mode, weakest link, minimum-action escape path, or optimal perturbation. Yet finite-amplitude susceptibility depends not only on the nearest route but on how much of perturbation space lies near dangerous directions. We cast this distinction as a fragility problem: for each perturbation direction, the failure distance is the smallest amplitude that crosses a prescribed boundary, and the fragility curve is the fraction of directions that fail below a given amplitude. Measuring this curve directly requires nonlinear trials over many directions; instead, we show that it is predicted, before any failure occurs, by the tail of a single pre-failure quantity: the boundary-normalized fragility gain computed from the linearized response. The breadth of the associated response spectrum sets how many near-dangerous pathways coexist beyond the strongest direction. We demonstrate the mechanism in a high-dimensional nonlinear non-normal network with the strongest directional gain held fixed: the system with broader response-channel breadth has a larger nonlinear fragility curve, isolating breadth from the worst direction. An independent scalar test in deterministic traffic breakdown confirms the predicted sign: response breadth lowers calibrated jam thresholds once the strongest response is matched, with residual margins screening but never reversing the effect. Response-spectrum breadth thus emerges as a pre-failure coordinate for finite-amplitude fragility beyond the strongest path.

cond-mat.stat-mech↗

arXiv2026

Path-Extrema Upper Bounds on Mean Entropy Production

Fluctuation relations imply the second-law inequality $\langle\Sigma_T\rangle\ge0$, but path extrema can also constrain how large the mean entropy production can be. For steady-state processes with entropy-production martingale $M_t=e^{-\Sigma_t}$, we show that knowing only the positive running maximum of $\Sigma_t$ gives no improvement over the trivial endpoint bound: rare negative entropy-production excursions can still carry the exponential weight required by the fluctuation relation. Using the running extrema $L_T=\inf M_t$ and $H_T=\sup M_t$, we derive a path-extrema upper envelope $\mathcal{U}_{\rm ext}$. The relaxed envelope problem ranks realized intervals by the entropy gain per martingale cost, $\ln(H_T/L_T)/(H_T-L_T)$, giving a continuous knapsack problem. The actual mean satisfies the exact identity $\langle\Sigma_T\rangle=\mathcal{U}_{\rm ext}-\mathcal{A}-\mathcal{C}$, where $\mathcal{A}$ is an allocation gap across realized envelopes and $\mathcal{C}$ is a curvature gap within each envelope. Thus path extrema set the upper envelope, while the two gaps quantify how actual dynamics allocate terminal outcomes across envelope classes and place terminal values within each realized envelope. This turns path-extrema information into a quantitative upper-bound theory for entropy production, complementary to the usual lower-bound role of fluctuation relations.

cond-mat.stat-mech↗

arXiv2026

Matrix-noise Jacobians in stochastic-calculus inference and optimal paths

Multiplicative noise makes stochastic dynamics depend on how the white-noise limit is interpreted. In multidimensional systems with matrix-valued noise amplitudes $\sigma(x)$, this dependence includes a local Jacobian contribution that is absent from the scalar examples most often used to build intuition. We formulate a finite-step path-likelihood framework for $\theta$-discretized diffusions and show that its short-time expansion isolates the scalar $J_\sigma=\partial_j\sigma_{ik}\partial_i\sigma_{jk}-(\partial_i\sigma_{ik})(\partial_l\sigma_{lk})$. For a specified noise-amplitude representation $\sigma$, this quantity vanishes in one-dimensional, scalar-isotropic, and strictly diagonal cases, but can survive when state-dependent noise directions mix different components. We then test its consequences using paired comparisons that hold the drift, diffusion matrix, interpolation point, and Gaussian increment term fixed. In Model A, removing only the off-diagonal determinant contribution produces a shift in the fitted stochastic prescription that vanishes when $J_\sigma=0$. In Model B, removing the corresponding state-dependent action term changes a stable optimized transition path. These results show that a genuinely matrix-noise part of the short-time path measure can survive the scalar cancellations familiar from simpler settings and produce measurable changes in fitted stochastic prescriptions and Onsager--Machlup paths.

cond-mat.stat-mech↗

arXiv2026

Weighted Phase-Space Paths for Exact Wigner Dynamics

A quantum state can be written in phase space, but the resulting object is not generally the probability density of a positive stochastic process on ordinary phase space. We spell this out for Wigner dynamics. If a positive phase-space process is required only to reproduce the Born density after integrating over momentum, the requirement fixes only an integrated current; the local drift and diffusion remain underdetermined. If one instead requires all Weyl-ordered expectation values, the phase-space object is fixed to be the Wigner function. For non-quadratic potentials the Wigner--Moyal generator contains higher-order, signed momentum-transfer terms, so it is not the Fokker--Planck generator of a positive Brownian diffusion. The exact Wigner function must therefore be reconstructed, in a stochastic representation, as a weighted empirical measure \[ \FW(\z,t)=\E_{\Pp}[W_t\delta(\z-\z_t)], \qquad \z=(q,p), \] rather than the unweighted density of sampled carrier trajectories. With classical Hamiltonian flow as the carrier, all nonclassical correction beyond classical transport sits in the Moyal residual and can be represented by signed weights or branching events. The same split defines a residual diagnostic that vanishes for quadratic Hamiltonians and measures what classical carrier transport misses in anharmonic dynamics. The formulation also gives a forward--reverse relation for signed Wigner path measures. The ratio of forward and reversed contributions separates into a positive magnitude factor and a sign factor. This sign records the parity of the Wigner interference contribution; it is not a thermodynamic entropy production.

quant-ph↗

arXiv2019

Dissipation of traffic congestion using agent-based car-following model with modified optimal velocity

We investigate dynamical properties of traffic flow using the stochastic car-following model with modified optimal velocity on circular road. The safety distance following the two-second rule and autonomous vehicles, acting as agents, obeying simple requirements are incorporated into the model. The dynamic safety distance increases in a light traffic condition where the average driving velocity is high, while decreases in a dense traffic condition in anticipation of slower traffic motion. The results show that the presence of the agents can enhance overall velocity and traffic current of the system, and postpone the traffic congestion. In a particular phase region, imposing a speed limit enables the system to leave the congested flow phase. The density-dependent speed limit in agent-free condition is obtained to achieve the optimal traffic flow.

nlin.AO↗

arXiv2017

Upper bound for the average entropy production based on stochastic entropy extrema

The second law of thermodynamics, which asserts the non-negativity of the average total entropy production of a combined system and its environment, is a direct consequence of applying Jensen's inequality to a fluctuation relation. It is also possible, through this inequality, to determine an upper bound of the average total entropy production based on the entropies along the most extreme stochastic trajectories. In this work, we construct an upper bound inequality of the average of a convex function over a domain whose average is known. When applied to the various fluctuation relations, the upper bounds of the average total entropy production are established. Finally, by employing the result of Neri, Roldán, and Jülicher [Phys. Rev. X 7, 011019 (2017)], we are able to show that the average total entropy production is bounded only by the total entropy production supremum, and vice versa, for a general non-equilibrium stationary system.

cond-mat.stat-mech↗

arXiv2015

Quantum Phase Estimation Algorithm for Finding Polynomial Roots

Quantum algorithm is an algorithm for solving mathematical problems using quantum systems encoded as information, which is found to outperform classical algorithms in some specific cases. The objective of this study is to develop a quantum algorithm for finding the roots of nth degree polynomials where n is any positive integer. In classical algorithm, the resources required for solving this problem increase drastically when n increases and it would be impossible to practically solve the problem when n is large. It was found that any polynomial can be rearranged into a corresponding companion matrix, whose eigenvalues are roots of the polynomial. This leads to a possibility to perform a quantum algorithm where the number of computational resources increase as a polynomial of n. In this study, we construct a quantum circuit representing the companion matrix and use eigenvalue estimation technique to find roots of polynomial.

quant-ph↗

arXiv2013

Random Walk on Lattices: Graph Theoretic Approach to Modeling Epitaxially Grown Thin Film

Immense interests in thin-film fabrication for industrial applications have driven both theoretical and computational aspects of modeling its growth with an aim to design and control film's surface morphology. Oftentimes, smooth surface is desirable and is experimentally achievable via molecular-beam epitaxy (MBE) growth technique with exceptionally low deposition flux. Adatoms on the film grown with such a method tend to have large diffusion length which can be computationally very costly when certain statistical aspects are demanded. We present a graph theoretic approach to modeling MBE grown thin film with long atomic mean free path. Using Markovian assumption and given a local diffusion bias, we derive the transition probabilities for a random walker to traverse from one lattice site to the others after a large, possibly infinite, number of hopping steps. Only computation with linear-time complexity is required for the surface morphology calculation without other probabilistic measures. The formalism is applied to simulate thin film growth on a two-dimensional flat substrate and around a screw dislocation under the modified Wolf--Villain diffusion rule. A rectangular spiral ridge is observed in the latter case with a smooth front feature similar to that obtained from simulations using the well-known multiple registration technique. An algorithm to compute the inverse of a class of sub-stochastic matrices is derived as a corollary.

cond-mat.stat-mech↗

arXiv2010

Growth instability due to lattice-induced topological currents in limited mobility epitaxial growth models

The energetically driven Ehrlich-Schwoebel (ES) barrier had been generally accepted as the primary cause of the growth instability in the form of quasi-regular mound-like structures observed on the surface of thin film grown via molecular beam epitaxy (MBE) technique. Recently the second mechanism of mound formation was proposed in terms of a topologically induced flux of particles originating from the line tension of the step edges which form the contour lines around a mound. Through large-scale simulations of MBE growth on a variety of crystalline lattice planes using limited mobility, solid-on-solid models introduced by Wolf-Villain and Das Sarma-Tamborenea in 2+1 dimensions, we propose yet another type of topological uphill particle current which is unique to some lattice, and has hitherto been overlooked in the literature. Without ES barrier, our simulations produce spectacular mounds very similar, in some cases, to what have been observed in many recent MBE experiments. On a lattice where these currents cease to exist, the surface appears to be scale-invariant, statistically rough as predicted by the conventional continuum growth equation.

cond-mat.mtrl-sci↗

arXiv2007

Study of size effects in thin films by means of a crystal plasticity theory based on DiFT

In a recent publication, we derived the mesoscale continuum theory of plasticity for multiple-slip systems of parallel edge dislocations, motivated by the statistical-based nonlocal continuum crystal plasticity theory for single-glide due to Yefimov et al. (2004b). In this dislocation field theory (DiFT) the transport equations for both the total dislocation densities and geometrically necessary dislocation densities on each slip system were obtained from the Peach-Koehler interactions through both single and pair dislocation correlations. The effect of pair correlation interactions manifested itself in the form of a back stress in addition to the external shear and the self-consistent internal stress. We here present the study of size effects in single crystalline thin films with symmetric double slip using the novel continuum theory. Two boundary value problems are analyzed: (1) stress relaxation in thin films on substrates subject to thermal loading, and (2) simple shear in constrained films. In these problems, earlier discrete dislocation simulations had shown that size effects are born out of layers of dislocations developing near constrained interfaces. These boundary layers depend on slip orientations and applied loading but are insensitive to the film thickness. We investigate stress response to changes in controlled parameters in both problems. Comparisons with previous discrete dislocation simulations are discussed.

cond-mat.mtrl-sci↗

arXiv2007

Statistical approach to dislocation dynamics: From dislocation correlations to a multiple-slip continuum plasticity theory

Due to recent successes of a statistical-based nonlocal continuum crystal plasticity theory for single-glide in explaining various aspects such as dislocation patterning and size-dependent plasticity, several attempts have been made to extend the theory to describe crystals with multiple slip systems using ad-hoc assumptions. We present here a mesoscale continuum theory of plasticity for multiple slip systems of parallel edge dislocations. We begin by constructing the Bogolyubov-Born-Green-Yvon-Kirkwood (BBGYK) integral equations relating different orders of dislocation correlation functions in a grand canonical ensemble. Approximate pair correlation functions are obtained for single-slip systems with two types of dislocations and, subsequently, for general multiple-slip systems of both charges. The effect of the correlations manifests itself in the form of an entropic force in addition to the external stress and the self-consistent internal stress. Comparisons with a previous multiple-slip theory based on phenomenological considerations shall be discussed.

cond-mat.stat-mech↗

arXiv2007

Stress-free states of continuum dislocation fields: Rotations, grain boundaries, and the Nye dislocation density tensor

We derive general relations between grain boundaries, rotational deformations, and stress-free states for the mesoscale continuum Nye dislocation density tensor. Dislocations generally are associated with long-range stress fields. We provide the general form for dislocation density fields whose stress fields vanish. We explain that a grain boundary (a dislocation wall satisfying Frank's formula) has vanishing stress in the continuum limit. We show that the general stress-free state can be written explicitly as a (perhaps continuous) superposition of flat Frank walls. We show that the stress-free states are also naturally interpreted as configurations generated by a general spatially-dependent rotational deformation. Finally, we propose a least-squares definition for the spatially-dependent rotation field of a general (stressful) dislocation density field.

cond-mat.dis-nn↗

arXiv2006

Mesoscale theory of grains and cells: crystal plasticity and coarsening

Solids with spatial variations in the crystalline axes naturally evolve into cells or grains separated by sharp walls. Such variations are mathematically described using the Nye dislocation density tensor. At high temperatures, polycrystalline grains form from the melt and coarsen with time: the dislocations can both climb and glide. At low temperatures under shear the dislocations (which allow only glide) form into cell structures. While both the microscopic laws of dislocation motion and the macroscopic laws of coarsening and plastic deformation are well studied, we hitherto have had no simple, continuum explanation for the evolution of dislocations into sharp walls. We present here a mesoscale theory of dislocation motion. It provides a quantitative description of deformation and rotation, grounded in a microscopic order parameter field exhibiting the topologically conserved quantities. The topological current of the Nye dislocation density tensor is derived from a microscopic theory of glide driven by Peach-Koehler forces between dislocations using a simple closure approximation. The resulting theory is shown to form sharp dislocation walls in finite time, both with and without dislocation climb.

cond-mat.mtrl-sci↗