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We demonstrate theoretically the possibility of spinodal de-wetting in heterostructures made of light--atom liquids (hydrogen, helium, and nitrogen) deposited on suspended graphene. Extending our theory of film growth on two-dimensional materials to include analysis of surface instabilities via the hydrodynamic Cahn--Hilliard-type equation, we characterize in detail the resulting spinodal de-wetting patterns. Both linear stability analysis and advanced computational treatment of the surface hydrodynamics show micron-sized (generally material and atom dependent) patterns of "dry" regions. The physical reason for the development of such instabilities on graphene can be traced back to the inherently weak van der Waals interactions between atomically thin materials and atoms in the liquid. Similar phenomena occur in doped graphene and other two-dimensional materials, such as monolayer dichalcogenides. Thus two-dimensional materials represent a universal theoretical and technological platform for studies of spinodal de-wetting., Comment: 11 pages, 10 figures

Stability properties of the well-known Fourier split-step method used to simulate a soliton and similar solutions of the nonlinear Dirac equations, known as the Gross--Neveu model, are studied numerically and analytically. Three distinct types of numerical instability that can occur in this case, are revealed and explained. While one of these types can be viewed as being related to the numerical instability occurring in simulations of the nonlinear Schr\"odinger equation, the other two types have not been studied or identified before, to the best of our knowledge. These two types of instability are {\em unconditional}, i.e. occur for arbitrarily small values of the time step. They also persist in the continuum limit, i.e. for arbitrarily fine spatial discretization. Moreover, one of them persists in the limit of an infinitely large computational domain. It is further demonstrated that similar instabilities also occur for other numerical methods applied to the Gross--Neveu soliton, as well as to certain solitons of another relativistic field theory model, the massive Thirring., Comment: 37 pages; 12 multi-panel figures

We study numerical (in)stability of the Method of characteristics (MoC) applied to a system of non-dissipative hyperbolic partial differential equations (PDEs) with periodic boundary conditions. We consider three different solvers along the characteristics: simple Euler (SE), modified Euler (ME), and Leap-frog (LF). The two former solvers are well known to exhibit a mild, but unconditional, numerical instability for non-dissipative ordinary differential equations (ODEs). They are found to have a similar (or stronger, for the MoC-ME) instability when applied to non-dissipative PDEs. On the other hand, the LF solver is known to be stable when applied to non-dissipative ODEs. However, when applied to non-dissipative PDEs within the MoC framework, it was found to have by far the strongest instability among all three solvers. We also comment on the use of the fourth-order Runge--Kutta solver within the MoC framework., Comment: 20 pages, 5 figures

We show that imposition of non-periodic, in place of periodic, boundary conditions (BC) can alter stability of modes in the Method of characteristics (MoC) employing certain ordinary-differential equation (ODE) numerical solvers. Thus, using non-periodic BC may render some of the MoC schemes stable for most practical computations, even though they are unstable for periodic BC. This fact contradicts a statement, found in some literature, that an instability detected by the von Neumann analysis for a given numerical scheme implies an instability of that scheme with arbitrary (i.e., non-periodic) BC. We explain the mechanism behind this contradiction. We also show that, and explain why, for the MoC employing some other ODE solvers, stability of the modes may be unaffected by the BC., Comment: 38 pages, 13 figures

We consider numerical instability that can be observed in simulations of localized solutions of the generalized nonlinear Schr\"odinger equation (NLS) by a split-step method where the linear part of the evolution is solved by a finite-difference discretization. Properties of such an instability cannot be inferred from the von Neumann analysis of the numerical scheme. Rather, their explanation requires tools of stability analysis of nonlinear waves, with numerically unstable modes exhibiting novel features not reported for "real" unstable modes of nonlinear waves. For example, modes that cause numerical instability of a standing soliton of the NLS are supported by the sides of the soliton rather than by its core. Furthermore, we demonstrate that both properties and analyses of the numerical instability may be substantially affected by specific details of the simulated solution; e.g., they are substantially different for standing and moving solitons of the NLS.

Prior research has shown that autocorrelation and variance in voltage measurements tend to increase as power systems approach instability. This paper seeks to identify the conditions under which these statistical indicators provide reliable early warning of instability in power systems. First, the paper derives and validates a semi-analytical method for quickly calculating the expected variance and autocorrelation of all voltages and currents in an arbitrary power system model. Building on this approach, the paper describes the conditions under which filtering can be used to detect these signs in the presence of measurement noise. Finally, several experiments show which types of measurements are good indicators of proximity to instability for particular types of state changes. For example, increased variance in voltages can reliably indicate the location of increased stress, while growth of autocorrelation in certain line currents is a reliable indicator of system-wide instability., Comment: 8 pages, 12 figures

This paper shows that the variance of load bus voltage magnitude in a small power system test case increases monotonically as the system approaches a Hopf bifurcation. This property can potentially be used as a method for monitoring oscillatory stability in power grid using high-resolution phasor measurements. Increasing variance in data from a dynamical system is a common sign of a phenomenon known as critical slowing down (CSD). CSD is slower recovery of dynamical systems from perturbations as they approach critical transitions. Earlier work has focused on studying CSD in systems approaching voltage collapse; In this paper, we investigate its occurrence as a power system approaches a Hopf bifurcation., Comment: 5 pages, 7 figures, IEEE Power & Energy Society General Meeting 2014

Many dynamical systems, including power systems, recover from perturbations more slowly as they approach critical transitions---a phenomenon known as critical slowing down. If the system is stochastically forced, autocorrelation and variance in time-series data from the system often increase before the transition, potentially providing an early warning of coming danger. In some cases, these statistical patterns are sufficiently strong, and occur sufficiently far from the transition, that they can be used to predict the distance between the current operating state and the critical point. In other cases CSD comes too late to be a good indicator. In order to better understand the extent to which CSD can be used as an indicator of proximity to bifurcation in power systems, this paper derives autocorrelation functions for three small power system models, using the stochastic differential algebraic equations (SDAE) associated with each. The analytical results, along with numerical results from a larger system, show that, although CSD does occur in power systems, its signs sometimes appear only when the system is very close to transition. On the other hand, the variance in voltage magnitudes consistently shows up as a good early warning of voltage collapse. Finally, analytical results illustrate the importance of nonlinearity to the occurrence of CSD., Comment: 14 Pages, 22 figures

High-frequency solutions of one or several Schr\"odinger-type equations are well known to differ very little from the plane wave solutions $\exp[\pm ik x]$. That is, the potential terms impact the envelope of a high-frequency plane wave by only a small amount. However, when such equations are solved by a finite-difference method, the highest-frequency solutions may, under certain conditions, turn out to be localized. In this letter we explain this numerical artifact., Comment: 4 pages, 6 figure panels (2 figures)

We analyze a numerical instability that occurs in the well-known split-step Fourier method on the background of a soliton. This instability is found to be very sensitive to small changes of the parameters of both the numerical grid and the soliton, unlike the instability of most finite-difference schemes. % on the background of a monochromatic wave, considered earlier in the literature. Moreover, the principle of ``frozen coefficients", in which variable coefficients are treated as ``locally constant" for the purpose of stability analysis, is strongly violated for the instability of the split-step method on the soliton background. Our analysis explains all these features. It is enabled by the fact that the period of oscillations of the unstable Fourier modes is much smaller than the width of the soliton., Comment: 28 pages, 5 figures This is the original manuscript submitted to the journal Numerical methods for PDEs. A revised version, in whose title the word "Stability" is changed to "Instability", is scheduled to appear in that journal at the end of 2010

The Conjugate Gradient method (CGM) is known to be the fastest generic iterative method for solving linear systems with symmetric sign definite matrices. In this paper, we modify this method so that it could find fundamental solitary waves of nonlinear Hamiltonian equations. The main obstacle that such a modified CGM overcomes is that the operator of the equation linearized about a solitary wave is not sign definite. Instead, it has a finite number of eigenvalues on the opposite side of zero than the rest of its spectrum. We present versions of the modified CGM that can find solitary waves with prescribed values of either the propagation constant or power. We also extend these methods to handle multi-component nonlinear wave equations. Convergence conditions of the proposed methods are given, and their practical implications are discussed. We demonstrate that our modified CGMs converge much faster than, say, Petviashvili's or similar methods, especially when the latter converge slowly., Comment: 44 pages, submitted to Physica D

Two accelerated imaginary-time evolution methods are proposed for the computation of solitary waves in arbitrary spatial dimensions. For the first method (with traditional power normalization), the convergence conditions as well as conditions for optimal accelerations are derived. In addition, it is shown that for nodeless solitary waves, this method converges if and only if the solitary wave is linearly stable. The second method is similar to the first method except that it uses a novel amplitude normalization. The performance of these methods is illustrated on various examples. It is found that while the first method is competitive with the Petviashvili method, the second method delivers much better performance than the first method and the Petviashvili method., Comment: To appear in Stud. Appl. Math. (28 pages, 6 figures)