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Stephen Gustafson

Publications and source records attributed to Stephen Gustafson.

At least 19 recordsLinked to original sources
arXiv2025

Saddle Point Configurations for Spherical Ferromagnets

We investigate saddle point configurations in spherical ferromagnets with perpendicular anisotropy. These are modeled by a micromagnetic energy functional on the unit sphere that leads to the emergence of the so-called curvature induced Dzyaloshinskii--Moriya interaction. For this functional we establish the existence of two distinct types of saddle points with zero mapping degree. We use a parabolic flow approach inspired by the harmonic map heat flow where for certain highly symmetric initial conditions we can rule out finite and infinite time blowup. Hence, we obtain solutions of the underlying Euler--Lagrange equation in the long time limit. This is in contrast to harmonic maps between two-spheres, where every map is a local minimizer of the energy functional.

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arXiv2023

Scattering and Blow-up for threshold even solutions to the nonlinear Schrödinger equation with repulsive delta potential at low frequencies

We consider the $L^2$-supercritical nonlinear Schrödinger equation with a repulsive Dirac delta potential in one dimensional space. In a previous work, we clarified the global dynamics of even solutions with the same action as the high-frequency ground state standing wave solutions. In that case, there are obvious non-scattering global solutions, i.e., the standing waves. In this paper, we show a scattering and blow-up dichotomy for threshold even solutions in the low-frequency case. We emphasize that this dichotomy still holds at the critical frequency between high and low.

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arXiv2023

Multi-solitons for the nonlinear Schrödinger equation with repulsive Dirac delta potential

We prove the existence of multi-soliton solutions for the nonlinear Schrödinger equation with repulsive Dirac delta potential and $L^2$-supercritical focusing nonlinear term. Our main contribution is to treat the unmoving part of the multi-solitons, which is the ground state of the equation. The linearized operator around it has two unstable eigenvalues. This is the main difference from NLS without potential, whose existence of multi-solitons is investigated by Côte, Martel, and Merle (2011).

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arXiv2023

Two-solitons with logarithmic separation for 1D NLS with repulsive delta potential

We consider the one-dimensional nonlinear Schrödinger equation with focusing, power nonlinearity, and a repulsive delta potential. We show that if the potential is not too strong, the construction by Nguy\~{ê}n (2019) of solutions converging strongly at time infinity to a pair of logarithmically separating solitons can be adapted to accommodate the effect of the potential. On the other hand, we show that if the potential is stronger, no such solutions exist.

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arXiv2023

Growth rates for anti-parallel vortex tube Euler flows in three and higher dimensions

We consider axisymmetric, swirl-free solutions of the Euler equations in three and higher dimensions, of generalized anti-parallel-vortex-tube-pair-type: the initial scalar vorticity has a sign in the half-space, is odd under reflection across the plane, is bounded and decays sufficiently rapidly at the axis and at spatial infinity. We prove lower bounds on the growth of such solutions in all dimensions, improving a lower bound proved by Choi and Jeong arXiv:2110.09079 in three dimensions.

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arXiv2022

Blow-up or Grow-up for the threshold solutions to the nonlinear Schrödinger equation

We consider the nonlinear Schrödinger equation with $L^{2}$-supercritical and $H^{1}$-subcritical power type nonlinearity. Duyckaerts and Roudenko and Campos, Farah, and Roudenko studied the global dynamics of the solutions with same mass and energy as that of the ground state. In these papers, finite variance is assumed to show the finite time blow-up. In the present paper, we remove the finite-variance assumption and prove a blow-up or grow-up result.

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arXiv2022

Threshold odd solutions to the nonlinear Schrödinger equation in one dimension

We consider odd solutions to the Schrödinger equation with the $L^2$-supercritical power type nonlinearity in one dimensional Euclidean space. It is known that the odd solution scatters or blows up if its action is less than twice as that of the ground state. In the present paper, we show that the odd solutions with the action as twice as that of the ground state scatter or blow up.

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arXiv2020

Co-rotational chiral magnetic skyrmions near harmonic maps

Chiral magnetic skyrmions are topological solitons, of significant physical interest, arising in ferromagnets described by a micromagnetic energy including a chiral (Dzyaloshinskii-Moriya) interaction term. We show that for small chiral interaction, the skyrmions on $\mathbb{R}^2$ with co-rotational symmetry are close to harmonic maps, and prove precise bounds on the differences. One application of these bounds is precise energy asymptotics. Another (pursued in a separate work) is an alternate, quantitative proof of the recent skyrmion stability result of Li-Melcher.

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arXiv2019

Solitary Waves and Dynamics for Subcritical Perturbations of Energy Critical NLS

We consider a perturbed energy critical focusing Nonlinear Schrödinger Equation in three dimensions. We construct solitary wave solutions for focusing subcritical perturbations as well as defocusing supercritical perturbations. The construction relies on the resolvent expansion, which is singular due to the presence of a resonance. Specializing to pure power focusing subcritical perturbations we demonstrate, via variational arguments, and for a certain range of powers, the existence of a ground state solitary wave, which is then shown to be the previously constructed solution. Finally, we present a dynamical theorem which characterizes the fate of radially-symmetric solutions whose initial data are below the action of the ground state. Such solutions will either scatter or blow-up in finite time depending on the sign of a certain function of their initial data.

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arXiv2017

Global solutions for the critical, higher-degree corotational harmonic map heat flow to $\mathbb{S}^2$

We study m-corotational solutions to the Harmonic Map Heat Flow from $\mathbb{R}^2$ to $\mathbb{S}^2$. We first consider maps of zero topological degree, with initial energy below the threshold given by twice the energy of the harmonic map solutions. For $m \geq 2$, we establish the smooth global existence and decay of such solutions via the {\it concentration-compactness} approach of Kenig-Merle, recovering classical results of Struwe by this alternate method. The proof relies on a profile decomposition, and the energy dissipation relation. We then consider maps of degree $m$ and initial energy above the harmonic map threshold energy, but below three times this energy. For $m \geq 4$, we establish the smooth global existence of such solutions, and their decay to a harmonic map (stability), extending results of Gustafson-Nakanishi-Tsai to higher energies. The proof rests on a stability-type argument used to rule out finite-time bubbling.

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arXiv2017

Global, decaying solutions of a focusing energy-critical heat equation in $\mathbb{R}^4$

We study solutions of the focusing energy-critical nonlinear heat equation $u_t = Δu - |u|^2u$ in $\mathbb{R}^4.$ We show that solutions emanating from initial data with energy and $\dot{H}^1-$norm below those of the stationary solution $W$ are global and decay to zero, via the "concentration-compactness plus rigidity" strategy of Kenig-Merle. First, such global solutions are shown to dissipate to zero, using a refinement of the small data theory and the $L^2$-dissipation relation. Finite-time blow-up is then ruled out using the backwards-uniqueness of Escauriaza, Seregin and Sverak in an argument similar to that of Kenig and Koch for the Navier-Stokes equations.

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arXiv2016

Stability of periodic waves of 1D cubic nonlinear Schr{ö}dinger equations

We study the stability of the cnoidal, dnoidal and snoidal elliptic functions as spatially-periodic standing wave solutions of the 1D cubic nonlinear Schr{ö}dinger equations. First, we give global variational characterizations of each of these periodic waves, which in particular provide alternate proofs of their orbital stability with respect to same-period perturbations, restricted to certain subspaces. Second, we prove the spectral stability of the cnoidal waves against same-period perturbations (in a certain parameter range), and provide an alternate proof of this (known) fact for the snoidal waves, which does not rely on complete integrability. Third, we give a rigorous version of a formal asymptotic calculation of Rowlands to establish the instability of a class of real-valued periodic waves in 1D, which includes the cnoidal waves of the 1D cubic focusing nonlinear Schr{ö}dinger equation, against perturbations with period a large multiple of their fundamental period. Finally, we develop a numerical method to compute the minimizers of the energy with fixed mass and momentum constraints. Numerical experiments support and complete our analytical results.

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arXiv2015

A Degenerate Edge Bifurcation in the 1D Linearized Nonlinear Schrodinger Equation

This work deals with the focusing Nonlinear Schrodinger Equation in one dimension with pure-power nonlinearity near cubic. We consider the spectrum of the linearized operator about the soliton solution. When the nonlinearity is exactly cubic, the linearized operator has resonances at the edges of the essential spectrum. We establish the degenerate bifurcation of these resonances to eigenvalues as the nonlinearity deviates from cubic. The leading-order expression for these eigenvalues is consistent with previous numerical computations.

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arXiv2013

On linear instability of solitary waves for the nonlinear Dirac equation

We consider the nonlinear Dirac equation, also known as the Soler model: $i\p\sb tψ=-iα\cdot \nabla ψ+m βψ-f(ψ\sp\ast βψ) βψ$, $ψ(x,t)\in\mathbb{C}^{N}$, $x\in\mathbb{R}^n$, $n\le 3$, $f\in C\sp 2(\R)$, where $α_j$, $j = 1,...,n$, and $β$ are $N \times N$ Hermitian matrices which satisfy $α_j^2=β^2=I_N$, $α_j β+βα_j=0$, $α_j α_k + α_k α_j =2 δ_{jk} I_N$. We study the spectral stability of solitary wave solutions $ϕ(x)e^{-iωt}$. We study the point spectrum of linearizations at solitary waves that bifurcate from NLS solitary waves in the limit $ω\to m$, proving that if $k>2/n$, then one positive and one negative eigenvalue are present in the spectrum of the linearizations at these solitary waves with $ω$ sufficiently close to $m$, so that these solitary waves are linearly unstable. The approach is based on applying the Rayleigh--Schroedinger perturbation theory to the nonrelativistic limit of the equation. The results are in formal agreement with the Vakhitov--Kolokolov stability criterion.

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arXiv2011

Global well-posedness for 2D radial Schrödinger maps into the sphere

We prove global well-posedness for a cubic, non-local Schrödinger equation with radially-symmetric initial data in the critical space $L^2(\R^2)$, using the framework of Kenig-Merle and Killip-Tao-Visan. As a consequence, we obtain a global well-posedness result for Schrödinger maps from $\R^2$ into $§^2$ (Landau-Lifshitz equation) with radially symmetric initial data (with no size restriction).

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arXiv2010

Stable Directions for Degenerate Excited States of Nonlinear Schrödinger Equations

We consider nonlinear Schrödinger equations, $i\partial_t ψ= H_0 ψ+ λ|ψ|^2ψ$ in $\mathbb{R}^3 \times [0,\infty)$, where $H_0 = -Δ+ V$, $λ=\pm 1$, the potential $V$ is radial and spatially decaying, and the linear Hamiltonian $H_0$ has only two eigenvalues $e_0 < e_1 <0$, where $e_0$ is simple, and $e_1$ has multiplicity three. We show that there exist two branches of small "nonlinear excited state" standing-wave solutions, and in both the resonant ($e_0 < 2e_1$) and non-resonant ($e_0 > 2e_1$) cases, we construct certain finite-codimension regions of the phase space consisting of solutions converging to these excited states at time infinity ("stable directions").

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