arXiv subjects
Publications and source records attributed to Hynek Kovarik.
In this paper we present Hardy type inequalities for magnetic Dirichlet forms with singular integral weights. We analyze the local and global optimality of the integral weight and discuss several examples in details. An application of our results to spectral estimates for magnetic Schrödinger operators is provided as well.
In this paper we consider three-dimensional Schrödinger operators with a simple threshold eigenvalue. We show, under certain assumptions, that when a small magnetic field is introduced, this eigenvalue turns into a resonance in the time-dependent sense. We find the leading term in the asymptotic expansion of the imaginary part of the resonance and discuss the principal differences with respect to resonances induced by weak electric fields obtained previously in the literature.
We prove a Cwikel-Lieb-Rozenblum type inequality for the number of negative eigenvalues of Pauli operators in dimension two. The resulting upper bound is sharp both in the weak as well as in the strong coupling limit. We also derive different upper bounds for magnetic Schrödinger operators. The nature of the two estimates depends on whether or not the spin-orbit coupling is taken into account.
In this note we consider achieving the largest principle eigenvalue of a Robin Laplacian on a bounded domain $Ω$ by optimizing the Robin parameter function under an integral constraint. The main novelty of our approach lies in establishing a close relation between the problem under consideration and the asymptotic behavior of the Dirichlet heat content of $Ω$. By using this relation we deduce a two-term asymptotic expansion of the principle eigenvalue and discuss several applications.
We obtain asymptotic resolvent expansions at the threshold of the essential spectrum for magnetic Schrödinger and Pauli operators in dimension three. These operators are treated as perturbations of the Laplace operator in $L^2(\mathbb{R}^3)$ and $L^2(\mathbb{R}^3;\mathbb{C}^2)$, respectively. The main novelty of our approach is to show that the relative perturbations, which are first order differential operators, can be factorized in suitably chosen auxiliary spaces. This allows us to derive the desired asymptotic expansions of the resolvents around zero. We then calculate their leading and sub-leading terms explicitly. Analogous factorization schemes for more general perturbations, including e.g.~finite rank perturbations, are discussed as well.
In this paper we present a new method of proof of Hardy type inequalities for two-dimensional quantum Hamiltonians with a magnetic field of finite flux. Our approach gives a quantitative lower bound on the best constant in these inequalities both for Schrödinger and Pauli operators. Pauli operators with Aharonov-Bohm magnetic field are discussed as well.
We consider eigenvalues of the Pauli operator in $\mathbb R^3$ embedded in the continuous spectrum. In our main result we prove the absence of such eigenvalues above a threshold which depends on the asymptotic behavior of the magnetic and electric field at infinity. We show moreover that the decay conditions on the magnetic and electric field are sharp. Analogous results are obtained for purely magnetic Dirac operators.
We study sufficient conditions for the absence of positive eigenvalues of magnetic Schrödinger operators in $\mathbb{R}^d,\, d\geq 2$. In our main result we prove the absence of eigenvalues above certain threshold energy which depends explicitly on the magnetic and electric field. A comparison with the examples of Miller--Simon shows that our result is sharp as far as the decay of the magnetic field is concerned. As applications, we describe several consequences of the main result for two-dimensional Pauli and Dirac operators, and two and three dimensional Aharonov--Bohm operators.
We study semigroups generated by two-dimensional relativistic Hamiltonians with magnetic field. In particular, for compactly supported radial magnetic field we show how the long time behaviour of the associated heat kernel depends on the flux of the field. Similar questions are addressed for Aharonov-Bohm type magnetic field.
For dimensions $N \geq 4$, we consider the Brézis-Nirenberg variational problem of finding \[ S(εV) := \inf_{0\not\equiv u\in H^1_0(Ω)} \frac{\int_Ω|\nabla u|^2 \, dx +ε\int_ΩV\, |u|^2 \, dx}{\left(\int_Ω|u|^q \, dx \right)^{2/q}}, \] where $q=\frac{2N}{N-2}$ is the critical Sobolev exponent and $Ω\subset \mathbb{R}^N$ is a bounded open set. We compute the asymptotics of $S(0) - S(εV)$ to leading order as $ε\to 0+$. We give a precise description of the blow-up profile of (almost) minimizing sequences and, in particular, we characterize the concentration points as being extrema of a quotient involving the Robin function. This complements the results from our recent paper in the case $N = 3$.
For a bounded open set $Ω\subset\mathbb R^3$ we consider the minimization problem $$ S(a+εV) = \inf_{0\not\equiv u\in H^1_0(Ω)} \frac{\int_Ω(|\nabla u|^2+ (a+εV) |u|^2)\,dx}{(\int_Ωu^6\,dx)^{1/3}} $$ involving the critical Sobolev exponent. The function $a$ is assumed to be critical in the sense of Hebey and Vaugon. Under certain assumptions on $a$ and $V$ we compute the asymptotics of $S(a+εV)-S$ as $ε\to 0+$, where $S$ is the Sobolev constant. (Almost) minimizers concentrate at a point in the zero set of the Robin function corresponding to $a$ and we determine the location of the concentration point within that set. We also show that our assumptions are almost necessary to have $S(a+εV) 0$.
We study the behaviour of Hardy-weights for a class of variational quasi-linear elliptic operators of $p$-Laplacian type. In particular, we obtain necessary sharp decay conditions at infinity on the Hardy-weights in terms of their integrability with respect to certain integral weights. Some of the results are extended also to nonsymmetric linear elliptic operators. Applications to various examples are discussed as well.
We consider a twisted quantum wave guide, and are interested in the spectral analysis of the associated Dirichlet Laplacian H. We show that if the derivative of rotation angle decays slowly enough at infinity, then there is an infinite sequence of discrete eigenvalues lying below the infimum of the essential spectrum of H, and obtain the main asymptotic term of this sequence.
We study three-body Schrödinger operators in one and two dimensions modelling an exciton interacting with a charged impurity. We consider certain classes of multiplicative interaction potentials proposed in the physics literature. We show that if the impurity charge is larger than some critical value, then three-body bound states cannot exist. Our spectral results are confirmed by variational numerical computations based on projecting on a finite dimensional subspace generated by a Gaussian basis.
Let $Ω\subset\mathbb{R}^N$, $N\ge 2,$ be a bounded domain with an outward power-like peak which is assumed not too sharp in a suitable sense. We consider the Laplacian $u\mapsto -Δu$ in $Ω$ with the Robin boundary condition $\partial_n u=αu$ on $\partialΩ$ with $\partial_n$ being the outward normal derivative and $α>0$ being a parameter. We show that for large $α$ the associated eigenvalues $E_j(α)$ behave as $E_j(α)\sim -ε_j α^ν$, where $ν>2$ and $ε_j>0$ depend on the dimension and the peak geometry. This is in contrast with the well-known estimate $E_j(α)=O(α^2)$ for the Lipschitz domains.
We consider a three-body one-dimensional Schrödinger operator with zero range potentials, which models a positive impurity with charge $κ> 0$ interacting with an exciton. We study the existence of discrete eigenvalues as $κ$ is varied. On one hand, we show that for sufficiently small $κ$ there exists a unique bound state whose binding energy behaves like $κ^4$, and we explicitly compute its leading coefficient. On the other hand, if $κ$ is larger than some critical value then the system has no bound states.
Let $Ω\subset\mathbb{R}^ν$, $ν\ge 2$, be a $C^{1,1}$ domain whose boundary $\partialΩ$ is either compact or behaves suitably at infinity. For $p\in(1,\infty)$ and $α>0$, define \[ Λ(Ω,p,α):=\inf_{\substack{u\in W^{1,p}(Ω)\\ u\not\equiv 0}}\dfrac{\displaystyle \int_Ω|\nabla u|^p \mathrm{d} x - α\displaystyle\int_{\partialΩ} |u|^p\mathrm{d}σ}{\displaystyle\int_Ω|u|^p\mathrm{d} x}, \] where $\mathrm{d}σ$ is the surface measure on $\partialΩ$. We show the asymptotics \[ Λ(Ω,p,α)=-(p-1)α^{\frac{p}{p-1}} - (ν-1)H_\mathrm{max}\, α+ o(α), \quad α\to+\infty, \] where $H_\mathrm{max}$ is the maximum mean curvature of $\partialΩ$. The asymptotic behavior of the associated minimizers is discussed as well. The estimate is then applied to the study of the best constant in a boundary trace theorem for expanding domains, to the norm estimate for extension operators and to related isoperimetric inequalities.
We study Schroedinger operators with Robin boundary conditions on exterior domains in $\R^d$. We prove sharp point-wise estimates for the associated semi-groups which show, in particular, how the boundary conditions affect the time decay of the heat kernel in dimensions one and two. Applications to spectral estimates are discussed as well.