Spectral Asymptotics for Waveguides with Perturbed Periodic Twisting

We consider the twisted waveguide $\Omega_\theta$, i.e. the domain obtained by the rotation of the bounded cross section $\omega \subset {\mathbb R}^{2}$ of the straight tube $\Omega : = \omega \times {\mathbb R}$ at angle $\theta$ which depends on the variable along the axis of $\Omega$. We study the spectral properties of the Dirichlet Laplacian in $\Omega_\theta$, unitarily equivalent under the diffeomorphism $\Omega_\theta \to \Omega$ to the operator $H_{\theta'}$, self-adjoint in ${\rm L}^2(\Omega)$. We assume that $\theta' = \beta - \epsilon$ where $\beta$ is a $2\pi$-periodic function, and $\epsilon$ decays at infinity. Then in the spectrum $\sigma(H_\beta)$ of the unperturbed operator $H_\beta$ there is a semi-bounded gap $(-\infty, {\mathcal E}_0^+)$, and, possibly, a number of bounded open gaps $({\mathcal E}_j^-, {\mathcal E}_j^+)$. Since $\epsilon$ decays at infinity, the essential spectra of $H_\beta$ and $H_{\beta - \epsilon}$ coincide. We investigate the asymptotic behaviour of the discrete spectrum of $H_{\beta - \epsilon}$ near an arbitrary fixed spectral edge ${\mathcal E}_j^\pm$. We establish necessary and quite close sufficient conditions which guarantee the finiteness of $\sigma_{\rm disc}(H_{\beta-\epsilon})$ in a neighbourhood of ${\mathcal E}_j^\pm$. In the case where the necessary conditions are violated, we obtain the main asymptotic term of the corresponding eigenvalue counting function. The effective Hamiltonian which governs the the asymptotics of $\sigma_{\rm disc}(H_{\beta-\epsilon})$ near ${\mathcal E}_j^\pm$ could be represented as a finite orthogonal sum of operators of the form $-\mu\frac{d^2}{dx^2} - \eta \epsilon$, self-adjoint in ${\rm L}^2({\mathbb R})$; here, $\mu > 0$ is a constant related to the so-called effective mass, while $\eta$ is $2\pi$-periodic function depending on $\beta$ and $\omega$.Comment: 29 pages, typos corrected, to appear in Journal of Spectral Theor

The Fate of the Landau Levels under Perturbations of Constant Sign

We show that the Landau levels cease to be eigenvalues if we perturb the 2D Schr\"odinger operator with constant magnetic field, by bounded electric potentials of fixed sign. We also show that, if the perturbation is not of fixed sign, then any Landau level may be an eigenvalue of arbitrary large, finite or infinite, multiplicity of the perturbed problem.Comment: This version corrects an error in the statement of Theorem 2. To appear in International Mathematics Research Notice

Discrete Spectrum of Quantum Hall Effect Hamiltonians I. Monotone Edge Potential

We consider the unperturbed operator $H_0 : = (-i \nabla - A)^2 + W$, self-adjoint in $L^2(\R^2)$. Here $A$ is a magnetic potential which generates a constant magnetic field $b>0$, and the edge potential $W$ is a non-decreasing non constant bounded function depending only on the first coordinate $x \in \R$ of $(x,y) \in \R^2$. Then the spectrum of $H_0$ has a band structure and is absolutely continuous; moreover, the assumption $\lim_{x \to \infty}(W(x) - W(-x)) < 2b$ implies the existence of infinitely many spectral gaps for $H_0$. We consider the perturbed operators $H_{\pm} = H_0 \pm V$ where the electric potential $V \in L^{\infty}(\R^2)$ is non-negative and decays at infinity. We investigate the asymptotic distribution of the discrete spectrum of $H_\pm$ in the spectral gaps of $H_0$. We introduce an effective Hamiltonian which governs the main asymptotic term; this Hamiltonian involves a pseudo-differential operator with generalized anti-Wick symbol equal to $V$. Further, we restrict our attention on perturbations $V$ of compact support and constant sign. We establish a geometric condition on the support of $V$ which guarantees the finiteness of the eigenvalues of $H_{\pm}$ in any spectral gap of $H_0$. In the case where this condition is violated, we show that, generically, the convergence of the infinite series of eigenvalues of $H_+$ (resp. $H_-$) to the left (resp. right) edge of a given spectral gap, is Gaussian.Comment: 32 page