# Seminars (PEPW04)

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Event When Speaker Title Presentation Material
PEPW04 22nd June 2015
10:00 to 11:00
On non-smooth functions of Wiener-Hopf operators
We discuss trace formulae for the operator \begin{equation*} f(PAP) - Pf(A)P, \end{equation*} where $A$ is a pseudo-differential operator on $L^2(\mathbb R^d)$ with a smooth or discontinuous symbol, and $P$ is a multiplication by the indicator of a piece-wise smooth domain in $\mathbb R^d$. The function $f$ is not supposed to be smooth. The obtained formulae generalise results obtained by H. Widom in the 80's. These results are used to study the entanglement entropy of free fermions at positive temperature both in the low and high temperature limits.
PEPW04 22nd June 2015
11:30 to 12:30
On the two-dimensional random walk in an isotropic random environment
This is joint work with Erich Baur (Lyon) and Ofer Zeitouni (Weizmann Institute).

We report on work in progress on the standard model of a random walk in random environment in the critical dimension 2. We investigate exit distributions from large sets which are supposed to be essentially the same as those for ordinary random walks. Random walks in random environment are well understood in dimension one, and for small disorder in dimensions above 2, but the two-dimensional case is largely open.

PEPW04 22nd June 2015
13:30 to 14:30
From the mesoscopic to microscopic scale in random matrix theory
Co-authors: Laszlo Erdos (IST), Horng Tzer Yau (Harvard), Jun Yin (Wisconsin Madison)

Eugene Wigner has envisioned that the distributions of the eigenvalues of large Gaussian random matrices are new paradigms for universal statistics of large correlated quantum systems. These random matrix eigenvalues statistics supposedly occur together with delocalized eigenstates. I will explain recent developments proving this paradigm for eigenvalues and eigenvectors of random matrices. This is achieved by bootstrap on scales, from mesoscopic to microscopic. Random walks in random environments, homogenization and the coupling method play a key role.

PEPW04 22nd June 2015
15:00 to 16:00
On fluctuations of eigenvalues of random band matrices
We consider the fluctuation of linear eigenvalue statistics of random band $n$ dimensional matrices whose bandwidth $b$ is assumed to grow with n in such a way that $b/n$ tends to zero. Without any additional assumptions on the growth of b we prove CLT for linear eigenvalue statistics for a rather wide class of test functions. Thus we remove the main technical restriction $n>>b>>n^{1/2}$ of all the papers, in which band matrices were studied before. Moreover, the developed method allows to prove automatically the CLT for linear eigenvalue statistics of the smooth test functions for almost all classical models of random matrix theory: deformed Wigner and sample covariance matrices, sparse matrices, diluted random matrices, matrices with heavy tales etc.
PEPW04 22nd June 2015
16:00 to 17:00
Primitive Pythagorean triples and "near" quasicrystals
Counting things is a great favorite of children, and mathematicians as well, whatever the things are. In this talk, I discuss an old counting problem on primitive Pythagorean triples in view of modern theory of quasicrystals.
PEPW04 23rd June 2015
10:00 to 11:00
An eigensystem approach to Anderson localization
Co-author: Alexander Elgart (Virginia Tech)

We introduce a new approach for proving localization (pure point spectrum with exponentially decaying eigenfunctions, dynamical localization) for the Anderson model at high disorder. In contrast to the usual strategy, we do not study finite volume Green's functions. Instead, we perform a multiscale analysis based on finite volume eigensystems, establishing localization of finite volume eigenfunctions with high probability. (Joint work with A. Elgart.)

PEPW04 23rd June 2015
11:30 to 12:30
An eigensystem approach to Anderson localization, part II
Co-author: Abel Klein (UC Irvine)

We introduce a new approach for proving localization (pure point spectrum with exponentially decaying eigenfunctions, dynamical localization) for the Anderson model at high disorder. In contrast to the usual strategy, we do not study finite volume Green's functions. Instead, we perform a multiscale analysis based on finite volume eigensystems, establishing localization of finite volume eigenfunctions with high probability. (Joint work with A. Klein.)

PEPW04 23rd June 2015
13:30 to 14:30
J Schenker Dissipative transport in the localized regime
Co-author: Jürg Fröhlich (ETH)

A quantum particle moving in a strongly disordered random environment is known to be subject to Anderson localization, which results in the complete suppression of transport. However, localization can be broken by a small perturbation, such as thermal noise from the environment, resulting in diffusive motion for the particle. I will discuss this phenomenon in two models in which the Schroedinger equation for a particle in the strongly localized regime is perturbed by (1) a time dependent fluctuating random potential and (2) a Lindblad operator incorporating the interaction with a heat bath in the Markov approximation. In each case, it can be proved that diffusive motion results with a strictly positive and finite diffusion constant. Furthermore, the diffusion constant tends continuously to zero at a calculable rate, as the strength of the perturbation is taken to zero. (Partially based on joint work with J. Fröhlich.)

PEPW04 23rd June 2015
15:00 to 16:00
Y Suhov New properties of entropy and their consequences
Co-authors: Salimeh Yasaei Sekeh (University of Sao Paulo at Sao Carlos, Brazil), Srefan Zohren (University of Oxford, UK)

Based on the concept of the weighted entropy (both classical and quantum), I will report a number of new results involving inequalities and convergence in a variety of context.

PEPW04 23rd June 2015
16:00 to 17:00
J Imbrie Level Spacing for Non-Monotone Anderson Models
PEPW04 24th June 2015
10:00 to 11:00
Pure point spectrum in the regime of zero Lyapunov exponents
Co-author: Anton Gorodetski (UC Irvine)

In the study of ergodic Schrodinger operators, a central role is played by the Lyapunov exponent of the associated Schrodinger cocycle. We discuss a construction showing that the regime of zero Lyapunov exponents can contain pure point spectrum.

PEPW04 24th June 2015
11:30 to 12:30
Invariance of IDS under Darboux transformation and its application
The integrated density of states (IDS) is a crucial quantity for studying spectral properties of ergodic Schroedinger opearators. Especially in one dimension it determines most of the spectral properties. On the other hand, in relation to completely integrable systems, Darboux transformation has been investigated from various points of views. In this talk the invariance of IDS under Darboux transformation will be shown, and as a byproduct the invariance of IDS under KdV flow will be remarked.
PEPW04 24th June 2015
13:30 to 14:30
Behavior of the spectrum of the periodic Schrodinger operators near the edges of the gaps
Co-author: Leonid Parnovski (UCL)

It is a common belief that generically all edges of the spectrum of periodic Schrodinger operators are non-degenerate, i.e. are attained by a single band function at finitely many points of quasi-momentum and represent a non-degenerate quadratic minimum or maximum. We present the construction which shows that all degenerate edges of the spectrum can be made non-degenerate under arbitrary small perturbation. The corresponding perturbation is found in the class of potentials with larger (but proportional) periods; thus the final operator is still periodic but the lattice of periods changes.

PEPW04 24th June 2015
15:00 to 16:00
$L_1$-Estimates for Eigenfunctions of the Dirichlet Laplacian
Co-authors: Michiel van den Berg (U Bristol), J\"urgen Voigt (TU Dresden)

For $d \in {\bf N}$ and $\Omega \ne \emptyset$ an open set in ${\bf R}^d$, we consider the eigenfunctions $\Phi$ of the Dirichlet Laplacian $-\Delta_\Omega$ of $\Omega$. We do {\it not} require $\Omega$ to be of finite volume. % If $\Phi$ is associated with an eigenvalue below the essential spectrum of $-\Delta_\Omega$, we provide estimates for the $L_1$-norm of $\Phi$ in terms of the $L_2$-norm of $\Phi$ and suitable spectral data of $-\Delta_\Omega$. The main idea in obtaining such estimates consists in finding a---sufficiently small---subset $\Omega' \subset \Omega$ where $\Phi$ is localized in the sense that $\Phi$ decays exponentially as one moves away from $\Omega'$. These $L_1$-estimates are then used in the comparison of the heat content of $\Omega$ at time $t>0$ and the heat trace at times $t' > 0$, where a two-sided estimate is established. \vskip.5em This is joint work with Michiel van den Berg (Bristol) and J\"urgen Voigt (Dresden), with improvements by Hendrik Vogt (Dresden).

PEPW04 25th June 2015
10:00 to 11:00
Spectral theory of the Schr?dinger operators on fractals

Spectral theory of the Schrodinger operators on fractals (Stanislav Molchanov UNC Charlotte)

Spectral properties of the Laplacian on the fractals as well as related topics (random walks on the fractal lattices, Brownian motion on the Sierpinski gasket etc.) are well understood. The next natural step is the analysis of the corresponding Schrodinger operators and not only with random ”ergodic” potentials (Anderson type Hamiltonians) but also with the classical potentials: fast decreasing, increasing or ”periodic” (in an appropriate sense) ones. The talk will present several results in this direction. They include a) Simon – Spencer type theorem (on the absence of a.c. spectrum) and localization theorem for the fractal nested lattices (Sierpinski lattice) b) Homogenization theorem for the random walks with the periodic intensities of the jumps c) Quasi-classical asymptotics and Bargman type estimates for the Schr?dinger operator with the decreasing gasket d) Bohr asymptotic formula in the case of the increasing to infinity potentials e) Random hierarchical operators, density of states and the non-Poissonian spectral statistics Some parts of the talk are based on joint research with my collaborators (Yu. Godin, A. Gordon, E. Ray, L. Zheng).

PEPW04 25th June 2015
11:30 to 12:30
P Hislop Eigenvalue statistics for random Schrodinger operators
Certain natural random variables associated with the local eigenvalue statistics for generalized Anderson-type random Schrodinger operators on the lattice and the continuum in the localization region are distributed according to compound Poisson distributions. In the lattice case the Levy measure of the associated distribution has finite support. Other properties of these random variables will be presented.
PEPW04 25th June 2015
13:30 to 14:30
Localisation and ageing in the parabolic Anderson model
The parabolic Anderson problem is the Cauchy problem for the heat equation on the d-dimensional integer lattice with random potential. It describes the behaviour of branching random walks in a random environment (represented by the potential) and is being actively studied by mathematical physicists. One of the most important situations is when the potential is time-independent and is a collection of independent identically distributed random variables. We discuss the intermittency effect occurring for such potentials and consisting in increasing localisation and randomisation of the solution. We also discuss the ageing behaviour of the model showing that the periods, in which the profile of the solutions remains nearly constant, are increasing linearly over time.
PEPW04 25th June 2015
15:00 to 16:00
The invariant measure for random walks on a strip
We explain the necessary and sufficient condition for existence of the invariant measure for the Markov chain on the space of ergodic environments in all regimes.
PEPW04 25th June 2015
16:00 to 17:00
Green's function asymptotic behavior near a non-degenerate spectral edge of a periodic operator
Co-authors: Minh Kha (Texas A&M University), Andrew Raich (University of Arkansas)

Green's function behavior near and at a spectral edge of a periodic operator is one of what was called by M. Birman and T. Suslina "threshold properties." I.e., it depends on the local behavior of the dispersion relation near the edge. The recent results are presented for the case of a non-degenerate spectral edge (which is conjectured to be the generic situation).

This is a joint work with Minh Kha (Texas A&M) and Andrew Raich (Univ. of Arkansas)

PEPW04 26th June 2015
10:00 to 11:00
Recent results and open problems on many-body localization
In the most recent decade the topic of many-body localization, understood as the absence of thermalization in interacting quantum many-body systems, has seen strong attention and rapid development in the physics literature. We will survey the relatively small number of mathematically rigorous results on MBL which have been obtained, in particular for disordered oscillator systems and some models of quantum spin chains. Specifically, we will consider manifestations of MBL such as absence of many-body transport, exponential decay of ground and thermal state correlations, as well as area laws for the entanglement of states. We will also mention some of the many open problems in this field.
PEPW04 26th June 2015
11:30 to 12:30
F Klopp Interacting quantum particles in a 1 D random background