Eigensystem multiscale analysis for the Anderson model via the Wegner estimate

Speaker: 

Abel Klein

Institution: 

UCI

Time: 

Thursday, October 10, 2019 - 2:00pm

Host: 

Location: 

RH 340P

We present a new approach to the  eigensystem multiscale analysis (EMSA) for the Anderson model that relies on the Wegner estimate.  The EMSA  treats all energies  of the finite volume operator in an energy interval at the same time,  simultaneously establishing localization of  all eigenfunctions with eigenvalues in the energy interval  with high probability.  It implies all the usual manifestations of localization (pure point spectrum with exponentially decaying eigenfunctions, dynamical localization)   for the Anderson model. The new method removes the restrictive level spacing hypothesis used in the previous versions of the EMSA, allowing  for single site probability distributions that are  H\"older continuous of order  $\alpha \in (0,1]$. (Joint work with Alex Elgart.)

Continuity of spectra and  spectral measure of quasi-periodic Schr\"odinger operators 

Speaker: 

Xin Zhao

Institution: 

Nanjing Universuty

Time: 

Thursday, October 3, 2019 - 2:00pm to 3:00pm

Location: 

rh 340p

In this talk, we first consider quasi-periodic Schr\"odinger operators with finitely differentiable potentials. If the potential is analytic, there are numerous results. But not every result holds if one replaces the analyticity with a smoothness condition. We will give some positive results in this aspect,  generalizing some interesting results in the analytic case to the finitely smooth case. This includes the global reducibility results, generalized Chamber's formula and their applications to the study of continuity of the spectra. Finally we will give a recent result on the continuity of spectral measure of multi frequency quasi-periodic Schr\"odinger operators with small analytic quasi-periodic potentials.

Ergodic Schrodinger operators in the infinite-measure setting

Speaker: 

Jake Fillman

Institution: 

Texas State University

Time: 

Thursday, October 24, 2019 - 2:00pm to 3:00pm

Location: 

RH 340P

We develop the basic spectral theory of ergodic Schrodinger operators when the underlying dynamics are given by a conservative
ergodic transformation of a \sigma-finite measure space. Some fundamental results, such as the Ishii--Pastur theorem carry over to the
infinite-measure setting. We also discuss some examples in which straightforward analogs of results from the probability-measure case do not hold. We will discuss some examples and some interesting open problems.

The talk is based on a joint work with M. Boshernitzan, D. Damanik, and M. Lukic.

On a universal limit conjecture for the nodal count statistics of quantum graphs

Speaker: 

Lior Alon

Institution: 

Technion

Time: 

Thursday, October 17, 2019 - 2:00pm to 3:00pm

Location: 

RH 340 P

 

 
 Understanding the statistical properties of Laplacian eigenfunctions in general and their nodal sets in particular, have an important role in the field of spectral geometry, and interest both mathematicians and physicists. A quantum graph is a system of a metric graph with a self-adjoint Schrodinger operator. It was proven for quantum graphs that the number of points on
 which each eigenfunction vanish (also known as the nodal count) is
 bounded away from the spectral position of the eigenvalue by the first Betti number of the graph. A remarkable result by Berkolaiko and Weyand showed that the nodal surplus is equal to a magnetic stability index of the corresponding eigenvalue. A similar result for discrete graphs holds as well proved first by Berkoliako and later by Colin deVerdiere.
 
 Both from the nodal count point of view and the magnetic point of view, it is interesting to consider the distribution of these indices over the spectrum. In our work, we show that such a density exists and defines a nodal count distribution. Moreover, this distribution is symmetric, which allows deducing the topology of a graph from its nodal count. Although for general graphs we can not a priori calculate the nodal count distribution, we proved that a certain family of graphs will have a binomial distribution. As a corollary, given any sequence of graphs from that family with an increasing number of cycles, the sequence of nodal count distributions, properly normalized, will converge to a normal distribution. 
A numerical study indicates that this property might be universal and led us to state the following conjecture. For every sequence of graphs with an increasing number of cycles, the corresponding sequence of properly normalized nodal count distributions will converge to a normal distribution. 
 In my talk, I will present our latest results extending the number
 of families of graphs for which we can prove the conjecture.
 
This talk is based on joint works with Ram Band (Technion) and Gregory Berkolaiko (Texas A&M)

Embedded Eigenvalues for multilayer quantum Graph Graphene

Speaker: 

Lee Fisher

Institution: 

LSU

Time: 

Friday, September 13, 2019 - 2:00pm to 3:00pm

Location: 

RH 306

 

 By judiciously constructing local defects in graph models of multi-layer graphene, bound states can be constructed at energies that lie within the continuous spectrum of the associated Schrödinger operator. The layers may be stacked in AA or AB fashion. A necessary condition for this construction is the reducibility of the Fermi surface for the multi-layer structure. This is achieved due to a special reduction of the complex dispersion relation to a function of a single polynomial "composite" function of the quasimomenta.
This is joint work with Wei Li and Stephen Shipman at LSU.

Nonstationary low-dimensional dynamics

Speaker: 

Victor Kleptsyn

Institution: 

IRM de Rennes

Time: 

Thursday, September 26, 2019 - 2:00pm to 3:00pm

Location: 

RH 340 P

We consider discrete Schr\"odinger operators on $\ell^2(\mathbb{Z})$ with bounded random but not necessarily identically distributed values of the potential. The distribution at a given site is not assumed to be absolutely continuous (or to contain an absolutely continuous component). We prove spectral localization (with exponentially decaying eigenfunctions) as well as dynamical localization for this model.

An important ingredient of the proof is a non-stationary analog of the Furstenberg Theorem on random matrix products,
which is also of independent interest. 

This is a joint project with A.Gorodetski.

Quantitative almost reducibility of quasiperiodic cocycles

Speaker: 

Lingrui Ge

Institution: 

Nanjing University

Time: 

Thursday, August 29, 2019 - 2:00pm to 3:00pm

Location: 

RH 306

 

Abstract: In this talk, I will give a brief introduction to several popular topics in the spectral theory of quasi-periodic Schrodinger operators. I will then talk about several sharp results we get recently on these topics (especially for almost Mathieu operator). Our results are based on quantitative almost reducibility, a method originally proposed by Dinaburg and Sinai. Finally I will explain the key points in developing and refining this method to get optimal results.

 

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