# Condensed Matter Seminar Series

## The Mesoscopic X-ray Edge Problem

### Dr. Martina Hentschel

*Dept.
of Physics, *Duke University

Dept. of Physics, University
of Regensburg (after April, 2004)

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##### Thursday February 26, 11:00 am, Room 234, Physics Building

* Host: Stephen Teitsworth *

Abstract: We study many-body
effects associated with a sudden perturbation in a mesoscopic system,
finding substantial differences from the bulk case. One example is the
sudden, localized perturbation caused by an x-ray exciting a core
electron into the conduction band. Here, Anderson orthogonality
catastrophe (AOC) competes with a many-body effect caused by the
interaction of the conduction electrons with the core hole. In the
bulk, this produces deviations from the naively expected
photoabsorption cross section in the form of a peaked or rounded
edge. For a coherent system with chaotic dynamics, such as a
nanoparticle or quantum dot, we use a random matrix model and find
substantial changes: (1) the finite number of particles leads to an
incomplete AOC, (2) the sample-to-sample fluctuations of the discrete
energy levels produce a distribution of AOC overlaps, and (3) most
importantly, the dipole matrix elements connecting the core and
conduction electrons are substantially modified. One of our key results
is that a photoabsorption cross section showing a rounded edge in the
bulk will change to a slightly peaked edge on average as the system
size is reduced to a mesoscopic (coherent) scale.

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