Solid-state group seminars archive
11.10.2016 15h (cajna soba) Jacek Herbrych (University of Crete),
Dynamical structure factor in disordered model of interacting
fermions
I will present the behavior of the dynamical structure factor S(q,w) in
the whole range of wavevectors q within the prototype one-dimensional
model of many-body localization (MBL). Extracted effective dynamical
conductivities and current-relaxation rates confirm strong
dependence on disorder but modest variation with q. Furthermore, I
will present an analytical self-consistent approximation based on
the perturbation theory to qualitatively account for the nontrivial
features of dynamical quantities at all q: the emergence of the
maximum in dynamical conductivities, nonanalytical low-omega variation
in the ergodic phase, and the transition to the nonergodic (MBL)
phase. Finite-size scaling also reveals the possibility of the
subdiffusive behavior in the ergodic regime.
Full abstract (.pdf)
1.3.2016 15h (cajna soba) Robert Triebl (Graz),
Breakdown of the bulk boundary correspondence in a strongly
correlated model Hamiltonian
In this talk I will give a short introduction to topology in general
and to the influence of strong correlations on a topological model
Hamiltonian in particular. The
first part will be about the definition of topological invariants, how
to compute them and what changes in case of strong interactions. Then
I will turn to the
Kane-Mele-Hubbard model, in particular focusing on the competition
between topological and magnetic order. Both bulk and ribbon Green's
functions have been calculated
using the variational cluster approach, employing a two-site dynamical
impurity approximation (DIA). The resulting invariants are compared to
the existence of gapless
edge states of the ribbon, where we use a site-dependent
antiferromagnetic Weiss field on the ribbon. It turns out that
spontaneous symmetry breaking occurs locally at
the edges already at much smaller interactions than in the middle of
the ribbon, leading to a gap in the edge spectral function. As a
consequence, the topological
invariant defined in the bulk may not correspond to the existence of
gapless edge states since time reversal invariance is locally broken
only at the edges. Eventually,
I will compare the results to Mean-Field approximations using in-plane
and out-of-plane magnetic moments, showing that the direction has a
huge influence on the correct
topological description.
Full abstract (.pdf)
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