dr. Erik G. C. P. van Loon  
evloon at itp dot unibremen dot de  
Room  NW1 O3040 
OttoHahnAllee 1  
University of Bremen  
28359, Bremen, Germany  
Telephone  +49 421 218 62041 
Since October 2018, I have been working in the group of prof. dr. Wehling at the University of Bremen. From December 2013 until September 2018, I was in the Theory of Condensed Matter group of prof. dr. Katsnelson, part of the Institute for Molecules and Materials of the Radboud University Nijmegen. On March 20 2018, I defended my PhD thesis Collective Phenomena in Strongly Correlated Systems. I finished my Master in the same group, having spent a year as an exchange student doing research in the group of prof. dr. Alexander Lichtenstein in Hamburg.
Research
I study the collective excitations in strongly correlated systems, especially in Hubbardlike models. Over the last two decades, Dynamical MeanField Theory (DMFT) has emerged as the best tool to describe the electronic structure of strongly correlated materials. It relies on mapping the full lattice problem with many degrees of freedom onto a selfconsisent impurity problem. This impurity problem is simpler and can be solved (numerically) exactly using CTQMC methods. It results in a local selfenergy, which is then used for the lattice electrons. This approach has been hugely succesful at describing the Hubbard model, in particular the oneparticle properties such as the density of states. DMFT can be used at arbitrary interaction strength, so the Mott transition from a weakly interacting metal to a strongly interacting insulator can be followed.
My work has focussed on the twoparticles properties (e.g. the susceptibility and dielectric function) of the Hubbard model. These are rather difficult to determine computationally. The DMFT impurity problem gives some information about their local components. However, often we are also interested in the full momentum dependence. For example, the momentum dependence is necessary for determining the dispersion of collective modes such as plasmons, magnons and zerosound modes. Many electrons at many different locations contribute to a collective mode, so the local description of DMFT is insufficient.
DMFT, with its good description of the local correlation, can be used as a starting point for a description of the momentum dependent susceptibility.
We are developing and implementing the dual boson approach, that incorporates nonlocal corrections to DMFT. An important aspect in these investigations is the issue of consistency: Approximate solutions can violate known exact properties of the system. For the case of the charge susceptibility, we have shown that the dual boson approach gives results satisfying the charge conservation law. This is in fact far from trivial and requires taking into account vertex corrections to all orders via the socalled ladder equation.
With this machinery, we have for the first time been able to study plasmons in the strongly correlated Hubbard model. In addition to this, we have used DMFTbased methods to investigate charge ordering in the extended Hubbard model, with applications to dipolar fermions in optical lattice, and the competition of electronelectron and electronphonon interaction in NbS2.
Publications

Dual Boson approach with instantaneous interaction

Bandwidth renormalization due to the intersite Coulomb interaction

Twoparticle Fermi liquid parameters at the Mott transition: Vertex divergences, Landau parameters, and incoherent response in dynamical meanfield theory

Fermionboson vertex within Dynamical MeanField Theory

Secondorder dual fermion approach to the Mott transition in the twodimensional Hubbard model

Confining graphene plasmons to the ultimate limit

Firstorder metalinsulator transitions in the extended Hubbard model due to selfconsistent screening of the effective interaction

Precursors of the insulating state in the squarelattice Hubbard model

The extended Hubbard model with attractive interactions

Competing Coulomb and electron–phonon interactions in NbS_{2}

Conservation in twoparticle selfconsistent extensions of dynamicalmeanfieldtheory

A comparison between methods of analytical continuation for bosonic functions

From local to nonlocal correlations: The Dual Boson perspective

Capturing nonlocal interaction effects in the Hubbard model: optimal mappings and limits of applicability

Interactiondriven Lifshitz transition with dipolar fermions in optical lattices

Double occupancy in dynamical meanfield theory and the Dual Boson approach

Selfconsistent Dual Boson approach to singleparticle and collective excitations in correlated systems

Ultralongrange order in the FermiHubbard model with longrange interactions

Thermodynamic consistency of the charge response in dynamical meanfield based approaches

Beyond extended dynamical meanfield theory: Dual boson approach to the twodimensional extended Hubbard model

Plasmons in Strongly Correlated Systems: Spectral Weight Transfer and Renormalized Dispersion

Collective charge excitations of strongly correlated electrons, vertex corrections, and gauge invariance
Popular Publications

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