
We are generally aiming at reaching a deeper understanding of realistic condensed
matter systems from a microscopic point of view. In this regard, the main
interest is on the electronic structure. As the latter shows in the most
fascinating cases a complex manyparticle character, exploring novel
approaches through the combination of bandstructure methods with manybody techniques is a
major guideline of our research.
Some general topics/frameworks are:

Density Functional Theory (DFT) in various flavors

Dynamical MeanField Theory (DMFT) in various flavors

Basis Sets for the correlated electronic structure problem (Wannier functions, etc.)

SlaveBoson Theory: Equilibrium and NonEquilibrium

Quantum MonteCarlo Methods

ClusterExpansion Technique and ClusterVariation Method
... and the fruitful combination thereof !
Recent HighlightsRigorous multiorbital implementation of rotationalinvariant slaveboson theory
and its application to Hund's physics
C. Piefke and F. Lechermann:
arXiv:1708.03191 (2017)
The theoretical and numerical description of correlated electron systems on a lattice proves
notoriously complicated. Meanfield approaches
such as dynamicalmean field theory (DMFT) provide valuable insight when the selfenergy has a
dominant local structure. But especially for larger orbital manifolds and complicated local
Hamiltonians, also the DMFT performance has still its limitations. Furthermore, the generalized
manybody representation renders the extraction of efficient lowenergy theories often difficult.
The rotationalinvariant slave boson (RISB) approach in its meanfield formulation enables a
simplified alternative access to correlated lattice electrons.
We present a thorough symmetryadapted advancement of RISB theory, suited to deal with manifest
multiorbital challenges. Illustrative
examples in view of Hund's physics in 3 and 5orbital problems, including crystalfield terms
as well as spinorbit interaction, are enclosed.



Oxygenvacancy driven electron localization and itinerancy in rutilebased TiO_{2}
F. Lechermann, W. Heckel, O. Kristanovski and S. Müller:
Phys. Rev. B 95, 195159 (2017)



Oxygendeficient TiO_{2} in the rutile structure as well as the Ti_{3}O_{5}
Magnéli phase is investigated within the charge selfconsistent combination of
density functional theory with dynamical meanfield theory. An isolated
oxygen vacancy in titanium dioxide is not sufficient to metallize the system at low
temperatures. In a semiconducting phase, an ingap state is identified at
ε_{IG}∼0.75 eV in excellent agreement with experimental
data. Bandlike impurity levels, resulting from a threefold vacancyTi coordination as well as
entangled (t_{2g},e_{g}) states, become localized due to sitedependent
electronic correlations.
Charge localization and strong orbital polarization occur in the vacancynear Ti ions, which
details can be modified by a variation of the correlated subspace. At higher oxygen vacancy
concentration, a correlated metal is stabilized in the Magnéli phase. A defect rutile
structure of identical stoichiometry shows key differences in the orbitalresolved character
and the spectral properties.

QuantumManyBody Intermetallics: Phase Stability of Fe_{3}Al and
SmallGap Formation in Fe_{2}VAl
O. Kristanovski, R. Richter, I. Krivenko, A. I. Lichtenstein and F. Lechermann:
Phys. Rev. B 95, 045114 (2017)
Various intermetallic compounds harbor subtle electronic correlation effects. To elucidate
this fact for the FeAl system, we perform a realistic manybody investigation based on
the combination of density functional theory with dynamical meanfield theory in a charge
selfconsistent manner. A better characterization and understanding of the phase stability of
bccbased D0_{3}Fe_{3}Al through an improved description of the
correlated charge density and the magneticenergy is achevied.
Upon replacement of one Fe sublattice by V, the Heusler compound Fe_{2}VAl is realized,
known to display badmetal behavior and increased specific heat. We here document a
chargegap opening at low temperatures in line with previous experimental work. The gap
structure does not match conventional band theory and is reminiscent of (pseudo)gap
charateristics of correlated oxides.



Unconventional electron states in δdoped SmTiO_{3}
F. Lechermann:
Sci. Rep. 7, 1565 (2017)



The Mottinsulating distorted perovskite SmTiO_{3}, doped with a single SrO layer
in a quantumwell architecture is studied by the combination of density functional theory
with dynamical meanfield theory. A rich correlated electronic structure in line with
recent experimental investigations is revealed by the given realistic manybody approach
to a largeunitcell oxide heterostructure. Coexistence of conducting and Mottinsulating
TiO_{2} layers prone to magnetic order gives rise to multiorbital electronic transport
beyond standard Fermiliquid theory. Hints towards a pseudogap opening due to
electronelectron scattering within a background of antiferromagnetic fluctuations are detected.

Electron dichotomy on the SrTiO_{3} defect surface augmented
by manybody effects
F. Lechermann, H. O. Jeschke, A. J. Kim, S. Backes and R. Valenti:
Phys. Rev. B 93, 121103(R) (2016)
In a common paradigm, the electronic structure of condensed matter is divided into
weakly and strongly correlated compounds. While conventional band theory usually works
well for the former class, manybody effects are essential for the latter.
Materials like the familiar SrTiO_{3} compound that bridge or even abandon this
characterization scheme are highly interesting. Here it is shown by means of combining
density functional theory with dynamicalmean field theory that oxygen
vacancies on the STO (001) surface give rise to a dichotomy of weaklycorrelated t_{2g}
lowenergy quasiparticles and localized 'ingap' states of dominant e_{g} character
with subtle correlation signature. We furthermore touch base with recent experimental work and
study the surface instability towards magnetic order.



Versatile approach to spin dynamics in correlated electron systems
M. Behrmann, A. I. Lichtenstein, M. I. Katsnelson and F. Lechermann:
Phys. Rev. B 94, 165120 (2016)



Timedependent spin phenomena in condensed matter are most often either described in the
weakly correlated limit of metallic Stoner/Slaterlike magnetism via band theory or in the
strongly correlated limit of Heisenberglike interacting spins in an insulator. However many
experimental studies, e.g. of (de)magnetization processes, focus on itinerant localmoment
materials such as transition metals and various of their compounds. We here present a general
theoretical framework that is capable of addressing correlated spin dynamics, also in the
presence of a vanishing charge gap. A realspace implementation of the timedependent
rotationalinvariant slave boson methodology allows to treat nonequilibrium spins numerically
fast and efficiently beyond linear response as well as beyond the bandtheoretical or Heisenberg limit.

Thermopower enhancement from engineering the Na_{0.7}CoO_{2}
interacting fermiology
R. Richter, D. Shopova, W. Xie, A. Weidenkaff and F. Lechermann:
arXiv:1601.04427 (2016)
The sodium cobaltate system Na_{x}CoO_{2} is a prominent representant of strongly
correlated materials with promising thermoelectric response. In a combined theoretical and experimental
study we show that by doping the Co site of the compound at x=0.7 with iron, a further increase of the
Seebeck coefficient is achieved. The Fe defects give rise to effective hole doping in the highthermopower
region of larger sodium content x. Originally filled hole pockets in the angularresolved spectral
function of the Fefree material shift to low energy when introducing Fe, leading to a multisheet
interacting Fermi surface. Because of the higher sensitivity of correlated materials to doping,
introducing adequate substitutional defects is thus a promising route to manipulate their thermopower.



Interface exchange processes in LaAlO_{3}/SrTiO_{3}
induced by oxygen vacancies
M. Behrmann and F. Lechermann:
Phys. Rev. B 92, 125148 (2015)



An understanding of the role of defects in oxide heterostructures is essential for
future functionalization of these novel materials systems. We study the impact
of oxygen vacancies (OVs) in variable concentration on orbital and spin exchange
in the LaAlO_{3}/SrTiO_{3} interface by first principles manybody theory as
well as realspace modelHamiltonian techniques. The relevance of the Hund's coupling
J_{H} for OVinduced correlated states is demonstrated. Strong orbital polarization
towards an effective e_{g} state with predominant local antiferromagnetic alignment on Ti
sites nearby OVs is contrasted by polarized t_{2g}(xy) states with ferromagnetic
tendencies in the defectfree regions. Different magnetic phases may be identified,
giving rise to distinct netmoment behavior at low and high vacancy concentration, with an
antiferromagneticpair region inbetween.

LowEnergy Model and ElectronHole Doping Asymmetry of
SingleLayer RuddlesdenPopper Iridates
A. Hampel, C. Piefke and F. Lechermann
Phys. Rev. B 92, 085141 (2015)
Starting from the firstprinciples band structure, the interplay
between local Coulomb interactions and spinorbit coupling in structurallyundistorted
Ba_{2}IrO_{4} is investigated by means of
rotationalinvariant slaveboson meanfield theory. The evolution from a threeband description
towards an anisotropic oneband (J=1/2) picture is traced. Singlesite and cluster selfenergies
are used to shed light on competing Slater and Mottdominated correlation regimes. We reveal a
clear asymmetry between electron and hole doping, notably in the nodal/antinodal Fermisurface
dichotomy at strong coupling. Electrondoped iridates appear comparable to holedoped cuprates due
to the different sign of the nextnearestneighbor hopping t'.



Towards Mott design by δdoping of strongly correlated titanates
F. Lechermann and M. Obermeyer:
New J. Phys. 17, 043026 (2015)



Doping the distortedperovskite Mott insulators LaTiO_{3} and GdTiO_{3}
with a single SrO layer
along the [001] direction gives rise to a rich correlated electronic structure.
A realistic superlattice study by means of the charge selfconsistent combination of density
functional theory with dynamical meanfield theory reveals layer and temperaturedependent
multiorbital metalinsulator transitions. An orbitalselective metallic layer at the interface
dissolves via an orbitalpolarized dopedMott state into an orbitalordered insulating regime
beyond the two conducting TiO_{2} layers. We find large differences in the
scattering behavior within the latter.
Allowing for spin ordering results in a further enrichment of the already sophisticated electronic
structure. A ferrimagnetic ordering, again with itinerant and Mottinsulating regimes, settles in
deltadoped GdTiO_{3}. It results from the subtle exchangeinteraction variations due to the
differences in structural distortions, orbital occupations as well as degree of itinerancy.
The present work renders it obvious that the electronic structure characteristics
in oxide heterostructures are in principle likely to cover the full plethora of manybody
condensed matter physics within a single (designed) compound. From another perspective, this
generates vast room for engineering and creating novel specific states of matter.

From Hubbard bands to spinpolaron excitations in the doped Mott
material Na_{x}CoO_{2 }
A. Wilhelm, F. Lechermann, H. Hafermann, M. I. Katsnelson and A. I. Lichtenstein:
Phys. Rev. B 91, 155114 (2015)
We investigate the excitation spectrum of strongly correlated sodium cobaltate within a
realistic manybody description beyond dynamical meanfield theory (DMFT). At lower doping
around x=0.3, rather close to Mottcritical halffilling, the singleparticle spectral
function of Na_{x}CoO_{2} displays an upper Hubbard band which is captured within
DMFT.
Momentumdependent selfenergy effects beyond DMFT become dominant at higher doping. Around
a doping level of x~0.67, the incoherent excitations give way to finiteenergy
spinpolaron excitations in close agreement with optics experiments. These excitations are a
direct consequence of the formation of bound states between quasiparticles and paramagnons
in the proximity to inplane ferromagnetic ordering.



Largeamplitude spin oscillations triggered by nonequilibrium strongly correlated
t_{2g} electrons
M. Behrmann and F. Lechermann:
Phys. Rev. B 91, 075110 (2015)
Whether femtosecond laser pulses change the spin orientation faster than the
timescale for spin procession (on the order hundreds of picoseconds), is a key question in
ultrafast (de)magnetization dynamics. Recent experiments indeed reveal intrinsic connections
between electronelectron interactions in strongly correlated materials and a femtosecond
spinorientation change. Therefore we here investigate (de)magnetization processes in
the multiorbital Hubbard model on the localcorrelation timescale (femtoseconds) by
focussing on t_{2g} electrons in a wider doping range. We reveal fillingdependant stable and
transient spinoscillations via interaction quenches from the antiferromagnetic or
paramagnetic ground state. Dynamic ultrafast spinorientation effects in prominent correlated
antiferromagnetic transitionmetal oxides are therefrom predicted.




