
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 Highlights Doping a Bad Metal:
Origin of Suppression of MetalInsulator Transition in
NonStoichiometric VO_{2}
P. Ganesh, Frank Lechermann, Ilkka Kylanpaa, Jaron Krogel, Paul R. C. Kent
and Olle Heinonen:
arXiv:1811.01145 (2018)



Rutile (R) phase VO_{2} is a quintessential example of a strongly correlated
badmetal, which undergoes a metalinsulator transition (MIT) concomitant with a
structural transition to a VV dimerized monoclinic phase below 340K. It has been
experimentally shown that one can control this transition by doping. In particular,
doping with oxygen vacancies has been shown to completely suppress this MIT
without any structural transition. We explain this suppression by elucidating the
influence of oxygen vacancies on the electronicstructure of the metallic R phase,
explicitly treating strong electronelectron correlations using dynamical meanfield theory
as well as diffusion Monte Carlo flavor of quantum Monte Carlo
techniques. We show that vacancies tend to change the V3d filling away from its nominal
halffilled value, with the e_{g}^{π} orbitals competing with the
otherwise dominant a_{1g} orbital. Loss of this near orbital polarization of the
a_{1g} orbital is associated with a weakening of electron correlations, especially
along the VV dimerization direction. This removes a chargedensity wave instability
along this direction above a critical doping concentration, which further suppresses the
metalinsulator transition.

Orbital Ordering of the Mobile and Localized Electrons at OxygenDeficient
LaAlO_{3}/SrTiO_{3} Interfaces
A. Chikina, F. Lechermann, M.A. Husanu, M. Caputo, C. Cancellieri, X. Wang, T. Schmitt,
M. Radovic and V. N. Strocov:
ACS Nano 12, 7927 (2018)
Interfacing different transitionmetal oxides opens a route to functionalizing their
rich interplay of electron, spin, orbital, and lattice degrees of freedom for electronic
and spintronic devices. Electronic and magnetic properties of SrTiO_{3}based
interfaces hosting a mobile twodimensional electron system (2DES) are strongly influenced
by oxygen vacancies, which form an electronic dichotomy, where strongly correlated localized
electrons in the ingap states (IGSs) coexist with noncorrelated delocalized 2DES. Here,
we use resonant softXray photoelectron spectroscopy to prove the eg character of the
IGSs, as opposed to the t_{2g} character of the 2DES in the paradigmatic
LaAlO_{3}/SrTiO_{3}
interface. Supported by a selfconsistent combination of density functional
theory and dynamical mean field theory calculations, this experiment identifies local
orbital reconstruction that goes beyond the conventional e_{g}vst_{2g}
band ordering. A hallmark of oxygendeficient LaAlO_{3}/SrTiO_{3} is a
significant hybridization of the e_{g} and
t_{2g} orbitals. Our findings provide routes for tuning the electronic
and magnetic properties of oxide interfaces through "defect engineering" with
oxygen vacancies.



Multiorbital nature of the spin fluctuations in Sr_{2}RuO_{4}
L. Boehnke, P. Werner and F. Lechermann:
EPL 122, 57001 (2018)



The spin susceptibility of strongly correlated Sr_{2}RuO_{4} is
known to display a rich structure in reciprocal space, with a prominent peak at
Q=(0.3,0.3,0). It is heavily debated, if the resulting
incommensurate spindensitywave fluctuations foster unconventional superconductivity
at low temperature or compete therewith. By means of density functional theory combined
with dynamical meanfield theory, we reveal the realistic multiorbital signature of the
(dynamic) spin susceptibility beyond existing weakcoupling approaches. The experimental
fluctuation spectrum up to 80 meV is confirmed by theory. Furthermore the peak at
Q is shown to carry nearly equal contributions from each of the
Ru(4d)t_{2g} orbitals, pointing to a concerted contribution of all relevant
correlated orbitals to the key feature of the resulting spin fluctuations.

Hidden Mott insulator in metallic PdCrO_{2}
F. Lechermann:
Phys. Rev. Materials 2, 085004 (2018)
There has been a longstanding debate on the coexistence between itinerant electrons
and localized spins in the PdCrO_{2} delafossite. By means of the charge
selfconsistent combination of density functional theory and dynamical meanfield
theory, it is corroborated that despite overall remarkable metallic response, the
CrO_{2} layers are indeed Mott insulating. The resulting kresolved spectral
function in the paramagnetic phase is in excellent agreement with available photoemission
data. Subtle coupling between the itinerant and Mottlocalized degrees of freedom is
revealed. Different doping scenarios are simulated in order to manipulate the electronic
states within the inert layers. In particular, oxygen vacancies prove effective in turning
the hidden Mott insulator into a strongly correlated itinerant subsystem. The present results
may open a new venue in research on quantum materials beyond canonical classification schemes.



Uncovering the mechanism of the impurityselective Mott transition in
paramagnetic V_{2}O_{3}
F. Lechermann, N. Bernstein, I. I. Mazin and R. Valenti:
Phys. Rev. Lett. 121, 106401 (2018)



While the phase diagrams of the one and multiorbital Hubbard model have been well studied,
the physics of real Mott insulators is often much richer, material dependent, and poorly
understood. In the prototype Mott insulator V_{2}O_{3},
chemical pressure was initially believed
to explain why the paramagneticmetal to antiferromagneticinsulator transition temperature
is lowered by Ti doping while Cr doping strengthens correlations, eventually rendering the
hightemperature phase paramagnetic insulating. However, this scenario has been recently shown
both experimentally and theoretically to be untenable. Based on full structural optimization,
we demonstrate via the charge selfconsistent combination of density functional theory and
dynamical meanfield theory that changes in the V_{2}O_{3} phase diagram are
driven by defectinduced
local symmetry breakings resulting from atomicsize and electron(hole) aspects of Cr(Ti)
doping. This finding emphasizes the high sensitivity of the Mott metalinsulator transition
to the local environment and the importance of accurately accounting for the oneelectron
starting Hamiltonian, since correlations crucially respond to it.

Rigorous symmetry adaptation of multiorbital rotationally
invariant slaveboson theory with application to Hund's rules physics
C. Piefke and F. Lechermann:
Phys. Rev. B 97, 125154 (2018)
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.


