Research Group:

Theory of Strongly Correlated Materials







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 many-particle character, exploring novel approaches through the combination of bandstructure methods with many-body techniques is a major guideline of our research.

Some general topics/frameworks are:
  • Density Functional Theory (DFT) in various flavors
  • Dynamical Mean-Field Theory (DMFT) in various flavors
  • Basis Sets for the correlated electronic structure problem (Wannier functions, etc.)
  • Slave-Boson Theory: Equilibrium and Non-Equilibrium
  • Quantum Monte-Carlo Methods
  • Cluster-Expansion Technique and Cluster-Variation Method
... and the fruitful combination thereof !

Recent Highlights

Interplay of charge-transfer and Mott-Hubbard physics approached by an
efficient combination of self-interaction correction and dynamical mean-field theory

F. Lechermann, W. Körner, D. F. Urban and C. Elsässer: arXiv:1902.07000 (2019)

Late transition-metal oxides with small charge-transfer energy raise issues for state-of-the-art correlated electronic structure schemes such as the combination of density functional theory (DFT) with dynamical mean-field theory (DMFT). The accentuated role of the oxygen valence orbitals in these compounds asks for an enhanced description of ligand-based correlations. Utilizing the rocksalt-like NiO as an example, we present an advancement of charge self-consistent DFT+DMFT by including self-interaction correction (SIC) applied to oxygen. This introduces explicit onsite O correlations as well as an improved treatment of intersite p-d correlations. Due to the efficient SIC incorporation in a pseudopotential form, the DFT+sicDMFT framework is an advanced but still versatile method to address the interplay of charge-transfer and Mott-Hubbard physics. We revisit the spectral features of stoichiometric NiO and reveal the qualitative sufficiency of local DMFT self-energies in describing spectral peak structures usually associated with explicit nonlocal processes. The semiconducting nature of LixNi1-xO, along with prominent in-gap states, is verified by the present theoretical study.

Doping a Bad Metal:
Origin of Suppression of Metal-Insulator Transition in Non-Stoichiometric VO2

P. Ganesh, F. Lechermann, I. Kylanpaa, J. Krogel, P. R. C. Kent and O. Heinonen: arXiv:1811.01145 (2018)

Rutile (R) phase VO2 is a quintessential example of a strongly correlated bad-metal, which undergoes a metal-insulator transition (MIT) concomitant with a structural transition to a V-V 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 electronic-structure of the metallic R phase, explicitly treating strong electron-electron correlations using dynamical mean-field theory as well as diffusion Monte Carlo flavor of quantum Monte Carlo techniques. We show that vacancies tend to change the V-3d filling away from its nominal half-filled value, with the egπ orbitals competing with the otherwise dominant a1g orbital. Loss of this near orbital polarization of the a1g orbital is associated with a weakening of electron correlations, especially along the V-V dimerization direction. This removes a charge-density wave instability along this direction above a critical doping concentration, which further suppresses the metal-insulator transition.

Orbital Ordering of the Mobile and Localized Electrons at Oxygen-Deficient LaAlO3/SrTiO3

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 transition-metal 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 SrTiO3-based interfaces hosting a mobile two-dimensional electron system (2DES) are strongly influenced by oxygen vacancies, which form an electronic dichotomy, where strongly correlated localized electrons in the in-gap states (IGSs) coexist with noncorrelated delocalized 2DES. Here, we use resonant soft-X-ray photoelectron spectroscopy to prove the eg character of the IGSs, as opposed to the t2g character of the 2DES in the paradigmatic LaAlO3/SrTiO3 interface. Supported by a self-consistent combination of density functional theory and dynamical mean field theory calculations, this experiment identifies local orbital reconstruction that goes beyond the conventional eg-vs-t2g band ordering. A hallmark of oxygen-deficient LaAlO3/SrTiO3 is a significant hybridization of the eg and t2g orbitals. Our findings provide routes for tuning the electronic and magnetic properties of oxide interfaces through "defect engineering" with oxygen vacancies.

Multi-orbital nature of the spin fluctuations in Sr2RuO4

L. Boehnke, P. Werner and F. Lechermann: EPL 122, 57001 (2018)

The spin susceptibility of strongly correlated Sr2RuO4 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 spin-density-wave fluctuations foster unconventional superconductivity at low temperature or compete therewith. By means of density functional theory combined with dynamical mean-field theory, we reveal the realistic multi-orbital signature of the (dynamic) spin susceptibility beyond existing weak-coupling 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)-t2g 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 PdCrO2

F. Lechermann: Phys. Rev. Materials 2, 085004 (2018)

There has been a long-standing debate on the coexistence between itinerant electrons and localized spins in the PdCrO2 delafossite. By means of the charge self-consistent combination of density functional theory and dynamical mean-field theory, it is corroborated that despite overall remarkable metallic response, the CrO2 layers are indeed Mott insulating. The resulting k-resolved spectral function in the paramagnetic phase is in excellent agreement with available photoemission data. Subtle coupling between the itinerant and Mott-localized 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 impurity-selective Mott transition in paramagnetic V2O3

F. Lechermann, N. Bernstein, I. I. Mazin and R. Valenti: Phys. Rev. Lett. 121, 106401 (2018)

While the phase diagrams of the one- and multi-orbital 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 V2O3, chemical pressure was initially believed to explain why the paramagnetic-metal to antiferromagnetic-insulator transition temperature is lowered by Ti doping while Cr doping strengthens correlations, eventually rendering the high-temperature 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 self-consistent combination of density functional theory and dynamical mean-field theory that changes in the V2O3 phase diagram are driven by defect-induced local symmetry breakings resulting from atomic-size and electron(hole) aspects of Cr(Ti) doping. This finding emphasizes the high sensitivity of the Mott metal-insulator transition to the local environment and the importance of accurately accounting for the one-electron starting Hamiltonian, since correlations crucially respond to it.

Rigorous symmetry adaptation of multiorbital rotationally invariant slave-boson 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. Mean-field approaches such as dynamical-mean field theory (DMFT) provide valuable insight when the self-energy 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 many-body representation renders the extraction of efficient low-energy theories often difficult. The rotational-invariant slave boson (RISB) approach in its mean-field formulation enables a simplified alternative access to correlated lattice electrons. We present a thorough symmetry-adapted advancement of RISB theory, suited to deal with manifest multi-orbital challenges. Illustrative examples in view of Hund's physics in 3- and 5-orbital problems, including crystal-field terms as well as spin-orbit interaction, are enclosed.

Oxygen-vacancy driven electron localization and itinerancy in rutile-based TiO2

F. Lechermann, W. Heckel, O. Kristanovski and S. Müller: Phys. Rev. B 95, 195159 (2017)

Oxygen-deficient TiO2 in the rutile structure as well as the Ti3O5 Magnéli phase is investigated within the charge self-consistent combination of density functional theory with dynamical mean-field theory. An isolated oxygen vacancy in titanium dioxide is not sufficient to metallize the system at low temperatures. In a semiconducting phase, an in-gap state is identified at εIG∼-0.75 eV in excellent agreement with experimental data. Band-like impurity levels, resulting from a threefold vacancy-Ti coordination as well as entangled (t2g,eg) states, become localized due to site-dependent electronic correlations. Charge localization and strong orbital polarization occur in the vacancy-near 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 orbital-resolved character and the spectral properties.

Quantum-Many-Body Intermetallics: Phase Stability of Fe3Al and Small-Gap Formation in Fe2VAl

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 Fe-Al system, we perform a realistic many-body investigation based on the combination of density functional theory with dynamical mean-field theory in a charge self-consistent manner. A better characterization and understanding of the phase stability of bcc-based D03-Fe3Al through an improved description of the correlated charge density and the magnetic-energy is achevied. Upon replacement of one Fe sublattice by V, the Heusler compound Fe2VAl is realized, known to display bad-metal behavior and increased specific heat. We here document a charge-gap 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 δ-doped SmTiO3

F. Lechermann: Sci. Rep. 7, 1565 (2017)

The Mott-insulating distorted perovskite SmTiO3, doped with a single SrO layer in a quantum-well architecture is studied by the combination of density functional theory with dynamical mean-field theory. A rich correlated electronic structure in line with recent experimental investigations is revealed by the given realistic many-body approach to a large-unit-cell oxide heterostructure. Coexistence of conducting and Mott-insulating TiO2 layers prone to magnetic order gives rise to multi-orbital electronic transport beyond standard Fermi-liquid theory. Hints towards a pseudogap opening due to electron-electron scattering within a background of antiferromagnetic fluctuations are detected.