Research Group:

Theory of Strongly Correlated Materials

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RESEARCH

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

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

A. Chikina, F. Lechermann, M.-A. Husanu, M. Caputo, C. Cancellieri, X. Wang, T. Schmitt,
M. Radovic, and V. N. Strocov: ACS Nano (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: arXiv:1805.01738 (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: arXiv:1801.08906 (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.

Electron dichotomy on the SrTiO3 defect surface augmented by many-body 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, many-body effects are essential for the latter. Materials like the familiar SrTiO3 compound that bridge or even abandon this characterization scheme are highly interesting. Here it is shown by means of combining density functional theory with dynamical-mean field theory that oxygen vacancies on the STO (001) surface give rise to a dichotomy of weakly-correlated t2g low-energy quasiparticles and localized 'in-gap' states of dominant eg 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)

Time-dependent spin phenomena in condensed matter are most often either described in the weakly correlated limit of metallic Stoner/Slater-like magnetism via band theory or in the strongly correlated limit of Heisenberg-like interacting spins in an insulator. However many experimental studies, e.g. of (de)magnetization processes, focus on itinerant local-moment 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 real-space implementation of the time-dependent rotational-invariant slave boson methodology allows to treat non-equilibrium spins numerically fast and efficiently beyond linear response as well as beyond the band-theoretical or Heisenberg limit.