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

Rigorous multi-orbital implementation of rotational-invariant slave-boson 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. 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.

Thermopower enhancement from engineering the Na0.7CoO2 interacting fermiology

R. Richter, D. Shopova, W. Xie, A. Weidenkaff and F. Lechermann: arXiv:1601.04427 (2016)

The sodium cobaltate system NaxCoO2 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 high-thermopower region of larger sodium content x. Originally filled hole pockets in the angular-resolved spectral function of the Fe-free material shift to low energy when introducing Fe, leading to a multi-sheet 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 LaAlO3/SrTiO3 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 LaAlO3/SrTiO3 interface by first principles many-body theory as well as real-space model-Hamiltonian techniques. The relevance of the Hund's coupling JH for OV-induced correlated states is demonstrated. Strong orbital polarization towards an effective eg state with predominant local antiferromagnetic alignment on Ti sites nearby OVs is contrasted by polarized t2g(xy) states with ferromagnetic tendencies in the defect-free regions. Different magnetic phases may be identified, giving rise to distinct net-moment behavior at low and high vacancy concentration, with an antiferromagnetic-pair region inbetween.

Low-Energy Model and Electron-Hole Doping Asymmetry of Single-Layer Ruddlesden-Popper Iridates

A. Hampel, C. Piefke and F. Lechermann Phys. Rev. B 92, 085141 (2015)

Starting from the first-principles band structure, the interplay between local Coulomb interactions and spin-orbit coupling in structurally-undistorted Ba2IrO4 is investigated by means of rotational-invariant slave-boson mean-field theory. The evolution from a three-band description towards an anisotropic one-band (J=1/2) picture is traced. Single-site and cluster self-energies are used to shed light on competing Slater- and Mott-dominated correlation regimes. We reveal a clear asymmetry between electron and hole doping, notably in the nodal/anti-nodal Fermi-surface dichotomy at strong coupling. Electron-doped iridates appear comparable to hole-doped cuprates due to the different sign of the next-nearest-neighbor hopping t'.

Towards Mott design by δ-doping of strongly correlated titanates

F. Lechermann and M. Obermeyer: New J. Phys. 17, 043026 (2015)

Doping the distorted-perovskite Mott insulators LaTiO3 and GdTiO3 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 self-consistent combination of density functional theory with dynamical mean-field theory reveals layer- and temperature-dependent multi-orbital metal-insulator transitions. An orbital-selective metallic layer at the interface dissolves via an orbital-polarized doped-Mott state into an orbital-ordered insulating regime beyond the two conducting TiO2 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 Mott-insulating regimes, settles in delta-doped GdTiO3. It results from the subtle exchange-interaction 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 many-body 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 spin-polaron excitations in the doped Mott material NaxCoO2

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 many-body description beyond dynamical mean-field theory (DMFT). At lower doping around x=0.3, rather close to Mott-critical half-filling, the single-particle spectral function of NaxCoO2 displays an upper Hubbard band which is captured within DMFT. Momentum-dependent self-energy effects beyond DMFT become dominant at higher doping. Around a doping level of x~0.67, the incoherent excitations give way to finite-energy spin-polaron 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 in-plane ferromagnetic ordering.

Large-amplitude spin oscillations triggered by nonequilibrium strongly correlated t2g 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 electron-electron interactions in strongly correlated materials and a femtosecond spin-orientation change. Therefore we here investigate (de)magnetization processes in the multi-orbital Hubbard model on the local-correlation timescale (femtoseconds) by focussing on t2g electrons in a wider doping range. We reveal filling-dependant stable and transient spin-oscillations via interaction quenches from the antiferromagnetic or paramagnetic ground state. Dynamic ultrafast spin-orientation effects in prominent correlated antiferromagnetic transition-metal oxides are therefrom predicted.