arXiv subjects

Frank Lechermann

Publications and source records attributed to Frank Lechermann.

At least 55 records · Page 3Linked to original sources

arXiv2018

Oxide Heterostructures from a Realistic Many-Body Perspective

Oxide heterostructures are a new class of materials by design, that open the possibility for engineering challenging electronic properties, in particular correlation effects beyond an effective single-particle description. This short review tries to highlight some of the demanding aspects and questions, motivated by the goal to describe the encountered physics from first principles. The state-of-the-art methodology to approach realistic many-body effects in strongly correlated oxides, the combination of density functional theory with dynamical mean-field theory, will be briefly introduced. Discussed examples deal with prominent Mott-band- and band-band-insulating type of oxide heterostructures, where different electronic characteristics may be stabilized within a single architectured oxide material.

cond-mat.mtrl-sci↗

arXiv2018

Hidden Mott insulator in metallic PdCrO$_2$

There has been a long-standing debate on the coexistence between itinerant electrons and localized spins in the PdCrO$_2$ 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 CrO$_2$ layers are indeed Mott insulating as suggested by previous experimental work. The resulting ${\bf k}$-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.

cond-mat.str-el↗

arXiv2018

Multi-orbital nature of the spin fluctuations in Sr$_2$RuO$_4$

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 ${\bf Q}_i$=$(0.3,0.3,0)$. It is still 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 ${\bf Q}_i$ is shown to carry nearly equal contributions from each of the Ru$(4d)$-$t_\mathrm{2g}$ orbitals, pointing to a cooperative effect resulting in the dominant spin fluctuations.

cond-mat.str-el↗

arXiv2018

Uncovering the mechanism of the impurity-selective Mott transition in paramagnetic V$_{2}$O$_{3}$

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 V$_{2}$O$_{3}$, 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 V$_{2}$O$_{3}$ phase diagram are driven by defect-induced local symmetry breakings resulting from dramatically different couplings of Cr and Ti dopants to the host system. 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 Hamiltonian, since correlations crucially respond to it.

cond-mat.str-el↗

arXiv2018

Rigorous symmetry adaptation of multiorbital rotationally invariant slave-boson theory with application to Hund's rules physics

The theory of correlated electron systems on a lattice proves notoriously complicated because of the exponential growth of Hilbert space. Mean-field approaches provide valuable insight when the self-energy has a dominant local structure. Additionally, the extraction of effective low-energy theories from the generalized many-body representation is highly desirable. In this respect, the rotational-invariant slave boson (RISB) approach in its mean-field formulation enables versatile access to correlated lattice problems. However in its original form, due to numerical complexity, the RISB approach is limited to about three correlated orbitals per lattice site. We thus present a thorough symmetry-adapted advancement of RISB theory, suited to efficiently deal with multi-orbital Hubbard Hamiltonians for complete atomic-shell manifolds. It is utilized to study the intriguing problem of Hund's physics for three- and especially five-orbital manifolds on the correlated lattice, including crystal-field terms as well as spin-orbit interaction. The well-known Janus-face phenomenology, i.e. strenghtening of correlations at smaller-to-intermediate Hubbard $U$ accompanied by a shift of the Mott transition to a larger $U$ value, has a stronger signature and more involved multiplet resolution for five-orbital problems. Spin-orbit interaction effectively reduces the critical local interaction strength and weakens the Janus-face behavior. Application to the realistic challenge of Fe chalcogenides underlines the subtle interplay of the orbital degrees of freedom in these materials

cond-mat.str-el↗

arXiv2018

The role of non-spherical double counting in DFT+DMFT: total energy and structural optimization of pnictide superconductors

A simple scheme for avoiding non-spherical double counting in the combination of density func- tional theory with dynamical mean-field theory (DFT+DMFT)is developed. It is applied to total- energy calculations and structural optimization of the pnictide superconductor LaFeAsO. The results are compared to a recently proposed "exact" double-counting formulation. Both schemes bring the optimized Fe-As interatomic distance close to the experimental value. This resolves the long stand- ing controversy between DFT+DMFT and experiment for the structural optimization of LaFeAsO.

cond-mat.mtrl-sci↗

arXiv2017

First-Principles Many-Body Investigation of Correlated Oxide Heterostructures: Few-Layer-Doped SmTiO$_3$

Correlated oxide heterostructures pose a challenging problem in condensed matter research due to their structural complexity interweaved with demanding electron states beyond the effective single-particle picture. By exploring the correlated electronic structure of SmTiO$_3$ doped with few layers of SrO, we provide an insight into the complexity of such systems. Furthermore, it is shown how the advanced combination of band theory on the level of Kohn-Sham density functional theory with explicit many-body theory on the level of dynamical mean-field theory provides an adequate tool to cope with the problem. Coexistence of band-insulating, metallic and Mott-critical electronic regions is revealed in individual heterostructures with multi-orbital manifolds. Intriguing orbital polarizations, that qualitatively vary between the metallic and the Mott layers are also encountered.

cond-mat.mtrl-sci↗

arXiv2017

Oxygen-vacancy driven electron localization and itinerancy in rutile-based TiO$_2$

Oxygen-deficient TiO$_2$ in the rutile structure as well as the Ti$_3$O$_5$ Magn{é}li phase is investigated within the charge self-consistent combination of density functional theory (DFT) with dynamical mean-field theory (DMFT). It is shown that an isolated oxygen vacancy (V$_{\rm O}$) in titanium dioxide is not sufficient to metallize the system at low temperatures. In a semiconducting phase, an in-gap state is identified at $\varepsilon_{\rm IG}^{\hfill}\sim -0.75\,$eV\, in excellent agreement with experimental data. Band-like impurity levels, resulting from a threefold V$_{\rm O}$-Ti coordination as well as entangled $(t_{2g},e_g)$ states, become localized due to site-dependent electronic correlations. Charge localization and strong orbital polarization occur in the V$_{\rm O}$-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 Magn{é}li phase. A V$_{\rm O}$-defect rutile structure of identical stoichiometry shows key differences in the orbital-resolved character and the spectral properties. Charge disproportionation is vital in the oxygen-deficient compounds, but obvious metal-insulator transitions driven or sustained by charge order are not identified.

cond-mat.mtrl-sci↗

arXiv2017

Unconventional electron states in $δ$-doped SmTiO$_3$

The Mott-insulating distorted perovskite SmTiO$_3$, 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 TiO$_2$ layers prone to magnetic order gives rise to multi-orbital electronic transport beyond standard Fermi-liquid theory. First hints towards a pseudogap opening due to electron-electron scattering within a background of ferromagnetic and antiferromagnetic fluctuations are detected.

cond-mat.str-el↗

arXiv2016

Quantum-Many-Body Intermetallics: Phase Stability of Fe$_3$Al and Small-Gap Formation in Fe$_2$VAl

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 D0$_3$-Fe$_3$Al 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 Fe$_2$VAl 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 in correlated oxides.

cond-mat.mtrl-sci↗

arXiv2016

Thermopower enhancement from engineering the Na$_{0.7}$CoO$_2$ interacting fermiology via Fe doping

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 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.

cond-mat.mtrl-sci↗

arXiv2016

Versatile Approach to the Spin Dynamics in Correlated Electron Systems

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.

cond-mat.str-el↗

arXiv2016

Magnetism, spin texture and in-gap states: Atomic specialization at the surface of oxygen-deficient SrTiO$_3$

Motivated by recent spin- and angular-resolved photoemission (SARPES) measurements performed on the two-dimensional electronic states confined near the (001) surface of SrTiO$_3$ in the presence of oxygen vacancies, we explore their spin structure by means of ab initio density functional theory (DFT) calculations of slabs. Relativistic nonmagnetic DFT calculations display Rashba-like spin winding with a splitting of a few meV and when surface magnetism on the Ti ions is in- cluded, bands become spin-split with an energy difference ~100 meV at the $Γ$ point, consistent with SARPES findings. While magnetism tends to suppress the effects of the relativistic Rashba interaction, signatures of it are still clearly visible in terms of complex spin textures. Furthermore, we observe an atomic specialization phenomenon, namely, two types of electronic contributions: one is from Ti atoms neighboring the oxygen vacancies that acquire rather large magnetic moments and mostly create in-gap states; another comes from the partly polarized t$_{2g}$ itinerant electrons of Ti atoms lying further away from the oxygen vacancy, which form the two-dimensional electron system and are responsible for the Rashba spin winding and the spin splitting at the Fermi surface.

cond-mat.str-el↗

arXiv2016

Electron dichotomy on the SrTiO$_3$ defect surface augmented by many-body effects

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 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 dynamical-mean field theory that oxygen vacancies on the STO (001) surface give rise to a dichotomy of weakly-correlated $t_{2g}$ low-energy quasiparticles and localized 'in-gap' 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.

cond-mat.mtrl-sci↗

arXiv2015

Hidden spin-orbital hexagonal ordering induced by strong correlations in LiVS$_2$

We present a first-principles many-body analysis of multi-orbital lattice susceptibilities in the metallic phase of the quasi-twodimensional compound LiVS$_2$. We base this on advanced correlated electronic structure methods for the $t_{2g}$-shell to reveal a highly entangled spin-orbital hexagonal ordering (SOHO) bringing about an inherently intersite order parameter for the trimerization transition, which eventually leads to an intriguing insulating phase at low temperature.

cond-mat.mtrl-sci↗

arXiv2015

Interface exchange processes in LaAlO$_3$/SrTiO$_3$ induced by oxygen vacancies

Understanding the role of defects in oxide heterostructures is crucial for future materials control and functionalization. We hence study the impact of oxygen vacancies (OVs) at variable concentrations on orbital- and spin exchange in the LaAlO$_3$/SrTiO$_3$ interface by first principles many-body theory and real-space model-Hamiltonian techniques. Intricate interplay between Hubbard $U$ and Hund's coupling $J_{\rm H}$ for OV-induced correlated states is demonstrated. Orbital polarization towards an effective $e_g$ state with predominant local antiferromagnetic alignment on Ti sites near OVs is contrasted with $t_{2g}(xy)$ states with ferromagnetic tendencies in the defect-free regions. Different magnetic phases are identified, giving rise to distinct net-moment behavior at low and high OV concentrations. This provides a theoretical basis for prospective tailored magnetism by defect manipulation in oxide interfaces.

cond-mat.mtrl-sci↗

arXiv2015

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

We study the correlated electronic structure of single-layer iridates based on structurally-undistorted Ba$_2$IrO$_4$. Starting from the first-principles band structure, the interplay between local Coulomb interactions and spin-orbit coupling 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 $t'$.

cond-mat.str-el↗

arXiv2015

Towards Mott design by $δ$-doping of strongly correlated titanates

Doping the distorted-perovskite 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 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 TiO$_2$ layers. We find large differences in the scattering behavior within the latter. Breaking the spin symmetry in $δ$-doped GdTiO$_3$ results in blocks of ferromagnetic itinerant and ferromagnetic Mott-insulating layers which are coupled antiferromagnetically.

cond-mat.mtrl-sci↗