arXiv subjects

Silke Biermann

Publications and source records attributed to Silke Biermann.

At least 37 records · Page 2Linked to original sources

arXiv2019

Mott and spin-Peierls physics in TiPO$_4$ under high pressure

TiPO$_4$ is a Mott insulator and one of few inorganic compounds featuring a spin-Peierls phase at low temperature. Recent experimental studies have suggested the presence of spin-Peierls dimerization also at ambient temperature though at high pressure. Here, we present a combined experimental and theoretical study of the energetics of the high-pressure phase. We analyse dimerization properties and their coupling to spin degrees of freedom. Most importantly, we argue that TiPO$_4$ resents a direct analogue to the celebrated binary transition metal oxide VO$_2$. TiPO$_4$ allows to assess spin-dimer physics in the high-pressure regime in a controlled fashion, having the potential to become an important model system representative of the class of dimerized transition metal oxides.

cond-mat.str-el↗

arXiv2019

Role of charge transfer in hybridization-induced spin transition in metal-organic molecules

The spin-crossover in organometallic molecules constitutes one of the most promising routes towards the realization of molecular spintronic devices. In this article, we explore the hybridization-induced spin-crossover in metal-organic complexes. We propose a minimal many-body model that captures the essence of the spin-state switching in a generic parameter space, thus providing insight into the underlying physics. Combining the model with density functional theory (DFT), we then study the spin-crossover in isomeric structures of Ni-porphyrin (Ni-TPP). We show that metal-ligand charge transfer plays a crucial role in the determination of the spin-state in Ni-TPP. Finally, we propose a spin-crossover mechanism based on mechanical strain, which does not require a switch between isomeric structures.

cond-mat.mes-hall↗

arXiv2019

The Dirac nodal line network in non-symmorphic rutile semimetal RuO$_2$

We employ angle resolved photoemission spectroscopy (ARPES) to investigate the Fermi surface of RuO$_2$. We find a network of two Dirac nodal lines (DNL) as previously predicted in theory, where the valence- and conduction bands touch along continuous lines in momentum space. In addition, we find evidence for a third DNL close to the Fermi level which appears robust despite the presence of significant spin orbit coupling. We demonstrate that the third DNL gives rise to a topologically trivial flat-band surface state (FBSS) at the (110) surface. This FBSS can be tuned by surface doping and presents an interesting playground for the study of surface chemistry and exotic correlation phenomena.

cond-mat.mes-hall↗

arXiv2019

Nature of the Doping-Driven Metal-Insulator Transition in Correlated Electron Systems with strong Hund's Exchange Coupling

We study the doping-driven Mott metal-insulator transition for multi-orbital Hubbard models with Hund's exchange coupling at finite temperatures. As in the single-orbital Hubbard model, the transition is of first-order within dynamical mean field theory, with a coexistence region where two solutions can be stabilized. We find, that in the presence of finite Hund's coupling, the insulating phase is connected to a badly metallic phase, which extends to surprisingly large dopings. While fractional power-law behavior of the self-energies on the Matsubara axis is found on both sides of the transition, a regime with frozen local moments develops only on the branch connected to the insulating phase.

cond-mat.str-el↗

arXiv2019

ARPES study of orbital characters, symmetry breakings and pseudogaps in doped and pure Sr2IrO4

Sr2IrO4 is characterized by a large spin-orbit coupling, which gives rise to bands with strongly entangled spin and orbital characters, called J=1/2 and J=3/2. We use light-polarization dependent ARPES to study directly the orbital character of these bands and fully map out their dispersion. We observe bands in very good agreement with our cluster dynamical mean-field theory calculations. We show that the J=1/2 band, the closest to the Fermi level Ef, is dominated by dxz character along kx and dyz along ky. This is actually in agreement with an isotropic J=1/2 character on average, but this large orbital dependence in k-space was mostly overlooked before. It gives rise to strong modulations of the ARPES intensity that we explain and carefully take into account to compare dispersions in equivalent directions of the Brillouin zone. Although the latter dispersions look different at first, suggesting possible symmetry breakings, they are found essentially similar, once corrected for these intensity variations. In particular, the pseudogap-like features close to the $X$ point appearing in the nearly metallic 15% Rh-doped Sr2IrO4 strongly depend on experimental conditions. We reveal that there is nevertheless an energy scale of 30meV below which spectral weight is suppressed, independent of the experimental conditions, which gives a reliable basis to analyze this behavior. We suggest it is caused by disorder.

cond-mat.str-el↗

arXiv2019

Spectral functions of Sr${}_2$IrO${}_4$: theory versus experiment

The spin-orbit Mott insulator Sr${}_2$IrO${}_4$ has attracted a lot of interest in recent years from theory and experiment due to its close connection to isostructural high-temperature copper oxide superconductors. Despite of not being superconducting its spectral features closely resemble those of the cuprates, including Fermi surface and pseudogap properties. In this article, we review and extend recent work in the theoretical description of the spectral function of pure and electron-doped Sr${}_2$IrO${}_4$ based on a cluster extension of dynamical mean-field theory ("oriented-cluster DMFT") and compare it to available angle-resolved photoemission data. Current theories provide surprisingly good agreement for pure and electron-doped Sr${}_2$IrO${}_4$, both in the paramagnetic and antiferromagnetic phases. Most notably, one obtains simple explanations for the experimentally observed steep feature around the $M$ point and the pseudo-gap-like spectral feature in electron-doped Sr${}_2$IrO${}_4$.

cond-mat.str-el↗

arXiv2018

Van Hove singularities, chemical pressure and phonons: an angle-resolved photoemission study of KFe$_2$As$_2$ and CsFe$_2$As$_2$

We report an angle-resolved photoemission spectroscopy (ARPES) study of KFe$_2$As$_2$ and CsFe$_2$As$_2$, revealing the existence of a van Hove singularity affecting the electronic properties. As a result of chemical pressure, we find a stronger three-dimensionality in KFe$_2$As$_2$ than in CsFe$_2$As$_2$, notably for the 3$d_{z^2}$ states responsible for the small three-dimensional hole-like Fermi surface pocket reported by quantum oscillations. Supported by first-principles calculations, our ARPES study shows that the van Hove singularity previously reported in KFe$_2$As$_2$ moves closer to the Fermi level under negative chemical pressure. This observation, which suggests that the large density-of-states accompanying the van Hove singularity contributes to the large Sommerfeld coefficient reported for the AFe$_2$As$_2$ (A = K, Rb, Cs) series, is also consistent with the evolution of the inelastic scattering revealed by transport under external pressure, thus offering a possible interpretation for the origin of the apparent change in the superconducting order parameter under pressure. We find that the coherent spectral weight decreases exponentially upon increasing temperature with a characteristic temperature $T^*$. We speculate how the low-energy location of the van Hove singularity and the presence of a low-energy peak in the phonon density-of-states can relate to the high-temperature crossover observed in various electronic and thermodynamic quantities.

cond-mat.supr-con↗

arXiv2018

Non-local Coulomb correlations in pure and electron-doped ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$: spectral functions, Fermi surface and pseudogap-like spectral weight distributions from oriented cluster dynamical mean field theory

We address the role of non-local Coulomb correlations and short-range magnetic fluctuations in the high-temperature phase of Sr$_2$IrO$_4$ within state-of-the-art spectroscopic and first-principles theoretical methods. Introducing a novel cluster dynamical mean field scheme, we compute momentum-resolved spectral functions, which we find to be in excellent agreement with angle-resolved photoemission spectra. We show that while short-range antiferromagnetic fluctuations are crucial to account for the electronic properties of the material even in the high-temperature paramagnetic phase, long-range magnetic order is not a necessary ingredient of the insulating state. Upon doping, an exotic metallic state is generated, exhibiting cuprate-like pseudo-gap spectral properties, for which we propose a surprisingly simple theoretical mechanism.

cond-mat.str-el↗

arXiv2017

Crystal field splittings in rare earth-based hard magnets: an ab initio approach

We apply the first-principles density functional theory + dynamical mean field theory framework to evaluate the crystal field splitting on rare earth sites in hard magnetic intermetallics. An atomic (Hubbard-I) approximation is employed for local correlations on the rare earth 4$f$ shell and self-consistency in the charge density is implemented. We reduce the density functional theory self-interaction contribution to the crystal field splitting by properly averaging the 4$f$ charge density before recalculating the one-electron Kohn-Sham potential. Our approach is shown to reproduce the experimental crystal field splitting in the prototypical rare earth hard magnet SmCo$_5$. Applying it to $R$Fe$_{12}$ and $R$Fe$_{12}X$ hard magnets ($R=$Nd, Sm and $X=$N, Li), we obtain in particular a large positive value of the crystal field parameter $A_2^0\langle r^2\rangle$ in NdFe$_{12}$N resulting in a strong out-of-plane anisotropy observed experimentally. The sign of $A_2^0\langle r^2\rangle$ is predicted to be reversed by substituting N with Li, leading to a strong out-of-plane anisotropy in SmFe$_{12}$Li. We discuss the origin of this strong impact of N and Li interstitials on the crystal field splitting on rare earth sites.

cond-mat.str-el↗

arXiv2017

Non-local Coulomb interactions on the triangular lattice in the high-doping regime: Spectra and charge dynamics from Extended Dynamical Mean Field Theory

We explore the two-dimensional extended Hubbard model on the triangular lattice in the high doping regime. On-site and nearest-neighbour repulsive interactions are treated in a non-perturbative way by means of Extended Dynamical Mean Field Theory. We compute the low-temperature phase diagram, displaying a metallic phase and a symmetry-broken phase for strong intersite repulsions. We describe the correlation effects on both single-particle and two-particle observables in the metallic phase. Whereas single-particle spectra feature a Hubbard satellite typical of strongly correlated systems, local susceptibilities remain close to their non-interacting limit, even for large on-site repulsions. We argue that this behaviour is typical of the strongly doped case. We also report a region in parameter space with negative static local screening.

cond-mat.str-el↗

arXiv2017

Renormalization of effective interactions in a negative charge-transfer insulator

We compute from first principles the effective interaction parameters appropriate for a low-energy description of the rare-earth nickelate LuNiO$_{3}$ involving the partially occupied $e_g$ states only. The calculation uses the constrained random-phase approximation and reveals that the effective on-site Coulomb repulsion is strongly reduced by screening effects involving the oxygen-$p$ and nickel-$t_{2g}$ states. The long-range component of the effective low-energy interaction is also found to be sizeable. As a result, the effective on-site interaction between parallel-spin electrons is reduced down to a small negative value. This validates effective low-energy theories of these materials proposed earlier. Electronic structure methods combined with dynamical mean-field theory are used to construct and solve an appropriate low-energy model and explore its phase diagram as a function of the on-site repulsion and Hund's coupling. For the calculated values of these effective interactions we find, in agreement with experiments, that LuNiO$_{3}$ is a metal without disproportionation of the $e_g$ occupancy when considered in its orthorhombic structure, while the monoclinic phase is a disproportionated insulator.

cond-mat.str-el↗

arXiv2017

Novel approaches to spectral properties of correlated electron materials: From generalized Kohn-Sham theory to screened exchange dynamical mean field theory

The most intriguing properties of emergent materials are typically consequences of highly correlated quantum states of their electronic degrees of freedom. Describing those materials from first principles remains a challenge for modern condensed matter theory. Here, we review, apply and discuss novel approaches to spectral properties of correlated electron materials, assessing current day predictive capabilities of electronic structure calculations. In particular, we focus on the recent Screened Exchange Dynamical Mean-Field Theory scheme and its relation to generalized Kohn-Sham theory. These concepts are illustrated on the transition metal pnictide BaCo$_2$As$_2$ and elemental zinc and cadmium.

cond-mat.str-el↗

arXiv2017

Is BaCr$_2$As$_2$ symmetrical to BaFe$_2$As$_2$ with respect to half $3d$ shell filling?

We have performed an angle-resolved photoemission spectroscopy study of BaCr$_2$As$_2$, which has the same crystal structure as BaFe$_2$As$_2$, a parent compound of Fe-based superconductors. We determine the Fermi surface of this material and its band dispersion over 5 eV of binding energy. Very moderate band renormalization (1.35) is observed for only two bands. We attribute this small renormalization to enhanced direct exchange as compared to Fe in BaFe$_2$As$_2$, and to a larger contribution of the $e_g$ orbitals in the composition of the bands forming the Fermi surface, leading to an effective valence count that is reduced by Fe $d$ - As $p$ hybridization.

cond-mat.supr-con↗

arXiv2017

Influence of Fock exchange in combined many-body perturbation and dynamical mean field theory

In electronic systems with long-range Coulomb interaction, the nonlocal Fock exchange term has a band-widening effect. While this effect is included in combined many-body perturbation theory and dynamical mean field theory schemes, it is not taken into account in standard extended DMFT (EDMFT) calculations. Here, we include this instantaneous term in both approaches and investigate its effect on the phase diagram and dynamically screened interaction. We show that the largest deviations between previously presented EDMFT and $GW$+EDMFT results originate from the nonlocal Fock term, and that the quantitative differences are especially large in the strong-coupling limit. Furthermore, we show that the charge-ordering phase diagram obtained in $GW$+EDMFT methods for moderate interaction values is very similar to the one predicted by dual boson methods that include the fermion-boson or four-point vertex.

cond-mat.str-el↗

arXiv2016

Coulomb Correlations in 4d and 5d Oxides from First Principles - or How Spin-Orbit Materials choose their Effective Orbital Degeneracies

The interplay of spin-orbit interactions and Coulomb correlations has become a hot topic in condensed matter theory. Here, we review recent advances in dynamical mean-field theory-based electronic structure calculations for iridates and rhodates. We stress the notion of the effective degeneracy of the compounds, which introduces an additional axis into the conventional picture of a phase diagram based on filling and on the ratio of interactions to bandwidth.

cond-mat.str-el↗

arXiv2016

Self-Optimized Superconductivity Attainable by Interlayer Phase Separation at Cuprate Interfaces

Stabilizing superconductivity at high temperatures and elucidating its mechanism have long been major challenges of materials research in condensed matter physics. Meanwhile, recent progress in nanostructuring offers unprecedented possibilities for designing novel functionalities. Above all, thin films of cuprate and iron-based high-temperature superconductors exhibit remarkably better superconducting characteristics (e.g. higher critical temperatures) than in the bulk, but the underlying mechanism is still not understood. Solving microscopic models suitable for cuprates, here, we demonstrate that at an interface between a Mott insulator and an overdoped non-superconducting metal, the superconducting amplitude stays always pinned at the optimum achieved in the bulk, independently of the carrier concentration in the metal. This is in contrast to the dome-like dependence in bulk superconductors, but consistent with the astonishing independence of the critical temperature from the carrier density x observed at interfaces of La2CuO4 and La2-xSrxCuO4. We furthermore identify a self-organization mechanism as responsible for the pinning at the optimum amplitude: An emergent electronic structure induced by interlayer phase separation eludes bulk phase separation and inhomogeneities that would kill superconductivity in the bulk. Thus, interfaces provide an ideal tool to enhance and stabilize superconductivity. This interfacial example opens up further ways of shaping superconductivity by suppressing competing instabilities, with direct perspectives for designing devices.

cond-mat.str-el↗

arXiv2016

Large effects of subtle electronic correlations on the energetics of vacancies in alpha-Fe

We study the effect of electronic Coulomb correlations on the vacancy formation energy in paramagnetic alpha-Fe within ab initio dynamical mean-field theory. The calculated value for the formation energy is substantially lower than in standard density-functional calculations and in excellent agreement with experiment. The reduction is caused by an enhancement of electronic correlations at the nearest neighbors of the vacancy. This effect is explained by subtle changes in the corresponding spectral function of the d-electrons. The local lattice relaxations around the vacancy are substantially increased by many-body effects.

cond-mat.str-el↗

arXiv2016

Hubbard interactions in iron-based pnictides and chalcogenides: Slater parametrization, screening channels and frequency dependence

We calculate the strength of the frequency-dependent on-site electronic interactions in the iron pnictides LaFeAsO, BaFe2As2, BaRu2As2, and LiFeAs and the chalcogenide FeSe from first principles within the constrained random phase approximation. We discuss the accuracy of an atomic-like parametrization of the two-index density-density interaction matrices based on the calculation of an optimal set of three independent Slater integrals, assuming that the angular part of the Fe-d localized orbitals can be described within spherical harmonics as for isolated Fe atoms. We show that its quality depends on the ligand-metal bonding character rather than on the dimensionality of the lattice: it is excellent for ionic-like Fe-Se (FeSe) chalcogenides and a more severe approximation for more covalent Fe-As (LaFeAsO, BaFe2As2) pnictides. We furthermore analyze the relative importance of different screening channels, with similar conclusions for the different pnictides but a somewhat different picture for the benchmark oxide SrVO3: the ligand channel does not appear to be dominant in the pnictides, while oxygen screening is the most important process in the oxide. Finally, we analyze the frequency dependence of the interaction. In contrast to simple oxides, in iron pnictides its functional form cannot be simply modeled by a single plasmon, and the actual density of modes enters the construction of an effective Hamiltonian determining the low-energy properties.

cond-mat.str-el↗