arXiv subjects

Steffen Backes

Publications and source records attributed to Steffen Backes.

24 records · Page 2Linked to original sources

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

Microscopic nature of correlations in multi-orbital AFe2As2 (A=K, Rb, Cs): Hund's coupling versus Coulomb repulsion

We investigate via LDA+DMFT (local density approximation combined with dynamical mean field theory) the manifestation of correlation effects in a wide range of binding energies in the hole-doped family of Fe-pnictides $A$Fe$_2$As$_2$ ($A={\rm K}$, Rb, Cs) as well as the fictitious FrFe$_2$As$_2$ and $a$-axis stretched CsFe$_2$As$_2$. This choice of systems allows for a systematic analysis of the interplay of Hund's coupling $J_H$ and on-site Coulomb repulsion $U$ in multi-orbital Fe-pnictides under negative pressure. With increasing ionic size of the alkali metal, we observe a non-trivial change in the iron $3d$ hoppings, an increase of orbitally-selective correlations and the presence of incoherent weight at high-binding energies that do not show the typical lower Hubbard-band behavior but rather characteristic features of a Hund's metal. This is especially prominent in $a$-stretched CsFe$_2$As$_2$. We also find that the coherent/incoherent electronic behavior of the systems is, apart from temperature, strongly dependent on $J_H$ and we provide estimates of the coherence scale $T^*$. We discuss these results in the framework of reported experimental observations.

cond-mat.str-el↗

arXiv2015

Origin of the superconducting state in the collapsed tetragonal phase of KFe2As2

Recently, KFe$_2$As$_2$ was shown to exhibit a structural phase transition from a tetragonal to a collapsed tetragonal phase under applied pressure of about $15~\mathrm{GPa}$. Surprisingly, the collapsed tetragonal phase hosts a superconducting state with $T_c \sim 12~\mathrm{K}$, while the tetragonal phase is a $T_c \leq 3.4~\mathrm{K}$ superconductor. We show that the key difference between the previously known non-superconducting collapsed tetragonal phase in AFe$_2$As$_2$ (A= Ba, Ca, Eu, Sr) and the superconducting collapsed tetragonal phase in KFe$_2$As$_2$ is the qualitatively distinct electronic structure. While the collapsed phase in the former compounds features only electron pockets at the Brillouin zone boundary and no hole pockets are present in the Brillouin zone center, the collapsed phase in KFe$_2$As$_2$ has almost nested electron and hole pockets. Within a random phase approximation spin fluctuation approach we calculate the superconducting order parameter in the collapsed tetragonal phase. We propose that a Lifshitz transition associated with the structural collapse changes the pairing symmetry from $d$-wave (tetragonal) to $s_\pm$ (collapsed tetragonal). Our DFT+DMFT calculations show that effects of correlations on the electronic structure of the collapsed tetragonal phase are minimal. Finally, we argue that our results are compatible with a change of sign of the Hall coefficient with pressure as observed experimentally.

cond-mat.supr-con↗

arXiv2014

Correlation effects in the tetragonal and collapsed tetragonal phase of CaFe2As2

We investigate the role of correlations in the tetragonal and collapsed tetragonal phases of CaFe2As2 by performing charge self-consistent DFT+DMFT (density functional theory combined with dynamical mean-field theory) calculations. While the topology of the Fermi surface is basically unaffected by the inclusion of correlation effects, we find important orbital-dependent mass renormalizations which show good agreement with recent angle-resolved photoemission (ARPES) experiments. Moreover, we observe a markedly different behavior of these quantities between the low-pressure tetragonal and the high-pressure collapsed tetragonal phase. We attribute these effects to the increased hybridization between the iron- and arsenic orbitals as one enters the collapsed tetragonal phase.

cond-mat.str-el↗

arXiv2014

Electronic structure and de Haas-van Alphen frequencies in KFe2As2 within LDA+DMFT

Recent density functional theory (DFT) calculations for KFe2As2 have shown to be insufficient to satisfactorily describe angle-resolved photoemission (ARPES) measurements as well as observed de Haas van Alphen (dHvA) frequencies. In the present work, we extend DFT calculations based on the full-potential linear augmented plane-wave method by dynamical mean field theory (DFT+DMFT) to include correlation effects beyond the local density approximation. Our results indicate that KFe2As2 is a moderately correlated metal with a mass renormalization factor of the Fe 3d orbitals between 1.6 and 2.7. Also, the obtained shape and size of the Fermi surface are in good agreement with ARPES measurements and we observe some topological changes with respect to DFT calculations like the opening of an inner hole cylinder at the Z point. As a result, our calculated dHvA frequencies differ greatly from existing DFT results and qualitatively agree with experimental data. On this basis, we argue that correlation effects are important to understand the -presently under debate- nature of superconducting state in KFe2As2.

cond-mat.str-el↗

arXiv2013

Finite temperature and pressure molecular dynamics for BaFe2As2

We study the temperature and pressure dependence of the structural and electronic properties of the iron pnictide superconductor BaFe2As2. We use density functional theory based Born-Oppenheimer molecular dynamics simulations to investigate the system at temperatures from T=5 K to 150 K and pressures from P=0 GPa to 30 GPa. When increasing the pressure at low temperature, we find the two transitions from an orthorhombic to a tetragonal and to a collapsed tetragonal structure that are also observed in zero temperature structure relaxations and in experiment. However, these transitions are considerably smeared out at finite temperature, whereas the critical pressure for the first transition increases with temperature. We also analyze the electronic structure of BaFe2As2 at finite temperature and work out differences between the time averaged band structure and Fermi surface at finite temperature compared to the known zero temperature results. Our results should be helpful for resolving some open issues in experimental reports for BaFe2As2 under high pressure.

cond-mat.str-el↗