We identify the driving mechanism of the gigantic Seebeck coefficient in FeSb$_2$ as the phonon-drag effect associated with an in-gap density of states that we demonstrate to derive from excess iron. We accurately model electronic and thermoelectric
transport coefficients and explain the so far ill-understood correlation of maxima and inflection points in different response functions. Our scenario has far-reaching consequences for attempts to harvest the spectacular powerfactor of FeSb$_2$.
We present a case study of the solar flare (SOL2001-10-02T17:31) that showed quasi-periodic pulsations (QPPs) in hard X-rays with two simultaneously excited periods, P_1 = 26-31 s and P_2 = 110 s. Complete evolution of the flare recorded by the Yohko
h telescopes, together with the patrol SOHO/EIT images, allowed us to identify magnetic structures responsible for particular periods and to propose an overall scenario which is consistent with the available observations. Namely, we suggest that emerging magnetic flux initiated the reconnection with legs of a large arcade of coronal loops that had been present in an active region for several days. The reconnection excited MHD oscillations in both magnetic structures simultaneously: period P_1 was generated in the emerging loop and in a loop being a result of the reconnection; period P_2 occurred in the arcade. Both resonators produced photons of different spectra. We anticipate that multiperiodicity in hard X-rays can be a common feature of flare hybrids, i.e. the events, in which magnetic structures of different sizes interact.
While second-order phase transitions always cause strong non-local fluctuations, their effect on spectral properties crucially depends on the dimensionality. For the important case of three dimensions, we show that the electron self-energy is well se
parable into a local dynamical part and static non-local contributions. In particular, our non-perturbative many-body calculations for the 3D Hubbard model at different fillings demonstrate that the quasi-particle weight remains essentially momentum-independent, also in the presence of overall large non-local corrections to the self-energy. Relying on this insight we propose a space-time-separated scheme for many-body perturbation theory that is up to ten times more efficient than current implementations. Besides these far-reaching implications for state-of-the-art electronic structure schemes, our analysis will also provide guidance to the quest of going beyond them.
While in strongly correlated materials one often focuses on local electronic correlations, the influence of non-local exchange and correlation effects beyond band-theory can be pertinent in systems with more extended orbitals. Thus in many compounds
an adequate theoretical description requires the joint treatment of local and non-local self-energies. Here, I will argue that this is the case for the iron pnictide and chalcogenide superconductors. As an approach to tackle their electronic structure, I will detail the implementation of the recently proposed scheme that combines the quasi-particle self-consistent GW approach with dynamical mean-field theory: QSGW+DMFT. I will showcase the possibilities of QSGW+DMFT with an application on BaFe2As2. Further, I will discuss the empirical finding that in pnictides dynamical and non-local correlation effects separate within the quasi-particle band-width.
We report the first comprehensive study of the high temperature form ($alpha$-phase) of iron disilicide. Measurements of the magnetic susceptibility, magnetization, heat capacity and resistivity were performed on well characterized single crystals. W
ith a nominal iron $d^6$ configuration, and a quasi-two dimensional crystal structure that strongly resembles that of LiFeAs, $alpha$-FeSi$_2$ is a potential candidate for unconventional superconductivity. Akin to LiFeAs, $alpha$-FeSi$_2$ does not develop any magnetic order, and we confirm its metallic state down to the lowest temperatures ($T$=1.8 K). However, our experiments reveal that paramagnetism and electronic correlation effects in $alpha$-FeSi$_2$ are considerably weaker than in the pnictides. Band theory calculations yield small Sommerfeld coefficients of the electronic specific heat $gamma=C_e/T$ that are in excellent agreement with experiment. Additionally, realistic many-body calculations further corroborate that quasi-particle mass enhancements are only modest in $alpha$-FeSi$_{2}$ . Remarkably, we find that the natural tendency to vacancy formation in the iron sublattice has little influence on the iron valence and the density of states at the Fermi level. Moreover, Mn doping does not significantly change the electronic state of the Fe ion. This suggests that the iron valence is protected against hole doping, and indeed the substitution of Co for Fe causes a rigid-band like response of the electronic properties. As a key difference from the pnictides, we identify the smaller inter-iron layer spacing, which causes the active orbitals near the Fermi level to be of a different symmetry in $alpha$-FeSi$_2$. This change in orbital character might be responsible for the lack of superconductivity in this system, providing constraints on pairing theories in the iron based pnictides and chalcogenides.
Understanding the Fermi surface and low-energy excitations of iron or cobalt pnictides is crucial for assessing electronic instabilities such as magnetic or superconducting states. Here, we propose and implement a new approach to compute the low-ener
gy properties of correlated electron materials, taking into account both screened exchange beyond the local density approximation and local dynamical correlations. The scheme allows us to resolve the puzzle of BaCo2As2, for which standard electronic structure techniques predict a ferromagnetic instability not observed in nature.
We investigate signatures of electronic correlations in the narrow-gap semiconductor FeGa$_3$ by means of electrical resistivity and thermodynamic measurements performed on single crystals of FeGa$_3$, Fe$_{1-x}$Mn$_x$Ga$_3$ and FeGa$_{3-y}$Zn$_y$, c
omplemented by a study of the 4$d$ analog material RuGa$_3$. We find that the inclusion of sizable amounts of Mn and Zn dopants into FeGa$_3$ does not induce an insulator-to-metal transition. Our study indicates that both substitution of Zn onto the Ga site and replacement of Fe by Mn introduces states into the semiconducting gap that remain localized even at highest doping levels. Most importantly, using neutron powder diffraction measurements, we establish that FeGa$_3$ orders magnetically above room temperature in a complex structure, which is almost unaffected by the doping with Mn and Zn. Using realistic many-body calculations within the framework of dynamical mean field theory (DMFT), we argue that while the iron atoms in FeGa$_3$ are dominantly in an $S=1$ state, there are strong charge and spin fluctuations on short time scales, which are independent of temperature. Further, the low magnitude of local contributions to the spin susceptibility advocates an itinerant mechanism for the spin response in FeGa$_3$. Our joint experimental and theoretical investigations classify FeGa$_3$ as a correlated band insulator with only small dynamical correlation effects, in which non--local exchange interactions are responsible for the spin gap of 0.4 eV and the antiferromagnetic order. We show that hole doping of FeGa$_3$ leads, within DMFT, to a notable strengthening of many--body renormalizations.
Many inorganic pigments contain heavy metals hazardous to health and environment. Much attention has been devoted to the quest for non-toxic alternatives based on rare-earth elements. The computation of colors from first principles is a challenge to
electronic structure methods however, especially for materials with localized f-orbitals. Here, starting from atomic positions only, we compute the color of the red pigment cerium fluorosulfide CeSF, as well as of mercury sulfide HgS (classic vermilion). Our methodology employs many-body theories to compute the optical absorption, combined with an intermediate length-scale modelization to assess how coloration depends on film thickness, pigment concentration and granularity. We introduce a quantitative criterion for the performance of a pigment. While for HgS this criterion is satisfied due to large transition matrix elements between wide bands, CeSF presents an alternative paradigm: the bright red color is shown to stem from the combined effect of the quasi two-dimensionality and the localized nature of 4f-states. Our work demonstrates the power of modern computational methods, with implications for the theoretical design of materials with specific optical properties.
We present the first dynamical implementation of the combined GW and dynamical mean field scheme (GW+DMFT) for first principles calculations of the electronic properties of correlated materials. The application to the ternary transition metal oxide S
rVO3 demonstrates that this schemes inherits the virtues of its two parent theories: a good description of the local low energy correlation physics encoded in a renormalized quasi-particle band structure, spectral weight transfer to Hubbard bands, and the physics of screening driven by long-range Coulomb interactions. Our data is in good agreement with available photoemission and inverse photoemission spectra; our analysis leads to a reinterpretation of the commonly accepted three-peak structure as originating from orbital effects rather than from the electron addition peak within the t2g manifold.
Iron based narrow gap semiconductors such as FeSi, FeSb2, or FeGa3 have received a lot of attention because they exhibit a large thermopower, as well as striking similarities to heavy fermion Kondo insulators. Many proposals have been advanced, howev
er, lacking quantitative methodologies applied to this problem, a consensus remained elusive to date. Here, we employ realistic many-body calculations to elucidate the impact of electronic correlation effects on FeSi. Our methodology accounts for all substantial anomalies observed in FeSi: the metallization, the lack of conservation of spectral weight in optical spectroscopy, and the Curie susceptibility. In particular we find a very good agreement for the anomalous thermoelectric power. Validated by this congruence with experiment, we further discuss a new physical picture of the microscopic nature of the insulator-to-metal crossover. Indeed, we find the suppression of the Seebeck coefficient to be driven by correlation induced incoherence. Finally, we compare FeSi to its iso-structural and iso-electronic homologue RuSi, and predict that partially substituted Fe(1-x)Ru(x)Si will exhibit an increased thermopower at intermediate temperatures.
