Thermopower of the Correlated Narrow Gap Semiconductor FeSi and Comparison to RuSi

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, however, 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.Comment: 14 pages. Proceedings of the Hvar 2011 Workshop on 'New materials for thermoelectric applications: theory and experiment

Momentum-resolved lattice dynamics of parent and electron-doped Sr2IrO4

The mixing of orbital and spin character in the wavefunctions of the 5d iridates has led to predictions of strong couplings between their lattice, electronic and magnetic degrees of freedom. As well as realising a novel spin-orbit assisted Mott-insulating ground state, the perovskite iridate Sr2IrO4 has strong similarities with the cuprate La2CuO4, which on doping hosts a charge density wave that appears intimately connected to high-temperature superconductivity. These phenomena can be sensitively probed through momentum-resolved measurements of the lattice dynamics, made possible by meV-resolution inelastic x-ray scattering. Here we report the first such measurements for both parent and electron-doped Sr2IrO4. We find that the low-energy phonon dispersions and intensities in both compounds are well described by the same non-magnetic DFT calculation. In the parent compound, no changes of the phonons upon magnetic ordering are discernible within the experimental resolution, and in the doped compound no anomalies are apparent due to charge density waves. These measurements extend our knowledge of the lattice properties of (Sr1-xLax)2IrO4, and constrain the couplings of the phonons to magnetic and charge order

High-pressure insulator-to-metal transition in Sr3Ir2O7 studied by x-ray absorption spectroscopy

High-pressure x-ray absorption spectroscopy was performed at the Ir L3 and L2 absorption edges of Sr3Ir2O7. The branching ratio of white-line intensities continuously decreases with pressure, reflecting a reduction in the angular part of the expectation value of the spin-orbit coupling operator, (L·S). Up to the high-pressure structural transition at 53 GPa, this behavior can be explained within a single-ion model, where pressure increases the strength of the cubic crystal field, which suppresses the spin-orbit induced hybridization of Jeff=3/2 and eg levels. We observe a further reduction of the branching ratio above the structural transition, which cannot be explained within a single-ion model of spin-orbit coupling and cubic crystal fields. This change in (L·S) in the high-pressure, metallic phase of Sr3Ir2O7 could arise from noncubic crystal fields or a bandwidth-driven hybridization of Jeff=1/2,3/2 states and suggests that the electronic ground state significantly deviates from the Jeff=1/2 limit

Data for "High-pressure insulator-to-metal transition in Sr3Ir2O7 studied by x-ray absorption spectroscopy"

Dataset for the article "High-pressure insulator-to-metal transition in Sr3Ir2O7 studied by x-ray absorption spectroscopy", C. Donnerer, M. Moretti Sala, S. Pascarelli, A. D. Rosa, S. N. Andreev, V. V. Mazurenko, T. Irifune, E. C. Hunter, R. S. Perry, and D. F. McMorrow, Phys. Rev. B (2018

Measuring the dzyaloshinskii-moriya interaction in a weak ferromagnet

Contains fulltext : 128403.pdf (preprint version ) (Open Access

The electronic structure of the high-symmetry perovskite iridate Ba2IrO4

We report angle-resolved photoemission (ARPES) measurements, density functional and model tight-binding calculations on Ba2IrO4 (Ba-214), an antiferromagnetic (T-N = 230 K) insulator. Ba-214 does not exhibit the rotational distortion of the IrO6 octahedra that is present in its sister compound Sr2IrO4 (Sr-214), and is therefore an attractive reference material to study the electronic structure of layered iridates. We find that the band structures of Ba-214 and Sr214 are qualitatively similar, hinting at the predominant role of the spin-orbit interaction in these materials. Temperature-dependent ARPES data show that the energy gap persists well above TN, and favor a Mott over a Slater scenario for this compound.open112625sciescopu