A flat band at the chemical potential of a Fe1.03Te0.94S0.06 superconductor observed by angle-resolved photoemission spectroscopy

The electronic structure of superconducting Fe1.03Te0.94S0.06 has been studied by angle-resolved photoemission spectroscopy (ARPES). Experimental band topography is compared to the calculations using the methods of Korringa-Kohn-Rostoker (KKR) with the coherent potential approximation (CPA) and the linearized augmented plane wave with local orbitals (LAPW+LO) method. The region of the Γ point exhibits two hole pockets and a quasiparticle peak close to the chemical potential (μ) with undetectable dispersion. This flat band with mainly d(z)(2) orbital character is most likely formed by the top of the outer hole pocket or is evidence of a third hole band. It may cover up to 3% of the Brillouin zone volume and should give rise to a Van Hove singularity. Studies performed for various photon energies indicate that at least one of the hole pockets has a two-dimensional character. The apparently nondispersing peak at μ is clearly visible for 40 eV and higher photon energies, due to an effect of the photoionization cross-section rather than band dimensionality. Orbital characters calculated by LAPW+LO for stoichiometric FeTe do not reveal the flat dz(2) band but are in agreement with the experiment for the other dispersions around Γ in Fe1.03Te0.94S0.06.

Evolution from a non-Fermi liquid Kondo lattice to intermediate valence behaviour in CeRhSn(1-x)In(x)

We present investigations of the magnetic and electric transport properties, specific heat, and electronic structure of the intermetallic and strongly correlated system of CeRhSn(1-x)In(x) compounds. The main goal of this paper is to determine the hybridization energy between the f electron and conduction electron states, V(cf), and its influence on the ground state properties of the system. The complementary experimental data are discussed on the basis of the Anderson model for a periodic Kondo lattice. CeRhSn is known as a non-Fermi liquid, while CeRhIn is a valence fluctuating system. We discuss the ground state properties of CeRhSn(1-x)In(x) and compare the results with those obtained for the doped Ce-based Kondo insulators.

Specific heat and magnetic susceptibility of CeNiSn doped with Rh

CeNiSn is known as a semimetallic system with a small pseudogap at the Fermi energy. We investigate the effect of Rh doping on the Kondo insulator CeNiSn by means of measurements of ac magnetic susceptibility and specific heat. We show that the formation of the Kondo insulator narrow gap in CeNi(1 - x)Rh(x)Sn is associated with disorder-induced f-electron localization. For doped CeNiSn with x ≤ 0.06, the electrical resistivity data follow an activation and variable range hopping behaviour at low T, consistent with weak disorder and localization, while C/T is large, which is not a common feature of Kondo insulators. For x > 0.06, the system is metallic and exhibits non-Fermi liquid behaviour with magnetic susceptibility χ ∼ T( - n) with n ∼ 0.4 and electrical resistivity ρ ∼ T.

Electronic structure, magnetic properties and electrical resistivity of the Fe(2)V(1-x)Ti(x)Al Heusler alloys: experiment and calculation

The aim of this work is to investigate electronic structure, magnetic properties and electrical resistivity of Fe(2)V(1-x)Ti(x)Al Heusler alloys. Numerical calculations give a pseudogap at the Fermi level for the majority-spin band of Fe(2)TiAl and a magnetic moment larger than 0.9 μ(B), whereas the ground state of Fe(2)VAl is calculated as a nonmagnetic semimetal with a very low total density of states at the Fermi level. In our calculations the remaining alloys of the Fe(2)V(1-x)Ti(x)Al series are nonmagnetic for x<0.1 and weakly magnetic for 0.1<x≤1. The magnetic moment μ of the series of Fe(2)V(1-x)Ti(x)Al compounds scales with the number of valence electrons and fits well to the Slater-Pauling curve. We also present a study of the electronic transport properties and magnetic susceptibility. The resistivities ρ(T) of Fe(2)VAl and Fe(2)V(0.9)Ti(0.1)Al are large and exhibit a negative temperature coefficient dρ/dT of the resistivity between 2 and 300 K. Below 20 K, ρ(T) also shows an activated character. The magnetic susceptibility of Fe(2)VAl and Fe(2)V(0.9)Ti(0.1)Al shows a maximum at ∼2 K which could reflect either the disorder effect or the hybridization gap, characteristic of Kondo insulators.