Electronic structure and transport in the potential Luttinger liquids CsNb3Br7S and RbNb3Br7S.

The crystal structures of ANb3Br7S (A = Rb and Cs) have been refined by single crystal X-ray diffraction, and are found to form highly anisotropic materials based on chains of the triangular Nb3 cluster core. The Nb3 cluster core contains seven valence electrons, six of them being assigned to Nb-Nb bonds within the Nb3 triangle and one unpaired d electron. The presence of this surplus electron gives rise to the formation of correlated electronic states. The connectivity in the structures is represented by one-dimensional [Nb3Br7S]- chains, containing a sulphur atom capping one face (µ3) of the triangular niobium cluster, which is believed to induce an important electronic feature. Several types of studies are undertaken to obtain deeper insight into the understanding of this unusual material: the crystal structure, morphology and elastic properties are analysed, as well the (photo-)electrical properties and NMR relaxation. Electronic structure (DFT) calculations are performed in order to understand the electronic structure and transport in these compounds, and, based on the experimental and theoretical results, we propose that the electronic interactions along the Nb chains are sufficiently one-dimensional to give rise to Luttinger liquid (rather than Fermi liquid) behaviour of the metallic electrons.

Formation of a Polar Structure in the Metallic Niobium Sulfide Nb4S3.

The synthesis of Nb4S3, a previously undiscovered binary sulfide, was achieved using Nb3Br7S as a precursor. Its structure is composed of Nb6S triangular prisms arranged in a polar (Imm2) configuration, with sulfur atoms lying in channels along the a axis. Electrical resistivity measurements and density functional theory calculations were used to determine that Nb4S3 is metallic and therefore a polar metal, with metallic bands occupied by electrons with primarily niobium character. The electrons near the Fermi level are so closely associated with the niobium sublattice that the sulfur atoms have positive Born effective charges, indicating that the electrostatic interactions between sulfur atoms are unscreened. Calculations of the dependence of the electron density on the sulfur atomic positions confirm that the metallic electrons do not screen the dipole-dipole interactions between sulfur atoms, which allows polarity and metallicity to coexist in Nb4S3. These findings suggest that applied electric fields might be able to reverse the polarity of thin films of Nb4S3.