Growth and Characterization of Rubidium Copper Sulfide

In this thesis I describe the synthesis and physical properties of a novel ternary compound RbCu4S3. The material is a bilayer variant of the familiar ThCr2Si2 structure (adopted, for instance, by several members of the iron-pnictide family of superconductors, including BaFe2As2 [1]). Interest in this quasi-low-dimensional compound revolves around its potential to harbor an electronic instability, for example a charge density wave. After establishing a suitable procedure to grow high quality single crystals, my ongoing experiments have probed the nature of the RbCu4S3 ground state via a combination of transport and thermodynamic measurements. The results of these experiments have been compared with band structure calculations performed using the WIEN2K software package. Additional x-ray diffraction and transmission electron microscopy (TEM) structural characterization has been performed by collaborators.

The calculated Fermi surface of RbCu4S3 reveals one three-dimensional pocket and two quasi-two-dimensional pockets with considerable potential for nesting. However, neither single crystal x-ray diffraction refinements nor high resolution TEM measurements indicate the presence of a lattice modulation in this material. The electrical resistivity is that of a good metal, with a very low residual resistivity and a correspondingly high residual resistance ratio (RRR[typical] = [rho](300K)/[rho] naught >=600). Such high quality samples permit the observation of quantum oscillatory effects in high magnetic fields, providing a direct probe of the Fermi surface morphology and the quasiparticle effective mass. Preliminary measurements of Shubnikov-de Haas oscillations in the magnetoresistance for fields oriented along the crystalline c-axis reveal several frequencies corresponding to different extremal orbits of the various Fermi surface pockets. Comparison with the calculated band structure leads to a tentative assignment for at least one of these orbits with the three-dimensional hole pocket, but additional angle-dependent measurements will be necessary to make an unambiguous comparison. Heat capacity measurements yield a Sommerfeld coefficient [gamma] = 13:1 +/- .04 mJ/mol K2, which is larger by a factor of ~ 1.6 over that estimated from our band structure calculations, presumably reflecting mass renormalization due to many-body effects. Taken together, the preponderance of the evidence suggests that RbCu4S3 does not suffer an electronic instability, although further experiments will be necessary to conclusively establish whether or not this is the case.