Condensed Matter Physics Seminar

Lance Cooper
University Of Illinois, Urbana Champaign

Monday, May 12, 2008
1:30 pm in Jadwin Hall, Room A09

Spectroscopic studies of the quantum phase transitions in TiSe2 and CuxTiSe2

Abstract: A major recent focus of condensed matter research concerns the relationship between diverse quantum phases - e.g., between antiferromagnetic or charge-density wave (CDW) and superconductor (SC) phases - in various strongly correlated materials such as the high Tc cuprates and the layered ruthenates. In this talk, I'll focus on the remarkable competition among novel phases observed as a function of both pressure and intercalation in a less-well-known system, 1T-TiSe2, which is a semimetal or small-gap semiconductor that develops a commensurate CDW with a 2a×2a×2c superlattice structure at temperatures below a second-order phase transition at TCDW ~ 200K.[1] Our low temperature, pressure-dependent inelastic light scattering studies of the critical ('soft') mode in 1T-TiSe2 indicate that lattice compression leads to quantum (T ~ 0) melting of the CDW phase through a novel incommensurate phase that may have hexatic order. More recently, Morosan et al. [1] discovered that lattice expansion of 1T-TiSe2 via copper intercalation (i.e., CuxTiSe2) leads to the suppression of the CDW transition temperature, and the emergence near x=0.04 of a SC phase having a maximum Tc of 4.15 K at x=0.08. Our light scattering studies of the soft mode in CuxTiSe2 provide evidence for x-dependent ‘quantum’ (T~0) mode softening in CuxTiSe2 that is consistent with a quantum critical point hidden in the superconductor phase of CuxTiSe2, suggesting that there is a coexistence of fluctuating CDW order and superconductivity in this system. [1]. E. Morosan et al., Nature Phys. 2, 544 (2006).