Condensed Matter Seminar: "Spectroscopy of Correlated Electronic States in Single Crystals of CuxTiSe2."

Speaker Goran Karapetrov, Drexel University
Date: 1/27/2017
Time: 1 p.m.

ESB 190


Physics - Condensed Matter

Event Type: Seminar/Symposium

TiSe2 is a member of transition metal dichalcogenide family of layered van der Waals materials that has attracted significant interest due to the intricate mechanism that drives its transition from a semiconducting to a charge density wave (CDW) phase. Intercalation of TiSe2 with copper donates electrons to the conduction band near the Fermi surface, producing a metallic phase at room temperature and a superconducting phase at low temperatures with highest superconducting critical temperature of 4.15K. The relationships between these phases are nontrivial, especially considering that both the CDW and superconducting phases rely on specific electronic and electron-phonon interactions.

We investigate the transient optical response of electronic states in CuxTiSe2as a function of temperature and copper doping from x=0 (semimetal and commensurate charge density wave phases) to x=0.08 (metallic and superconducting phases). We find that the cooperative driving mechanisms for the CDW, the excitonic insulator mechanism and the soft phonon mode, decouple at x=0.04, where fluctuations of a quantum critical point were observed in the folded Se-4p band. We also demonstrate a loss of coherence in the A1g phonon signal with increased copper intercalation of the parent lattice, indicating a loss of long-range lattice order. These findings provide compelling evidence that TiSe2 undergoes a quantum phase transition upon Cu intercalation from a state of commensurate charge order without superconductivity to a state with a different symmetry in which new charge order coexists with the superconducting phase.

Superconductivity in TiSe2 is very intriguing by itself as it appears with copper or palladium intercalation or under pressure in the intrinsic system. We will discuss the results obtained in CuxTiSe2 system and the conclusions drawn from complex set of STM/STS and thermodynamic bulk measurements on single crystals.

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