CME Seminar - TODAY - Utpal Chatterjee, University of Virginia - 11:30am, Smith Seminar Room (1080 PRB)

[Longbrake, Patricia]

Please join us today for the CME Seminar presented by Utpal Chatterjee from the University of Virginia at 11:30am in the Smith Seminar Room (1080 PRB).

"Phase incoherence driven melting of CDW order across quantum and thermal phase transitions in intercalated 2H-NbSe2 system"

Charge density waves (CDWs) and superconductivity are canonical examples of symmetry breaking in materials. Both are characterized by a complex order parameter - namely an amplitude and a phase. In the limit of weak coupling and in the absence of disorder, the formation of pairs (electron-electron for superconductivity, electron-hole for CDWs) and the establishment of macroscopic phase coherence both occur at the transition temperature Tc that marks the onset of long-range order. But, the situation may be different at strong coupling or in the presence of disorder. We have performed extensive experimental investigations on pristine and intercalated samples of 2H-NbSe2, a CDW material with strong electron-phonon coupling, using a combination of structural (X-ray), spectroscopic (photoemission and tunnelling) and transport probes. We find that Tc(d) is suppressed as a function of the intercalation-concentration d and eventually vanishes at a critical value of d=dc leading to quantum phase transition (QPT). Our integrated approach provides clear signatures that the phase of the order parameter becomes incoherent at the quantum/ thermal phase transition, although the amplitude remains finite over an extensive region above Tc or beyond dc. This leads to the persistence of a gap in the electronic spectra in the absence of long-range order, a phenomenon strikingly similar to the so-called pseudogap in completely different systems such as high temperature superconductors, disordered superconducting thin films and cold atoms.

Thank you,
Trisch
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