[PAST EVENT] Physics Colloquium

April 22, 2016
4pm - 5pm
Small Hall, Room 111
300 Ukrop Way
Williamsburg, VA 23185Map this location
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 drastically 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 transition metal dichalcogenide 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(?) is suppressed as a function of the
intercalation-concentration ? and eventually vanishes at a critical value of ?=?c 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 ?c. 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.