Similar Pseudogap Physics in Intercalated Transition Metal Dichalcogenide CDW Materials and Cuprate HTSCs

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(δ)  is  suppressed  as  a  function  of  the
intercalation-concentration δ and eventually vanishes at a critical value of δ=δc 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 δc. 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