Mid-Atlantic Section Fall Meeting 2020
Volume 65, Number 20
Friday–Sunday, December 4–6, 2020;
Virtual
Session K05: Electronic Structure Theory
11:30 AM–1:30 PM,
Sunday, December 6, 2020
Chair: Jedediah Pixley, Rutgers University
Abstract: K05.00002 : Critical charge fluctuations and emergent coherence in a strongly correlated excitonic insulator*
12:06 PM–12:42 PM
Preview Abstract
Abstract
Author:
Pavel Volkov
(Rutgers University)
Attraction between electrons and holes in semiconductors or semimetals can
drive a transition to a macroscopically coherent state, characterized by a
proliferation of particle-hole pairs -- the excitonic insulator (EI). With
only a few candidate materials known, formation of the EI breaks lattice
symmetries, which makes it challenging to distinguish it from a structural
transition. Recently, the attention has been attracted to the possible EI
transition in the candidate material Ta$_{\mathrm{2}}$NiSe$_{\mathrm{5}}$
[1]; however, a structural origin of the transition has been also proposed
[2]. I will present the results of an experimental study of
Ta$_{\mathrm{2}}$NiSe$_{\mathrm{5\thinspace }}$by means of
polarization-resolved Raman scattering, which allows to selectively probe
the excitations with the symmetry of the order parameter, yielding access to
the critical soft mode of the transition. We reveal an overdamped electronic
collective mode, consistent with excitonic fluctuations in a semimetal, that
softens close to the transition temperature. At the same time, the optical
phonons do not soften, ruling out their role in the transition. Furthermore,
on cooling, an emergence of a many-body gap is observed with signatures of
coherent superpositions of band states at the gap edge. Its temperature
dependence shows strong departures from mean-field theory, bearing analogy
with that observed in strongly coupled fermionic superfluids. Finally, I
will present the results on the evolution of the transition in the presence
of gradual chemical substitution of Se by S, which allows to probe the
effect of band structure changes on the EI.
[1] Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009); Y.F. Lu et al.,
Nat. Commun. 8, 14408 (2017)
[2] E. Baldini et al., arXiv:2007.02909; A. Subedi, Phys. Rev. Materials 4,
083601 (2020).
[3] P. A. Volkov et al., arXiv:2007.07344
*Work was performed in collaboration with M. Ye, H. Lohani, I. Feldman, A. Kanigel, and G. Blumberg. Research at Rutgers was supported by NSF Grant No. DMR-1709161.