Interplay of broken symmetry and delocalized excitations in the insulating state of 1T-TaS_2
arxiv(2022)
摘要
Coexistence of localized and extended excitations is central to the
macroscopic properties of correlated materials. For 5d transition metal
compounds, electron correlations alone generally do not lead to a
metal-insulator (Mott) transition, with insulating behavior usually resulting
from their coupling with magnetic ordering and/or structural distortions.
1T-TaS_2 is a prototypical example of such correlated insulating behavior,
with a high-symmetry metallic phase transforming into a distorted, charge
density wave (CDW) insulating state at low temperatures. The relevance of the
localized electron physics at play in 3d compounds to these 5d transition metal
compounds remains an open question. We resolved this standing controversy in
1T-TaS_2 combining resonant inelastic X-ray spectroscopy and
first-principles calculations. We observed five electronic excitations arising
from the interband transitions of the Ta 5d orbitals and the S 3p ligand state,
with none of the excitations on the order of the Mott gap. These excitations
cannot be explained within the framework of standard multiplet calculations
that assume a localized wavefunction, but instead, are captured by a band
theory framework accounting for the low symmetry of the crystal field in the
CDW state. Our findings suggest that the electronic property of 1T-TaS_2 is
dominated by both plasmonic quasiparticles and inter-band transitions
associated with a Drude-type response, with no resonance associated with a
putative Mott transition. Our discovery provides new insights into the electron
localization and the onset of insulating behavior in 5d transition metal
materials.
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