Doping-controlled transition from excitonic insulator to semimetal in Ta${}_2$NiSe${}_5$

Abstract

Excitonic insulator (EI) is an intriguing insulating phase of matter, where electrons and holes are bonded into pairs, so called excitons, and form a phase-coherent state via Bose-Einstein Condensation (BEC). Its theoretical concept has been proposed several decades ago, but the followed research is very limited, due to the rare occurrence of EI in natural materials and the lack of manipulating method of excitonic condensation. In this paper, we report the realization of a doping-controlled EI-to-semi-metal transition in Ta${}_2$NiSe${}_5$ using $in$-$situ$ potassium deposition. Combining with angle-resolved photoemission spectroscopy (ARPES), we delineate the evolution of electronic structure through the EI transition with unprecedented precision. The results not only show that Ta${}_2$NiSe${}_5$ (TNS) is an EI originated from a semi-metal non-interacting band structure, but also resolve two sequential transitions, which could be attributed to the phase-decoherence and pair-breaking respectively. Our results unveil the Bardeen-Cooper-Schrieffer (BCS)-BEC crossover behavior of TNS and demonstrate that its band structure and excitonic binding energy can be tuned precisely via alkali-metal deposition. This paves a way for investigations of BCS-BEC crossover phenomena, which could provide insights into the many-body physics in condensed matters and other many-body systems.

arXiv comment5 pages, 4 figures, See Supplemental Material at http://link.aps.org/supplemental/ 10.1103/PhysRevB.102.161116

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• A New Era of Excitonic Insulators
• Spontaneous gap opening and potential excitonic states in an ideal Dirac semimetal Ta${}_2$Pd${}_3$Te${}_5$
• Terahertz parametric amplification as a reporter of exciton condensate dynamics
• A gate-tunable quantum phase transition in a topological excitonic insulator
• Tunneling-tip-induced collapse of the charge gap in the excitonic insulator Ta${}_2$NiSe${}_5$
• Thermal transport signatures of the excitonic transition and associated phonon softening in the layered chalcogenide Ta${}_2$NiSe${}_5$
• Pressure suppression of the excitonic insulator state in Ta2NiSe5 observed by optical conductivity
• Theory of time-crystalline behaviour mediated by phonon squeezing in Ta2NiSe5
• Carrier-Density Control of the Quantum-Confined 1$T$-TiSe${}_2$ Charge-Density-Wave
• Gate tuning of coupled electronic and structural phase transition in atomically thin Ta${}_2$NiSe${}_5$
• Strong long-wavelength electron-phonon coupling in Ta${}_2$Ni(Se,S)${}_5$
• Insensitivity of the striped charge-orders in IrTe${}_2$ to alkali surface doping implies their structural origin
• Comment on "Direct observation of excitonic instability in Ta${}_2$NiSe${}_5$" [arXiv:2007.08212 (2020)]
• Excitonic Insulator and the Extended Falicov--Kimball Model Away from Half-Filling