Exchange coupling-mediated broken symmetries in Ta<sub>2</sub>NiSe<sub>5</sub> revealed from quadrupolar circular photogalvanic effect.
basic_science · Level V
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- Record sourced from PubMed, PMID 35171672.
- Also identified by DOI 10.1126/sciadv.abl9020 and PMC identifier 8849275.
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Abstract
In low-electron density materials, interactions can lead to highly correlated quantum states of matter. Ta<sub>2</sub>NiSe<sub>5</sub>, an excitonic insulator (EI) candidate, exists in a novel broken-symmetry phase below 327 K, characterized by robust exchange interaction and electron-lattice coupling. We study this phase of Ta<sub>2</sub>NiSe<sub>5</sub> using the quadrupole circular photogalvanic effect (QCPGE). Light-matter interaction in Ta<sub>2</sub>NiSe<sub>5</sub> mediated by electric quadrupole/magnetic dipole coupling produces helicity-dependent DC response even with centrosymmetry, making it particularly sensitive to certain other broken symmetries. We show that the exchange interaction in Ta<sub>2</sub>NiSe<sub>5</sub> can lead to a triclinic structure with a broken <i>C</i><sub>2</sub> symmetry. Our results provide an incisive probe of the symmetries of the low-temperature phase of Ta<sub>2</sub>NiSe<sub>5</sub> and add new symmetry constraints to the identification of a strongly correlated EI phase. The high sensitivity of QCPGE to subtle symmetry breaking in centrosymmetric systems will enable its use in studying other complex crystalline systems.