Exchange coupling-mediated broken symmetries in Ta 2 NiSe 5 revealed from quadrupolar circular photogalvanic effect.

In low-electron density materials, interactions can lead to highly correlated quantum states of matter. Ta 2 NiSe 5 , 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 2 NiSe 5 using the quadrupole circular photogalvanic effect (QCPGE). Light-matter interaction in Ta 2 NiSe 5 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 2 NiSe 5 can lead to a triclinic structure with a broken C 2 symmetry. Our results provide an incisive probe of the symmetries of the low-temperature phase of Ta 2 NiSe 5 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.