Exchange coupling–mediated broken symmetries in Ta <sub>2</sub> NiSe <sub>5</sub> revealed from quadrupolar circular photogalvanic effect
Harshvardhan Jog, Luminita Harnagea, E. J. Melé, Ritesh Agarwal
Abstract
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.