Magnetic field control of the excitonic transition in Ta$_2$NiSe$_5$

Abstract

The formation of excitonic insulator phases in quantum materials is often masked by structural distortions caused by the coupling between electronic and phononic order parameters. Here we show that the candidate material Ta$_2$NiSe$_5$ is characterized by a metastable excitonic insulating phase that is decoupled from the lattice, and that can be stabilized for sufficiently high applied magnetic fields. By considering the interplay between the excitonic and structural instabilities, we predict a magnetic field induced transition from the low-temperature structurally distorted semiconducting phase to an undistorted excitonic insulator phase with ground state loop currents. Before the transition, the existence of a latent excitonic phase can be detected by the magnetic field softening of the phonon mode associated with the structural distortion. These results highlight an unbiased route towards the disentanglement of the coupled excitonic-structural transition in Ta$_2$NiSe$_5$, and uncover a general mechanism for magnetic field control of competing phases in quantum materials.