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.

Figures (5)

• Figure 1: (a) Ta-Ni-Ta chain structure in the high-temperature orthorombic phase. The orange links schematically represent the charge distribution on the bonds. Shaded area highlights the unit cell. The blue arrows indicates the Ta-shear mode distortion. $a \simeq 3.5~{\rm \buildrel_{\circ}\over{\mathrm{A}}}$ and $b \simeq 3.9~{\rm \buildrel_{\circ}\over{\mathrm{A}}}$ are the lattice parameters of the chain. (b) Low-energy band structure for the high-temperature phase tight-binding model. Blue/red fat lines indicate the Ta/Ni characters of the bands. (c) Zero temperature phase diagram as a function of the perpendicular field $B$, and of the electron-phonon coupling $g$. The hatched area indicates a coexistence region.
• Figure 2: (a)-(b) Real $\Psi_{\rm R}$ and imaginary $\Psi_{\rm I}$ parts of the excitonic order parameter obtained using seeds $\Psi_{\rm seed} = 0.2e^{i \varphi_{\rm seed}}$ with $\varphi_{\rm seed}=0$ (a) and $\varphi_{\rm seed}=\pi/2$ (b). (c) Condensation energies for the $\varphi=0$ and $\varphi = \pi/2$ phases. (d) Relative energy difference $\Delta E_c = E_c(\varphi=0) - E_c(\varphi=\pi/2)$ normalized to $E_c(0)$. (e)-(f) Low-energy bands in the $\varphi=0$ (e) and $\varphi=\pi/2$ (f) cases at $V=1.0~{\rm eV}$ (g)-(h) Sketches of the charge density and the bond current distributions in the $\varphi=0$ (g) and $\varphi=\pi/2$ (h) phases.
• Figure 3: (a) Condensation energy of the CD and LC phases as a function of the dimensional electron-phonon coupling. (b) Displacement of the Ta-atom with respect to the equilibrium position in the orthorombic phase. For the LC phase, the dashed lines indicate that, for the corresponding values of $g$, the solution is unstable.
• Figure 4: (a) Orbital magnetic moment as a function of the applied magnetic field $B$, and for different values of $g$. The dashed magenta lines corresponds to the magnetic moment of the LC phase obtained by imposing $\delta_{\rm Ta} =0$. (b) Static displacement of the Ta-atoms as a function of the applied fields and the same values of $g$ shown in (a).
• Figure 5: Phonon frequency as a function of the magnetic field and different values of electron-phonon coupling, and $V=1.0~{\rm eV}$. For each $g$, the green and magenta symbols corresponds, respectively, to the structurally distorted semiconductor and LC excitonic insulator phase. The dots and the vertical dashed lines indicate the critical fields $B_c$.