Phase Diagrams of the Excitonic Insulator State: Analyzing the Excitonic Susceptibility

In this paper, the formation of the excitonic insulator state in the rare-earth chalcogenides hasbeen investigated through the extended Falicov-Kimball model. Adapting the unrestricted Hartree-Fockapproximation, we have derived a set of explicitly self-consistent equations determining expectationvalues and the excitonic susceptibility in the system. Analyzing the excitonic susceptibility, we haveestablished phase diagrams of the excitonic insulator state depending on the model parameters. The phasestructures confirmed the excitonic insulator state is found at low temperature and between two criticalvalues of the Coulomb interaction. The effect of the external pressure on the formation of the excitonicinsulator state is also shown.

Electrical Properties in Ta2NiSe5 Film and van der Waals Heterojunction

Ternary Ta2NiSe5 is a novel electronic material having the property of an excitonic insulator at room temperature. The electrical properties of Ta2NiSe5 have not been elucidated in detail. We discuss the electronic properties in Ta2NiSe5 films and the formation of heterojunctions. Hall effect measurements showed p-type conductivity. The activation energies estimated from the temperature dependence of the carrier concentration were seen to be 0.17 eV and 0.12 eV, at approximately 300 and 400 K, respectively. It was observed that carrier generation behavior changes at the critical temperature of the excitonic insulator state (328 K). The temperature dependence of the Hall mobility below the critical temperature nearly follows the bell-shaped curves for conventional semiconductor materials. A MoS2/Ta2NiSe5 van der Waals heterojunction was fabricated using the transfer method. Rectification characteristics, which depend on the gate bias voltage, were obtained. The barrier height at the MoS2/Ta2NiSe5 heterointerface and the on/off ratio could be modulated by applying a gate bias voltage, suggesting that the carrier transport was exhibited in band-to-band flow. Our demonstration suggests that the knowledge of Ta2NiSe5 increased as an electronic material, and diode performance was successfully achieved for the electronic device applications.

Common microscopic origin of the phase transitions in Ta2NiS5 and the excitonic insulator candidate Ta2NiSe5

The structural phase transition in Ta2NiSe5 has been envisioned as driven by the formation of an excitonic insulating phase. However, the role of structural and electronic instabilities on crystal symmetry breaking has yet to be disentangled. Meanwhile, the phase transition in its complementary material Ta2NiS5 does not show any experimental hints of an excitonic insulating phase. We present a microscopic investigation of the electronic and phononic effects involved in the structural phase transition in Ta2NiSe5 and Ta2NiS5 using extensive first-principles calculations. In both materials the crystal symmetries are broken by phonon instabilities, which in turn lead to changes in the electronic bandstructure also observed in the experiment. A total energy landscape analysis shows no tendency towards a purely electronic instability and we find that a sizeable lattice distortion is needed to open a bandgap. We conclude that an excitonic instability is not needed to explain the phase transition in both Ta2NiSe5 and Ta2NiS5.

Dipolar excitonic insulator in a moiré lattice

Two-dimensional (2D) moiré materials provide a highly-controllable solid-state platform for studies of correlated quantum phenomena. To date, experimental studies have focused on the correlated electronic states; the correlated bosonic states in moiré materials have remained practically unexplored. Here, we report a correlated dipolar excitonic insulator--a charge insulating state driven by exciton formation--in a Coulomb-coupled WSe2 monolayer and WSe2/WS2 moiré bilayer at total hole doping density equal to the moiré density. The system is a Mott insulator when all the holes reside in the moiré layer. Under an out-of-plane electric field, the holes can be continuously transferred to the WSe2 monolayer, but remain strongly bound to the empty moiré sites. This is effectively an interlayer exciton fluid in the moiré lattice under a particle-hole transformation. We identify the phase space and determine the charge gap energy of the excitonic insulating state by optical spectroscopy and capacitance measurements, respectively. We further observe the emergence of local magnetic moments in the WSe2 monolayer induced by the strong interlayer Coulomb correlation. Our result paves the path for realizing correlated bosonic quantum phenomena described by the Bose-Hubbard model in a solid-state system.