Zero-gap semiconductor to excitonic insulator transition in Ta2NiSe5

The excitonic insulator is a long conjectured correlated electron phase of narrow-gap semiconductors and semimetals, driven by weakly screened electron–hole interactions. Having been proposed more than 50 years ago, conclusive experimental evidence for its existence remains elusive. Ta₂NiSe₅ is a narrow-gap semiconductor with a small one-electron bandgap E_G of <50 meV. Below T_C=326 K, a putative excitonic insulator is stabilized. Here we report an optical excitation gap E_op ∼0.16 eV below T_C comparable to the estimated exciton binding energy E_B. Specific heat measurements show the entropy associated with the transition being consistent with a primarily electronic origin. To further explore this physics, we map the T_C–E_G phase diagram tuning E_G via chemical and physical pressure. The dome-like behaviour around E_G∼0 combined with our transport, thermodynamic and optical results are fully consistent with an excitonic insulator phase in Ta₂NiSe₅.

The nature of an insulating phase in Ta₂NiSe₅ is an open question. Here, Lu et al. report transport, thermodynamic and optical evidences being fully consistent with an excitonic insulator phase in this material.

Quantum oscillations in the anomalous phase in Sr3Ru2O7

We report measurements of quantum oscillations detected in the putative nematic phase of Sr3Ru2O7. Improvements in sample purity enabled the resolution of small amplitude de Haas-van Alphen (dHvA) oscillations between two first order metamagnetic transitions delimiting the phase. Two distinct frequencies were observed, whose amplitudes follow the normal Lifshitz-Kosevich profile. Variations of the dHvA frequencies are explained in terms of a chemical potential shift produced by reaching a peak in the density of states, and an anomalous field dependence of the oscillatory amplitude provides information on domains.