Uniaxial-Pressure Boosts Excitonic Fluctuations

Studying the properties of Ta₂NiSe₅ to test the hypothesis that its ground state is an excitonic insulator has been a decade-long research project combining the efforts of our research groups from two departments at the Max Planck Institute for Solid State Research: Solid State Spectroscopy (Bernhard Keimer) and Quantum Materials (Hidenori Takagi), together with the Scientific Facility Crystal Growth. Our recent experiments on Raman scattering under uniaxial compressive strain reveal enhanced excitonic fluctuations accompanied by reduced monoclinic distortions, supporting the excitonic mechanism of the phase transition in Ta₂NiSe₅.

Research on Ta₂NiSe₅ has attracted considerable attention due to its phase transition, presumably associated with exciton condensation, which occurs below 326 K. This transition is characterized by the opening of an energy gap and a reduction in crystal symmetry [1]. The primary cause of this transition is debated to be either electron–hole correlations, consistent with the excitonic-insulator hypothesis, or structural instability, as both factors are thought to play a cooperative role.

Using THz-to-UV spectroscopic ellipsometry, we have discovered an unusual set of excitonic Fano resonances in Ta₂NiSe₅ [2–3], which emerge at the absorption edge as the gap opens. The resonances are optically generated in self-trapped states due to strong interactions with phonons. Their giant spectral weight suggests that the resulting exciton–polaron complexes are highly extended and strongly overlapping along the Ta–Ni chain direction, thereby enhancing excitonic fluctuations while preserving the translational symmetry of the lattice.

The chain structure of Ta₂NiSe₅ enables effective control over its electronic and lattice properties through uniaxial strain. In our recent study, polarization-resolved Raman spectroscopy has demonstrated that the electronic and structural order parameters respond differently to uniaxial pressure applied along the Ta–Ni chains [4]. Compressive strain reduces monoclinic distortions while boosting excitonic fluctuations. This contrasting behavior suggests that collective many-body effects may enhance excitonic fluctuations even when monoclinic distortions are diminished by uniaxial pressure, supporting the excitonic origin of the phase transition. Within the context of our previous hypothesis, the wave function of a weakly bound exciton involves a large polaron region with coherent electronic polarization oscillations, suggesting that such cavity-coupled resonant states could significantly influence the condensation behavior.

[1] Y.F. Lu, H. Kono, T.I. Larkin, A.W. Rost, T. Takayama, A.V. Boris, B. Keimer, and H. Takagil., Nature Commun. 8, 14408 (2017).
[2] T.I. Larkin, A.N. Yaresko, D. Pröpper, K.A. Kikoin, Y.F. Lu, T. Takayama, Y.-L. Mathis, A.W. Rost, H. Takagi, B. Keimer and A.V. Boris, Phys. Rev. B 95, 195144 (2017).
[3] T.I. Larkin, R.D. Dawson, M. Höppner, T. Takayama, M. Isobe, Y.-L. Mathis, H. Takagi, B. Keimer, and A.V. Boris, Phys. Rev. B 98, 125113 (2018).
[4] X. Shi, Y.-S. Zhang, D. Huang, M. Isobe, H. Takagi, B. Keimer, and A. V. Boris, Phys. Rev. Lett, https://doi.org/10.1103/jysr-2dk1