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1. The spontaneous symmetry breaking in Ta ₂ NiSe ₅ is structural in nature

   https://doi.org/10.1073/pnas.2221688120  

   Baldini, Edoardo; Zong, Alfred; Choi, Dongsung; Lee, Changmin; Michael, Marios H.; Windgaetter, Lukas; Mazin, Igor I.; Latini, Simone; Azoury, Doron; Lv, Baiqing; et al (April 2023, Proceedings of the National Academy of Sciences)

   The excitonic insulator is an electronically driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta 2 NiSe 5 being the most promising. Here, we test this scenario by using an ultrashort laser pulse to quench the broken-symmetry phase of this transition metal chalcogenide. Tracking the dynamics of the material’s electronic and crystal structure after light excitation reveals spectroscopic fingerprints that are compatible only with a primary order parameter of phononic nature. We rationalize our findings through state-of-the-art calculations, confirming that the structural order accounts for most of the gap opening. Our results suggest that the spontaneous symmetry breaking in Ta 2 NiSe 5 is mostly of structural character, hampering the possibility to realize quasi-dissipationless energy transport. 

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2. Phase‐Change Materials for Volatile Threshold Resistive Switching and Neuronal Device Applications

   https://doi.org/10.1002/advs.202503209  

   Chen, Huandong; Ravichandran, Jayakanth (November 2025, Advanced Science)

   Abstract Volatile threshold resistive switching and neuronal oscillations in phase‐change materials, specifically those undergoing ‘metal‐to‐insulator’ transitions, offer unique attributes such as fast and low‐field volatile switching, tunability, and stochastic dynamics. These characteristics are particularly promising for emulating neuronal behaviors and solving complex computational problems. In this review, we summarize recent advances in the development of volatile resistive switching devices and neuronal oscillators based on three representative materials with coincident electronic and structural phase transitions, at different levels of technological readiness: the well‐studied correlated oxide VO₂, the charge‐density‐wave transition metal dichalcogenide 1T‐TaS₂, and the emerging phase‐change complex chalcogenide BaTiS₃. We discuss progresses from the perspective of materials development and device implementation. Finally, we emphasize the major challenges that must be addressed for practical applications of these phase‐change materials and provides outlook on the future research directions in this rapidly evolving field. 

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3. Intercalation‐Induced Phase Transitions in Ferroelectric α‐In ₂ Se ₃

   https://doi.org/10.1002/advs.202513712  

   He, Xin; Gong, Zhihao; Wang, Tao; Wang, Baoyu; Liu, Chen; Wang, Ding; Ma, Yinchang; Feng, Pu; Zhang, Chenhui; Hu, Weijin; et al (December 2025, Advanced Science)

   Abstract Specific ions can be intercalated into functional materials using the electrolyte gating technique, which has been widely used to regulate channel conductance in transistors and develop low‐power neuromorphic devices. However, in these devices, fundamental exploration of ion intercalation‐induced structural phase transitions remains largely overlooked and rarely explored. Here, the lithium‐based electrolyte gating technique is used to probe the collective interactions between ions, lattices, and electrons in a van der Waals ferroelectric semiconductor α‐In₂Se₃. Using a polymer electrolyte as the lithium‐ion reservoir and α‐In₂Se₃as the channel material, the intercalated lithium concentration via a gate electric field is modulated. This manipulation drives a phase transition in α‐In₂Se₃from a ferroelectric semiconductor to a dirty metal and finally to a metal, accompanied by a structural transformation. Concurrently, with enhanced intercalation, the ferroelectric hysteresis window progressively narrows and eventually disappears, indicating the evolution from switchable to non‐switchable polarization. This study represents a promising platform for the artificial construction of correlated material systems, enabling a systematic investigation into the interaction of ferroelectricity and electronic conduction using ion intercalation. 

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4. Pressure-induced phase transitions and superconductivity in a quasi–1-dimensional topological crystalline insulator α-Bi ₄ Br ₄

   https://doi.org/10.1073/pnas.1909276116  

   Li, Xiang; Chen, Dongyun; Jin, Meiling; Ma, Dashuai; Ge, Yanfeng; Sun, Jianping; Guo, Wenhan; Sun, Hao; Han, Junfeng; Xiao, Wende; et al (September 2019, Proceedings of the National Academy of Sciences)

   Significance The quasi–1-dimensional bismuth bromide, α-Bi₄Br₄, has been predicted to be a rotational symmetry-protected topological crystalline insulator. The structural study under high pressure indicates that the α-Bi₄Br₄phase is stable up to 4.3 GPa. There is a rich phase diagram of physical properties under high pressure in the α-Bi₄Br₄phase (i.e., a pressure-induced insulator–metal transition and, most importantly, a superconductive phase near the boundary of the insulator–metal transition). These findings help to answer questions, such as whether it is possible for the symmetry-protected electrons to form Cooper pairs. The α-Bi₄Br₄undergoes a pressure-induced structural transition above 4.3 GPa to a triclinicP-1 phase, which is another superconductive phase. 

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5. Directly measuring the structural transition pathways of strain-engineered VO ₂ thin films

   https://doi.org/10.1039/d0nr04776g  

   Evlyukhin, Egor; Howard, Sebastian A.; Paik, Hanjong; Paez, Galo J.; Gosztola, David J.; Singh, Christopher N.; Schlom, Darrell G.; Lee, Wei-Cheng; Piper, Louis F. (September 2020, Nanoscale)

   null (Ed.)

   Epitaxial films of vanadium dioxide (VO 2 ) on rutile TiO 2 substrates provide a means of strain-engineering the transition pathways and stabilizing of the intermediate phases between monoclinic (insulating) M1 and rutile (metal) R end phases. In this work, we investigate structural behavior of epitaxial VO 2 thin films deposited on isostructural MgF 2 (001) and (110) substrates via temperature-dependent Raman microscopy analysis. The choice of MgF 2 substrate clearly reveals how elongation of V–V dimers accompanied by the shortening of V–O bonds triggers the intermediate M2 phase in the temperature range between 70–80 °C upon the heating–cooling cycles. Consistent with earlier claims of strain-induced electron correlation enhancement destabilizing the M2 phase our temperature-dependent Raman study supports a small temperature window for this phase. The similarity of the hysteretic behavior of structural and electronic transitions suggests that the structural transitions play key roles in the switching properties of epitaxial VO 2 thin films. 

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