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Electron-hole bound pairs, or excitons, are common excitations in semiconductors. They can spontaneously form and condense into a new insulating ground state—the so-called excitonic insulator—when the energy of electron-hole Coulomb attraction exceeds the band gap. In the presence of electron-phonon coupling, a periodic lattice distortion often concomitantly occurs. However, a similar structural transition can also be induced by electron-phonon coupling itself, therefore hindering the clean identification of bulk excitonic insulators (e.g., which instability is the driving force of the phase transition). Using high-resolution synchrotron x-ray diffraction and angle-resolved photoemission spectroscopy, we identify key electron-phonon coupling effects in a leading excitonic insulator candidate Ta 2 NiSe 5 . These include an extensive unidirectional lattice fluctuation and an electronic pseudogap in the normal state, as well as a negative electronic compressibility in the charge-doped broken-symmetry state. In combination with first principles and model calculations, we use the normal state electronic spectra to quantitatively determine the electron-phonon interaction vertex g and interband Coulomb interaction V in the minimal lattice model, the solution to which captures the experimental observations. Moreover, we show how the Coulomb and electron-phonon coupling effects can be unambiguously separated based on the solution to quantified microscopic models. Finally, we discuss how the strong lattice fluctuations enabled by low dimensionality relate to the unique electron-phonon interaction effects beyond the textbook Born-Oppenheimer approximation. 

New properties and exotic quantum phenomena can form due to periodic nanotextures, including Moire patterns, ferroic domains, and topologically protected magnetization and polarization textures. Despite the availability of powerful tools to characterize the atomic crystal structure, the visualization of nanoscale strain-modulated structural motifs remains challenging. Here, we develop nondestructive real-space imaging of periodic lattice distortions in thin epitaxial films and report an emergent periodic nanotexture in a Mott insulator. Specifically, we combine iterative phase retrieval with unsupervised machine learning to invert the diffuse scattering pattern from conventional X-ray reciprocal-space maps into real-space images of crystalline displacements. Our imaging in PbTiO₃/SrTiO₃superlattices exhibiting checkerboard strain modulation substantiates published phase-field model calculations. Furthermore, the imaging of biaxially strained Mott insulator Ca₂RuO₄reveals a strain-induced nanotexture comprised of nanometer-thin metallic-structure wires separated by nanometer-thin Mott-insulating-structure walls, as confirmed by cryogenic scanning transmission electron microscopy (cryo-STEM). The nanotexture in Ca₂RuO₄film is induced by the metal-to-insulator transition and has not been reported in bulk crystals. We expect the phasing of diffuse X-ray scattering from thin crystalline films in combination with cryo-STEM to open a powerful avenue for discovering, visualizing, and quantifying the periodic strain-modulated structures in quantum materials.

 

During a band-gap-tuned semimetal-to-semiconductor transition, Coulomb attraction between electrons and holes can cause spontaneously formed excitons near the zero-band-gap point, or the Lifshitz transition point. This has become an important route to realize bulk excitonic insulators – an insulating ground state distinct from single-particle band insulators. How this route manifests from weak to strong coupling is not clear. In this work, using angle-resolved photoemission spectroscopy (ARPES) and high-resolution synchrotron x-ray diffraction (XRD), we investigate the broken symmetry state across the semimetal-to-semiconductor transition in a leading bulk excitonic insulator candidate system Ta₂Ni(Se,S)₅. A broken symmetry phase is found to be continuously suppressed from the semimetal side to the semiconductor side, contradicting the anticipated maximal excitonic instability around the Lifshitz transition. Bolstered by first-principles and model calculations, we find strong interband electron-phonon coupling to play a crucial role in the enhanced symmetry breaking on the semimetal side of the phase diagram. Our results not only provide insight into the longstanding debate of the nature of intertwined orders in Ta₂NiSe₅, but also establish a basis for exploring band-gap-tuned structural and electronic instabilities in strongly coupled systems.

 

Actinide materials exhibit strong spin–lattice coupling and electronic correlations, and are predicted to host new emerging ground states. One example is piezomagnetism and magneto-elastic memory effect in the antiferromagnetic Mott-Hubbard insulator uranium dioxide, though its microscopic nature is under debate. Here, we report X-ray diffraction studies of oriented uranium dioxide crystals under strong pulsed magnetic fields. In the antiferromagnetic state a [888] Bragg diffraction peak follows the bulk magnetostriction that expands under magnetic fields. Upon reversal of the field the expansion turns to contraction, before the [888] peak follows the switching effect and piezomagnetic ‘butterfly’ behaviour, characteristic of two structures connected by time reversal symmetry. An unexpected splitting of the [888] peak is observed, indicating the simultaneous presence of time-reversed domains of the 3-k structure and a complex magnetic-field-induced evolution of the microstructure. These findings open the door for a microscopic understanding of the piezomagnetism and magnetic coupling across strong magneto-elastic interactions.

 

Ferroquadrupole order associated with local$4f$atomic orbitals of rare-earth ions is a realization of electronic nematic order. However, there are relatively few examples of intermetallic materials which exhibit continuous ferroquadrupole phase transitions, motivating the search for additional materials that fall into this category. Furthermore, it is not clear a priori whether experimental approaches based on transport measurements which have been successfully used to probe the nematic susceptibility in materials such as the Fe-based superconductors will be as effective in the case of$4f$intermetallic materials, for which the important electronic degrees of freedom are local rather than itinerant and are consequently less strongly coupled to the charge-carrying quasiparticles near the Fermi energy. In the present work, we demonstrate that the intermetallic compound${\mathrm{Y}\mathrm{b}\mathrm{R}\mathrm{u}}_2{\mathrm{G}\mathrm{e}}_2$exhibits a tetragonal-to-orthorhombic phase transition consistent with ferroquadrupole order of the Yb ions and go on to show that elastoresistivity measurements can indeed provide a clear window on the diverging nematic susceptibility in this system. This material provides an arena in which to study the causes and consequences of electronic nematicity.