Jiandong Guo

Conventional (001) compressive strain typically exacerbates octahedral buckling, driving Hund's metals like SrRuO3 into strongly correlated regimes. Here, we circumvent this via (111)-oriented SrRuO3 epitaxial films. Unlike conventional Poisson expansion, the compressed (111) lattice clamps the out-of-plane parameter and suppresses the intrinsic c+ octahedral rotation. The anomalous unbuckling straightens Ru-O-Ru bonds and dilutes effective Hund–Hubbard correlations. O K-edge x-ray absorption spectroscopy confirms this dilution, revealing a direct spectral weight transfer from incoherent multiplet states to the coherent quasiparticle pole. Macroscopically, the Fermi liquid coherence temperature is enhanced to 40 K under −1.8% strain. Furthermore, magnetotransport is consistent with the preservation of the intrinsic momentum-space Berry curvature and ferromagnetic ground state. Ultimately, this (111) geometric unbuckling provides a quantitative pathway to decouple electron correlation and band topology in complex oxides.

Wurtzite AlScN films have emerged as promising ferroelectric layers for next-generation memory and piezoelectric devices due to their high remanent polarization, excellent thermal stability (up to 1 100 °C), scalability to nanometer thickness, and back-end-of-line complementary metal–oxide–semiconductor compatibility. Although numerous efforts have been devoted to synthesizing AlScN epitaxial films, wafer-scale epitaxy (i.e., >2 inches) of AlScN ferroelectric films at very low substrate temperatures (i.e., 0.1°). These AlScN films have high uniformity across the entire 4-inch wafer. More interestingly, the TiN buffer layer can facilitate low-temperature (100 °C) epitaxial growth of AlScN films due to a reduced lattice mismatch, achieving the lowest epitaxy temperature reported thus far. Our work establishes a scalable and low-thermal-budget pathway for synthesizing high-quality AlScN epitaxial films, which can significantly advance the development of AlScN-based electronic devices.

Atomic-scale two-dimensional (2D) ferroelectrics are garnering immense research interest for powering next-generation high-performance nanoelectronics. Herein, we report the discovery of robust room-temperature out-of-plane ferroelectricity in thin 2D β-Bi2SeO5 films, with the ferroelectric response persisting even down to the monolayer limit. The ferroelectricity of a 10 nm thick β-Bi2SeO5 thin film was also unambiguously revealed by ferroelectric analysis using a ferroelectric analyzer, achieving an out-of-plane polarization of 10 μC/cm2. Furthermore, a pronounced in-plane ferroelectricity and photoelectric effect were also observed, along with a strong coupling between them. First-principles density functional theory calculations identify the displacement and distortion of [SeO3] tetrahedra under external electric fields as the primary origin of the observed out-of-plane and in-plane ferroelectric polarization in β-Bi2SeO5 films. These findings open up new possibilities for using β-Bi2SeO5 thin films in future nanoscale electronic and quantum devices.

The acoustic surface plasmon (ASP) with a low and linear dispersion relation is usually attributed to the strong Coulomb screening of surface states by bulk electrons, offering a promising avenue for studying fundamental quantum phenomena such as pairing glue for unconventional superconductivity and practical plasmonic applications. However, whether ASPs can persist in systems with reduced electronic screening remains unclear. Topological semimetals feature linear band touching, where the low density of states near the nodes leads to insufficient screening of the long-range Coulomb interactions and give rise to intriguing physical properties, providing a natural platform for exploring ASPs under weak-screening conditions. Here, using high-resolution electron energy loss spectroscopy with precise beam energy tuning, we unambiguously observe an ASP mode over a wide energy-momentum range on prototypical nodal-line semimetals, ZrSiS and ZrSiSe. We further find the ASP mode originates from the interplay between the floating surface states and substantially lower bulk charge carrier densities than conventional metals. Our results establish the physical foundation for the realization and application of ASPs on the surface of nodal-line semimetals.

Berry curvature in transition metal dichalcogenides (TMDs) critically governs spin‐valley degrees of freedom, nonlinear/quantum electrical transport, and magnetoelectric coupling, underpinning emergent functionalities in valleytronic, magnetoelectric, and optoelectronic devices. Self‐intercalated Cr _{1+ δ} Te ₂ , as a representative magnetic semimetal among TMDs, enables controlled the Berry curvature due to its rigid band shift under external stimuli. In this work, we study the thickness‐tuned Berry curvature, manifested by intrinsic anomalous Hall effect (AHE), in van der Waals (vdW) epitaxial Cr ₃ Te ₄ ( δ = 0.5) ultrathin films. At low temperatures, the intrinsic AHE of Cr ₃ Te ₄ ultrathin film has a negative‐polarity below 13 monolayers and positive‐polarity for 22 monolayers. This sign reversal, indicating the Berry curvature modulation, could be attributed to the Fermi level shift caused by Te(CrTe ₂ )‐termination at the surface and interface. These findings highlight the crucial role of controlling the electronic structure of vdW materials at atomic scale and pave the way for the design of high‐density, miniaturized topological spintronic devices.

Crystal structure engineering is pivotal for tailoring material properties, particularly in atomically thin two-dimensional (2D) transition-metal chalcogenides that exhibit rich, crystal-phase-dependent electronic, optical, and magnetic properties. However, their crystal-phase-controlled synthesis remains a critical challenge that limits their practical applications. Here, we report the phase-controlled synthesis of atomically thin chromium telluride films, including CrTe₃, CrTe₂, Cr_1+δTe₂, and CrTe, on graphene/SiC substrates via precise tuning of molecular beam epitaxy parameters. The growth mechanism is further revealed. We have systematically characterized the different structures and electronic, chemical, and magnetic properties of these films using scanning tunneling microscopy/spectroscopy, scanning transmission electron microscopy, density functional theory calculations, X-ray photoelectron/absorption spectroscopy, and circular-polarized X-ray photoemission electron microscopy. Heterostructures of different chromium telluride phases are further fabricated, demonstrating band engineering enabled by phase control of chromium tellurides.

The pyrochlore iridates R ₂ Ir ₂ O ₇ have emerged as a unique playground for exploring exotic quantum phenomena arising from the intricate interplay of strong spin-orbit coupling, electron correlations, and geometric frustration. While bulk crystals of these materials have revealed a rich landscape of correlated and topological states, recent breakthroughs in epitaxial thin-film synthesis and heterostructure engineering unlocked an entirely new dimension of discovery. This brief Perspective reviews recent advancements highlighting how new tuning knobs such as dimensional confinement, epitaxial strain, and interfacial coupling can be used to manipulate the delicate balance of competing interactions. We discuss several key discoveries enabled by this approach including the realization of the magnetic Weyl semimetal phase in (111) oriented films, strain-engineered magnetic multipolar orders, the emergence of a chiral spin liquid-like state in the quasi-2D limit, and the discovery of novel electronically anisotropic states at interfaces between pyrochlore iridates and other quantum materials, such as spin ice pyrochlores. These findings showcase that low-dimensional pyrochlore iridates provide ample opportunities for both theory and experiment to unravel, control and ultimately design novel quantum states of matter. We conclude by outlining key open questions and future directions ranging from the synthesis of new heterostructures to the application of advanced probes and the exploration of non-equilibrium phenomena.

Surface plasmons (SPs) in refractory materials like titanium nitride (TiN) offer promising alternatives to noble metals for plasmonic applications due to their robustness and CMOS compatibility. However, the dispersion behavior of SPs at the surface of TiN in the non-retarded regime remains poorly understood. Here, we employ momentum-resolved high-resolution electron energy loss spectroscopy to investigate the collective electronic excitations of atomically flat TiN(111) thin films treated by Ar ⁺ sputtering and annealing. Two distinct excitations are observed: a SP mode near 2 eV and a defect-induced interband transition (IT) mode around 1 eV. The SP exhibits an anisotropic, anomalous dispersion: negative at small momenta ( q < 0.03 Å ^-1 along ΓM and q < 0.025 Å ^-1 along ΓK), transitioning to positive at larger momenta. The spectral weight of the IT shows an opposite momentum dependence to the SP energy. Assisted by a Drude-Lorentz dielectric model incorporating the momentum-dependent IT oscillator strength, we demonstrate that the anomalous SP dispersion arises from q-dependent screening by defect states. These findings highlight defect engineering as a strategy for tailoring plasmonic properties in refractory nitrides, with implications for integrated photonics and sensing applications.

Excitonic band structure is critical for investigating exciton dynamics. Theoretically, quantum effects from exchange scattering between electron-hole pairs significantly modulate exciton dispersion. Here, we report the direct observation of dimensionality-dependent exciton dispersion in a single-band Mott insulator Nb_{3}Cl_{8} through high-resolution electron energy loss spectroscopy. In the high-temperature phase, the exciton in Nb_{3}Cl_{8} hosts an exceptionally large binding energy, and exhibits clear quasi-two-dimensional massless linear dispersion. In contrast, in the low-temperature phase, the exciton splits into two bands, both displaying three-dimensional parabolic dispersion. These dramatic changes in the exciton dispersion stem from the dimensional mutation driven by a substantial enhancement of interlayer coupling across the phase transition. This Letter provides a clear and typical example of how exciton behavior evolves with dimensionality.