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Abstract Photonics in the frequency range of 5–15 terahertz (THz) potentially open a new realm of quantum materials manipulation and biosensing. This range, sometimes called “the new terahertz gap”, is traditionally difficult to access due to prevalent phonon absorption bands in solids. Low‐loss phonon–polariton materials may realize sub‐wavelength, on‐chip photonic devices, but typically operate in mid‐infrared frequencies with narrow bandwidths and are difficult to manufacture on a large scale. Here, for the first time, quantum paraelectric SrTiO₃enables broadband surface phonon–polaritonic devices in 7–13 THz. As a proof of concept, polarization‐independent field concentrators are designed and fabricated to locally enhance intense, multicycle THz pulses by a factor of 6 and increase the spectral intensity by over 90 times. The time‐resolved electric field inside the concentrators is experimentally measured by THz‐field‐induced second harmonic generation. Illuminated by a table‐top light source, the average field reaches 0.5 GV m⁻¹over a large volume resolvable by far‐field optics. These results potentially enable scalable THz photonics with high breakdown fields made of various commercially available phonon–polariton crystals for studying driven phases in quantum materials and nonlinear molecular spectroscopy. 

Abstract Piezoelectricity in low‐dimensional materials and metal–semiconductor junctions has attracted recent attention. Herein, a 2D in‐plane metal–semiconductor junction made of multilayer 2H and 1T′ phases of molybdenum(IV) telluride (MoTe₂) is investigated. Strong piezoelectric response is observed using piezoresponse force microscopy at the 2H–1T′ junction, despite that the multilayers of each individual phase are weakly piezoelectric. The experimental results and density functional theory calculations suggest that the amplified piezoelectric response observed at the junction is due to the charge transfer across the semiconducting and metallic junctions resulting in the formation of dipoles and excess charge density, allowing the engineering of piezoelectric response in atomically thin materials. 

Abstract Enhancing electron correlation in a weakly interacting topological system has great potential to promote correlated topological states of matter with extraordinary quantum properties. Here, the enhancement of electron correlation in a prototypical topological metal, namely iridium dioxide (IrO₂), via doping with 3d transition metal vanadium is demonstrated. Single‐crystalline vanadium‐doped IrO₂nanowires are synthesized through chemical vapor deposition where the nanowire yield and morphology are improved by creating rough surfaces on substrates. Vanadium doping leads to a dramatic decrease in Raman intensity without notable peak broadening, signifying the enhancement of electron correlation. The enhanced electron correlation is further evidenced by transport studies where the electrical resistivity is greatly increased and follows an unusual dependence on the temperature (T). The lattice thermal conductivity is suppressed by an order of magnitude via doping even at room temperature where phonon‐impurity scattering becomes less important. Density functional theory calculations suggest that the remarkable reduction of thermal conductivity arises from the complex phonon dispersion and reduced energy gap between phonon branches, which greatly enhances phase space for phonon–phonon Umklapp scattering. This work demonstrates a unique system combining 3d and 5d transition metals in isostructural materials to enrich the system with various types of interactions. 

Abstract The ability to reconfigure spin structure and spin‐photon interactions by an external electric field is a prerequisite for seamless integration of opto‐spintronics into modern electronics. In this study, the use of electric field on the tuning of circular photo galvanic effect in a quasi‐2D oxyhalide perovskite Bi₄NbO₈Cl is reported. The electrical transport measurements are applied to study the switching characteristics of the microsheet devices. The electric field is used to tune the nanoscale devices and an optical orientation approach is applied to understand the field‐tuned spin‐polarized band structures. It is found that the circular photogalvanic current can be erased and re‐created by poling, indicating the electric‐field‐based control over spin structure. The study enriches the basic understanding of the symmetry‐regulated optoelectronic response in ferroelectrics with spin‐orbit coupling. 

Abstract Halide perovskites are revolutionizing the renewable energy sector owing to their high photovoltaic efficiency, low manufacturing cost, and flexibility. Their remarkable mobility and long carrier lifetime are also valuable for information technology, but fundamental challenges like poor stability under an electric field prevent realistic applications of halide perovskites in electronics. Here, it is discovered that valleytronics is a promising route to leverage the advantages of halide perovskites and derivatives for information storage and processing. The synthesized all‐inorganic lead‐free perovskite derivative, Cs₃Bi₂I₉, exhibits strong light–matter interaction and parity‐dependent optically addressable valley degree of freedom. Robust optical helicity in all odd‐layer‐number crystals with inversion symmetry breaking is observed, indicating excitonic coherence extending well beyond 11 layers. The excellent optical and valley properties of Cs₃Bi₂I₉arise from the unique parallel bands, according to first principles calculations. This discovery points to new materials design principles for scalable valleytronic devices and demonstrates the promise of perovskite derivatives beyond energy applications. 

Time-reversal symmetry (TRS) is pivotal for materials’ optical, magnetic, topological, and transport properties. Chiral phonons, characterized by atoms rotating unidirectionally around their equilibrium positions, generate dynamic lattice structures that break TRS. Here, we report that coherent chiral phonons, driven by circularly polarized terahertz light pulses, polarize the paramagnetic spins in cerium fluoride in a manner similar to that of a quasi-static magnetic field on the order of 1 tesla. Through time-resolved Faraday rotation and Kerr ellipticity, we found that the transient magnetization is only excited by pulses resonant with phonons, proportional to the angular momentum of the phonons, and growing with magnetic susceptibility at cryogenic temperatures. The observation quantitatively agrees with our spin-phonon coupling model and may enable new routes to investigating ultrafast magnetism, energy-efficient spintronics, and nonequilibrium phases of matter with broken TRS.