Meng Zhao

After decades of research, symmetry breaking in high-temperature cuprate superconductors remains a key issue to resolve and is relevant to understanding their exotic quantum phases. In the prototypical cuprate superconductor, Bi₂Sr₂CaCu₂O_8+δ (Bi2212), the possible symmetry breaking has been mostly examined microscopically with scanning tunneling microscopy and photoemission spectroscopy. However, macroscopic evidence and the direct implications for electronic transport have remained elusive. Using superconductivity-enhanced nonreciprocal transport, we report macroscopic evidence of inversion symmetry breaking in Bi2212. While the inversion symmetry breaking is subtle, its effect on nonreciprocal transport is significantly enhanced by the vortex motion during the superconducting transition, leading to a robust manifestation of inversion symmetry breaking in macroscopic transport. Combining angle-resolved nonreciprocal transport and 3D tight-binding model calculations, we derive that the inversion symmetry breaking is due to subtle crystal distortions that give rise to both in-plane and out-of-plane polar axes. Our work not only establishes nonreciprocal transport as a sensitive macroscopic probe to provide electrical transport-based evidence of fine symmetry breaking in Bi2212, but also paves the way for novel device applications such as high-temperature superconducting diodes and superconducting spintronics.

Extreme light confinement down to the atomic scale has been theoretically predicted for ultrathin, Ta-based transition metal dichalcogenides (TMDs). In this work, we report the observation of highly confined plasmons in 2H-TaS₂ monolayers and bilayers via momentum-resolved electron energy loss spectroscopy (q-EELS), with a resolution of 0.0056 Å⁻¹. Momentum-dispersed two-dimensional (2D) plasmon resonances were found to exhibit a lateral confinement ratio up to 300 at large wave vectors of q = 0.15 Å⁻¹ and slow light behaviour with a group velocity ~10^-4c. Moreover, we observed a transition from 2D to 3D Coulomb interaction in the high-momentum regime, equivalent to light confinement volumes of 1-2 nm³. Remarkably, the resonant modes do not enter the electron-hole continuum, potentially enabling even further enhanced optical field confinements for this material at cryogenic temperatures.

The α-phase molybdenum trioxide (α-MoO₃), a biaxial hyperbolic van der Waals (vdW) crystal, supports highly confined and anisotropic phonon polaritons (PhPs), positioning it as a superior platform for mid-infrared light manipulation. The performance of PhP-based devices critically depends on the properties of α-MoO₃ flakes, including their thickness, roughness, and pattern geometry. However, conventional patterning techniques, such as ion beam milling and plasma etching, often introduce doping artifacts and surface damage, resulting in high PhP losses. In this work, we develop a hot-water-based technique for the crystallographic engineering of α-MoO₃, leveraging its anisotropic etching properties for surface polishing and nanopatterning. This method exploits the notably higher etching rate along intralayer directions ([100], [001]) compared to the interlayer direction ([010]). Consequently, a 24% enhancement in PhP lifetime was observed in RIE-treated α-MoO₃ flakes after hot water polishing, with no measurable change in material thickness. To further validate this technology, we fabricated various two-dimensional PhP manipulation devices using standard nanopatterning and thinning processes, followed by chemical-free hot water anisotropic crystallographic etching. This approach enabled the creation of nanoresonators, lenses, nanocavities, and unidirectional emitters with sharp edges precisely aligned along the crystallographic planes. Our crystallographic engineering approach unlocks precise control of surface waves at the nanoscale, facilitating the development of photonic devices for cutting-edge nanophotonic and nanoscale sensing applications.

P-block metal monochalcogenides (MX) adopting black phosphorus (BP)-like structures are promising electrocatalysts due to their abundant exposed metal sites and tunable electronic structures. However, their practical application is limited by structural instability arising from lone-pair electron-induced structural distortions, along with an inherent orbital symmetry mismatch with the frontier orbitals of small molecules (e.g., CO₂), reducing the activation efficiency. Here, we report a noninvasive doping strategy to overcome both structural instability and orbital symmetry mismatch in p-block metal monochalcogenides for efficient CO₂ electroreduction, through engineering a periodic van der Waals (vdW) superlattice, known as a misfit superlattice. These vdW superlattices with tunable sublayer ratios contain the catalytically active p-electron-rich MX sublayers and conductive transition metal dichalcogenide current collectors. Taking [BiS]₁[TaS₂]₁ as a proof-of-concept, the presence of noninvasive vdW doping and ionic interactions between sublayers is crucial for modulating their electronic structures and stabilizing BiS sublayers by transforming the Bi into a higher valence state of Bi^(2+δ). Concurrently, interlayer noninvasive vdW doping induces uneven electron redistribution in Bi's p-orbitals, breaking its orbital symmetry mismatch with the LUMO of CO₂, thereby reducing the CO₂ activation barrier. In situ characterization and theoretical calculations reveal that the optimized Bi sites exhibit moderate adsorption for the *OCHO, endowing the superlattice with exceptional selectivity (>90%) for formate in CO₂ electroreduction. This work advances vdW superlattice engineering as a versatile platform for synergistically stabilizing layered p-block materials and tailoring their sublayer interactions and orbital symmetry alignment by leveraging noninvasive vdW doping, achieving optimal catalytic performance for the efficient electrochemical conversion of small molecules.

Emergent quantum phenomena in two-dimensional van der Waal (vdW) magnets are largely governed by the interplay between exchange and Coulomb interactions. The ability to precisely tune the Coulomb interaction enables the control of spin-correlated flat-band states, band gap, and unconventional magnetism in such strongly correlated materials. Here, we demonstrate a gate-tunable renormalization of spin-correlated flat-band states and bandgap in magnetic chromium tribromide (CrBr₃) monolayers grown on graphene. Our gate-dependent scanning tunneling spectroscopy (STS) studies reveal that the interflat-band spacing and bandgap of CrBr₃ can be continuously tuned by 120 and 240 meV, respectively, via electrostatic injection of carriers into the hybrid CrBr₃/graphene system. This can be attributed to the self-screening of CrBr₃ arising from the gate-induced carriers injected into CrBr₃, which dominates over the weakened remote screening of the graphene substrate due to the decreased carrier density in graphene. Precise tuning of the spin-correlated flat-band states and bandgap in 2D magnets via electrostatic modulation of Coulomb interactions not only provides effective strategies for optimizing the spin transport channels but also may exert a crucial influence on the exchange energy and spin-wave gap, which could raise the critical temperature for magnetic order.

Two-dimensional (2D) materials including graphene, transition metal dichalcogenides, black phosphorus, MXenes, and semimetals have attracted extensive and widespread interest over the past years for their many intriguing properties and phenomena, underlying physics, and great potential for applications. The vast library of 2D materials and their heterostructures provides a diverse range of electrical, photonic, mechanical, and chemical properties with boundless opportunities for photonics and plasmonic devices. The infrared (IR) regime, with wavelengths across 0.78 μm to 1000 μm, has particular technological significance in industrial, military, commercial, and medical settings while facing challenges especially in the limit of materials. Here, we present a comprehensive review of the varied approaches taken to leverage the properties of the 2D materials for IR applications in photodetection and sensing, light emission and modulation, surface plasmon and phonon polaritons, non-linear optics, and Smith-Purcell radiation, among others. The strategies examined include the growth and processing of 2D materials, the use of various 2D materials like semiconductors, semimetals, Weyl-semimetals and 2D heterostructures or mixed-dimensional hybrid structures, and the engineering of light-matter interactions through nanophotonics, metasurfaces, and 2D polaritons. Finally, we give an outlook on the challenges in realizing high-performance and ambient-stable devices and the prospects for future research and large-scale commercial applications.

In the broad spectral range, near‐infrared (NIR) plasmonics find applications in telecommunication, energy harvesting, sensing, and more, all of which would benefit from an electrostatically controllable NIR plasmon source. However, it is difficult to control bulk NIR plasmonics directly with electrostatics because of the strong electric‐field screening effect and high carrier concentration required to support NIR plasmons. Here, this constraint is overcome and the observation of NIR plasmonic resonances that can be modulated electrostatically over a range of ≈360 cm^–1 in few‐layer NbSe₂ gratings is reported, thanks to the enhanced electrostatics of atomically thin 2D materials and the high‐quality film produced by a solution method. NbSe₂ plasmons also render strong field confinement due to their atomic thickness and provide an extra degree of resonance frequency modulation from the layered structure. This study identifies metallic 2D materials as promising (easily produced and well‐performing) candidates to extend electrostatically tunable plasmonics to the technologically important NIR range.

We demonstrate a simple, cost‐effective method to enhance the photoluminescence intensity of monolayer MoS ₂ . A hexagonal symmetric Au metasurface, made by polystyrene nanosphere lithography and metal coating, is developed to enhance the photoluminescence intensity of monolayer MoS ₂ . By using nanospheres of different sizes, the localized surface plasmon resonances of the Au metasurfaces can be effectively tuned. By transferring monolayer MoS ₂ onto the Au metasurface, the photoluminescence signal of the monolayer MoS ₂ can be significantly enhanced up to 12‐fold over a square‐centimeter area. The simple, large‐area, cost‐effective fabrication technique could pave a new way for plasmon‐enhanced light‐mater interactions of atomically thin two‐dimensional materials.