Abstract: Monolayer hexagonal boron nitride (hBN) has been widely considered as a fundamental building block for two–dimensional (2D) heterostructures and devices. However, the controlled and scalable synthesis of hBN and its 2D heterostructures has remained a daunting challenge. Here, we propose and further demonstrate a hBN/graphene (hBN/G) interface–mediated growth process for the controlled synthesis of high–quality monolayer hBN. We discover that the in–plane hBN/G interface can be precisely controlled, enabling the scalable epitaxy of unidirectional monolayer hBN on graphene, which exhibits a uniform moiré superlattice consistent with single–domain hBN, aligned to the underlying graphene lattice. Furthermore, we identify that the deep–ultraviolet emission at 6.12 eV stems from the 1s–exciton state of monolayer hBN with a giant renormalized direct bandgap on graphene. This work provides a viable path for the controlled synthesis of ultraclean, wafer–scale, atomically ordered 2D quantum materials, as well as the fabrication of 2D quantum electronic and optoelectronic devices.

Full text available from Advanced Materials

Abstract

Compelling evidence suggests distinct correlated electron behavior may exist only in clean 2D materials such as 1T-TaS₂. Unfortunately, experiment and theory suggest that extrinsic disorder in free standing 2D layers disrupts correlation-driven quantum behavior. Here we demonstrate a route to realizing fragile 2D quantum states through endotaxial polytype engineering of van der Waals materials. The true isolation of 2D charge density waves (CDWs) between metallic layers stabilizes commensurate long-range order and lifts the coupling between neighboring CDW layers to restore mirror symmetries via interlayer CDW twinning. The twinned-commensurate charge density wave (tC-CDW) reported herein has a single metal–insulator phase transition at ~350 K as measured structurally and electronically. Fast in-situ transmission electron microscopy and scanned nanobeam diffraction map the formation of tC-CDWs. This work introduces endotaxial polytype engineering of van der Waals materials to access latent 2D ground states distinct from conventional 2D fabrication.

Full text available from nature communications

Abstract

Since the discovery of high-temperature superconductivity in copper oxide materials¹, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials². One prime materials platform has been the rare-earth nickelates and, indeed, superconductivity was recently discovered in the doped compound Nd_0.8Sr_0.2NiO₂ (ref. ³). Undoped NdNiO₂ belongs to a series of layered square-planar nickelates with chemical formula Nd_n+1Ni_nO_2n+2 and is known as the ‘infinite-layer’ (n = ∞) nickelate. Here we report the synthesis of the quintuple-layer (n = 5) member of this series, Nd₆Ni₅O₁₂, in which optimal cuprate-like electron filling (d^8.8) is achieved without chemical doping. We observe a superconducting transition beginning at ~13 K. Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd₆Ni₅O₁₂ interpolates between cuprate-like and infinite-layer nickelate-like behaviour. In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors that can be tuned via both doping and dimensionality.

Full text available from Nature Materials

Abstract:

With the explosion of Internet traffic, the rise of large data centers, and smart technologies on the horizon, forecasts of the global energy consumption from information, and communications technologies are expected to rise from ~ 8% in 2020 to ~ 21% in 2030. The future demand will challenge the supply of electricity and has technology makers looking for ways to improve the efficiency of information and communication devices. In recent years, advances in magnetoelectric and multiferroic materials now provide the basis for nonvolatile spin-based logic and memory elements that have a projected energy efficiency orders of magnitude larger than the complementary metal-oxide semiconductor transistor. The possibilities are exciting, yet significant challenges remain. This article summarizes key materials, recent advancements, and current challenges in electric-field-controlled magnetism for realizing these potentially transformational devices. A perspective and potential considerations are given.

Full text available from MRS Bulletin

Abstract:

Narrowband terahertz (THz) radiation is crucial for high-resolution spectral identification, but a narrowband THz source driven by a femtosecond (fs) laser has remained scarce. Here, it is computationally predicted that a metal/dielectric/magnetoelastic heterostructure enables converting a fs laser pulse into a multicycle THz pulse with a narrow linewidth down to ∼1.5 GHz, which is in contrast to the single-cycle, broadband THz pulse from the existing fs-laser-excited emitters. It is shown that such narrowband THz pulse originates from the excitation and long-distance transport of THz spin waves in the magnetoelastic film, which can be enabled by a short strain pulse obtained from fs laser irradiation of the metal film when the thicknesses of the metal and magnetoelastic films both fall into a specific range. These results therefore reveal an approach to achieving optical generation of narrowband THz pulse based on heterostructure design, which also has implications in the design of THz magnonic devices.

Full text available from ACS Publications