Controlled alignment of supermoiré lattice in double-aligned graphene heterostructures

Interlayer Phonon Coupling and Enhanced Electron-Phonon Interactions in Doubly Aligned hBN/Graphene/hBN Heterostructures

Engineering the band structure via moiré superlattices plays a crucial role in tailoring the electronic and phononic spectra of hBN/graphene heterostructures, enabling a range of emergent properties. While moiré heterostructures have been extensively studied through transport measurements to investigate electronic spectra, their influence on the phononic spectrum, particularly on phonon-phonon and electron-phonon interactions, remains less explored. In this study, we examine the temperature-dependent (8 K-300 K) frequency and line width responses of the phonon near the K-point of graphene in hBN/graphene/hBN heterostructures for nonaligned, partially aligned, singly aligned, and doubly aligned configurations. The nonaligned samples, where the graphene is rotated by 30° with respect to both top and bottom hBN, exhibit pristine graphene behavior, characterized by minimal frequency variation with temperature and a typical line width increase with increasing temperature. In contrast, doubly aligned samples, where graphene and both hBN are perfectly aligned, display anomalous behavior, with the Raman frequency decreasing linearly and the lifetime increasing with increasing temperature. This anomalous anharmonic response could not be explained by the existing models considering only intralayer (within the graphene) phonon-phonon interactions, but rather indicates the role of strong interlayer phonon-phonon coupling (between hBN and graphene phonons), hitherto not observed. Furthermore, the enhanced electron-phonon interactions due to the resonant condition of phonon decay into electronic channels of doubly aligned hBN/graphene/hBN heterostructures explain the observed line width behavior. Our findings demonstrate the ability to engineer phonon-phonon and electron-phonon interactions through the precise alignment of hBN and graphene lattices, with implications for thermal management and carrier transport optimization in hBN/graphene/hBN heterostructures.

Massive Transfer and Assembly of Microscale Superlubric Materials

Structural superlubricity (SSL) offers a revolutionary solution to the challenges of friction and wear. However, current transfer methods for superlubric materials rely on probe-based techniques that are limited to individual, one-by-one transfers. Moreover, the maximum achievable scale of SSL is constrained by the single-crystal size and defect distribution of the material. To enable the mass production of devices and the scaling of SSL contact areas, scalable transfer and assembly techniques are critically needed. Here, we introduce a batch "slide-and-lift" dry transfer technique that leverages the sliding motion of polydimethylsiloxane stamps to modulate adhesion at van der Waals interfaces, enabling the simultaneous transfer of hundreds of sliders. This technique accommodates sliders of various sizes and shapes while ensuring their surfaces remain ultraclean and defect-free. Transferred slider arrays are successfully released onto various substrates, maintaining their superlubric properties. Furthermore, these transferred sliders are assembled to achieve larger-scale SSL through multiphoton polymerization printing, where connected microscale sliders form a basic unit that can theoretically be scaled to any size and shape for SSL applications. Our approach facilitates the development of SSL-based devices and the realization of macroscale SSL. Additionally, it may inspire novel sliding-based transfer methods for two-dimensional materials by leveraging their inherent sliding characteristics.

Soft-matter-induced orderings in a solid-state van der Waals heterostructure

Deoxyribose nucleic acid (DNA), a type of soft matter, is often considered a promising building block to fabricate and investigate hybrid heterostructures with exotic functionalities. However, at this stage, investigations on DNA-enabled nanoelectronics have been largely limited to zero-dimensional (0D) and/or one-dimensional (1D) structures. Exploring their potential in higher dimensions, particularly in combination with hard matter solids such as van der Waals (vdW) two-dimensional (2D) materials, has proven challenging. Here, we show that 2D tessellations of DNA origami thin films, with a lateral size over 10 μm, can function as a sufficiently stiff substrate (Young's modulus of  ~4 GPa). We further demonstrate a two-dimensional soft-hard interface of matter (2D-SHIM), in which vdW layers are coupled to the 2D tessellations of DNA origami. In such 2D-SHIM, the DNA film can then serve as a superlattice due to its sub-100 nm sized pitch of the self-assemblies, which modulates the electronic states of the hybrid system. Our findings open up promising possibilities for manipulating the electronic properties in hard matter using soft matter as a super-structural tuning knob, which may find applications in next generation nanoelectronics.

Magnetic Bloch states at integer flux quanta induced by super-moiré potential in graphene aligned with twisted boron nitride

Two-dimensional electron systems in both magnetic fields and periodic potentials are described by the Hofstadter butterfly, a fundamental problem of solid-state physics. While moiré systems provide a powerful method to realize this type of spectrum, previous experiments have been limited to fractional flux quanta regime, due to the difficulty of building ~ 50 nm periodic modulations. Here, we demonstrate a super-moiré strategy to overcome this challenge. By aligning monolayer graphene (G) with 1.0° twisted hexagonal boron nitride (t-hBN), a 63.2 nm bichromatic G/t-hBN super-moiré is constructed, made possible by exploiting the electrostatic nature of t-hBN potential. Under magnetic field B , magnetic Bloch states at ϕ / ϕ 0 = 1 - 9 are achieved and observed as integer Brown-Zak oscillations, expanding the flux quanta from fractions to integers. Theoretical analysis reproduces these experimental findings. This work opens promising avenues to study unexplored Hofstadter butterfly, explore emergent topological order at integer flux quanta and engineer long-wavelength periodic modulations.

Adhesion and Reconstruction of Graphene/Hexagonal Boron Nitride Heterostructures: A Quantum Monte Carlo Study

We investigate interlayer adhesion and relaxation at interfaces between graphene and hexagonal boron nitride (hBN) monolayers in van der Waals heterostructures. The adhesion potential between graphene and hBN is calculated as a function of local lattice offset using diffusion quantum Monte Carlo methods, which provide an accurate treatment of van der Waals interactions. Combining the adhesion potential with elasticity theory, we determined the relaxed structures of graphene and hBN layers at interfaces, finding no metastable structures. The adhesion potential is well described by simple Lennard-Jones pair potentials that we parametrize using our quantum Monte Carlo data. Encapsulation of graphene between near-aligned crystals of hBN gives rise to a moiré pattern whose period is determined by the misalignment angle between the hBN crystals superimposed over the moiré superlattice previously studied in graphene on an hBN substrate. We model minibands in such supermoiré superlattices and find them to be sensitive to the 180° rotation of one of the encapsulating hBN crystals. We find that monolayer and bilayer graphene placed on a bulk hBN substrate and bulk hBN/graphene/bulk hBN systems do not relax to adopt a common lattice constant. The energetic balance is much closer for free-standing monolayer graphene/hBN bilayers and hBN/graphene/hBN trilayers. The layers in an alternating stack of graphene and hBN are predicted to strain to adopt a common lattice constant, and hence, we obtain a stable three-dimensional crystal with a distinct electronic structure.

Topological Flat Bands in Graphene Super-Moiré Lattices

Moiré-pattern-based potential engineering has become an important way to explore exotic physics in a variety of two-dimensional condensed matter systems. While these potentials have induced correlated phenomena in almost all commonly studied 2D materials, monolayer graphene has remained an exception. We demonstrate theoretically that a single layer of graphene, when placed between two bulk boron nitride crystal substrates with the appropriate twist angles, can support a robust topological ultraflat band emerging as the second hole band. This is one of the simplest platforms to design and exploit topological flat bands.

Tunable Spin-Polarized States in Graphene on a Ferrimagnetic Oxide Insulator

Spin-polarized two-dimensional (2D) materials with large and tunable spin-splitting energy promise the field of 2D spintronics. While graphene has been a canonical 2D material, its spin properties and tunability are limited. Here, this work demonstrates the emergence of robust spin-polarization in graphene with large and tunable spin-splitting energy of up to 132 meV at zero applied magnetic fields. The spin polarization is induced through a magnetic exchange interaction between graphene and the underlying ferrimagnetic oxide insulating layer, Tm3 Fe5 O12 , as confirmed by its X-ray magnetic circular dichroism (XMCD). The spin-splitting energies are directly measured and visualized by the shift in their Landau-fan diagram mapped by analyzing the measured Shubnikov-de-Haas (SdH) oscillations as a function of applied electric fields, showing consistent fit with the first-principles and machine learning calculations. Further, the observed spin-splitting energies can be tuned over a broad range between 98 and 166 meV by field cooling. The methods and results are applicable to other 2D (magnetic) materials and heterostructures, and offer great potential for developing next-generation spin logic and memory devices.