Explore the vast range of titles available.

A properly strained graphene monolayer or bilayer is expected to harbour periodic pseudo-magnetic fields with high symmetry, yet to date, a convincing demonstration of such pseudo-magnetic fields has been lacking, especially for bilayer graphene. Here, we report a definitive experimental proof for the existence of large-area, periodic pseudo-magnetic fields, as manifested by vortex lattices in commensurability with the moiré patterns of low-angle twisted bilayer graphene. The pseudo-magnetic fields are strong enough to confine the massive Dirac electrons into circularly localized pseudo-Landau levels, as observed by scanning tunneling microscopy/spectroscopy, and also corroborated by tight-binding calculations. We further demonstrate that the geometry, amplitude, and periodicity of the pseudo-magnetic fields can be fine-tuned by both the rotation angle and heterostrain. Collectively, the present study substantially enriches twisted bilayer graphene as a powerful enabling platform for exploration of new and exotic physical phenomena, including quantum valley Hall effects and quantum anomalous Hall effects. Precisely strained graphene layers can enable observation of periodic pseudo-magnetic fields with high symmetry. Here, the authors report experimental tuning of large area periodic pseudo-magnetic fields within twisted bilayer graphene and massive Dirac electrons having circularly localized pseudo-Landau levels.

Dirac fermions with highly dispersive linear bands 1 – 3 are usually considered weakly correlated due to the relatively large bandwidths ( W ) compared to Coulomb interactions ( U ). With the discovery of nodal-line semimetals, the notion of the Dirac point has been extended to lines and loops in momentum space. The anisotropy associated with nodal-line structure gives rise to greatly reduced kinetic energy along the line. However, experimental evidence for the anticipated enhanced correlations in nodal-line semimetals is sparse. Here, we report on prominent correlation effects in a nodal-line semimetal compound, ZrSiSe, through a combination of optical spectroscopy and density functional theory calculations. We observed two fundamental spectroscopic hallmarks of electronic correlations: strong reduction (1/3) of the free-carrier Drude weight and also the Fermi velocity compared to predictions of density functional band theory. The renormalization of Fermi velocity can be further controlled with an external magnetic field. ZrSiSe therefore offers the rare opportunity to investigate correlation-driven physics in a Dirac system. What happens to topological materials when their electrons are strongly interacting is an open question. Shao and others demonstrate that ZrSiSe is a material that can address this as it has a topological band structure and non-trivial correlations.

Transition metal dichalcogenide (TMD) nanotubes offer a unique platform to explore the properties of TMD materials at the one-dimensional limit. Despite considerable efforts thus far, the direct growth of TMD nanotubes with controllable chirality remains challenging. Here we demonstrate the direct and facile growth of high-quality WS 2 and WSe 2 nanotubes on Si substrates using catalytic chemical vapour deposition with Au nanoparticles. The Au nanoparticles provide unique accommodation sites for the nucleation of WS 2 or WSe 2 shells on their surfaces and seed the subsequent growth of nanotubes. We find that the growth mode of nanotubes is sensitive to the temperature. With careful temperature control, we realize ~79% WS 2 nanotubes with single chiral angles, with a preference of 30° (~37%) and 0° (~12%). Moreover, we demonstrate how the geometric, electronic and optical properties of the synthesized WS 2 nanotubes can be modulated by the chirality. We anticipate that this approach using Au nanoparticles as catalysts will facilitate the growth of TMD nanotubes with controllable chirality and promote the study of their interesting properties and applications. The direct and facile growth of WS 2 and WSe 2 nanotubes with controllable chirality is realized using catalytic chemical vapour deposition with Au nanoparticles.

Twisted graphene multilayers exhibit strongly correlated insulating states and superconductivity due to the presence of ultraflat bands near the charge neutral point. In this paper, the response of ultraflat bands to lattice relaxation and a magnetic field in twisted trilayer graphene (tTLG) with different stacking arrangements is investigated by using a full tight-binding model. We show that lattice relaxations are indispensable for understanding the electronic properties of tTLG, in particular, of tTLG in the presence of mirror symmetry. Lattice relaxations renormalize the quasiparticle spectrum near the Fermi energy and change the localization of higher energy flat bands. Furthermore, different from the twisted bilayer graphene, the Hofstadter butterfly spectrum can be realized at laboratory accessible strengths of magnetic field. Our work verifies tTLG as a more tunable platform than the twisted bilayer graphene in strongly correlated phenomena.

The discovery of magnetism in ultrathin crystals opens up opportunities to explore new physics and to develop next-generation spintronic devices. Nevertheless, two-dimensional magnetic semiconductors with Curie temperatures higher than room temperature have rarely been reported. Ferrites with strongly correlated d -orbital electrons may be alternative candidates offering two-dimensional high-temperature magnetic ordering. This prospect is, however, hindered by their inherent three-dimensional bonded nature. Here, we develop a confined-van der Waals epitaxial approach to synthesizing air-stable semiconducting cobalt ferrite nanosheets with thickness down to one unit cell using a facile chemical vapor deposition process. The hard magnetic behavior and magnetic domain evolution are demonstrated by means of vibrating sample magnetometry, magnetic force microscopy and magneto-optical Kerr effect measurements, which shows high Curie temperature above 390 K and strong dimensionality effect. The addition of room-temperature magnetic semiconductors to two-dimensional material family provides possibilities for numerous novel applications in computing, sensing and information storage. Van der Waals crystals allow for magnetism down to the monolayer limit, however, this magnetism, and frequently the material itself, is fragile. Ferrites, conversely, have robust material stability and magnetic order, but are three dimensional. Here the authors succeed in creating a single unit cell thickness of Cobalt Ferrite via chemical vapour deposition, with hard magnetic properties, and curie temperature exceeding room temperature.

Twisted bilayer graphene (TBG) has taken the spotlight in the condensed matter community since the discovery of correlated phases. In this work, we study heterostructures of TBG and hexagonal boron nitride (hBN) using an atomistic tight-binding model together with semi-classical molecular dynamics to consider relaxation effects. The hBN substrate has significant effects on the band structure of TBG even in the case where TBG and hBN are not aligned. Specifically, the substrate induces a large mass gap and strong pseudo-magnetic fields that break the layer degeneracy. Interestingly, such degeneracy can be recovered with a second hBN layer. Finally, we develop a continuum model that describes the tight-binding band structure. Our results show that a real-space tight-binding model in combination with semi-classical molecular dynamics is a powerful tool to study the electronic properties of moiré heterostructures, and to explain experimental results in which the effect of the substrate plays an important role.

The discovery of various primary ferroic phases in atomically-thin van der Waals crystals have created a new two-dimensional wonderland for exploring and manipulating exotic quantum phases. It may also bring technical breakthroughs in device applications, as evident by prototypical functionalities of giant tunneling magnetoresistance, gate-tunable ferromagnetism and non-volatile ferroelectric memory etc. However, two-dimensional multiferroics with effective magnetoelectric coupling, which ultimately decides the future of multiferroic-based information technology, has not been realized yet. Here, we show that an unconventional magnetoelectric coupling mechanism interlocked with heterogeneous ferrielectric transitions emerges at the two-dimensional limit in van der Waals multiferroic CuCrP 2 S 6 with inherent antiferromagnetism and antiferroelectricity. Distinct from the homogeneous antiferroelectric bulk, thin-layer CuCrP 2 S 6 under external electric field makes layer-dependent heterogeneous ferrielectric transitions, minimizing the depolarization effect introduced by the rearrangements of Cu + ions within the ferromagnetic van der Waals cages of CrS 6 and P 2 S 6 octahedrons. The resulting ferrielectric phases are characterized by substantially reduced interlayer magnetic coupling energy of nearly 50% with a moderate electric field of 0.3 V nm −1 , producing widely-tunable magnetoelectric coupling which can be further engineered by asymmetrical electrode work functions. Two-dimensional multiferroics with effective magnetoelectric coupling has not been realized. Here, the authors find a magnetoelectric coupling mechanism in two-dimensional CuCrP 2 S 6 interlocked with heterogeneous ferrielectric state.

All-perovskite tandem solar cells offer great promise for achieving low levelized cost of electricity, but their performance remains limited by insufficient near-infrared photon absorption in narrow bandgap tin-lead (Sn-Pb) subcells. Micron-thick Sn-Pb layers are essential for maximizing absorption, yet high-concentration precursor solutions often cause non-uniform crystallization, stoichiometric imbalance and limited carrier diffusion lengths. Here we identify the root cause of these limitations as the insufficient coordination of tin(II) iodide (SnI 2 ) in conventional dimethylformamide (DMF)/dimethyl sulfoxide (DMSO) binary solvent system at high precursor concentrations, resulting in Sn-rich colloids that nucleate detrimental Sn-rich phases in final films. To address this, we develop a ternary solvent system that fully coordinates with SnI 2 , suppressing Sn-rich phases and enabling stoichiometric, micron-thick Sn-Pb films with carrier diffusion lengths of ~11 μm. The enhanced Sn-Pb absorber achieves efficiencies of 24.2% in single-junction cells and 29.3% in tandem devices, along with significantly improved long-term operational stability. The performance of all-perovskite tandem solar cells remains limited by the insufficient infrared photon absorption in the narrow bandgap subcells. Here, the authors develop a ternary solvent system to suppress tin-rich phases and achieve an efficiency of 29.3% in operationally stable devices.

Understanding how symmetries encode optical polarization information into selection rules in molecules and materials is important for their optoelectronic applications including spectroscopic analysis, display technology, and quantum computation. Here, we extend polarization-dependent selection rules from atoms to solid-state systems with various point groups with the help of the rotational operator for circular polarization and the twofold rotational operator (or reflection operator) for linear polarization. We use these new selection rules to study the optical properties of twisted bilayer graphene quantum dots (TBGQDs), which inherit advantages of graphene quantum dot including its ultrathin thickness, excellent biocompatibility, and shape- and size-tunable optical absorption or emission. We study how the electronic structures and optical properties of TBGQDs rely on size, shape, twist angle, and correlation effects for TBGQDs with 10 different point groups for which we obtain an optical selection rule database. We show how current operator matrix elements identify the generalized polarization-dependent selection rules. Our results show that both the electronic and optical band gaps follow power-law size scalings with a dominant role of the twist angle. We derive an atlas of optical conductivity spectra for both size and twist angle in TBGQDs. As a result of quantum confinement effects, in the atlas a new type of optical conductivity features emerges with multiple discrete absorption frequencies ranging from infrared to ultraviolet energy, allowing for applications in photovoltaic devices and photodetectors. The atlas and size scaling provide a full structure–symmetry-function interrelation and hence offer an excellent basis for the geometrical manipulation of optical properties of TBGQDs as building blocks for novel integrated carbon optoelectronics.

Dodecagonal bilayer graphene quasicrystal has 12-fold rotational order but lacks translational symmetry which prevents the application of band theory. In this paper, we study the electronic and optical properties of graphene quasicrystal with large-scale tight-binding calculations involving more than ten million atoms. We propose a series of periodic approximants which reproduce accurately the properties of quasicrystal within a finite unit cell. By utilizing the band-unfolding method on the smallest approximant with only 2702 atoms, the effective band structure of graphene quasicrystal is derived. The features, such as the emergence of new Dirac points (especially the mirrored ones), the band gap at \\[M\\] point and the Fermi velocity are all in agreement with recent experiments. The properties of quasicrystal states are identified in the Landau level spectrum and optical excitations. Importantly, our results show that the lattice mismatch is the dominant factor determining the accuracy of layered approximants. The proposed approximants can be used directly for other layered materials in honeycomb lattice, and the design principles can be applied for any quasi-periodic incommensurate structures.