Direct Evidence of Klein and Anti-Klein Tunneling of Graphitic Electrons in a Corbino Geometry.

Transport measurement of electron optics in monolayer graphene p-n junction devices has been traditionally studied with negative refraction and chiral transmission experiments in Hall bar magnetic focusing setups. We show direct signatures of Klein (monolayer) and anti-Klein (bilayer) tunneling with a circular "edgeless" Corbino geometry made out of gated graphene p-n junctions. Noticeable in particular is the appearance of angular sweet spots (Brewster angles) in the magnetoconductance data of bilayer graphene, which minimizes head-on transmission, contrary to conventional Fresnel optics or monolayer graphene which show instead a sharpened collimation of transmission paths.

Evidence of the fractional quantum spin Hall effect in moiré MoTe.

Quantum spin Hall (QSH) insulators are two-dimensional electronic materials that have a bulk band gap similar to an ordinary insulator but have topologically protected pairs of edge modes of opposite chiralities. So far, experimental studies have found only integer QSH insulators with counter-propagating up-spins and down-spins at each edge leading to a quantized conductance G = e/h (with e and h denoting the electron charge and Planck's constant, respectively). Here we report transport evidence of a fractional QSH insulator in 2.

Thermodynamic evidence of fractional Chern insulator in moiré MoTe.

Chern insulators, which are the lattice analogues of the quantum Hall states, can potentially manifest high-temperature topological orders at zero magnetic field to enable next-generation topological quantum devices. Until now, integer Chern insulators have been experimentally demonstrated in several systems at zero magnetic field, whereas fractional Chern insulators have been reported in only graphene-based systems under a finite magnetic field. The emergence of semiconductor moiré materials, which support tunable topological flat bands, provides an opportunity to realize fractional Chern insulators.

Switchable moiré potentials in ferroelectric WTe/WSe superlattices.

Moiré materials with superlattice periodicity many times the atomic length scale have shown strong electronic correlations and band topology with unprecedented tunability. Non-volatile control of the moiré potentials could allow on-demand switching of superlattice effects but has remained challenging to achieve. Here we demonstrate the switching of the correlated and moiré band insulating states, and the associated nonlinear anomalous Hall effect, by the ferroelectric effect.

Exciton density waves in Coulomb-coupled dual moiré lattices.

Strongly correlated bosons in a lattice are a platform that can realize rich bosonic states of matter and quantum phase transitions. While strongly correlated bosons in a lattice have been studied in cold-atom experiments, their realization in a solid-state system has remained challenging. Here we trap interlayer excitons-bosons composed of bound electron-hole pairs, in a lattice provided by an angle-aligned WS/bilayer WSe/WS multilayer.

Electrical conductivity in a non-covalent two-dimensional porous organic material with high crystallinity.

Electroactive macrocycle building blocks are a promising route to new types of functional two-dimensional porous organic frameworks. Our strategy uses conjugated macrocycles that organize into two dimensional porous sheets non-covalent van der Waals interactions, to make ultrathin films that are just one molecule thick. In bulk, these two-dimensional (2D) sheets stack into a three-dimensional van der Waals crystal, where relatively weak alkyl-alkyl interactions constitute the interface between these sheets.

Edge channels of broken-symmetry quantum Hall states in graphene visualized by atomic force microscopy.

The quantum Hall (QH) effect, a topologically non-trivial quantum phase, expanded the concept of topological order in physics bringing into focus the intimate relation between the "bulk" topology and the edge states. The QH effect in graphene is distinguished by its four-fold degenerate zero energy Landau level (zLL), where the symmetry is broken by electron interactions on top of lattice-scale potentials. However, the broken-symmetry edge states have eluded spatial measurements.

Visualization of moiré superlattices.

Moiré superlattices in van der Waals heterostructures have given rise to a number of emergent electronic phenomena due to the interplay between atomic structure and electron correlations. Indeed, electrons in these structures have been recently found to exhibit a number of emergent properties that the individual layers themselves do not exhibit. This includes superconductivity, magnetism, topological edge states, exciton trapping and correlated insulator phases.

High-Quality Electrostatically Defined Hall Bars in Monolayer Graphene.

Realizing graphene's promise as an atomically thin and tunable platform for fundamental studies and future applications in quantum transport requires the ability to electrostatically define the geometry of the structure and control the carrier concentration, without compromising the quality of the system. Here, we demonstrate the working principle of a new generation of high-quality gate-defined graphene samples, where the challenge of doing so in a gapless semiconductor is overcome by using the ν = 0 insulating state, which emerges at modest applied magnetic fields. In order to verify that the quality of our devices is not compromised, we compare the electronic transport response of different sample geometries, paying close attention to fragile quantum states, such as the fractional quantum Hall states that are highly susceptible to disorder.

Quantitative analysis of 17 amino acids in tobacco leaves using an amino acid analyzer and chemometric resolution.

A method was developed for quantifying 17 amino acids in tobacco leaves by using an A300 amino acid analyzer and chemometric resolution. In the method, amino acids were eluted by the buffer solution on an ion-exchange column. After reacting with ninhydrin, the derivatives of amino acids were detected by ultraviolet detection.

Step-by-step fracture of two-layer stacked graphene membranes.

Layer-by-layer assembly of graphene has been proven to be an effective way to improve its mechanical properties, but its fracture mechanism, which is crucial for practical device applications, is still not clear and has not been fully studied yet. By consecutive stacking of two graphene monolayers, we fabricate two-layer stacked graphene membranes with a clean interface between the two layers. Fracture behavior of the two-layer stacked graphene membranes is studied using nanoindentation performed by atomic force microscopy.