Orientation birefringence and its wavelength dispersion are studied for hot-drawn films of cellulose esters such as cellulose triacetate (CTA), cellulose diacetate (CDA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB). The orientation birefringence of the cellulose esters is not proportional to the orientation function, indicating that the stress-optical law is not applicable for the cellulose esters. Furthermore, CTA shows negative birefringence, and the magnitude of the absolute value decreases with increasing the wavelength. On the contrary, CAP, CAB, and CDA show positive orientation birefringence that increases with the wavelength. The extraordinary wavelength dispersion of the orientation birefringence for CAP, CAB, and CDA is attributed to the difference in polarizability anisotropy of ester groups. Furthermore, the wavelength dependence of the orientation birefringence for CAP is dependent on the draw ratio and draw temperature, demonstrating that the contribution of polarizability anisotropy from each ester group to the orientation birefringence varies with the draw ratio and temperature. Moreover, the molecular weight and film-processing method also affect the orientation birefringence.

Bubbles at the interface of two-dimensional layered materials in van der Waals heterostructures cause deterioration in the quality of materials, thereby limiting the size and design of devices. In this paper, we report a simple all-dry transfer technique, with which the bubble formation can be avoided. As a key factor in the technique, a contact angle between a picked-up flake on a viscoelastic polymer stamp and another flake on a substrate was introduced by protrusion at the stamp surface. Using this technique, we demonstrated the fabrication of high-quality devices on the basis of graphene/hexagonal boron nitride heterostructures with a large bubble-free region. Additionally, the technique can be used to remove unnecessary flakes on a substrate under an optical microscopic scale. Most importantly, it improves the yield and throughput for the fabrication process of high-quality van der Waals heterostructure-based devices.

No abstract

Graphene superlattices have recently been attracting growing interest as an emergent class of quantum metamaterials. In this paper, we report the observation of nonlocal transport in bilayer graphene (BLG) superlattices encapsulated between two hexagonal boron nitride (hBN) layers, which formed hBN/BLG/hBN moiré superlattices. We then employed these superlattices to detect a long-range charge-neutral valley current using an all-electrical method. The moiré superlattice with broken inversion symmetry leads to a "hot spot" at the charge-neutral point (CNP), and it harbors satellites of the CNP. We observed nonlocal resistance on the order of 1 kΩ, which obeys a scaling relation. This nonlocal resistance evolves from an analog of the quantum Hall effect but without magnetic field/time-reversal symmetry breaking, which is associated with a hot-spot-induced topological valley current. This study should pave the way to developing a Berry-phase-sensitive probe to detect hot spots in gapped Dirac materials with inversion-symmetry breaking. _____________________________ Graphene superlattices are an emergent class of quantum metamaterials with considerable promise. In this letter, we explore the transport properties of bilayer graphene (BLG) superlattices, focusing on the topological current associated with a valley degree of freedom. A valley in an energy band implies a degenerate local minimum (maximum) in the conduction (valence) band and is referred to as K and K' in the case of graphene, which correspond to valley pseudospin states [1]. Graphene is associated with two valley pseudospin states, which transform into each other under spatial inversion. The broken inversion symmetry induces valleycontrasted physics through the emergence of a "hot spot" for each valley, which leads to the generation of valley Hall currents owing to the finite Berry curvature in the energy band [1,2]. The nonlocal transport is a Berry-phase-sensitive probe to detect such a hot spot in graphene superlattices [3,4,5]. More recently, quantum valley current was detected through the nonlocal resistance in the quantum limit [5]. This is in analogy with quantum Hall effect but without magnetic-field/time-reversal symmetry breaking, which is associated with hot-spot-induced

Graphene nanomesh (GNM) is formed by patterning graphene with nanometer-scale pores separated by narrow necks. GNMs are of interest due to their potential semiconducting characteristics when quantum confinement in the necks leads to an energy gap opening. GNMs also have potential for use in phonon control and water filtration. Furthermore, physical phenomena, such as spin qubit, are predicted at pitches below 10 nm fabricated with precise structural control. Current GNM patterning techniques suffer from either large dimensions or a lack of structural control. This work establishes reliable GNM patterning with a sub-10 nm pitch and an < 4 nm pore diameter by the direct helium ion beam milling of suspended monolayer graphene. Due to the simplicity of the method, no postpatterning processing is required. Electrical transport measurements reveal an effective energy gap opening of up to ∼450 meV. The reported technique combines the highest resolution with structural control and opens a path toward GNM-based, room-temperature semiconducting applications.

Placing a sensor close to the target at the nano-level is a central challenge in quantum sensing. We demonstrate magnetic field imaging with a boron vacancy (VB−) defects array in hexagonal boron nitride with a few 10 nm thickness. VB− sensor spots with a size of (100 nm)2 are arranged periodically with nanoscale accuracy using a helium ion microscope and attached tightly to a gold wire. The sensor array allows us to visualize the magnetic field induced by the current in the straight micro wire with a high spatial resolution. Each sensor exhibits a practical sensitivity of 73.6 μT/Hz0.5, suitable for quantum materials research. Our technique of arranging VB− quantum sensors periodically and tightly on measurement targets will maximize their potential.

No abstract

Germanene, a 2D honeycomb germanium crystal, is grown at graphene/Ag(111) and hexagonal boron nitride (h‐BN)/Ag(111) interfaces by segregating germanium atoms. A simple annealing process in N2 or H2/Ar at ambient pressure leads to the formation of germanene, indicating that an ultrahigh‐vacuum condition is not necessary. The grown germanene is stable in air and uniform over the entire area covered with a van der Waals (vdW) material. As an important finding, it is necessary to use a vdW material as a cap layer for the present germanene growth method since the use of an Al2O3 cap layer results in no germanene formation. The present study also proves that Raman spectroscopy in air is a powerful tool for characterizing germanene at the interfaces, which is concluded by multiple analyses including first‐principles density functional theory calculations. The direct growth of h‐BN‐capped germanene on Ag(111), which is demonstrated in the present study, is considered to be a promising technique for the fabrication of future germanene‐based electronic devices.