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.

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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

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