Takuya Iwasaki

Negatively charged boron vacancy (VB−) defects in hexagonal boron nitride (hBN) are promising for nanoscale-proximity quantum sensing. To evaluate their performance, it is important to characterize the spin-coherence times T2* and T2. In this study, we realized subgigahertz Rabi oscillations of VB− using an isotopically enriched h10B15N thin film directly stamped onto a narrow gold wire. Using these strong microwave pulses, we performed Ramsey interference and Hahn-echo measurements. The Ramsey interference signal showed Gaussian-like decay, yielding T2*=13.8 ± 0.5 ns. The Hahn-echo measurement gave T2=108.7 ± 5.5 ns and a stretch factor of αHE=1.25 ± 0.11. These results experimentally clarify the spin-coherence properties of VB− and provide an effective method for evaluating the coherence of spin defects in van der Waals thin films with broad resonance linewidths.

Hydrogen-terminated diamond field-effect transistors (FETs) using a hexagonal boron nitride (h-BN) gate insulator were fabricated on a diamond surface with reduced surface roughness in the direction of source/drain electrodes. The diamond surface was prepared on a mesa structure using chemical vapor deposition with a low methane concentration. The hydrogen-terminated surface was laminated with the h-BN gate insulator without air exposure to prevent the adsorption of atmospheric surface acceptors. The hydrogen-terminated diamond FET exhibited a high mobility of ≈1000 cm2 V−1 s−1 at room temperature. We performed theoretical analysis on the temperature and carrier density dependences of mobility, which suggested that Coulomb and surface roughness scattering were effectively reduced. The high mobility obtained in this study indicates the high potential of diamond as a semiconducting material. This study can contribute to the future development of diamond devices.

We review the fabrication and transport characterization of hexagonal boron nitride (hBN)/Bernal bilayer graphene (BLG) moiré superlattices. Due to the moiré effect, the hBN/BLG moiré superlattices exhibit an energy gap at the charge neutrality point (CNP) even in the absence of a perpendicular electric field. In BLG, the application of a perpendicular electric field tunes the energy gap at the CNP, which contrasts with single-layer graphene and is similar to the family of rhombohedral multilayer graphene. The hBN/BLG moiré superlattice is associated with non-trivial energy-band topology and a narrow energy band featuring a van Hove singularity. By employing a dual-gated device structure where both the perpendicular displacement field and the carrier density are individually controllable, systematic engineering of the energy-band structure can be achieved. The data presented here demonstrate the universality and diversity in the physics of hBN/BLG moiré superlattices.

We review the fabrication and transport characterization of hexagonal boron nitride (hBN)/Bernal bilayer graphene (BLG) moiré superlattices. Due to the moiré effect, the hBN/BLG moiré superlattices exhibit an energy gap at the charge neutrality point (CNP) even in the absence of a perpendicular electric field. In BLG, the application of a perpendicular electric field tunes the energy gap at the CNP, which contrasts with single-layer graphene and is similar to the family of rhombohedral multilayer graphene. The hBN/BLG moiré superlattice is associated with non-trivial energy-band topology and a narrow energy band featuring a van Hove singularity. By employing a dual-gated device structure where both the perpendicular displacement field and the carrier density are individually controllable, systematic engineering of the energy-band structure can be achieved. The data presented here demonstrate the universality and diversity in the physics of hBN/BLG moiré superlattices.

We report on the fabrication and characterization of dual-gated hexagonal boron nitride (hBN)/bilayer-graphene (BLG) superlattices. Due to the moire effect, the hBN/BLG superlattice harbors an energy gap at the charge neutral point (CNP) and the satellites even without a perpendicular electric field. In BLG, moreover, the application of a perpendicular electric field tunes the energy gap, which contrasts with the single-layer graphene (SLG) and is linked to the family of rhombohedral multilayer graphene. Therefore, the hBN/BLG superlattice is accompanied with non-trivial energy-band topology and a narrow energy band with van Hove singularities. By the dual gating, systematic engineering of the energy-band structure can be performed and the carrier concentration is fine-tunable. This review is an extended version of the talk based on ref. [1], which is also a supplementary to the ref. T. Iwasaki, Y. Morita, K. Watanabe, T. Taniguchi, Phys. Rev. B106, 165134 (2022) and Phys. Rev. B109, 075409 (2024). The data show the universality and diversity in the physics of the hBN/BLG superlattices.

Diamond electronic devices have attracted significant interest owing to their excellent semiconducting properties. We recently demonstrated that eliminating surface-transfer doping enhances carrier mobility and achieves normally off behavior in diamond field-effect transistors (FETs) with a hexagonal boron nitride (h-BN) gate insulator. In our previous study, the gate electrode was overlapped onto the source/drain electrodes to prevent the increase in access resistance caused by excluding surface-transfer doping. However, it is known that gate overlap increases parasitic capacitance and gate leakage current. In this study, we developed a technique for self-aligning the gate electrode with the edge of h-BN using oblique-angle deposition. The diamond FET with a self-aligned gate electrode exhibits optimal FET characteristics, including high mobility of ≈ 400 cm2V−1s−1, low sheet resistance of 2.4 kΩ, and output characteristics demonstrating pinch-off behavior. Furthermore, the capacitance-voltage characteristics clearly indicate distinct ON and OFF states, validating the efficacy of this technique. This method enables the fabrication of diamond/h-BN FETs with no gate overlap and without increasing access resistance, making it a promising approach for developing high-speed, low-loss diamond FETs with a wide range of applications.

In-memory computing may make it possible to realize non-von Neumann computing because the logic circuits are unified in the memory units. We investigated two types of in-memory logic operations, namely, two-input logic circuits and multifunctional artificial synapses. These were realized in a dual-gate antiambipolar transistor (AAT) with a ReS₂/WSe₂ heterojunction, in which polystyrene with a zinc phthalocyanine core (ZnPc-PS₄) was incorporated as a memory layer. Here, reconfigurability is a key concept for both types of device operations and was achieved by merging the Λ-shaped transfer curve of the AAT and the nonvolatile memory effect of ZnPc-PS₄. First, we achieved electrically reconfigurable two-input logic circuits. Versatile logic circuits such as AND, OR, NAND, NOR, and XOR circuits were demonstrated by taking advantage of the Λ-shaped transfer curve of the dual-gate AAT. Importantly, the nonvolatile memory function provided the electrical switching of the individual circuits between AND/OR, NAND/NOR, and XOR/NAND circuits with constant input signals. Second, the memory effect was applied to multifunctional artificial synapses. The inhibitory/excitatory and long-term potentiation/depression synaptic operations were electrically reconfigured simply by controlling one parameter (readout voltage), making three distinct responses possible even with the same presynaptic signals. These findings provide hints that may lead to the realization of new in-memory computing architectures beyond the current von Neumann computers.

A direct focused He⁺ beam direct machining is presented to fabricate solid-state nano-disk at the surface of a graphene multilayer micro-flake deposited on an Au/Ti/sapphire surface. At irradiation doses larger than 5.0 × 10¹⁷ ions cm⁻² and with a beam size well below 1 nm, graphene disks down to 20 nm in diameter have been machined with for nano-disk down to 50 nm in diameter, a central hole for preparing the positioning of a rotation axle. The local heat generated by this irradiation is inducing a partial graphene amorphization and deformation, leading to a complete graphene nano-disk vaporization at doses larger than 5 × 10¹⁸ ions cm⁻². A dry transfer printing technique followed by a graphene surface cleaning was used to transfer the nano-disks from its initial surface to a fresh and clean surface. Tapping mode atomic force micrograph have been recorded to follow the vaporization as a function of the He⁺ dose to confirm the graphene solid-state nano-disk fabrication limit to about 20 nm with this process.