arXiv subjects

Yoshifumi Morita

Publications and source records attributed to Yoshifumi Morita.

11 recordsLinked to original sources

arXiv2025

Hexagonal boron nitride/bilayer graphene moiré superlattices in the Dirac-material family: energy-band engineering and carrier doping by dual gating

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.

cond-mat.mes-hall↗

arXiv2024

Gapped Dirac materials and quantum valley currents in dual-gated hBN/bilayer-graphene heterostructures

In gapped Dirac materials, the topological current associated with each valley can flow in opposite directions creating long-range charge-neutral valley currents. We report valley currents in hBN/bilayer-graphene heterostructures with an energy gap, which is tunable by a perpendicular electric (displacement) field in a dual-gated structure. We observed significant nonlocal resistance, consistent with the scaling theory of the valley Hall effect. In the low-temperature limit, the nonlocal resistance approaches a saturated value near the "quantum limit," indicating the emergence of quantum valley currents.

cond-mat.mes-hall↗

arXiv2024

Bilayer Graphene on hexagonal Boron Nitride and the Family of quantum MetaMaterial

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.

cond-mat.mes-hall↗

arXiv2022

Dual-gated hBN/bilayer-graphene superlattices and the transitions between the insulating phases at the charge neutrality point

We report on transport properties in dual-gated hexagonal boron nitride (hBN)/bilayer-graphene (BLG) superlattices. Here, BLG is nontwisted, i.e., plain. This paper focuses on the charge neutrality point (CNP) for a plain BLG. Under a perpendicular magnetic field, transitions between two insulating phases at the CNP are detected by varying a displacement field with the study on the resistance-temperature characteristics and the magnetoresistance. This work opens avenues for exploring the global phase diagram of the hBN/BLG superlattices beyond the CNP.

cond-mat.mes-hall↗

arXiv2021

Electron transport in folded bilayer-bilayer graphene/hexagonal boron nitride superlattices under high magnetic fields

Employing graphene as a template, we fabricate moiré superlattices by stacking bilayer or folded bilayer-bilayer graphene (BLG or fBBLG) and hexagonal boron nitride (hBN), i.e., hBN/BLG/hBN or hBN/fBBLG/hBN stacks, with a small twist angle between the graphene and one of the two hBN layers. Because of the modulation due to the hBN, higher-generation Dirac points can emerge with a narrow bandwidth and van Hove singularities. In the moiré superlattice devices, we can therefore access the higher-generation Dirac points by in-situ gate tuning. This study is based on our previous paper (Appl. Phys. Express 13, 035003 (2020)). Here we show more extended data by applying high magnetic fields up to ~24 T. We also comment on the temperature dependence of the resistivity and magnetoresistance with reference to the 'plain' BLG data for a comparative study.

cond-mat.mes-hall↗

arXiv2020

Single-Carrier Transport in Graphene/hBN Superlattices

Graphene/hexagonal boron nitride (hBN) moiré superlattices have attracted interest for use in the study of many-body effects and fractal physics in Dirac fermion systems. Many exotic transport properties have been intensively examined in such superlattices, but previous studies have not focused on single-carrier transport. The investigation of the single-carrier behavior in these superlattices would lead to an understanding of the transition of single-particle/correlated phenomena. Here, we show the single-carrier transport in a high-quality bilayer graphene/hBN superlattice-based quantum dot device. We demonstrate remarkable device controllability in the energy range near the charge neutrality point (CNP) and the hole-side satellite point. Under a perpendicular magnetic field, Coulomb oscillations disappear near the CNP, which could be a signature of the crossover between Coulomb blockade and quantum Hall regimes. Our results pave the way for exploring the relationship of single-electron transport and fractal quantum Hall effects with correlated phenomena in two-dimensional quantum materials.

cond-mat.mes-hall↗

arXiv2019

Observation of superconductivity in bilayer graphene/hexagonal boron nitride superlattices

A class of low-dimensional superconductivity (SC), such as most of "atomic-layer" SCs, has survived only under certain circumstances, implying a role of the substrate. Moreover, in some recent SC discoveries at heterogeneous interfaces, SC was buried in bulk solids and ex situ. Genuine atomic-layer SC is difficult to access. Here we report a novel route to atomic-layer SC in graphene superlattices. Our device comprises stacked non-twisted bilayer graphene (BLG) and hexagonal boron nitride (hBN), i.e., hBN/BLG/hBN Moiré superlattices. Upon in situ electrostatic doping, we observe an SC dome with a critical temperature up to $T_{\rm{BKT}} = 14 \rm{K}$, corresponding to the confinement of vortices. We believe that SC via doping Dirac materials is ubiquitous in condensed matter and that this study paves a way toward the design of a new SC family.

cond-mat.supr-con↗

arXiv2019

Topological Valley Currents in Bilayer Graphene/Hexagonal Boron Nitride Superlattices

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 with Berry curvature accumulating at the charge neutral point (CNP), and it harbors satellites of the CNP. We observed nonlocal resistance on the order of 1 $\text{k}Ω$, which obeys a scaling relation. This nonlocal resistance evolves from 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.

cond-mat.mes-hall↗

arXiv2019

Fabry-Pérot resonances and a crossover to the quantum Hall regime in ballistic graphene quantum point contacts

We report on the observation of quantum transport and interference in a graphene device that is attached with a pair of split gates to form an electrostatically-defined quantum point contact (QPC). In the low magnetic field regime, the resistance exhibited Fabry-Pérot (FP) resonances due to np'n (pn'p) cavities formed by the top gate. In the quantum Hall (QH) regime with a high magnetic field, the edge states governed the phenomena, presenting a unique condition where the edge channels of electrons and holes along a p-n junction acted as a solid-state analogue of a monochromatic light beam. We observed a crossover from the FP to QH regimes in ballistic graphene QPC under a magnetic field with varying temperatures. In particular, the collapse of the QH effect was elucidated as the magnetic field was decreased. Our high-mobility graphene device enabled observation of such quantum coherence effects up to several tens of kelvins. The presented device could serve as one of the key elements in future electronic quantum optic devices.

cond-mat.mes-hall↗

arXiv2013

Teleoperation System Using Past Image Records Considering Narrow Communication Band

Teleoperation is necessary when the robot is applied to real missions, for example surveillance, search and rescue. We proposed teleoperation system using past image records (SPIR). SPIR virtually generates the bird's-eye view image by overlaying the CG model of the robot at the corresponding current position on the background image which is captured from the camera mounted on the robot at a past time. The problem for SPIR is that the communication bandwidth is often narrow in some teleoperation tasks. In this case, the candidates of background image of SPIR are few and the position of the robot is often delayed. In this study, we propose zoom function for insufficiency of candidates of the background image and additional interpolation lines for the delay of the position data of the robot. To evaluate proposed system, an outdoor experiments are carried out. The outdoor experiment is conducted on a training course of a driving school.

cs.RO↗

arXiv1997

Near Critical States of Random Dirac Fermions

Random Dirac fermions in a two-dimensional space are studied numerically. We realize them on a square lattice using the $π$-flux model with random hopping. The system preserves two symmetries, the time-reversal symmetry and the symmetry denoted by ${{\cal H},γ}=0$ with a $4\times 4$ matrix $γ$ in an effective field theory. Although it belongs to the orthogonal ensemble, the zero-energy states do not localize and become critical. The density of states vanishes at zero energy as $\sim E^α$ and the exponent $α$ changes with strength of the randomness, which implies the existence of the critical line. Rapid growth of the localization length near zero energy is suggested and the eigenstates near zero energy exhibit anomalous behaviour which can be interpreted as a critical slowing down in the available finite-size system. The level-spacing distributions close to zero energy deviate from both the Wigner surmise and the Poissonian, and exhibit critical behaviour which reflects the existence of critical states at zero energy.

cond-mat↗