arXiv subjects

Tomoki Machida

Publications and source records attributed to Tomoki Machida.

At least 19 recordsLinked to original sources

arXiv2026

Janus-induced atomic reconstruction amplifies twist-angle modulation of interlayer thermal transport in moir\'e bilayers

In two-dimensional moir\'e bilayers, atomic reconstruction, the spontaneous structural relaxation toward energy-minimizing stacking registries in the near-commensurate regime, can strongly modify local stacking and interlayer coupling, providing a possibility to significantly control phonon-mediated properties. Here we show that the twist-angle dependence of interlayer thermal conductance can be modified by introducing Janus-induced mirror-symmetry breaking into bilayer MoS2. The intrinsic out-of-plane dipole in MoSSe/MoS2 bilayers leads to frictionless interface, and reduces the lattice deformation energy, thereby promoting atomic-reconstruction into locally distorted aperiodic moir\'e patterns. These features weaken interlayer coupling and suppress phonon transmission across the interface, leading to an anomalously strong twist-angle dependence of thermal conductance, with a pronounced minimum at small twist angles and a reduction rate nearly one order of magnitude larger than that of twisted bilayer MoS2. Our results demonstrate that interlayer thermal transport is modified by atomic reconstruction, highlighting Janus-induced mirror-symmetry breaking as an effective way to promoting phonon engineering in two-dimensional moir\'e structures.

cond-mat.mes-hall↗

arXiv2026

Raman spectroscopy of van der Waals topological magnet GdGaI

We report polarization-resolved Raman spectroscopy of a van der Waals compound GdGaI that is a candidate for excitonic insulators. By combining the symmetry analysis with density functional theory calculations, we identify six Raman-active phonons. The spectra exhibit only the expected anharmonic hardening down to 4 K: no additional peaks, no soft modes, and no signatures of zone folding are observed. This result indicates that any lattice distortion is below our experimental sensitivity, supporting an electronically driven origin for the band reconstruction reported by angle-resolved photoemission spectroscopy rather than an electron-phonon-driven mechanism. Moreover, we observe a pronounced circular dichroism of the $A_{1g}$ modes under an out-of-plane magnetic field. Based on symmetry considerations, we attribute this dichroic response to chiral $A_{1g}$ phonons with opposite angular momenta generated by spin-phonon coupling in the time-reversal-broken state. The temperature evolution of the degree of circular polarization further suggests that circularly polarized Raman spectroscopy detects the emergence of short-range antiferromagnetic correlations. Our results highlight GdGaI as a promising platform in which excitonic order, magnetism, and circularly polarized phonons can be intertwined, and demonstrate that circular-polarization Raman provides a sensitive probe of spin-phonon coupling in excitonic systems.

cond-mat.mtrl-sci↗

arXiv2026

Twist-configured moire-moire reconstruction governs diverse commensurate double-moire phases in twisted bilayer graphene on h-BN

The coexistence of multiple moire lattices in van der Waals heterostructures raises a fundamental question: how do distinct moire patterns interact and reconstruct? Here, we investigate twisted bilayer graphene (tBG) on hexagonal boron nitride (h-BN), where tBG and graphene/h-BN moire structures coexist, using conductive atomic force microscopy combined with continuum-model simulations. We show that reconstruction between these moire lattices-moire-moire reconstruction-manifests across multiple length scales, giving rise to diverse commensurate double-moire phases. Locally, the stacking registry between the two moire lattices is uniquely selected by the global twist configuration (helical or alternate), mediated by rotational relaxation of the shared graphene layer. This registry, together with twist angle and strain, governs commensurate domains from C3z-symmetric to strained symmetry-modified structures. These results establish moire-moire reconstruction as a general framework for engineering structural and electronic order -- including theoretically predicted topological flat bands below the magic angle -- in multilayer moire materials.

cond-mat.mtrl-sci↗

arXiv2026

One-Dimensional Electronic States in a Moir\'e Superlattice of Twisted Bilayer WTe2

One-dimensional (1D) moir\'e superlattices provide a new route to engineering reduced-dimensional electronic states in van der Waals materials, yet their electronic structure and microscopic origin remain largely unexplored. Here, we investigate the structural relaxation and electronic properties of a 1D moir\'e superlattice formed in twisted bilayer 1T$'$-WTe$_2$ using density functional theory calculations, complemented by high-angle annular dark-field scanning transmission electron microscopy. We show that lattice relaxation strongly reconstructs the moir\'e stripes, leading to stacking-dependent stripe widths that are in excellent agreement with experimental observations. The relaxed structure hosts quasi-one-dimensional electronic bands near the Fermi level, characterized by strong dispersion along the stripe direction and nearly flat dispersion in the perpendicular direction. By comparing the full bilayer with isolated relaxed layers, we establish that these 1D electronic states are governed predominantly by an intralayer moir\'e potential induced by in-plane lattice relaxation, rather than by interlayer hybridization. We extract this position-dependent moir\'e potential directly from DFT calculations and construct an effective tight-binding model that reproduces both the band dispersion and the real-space localization of the electronic wave functions. Our results identify lattice relaxation as the key mechanism underlying 1D electronic states in 1D moir\'e superlattices. %and establish twisted bilayer WTe$_2$ as a promising platform for exploring emergent one-dimensional moir\'e physics. The framework developed here provides a unified theoretical basis for realizing and exploring one-dimensional moir\'e physics in a broad class of anisotropic two-dimensional materials.

cond-mat.mes-hall↗

arXiv2026

Emergent Cooperative Superstructures via Order-Disorder Kinetics in Molecule-Intercalated NbSe2

The design of quantum states at heterointerfaces has enabled a variety of emergent phenomena. Among them, molecular intercalation superlattices have attracted attention as tunable hybrid materials, formed by inserting organic molecules into van der Waals crystals, where molecular structure and chemistry provide new degrees of freedom. Traditionally, the intercalated molecules have been regarded as inactive spacers, while possible molecular ordering and its impact on the host lattice have remained largely unexplored. Here, we report the discovery of a cooperative superstructure (CSS) phase in molecule intercalated NbSe2, where ordering of the guest molecules induce a concomitant superstructure in the NbSe2 host lattice, characterized by a moir\'e structure due to incommensurability between the molecular layer and the inorganic lattice. Synchrotron X-ray diffraction reveals the emergence of CSS phase, accompanied by crystal symmetry lowering. Complementary resistivity and thermal-quench measurements show that the transition is governed by unusually slow order-disorder kinetics, so that the CSS phase can be selectively accessed under standard laboratory cooling rates. This kinetic behavior arises from slow molecular dynamics coupled to the host lattice, contrasting with fast charge or magnetic ordering in inorganic solids. Our findings establish molecular ordering as a route for engineering heterointerfaces, enabling thermally programmable superstructures.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic-field-induced insulating behavior in black phosphorus under pressure

We investigated the out-of-plane magnetoresistance of pressurized black phosphorus (BP) with a longitudinal field configuration. Despite the absence of the Lorentz force in the present configuration, we observed a significant enhancement of magnetoresistance marked with a clear onset field in both the semiconducting (1.1 GPa) and semimetallic (1.3 GPa) phases. The insulating behavior observed near the semiconductor-semimetal transitio pressure is possibly associated with emergence of an excitonic phase, which has been suggested in a recent theoretical study. BP under finely tuned pressure can be a candidate to realize the field-induced electronic phase transition in a moderate magnetic field below 9 T.

cond-mat.str-el↗

arXiv2025

Direct observation of band structure modifications from monolayer WSe2 to Janus WSSe

Janus monolayer transition metal dichalcogenides (TMDs), created by post-growth substitution of the top chalcogen layer, represent a new direction for engineering 2D crystal properties. However, their rapid ambient degradation and the difficulty of obtaining large-area monolayer samples have limited the available experimental probes, leaving their detailed electronic structure near the Fermi level largely unexplored. In this work, by performing micro-focused angle-resolved photoemission spectroscopy ({\mu}-ARPES) on an identical sample transformed from monolayer WSe2 to Janus WSSe via a H2 plasma-assisted chalcogen-exchange method, we reveal the evolution of its electronic band structure. We observe ARPES signature consistent with the Rashba-type spin splitting due to broken horizontal mirror symmetry, and a significant upward shift of the highest valence band at the {\Gamma}-point by approximately 160 meV. These direct observations clarify the key electronic modifications that govern the material's properties and provide a pathway for band engineering in Janus TMDs.

cond-mat.mtrl-sci↗

arXiv2024

Tunneling photo-thermoelectric effect in monolayer graphene/bilayer hexagonal boron nitride/bilayer graphene asymmetric van der Waals tunnel junctions

Graphene is known to exhibit a pronounced photo-thermoelectric effect (PTE) in its in-plane carrier transport and attracting attention toward various optoelectronic applications. In this study, we demonstrate an out-of-plane PTE by utilizing electron tunneling across a barrier, namely, the tunneling photo-thermoelectric effect (TPTE). This was achieved in a monolayer graphene (MLG)/bilayer hexagonal boron nitride (h-BN)/bilayer graphene (BLG) asymmetric tunnel junction. MLG and BLG exhibit different cyclotron resonance (CR) optical absorption energies when their energies are Landau quantized under an out-of-plane magnetic field. We tuned the magnetic field under mid-infrared (MIR) irradiation to bring MLG into CR conditions, whereas BLG was not in CR. The CR absorption in the MLG generates an electron temperature difference between the MLG and BLG, and induces an out-of-plane TPTE voltage across the h-BN tunnel barrier. The TPTE exhibited a unique dependence on the Fermi energy of the MLG, which differed from that of the in-plane PTE of the MLG. The TPTE signal was large when the Fermi energy of the MLG was tuned near the phase transition between the quantum Hall state (QHS) and non-QHS, that is, the transition between carrier localization and delocalization. The TPTE provides another degree of freedom for probing the electronic and optoelectronic properties of two-dimensional material heterostructures.

cond-mat.mes-hall↗

arXiv2023

Ultrafast Control of Crystal Structure in a Topological Charge-Density-Wave Material

Optical control of crystal structures is a promising route to change physical properties including topological nature of a targeting material. Time-resolved X-ray diffraction measurements using the X-ray free-electron laser are performed to study the ultrafast lattice dynamics of VTe$_2$, which shows a unique charge-density-wave (CDW) ordering coupled to the topological surface states as a first-order phase transition. A significant oscillation of the CDW amplitude mode is observed at a superlattice reflection as well as Bragg reflections. The frequency of the oscillation is independent of the fluence of the pumping laser, which is prominent to the CDW ordering of the first-order phase transition. Furthermore, the timescale of the photoinduced 1$T^{\prime\prime}$ to 1$T$ phase transition is independent of the period of the CDW amplitude mode.

cond-mat.str-el↗

arXiv2022

Observation of phonon Poiseuille flow in isotopically-purified graphite ribbons

In recent times, the unique collective transport physics of phonon hydrodynamics motivates theoreticians and experimentalists to explore it in micro- and nanoscale and at elevated temperatures. Graphitic materials have been predicted to facilitate hydrodynamic heat transport with their intrinsically strong normal scattering. However, owing to the experimental difficulties and vague theoretical understanding, the observation of phonon Poiseuille flow in graphitic systems remains challenging. In this study, based on a microscale experimental platform and the pertinent occurrence criterion in anisotropic solids, we demonstrate the phonon Poiseuille flow in a 5 {\mu}m-wide suspended graphite ribbon with purified 13C isotope concentration. Our observation is well supported by our theoretical model based on a kinetic theory with fully first-principles inputs. Thus, this study paves the way for deeper insight into phonon hydrodynamics and cutting-edge heat manipulating applications.

cond-mat.mes-hall↗

arXiv2020

Selective Etching of Hexagonal Boron Nitride by High-Pressure CF4 Plasma for Individual One-dimensional Ohmic Contacts to Graphene Layers

We describe a technique for making one-dimensional ohmic contacts to individual graphene layers encapsulated in hexagonal boron nitride (h-BN) using CF4 and O2 plasmas. The high etch selectivity of h-BN against graphene (>1000) is achieved by increasing the plasma pressure, which enables etching of h-BN, while graphene acts as an etch stop to protect underlying h-BN. A low-pressure O2 plasma anisotropically etches graphene in the vertical direction, which exposes graphene edges at h-BN sidewalls. Despite the O2 plasma bombardment, the lower h-BN layer functions as an insulating layer. Thus, this method allows us to pattern metal electrodes on h-BN over a second graphene layer. Subsequent electron-beam lithography and evaporation fabricate metal contacts at the graphene edges that are active down to cryogenic temperatures. This fabrication method is demonstrated by the preparation of a graphene Hall bar with a graphite back-gate and double bilayer-graphene Hall bar devices. The high flexibility of the device geometries enabled by this method creates access to a variety of experiments on electrostatically coupled graphene layers.

cond-mat.mtrl-sci↗

arXiv2020

Dark-state impact on the exciton recombination of WS2 monolayers as revealed by multi-time-scale pump-and-probe spectroscopy

The luminescence yield of transition metal dichalcogenide monolayers frequently suffers from the formation of long-lived dark states, which include excitons with intervalley charge carriers, spin-forbidden transitions, and a large center-of-mass momentum located outside the light cone of dispersion relations. Efficient relaxation from bright exciton states to dark states suppresses the quantum yield of photon emission. In addition, the radiative recombination of excitons is heavily influenced by Auger-type exciton-exciton scattering, which yields another nonradiative relaxation channel at room temperature. Here, we show that Auger-type scattering is promoted not only between (bright) excitons but also between excitons and long-lived dark states. We studied the luminescence dynamics of monolayer WS2 capped with hexagonal BN over broad time ranges of picoseconds to milliseconds using carefully designed pump-and-probe techniques. We observed that luminescence quenching associated with Auger-type scattering occurs on 1-100 microsecond time scales, which thus correspond to the lifetimes of the relevant dark states. The broad distribution of the measured lifetimes implies the impact of various types of long-lived states on the exciton annihilation process.

cond-mat.mes-hall↗

arXiv2019

Deep-Learning-Based Image Segmentation Integrated with Optical Microscopy for Automatically Searching for Two-Dimensional Materials

Deep-learning algorithms enable precise image recognition based on high-dimensional hierarchical image features. Here, we report the development and implementation of a deep-learning-based image segmentation algorithm in an autonomous robotic system to search for two-dimensional (2D) materials. We trained the neural network based on Mask-RCNN on annotated optical microscope images of 2D materials (graphene, hBN, MoS2, and WTe2). The inference algorithm is run on a 1024 x 1024 px2 optical microscope images for 200 ms, enabling the real-time detection of 2D materials. The detection process is robust against changes in the microscopy conditions, such as illumination and color balance, which obviates the parameter-tuning process required for conventional rule-based detection algorithms. Integrating the algorithm with a motorized optical microscope enables the automated searching and cataloging of 2D materials. This development will allow researchers to utilize unlimited amounts of 2D materials simply by exfoliating and running the automated searching process.

eess.IV↗

arXiv2019

Detection of cyclotron resonance using photo-induced thermionic emission at graphene/MoS2 van der Waals interface

We demonstrate the detection of cyclotron resonance in graphene by using a photo-induced thermionic emission mechanism at the graphene/MoS2 van der Waals (vdW) Schottky junction. At cyclotron resonance in Landau-quantized graphene, the infrared light is absorbed and an electron-hole pair is generated. When the energy of a photoexcited electron exceeds the band offset energy at the graphene/MoS2 interface, the electron transfer occurs from graphene to the conduction band of MoS2, and the hole remains in graphene. This creates an electron-hole separation at the graphene/MoS2 interface at cyclotron resonance and a photovoltage is generated. The proposed method is an infrared photodetection technique through out-of-plane transport at the vdW junction, which is distinct from the previously reported methods that use in-plane transport in graphene for electronic detection of the cyclotron resonance. Despite the simple structure of our device with a single-vdW junction, our method exhibits a very high sensitivity of about 10^6 V/W, which shows an improvement of three orders of magnitude over the previously reported values. Therefore, the proposed method displays a high potential for cyclotron resonance-based infrared photodetector applications.

cond-mat.mes-hall↗

arXiv2019

Photo-Nernst detection of cyclotron resonance in partially irradiated graphene

Cyclotron resonance of a Landau-quantized graphene can absorb significant amount of infrared light. However, application of this phenomenon to the photodetector had been limited due to the lack of efficient photon to charge conversion scheme. Here, we demonstrate the detection of cyclotron resonance in a partially metal-masked monolayer graphene two-terminal device using photo-Nernst effect. Due to the presence of the mask, incident infrared light is irradiated on only one-half of the graphene channel. This partial irradiation creates a temperature gradient perpendicular to the graphene channel. In the presence of an external magnetic field, thermopower is generated perpendicular to the temperature gradient due to the Nernst effect. Consequently, photo-Nernst voltage is generated along the graphene channel, which can be detected from the contacts on both ends of the channel. We demonstrate selective detection of the photo-Nernst effect while minimizing the other photovoltaic contributions, such as the photo-Seebeck effect. We investigate the dependence of the photo-Nernst effect on magnetic field and excitation wavelength, which reveals a significant enhancement of photo-Nernst signal at the cyclotron resonance conditions in graphene. Our finding could facilitate the realization of far-infrared light detector using cyclotron resonance of graphene.

cond-mat.mes-hall↗

arXiv2019

Dry release transfer of graphene and few-layer h-BN by utilizing thermoplasticity of polypropylene carbonate for fabricating edge-contact-free van der Waals heterostructures

The dry release transfer of two-dimensional (2D) materials such as graphene, h-BN, and TMDs is a versatile method for fabricating high-quality van der Waals heterostructures. Up until now, polydimethylpolysiloxane (PDMS) sheets have been widely used for the dry release transfer of TMD materials. However, this method has been known to have limitations that make it difficult to transfer few-layer-thick graphene and h-BN because of the difficulty to fabricate these materials on PDMS. As an alternative method, we demonstrate the dry release transfer of single- and bi-layer graphene and few-layer h-BN in this study by utilizing poly(propylene) carbonate (PPC) films. Because of the strong adhesion between PPC and 2D materials around room temperature, we demonstrate that single- to few-layer graphene, as well as few-layer h-BN, can be fabricated on a spin-coated PPC film/290-nm-thick SiO2/Si substrate via the mechanical exfoliation method. In addition, we show that these few-layer crystals are clearly distinguishable using an optical microscope with the help of optical interference. Because of the thermoplastic properties of PPC film, the adhesion force between the 2D materials and PPC significantly decreases at about 70 {\deg}C. Therefore, we demonstrate that single- to few-layer graphene, as well as few-layer h-BN flakes, on PPC can be easily dry-transferred onto another h-BN substrate. This method enables a multilayer van der Waals heterostructure to be constructed with a minimum amount of polymer contamination. We demonstrate the fabrication of encapsulated h-BN/graphene/h-BN devices and graphene/few-layer h-BN/graphene vertical-tunnel-junction devices using this method. Since devices fabricated by this method do not require an edge-contact scheme, our finding provide a simples method for constructing high-quality graphene and h-BN-based van der Waals heterostructures.

physics.app-ph↗

arXiv2018

Optical coupling between atomically-thin black phosphorus and a two dimensional photonic crystal nanocavity

Atomically-thin black phosphorus (BP) is an emerging two dimensional (2D) material exhibiting bright photoluminescence in the near infrared. Coupling its radiation to photonic nanostructures will be an important step toward the realization of 2D material based nanophotonic devices that operate efficiently in the near infrared, which includes the technologically important optical telecommunication wavelength bands. In this letter, we demonstrate the optical coupling between atomically-thin BP and a 2D photonic crystal nanocavity. We employed a home-build dry transfer apparatus for placing a thin BP flake on the surface of the nanocavity. Their optical coupling was analyzed through measuring cavity mode emission under optical carrier injection at room temperature.

cond-mat.mes-hall↗

arXiv2018

Photo-thermoelectric detection of cyclotron resonance in asymmetrically carrier-doped graphene two-terminal device

Graphene is known to show a significant photo-thermoelectric effect that can exceed its photovoltaic contribution. Here, by utilizing this effect, we demonstrate a photovoltage measurement of cyclotron resonance in a double-back-gated h-BN/graphene/h-BN two-terminal device. A graphite local bottom gate was fabricated in addition to a p-doped Si global back gate. By tuning the two gate voltages, an in-plane graphene junction having an asymmetric carrier-doping profile was created. With the help of this asymmetric structure, the photo-thermoelectric voltage generated in the vicinity of the metal-electrode/graphene junction was detected. At a low temperature and in the presence of a magnetic field, a photo-induced voltage was measured under the irradiation of an infrared laser (Wavelength= 9.28 to 10.61 micron). We observed a strong enhancement of the photovoltage signal under the cyclotron resonance condition, at which the energy of excitation coincides with a transition between Landau levels. These results highlight the possibility of using the photo-thermoelectric effect in graphene for THz photo-detection.

cond-mat.mes-hall↗