arXiv subjects

Satoru Masubuchi

Publications and source records attributed to Satoru Masubuchi.

At least 19 recordsLinked to original sources

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↗

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 μm-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

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

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

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

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 °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

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↗

arXiv2018

Heat transfer at the van der Waals interface between graphene and NbSe2

Graphene has been widely used to construct low-resistance van der Waals (vdW) contacts to other two-dimensional (2D) materials. However, a rise of electron temperature of the graphene under a current flow has not been seriously considered in many applications. Owing to its small electronic heat capacity and electron-phonon coupling, electron temperature of the graphene can be increased easily by the application of current. The heat generated within the graphene is transferred to the contacted 2D materials through the vdW interface and potentially influences their properties. Here, we compare the superconducting critical currents of an NbSe2 flake for two different methods of current application: with a Au/Ti electrode fabricated by thermal evaporation and with a graphene electrode contacted to the NbSe2 flake through a vdW interface. The influence of the heat transfer from the graphene to NbSe2 is detected through the change of the superconductivity of NbSe2. We found that the critical current of NbSe2 significantly reduces when the current is applied with the graphene electrode compared to that from the conventional Au/Ti electrode. Further, since the electron heating in graphene exhibits ambipolar back-gate modulation, we demonstrate the electric field modulation of the critical current in NbSe2 when the current is applied with graphene electrode. These results are attributed to the significant heat transfer from the graphene electrode to NbSe2 through vdW interface.

cond-mat.mes-hall↗

arXiv2017

Imaging bulk and edge transport near the Dirac point in graphene moiré superlattices

Van der Waals structures formed by aligning monolayer graphene with insulating layers of hexagonal boron nitride exhibit a moiré superlattice that is expected to break sublattice symmetry. Despite an energy gap of several tens of millielectron volts opening in the Dirac spectrum, electrical resistivity remains lower than expected at low temperature and varies between devices. While subgap states are likely to play a role in this behavior, their precise nature is unclear. We present a scanning gate microscopy study of moiré superlattice devices with comparable activation energy but with different charge disorder levels. In the device with higher charge impurity (~${10}^{10}$ $cm^{-2}$) and lower resistivity (~$10$ $kΩ$) at the Dirac point we observe current flow along the graphene edges. Combined with simulations, our measurements suggest that enhanced edge doping is responsible for this effect. In addition, a device with low charge impurity (~$10^9$ $cm^{-2}$) and higher resistivity (~$100$ $kΩ$) shows subgap states in the bulk, consistent with the absence of shunting by edge currents.

cond-mat.mes-hall↗

arXiv2017

Exfoliation and van der Waals heterostructure assembly of intercalated ferromagnet Cr1/3TaS2

Ferromagnetic van der Waals (vdW) materials are in demand for spintronic devices with all-two-dimensional-materials heterostructures. Here, we demonstrate mechanical exfoliation of magnetic-atom-intercalated transition metal dichalcogenide Cr1/3TaS2 from its bulk crystal; previously such intercalated materials were thought difficult to exfoliate. Magnetotransport in exfoliated tens-of-nanometres-thick flakes revealed ferromagnetic ordering below its Curie temperature TC ~ 110 K as well as strong in-plane magnetic anisotropy; these are identical to its bulk properties. Further, van der Waals heterostructure assembly of Cr1/3TaS2 with another intercalated ferromagnet Fe1/4TaS2 is demonstrated using a dry-transfer method. The fabricated heterojunction composed of Cr1/3TaS2 and Fe1/4TaS2 with a native Ta2O5 oxide tunnel barrier in between exhibits tunnel magnetoresistance (TMR), revealing possible spin injection and detection with these exfoliatable ferromagnetic materials through the vdW junction.

cond-mat.mes-hall↗

arXiv2017

Inter-subband Landau level couplings induced by in-plane magnetic fields in trilayer graphene

We observed broken-symmetry quantum Hall effects and level crossings between spin- and valley- resolved Landau levels (LLs) in Bernal stacked trilayer graphene. When the magnetic field was tilted with respect to sample normal from $0^{\circ}$ to $66^\circ$, the LL crossings formed at intersections of zeroth and second LLs from monolayer-graphene-like and bilayer-graphene-like subbands, respectively, exhibited a sequence of transitions. The results indicate the LLs from different subbands are coupled by in-plane magnetic fields ($B_{\parallel}$), which was explained by developing the tight-binding model Hamiltonian of trilayer graphene under $B_{\parallel}$.

cond-mat.mes-hall↗

arXiv2017

N- and p-type carrier injections into WSe2 with van der Waals contacts of two-dimensional materials

We demonstrated n-type and p-type carrier injections into a transition metal dichalcogenide (TMD) WSe2 using van der Waals (vdW) contacts of two-dimensional (2D) materials: graphite for an n-type contact and NbSe2 for a p-type contact. Instead of conventional methods such as the evaporation of metals on TMD, 2D metals were transferred onto WSe2 in order to form van der Waals contacts. With these contacts, we demonstrated a small Schottky barrier height for both carrier polarities. Our finding reveals the potential of a high-performance vdW metal/semiconductor contact for use in electronics applications.

cond-mat.mes-hall↗

arXiv2017

Dirac fermion reflector by ballistic graphene sawtooth-shaped npn junctions

We have realized a Dirac fermion reflector in graphene by controlling the ballistic carrier trajectory in a sawtooth-shaped npn junction. When the carrier density in the inner p-region is much larger than that in the outer n-regions, the first straight np interface works as a collimator and the collimated ballistic carriers can be totally reflected at the second zigzag pn interface. We observed clear resistance enhancement around the np+n regime, which is in good agreement with the numerical simulation. The tunable reflectance of ballistic carriers could be an elementary and important step for realizing ultrahigh-mobility graphene field effect transistors utilizing Dirac fermion optics in the near future.

cond-mat.mes-hall↗

arXiv2016

Spin injection into multilayer graphene from highly spin-polarized Co2FeSi Heusler alloy

We demonstrate electrical spin injection into multilayer graphene (MLG) in a lateral spin valve device from a highly spin-polarized Co2FeSi (CFS) Huesler electrode. Exfoliated MLG was transferred onto pre-patterned epitaxial CFS wires grown on an Si(111) substrate by a polymer-based transfer method. This method enabled us to fabricate multiple single-crystal CFS electrodes in contact with MLG. Electrical spin injection from CFS to MLG was detected through non-local magnetoresistance (MR) measurement. A non-local spin signal of 430 ohm was observed; this is the largest value among all reported non-local MR values in graphene-based devices.

cond-mat.mtrl-sci↗

arXiv2015

Imaging ballistic carrier trajectories in graphene using scanning gate microscopy

We use scanning gate microscopy to map out the trajectories of ballistic carriers in high-mobility graphene encapsulated by hexagonal boron nitride and subject to a weak magnetic field. We employ a magnetic focusing geometry to image carriers that emerge ballistically from an injector, follow a cyclotron path due to the Lorentz force from an applied magnetic field, and land on an adjacent collector probe. The local electric field generated by the scanning tip in the vicinity of the carriers deflects their trajectories, modifying the proportion of carriers focused into the collector. By measuring the voltage at the collector while scanning the tip, we are able to obtain images with arcs that are consistent with the expected cyclotron motion. We also demonstrate that the tip can be used to redirect misaligned carriers back to the collector.

cond-mat.mes-hall↗