arXiv subjects

Tomoki Machida

Publications and source records attributed to Tomoki Machida.

At least 37 records · Page 2Linked to original sources

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↗

arXiv2015

Construction of van der Waals magnetic tunnel junction using ferromagnetic layered dichalcogenide

We investigate the micromechanical exfoliation and van der Waals (vdW) assembly of ferromagnetic layered dichalcogenide Fe0.25TaS2. The vdW interlayer coupling at the Fe-intercalated plane of Fe0.25TaS2 allows exfoliation of flakes. A vdW junction between the cleaved crystal surfaces is constructed by dry transfer method. We observe tunnel magnetoresistance in the resulting junction under an external magnetic field applied perpendicular to the plane, demonstrating spin-polarized tunneling between the ferromagnetic layered material through the vdW junction.

cond-mat.mes-hall↗

arXiv2015

Electric field modulation of Schottky barrier height in graphene/MoSe2 van der Waals heterointerface

We demonstrate a vertical field-effect transistor based on a graphene/MoSe2 van der Waals (vdW) heterostructure. The vdW interface between the graphene and MoSe2 exhibits a Schottky barrier with an ideality factor of around 1.3, suggesting a high-quality interface. Owing to the low density of states in graphene, the position of the Fermi level in the graphene can be strongly modulated by an external electric field. Therefore, the Schottky barrier height at the graphene/MoSe2 vdW interface is also modulated. We demonstrate a large current ON-OFF ratio of 10^5. These results point to the potential high performance of the graphene/MoSe2 vdW heterostructure for electronics applications.

cond-mat.mes-hall↗

arXiv2015

Influence of the density of states of graphene on the transport properties of graphene/MoS2/metal vertical field-effect transistors

We performed detailed studies of the current-voltage characteristics in graphene/MoS2/metal vertical field-effect transistors. Owing to its low density of states, the Fermi level in graphene is very sensitive to its carrier density and thus the external electric field. Under the application of a bias voltage VB between graphene and the metal layer in the graphene/MoS2/metal heterostructure for driving current through the van der Waals interface, the electric field across the MoS2 dielectric induces a shift in the Fermi level of graphene. When the Fermi level of graphene coincides with the Dirac point, a significant nonlinearity appears in the measured I-V curve, thus enabling us to perform spectroscopy of the Dirac point. By detecting the Dirac point for different back-gate voltages, we revealed that the capacitance of the nanometer-thick MoS2 layer can be determined from a simple DC transport measurement.

cond-mat.mes-hall↗

arXiv2015

Edge-channel interferometer at the graphene quantum Hall pn junction

We demonstrate a quantum Hall edge-channel interferometer in a high-quality graphene pn junction under a high magnetic field. The co-propagating p and n quantum Hall edge channels traveling along the pn interface functions as a built-in Aharanov-Bohm-type interferometer, the interferences in which are sensitive to both the external magnetic field and the carrier concentration. The trajectories of peak and dip in the observed resistance oscillation are well reproduced by our numerical calculation that assumes magnetic flux quantization in the area enclosed by the co-propagating edge channels. Coherent nature of the co-propagating edge channels are confirmed by the checkerboard-like pattern in the dc-bias and magnetic-field dependences of the resistance oscillations.

cond-mat.mes-hall↗

arXiv2015

Modulation of Schottky barrier height in graphene/MoS2/metal vertical heterostructure with large current ON-OFF ratio

Detail transport properties of graphene/MoS2/metal vertical heterostructure have been investigated. The van der Waals interface between the graphene and MoS2 exhibits Schottky barrier. The application of gate voltage to the graphene layer enables us to modulate the Schottky barrier height; thus gives rise to the control of the current flow across the interface. By analyzing the temperature dependence of the conductance, the modulation of Schottky barrier height Δϕ has been directly determined. We observed significant MoS2 layer number dependence of Δϕ. Moreover, we demonstrate that the device which shows larger Δϕ exhibits larger current modulation; this is consistent with the fact that the transport of these devices is dominated by graphene/MoS2 Schottky barrier.

cond-mat.mes-hall↗

arXiv2014

Tunneling transport in a few monolayer-thick WS2/graphene heterojunction

This paper demonstrates the high-quality tunnel barrier characteristics and layer number controlled tunnel resistance of a transition metal dichalcogenide (TMD) measuring just a few monolayers in thickness. Investigation of vertical transport in WS2 and MoS2 TMDs in graphene/TMD/metal heterostructures revealed that WS2 exhibits tunnel barrier characteristics when its thickness is between 2 to 5 monolayers, whereas MoS2 experiences a transition from tunneling to thermionic emission transport with increasing thickness within the same range. Tunnel resistance in a graphene/WS2/metal heterostructure therefore increases exponentially with the number of WS2 layers, revealing the tunnel barrier height of WS2 to be 0.37 eV.

cond-mat.mes-hall↗

arXiv2014

Large current modulation in exfoliated-graphene/MoS2/metal vertical heterostructures

Graphene-based vertical field effect transistors have attracted considerable attention in the light of realizing high-speed switching devices; however, the functionality of such devices has been limited by either their small ON-OFF current ratios or ON current densities. We fabricate a graphene/MoS2/metal vertical heterostructure by using mechanical exfoliation and dry transfer of graphene and MoS2 layers. The van der Waals interface between graphene and MoS2 exhibits a Schottky barrier, thus enabling the possibility of well-defined current rectification. The height of the Schottky barrier can be strongly modulated by an external gate electric field owing to the small density of states of graphene. We obtain large current modulation exceeding 10^5 simultaneously with a large current density of ~10^4 A/cm^2 , thereby demonstrating the superior performance of the exfoliated-graphene/MoS2/metal vertical field effect transistor

cond-mat.mes-hall↗

arXiv2014

Cubic Rashba spin-orbit interaction of two-dimensional hole gas in strained-Ge/SiGe quantum well

The spin-orbit interaction (SOI) of the two-dimensional hole gas in the inversion symmetric semiconductor Ge is studied in a strained-Ge/SiGe quantum well structure. We observed weak anti-localization (WAL) in the magnetoconductivity measurement, revealing that the WAL feature can be fully described by the k-cubic Rashba SOI theory. Furthermore, we demonstrated electric field control of the Rashba SOI. Our findings reveal that the heavy hole (HH) in strained-Ge is a purely cubic-Rashba-system, which is consistent with the spin angular momentum mj = +-3/2 nature of the HH wave function.

cond-mat.mes-hall↗

arXiv2013

Electrical spin injection into graphene through monolayer hexagonal boron nitride

We demonstrate electrical spin injection from a ferromagnet to a bilayer graphene (BLG) through a monolayer (ML) of single-crystal hexagonal boron nitride (h-BN). A Ni81Fe19/ML h-BN/BLG/h-BN structure is fabricated using a micromechanical cleavage and dry transfer technique. The transport properties across the ML h-BN layer exhibit tunnel barrier characteristics. Spin injection into BLG has been detected through non local magnetoresistance measurements.

cond-mat.mes-hall↗

arXiv2013

Observation of finite excess noise in the voltage-biased quantum Hall regime as a precursor for breakdown

We performed noise measurements in a two-dimensional electron gas to investigate the nonequilibrium quantum Hall effect (QHE) state. While excess noise is perfectly suppressed around the zero-biased QHE state reflecting the dissipationless electron transport of the QHE state, considerable finite excess noise is observed in the breakdown regime of the QHE. The noise temperature deduced from the excess noise is found to be of the same order as the energy gap between the highest occupied Landau level and the lowest empty one. Moreover, unexpected finite excess noise is observed at a finite source-drain bias voltagesmaller than the onset voltage of the QHE breakdown, which indicates finite dissipation in the QHE state and may be related to the prebreakdown of the QHE.

cond-mat.mes-hall↗