arXiv subjects

M. Shiraishi

Publications and source records attributed to M. Shiraishi.

35 records · Page 2Linked to original sources

arXiv2023

Electrical transport properties of atomically thin WSe2 using perpendicular magnetic anisotropy metal contacts

Tungsten diselenide, WSe2 shows excellent properties and become very promising material among two dimensional semiconductors. Wide band gap and large spin-orbit coupling along with naturally lacking inversion symmetry in the monolayer WSe2 make it efficient material for spintronics, optoelectronics and valleytronics applications. In this work, we report electrical transport properties of monolayer WSe2 based field effect transistor with most needed multilayer Co/Pt ferromagnetic electrodes exhibiting perpendicular magnetic anisotropy. We studied contacts behaviour by performing I-V curve measurements and estimating Schottky barrier heights (SBHs). SBHs estimated from experimental data are found to be comparatively small, without using any tunnel barrier. This work expands the current understanding of WSe2 based devices and gives insight into the electrical behaviour of Co/Pt metal contacts, which can open great possibilities for spintronic/valleytronic applications.

cond-mat.mtrl-sci↗

arXiv2023

Ferroic Berry Curvature Dipole in a Topological Crystalline Insulator at Room Temperature

The physics related to Berry curvature is now a central research topic in condensed matter physics. The Berry curvature dipole (BCD) is a significant and intriguing condensed matter phenomenon that involves inversion symmetry breaking. However, the creation and controllability of BCDs have so far been limited to far below room temperature (RT) and non-volatile (i.e., ferroic) BCDs have not yet been discovered, hindering further progress in topological physics. In this work, we demonstrate a switchable and non-volatile BCD effect at RT in a topological crystalline insulator, Pb1-xSnxTe (PST), which is attributed to ferroic distortion. Surprisingly, the magnitude of the ferroic BCD is several orders of magnitude greater than that of the non-ferroic BCDs that appear, for example, in transition metal dichalcogenides. The discovery of this ferroic and extraordinarily large BCD in PST could pave the way for further progress in topological material science and the engineering of novel topological devices.

cond-mat.mes-hall↗

arXiv2022

Polarization angle requirements for CMB B-mode experiments. Application to the LiteBIRD satellite

A methodology to provide the polarization angle requirements for different sets of detectors, at a given frequency of a CMB polarization experiment, is presented. The uncertainties in the polarization angle of each detector set are related to a given bias on the tensor-to-scalar ratio $r$ parameter. The approach is grounded in using a linear combination of the detector sets to obtain the CMB polarization signal. In addition, assuming that the uncertainties on the polarization angle are in the small angle limit (lower than a few degrees), it is possible to derive analytic expressions to establish the requirements. The methodology also accounts for possible correlations among detectors, that may originate from the optics, wafers, etc. The approach is applied to the LiteBIRD space mission. We show that, for the most restrictive case (i.e., full correlation of the polarization angle systematics among detector sets), the requirements on the polarization angle uncertainties are of around 1 arcmin at the most sensitive frequency bands (i.e., $\approx 150$ GHz) and of few tens of arcmin at the lowest (i.e., $\approx 40$ GHz) and highest (i.e., $\approx 400$ GHz) observational bands. Conversely, for the least restrictive case (i.e., no correlation of the polarization angle systematics among detector sets), the requirements are $\approx 5$ times less restrictive than for the previous scenario. At the global and the telescope levels, polarization angle knowledge of a few arcmins is sufficient for correlated global systematic errors and can be relaxed by a factor of two for fully uncorrelated errors in detector polarization angle. The reported uncertainty levels are needed in order to have the bias on $r$ due to systematics below the limit established by the LiteBIRD collaboration.

astro-ph.CO↗

arXiv2021

Giant spin Hall angle in the Heusler alloy Weyl ferromagnet Co$_2$MnGa

Weyl semimetals are playing a major role in condensed matter physics due to exotic topological properties, and their coexistence with ferromagnetism may lead to enhanced spin-related phenomena. Here, the inverse spin Hall effect (ISHE) in the ferromagnetic Weyl-semimetal Heusler alloy Co$_2$MnGa was investigated at room temperature by means of electrical spin injection in lateral spin valve structures. Spin transport properties such as spin polarization and spin diffusion length in this material were precisely extracted in order to estimate the spin Hall angle $θ_{\textrm{SH}}$, which was found to be $-0.19\pm0.04$ and is among the highest reported for a ferromagnet. Although this value is on the same order of magnitude of known heavy metals, the significantly higher resistivity of Co$_2$MnGa implies an improvement on the magnitude of detection voltages, while its ferromagnetic nature allows controlling the intensity of SHE through the magnetization direction. It was also shown that Onsager's reciprocity does not hold for this system, which is in part attributable to a different spin-dependent Hall conductivity for spin-up and spin-down carriers.

cond-mat.mtrl-sci↗

arXiv2020

Detection of Ferromagnetic Resonance from 1 nm-thick Co

To explore the further possibilities of nanometer-thick ferromagnetic films (ultrathin ferromagnetic films), we investigated the ferromagnetic resonance (FMR) of 1 nm-thick Co film. Whilst an FMR signal was not observed for the Co film grown on a SiO2 substrate, the insertion of a 3 nm-thick amorphous Ta buffer layer beneath the Co enabled the detection of a salient FMR signal, which was attributed to the smooth surface of the amorphous Ta. This result implies the excitation of FMR in an ultrathin ferromagnetic film, which can pave the way to controlling magnons in ultrathin ferromagnetic films.

cond-mat.mtrl-sci↗

arXiv2020

Over 1% magnetoresistance ratio at room temperature in non-degenerate silicon-based lateral spin valves

To augment the magnetoresistance (MR) ratio of n-type non-degenerate Si-based lateral spin valves (Si-LSVs), we modify the doping profile in the Si layer and introduce a larger local strain into the Si channel by changing a capping insulator. The highest MR ratio of 1.4% is achieved in the Si-LSVs through these improvements, with significant roles played by a reduction in the resistance-area product of the ferromagnetic contacts and an enhancement of the momentum relaxation time in the Si channel.

physics.app-ph↗

arXiv2020

Sizable spin-transfer torque in Bi/Ni80Fe20 bilayer film

The search for efficient spin conversion in Bi has attracted great attention in spin-orbitronics. In the present work, we employ spin-torque ferromagnetic resonance to investigate spin conversion in Bi/Ni80Fe20(Py) bilayer films with continuously varying Bi thickness. In contrast with previous studies, sizable spin-transfer torque (i.e., a sizable spin-conversion effect) is observed in Bi/Py bilayer film. Considering the absence of spin conversion in Bi/yttrium-iron-garnet bilayers and the enhancement of spin conversion in Bi-doped Cu, the present results indicate the importance of material combinations to generate substantial spin-conversion effects in Bi.

cond-mat.mes-hall↗

arXiv2019

Monolayer MoS2 field effect transistor with low Schottky barrier height with ferromagnetic metal contacts

Two-dimensional MoS2 has emerged as promising material for nanoelectronics and spintronics due to its exotic properties. However, high contact resistance at metal semiconductor MoS2 interface still remains an open issue. Here, we report electronic properties of field effect transistor devices using monolayer MoS2 channels and permalloy (Py) as ferromagnetic (FM) metal contacts. Monolayer MoS2 channels were directly grown on SiO2/Si substrate via chemical vapor deposition technique. The increase in current with back gate voltage shows the tunability of FET characteristics. The Schottky barrier height (SBH) estimated for Py/MoS2 contacts is found to be +28.8 meV (zero-bias), which is the smallest value reported so-far for any direct metal (magnetic or non-magnetic)/monolayer MoS2 contact. With the application of gate voltage (+10 V), SBH shows a drastic reduction down to a value of -6.8 meV. The negative SBH reveals ohmic behavior of Py/MoS2 contacts. Low SBH with controlled ohmic nature of FM contacts is a primary requirement for MoS2 based spintronics and therefore using directly grown MoS2 channels in the present study can pave a path towards high performance devices for large scale applications.

physics.app-ph↗

arXiv2019

Gate-tunable spin exclusive or operation in a silicon-based spin device at room temperature

Room temperature operation of a spin exclusive or (XOR) gate was demonstrated in lateral spin valve devices with nondegenerate silicon (Si) channels. The spin XOR gate is a fundamental part of the magnetic logic gate (MLG) that enables reconfigurable and nonvolatile NAND or OR operation in one device. The device for the spin XOR gate consists of three iron (Fe)/cobalt (Co)/magnesium oxide (MgO) electrodes, i.e., two input and one output electrodes. Spins are injected into the Si channel from the input electrodes whose spin angular momentum corresponds to the binary input 1 or 0. The spin drift effect is controlled by a lateral electric field in the Si channel to adjust the spin accumulation voltages under two different parallel configurations, corresponding to (1, 1) and (0, 0), so that they exhibit the same value. As a result, the spin accumulation voltage detected by the output electrode exhibits three different voltages, represented by an XOR gate. The one-dimensional spin drift-diffusion model clearly explains the obtained XOR behavior. Charge current detection of the spin XOR gate is also demonstrated. The detected charge current has a maximum of 0.94 nA, the highest value in spin XOR gates reported thus far. Furthermore, gate voltage modulation of the spin XOR gate is also demonstrated, which enables operation of multiple MLG devices.

physics.app-ph↗

arXiv2018

Induced spin-orbit coupling in silicon thin films by bismuth doping

We demonstrate an enhancement of the spin-orbit coupling in silicon (Si) thin films by doping with bismuth (Bi), a heavy metal, using ion implantation. Quantum corrections to conductance at low temperature in phosphorous-doped Si before and after Bi implantation is measured to probe the increase of the spin-orbit coupling, and a clear modification of magnetoconductance signals is observed: Bi doping changes magnetoconductance from weak localization to the crossover between weak localization and weak antilocalization. The elastic diffusion length, phase coherence length and spin-orbit coupling length in Si with and without Bi implantation are estimated, and the spin-orbit coupling length after the Bi doping becomes the same order of magnitude (Lso = 54 nm) with the phase coherence length (Lϕ = 35 nm) at 2 K. This is an experimental proof that the spin-orbit coupling strength in Si thin film is tunable by doping with heavy metals.

cond-mat.mes-hall↗

arXiv2016

Gate-tunable spin-charge conversion and a role of spin-orbit interaction in graphene

The small spin-orbit interaction of carbon atoms in graphene promises a long spin diffusion length and potential to create a spin field-effect transistor. However, for this reason, graphene was largely overlooked as a possible spin-charge conversion material. We report electric gate tuning of the spin-charge conversion voltage signal in a single-layer graphene. Using spin pumping from yttrium iron garnet ferrimagnetic insulator and ionic liquid top gate we determined that the inverse spin Hall effect is the dominant spin-charge conversion mechanism in a single-layer graphene. From the gate dependence of the electromotive force we showed dominance of the intrinsic over Rashba spin-orbit interaction: a long-standing question in graphene research.

cond-mat.mtrl-sci↗

arXiv2015

Precise determination of two-carrier transport properties in the topological insulator TlBiSe$_2$

We report the electric transport study of the three-dimensional topological insulator TlBiSe$_2$. We applied a newly developed analysis procedure and precisely determined two-carrier transport properties. Magnetotransport properties revealed a multicarrier conduction of high- and low-mobility electrons in the bulk, which was in qualitative agreement with angle-resolved photoemission results~[K. Kuroda $et~al.$, Phys. Rev. Lett. $\bm{105}$, 146801 (2010)]. The temperature dependence of the Hall mobility was explained well with the conventional Bloch-Gr{ü}neisen formula and yielded the Debye temperature $\varTheta_{\rm{D}}=113 \pm 14$~K. The results indicate that the scattering of bulk electrons is dominated by acoustic phonons.

cond-mat.mtrl-sci↗

arXiv2014

Surface Shubnikov-de Hass oscillations and non-zero Berry phases of the topological hole conduction in Tl$_{1-x}$Bi$_{1+x}$Se$_2$

We report the observation of two-dimensional Shubnikov-de Hass (SdH) oscillations in the topological insulator Tl$_{1-x}$Bi$_{1+x}$Se$_2$. Hall effect measurements exhibited electron-hole inversion in samples with bulk insulating properties. The SdH oscillations accompanying the hole conduction yielded a large surface carrier density of $n_{\rm{s}}=5.1 \times10^{12}$/cm$^2$, with the Landau-level fan diagram exhibiting the $π$ Berry phase. These results showed the electron-hole reversibility around the in-gap Dirac point and the hole conduction on the surface Dirac cone without involving the bulk metallic conduction.

cond-mat.mes-hall↗

arXiv2013

Giant enhancement of spin pumping efficiency using Fe3Si ferromagnet

Spincurrentronics, which involves the generation, propagation and control of spin currents, has attracted a great deal of attention because of the possibility of realizing dissipation-free information propagation. Whereas electrical generation of spin currents originally made the field of spincurrentronics possible, and significant advances in spin-current devices has been made, novel spin-current-generation approaches such as dynamical methods have also been vigorously investigated. However, the low spin-current generation efficiency associated with dynamical methods has impeded further progress towards practical spin devices. Here we show that by introducing a Heusler-type ferromagnetic material, Fe3Si, pure spin currents can be generated about twenty times more efficiently using a dynamical method. This achievement paves the way to the development of novel spin-based devices.

cond-mat.mtrl-sci↗

arXiv2011

Observation of weak temperature dependence of spin diffusion length in highly-doped Si by using a non-local 3-terminal method

We conduct an experimental investigation of temperature dependence of spin diffusion length in highly-doped n-type silicon by using a non-local 3-terminal method. Whereas an effect of spin drift is not ignorable to bias- and temperature-dependence of spin signals in non-metallic systems except for the case of a non-local 4-terminal method, it is not fully conclusive how the spin drift affects spin transport properties in highly-doped Si in a non-local 3-terminal method that is often used in Si spintronics. Here, we report on temperature dependence of spin diffusion length in the Si, and it is clarified that the spin transport is less affected by an external electric field.

cond-mat.mtrl-sci↗

arXiv2010

Temperature dependence of spin diffusion length in silicon by Hanle-type spin precession

The Hanle-type spin precession method was carried out associated with non-local magnetoresistance measurement using a highly doped (5\times10^19) silicon channel. The spin diffusion length obtained by the Hanle-method is in good agreement with that by the gap dependence of non-local signals. We have evaluated the interface and bulk channel effects separately, and it was demonstrated that the major source of temperature dependence of non-local signals originates from the spin polarization reduction at interface between the tunnel barrier and silicon.

cond-mat.mtrl-sci↗

arXiv2005

Electronic mean free path in as-produced and purified single-wall carbon nanotubes

The effect of purification on room temperature electronic transport properties of single-wall carbon nanotubes (SWNT) was studied by submerging samples into liquid mercury. The conductance plots of purified SWNTs showed plateaus, indicating weak dependence of the electrical resistance on the length of the tube connecting the electrodes, providing evidence of quasi-ballistic conduction in SWNTs. The electronic mean free path of the purified SWNTs reached a few microns, which is longer than that of the as-produced SWNTs, and which is consistent with the calculation based on the scattering by acoustic phonons.

cond-mat.mtrl-sci↗