arXiv subjects

S. Katsumoto

Publications and source records attributed to S. Katsumoto.

11 recordsLinked to original sources

arXiv2019

Laser-beam patterned topological insulating states on thin semiconducting MoS2

Identifying the two-dimensional (2D) topological insulating (TI) state in new materials and its control are crucial aspects towards the development of voltage-controlled spintronic devices with low power dissipation. Members of the 2D transition metal dichalcogenides (TMDCs) have been recently predicted and experimentally reported as a new class of 2D TI materials, but in most cases edge conduction seems fragile and limited to the monolayer phase fabricated on specified substrates. Here, we realize the controlled patterning of the 1T'-phase embedded into the 2H-phase of thin semiconducting molybdenum-disulfide (MoS2) by laser beam irradiation. Integer fractions of the quantum of resistance, the dependence on laser-irradiation conditions, magnetic field, and temperature, as well as the bulk gap observation by scanning tunneling spectroscopy and theoretical calculations indicate the presence of the quantum spin Hall phase in our patterned 1T' phases.

cond-mat.mes-hall↗

arXiv2018

Evidence for a quantum-spin-Hall phase in graphene decorated with Bi2Te3 nanoparticles

Realization of the quantum-spin-Hall effect in graphene devices has remained an outstanding challenge dating back to the inception of the field of topological insulators. Graphene's exceptionally weak spin-orbit coupling -stemming from carbon's low mass- poses the primary obstacle. We experimentally and theoretically study artificially enhanced spin-orbit coupling in graphene via random decoration with dilute Bi2Te3 nanoparticles. Remarkably, multi-terminal resistance measurements suggest the presence of helical edge states characteristic of a quantum-spin-Hall phase; the magnetic-field and temperature dependence of the resistance peaks, X-ray photoelectron spectra, scanning tunneling spectroscopy, and first-principles calculations further support this scenario. These observations highlight a pathway to spintronics and quantum-information applications in graphene-based quantum-spin-Hall platforms.

cond-mat.mes-hall↗

arXiv2018

Spin-orbit interaction in Pt or Bi2Te3 nanoparticle-decorated graphene realized by a nanoneedle method

The introduction of spin-orbit interactions (SOIs) and the subsequent appearance of a two-dimensional (2D) topological phase are crucial for voltage-controlled and zero-emission energy spintronic devices. In contrast, graphene basically lacks SOIs due to the small mass of the carbon atom, and appropriate experimental reports for SOIs are rare. Here, we control small-amount (cover ratios < 8%) random decoration of heavy nanoparticles [platinum (Pt) or bismuth telluride (Bi2Te3)] onto mono-layer graphene by developing an original nanoneedle method. X-ray photoelectron spectra support low-damage and low-contamination decoration of the nanoparticles, suggesting the presence of Bi-C and Te-C coupling orbitals. In the samples, we find particle-density-dependent non-local resistance (RNL) peaks, which are attributed to the (inverse) spin Hall effect (SHE) arising from SOI with energies as large as about 30 meV. This is a larger value than in previous reports and supported by scanning tunneling spectroscopy. The present observation should lead to topological phases of graphene, which can be introduced by random decoration with controlled small amounts of heavy nanoparticles, and their applications.

cond-mat.mes-hall↗

arXiv2013

Adiabatic measurements of magneto-caloric effects in pulsed high magnetic fields up to 55 T

Magneto-caloric effects (MCEs) measurement system in adiabatic condition is proposed to investigate the thermodynamic properties in pulsed magnetic fields up to 55 T. With taking the advantage of the fast field- sweep rate in pulsed field, adiabatic measurements of MCEs were carried out at various temperatures. To obtain the prompt response of the thermometer in the pulsed field, a thin film thermometer is grown directly on the sample surfaces. The validity of the present setup was demonstrated in the wide temperature range through the measurements on Gd at about room temperature and on Gd3Ga5O12 at low temperatures. The both results show reasonable agreement with the data reported earlier. By comparing the MCE data with the specific heat data, we could estimate the entropy as functions of magnetic field and temperature. The results demonstrate the possibility that our approach can trace the change in transition temperature caused by the external field.

cond-mat.mtrl-sci↗

arXiv2009

Measurement of diffusion thermopower in the quantum Hall systems

We have measured diffusion thermopower in a two-dimensional electron gas at low temperature ($T$=40 mK) in the field range 0 $<B<$ 3.4 T, by employing the current heating technique. A Hall bar device is designed for this purpose, which contains two crossing Hall bars, one for the measurement and the other used as a heater, and is equipped with a metallic front gate to control the resistivity of the areas to be heated. In the low magnetic field regime ($B\leq$ 1 T), we obtain the transverse thermopower $S_{yx}$ that quantitatively agrees with the $S_{yx}$ calculated from resistivities using the generalized Mott formula. In the quantum Hall regime ($B\geq$ 1T), we find that $S_{yx}$ signal appears only when both the measured and the heater area are in the resistive (inter-quantum Hall transition) region. Anomalous gate-voltage dependence is observed above $\sim$1.8 T, where spin-splitting in the measured area becomes apparent.

cond-mat.mes-hall↗

arXiv2007

Dynamic nuclear polarization and Knight shift measurements in a breakdown regime of integer quantum Hall effect

Nuclear spins are polarized electrically in a breakdown regime of an odd-integer quantum Hall effect (QHE). Electron excitation to the upper Landau subband with the opposite spin polarity flips nuclear spins through the hyperfine interaction. The polarized nuclear spins reduce the spin-splitting energy and accelerate the QHE breakdown. The Knight shift of the nuclear spins is also measured by tuning electron density during the irradiation of radio-frequency magnetic fields.

cond-mat.mes-hall↗

arXiv2007

Electrical coherent control of nuclear spins in a breakdown regime of quantum Hall effect

Using a conventional Hall-bar geometry with a micro-metal strip on top of the surface, we demonstrate an electrical coherent control of nuclear spins in an AlGaAs/GaAs semiconductor heterostructure. A breakdown of integer quantum Hall (QH) effect is utilized to dynamically polarize nuclear spins. By applying a pulse rf magnetic field with the metal strip, the quantum state of the nuclear spins shows Rabi oscillations, which is detected by measuring longitudinal voltage of the QH conductor.

cond-mat.mes-hall↗

arXiv2004

Photo-induced demagnetization and polaron binding energy increase observed by mid-infrared pump-probe spectroscopy in ferromagnetic Ga$_{0.94}$Mn$_{0.06}$As

Time-resolved transmittance measurements performed on Ga$_{0.94}$Mn$_{0.06}$As in the vicinity of the Mn-induced mid-infrared absorption band are presented. Upon photo-excitation, a slow increase (hundreds of ps timescale) of the differential transmittance is observed and found to be directly related to demagnetization. The temporal profiles of the transmittance and of the demagnetization measured by time-resolved magneto-optical Kerr spectroscopy are found to coincide. Well below the Curie temperature, the maximum amplitude of the slow component of the differential transmittance as a function of the probe energy is on the rising edge of the linear absorption peak, suggesting that ferromagnetic ordering can be explained by a coupling of the Mn local spins through bound magnetic polarons.

cond-mat.mtrl-sci↗

arXiv2004

Ultrahigh-Field Hole Cyclotron Resonance Absorption in InMnAs Films

We have carried out an ultrahigh-field cyclotron resonance study of p-type In1-xMnxAs films, with Mn composition x ranging from 0% to 2.5%, grown on GaAs by low-temperature molecular-beam epitaxy. Pulsed magnetic fields up to 500 T were used to make cyclotron resonance observable in these low-mobility samples. The clear observation of hole cyclotron resonance is direct evidence of the existence of a large number of itinerant, effective-mass-type holes rather than localized d-like holes. It further suggests that the p-d exchange mechanism is more favorable than the double exchange mechanism in this narrow gap InAs-based dilute magnetic semiconductor. In addition to the fundamental heavy-hole and light-hole cyclotron resonance absorption appearing near the high-magnetic-field quantum limit, we observed many inter-Landau-level absorption bands whose transition probabilities are stronglydependent on the sense of circular polarization of the incident light.

cond-mat.mtrl-sci↗

arXiv2003

Transport in two-dimensional electron gas narrow channel with a magnetic field gradient

We have investigated distinctive transport phenomena in two-dimensional electron gas (2DEG) narrow channel subjected to a large magnetic field gradient, a unique situation created in a ferromagnet/2DEG hybrid structure. The so-called snake orbits which propagate in the direction determined by the sign of field gradient, give rise to conductivity enhancement and rectification effect with respect to the direction of DC bias current.

cond-mat.mes-hall↗

arXiv2003

Observation of the spin-charge thermal isolation of ferromagnetic Ga_{0.94}Mn_{0.06}As by time-resolved magneto-optical measurement

The dynamics of magnetization under femtosecond optical excitation is studied in a ferromagnetic semiconductor Ga_{0.94}Mn_{0.06}As with a time-resolved magneto-optical Kerr effect measurement with two color probe beams. The transient reflectivity change indicates the rapid rise of the carrier temperature and relaxation to a quasi-thermal equilibrium within 1 ps, while a very slow rise of the spin temperature of the order of 500ps is observed. This anomalous behavior originates from the thermal isolation between the charge and spin systems due to the spin polarization of carriers (holes) contributing to ferromagnetism. This constitutes experimental proof of the half-metallic nature of ferromagnetic Ga_{0.94}Mn_{0.06}As arising from double exchange type mechanism originates from the d-band character of holes.

cond-mat.str-el↗