arXiv subjects

Roland Kawakami

Publications and source records attributed to Roland Kawakami.

8 recordsLinked to original sources

arXiv2026

Stabilizing Magnetic Bubble Domains in Epitaxial 2D Magnet/Topological Insulator Heterostructures through Interfacial Interactions

Epitaxial heterostructures of two-dimensional van der Waals magnets and topological insulators offer a powerful platform for probing interfacial spin interactions that govern magnetic textures in low-dimensional quantum systems, while simultaneously enabling highly efficient, atomically thin spin-orbit-torque memory and computing architectures. Despite this promise, the fundamental role of these interfacial interactions in determining magnetic domain-phase stability remain largely uncharted. Here, we perform scanning transmission X-ray microscopy to image nanoscale magnetic textures in epitaxial Fe3GeTe2 Bi2Te3 heterostructures, enabled by a thermal-release-tape dry transfer process onto X-ray transparent silicon-rich nitride membranes. Under zero-field-cooled conditions, we observe robust bubble domain phases from 75 to 165 K, and across different number of folds of the multilayer Fe3GeTe2 Bi2Te3 heterostructures. This is in stark contrast with exfoliated single-crystal Fe3GeTe2 flakes, where ZFC stripe domains are observed for flakes thicker than 20 nm and no domains have been reported for thin flakes less than 15 nm. First-principles calculations and micromagnetic simulations reveal that interfacial coupling to Bi2Te3 modifies the magnetic anisotropy and introduces interfacial Dzyaloshinskii-Moriya interaction, shifting the magnetic phase space towards bubble-domain stabilization without field-cooling. Together, our results offer a new strategy for phase-selective control of magnetic domains through interfacial engineering.

cond-mat.mtrl-sci↗

arXiv2024

Revealing the EuCd_{2}As_{2} Semiconducting Band Gap via n-type La-Doping

EuCd_{2}As_{2} has attracted considerable interest as one of the few magnetic Weyl semimetal candidate materials, although recently there have been emerging reports that claim it to have a semiconducting electronic structure. To resolve this debate, we established the growth of n-type EuCd_{2}As_{2} crystals, to directly visualize the nature of the conduction band using angle resolve photoemission spectroscopy (ARPES). We show that La-doping leads to n-type transport signatures in both the thermopower and Hall effect measurements, in crystals with doping levels at 2 - 6 x 10^{17} e^{-} cm^{-3}. Both p-type and n-type doped samples exhibit antiferromagnetic ordering at 9 K. ARPES experiments at 6 K clearly show the presence of the conduction band minimum at 0.8 eV above the valence band maximum, which is further corroborated by the observation of a 0.71 - 0.72 eV band gap in room temperature diffuse reflectance absorbance measurements. Together these findings unambiguously show that EuCd_{2}As_{2} is indeed a semiconductor with a substantial band gap and not a topological semimetal.

cond-mat.mtrl-sci↗

arXiv2023

Quantum Octets in Air Stable High Mobility Two-Dimensional PdSe2

Two-dimensional (2D) materials have drawn immense interest in scientific and technological communities, owing to their extraordinary properties that are profoundly altered from their bulk counterparts and their enriched tunability by gating, proximity, strain, and external fields. For digital applications, an ideal 2D material would have high mobility, air stability, sizable band gap, and be compatible with large-scale synthesis. Here we demonstrate air-stable field-effect transistors using atomically thin few-layer PdSe2 sheets that are sandwiched between hexagonal BN (hBN), with record high saturation current >350μA/μm, and field effect mobilities 700 and 10,000 cm2/Vs at 300K and 2K, respectively. At low temperatures, magnetotransport studies reveal unique octets in quantum oscillations, arising from 2-fold spin and 4-fold valley degeneracies, which can be broken by in-plane and out-of-plane magnetic fields toward quantum Hall spin and orbital ferromagnetism.

cond-mat.mtrl-sci↗

arXiv2020

Atomic scale visualization of topological spin textures in the chiral magnet MnGe

Spin polarized scanning tunneling microscopy is used to directly image topological magnetic textures in thin films of MnGe, and to correlate the magnetism with structure probed at the atomic-scale. Our images indicate helical stripe domains, each characterized by a single wavevector Q, and their associated helimagnetic domain walls, in contrast to the 3Q magnetic state seen in the bulk. Combining our surface measurements with micromagnetic modeling, we deduce the three-dimensional orientation of the helical wavevectors and gain detailed understanding of the structure of individual domain walls and their intersections. We find that three helical domains meet in two distinct ways to produce either a "target-like" or a "pi-like" topological spin texture, and correlate these with local strain on the surface. We further show that the target-like texture can be reversibly manipulated through either current/voltage pulsing or applied magnetic field, a promising step toward future applications.

cond-mat.mes-hall↗

arXiv2016

Nanosecond spin relaxation times in single layer graphene spin valves with hexagonal boron nitride tunnel barriers

We present an experimental study of spin transport in single layer graphene using atomic sheets of hexagonal boron nitride (h-BN) as a tunnel barrier for spin injection. While h-BN is expected to be favorable for spin injection, previous experimental studies have been unable to achieve spin relaxation times in the nanosecond regime, suggesting potential problems originating from the contacts. Here, we investigate spin relaxation in graphene spin valves with h-BN barriers and observe room temperature spin lifetimes in excess of a nanosecond, which provides experimental confirmation that h-BN is indeed a good barrier material for spin injection into graphene. By carrying out measurements with different thicknesses of h-BN, we show that few layer h-BN is a better choice than monolayer for achieving high non-local spin signals and longer spin relaxation times in graphene.

cond-mat.mes-hall↗

arXiv2015

Molecular beam epitaxy growth of SrO buffer layers on graphite and graphene for the integration of complex oxides

We report the successful growth of high-quality SrO films on highly-ordered pyrolytic graphite (HOPG) and single-layer graphene by molecular beam epitaxy. The SrO layers have (001) orientation as confirmed by x-ray diffraction (XRD) while atomic force microscopy measurements show continuous pinhole-free films having rms surface roughness of <1.5 Å. Transport measurements of exfoliated graphene after SrO deposition show a strong dependence between the Dirac point and Sr oxidation. Subsequently, the SrO is leveraged as a buffer layer for more complex oxide integration via the demonstration of (001) oriented SrTiO3 grown atop a SrO/HOPG stack.

cond-mat.mtrl-sci↗

arXiv2014

Direct comparison of graphene devices before and after transfer to different substrates

The entire graphene field-effect-transistor (FET) devices first fabricated on SiO2/Si are peeled from the surface and placed on a different wafer. Both longitudinal and transverse resistivity measurements of the devices before and after the transfer are measured to calculate the mobility for a direct comparison. After transferred to different SiO2/Si wafers, the mobility generally is comparable and the defect density does not show any significant increase, which indicates the degradation due to the transfer process itself is minimal. The same method can be applied to transfer graphene devices to any arbitrary substrates (e.g. SrTiO3 or STO). The transfer method developed here not only eliminates the need to locate single-layer graphene on non-SiO2/Si substrates for patterning, but also provides a convenient way to study the effects of various substrates on graphene electronic properties.

cond-mat.mes-hall↗

arXiv2011

Reconfigurable nanoelectronics using graphene based spintronic logic gates

This paper presents a novel design concept for spintronic nanoelectronics that emphasizes a seamless integration of spin-based memory and logic circuits. The building blocks are magneto-logic gates based on a hybrid graphene/ferromagnet material system. We use network search engines as a technology demonstration vehicle and present a spin-based circuit design with smaller area, faster speed, and lower energy consumption than the state-of-the-art CMOS counterparts. This design can also be applied in applications such as data compression, coding and image recognition. In the proposed scheme, over 100 spin-based logic operations are carried out before any need for a spin-charge conversion. Consequently, supporting CMOS electronics requires little power consumption. The spintronic-CMOS integrated system can be implemented on a single 3-D chip. These nonvolatile logic circuits hold potential for a paradigm shift in computing applications.

cond-mat.mtrl-sci↗