First-principles study on scanning tunneling microscopy images of hydrogen-terminated Si(110) surfaces

Scanning tunneling microscopy images of hydrogen-terminated Si(110) surfaces are studied using first-principles calculations. Our results show that the calculated filled-state images and local density of states are consistent with recent experimental results, and the empty-state images appear significantly different from the filled-state ones. To elucidate the origin of this difference, we examined in detail the local density of states, which affects the images, and found that the bonding and antibonding states of surface silicon atoms largely affect the difference between the filled- and empty-state images.Comment: 4 pages, and 4 figure

Atomic-scale flattening of SiC surfaces by electroless chemical etching in HF solution with Pt catalyst

The authors present a method for flattening SiC surfaces with Pt as a catalyst in HF solution. The mechanism for flattening SiC surfaces is discussed. The flattened 4H-SiC (0001) surface is composed of alternating wide and narrow terraces with single-bilayer-height steps, which are induced by the rate difference of the catalytic reactions between adjacent terraces. Scanning tunneling microscopy images reveal a 1×1 phase on the terraces. The 1×1 phase is composed of coexisting of F- and OH-terminated Si atoms, which originate from the polarization of the underlying Si-C bonds. © 2007 American Institute of Physics.Kenta Arima, Hideyuki Hara, et al. "Atomic-scale flattening of SiC surfaces by electroless chemical etching in HF solution with Pt catalyst", Appl. Phys. Lett. 90(20), 202106 (2007) https://doi.org/10.1063/1.2739084

Pressure-induced volumetric negative thermal expansion in CoZr2 superconductor

We investigate the thermal expansion and superconducting properties of a CuAl2-type (tetragonal) superconductor CoZr2 under high pressures. We perform high-pressure synchrotron X-ray diffraction in a pressure range of 2.9 GPa < P < 10.4 GPa and discover that CoZr2 exhibits volumetric negative thermal expansion under high pressures. Although the uniaxial positive thermal expansion (PTE) along the a-axis is observed under ambient pressure, that is suppressed by pressure, while the large uniaxial negative thermal expansion (NTE) along the c-axis is maintained under the pressure regime. As a result of a combination of the suppressed uniaxial PTE along the a-axis and uniaxial NTE along the c-axis, volumetric negative thermal expansion is achieved under high pressure in CoZr2. The mechanisms of volumetric NTE would be based on the flexible crystal structure caused by the soft Co-Co bond as seen in the iso-structural compound FeZr2, which exhibits uniaxial NTE along the c-axis. We also perform high-pressure electrical resistance measurements of CoZr2 to confirm the presence of superconductivity under the examined pressure regime in the range of 0.03 GPa < P < 41.9 GPa. We confirm the presence of superconductivity under all pressures and observe dome-like shape pressure dependence of superconducting transition temperature. Because of the coexistence of two phenomena, which are volumetric NTE and superconductivity, in CoZr2 under high pressure, the coexistence would be achievable under ambient pressure by tuning chemical compositions after our present observation.Comment: 22 pages, 7 figures, supporting informatio

X-ray study of ferroic octupole order producing anomalous Hall effect

放射光でついに見えた磁気オクタポール --熱を電気に変える新たな担い手--. 京都大学プレスリリース. 2021-09-27.Recently found anomalous Hall, Nernst, magnetooptical Kerr, and spin Hall effects in the antiferromagnets Mn₃X (X = Sn, Ge) are attracting much attention for spintronics and energy harvesting. Since these materials are antiferromagnets, the origin of these functionalities is expected to be different from that of conventional ferromagnets. Here, we report the observation of ferroic order of magnetic octupole in Mn₃Sn by X-ray magnetic circular dichroism, which is only predicted theoretically so far. The observed signals are clearly decoupled with the behaviors of uniform magnetization, indicating that the present X-ray magnetic circular dichroism is not arising from the conventional magnetization. We have found that the appearance of this anomalous signal coincides with the time reversal symmetry broken cluster magnetic octupole order. Our study demonstrates that the exotic material functionalities are closely related to the multipole order, which can produce unconventional cross correlation functionalities

Discovery of the Fastest Early Optical Emission from Overluminous SN Ia 2020hvf: A Thermonuclear Explosion within a Dense Circumstellar Environment

Ia型超新星の爆発直後の閃光を捉えることに成功 --特異な爆発に至る恒星進化の謎に迫る--. 京都大学プレスリリース. 2021-12-10.In this Letter we report a discovery of a prominent flash of a peculiar overluminous Type Ia supernova, SN 2020hvf, in about 5 hr of the supernova explosion by the first wide-field mosaic CMOS sensor imager, the Tomo-e Gozen Camera. The fast evolution of the early flash was captured by intensive intranight observations via the Tomo-e Gozen high-cadence survey. Numerical simulations show that such a prominent and fast early emission is most likely generated from an interaction between 0.01 M⊙ circumstellar material (CSM) extending to a distance of ∼10¹³ cm and supernova ejecta soon after the explosion, indicating a confined dense CSM formation at the final evolution stage of the progenitor of SN 2020hvf. Based on the CSM–ejecta interaction-induced early flash, the overluminous light curve, and the high ejecta velocity of SN 2020hvf, we suggest that the SN 2020hvf may originate from a thermonuclear explosion of a super-Chandrasekhar-mass white dwarf (“super-MCh WD”). Systematical investigations on explosion mechanisms and hydrodynamic simulations of the super-MCh WD explosion are required to further test the suggested scenario and understand the progenitor of this peculiar supernova

Water adsorption, solvation and deliquescence of alkali halide thin films on SiO2 studied by ambient pressure X-ray photoelectron spectroscopy

The adsorption of water on KBr thin films evaporated onto SiO2 was investigated as a function of relative humidity (RH) by ambient pressure X-ray photoelectron spectroscopy. At 30percent RH adsorbed water reaches a coverage of approximately one monolayer. As the humidity continues to increase, the coverage of water remains constant or increases very slowly until 60percent RH, followed by a rapid increase up to 100percent RH. At low RH a significant number of the Br atoms are lost due to irradiation damage. With increasing humidity solvation increases ion mobility and gives rise to a partial recovery of the Br/K ratio. Above 60percent RH the increase of the Br/K ratio accelerates. Above the deliquescence point (85percent RH), the thickness of the water layer continues to increase and reaches more than three layers near saturation. The enhancement of the Br/K ratio at this stage is roughly a factor 2.3 on a 0.5 nm KBr film, indicating a strong preferential segregation of Br ions to the surface of the thin saline solution on SiO2