We study the effects of hydrostatic pressure (HP) compression on the superconducting transition of severely strained Nb samples, whose grain sizes are reduced to the submicrometer level. Engineered granularity by high-pressure torsion (HPT) treatment changes the strength of coupling between submicrometer-scale grains and introduces lattice strain. We attempt to utilize the initially accumulated shear strain in the starting material for increasing the superconducting transition temperature Tc under HP compression. The HP effects on non-strained Nb have already been investigated in the pressure regime over 100 GPa by Struzhkin et al. [Phys. Rev. Lett. 79, 4262 (1997)], and Tc reportedly exhibited an increase from 9.2 to 9.9 K at approximately 10 GPa. (1) Slightly strained Nb in the HPT treatment exhibits the increase in Tc under HP due to the strengthening of the intergrain coupling, so the pressure scale of the pressure response observed by Struzhkin et al. is reduced to approximately one-seventh at the maximum. (2) Prominently strained Nb in the HPT treatment exhibits the increase in Tc under HP due to a reduction in structural symmetry at the unit-cell level: In a Nb sample subjected to HPT (6 GPa, 10 revolutions), Tc exceeds 9.9 K at approximately 2 GPa. According to our first-principle calculations, the reduction in the structural symmetry affords an increase in the density of states at the Fermi energy, thereby yielding a prominent increase in Tc at low pressures.
Hydrogen absorption and desorption through diffusive processes and the accompanying phase transformations in hydrogen storage materials have been investigated by x-ray diffraction, thermogravimetry, pressure-composition-temperature isotherms, and electrical resistance measurements. We use time- and temperature-dependent magnetic measurements to gain a detailed understanding of these phenomena in palladium hydride PdH0.64 prepared by severe plastic deformation via the high-pressure torsion method. The concept of our method is pursuing an increase in the ratio of the paramagnetic α-phase Pd+H to the diamagnetic β-phase PdH. An antiferromagnetic correlation appears between the paramagnetic moments of Pd at a low temperature and a huge discrete change in magnetization appears due to collective H desorption above room temperature. It is also verified that H diffusion and its desorption can be changed with the accumulation of the severe lattice strain.
In fifth-group element superconductors V, Nb, and Ta, the increase in superconducting transition temperature (T c) was attempted by using both high-pressure torsion (HPT) and additional hydrostatic pressure (HP) compression. The former brings about the grain refinement and strain accumulation in the unit-cell level. The additional compression for severely strained superconductors triggers strengthening intergrain-contact and/or structural deformation in the unit-cell level. The manner of the appearance of the above two effects depends on the kind of elements: First, in V, there is no prominent effect of HPT, comparing to the hydrostatic compression effects on its non-strained material. Next, in Ta, the effect of strengthening intergrain-contact appears at small hydrostatic compression, resulting in temporal increase in T c. Finally, Nb exhibits prominent increase in T c by both effects and, in particular, the structural deformation in the unit-cell level promotes the increase in T c. Thus, the accumulation of residual strain in the level of starting material can be a promising work to manipulate T c under HP compression.
This study focuses on the problem of eye irritation when measuring steady-state visual evoked potentials (SSVEPs) using a brain–computer interface and aims to clarify experimentally visual stimulus signals that do not cause discomfort to users. To this end, a method is proposed that introduces a flash stimulus in which the color is changed by changing its hue. This reduces the change in brightness while providing a color change, thereby facilitating visual stimulation with less discomfort. In experiments conducted, flash stimuli of the primary colors red, green, and blue and colors with different hues of 5–45° from these primary colors were generated to investigate the algorithm accuracy of SSVEP and discomfort. Subjective questionnaire and CFF values, which are ophthalmic parameters, were obtained for the subjects and compared to the discrimination rate. As a result of the comparison, it was confirmed that the fatigue level of the visual stimulus generated by the proposed hue change was lower than that of the conventional black-and-white stimulus. It was also confirmed that the combination of the hue difference and frequency could obtain the same discrimination rate as the conventional method.
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