Masaaki Araidai

First-principles calculations for underdoped La_2−xSr_xCuO₄ (LSCO) have revealed a Fermi surface consisting of spin-triplet (KS) particles at the antinodal Fermi-pockets and spin-singlet (SS) particles at the nodal Fermi-arcs in the presence of AF local order. By performing a unique method of calculating the electronic-spin state of overdoped LSCO and by measurement of the spin-correlation length by neutron inelastic scattering, the origin of the phase-diagram, including the pseudogap phase in the high temperature superconductor, Sr-doped copper-oxide LSCO, has been elucidated. We have theoretically solved the long-term problem as to why the angle-resolved photoemission spectroscopy (ARPES) has not been able to observe Fermi pockets in the Fermi surface of LSCO. As a result, we show that the pseudogap region is bounded below the characteristic temperature T*(x) and above the superconducting transition temperature T_c(x) in the T vs. x phase diagram, where both the AF order and the KS particles in the Fermi pockets vanish at T*(x), whilst KS particles contribute to d-wave superconductivity below T_c. We also show that the relationship T*(x_c) = T_c(x_c) holds at x_c = 0.30, which is consistent with ARPES experiments. At T*(x), a phase transition occurs from the pseudogap phase to an unusual metallic phase in which only the SS particles exist.

Potassium-ion electrets, which mainly consist of amorphous silica and permanently store negative charge, are vital elements of small autonomous vibration-powered generators. However, negative charge decay is sometimes observed to cause issues for applications. In this study, we propose an improved guideline for the fabrication of potassium-ion electrets by first-principles calculations based on density functional theory, which shows that yield reduction can be suppressed by additional oxidation. Moreover, we experimentally confirm that the additional oxidation step effectively extends the lifetime of potassium-ion electrets.

We investigated the effect of interfacial nitrogen (N) defects on tunnel magnetoresistance (TMR) in Fe/MgO/Fe magnetic tunnel junctions (MTJs) which are the basic building block of magnetoresistive random access memory. The N atoms are predicted to originate from the SiN covering for antioxidation. It was found from first-principles quantum-transport calculations that the N defects significantly worsen the TMR. This is particularly evident in the MTJ models with an additional N atom at the MgO/Fe interface, because a conduction channel appears in the antiparallel magnetization configuration due to the N defects. The TMR is directly related to the read error rate of data and the scaling of the memory cell. Therefore, the prevention of nitrogen contamination during the manufacturing processes is a prerequisite for maintaining high performance.

Potassium-ion electrets, which are key elements in vibration-powered microelectromechanical generators, can store negative charge almost permanently. However, our experiments show that the charge storing capability of potassium ion electrets is severely degraded when a SiC plate instead of the usual SiO ₂ plate is used in the fabrication process, indicating the negative influence of carbon incorporation. We theoretically studied the atomic and electronic structures of amorphous silica (a-SiO ₂ ) with and without potassium and carbon atoms using first-principles molecular-dynamics calculations. Our calculations show that negative charge accumulates in fivefold-coordinated Si atoms without carbon contamination. However, carbon contamination of the potassium-ion electret causes oxygen deficiencies in SiO ₂ , hindering the formation of fivefold-coordinated Si atoms. As a result, carbon contamination causes degradation of the negative charge storing capability of potassium-ion electrets.

To confirm the feasibility of a theoretically proposed method of forming free-standing germanene [Araidai et al., J. Appl. Phys. 128 , 125301 (2020).], we experimentally investigated the hydrogen desorption properties of hydrogen-terminated germanane (GeH) flakes. Thermal desorption spectroscopy analysis revealed that hydrogen desorption occurred during heating under an ultrahigh vacuum environment corresponding to a mass loss of 1.0 wt%. Moreover, we have found that using an ultrahigh vacuum, ambient and short-time annealing for hydrogen desorption is a key to sustain the crystal structures.

We have obtained single phase monolayer germanene on aluminum (111) thin films grown on a germanium (111) template by atomic segregation epitaxy, a preparation method differing from molecular beam epitaxy used in previous works. This 2 × 2 reconstructed germanene phase matching an Al(111)3 × 3 supercell has been prepared in large areas upon annealing at 430 °C. Detailed studies have been carried out using scanning tunneling microscopy (STM), low-energy electron diffraction, Auger electron spectroscopy, and synchrotron radiation photoemission spectroscopy. First-principles calculations based on the density function theory along with atomic-scale STM images reveal the atomic structure with one protruding Ge atom per 2 × 2 germanene supercell and a characteristic dispersing band originating from the germanene sheet slightly coupled to the first layer Al atoms underneath. Instead, upon annealing at lower temperatures, multi-phase regions comprise twisted germanene domains in correspondence with an Al(111)√7×√7 R ± 19.1° superstructure as obtained in previous studies.