arXiv subjects

Taro Moriwaki

Publications and source records attributed to Taro Moriwaki.

6 recordsLinked to original sources

arXiv2025

High-pressure electronic states in semiconductors studied by infrared spectroscopy: metallization and band gap tuning in Mg$_2$Si, InAs and InSb

In this article, a brief introduction is first given on infrared studies of materials at high pressures using a diamond anvil cell. Then, our recent results of high-pressure infrared studies are described for Mg$_2$Si, InAs, and InSb. For Mg$_2$Si, pressure-induced metallization at pressures near 10 GPa were clearly demonstrated for both carrier-doped and undoped Mg$_2$Si by large increases of reflectivity. For InAs and InSb, their band gap ($E_g$) increased rapidly and almost linearly with pressure with linear coefficients of $dE_g/dP$=84.6 and 112 meV/GPa, respectively. Obtained values of $E_g$ versus lattice parameter at high pressures are compared with those for other IIl-V semiconductors at ambient pressure, giving unique insight into effects of physical and chemical pressures on $E_g$. Above the structural transition pressures of 7 and 3 GPa for InAs and InSb, respectively, they exhibit highly metallic characteristics accompanied by high reflectivity.

cond-mat.mtrl-sci↗

arXiv2025

Magneto-optical spectra of an organic antiferromagnet as a candidate for an altermagnet

We have measured the magneto-optical Kerr effect (MOKE) in an orthorhombic organic antiferromagnet $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl, which is a candidate for an altermagnet. From the Maxwell equations, we derived matrix-type general formulae describing the optical propagation and reflection for arbitrary crystals. These formulae enabled us to correctly measure and obtain the off-diagonal optical responses of $κ$-Cl. The MOKE of $κ$-Cl appeared at around the Néel temperature and exhibited a nonlinear field dependence in the antiferromagnetic phase. These nonlinear field dependence eliminates a simple origin due to the net canted magnetization. The obtained off-diagonal optical conductivity spectra for almost entire $π$-electron band clearly shows three features. One is the large peaks at the spectral ends due to the magnetic origin with the large energy scale compared to the very small spin-orbit interaction in $κ$-Cl. The others are the middle region spectra proportional to the diagonal conductivities, possibly related to the symmetric piezomagnetic effect and the standard antisymmetric origins. These results suggest the altermagnetic response of $κ$-Cl. We also discuss the analogy between the magnetization in ferromagnets and the net magnetization and Néel vector with respect to the magneto-optical configurations.

cond-mat.str-el↗

arXiv2021

Hybridization-gap Formation and Superconductivity in the Pressure-induced Semimetallic Phase of the Excitonic Insulator Ta$_2$NiSe$_5$

The excitonic insulator Ta$_2$NiSe$_5$ experiences a first-order structural transition under pressure from rippled to flat layer-structure at Ps = 3 GPa, which drives the system from an almost zero-gap semiconductor to a semimetal. The pressure-induced semimetal, with lowering temperature, experiences a transition to another semimetal with a partial-gap of 0.1-0.2 eV, accompanied with a monoclinic distortion analogous to that occurs at the excitonic transition below Ps. We argue that the partial-gap originates primarily from a symmetry-allowed hybridization of Ta-conduction and Ni-valence bands due to the lattice distortion, indicative of the importance of electron-lattice coupling. The transition is suppressed with increasing pressure to Pc = 8 GPa. Superconductivity with a maximum Tsc = 1.2 K emerges around Pc, likely mediated by strongly electron-coupled soft phonons. The electron-lattice coupling is as important ingredient as the excitonic instability in Ta2NiSe5.

cond-mat.str-el↗

arXiv2013

Optical Conductivity Measurement of a Dimer-Mott to Charge-Order Phase Transition in a Two-Dimensional Quarter-Filled Organic Salt Compound

We report a novel insulator-insulator transition arising from the internal charge degrees of freedom in the two-dimensional quarter-filled organic salt beta-(meso-DMBEDT-TTF)2PF6. The optical conductivity spectra above Tc = 70 K display a prominent feature of the dimer-Mott insulator, characterized by a substantial growth of a dimer peak near 0.6 eV with decreasing temperature. The dimer-peak growth is rapidly quenched as soon as a peak of the charge order shows up below Tc, indicating a competition between the two insulating phases. Our infrared imaging spectroscopy has further revealed a spatially competitive electronic phases far below Tc, suggesting a nature of quantum phase transition driven by material-parameter variations.

cond-mat.str-el↗

arXiv2008

Metallic Pattern Fabrication in Organic Mott Insulating Crystal by Local X-Ray Irradiation

We have fabricated a metallic structure in an organic Mott insulator $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl. The periodic metallic domains of approximately 90$\times$90 $μ$m$^2$ obtained by X-ray irradiation through a molybdenum mesh mask are visualized by scanning microregion infrared reflectance spectroscopy technique. No deterioration by irradiation is found in a range of nanometer to micrometer scales. We demonstrate that the present processing method is applicable for the fabrication of molecular electronic devices.

cond-mat.str-el↗

arXiv2003

Infrared study of spin crossover Fe-picolylamine complex

Infrared (IR) absorption spectroscopy has been used to probe the evolution of microscopic vibrational states upon the temperature- and photo-induced spin crossovers in [Fe(2-picolylamine)3]Cl2EtOH (Fe-pic). To overcome the small sizes and the strong IR absorption of the crystal samples used, an IR synchrotron radiation source and an IR microscope have been used. The obtained IR spectra of Fe-pic show large changes between high-spin and low-spin states for both the temperature- and the photo- induced spin crossovers. Although the spectra in the temperature- and photo-induced high-spin states are relatively similar to each other, they show distinct differences below 750 cm-1. This demonstrates that the photo-induced high-spin state involves microscopically different characters from those of the temperature-induced high-spin state. The results are discussed in terms of local pressure and structural deformations within the picolylamine ligands, and in terms of their possible relevance to the development of macroscopic photo-induced phase in Fe-pic.

cond-mat.soft↗