arXiv subjects

S. Kimura

Publications and source records attributed to S. Kimura.

80 records · Page 5Linked to original sources

arXiv2006

Magnetic behavior of a spin-1 dimer: model system for homodinuclear nickel (II) complexes

Magnetic behavior of a spin-1 Heisenberg dimer is analysed in dependence on both uniaxial single-ion anisotropy and XXZ exchange anisotropy in a zero- as well as non-zero longitudinal magnetic field. A complete set of eigenfunctions and eigenvalues of the total Hamiltonian is presented together with an exact analytical expression for the Gibbs free energy, longitudinal magnetization, longitudinal and transverse susceptibility. The obtained theoretical results are compared with the relevant experimental data of [Ni2(Medpt)2(ox)(H2O)2](ClO4)2.2H2O (Medpt = methyl-bis(3-aminopropyl)amine).

cond-mat.mtrl-sci↗

arXiv2005

Influence of Chaos on the fusion enhancement by electron screening

We study the effect of screening by bound electrons in low energy nuclear reactions. We use molecular dynamics to simulate the reactions involving many electrons: D+$d$, D+D, $^3$He+$d$, $^3$He+D, $^6$Li+$d$, $^6$Li+D, $^7$Li+$p$, $^7$Li+H. Quantum effects corresponding to the Pauli and Heisenberg principles are enforced by constraints in terms of the phase space occupancy. In addition to the well known adiabatic and sudden limits, we propose a new "dissipative limit" which is expected to be important not only at high energies but in the extremely low energy region. The dissipative limit is associated with the chaotic behavior of the electronic motion. It affects also the magnitude of the enhancement factor. We discuss also numerical experiments using polarized targets. The derived enhancement factors in our simulation are in agreement with those extracted within the $R$-matrix approach.

nucl-th↗

arXiv2005

Infrared spectroscopy under multi-extreme conditions: Direct observation of pseudo gap formation and collapse in CeSb

Infrared reflectivity measurements of CeSb under multi-extreme conditions (low temperatures, high pressures and high magnetic fields) were performed. A pseudo gap structure, which originates from the magnetic band folding effect, responsible for the large enhancement in the electrical resistivity in the single-layered antiferromagnetic structure (AF-1 phase) was found at a pressure of 4 GPa and at temperatures of 35 - 50 K. The optical spectrum of the pseudo gap changes to that of a metallic structure with increasing magnetic field strength and increasing temperature. This change is the result of the magnetic phase transition from the AF-1 phase to other phases as a function of the magnetic field strength and temperature. This result is the first optical observation of the formation and collapse of a pseudo gap under multi-extreme conditions.

cond-mat.str-el↗

arXiv2005

Indirect and direct energy gaps in the Kondo semiconductor YbB12

Optical conductivity [$σ(ω)$] of the Kondo semiconductor YbB$_{12}$ has been measured over wide ranges of temperature ($T$=8$-$690 K) and photon energy ($\hbar ω\geq$ 1.3 meV). The $σ(ω)$ data reveal the entire crossover of YbB$_{12}$ from a metallic electronic structure at high $T$ into a semiconducting one at low $T$. Associated with the gap development in $σ(ω)$, a clear onset is newly found at $\hbarω$=15 meV for $T \leq$ 20 K. The onset energy is identified as the gap width of YbB$_{12}$ appearing in $σ(ω)$. This gap in σ(ω)$ is interpreted as the indirect gap, which has been predicted in the band model of Kondo semiconductor. On the other hand, the strong mid-infrared (mIR) peak observed in $σ(ω)$ is interpreted as arising from the direct gap. The absorption coefficient around the onset and the mIR peak indeed show characteristic energy dependences expected for indirect and direct optical transitions in conventional semiconductors.

cond-mat.str-el↗

arXiv2005

The origin of the phase separation in partially deuterated $κ$-(ET)$_2$Cu[N(CN)$_2$]Br studied by infrared magneto-optical imaging spectroscopy

The direct observation of the phase separation between the metallic and insulating states of 75 %-deuterated $κ$-(ET)$_2$Cu[N(CN)$_2$]Br ($d33$) using infrared magneto-optical imaging spectroscopy is reported, as well as the associated temperature, cooling rate, and magnetic field dependencies of the separation. The distribution of the center of spectral weight ($< ω>$) of $d33$ did not change under any of the conditions in which data were taken and was wider than that of the non-deuterated material. This result indicates that the inhomogenity of the sample itself is important as part of the origin of the metal - insulator phase separation.

cond-mat.str-el↗

arXiv2004

Carrier-Induced Magnetic Circular Dichloism in the Magnetoresistive Pyrochlore Tl2Mn2O7

Infrared magnetic circular dichloism (MCD), or equivalently magneto-optical Kerr effect, has been measured on the Tl2Mn2O7 pyrochlore, which is well known for exhibiting a large magnetoresistance around the Curie temperature T_C ~ 120 K. A circularly polarized, infrared synchrotron radiation is used as the light source. A pronounced MCD signal is observed exactly at the plasma edge of the reflectivity near and below T_c. However, contrary to the conventional behavior of MCD for ferromagnets, the observed MCD of Tl2Mn2O7 grows with the applied magnetic field, and not scaled with the internal magnetization. It is shown that these results can be basically understood in terms of a classical magnetoplasma resonance. The absence of a magnetization-scaled MCD indicates a weak spin-orbit coupling of the carriers in Tl2Mn2O7. We discuss the present results in terms of the microscopic electronic structures of Tl2Mn2O7.

cond-mat.str-el↗

arXiv2001

Charge Dynamics in the Colossal Magneto-Resistance Pyrochlore Tl2Mn2O7

Optical conductivity data s(w) of the colossal magneto-resistance (CMR) pyrochlore Tl2Mn2O7 are presented as functions of temperature (T) and externalmagnetic field (B). Upon cooling and upon applying B near the Curietemperature, where the CMR manifests itself, s(w) shows a clear transition from an insulatorlike to a metallic electronic structure as evidenced by theemergence of a pronounced Drude-like component below 0.2 eV. Analyses on thes(w) spectra show that both T- and B-induced evolutions of the electronicstructure are very similar to each other, and that they are universally related to the development of macroscopic magnetization (M). In particular, the effective carrier density obtained from s(w) scales with M^2 over wide ranges of T and B. The contributions to the CMR from the carrier effective mass and scattering time are also evaluated from the data.

cond-mat.str-el↗

arXiv1998

Optical conductivity of the Kondo insulator YbB_12: Gap formation and low-energy excitations

Optical reflectivity experiments have been conducted on single crystals of the Kondo insulator YbB_12 in order to obtain its optical conductivity, σ(ω). Upon cooling below 70 K, a strong supression of σ(ω) is seen in the far-infrared region, indicating the opening of an energy gap of ~ 25 meV. This gap development is coincident with a rapid decrease in the magnetic susceptibility, which shows that the gap opening has significant influence on magnetic properties. A narrow, asymmetric peak is observed at ~40 meV in σ(ω), which is attributed to optical transitions between the Yb 4f-derived states across the gap. In addition, a broad peak is observed at ~0.25 eV. This peak is attributed to transitions between Yb 4f-derived states and p-d band, and is reminiscent of similar peaks previously observed for rare-earth hexaborides.

cond-mat.str-el↗