Yoko Miura

Studies on Effects of Impurity Doping and NMR Measurements of La 1111 and/or Nd 1111 Fe-Pnictide Superconductors

Measurements of the electrical resistivity ρ, Hall coefficient R H , thermoelectric power S , and the electronic specific heat coefficient γ have been carried out for samples of LnFe 1- y M y AsO 1- x F x (Ln=La, Nd; M=Co, Mn; x =0.11) obtained by doping M atoms into the superconducting LnFeAsO 1- x F x (Ln 1111) system. The NMR longitudinal relaxation rate 1/ T 1 has also been measured for samples of LaFe 1- y Co y AsO 1- x F x with various x values. Co atoms doped into the superconducting LnFeAsO 1- x F x are nonmagnetic, and the rate of T c -suppression |d T c /d x | by Co atoms has been found to be too small to be explained by the pair-breaking effect expected for the S ± superconducting symmetry proposed as the most probable symmetry for the system. This result throws a serious doubt on whether the symmetry is realized in the system. Instead of the pair breaking, two mechanisms of T c suppression by the doped impurities have been found: One is the electron localization, which appears when the sheet r...

New-Type Phase Transition of Li2RuO3 with Honeycomb Structure

A new-type structural transition has been found in Li 2 RuO 3 with a honeycomb lattice of edge-sharing RuO 6 octahedra. With decreasing temperature T , the electrical resistivity exhibits an anomalous increase at T = T c ∼540 K, suggesting the (metal-to-insulator)-like transition, and the magnetic susceptibility also shows a sharp decrease. Detailed structure analyses have revealed that the high-temperature space group C 2/ m changes to P 2 1 / m at T c . The most striking fact is that a significant reduction of the bond lengths is found between two of the six Ru–Ru pairs of the hexagon in the low-temperature phase, indicating a new-type phase transition driven by the formation of the molecular orbits of these Ru–Ru pairs.

Spin-gap behavior of Na3Cu2SbO6 with distorted honeycomb structure

The spin-gap behavior observed in the temperature dependences of the magnetic susceptibility χ and the specific heat C of Na 3 Cu 2 SbO 6 with a (distorted) honeycomb structure is analyzed in detail. The behavior is commonly observed in similar systems A 3 Cu 2 SbO 6 and Na 2 Cu 2 TeO 6 (A=Na, Li), and can be understood by considering both the significant Jahn–Teller distortion in the compounds and the characteristics of the shape of the electron orbits, in which spins exist. The behavior is contrasted with the antiferromagnetic transition in systems of Na 3 T 2 SbO 6 and Na 2 T 2 TeO 6 (T=Co, Ni). We point out that the antiferromagnetic–ferromagnetic alternating chain model successfully describes the observed magnetic behavior of Na 3 Cu 2 SbO 6 .

Magnetic Excitations of Spin-Gap System Na3Cu2SbO6 with Distorted Honeycomb Structure

Magnetic excitation spectra of quantum spins of Na 3 Cu 2 SbO 6 with a distorted honeycomb structure have been measured by neutron inelastic scattering. The intensity distribution and the dispersion curves of the excitations indicate that the spin system can be well understood by considering one-dimensional spin chains formed of Cu 2+ ions on a lattice of alternating the shorter and longer spacings along the b -axis. The exchange interactions between the neighboring spins with the shorter and longer spacings correspond to ferromagnetic and antiferromagnetic interactions, respectively. These findings are consistent with those we reported previously on the basis of macroscopic measurements. The observed gap Δ G is 8.9 meV and the energies of the antiferromagnetic and ferromagnetic interactions have been found to be 13.9 and -12.5 meV, respectively.

Effects of Geometrical Spin Frustration on Triangular Spin Tubes Formed in CsCrF4 and α-KCrF4

We conducted X-ray diffraction, magnetic susceptibility, heat capacity, high-field magnetization, and electron spin resonance experiments on high-quality polycrystalline S =3/2 compounds consisting of equilateral and non-equilateral triangular spin tubes in CsCrF 4 and α-KCrF 4 , respectively. In both compounds, geometrical spin frustration in the triangular planes coexists with one-dimensionality along the tubes. In CsCrF 4 , no magnetic long-range order appeared above 1.5 K, which agrees with a prior report [H. Manaka et al. : J. Phys. Soc. Jpn. 78 (2009) 093701]. In a low temperature ( T ) regime, T -linear component of heat capacity appeared and magnetization curves showed finite initial slopes. As a result, CsCrF 4 consists of a gapless spin-liquid state encompassing resonating spin-singlet pairs not only in each equilateral triangle but also along the tubes. In contrast, for α-KCrF 4 , successive magnetic phase transitions occurred at T N1 = 2.5(1) K and at T N2 = 4.0(1) K because superexchange inte...

Thermal contact of joints between different kinds of metals at low temperatures

It was found from electrical measurements at 4.2 K that good thermal contact between different kinds of metals can be obtained much more effectively by fastening two pieces with a screw of a large diameter rather than by welding. The thermal conduction of a threaded joint between silver and platinum has been achieved equal to that of copper‐to‐copper joints.

Structural Transition of Li2RuO3 Induced by Molecular-Orbit Formation

A pseudo honeycomb system Li 2 RuO 3 exhibits a second-order-like transition at temperature T = T c ∼540 K to a low- T nonmagnetic phase with a significant lattice distortion forming Ru–Ru pairs. For this system, we have calculated the band structure, using the generalized gradient approximation (GGA) in both the high- and low- T phases, and found that the results of the calculation can naturally explain the insulating behavior observed in the low- T phase. The detailed characters of the Ru 4 d t 2g bands obtained by the tight-binding fit to the calculated dispersion curves show clear evidence that the structural transition is driven by the formation of the Ru–Ru molecular orbit, as proposed in our previous experimental studies.

Development of birefringence imaging analysis method for observing cubic crystals in various phase transitions

Optical birefringence imaging systems demonstrate a high potential for comprehensively investigating various phase transitions. To completely demonstrate such abilities, the temperature dependence of birefringence (Δn) was measured in Δn ≃ 0 materials (i.e., cubic crystals with imperfect crystallization) via a background subtraction method. As a result, highly accurate birefringence imaging at 384 × 288 pixels was obtained using phase transition processes as well as varying temperatures visually characterized by the spatial distribution of not only the retardance level but also the optical fast-axis azimuth.

Studies of Crystal Structure and Spin State in Diluted Triangular Spin Tube KCr1-xAlxF4

We performed X-ray diffraction and magnetization measurements on diluted triangular spin tubes made of ?-KCr1-xAlxF4. For ?-KCrF4 (x = 0), successive antiferromagnetic phase transitions occurred at TN1 = 2.5(1) K and TN2 = 4.0(1) K, because superexchange interactions in each non-equilateral triangle lost their equilibrium below TN2. For 0 < x ? 0.10, the complex crystal structures are retained. When x increases, TN2 gradually decreases, but frustration factors indicating the magnitude of the geometrical spin frustration in each triangular plane increase. Because spin-flop transition fields dramatically decrease with increasing x, the substitution of Al3+ for Cr3+ creates isotropic magnetic states. We suspect that the spin structure is strongly influenced by the slight distortion of CrF6 octahedra caused by a magnetoelectric linear effect.

Nonmagnetic impurity effect on equilateral triangle spin tubes CsCrF4

A chemical impurity substitution in gapped spin-liquid ground states often leads to antiferromagnetic long-range order. However, few experimental studies have been conducted on the substitution effect in gapless spin-liquid ground states. We performed X-ray diffraction and magnetic susceptibility experiments on equilateral triangle spin tubes composed of CsCr1−xAlxF4 (x = 0~0.06). Because the gapless spin-liquid ground state is realized in CsCrF4 (CsCr1−xAlxF4 with x = 0), for x > 0, we expected competition between the geometrical spin frustration in each triangular plane and the impurity-induced antiferromagnetic long-range order. We verified that for x = 0~0.06 no anomaly that indicates an antiferromagnetic long-range order appeared and that the gapless spin-liquid ground state was robust with respect to the effect of nonmagnetic impurity; we suggested that this is because geometrical spin frustration exists in the equilateral triangular plane.