arXiv subjects

T. Moyoshi

Publications and source records attributed to T. Moyoshi.

5 recordsLinked to original sources

arXiv2020

Field-induced spin reorientation in the antiferromagnetic Dirac material EuMnBi$_2$ revealed by neutron and resonant x-ray diffraction

Field-dependent magnetic structure of a layered Dirac material EuMnBi$_2$ was investigated in detail by the single crystal neutron diffraction and the resonant x-ray magnetic diffraction techniques. On the basis of the reflection conditions in the antiferromagnetic phase at zero field, the Eu moments were found to be ordered ferromagnetically within the $ab$ plane and stacked antiferromagnetically along the $c$ axis in the sequence of up-up-down-down. Upon the spin-flop transition under the magnetic field parallel to the $c$ axis, the Eu moments are reoriented from the $c$ to the $a$ or $b$ directions forming two kinds of spin-flop domains, whereas the antiferromagnetic structure of the Mn sublattice remains intact as revealed by the quantitative analysis of the change in the neutron diffraction intensities. The present study provides a concrete basis to discuss the dominant role of the Eu sublattice on the enhanced two-dimensionality of the Dirac fermion transport in EuMnBi$_2$.

cond-mat.mtrl-sci↗

arXiv2009

NMR Studies on the Superconducting Symmetry of Iron Pnictide Systems

NMR longitudinal relaxation rates 1/T1 and Knight shifts K have been measured for superconducting samples of LaFe1-yCoyAsO1-xFx with y=0.0 and 0.0075 and for a nonsuperconducting metallic sample with y=0.1, where the x values are always fixed at 0.11. The temperature (T) dependence the relaxation rates 1/T1 of the superconducting samples has been found to be markedly different from the behavior 1/T1 T^(2.5-3.0) reported by many groups in the entire T range measured (from the temperature immediately below the superconducting transition temperature Tc down to (0.1-0.2)Tc). The nonexistence of the coherence peak has also been found. Based on the results of the measurements and other kinds of existing data, arguments are presented on the superconducting symmetry of the Fe pnictide systems, where the several points which cannot be easily understood by existing theories, are pointed out. Results of the measurements on the nonsuperconducting metallic samples are also presented.

cond-mat.supr-con↗

arXiv2008

Distinct transport behaviors of LaFe1-yCoyAsO1-xFx (x=0.11) between the superconducting and nonsuperconducting metallic y regions divided by y ~ 0.05

Electrical resistivities, Hall coefficients and thermoelectric powers have been measured for polycrystalline samples of LaFe1-yCoyAsO1-xFx (x=0.11) with various values of y. The results show that there exists clear distinction of these transport behaviors between the superconducting and nonsuperconducting metallic regions of y divided by the boundary value yc~0.05. We have found that the behaviors in both regions are very similar to those of high-Tc Cu oxides in the corresponding phases. If they reflect, as in the case of Cu oxides, effects of strong magnetic fluctuations, the energy scale of the fluctuations is considered to be smaller than that of the high Cu oxides by a factor of ~1/2. Arguments on the electronic nature and superconducting symmetry are presented on the basis of the observed small rate of the Tc suppression rate by the Co doping.

cond-mat.supr-con↗

arXiv2006

Magnetic and Metal-Insulator Transitions in beta-Na0.5CoO2 and gamma-K0.5CoO2 -NMR and Neutron Diffraction Studies-

Co-oxides beta-Na0.5CoO2 and gamma-K0.5CoO2 have been prepared by the Na de-intercalation from alpha-NaCoO2 and by the floating-zone method, respectively. It has been found that successive phase transitions take place at temperatures Tc1 and Tc2 in both systems. The appearance of the internal magnetic field at Tc1 with decreasing temperature T indicates that the antiferromagnetic order exists at T < Tc1, as in gamma-Na0.5CoO2. For beta-Na0.5CoO2, the transition temperatures and the NMR parameters determined from the data taken for magnetically ordered state are similar to those of gamma-Na0.5CoO2, indicating that the difference of the stacking ways of the CoO2 layers between these systems do not significantly affect their physical properties. For gamma-K0.5CoO2, the quantitative difference of the physical quantities are found from those of beta- and gamma-Na0.5CoO2. The difference between the values of Tci (i = 1 and 2) of these systems might be explained by considering the distance between CoO2 layers.

cond-mat.str-el↗

arXiv2003

Impurity Effects on the Superconducting Transition Temperature of NazCoO2.yH2O

Superconducting transition temperature Tc of NazCo1-xMxO2.yH2O has been studied for M=Ir and Ga, which can substitute for Co. It has been found that Tc is suppressed by the M-atom doping. The decreasing rates |dTc/dx| are of the order of 1 K/% and too small to be explained by the pair breaking mechanism for the anisotropic order parameters by non-magnetic impurities. Brief arguments are given on possible origins of the Tc suppression.

cond-mat.supr-con↗