[en] Highlights: • A_x(C₂H₈N₂)_yFe_2−zSe₂ (A = Li, Na) with T_c = 45 K has been synthesized. • The c-axes of A_x(C₂H₈N₂)_yFe_2−zSe₂ (A = Li, Na) are as long as about 21 Å. • T_c decreases with increasing post-annealing temperature. • T_c of intercalated FeSe rises with increasing distance between the neighboring Fe layers. • T_c of intercalated FeSe tends to be saturated at about 45 K. - Abstract: New iron-based intercalation superconductors A_x(C₂H₈N₂)_yFe_2−zSe₂ (A = Li, Na) with T_c = 45 K have successfully been synthesized via intercalation of dissolved alkaline metal in ethylenediamine. The c-axis lengths of A_x(C₂H₈N₂)_yFe_2−zSe₂ (A = Li, Na) are 20.74(7) Å and 21.9(1) Å, respectively, and are about 50% larger than that of K_xFe₂Se₂, indicating that not only alkaline metal but also ethylenediamine is intercalated between the Se–Se layers of FeSe. It seems that the high-T_c of A_x(C₂H₈N₂)_yFe_2−zSe₂ (A = Li, Na) is caused by the possible two-dimensional electronic structure due to the large c-axis length. Through the post-annealing in an evacuated glass tube, it has been found that T_c decreases with increasing post-annealing temperature and that deintercalation of EDA from the as-intercalated sample takes place at low temperatures below 250 °C

[en] To investigate the spin state of azurite, Cu_3(CO_3)_2(OH)_2, we have measured the thermal conductivity along the c-axis, κ_c, perpendicular to the spin diamond chains. It has been found that the temperature dependence of κ_c shows a broad peak at ∼100 K, which is explained as being due to the strong phonon scattering by the strong spin fluctuation owing to the spin frustration at low temperatures below ∼100 K. Furthermore, it has been found that the temperature dependence of κ_c shows another peak at low temperatures below 20 K and that κ_c is suppressed by the application of magnetic field along the c-axis at low temperatures below ∼35 K. In high magnetic fields above ∼8 T at low temperatures below ∼6 K, it has been found that κ_c increases with increasing field. The present results have indicated that both spin-singlet dimers with a spin gap of ∼35 K and antiferromagnetically correlated spin chains with the antiferromagnetic exchange interaction of ∼5.4 K are formed at low temperatures, which is consistent with the recent conclusion by Jeschke et al. that the ground state of spins in azurite in zero field is a spin-fluid one. In addition, a new quantum critical line in magnetic fields at temperatures above 3 K has been proposed to exist. (author)

[en] Using the solvothermal method, we have successfully synthesized new MoSe₂-based intercalation superconductors A_x(C₂H₈N₂)_yMoSe₂ (A = Ca, Sr, Ba; x = 1) with T_c = 6.0, 6.4, and 7.2 K, respectively, through the co-intercalation of alkaline-earth metals and ethylenediamine into semiconducting 2H-MoSe₂. It has been concluded that the T_c values in the MoSe₂-based intercalation superconductors are clearly related to the electronic density of states (EDOS) at the Fermi level. Moreover, it has been found that the T_c value increases with increasing ionic radius of A in A_x(C₂H₈N₂)_yMoSe₂ (A = Ca, Sr, Ba; x = 1), which is speculated to be due to the increase in the a-axis length and the resulting increase in the EDOS at the Fermi level. The solvothermal method is useful for the synthesis of both alkaline-earth metal- and organic molecule-intercalated superconductors. (author)

[en] New FeSe-based intercalation superconductors, A_x(C_8H_1_1N)_yFe_1_-_zSe (A = Li, Na), with T_c = 39-44 K have been successfully synthesized via the intercalation of alkali metals and 2-phenethylamine into FeSe. The interlayer spacings, namely, the distances between neighboring Fe layers, d, of A_x(C_8H_1_1N)_yFe_1_-_zSe (A = Li, Na) are 19.04(6) and 18.0(1) Å, respectively. These d values are the largest among those of the FeSe-based intercalation compounds and are understood to be due to the intercalation of two molecules of 2-phenethylamine in series perpendicular to the FeSe layers. It appears that the relationship between T_c and d in the FeSe-based intercalation superconductors is not domic but T_c is saturated at ∼45 K, which is comparable to the T_c values of single-layer FeSe films, for d ≥ 9 Å. (author)

[en] Post-annealing effects on the crystal structure and superconductivity of the lithium- and hexamethylenediamine (HMDA)-intercalated superconductor Li_x(C_6H_1_6N_2)_yFe_2_-_zSe_2 have been investigated. Through the post-annealing, a two-step reduction of the interlayer spacing between neighboring Fe layers, d, has been observed. It has been found that a new phase of Li_x(C_6H_1_6N_2)_yFe_2_-_zSe_2 with d = 10.30(2) Å and T_c = 41 K different from the as-intercalated phase is stabilized owing to the possible stable inclination of HMDA intercalated between FeSe layers. This result supports the domic relation between T_c and d in the FeSe-based intercalation superconductors. The reason why T_c increases with a decrease in d through the post-annealing is discussed. (author)

[en] The superconductivity and intercalation state in the lithium- and hexamethylenediamine (HMDA)-intercalated superconductor Li_x(C_6H_1_6N_2)_yFe_2_-_zSe_2 have been investigated from powder x-ray diffraction, thermogravimetric, and magnetic susceptibility measurements, changing the intercalation temperature T_i and the Li content x. Both Li and HMDA have been co-intercalated stably up to x = 2 roughly in a molar ratio of x : y = 2 : 1. In the case of T_i = 45degC, it has been found that both Li and HMDA are co-intercalated locally at the edge of FeSe crystals, indicating that both Li and HMDA are hard to diffuse into the inside of FeSe crystals at 45degC. In the case of T_i = 100degC, on the other hand, it has been found that both Li and HMDA diffuse into the inside of FeSe crystals, so that T_c tends to increase with increasing x from ∼30 K at x = 1 up to 38 K at x = 2 owing to the increase in the number of electron carriers doped from Li into the FeSe layers. (author)

[en] We have succeeded in synthesizing electron-doped polycrystalline bulk samples of T′-La_1.8Eu_0.2CuO_4-yF_y (y = 0-0.15) by the fluorination of undoped (Ce-free) T′-La_1.8Eu_0.2CuO₄ using NH₄F. The magnetic susceptibility measurements have revealed that the superconducting transition temperature, T_c, increases with increasing y, exhibits the maximum of 23 K at y = 0.025, and decreases. The dome-like dependence of T_c on the doped carrier concentration in the T′-type (La,Eu)-based cuprates is explained in terms of the pairing mediated by spin fluctuations based on the d-p model calculation. (author)

[en] We have attempted to synthesize the Bi-2212 phase of BiPbSr_2-x Ba_x RECu₂O_8+δ (RE = Nd, Sm, Gd, Y, Yb) by the conventional solid-state reaction method. The completely Ba-substituted sample with x = 1 has been successfully synthesized only for Nd. As the ionic radius of RE decreases, the solubility limit of Ba decreases. It has been found that the substitution of Ba for Sr requires the expansion of both the Bi-O and RE layers as well as the synthesis in a reducing atmosphere to suppress the formation of BaBiO₃-related phases. Unfortunately, the hole-doping to semiconducting BaPbBa₂NdCu₂O_8+δ has not been successful. (author)

[en] We have measured the temperature dependences of the thermal conductivity of Ba₂Cu_3−xM_xO₄Cl₂ (M = Pd, Ni, x=0, 0.03 ) single crystals including two-dimensional (2D) Cu₃O₄ planes consisting of a strong 2D spin network of Cu_A²⁺ spins and a weak 2D spin network of Cu_B²⁺ spins. It has been found that the thermal conductivity due to spins, κ_spin, exists in the thermal conductivity parallel to the Cu₃O₄ plane owing to the strong 2D spin network of Cu_A²⁺ spins and exhibits a broad peak around room temperature. The maximum value of κ_spin is ∼7 W/Km and comparable with that in Nd₂CuO₄ with almost the same 2D spin network of Cu²⁺ spins as Cu_A²⁺ spins in Ba₂Cu₃O₄Cl₂. The κ_spin has been found to be suppressed by 1% impurities on account of the decrease in the mean free path of magnetic excitations, suggesting that κ_spin is expected to be enhanced in 2D quantum spin systems such as Ba₂Cu₃O₄Cl₂ by reducing the amount of impurities in a single crystal. Moreover, it has concluded that the frustration between Cu_A²⁺ and Cu_B²⁺ spins little affects the existence of κ_spin. (author)

[en] Recently, hole-doped superconducting cuprates with the T'-structure La_1.8−xEu_0.2Sr_xCuO₄ (LESCO) have attracted a lot of attention. We have performed x-ray photoemission and absorption spectroscopy measurements on as-grown and reduced T'-LESCO. Results show that electrons and holes were doped by reduction annealing and Sr substitution, respectively. However, it is shown that the system remains on the electron-doped side of the Mott insulator or that the charge-transfer gap is collapsed in the parent compound. (author)