Crystal growth and characterization of the antiperovskite superconductor MgC1-xNi3-y

By varying the parameters controlling the growth of crystals, including the thermodynamic variables, such as temperature, pressure, and reagent composition and the kinetic factors, namely reaction time and cooling rate, we found the most appropriate conditions for the reproducible growth of the nonstoichiometric antiperovskite superconductors MgC1-xNi3-y. Bulk single crystals of MgC1-xNi3-y were grown by a self-flux method at 3 GPa and 1700 C using a mixture of Mg, C, and Ni powders in a molar ratio 1:1.25:3. The as-grown black colored crystals, mechanically extracted from solidified lump, exhibit various irregular three dimensional shapes, with flat surfaces and maximum dimensions up to ~ (1-1.2) x (0.8-1.0) x (0.4-0.6) mm3. Single-crystal x-ray diffraction refinement confirmed the high structural perfection of the grown crystals (Space group Pm-3m, No 221, Z = 1, a = 3.7913(1) {\AA}, and V = 54.5(1) {\AA}3), but also the presence of deficiencies on the C and Ni sites. Temperature-dependent magnetization measurements showed a single-phase behaviour with a critical temperature (Tc) ranging between 6.3 and 6.8 K due to the slightly different C and Ni stoichiometries of MgC1-xNi3-y crystals. The growth of relatively large crystals reported here could provide a helpful guidance for further syntheses of various 3d-based antiperovskite intermetallics under high pressure.Comment: 4 tables, 5 figure

Study of the in-plane magnetic penetration depth in the cuprate superconductor Ca_2-xNa_xCuO_2Cl_2: role of the apical sites

A study of the in-plane magnetic penetration depth \lambda_ab in a series of the cuprate superconductors Ca_2-xNa_xCuO_2Cl_2 (Na-CCOC) with Na content x=0.11, 0.12, 0.15, 0.18, and 0.19 is reported. The zero temperature values of \lambda_ab(0) were obtained by means of the muon-spin rotation technique, as well as from measurements of the intrinsic susceptibility \chi^int(0) by using the procedure developed by Kanigel et al. [Phys.Rev.B 71, 224511 (2005)]. \lambda_ab at T=0K was found to increase with decreasing doping from \lambda_ab(0)=316(19)nm for the x=0.19 sample to \lambda_ab(0)=430(26)nm for the x=0.11 one. From a comparison of the present Na-CCOC data with those of Bi2201 and La214 cuprate superconductors it is concluded that substitution of the apical oxygen by chlorine decreases the coupling between the superconducting CuO_2 planes, leading to an enhancement of the two-dimensional properties of Na-CCOC.Comment: 8 pages, 7 figure

High-pressure growth and characterization of bulk MnAs single crystals

Bulk single crystals of manganese arsenide (MnAs) were grown from melt at 1 GPa and 1100 {\deg}C by using a cubic-anvil, high-pressure, and high-temperature technique. The as-grown black colored crystals extracted from solidified lump exhibit a plate-like morphology, with flat surfaces and maximum dimensions up to ~ 3 x 2 x 0.5 mm3. The hexagonal crystal structure at room temperature was confirmed by X-ray diffraction [B81, space group P63/mmc, No 194, Z = 2, a = 3.7173(4) {\AA}, b = 3.7173(4) {\AA}, c = 5.7054(8) {\AA}, and V = 68.277(16) {\AA}3]. Temperature-dependent magnetization measurements reveal the occurrence of a first-order ferro- to paramagnetic transition at Tc = 318.5 K accompanied by a hysteresis of ~ 9 K. The successful growth of relatively large crystals reported here might be extended to various substituted analogues of MnAs, thus opening new possibilities for further exploration of this interesting system.Comment: arXiv admin note: text overlap with arXiv:1705.1049

Anharmonicity and self-energy effects of the E2g phonon in MgB2

We present a Raman scattering study of the E2g phonon anharmonicity and of superconductivity induced self-energy effects in MgB2 single crystals. We show that anharmonic two phonon decay is mainly responsible for the unusually large linewidth of the E2g mode. We observe ~ 2.5 % hardening of the E2g phonon frequency upon cooling into the superconducting state and estimate the electron-phonon coupling strength associated with this renormalization.Comment: 4 pages, 3 figures, accepted to PR

Normal state bottleneck and nematic fluctuations from femtosecond quasi-particle relaxation dynamics in Sm(Fe,Co)AsO

We investigate temperature and fluence dependent dynamics of the photoexcited quasi-particle relaxation and low-energy electronic structure in electron-doped 1111-structure Sm(Fe_{0.93}Co_{0.07})AsO single crystal. We find that the behavior is qualitatively identical to the 122-structure Ba(Fe,Co)_{2}As_{2} including the presence of a normal state pseudogap and a marked 2-fold symmetry breaking in the tetragonal phase that we relate to the electronic nematicity. The 2-fold symmetry breaking appears to be a general feature of the electron doped iron pnictides

Critical current anisotropy in Nd-1111 single crystals and the infuence of neutron irradiation

We report on angle-resolved magnetization measurements on NdFeAsO$_{0.65}$F$_{0.35}$ (Nd-1111) single crystals. The field dependence of the critical current density, $J_c$, is non-monotonous in these crystals at all orientations and temperatures due to the fishtail effect, which strongly influences the angular dependence of $J_c$. The currents decrease as the field is tilted from the crystallographic c-axis at low fields, but increase at high fields. A peak occurs in the angular dependence of $J_c$ at intermediate fields. The critical currents are significantly enhanced after irradiation with fast neutrons and the fishtail disappears. The different current anisotropies at low and high fields, however, persist. We discuss the data in the framework of the anisotropic scaling approach and propose a transition from dominant pinning by large defects of low density at low fields to pinning by small defects of high density at high fields in the pristine crystal. Strong pinning dominates at all fields after the irradiation, and the angular dependence of $J_c$ can be described by anisotropic scaling only after an appropriate extension to this pinning regime