S. Matsuishi

Magnetic order in the CaFe1-xCoxAsF (x=0.00,0.06,0.12) superconducting compounds

A Neutron Powder Diffraction (NPD) experiment has been performed to investigate the structural phase transition and magnetic order in CaFe1-xCoxAsF superconductor compounds (x = 0, 0.06, 0.12). The parent compound CaFeAsF undergoes a tetragonal to orthorhombic phase transition at 134(3) K, while the magnetic order in form of a spin-density wave (SDW) sets in at 114(3) K. The antiferromagnetic structure of the parent compound has been determined with a unique propagation vector k = (1,0,1) and the Fe saturation moment of 0.49(5)uB aligned along the long a-axis. With increasing Co doping, the long range antiferromagnetic order has been observed to coexist with superconductivity in the orthorhombic phase of the underdoped CaFe0.94Co0.06AsF with a reduced Fe moment (0.15(5)uB). Magnetic order is completely suppressed in optimally doped CaFe0.88Co0.12AsF. We argue that the coexistence of SDW and superconductivity might be related to mesoscopic phase separation.

Detection of Antiferromagnetic Ordering in Heavily Doped LaFeAsO_{1-x}H_{x} Pnictide Superconductors Using Nuclear-Magnetic-Resonance Techniques

We studied double superconducting (SC) domes in LaFeAsO(1-x)H(x) by using 75As and 1H nuclear-magnetic-resonance techniques and unexpectedly discovered that a new antiferromagnetic (AF) phase follows the double SC domes on further H doping, forming a symmetric alignment of AF and SC phases in the electronic phase diagram. We demonstrated that the new AF ordering originates from the nesting between electron pockets, unlike the nesting between electron and hole pockets, as seen in the majority of undoped pnictides. The new AF ordering is derived from the features common to high-Tc pnictides; however, it has not been reported so far for other high-Tc pnictides because of their poor electron doping capability.

Effects of magnetic doping and temperature dependence of phonon dynamics in CaFe1-xCoxAsF compounds (x=0, 0.06, and 0.12)

We report detailed measurements of composition as well as temperature dependence of the phonon density-of-states in a new series of FeAs compounds with composition CaFe1\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12). The composition as well as temperature dependence of phonon spectra for CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) compounds have been measured using time of flight IN4C and IN6 spectrometers at ILL, France. The comparison of phonon spectra at 300 K in these compounds shows that acoustic phonon modes up to 12 meV harden in the doped compounds in comparison to the parent CaFeAsF. While intermediate energy phonon modes from 15 meV to 25 meV are also found to shift towards high energies only in the 12 % Co doped CaFeAsF compound. The experimental results for CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) are quite different from our previous phonon studies on parent and superconducting MFe2As2 (M=Ba, Ca, Sr) where low-energy acoustic phonon modes do not react with doping, while the phonon spectra in the intermediate range from 15 to 25 K are found to soften in these compounds. We argue that stronger spin phonon interaction play an important role for the emergence of superconductivity in these compounds. The lattice dynamics of CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) compounds is also investigated using the ab-initio as well as shell model phonon calculations. We show that the nature of the interaction between the Ca and the Fe-As layers in CaFeAsF compounds is quite different compared with our previous studies on CaFe2As2.

Observation of Jonscher law in ac hopping conduction of the electron-doped nanoporous crystal 12CaO·7Al2O3 in a THz frequency range

We have performed terahertz time-domain spectroscopy of carrier-doped nanoporous crystal

Phonon spectra in CaFe2As2 and Ca0.6Na0.4Fe2As2: Measurement of the pressure and temperature dependence and comparison with ab initio and shell model calculations

12\mathrm{Ca}\mathrm{O}∙7{\mathrm{Al}}_{2}{\mathrm{O}}_{3}

Experimental evidence of T c enhancement without the influence of spin fluctuations: NMR study on LaFeAsO 1 − x H x under a pressure of 3.0 GPa

showing the Mott variable range hopping at room temperature. The real part of the dielectric constant clearly demonstrates the nature of localized carriers. The frequency dependence of both the real and imaginary parts of the dielectric constant can be simply explained by assuming two contributions: a dielectric response by the parent compound with no carriers and an ac hopping conduction with the Jonscher law generally reported up to GHz range. The possible obedience to the Jonscher law in the THz range suggests a relaxation time of the hopping carriers much faster than

Pressure dependence of the low-temperature crystal structure and phase transition behavior of CaFeAsF and SrFeAsF: A synchrotron x-ray diffraction study

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Possible Polaron Effect in Complex Terahertz Conductivity of Electron-Doped Nanoporous Crystal 12CaO·7Al2O3

in the carrier-doped

Homogeneous coexistence of superconducting and spin-density-wave states in CaFe_{1-x}Co_{x}AsF as seen via nuclear magnetic resonance

12\mathrm{Ca}\mathrm{O}∙7{\mathrm{Al}}_{2}{\mathrm{O}}_{3}

Multiple-pseudogap phases in the hydrogen-doped LaFeAsO system

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