We have successfully grown high-quality single crystals of SrFe 2 As 2 and A 0.6 K 0.4 Fe 2 As 2 ͑A = Sr, Ba͒ using flux method. The resistivity, specific heat, and Hall coefficient have been measured. For parent compound SrFe 2 As 2 , an anisotropic resistivity with c / ab as large as 130 is obtained at low temperatures. A sharp drop in both in-plane and out-plane resistivities due to the spin-density-wave ͑SDW͒ instability is observed below 200 K. The angular dependence of in-plane magnetoresistance shows twofold symmetry with field rotating within ab plane below SDW transition temperature. This is consistent with a stripe-type spin ordering in SDW state. In K-doped A 0.6 K 0.4 Fe 2 As 2 ͑A = Sr, Ba͒, the SDW instability is suppressed and the superconductivity appears with T c above 35 K. The rather low anisotropy in upper critical field between H ʈ ab and H ʈ c indicates that interplane coupling plays an important role in hole-doped Fe-based superconductors.The recent discovery of superconductivity with transition temperature T c ϳ 26 K in LaFeAsO 1−x F x has generated tremendous interest in scientific community. 1 Shortly after this discovery, the T c was raised to 41-55 K by replacing La by rare-earth Ce, Sm, Pr, Nd, etc., making those systems with T c exceeding 50 K. 2-5 The undoped quaternary compounds crystallize in a tetragonal ZrCuSiAs-type structure, which consists of alternate stacking of edge-sharing Fe 2 As 2 tetrahedral layers and La 2 O 2 tetrahedral layers along c axis. Very recently, superconductivity with T c of up to 38 K was discovered in AFe 2 As 2 ͑A = Ba, Sr, Ca͒ upon K or Na doping. 6-10 AFe 2 As 2 compounds crystallize in a tetragonal ThCr 2 Si 2 -type structure with identical Fe 2 As 2 tetrahedral layers as in LaFeAsO, but separated by single elemental A layers. These compounds contain no oxygen in A layers. The simpler structure of AFe 2 As 2 system makes it more suitable for research of intrinsic physical properties of Fe-based compounds.Except for a relatively high transition temperature, the system displays many interesting properties. The existence of a spin-density-wave ͑SDW͒ instability in parent LaFeAsO ͑Ref. 11͒ was indicated by specific heat, optical measurements, and first-principles calculations, and subsequently confirmed by neutron-scattering, 12 NMR, 13 sR, 14 and Mössbauer 15 spectroscopic measurements. The superconductivity only appears when SDW instability was suppressed by doping carriers or applying pressure. The competition between superconductivity and SDW instability was identified in other rare-earth substituted systems. 2,16,17 Besides the SDW instability, structural distortions from tetragonal to monoclinic were also observed for both ReFeAsO ͑Re = rare earth͒ and AFe 2 As 2 ͑A = Ba, Sr, Ca͒. 18-23 The structural transition temperatures were found to occur at slightly higher than SDW transition temperature in LaFeAsO, 12 but the two transitions occur simultaneously in AFe 2 As 2 ͑A = Ba, Sr, Ca͒. 18,19,24 The band-structure calculation and neutronscattering exp...

Resistance measurements of a "111" type superconductor Li x FeAs ͑x = 0.8, 1.0, 1.1͒ were performed at high pressure. The superconductivity transition temperature ͑T c ͒ is found to decrease almost linearly with increasing pressure and the pressure derivative, dT c / dP, becomes smaller with increasing Li content. Electron spin resonance experiment at ambient pressure on a stoichometric sample shows gradual decrease in the spin moment and the loss of coherence of spin coupling as the temperature is lowered. A very weak Fe local moment is observed when the temperature is lowered to T c. In situ synchrotron radiation powder x-ray diffraction at high pressure shows changes in the resistivity are not related to a structural transition.

The low and glasslike thermal conductivity of metal-doped semiconductor clathrate compounds makes them potentially high-efficiency thermoelectric materials. The cause of this unique and remarkable property has been postulated to be due to resonant scattering of lattice phonons by localized vibrations of the dopants. We present theoretical evidence in support of this hypothesis through the analysis of electronic and vibrational interactions between dopant atoms with the host framework. In particular, the contrasting behavior of two clathrates: the glasslike thermal conductivity in Na8Si46 and the normal behavior in Cs8Sn44 can be rationalized.

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