N. Ni

Pressure-induced volume-collapsed tetragonal phase of CaFe2As2 as seen via neutron scattering

Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe2As2 undergoes a pressure-induced transition to a nonmagnetic volume collapsed tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction. © 2008 The American Physical Society.

Structural transition and anisotropic properties of single-crystalline SrFe2As2

Plate-like single crystals of SrFe2As2 as large as 3x3x0.5 mm3 have been grown out of Sn flux. The SrFe2As2 single crystals show a structural phase transition from a high temperature tetragonal phase to a low temperature orthorhombic phase at To = 198 K, and do not show any sign of superconductivity down to 1.8 K. The structural transition is accompanied by an anomaly in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In an intermediate temperature range from 198 K to 160 K, single crystal X-ray diffraction suggests a coexistence of the high-temperature tetragonal and the low-temperature orthorhombic phases.

Temperature versus doping phase diagrams for Ba(Fe1―xTMx)2As2(TM=Ni, Cu, Cu/Co) single crystals

Microscopic, structural, transport and thermodynamic measurements of single crystalline Ba(Fe1-xTMx)2As2 (TM = Ni and Cu) series, as well as two mixed TM = Cu / Co series, are reported. All the transport and thermodynamic measurements indicate that the structural and magnetic phase transitions at 134 K in pure BaFe2As2 are monotonically suppressed and increasingly separated in a similar manner by these dopants. In the Ba(Fe1-xNix)2As2 (x =< 0.072), superconductivity, with Tc up to 19 K, is stabilized for 0.024 =< x =< 0.072. In the Ba(Fe1-xCux)2As2 (x =< 0.356) series, although the structural and magnetic transitions are suppressed, there is only a very limited region of superconductivity: a sharp drop of the resistivity to zero near 2.1 K is found only for the x = 0.044 samples. In the Ba(Fe1-x-yCoxCuy)2As2 series, superconductivity, with Tc values up to 12 K (x ~ 0.022 series) and 20 K (x ~ 0.047 series), is stabilized. Quantitative analysis of the detailed temperature-dopant concentration (T-x) and temperature-extra electrons (T-e) phase diagrams of these series shows that there exists a limited range of the number of extra electrons added, inside which the superconductivity can be stabilized if the structural and magnetic phase transitions are suppressed enough. Moreover, comparison with pressure-temperature phase diagram data, for samples spanning the whole doping range, further reenforces the conclusion that suppression of the structural / magnetic phase transition temperature enhances Tc on the underdoped side, but for the overdoped side Tcmax is determined by e. Therefore, by choosing the combination of dopants that are used, we can adjust the relative positions of the upper phase lines (structural and magnetic phase transitions) and the superconducting dome to control the occurrence and disappearance of the superconductivity in transition metal, electron-doped BaFe2As2.

Variation of the magnetic ordering in GdT2Zn20 (T=Fe, Ru, Os, Co, Rh and Ir) and its correlation with the electronic structure of isostructural YT2Zn20

Magnetization, resistivity, and specific heat measurements were performed on solution-grown single crystals of six GdT{sub 2}Zn{sub 20} (T=Fe, Ru, Os, Co, Rh, and Ir) compounds, as well as on their Y analogs. For the Gd compounds, the Fe column members manifest a ferromagnetic (FM) ground state (with an enhanced Curie temperature T{sub c} for T=Fe and Ru), whereas the Co column members manifest an antiferromagnetic (AFM) ground state. Thermodynamic measurements on YT{sub 2}Zn{sub 20} revealed that the enhanced T{sub c} for GdFe{sub 2}Zn{sub 20} and GdRu{sub 2}Zn{sub 20} can be understood within the framework of Heisenberg moments embedded in a nearly ferromagnetic Fermi liquid. Furthermore, electronic structure calculations indicate that this significant enhancement is due to a large transition metal partial density of states at the Fermi level that places these compounds close to the Stoner FM criterion. The change from FM to AFM ordering (between the Fe and Co column materials) is associated with the filling of electronic states with two additional electrons/f.u. The degree of this sensitivity is addressed by the studies of the pseudoternary compounds Gd(Fe{sub x}Co{sub 1-x}){sub 2}Zn{sub 20} and Y(Fe{sub x}Co{sub 1-x}){sub 2}Zn{sub 20}, which clearly reveal the effect of 3d-band filling on their magneticmorexa0» properties.«xa0less

Effect of pressure on the structural phase transition and superconductivity in (Ba1-xKx)Fe2As2 (x=0 and 0.45) and Srfe2As2 single crystals

The effects of pressure up to

First-order structural phase transition in CaFe2As2

sim 20

Suppression of antiferromagnetic spin fluctuations in the collapsed phase of CaFe2As2

kbar, on the structural phase transition of SrFe

First order structural phase transition in CaFe

_2

_2

As

As

_2