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 non-magnetic, 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. Two recently discovered [1,2,3,4] series of high transition temperature (high-T c ) superconductors originate from the parent systems RFeAsO (R = rare earth) and AFe 2 As 2 (A = alkaline earth metal), which are tetragonal at room temperature but undergo an orthorhombic distortion in the range 100-220 K that is associated with the onset of antiferromagnetic order [5,6,7,8,9,10,11]. Tuning the system via element substitution [2,3,4,12,13,14] or oxygen deficiency [15,16] suppresses the magnetic order and structural distortion in favor of superconductivity (T c 's up to 55 K), with an overall behavior strikingly similar to the high-T c copper oxide family of superconductors.The recent report [17] of pressure-induced superconductivity in the parent CaFe 2 As 2 compound opens an alternative path to superconductivity. Pressure suppresses the distinct resistivity signature of the hightemperature structural and magnetic phase transition from 170 K at ambient pressure [18] to 128 K at 0.35 GPa [17]. Superconductivity emerges with T c up to 12 K for pressures between 0.23 GPa and 0.86 GPa [17]. The pressure-induced superconductivity in CaFe 2 As 2 was confirmed [19] and followed by observations of superconductivity for BaFe 2 As 2 and SrFe 2 As 2 at significantly higher pressures [20]. In CaFe 2 As 2 , a second hightemperature phase transition is observed above 0.55 GPa and 104 K by anomalies in the resistivity [17]. However, the nature of the phase at temperatures below this transition and its relation to the ambient-pressure tetragonal, orthorhombic and pressure-induced superconducting phases are as yet unknown.Neutron scattering experiments on CaFe 2 As 2 were performed to elucidate these issues. Special attention was paid to maintain experimental conditions closest to the reported macroscopic measurements and under welldefined hydrostatic pressure. Therefore, the experiments were performed on a polycrystalline sample prepared out of approximately 1.75 grams of single crystalline CaFe 2 As 2 material grown using the procedure described in references [18] and [21]. The temperature profile for preparing this material was slightly modified (heating to 1100 • C and cooling over 50 hours to 600 • C) to inhibit the formation of the reported[18] needle-shaped impurity phase. Temperature-dependent resistance measurements on these crystals reproduced the data presented in references [17] and [18]. The single crystals ( 300 pieces) were loaded with attempted random orientation into a He-gas pressure cel...

Neutron diffraction measurements of a high quality single crystal of CaFe 2 As 2 are reported. A sharp transition was observed between the high temperature tetragonal and low temperature orthorhombic structures at T S = 172.5K (on cooling) and 173.5K (on warming). Coincident with the structural transition we observe a rapid, but apparently continuous, ordering of the Fe moments, in a commensurate antiferromagnetic structure is observed, with a saturated moment of 0.80(5)μ B /Fe directed along the orthorhombic aaxis. The hysteresis of the structural transition is 1K between cooling and warming and is consistent with previous thermodynamic, transport and single crystal x-ray studies. The temperature onset of magnetic ordering shifts rigidly with the structural transition providing the clearest evidence to date of the coupling between the structural and magnetic transitions in this material and the broader class of iron arsenides.

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The iron chalcogenide Fe(1+y)(Te(1-x)Se(x)) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe(1+y)Te exhibits antiferromagnetic order with an in-plane magnetic wave vector (pi,0) (ref. 6). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave vector (pi,pi) that connects hole and electron parts of the Fermi surface. Despite these differences, both the pnictide and chalcogenide Fe superconductors exhibit a superconducting spin resonance around (pi,pi) (refs 9, 10, 11). A central question in this burgeoning field is therefore how (pi,pi) superconductivity can emerge from a (pi,0) magnetic instability. Here, we report that the magnetic soft mode evolving from the (pi,0)-type magnetic long-range order is associated with weak charge carrier localization. Bulk superconductivity occurs as magnetic correlations at (pi,0) are suppressed and the mode at (pi, pi) becomes dominant for x>0.29. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors.

The discovery of a new family of high-T(C) materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked; however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2.