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...

We describe a novel cold neutron spectrometer under development at NIST optimized for wave vector resolved spectroscopy with incident energies between 2.1 meV and 20 meV and energy resolution from 0.05 meV (E i = 2.1 meV) to 3.0 meV (E i = 20 meV). By using a 1428 cm 2 double focusing PG (0 0 2) monochromator close to the National Institute of Standards and Technology (NIST) cold neutron source the instrument provides up to 5 × 10 8 neutrons cm −2 s −1 on a 8 cm 2 sample area. The measured performance is consistent with Monte Carlo simulations. The monochromating system, which includes radial collimators, three filters and a variable beam aperture, offers considerable flexibility in optimizing Q-resolution, energy resolution and intensity. The detector system will consist of an array of 20 channels which combined will subtend a solid angle of 0.2 sr. This is approximately a factor of 40 more than a conventional triple axis spectrometer. Each detector channel contains a vertically focusing double crystal analyzer system (DXAL) actuated by a single stepping motor. We find identical integrated reflectivity at approximately 10% coarser energy resolution for the 130 mosaic double bounce analyzer as compared to a conventional 25 analyzer at the same energy. The vertical focusing of the DXAL allows for smaller detectors for enhanced signal to noise with 8 • vertical acceptance. Options for post sample collimators and filters provide flexibility in the choice of scattered beam energy and wavevector resolution.

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