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...

Single crystal neutron and high-energy x-ray diffraction have identified the phase lines corresponding to transitions between the ambient-pressure tetragonal (T), the antiferromagnetic orthorhombic (O) and the nonmagnetic collapsed tetragonal (cT) phases of CaFe2As2. We find no evidence of additional structures for pressures up to 2.5 GPa (at 300 K). Both the T-cT and O-cT transitions exhibit significant hysteresis effects and we demonstrate that coexistence of the O and cT phases can occur if a non-hydrostatic component of pressure is present. Measurements of the magnetic diffraction peaks show no change in the magnetic structure or ordered moment as a function of pressure in the O phase and we find no evidence of magnetic ordering in the cT phase. Band structure calculations show that the transition results in a strong decrease of the iron 3d density of states at the Fermi energy, consistent with a loss of the magnetic moment.PACS numbers: 61.50. Ks, 61.05.fm, 74.70.Dd The discovery 1,2 of pressure-induced superconductivity in CaFe 2 As 2 has opened an exciting new avenue for investigations of the relationship between magnetism, superconductivity, and lattice instabilities in the iron arsenide family of superconductors. Features found in the compositional phase diagrams of the iron arsenides, 3 such as a superconducting region at low temperature and finite doping concentrations, are mirrored in the pressure-temperature phase diagrams. Superconductivity appears at either a critical doping, or above some critical pressure in the AFe 2 As 2 (A=Ba, Sr, Ca) or '122' family of compounds, raising questions regarding the role of both electronic doping and pressure, especially in light of the recent observation of pressure induced superconductivity in the related compound, LaFeAsO.4 Does doping simply add charge carriers, or are changes in the chemical pressure, upon doping, important as well? What subtle, or striking, modifications in structure or magnetism occur with doping or pressure, and how are they related to superconductivity? Similar to other members of the AFe 2 As 2 (A=Ba, Sr) family, 5,6,7,8 at ambient pressure CaFe 2 As 2 undergoes a transition from a non-magnetically ordered tetragonal (T) phase (a = 3.879(3)Å, c = 11.740(3)Å) to an antiferromagnetic (AF) orthorhombic (O) phase (a = 5.5312(2)Å, b = 5.4576(2)Å, c = 11.683(1)Å) below approximately 170 K.9,10 In the O phase, Fe moments order in the so called AF2 structure 11 with moments directed along the aaxis of the orthorhombic structure.10 Neutron powder diffraction measurements 12 of CaFe 2 As 2 under hydrostatic pressure found that for p>0.35 GPa (at T=50 K), the antiferromagnetic O phase transforms to a new, non-magnetically ordered, collapsed tetragonal (cT) structure (a = 3.9792(1)Å, c = 10.6379(6)Å) with a dramatic decrease in both the unit cell volume (5%) and the c/a ratio (11%). The transition to the cT phase occurs in close proximity to the pressure at which superconductivity is first observed. 1 Total energy calculations based on this cT s...

A neutron scattering technique was developed to measure the density of heavy water confined in a nanoporous silica matrix in a temperature-pressure range, from 300 to 130 K and from 1 to 2,900 bars, where bulk water will crystalize. We observed a prominent hysteresis phenomenon in the measured density profiles between warming and cooling scans above 1,000 bars. We interpret this hysteresis phenomenon as support (although not a proof) of the hypothetical existence of a first-order liquid-liquid phase transition of water that would exist in the macroscopic system if crystallization could be avoided in the relevant phase region. Moreover, the density data we obtained for the confined heavy water under these conditions are valuable to large communities in biology and earth and planetary sciences interested in phenomena in which nanometer-sized water layers are involved.confined water | equation of state | liquid-liquid critical phenomenon I n many biological and geological systems, water resides in pores of nanoscopic dimensions, or close to hydrophilic or hydrophobic surfaces, comprising a layer of water, one or two molecules thick, with properties often different from the bulk. Such "confined" or "interfacial" water has attracted considerable attention, due to its fundamental importance in many processes, such as protein folding, concrete curing, corrosion, molecular and ionic transport, etc. (1-3). However, our understanding of the numerous physicochemical anomalies of confined water, and indeed of bulk water, is still incomplete. Basic gaps persist, among which the most interesting one is the origin of the unusual behavior of water in the supercooled region where water remains in the liquid state below the melting point (4-7). Recent studies have aimed at explaining anomalies such as the density maximum and minimum (8-10), and the apparent divergence of the thermodynamic response functions at 228 K at ambient pressure (11). The three major hypothesized scenarios currently under scrutiny are the "singularity-free (SF) scenario" (12, 13), the "liquidliquid critical point (LLCP) scenario" (14, 15), and the "critical point-free (CPF) scenario" (16). It is hypothesized, by all these three scenarios, that in the low temperature range bulk water is composed of a mixture of two structurally distinct liquids: the low-density liquid (LDL) and the high-density liquid (HDL). They are respectively the thermodynamic continuation of the low-density amorphous ice (LDA) and high-density amorphous ice (HDA) into the liquid state. Evidence of a first-order phase transition between LDA and HDA has been reported since 1985 (17-20). Subsequently, several experimental findings have been interpreted as support of the hypothetical existence of two different structural motifs of liquid water (21-27). However, some of the interpretations have been questioned (28,29). So far, direct evidence of a first-order liquid-liquid phase transition between LDL and HDL, as a thermodynamic extension of the first-order transition established in the am...