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.

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...

Extensive X-ray and neutron scattering experiments and additional transmission electron microscopy results reveal the partial decomposition of Nd2−xCexCuO 4±δ (NCCO) in a low-oxygenfugacity environment such as that typically realized during the annealing process required to create a superconducting state. Unlike a typical situation in which a disordered secondary phase results in diffuse powder scattering, a serendipitous match between the in-plane lattice constant of NCCO and the lattice constant of one of the decomposition products, (Nd,Ce)2O3, causes the secondary phase to form an oriented, quasi-two-dimensional epitaxial structure. Consequently, diffraction peaks from the secondary phase appear at rational positions (H, K, 0) in the reciprocal space of NCCO. Additionally, because of neodymium paramagnetism, the application of a magnetic field increases the low-temperature intensity observed at these positions via neutron scattering. Such effects may mimic the formation of a structural superlattice or the strengthening of antiferromagnetic order of NCCO, but the intrinsic mechanism may be identified through careful and systematic experimentation. For typical reduction conditions, the (Nd,Ce)2O3 volume fraction is ∼ 1%, and the secondary-phase layers exhibit long-range order parallel to the NCCO CuO2 sheets and are 50 − 100 A thick. The presence of the secondary phase should also be taken into account in the analysis of other experiments on NCCO, such as transport measurements.

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CaFe2As2 single crystals under uniaxial pressure applied along the c axis exhibit the coexistence of several structural phases at low temperatures. We show that the room temperature tetragonal phase is stabilized at low temperatures for pressures above 0.06 GPa, and its weight fraction attains a maximum in the region where superconductivity is observed under applied uniaxial pressure. Simultaneous resistivity measurements strongly suggest that this phase is responsible for the superconductivity in CaFe2As2 found below 10 K in samples subjected to non-hydrostatic pressure conditions. , the structural and magnetic transitions are suppressed and SC is observed with T C as high as 55 K 9 . One of the most interesting anomalies in the AEFe 2 As 2 family is found in CaFe 2 As 2 under pressure as discussed in a recent review 4 . At ambient pressure, CaFe 2 As 2 undergoes a first order transition from a high temperature tetragonal (T ) phase (ThCr 2 Si 2 structure) to a structure with orthorhombic (O) symmetry at T T O = 172 K 10 concomitant with an AF transition 6 . Upon the application of modest pressures, using liquid media self-clamping cells, the structural and AF transitions were rapidly suppressed and SC was observed for P ≥ 0.23 GPa and T ≤ 12 K 8,11 . SC has also been observed in electrical resistance measurements of samples under uniaxial pressure 12 . Neutron powder diffraction measurements, using a He gas pressure cell to ensure hydrostatic pressure conditions, revealed a volume-collapsed tetragonal (cT ) phase in this pressure range, below ≈ 100 K 13 . Although the onset of SC seemed to be closely related to the appearance of the cT phase, more recent transport measurements under hydrostatic pressure conditions (He-gas cell) have revealed that neither the ambient pressure O phase (below T T O ) nor the cT phase support SC 14 . These measurements along with an extended structural study by single crystal neutron diffraction 15 , demonstrated that the electronic, magnetic and structural transitions are sharp and clearly defined under hydrostatic pressure.Measurements done using a frozen liquid medium, in contrast, manifest a significant non-hydrostatic component upon the transition to the cT phase resulting in a low temperature multi-crystallographic-phase state that includes both the O and cT phases among, perhaps, other as yet unidentified phases. This is consistent with reports of the coexistence between static magnetic order and SC as inferred from µSR experiments 16 and recent NMR experiments 17 . Nevertheless, the puzzle remains: Which phase(s) is(are) responsible for SC in CaFe 2 As 2 under pressure? Does the orthorhombic phase support both superconductivity and magnetic ordering or, as speculated in Ref.4 , is SC associated with some residual untransformed T phase? Is SC to be found in this, as yet undiscovered phase at the boundary between the O and cT phases 12,14,18 ? To investigate these issues we have performed single crystal neutron diffraction measurements on CaFe 2 As 2 under uniaxial pressure....

Neutron diffraction was used to determine the crystal structure and magnetic ordering pattern of a La 2 CuO 4 single crystal, with and without applied magnetic field. A previously unreported, subtle monoclinic distortion of the crystal structure away from the orthorhombic space group Bmab was detected. The distortion is also present in lightly Sr-doped crystals. A refinement of the crystal structure shows that the deviation from orthorhombic symmetry is predominantly determined by displacements of the apical oxygen atoms. An in-plane magnetic field is observed to drive a continuous reorientation of the copper spins from the orthorhombic b-axis to the c-axis, directly confirming predictions based on prior magnetoresistance and Raman scattering experiments. A spin-flop transition induced by a caxis oriented field previously reported for non-stoichiometric La 2 CuO 4 is also observed, but the transition field (11.5 T) is significantly larger than that in the previous work.