Jerel L. Zarestky

Coexistence of competing antiferromagnetic and superconducting phases in the underdoped Ba(Fe0.953Co0.047)2As2 compound using x-ray and neutron scattering techniques.

Neutron and x-ray diffraction studies show that the simultaneous first-order transition to an orthorhombic and antiferromagnetic (AFM) ordered state in BaFe2As2 splits into two transitions with Co doping. For Ba(Fe0.953Co0.047)2As2, a tetragonal-orthorhombic transition occurs at TS = 60 K, followed by a second-order transition to AFM order at TN = 47 K. Superconductivity (SC) occurs in the orthorhombic state below TC = 15 K and coexists with AFM. Below TC, the static Fe moment is reduced and a 4 meV spin gap develops indicating competition between coexisting SC and AFM order.

Unconventional pairing in the iron arsenide superconductors

We use magnetic long-range order as a tool to probe the Cooper-pair wave function in the iron arsenide superconductors. We show theoretically that antiferromagnetism and superconductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state. The observation of coexistence in

Magnetic structure of DyNi2B2C

\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}

A comparative study of the post-quench behaviour of Cu–Al–Be and Cu–Zn–Al shape memory alloys

then demonstrates unconventional pairing in this material. The detailed agreement between theory and neutron-diffraction experiments, in particular, for the unusual behavior of the magnetic order below

Anisotropic three-dimensional magnetism in CaFe2As2

{T}_{c}

Flux growth at ambient pressure of millimeter-sized single crystals of LaFeAsO, LaFeAsO1−xFx, and LaFe1−xCoxAsO

, demonstrates the robustness of our conclusions. Our findings strongly suggest that superconductivity is unconventional in all members of the iron arsenide family.

Incommensurate Spin-Density Wave Order in Electron-Doped BaFe2As2 Superconductors

Abstract Neutron diffraction techniques have been used to study the magnetic structure of DyNi2B2C. The measurements, performed on single crystals of this compound depleted in the neutron absorbing B10 nuclei, show that below approximately 10 K the compound is a simple collinear antiferromagnet. The moments are aligned ferromagnetically in each rare-earth carbon layer perpendicular to the c-axis with the magnetic moments of two consecutive layers aligned in opposite directions. In contrast to the case of both HoNi2B2C and ErNi2B2C no modulation along the a∗ direction was observed in this compound.

Probing spin frustration in high-symmetry magnetic nanomolecules by inelastic neutron scattering

Abstract This paper reports a comparative investigation of the effect of quenching on the Cu–Al–Be and Cu–Zn–Al shape memory alloys by the use of several experimental techniques. In a first stage, the order–disorder transitions in these alloys have been characterized by means of modulated calorimetry. Results have proved that the A2⇋DO3 transition in Cu–Al–Be is first order with a latent heat of 1160 J/mol; the B2⇋L21 transition in Cu–Zn–Al is second order, and a peak in the specific-heat vs temperature curve has been observed. Secondly, the post-quench behaviour of these alloys, when subjected to some of the typical heat treatments used to stabilize the β phase, has also been studied by means of neutron diffraction, positron annihilation and highly sensitive calorimetry. A different post-quench time evolution of the martensitic transition temperatures has been found for the two alloys. For Cu–Al–Be, this evolution has been shown to be correlated with positron annihilation data, while, for Cu–Zn–Al, a correlation with neutron diffraction data has been established. These results show that the measured shifts in the transition temperatures induced by a quench are mostly due to an excess of vacancies in the case of Cu–Al–Be, and to an incomplete degree of L21 atomic order in Cu–Zn–Al.

Paramagnetic Spin Correlations in CaFe2As2 Single Crystals

Inelastic neutron scattering measurements of the magnetic excitations in CaFe2As2 indicate that the spin wave velocity in the Fe layers is exceptionally large and similar in magnitude to the cuprates. However, the spin wave velocity perpendicular to the layers is at least half as large that in the layer, so that the magnetism is more appropriately categorized as anisotropic three-dimensional, in contrast to the two-dimensional cuprates. Exchange constants derived from band structure calculations predict spin wave velocities that are consistent with the experimental data.

Spin-waves in Antiferromagnetic Single-crystal LiFePO4

Millimeter-sized single crystals of LaFeAsO, LaFeAsO1−xFx, and LaFe1−xCoxAsO were grown in NaAs flux at ambient pressure. The detailed growth procedure and crystal characterizations are reported. The as-grown crystals have typical dimensions of 3×4×0.05–0.3 mm3 with the crystallographic c-axis perpendicular to the plane of the platelike single crystals. Various characterizations confirmed the high quality of our LaFeAsO crystals. Co and F were introduced into the lattice leading to superconducting LaFe1−xCoxAsO and LaFeAsO1−xFx single crystals, respectively. This growth protocol is expected to be broadly applicable to grow other RMAsO (R=rare earth, M=transition metal) compounds.