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Kreyssig, A

Publications and source records attributed to Kreyssig, A.

13 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Crossover from spin waves to diffusive spin excitations in underdoped Ba(Fe1-xCox)2 As2

Using inelastic neutron scattering, we show that the onset of superconductivity in underdoped Ba(Fe1-xCox)2As2 coincides with a crossover from well-defined spin waves to overdamped and diffusive spin excitations. This crossover occurs despite the presence of long-range stripe antiferromagnetic order for samples in a compositional range from x=0.04 to 0.055, and is a consequence of the shrinking spin-density wave gap and a corresponding increase in the particle-hole (Landau) damping. The latter effect is captured by a simple itinerant model relating Co doping to changes in the hot spots of the Fermi surface. We argue that the overdamped spin fluctuations provide a pairing mechanism for superconductivity in these materials.

36 MATERIALS SCIENCE↗
DOE OSTI2012

High-energy X-ray diffraction studies of i-Sc[subscript 12]Zn[subscript 88]

Although quasicrystals form in a wide variety of ternary and quaternary metallic alloys, examples of stable binary icosahedral quasicrystals are quite rare. Indeed, it has been a decade since the discovery of icosahedral phases in Yb-Cd and Ca-Cd. We have discovered millimeter-sized facetted grains of i-Sc{sub 12}Zn{sub 88} with icosahedral (pentagonal dodecahedral and rhombic triacontahedral) morphologies in solution-grown samples. Structural characterization of the bulk icosahedral phase was accomplished through single-grain high-energy X-ray diffraction. For both growth morphologies, all diffraction peaks could be indexed by a primitive (P-type) icosahedral phase. The two types of morphology do, however, present interesting differences in their respective degrees of quasicrystalline order.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Effect of tensile stress on the in-plane resistivity anisotropy in BaFe[subscript 2]As[subscript 2]

The effect of uniaxial tensile stress and the resultant strain on the structural/magnetic transition in the parent compound of the iron arsenide superconductor BaFe{sub 2}As{sub 2} is characterized by temperature-dependent electrical resistivity, x-ray diffraction, and quantitative polarized light imaging. We show that strain induces a measurable uniaxial structural distortion above the first-order magnetic transition and significantly smears the structural transition. This response is different from that found in another parent compound, SrFe{sub 2}As{sub 2}, where the coupled structural and magnetic transitions are strongly first order. This difference in the structural responses explains the in-plane resistivity anisotropy above the transition in BaFe{sub 2}As{sub 2}. This conclusion is supported by the Ginzburg-Landau-type phenomenological model for the effect of the uniaxial strain on the resistivity anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

In-plane anisotropy of electrical resistivity in strain-detwinned SrFe[subscript 2]As[subscript 2]

Intrinsic, in-plane anisotropy of electrical resistivity was studied on mechanically detwinned single crystals of SrFe{sub 2}As{sub 2} above and below the temperature of the coupled structural/magnetic transition, T{sub TO}. Resistivity is smaller for electrical current flow along the orthorhombic a{sub o} direction (direction of antiferromagnetically alternating magnetic moments) and is larger for transport along the b{sub o} direction (direction of ferromagnetic chains), which is similar to CaFe{sub 2}As{sub 2} and BaFe{sub 2}As{sub 2} compounds. A strongly first-order structural transition in SrFe{sub 2}As{sub 2} was confirmed by high-energy x-ray measurements, with the transition temperature and character unaffected by moderate strain. For small strain levels, which are just sufficient to detwin the sample, we find a negligible effect on the resistivity above T{sub TO}. With the increase of strain, the resistivity anisotropy starts to develop above T{sub TO}, clearly showing the relation of anisotropy to an anomalously strong response to strain. Our study suggests that electronic nematicity cannot be observed in the FeAs-based compounds in which the structural transition is strongly first order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Short- and medium-range order in Zr[subscript 80]Pt[subscript 20] liquids

The atomic structures in equilibrium and supercooled liquids of Zr{sub 80}Pt{sub 20} were determined as a function of temperature by in situ high-energy synchrotron diffraction studies of the levitated liquids (containerless processing) using the beamline electrostatic levitation (BESL) technique. The presence of a pronounced pre-peak at q - 1.7 {angstrom}{sup -1} in the static structure factor indicates medium-range order (MRO) in the liquid. The position and intensity of the pre-peak remain constant with cooling, indicating that the MRO is already present in the liquid above its melting temperature. An analysis of the liquid atomic structures obtained using the Reverse Monte Carlo method utilizing both the structure factor S(q) from x-ray diffraction experiments and the partial pair-correlation functions from ab initio molecular dynamics simulations show that the pre-peak arises from a Pt-Pt correlation that can be identified with icosahedral short-range order around the Pt atoms. The local atomic ordering is dominated by icosahedral-like structures, raising the nucleation barrier between the liquid and these phases, thus assisting glass formation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Uniaxial-strain mechanical detwinning of CaFe[subscript 2]As[subscript 2] and BaFe[subscript 2]As[subscript 2] crystals: Optical and transport study

The parent compounds of iron-arsenide superconductors, AFe{sub 2}As{sub 2} (A=Ca, Sr, Ba), undergo a tetragonal to orthorhombic structural transition at a temperature T{sub TO} in the range 135-205 K depending on the alkaline-earth element. Below T{sub TO} the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning CaFe{sub 2}As{sub 2} and BaFe{sub 2}As{sub 2}. The detwinning is nearly complete, as demonstrated by polarized light imaging and synchrotron x-ray measurements, and reversible, with twin pattern restored after strain release. Electrical resistivity measurements in the twinned and detwinned states show that resistivity, {rho}, decreases along the orthorhombic a{sub o} axis but increases along the orthorhombic b{sub o} axis in both compounds. Immediately below T{sub TO} the ratio {rho}{sub bo}/{rho}{sub ao} = 1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to CaFe{sub 2}As{sub 2}, BaFe{sub 2}As{sub 2} reveals an anisotropy in the nominally tetragonal phase, suggesting that either fluctuations play a larger role above T{sub TO} in BaFe{sub 2}As{sub 2} than in CaFe{sub 2}As{sub 2} or that there is a higher temperature crossover or phase transition.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

High-energy X-ray diffraction studies of i-Sc12Zn88

Although quasicrystals form in a wide variety of ternary and quaternary metallic alloys, examples of stable binary icosahedral quasicrystals are quite rare. Indeed, it has been a decade since the discovery of icosahedral phases in Yb-Cd and Ca-Cd. We have discovered millimeter-sized facetted grains of i-Sc12Zn88 with icosahedral (pentagonal dodecahedral and rhombic triacontahedral) morphologies in solution-grown samples. Structural characterization of the bulk icosahedral phase was accomplished through single-grain high-energy X-ray diffraction. For both growth morphologies, all diffraction peaks could be indexed by a primitive (P-type) icosahedral phase. The two types of morphology do, however, present interesting differences in their respective degrees of quasicrystalline order.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2010

Solution growth of a binary icosahedral quasicrystal of Sc[subscript 12]Zn[subscript 88]

We report the discovery of a binary icosahedral phase in a Sc-Zn alloy obtained through solution-growth, producing millimeter-sized, facetted, single grain quasicrystals that exhibit different growth morphologies, pentagonal dodecahedra, and rhombic triacontahedra, under only marginally different growth conditions. These two morphologies manifest different degrees of quasicrystalline order. The discovery of i-Sc{sub 12}Zn{sub 88} suggests that a re-examination of binary phase diagrams at compositions close to crystalline approximant structures may reveal other binary quasicrystalline phases.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Magnetic structure of Gd[subscript 5]Ge[subscript 4]

Gd{sub 5}Ge{sub 4} crystallizes in the orthorhombic space group Pnma, and orders antiferromagnetically below the Neel temperature T{sub N} {approx} 127 K. We have employed x-ray resonant magnetic scattering to elucidate the details of the magnetic structure. The magnetic unit cell is the same as the chemical unit cell. From azimuth scans and the Q dependence of the magnetic scattering, all three Gd sites in the structure were determined to be in the same magnetic space group Pnma. The magnetic moments are primarily aligned along the c axis and the c components of the magnetic moments at the three different sites are equal. The ferromagnetic Gd-rich slabs are stacked antiferromagnetically along the b direction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗