B. C. Sales

Structural disorder and thermal conductivity of the semiconducting clathrate Sr8Ga16Ge30

Abstract The temperature dependence of the atomic displacement parameters for Sr8Ga16Ge30 determined from refinements of neutron powder and single-crystal diffraction data shows that the anomalously large values for one of the two unique Sr atoms persist from 295 to 11 K. Its position is better described by a fractionally occupied four-fold split site, but the rms displacement remains the largest of all of the atoms in the structure. Difference Fourier maps of this Sr site show a residual nuclear density with lobes in the directions of the split-atom positions. The Ga and Ge atoms appear to be fully disordered on the three distinct framework sites. The measured atomic displacement parameters are used to derive estimates of the following thermodynamic related quantities: Debye temperature, 271 K; mean velocity of sound, 2600 m/s; temperature of the Einstein “rattler”, 85 K; mean free path of heat-carrying phonons, 5.36 A; and lattice thermal conductivity, 0.008 W/cm-K.

Two-dimensional resonant magnetic excitation in BaFe1.84Co0.16As2.

Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wave vectors (1/2 1/2 L) in tetragonal notation. On cooling below T_{C}, a clear resonance peak is observed at this wave vector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) k_{B}T_{C}. This is in good agreement with the canonical value of 5 k_{B}T_{C} observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c axis, indicating that the magnetic fluctuations are two dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.

Contrasting spin dynamics between underdoped and overdoped Ba(Fe1-xCox)2As2.

We report the first NMR investigation of spin dynamics in the overdoped nonsuperconducting regime of Ba(Fe1-xCox)2As2 up to x=0.26. We demonstrate that the absence of interband transitions with large momentum transfer Q{AF} approximately (pi/a,0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x greater than or approximately 0.15. Our experimental results provide direct evidence for a correlation between T{c} and the strength of Q{AF} antiferromagnetic spin fluctuations.

MAGNETIC EXCITATIONS IN THE S = 1/2 ALTERNATING CHAIN COMPOUND (VO)2P2O7

Magnetic excitations in an array of (VO)_2P_2O_7 single crystals have been measured using inelastic neutron scattering. Until now, (VO)_2P_2O_7 has been thought of as a two-leg antiferromagnetic Heisenberg spin ladder with chains running in the a-direction. The present results show unequivocally that (VO)_2P_2O_7 is best described as an alternating spin-chain directed along the crystallographic b-direction. In addition to the expected magnon with magnetic zone-center energy gap \Delta = 3.1 meV, a second excitation is observed at an energy just below 2\Delta. The higher mode may be a triplet two-magnon bound state.

Structural disorder and magnetism of the semiconducting clathrate Eu8Ga16Ge30

Abstract The temperature dependence of the atomic displacement parameters for an isotopically enriched Eu8Ga16Ge30 crystal determined from refinements of neutron diffraction data show the anomalously large values for the Eu atom in the large cage persist from 295 to 12 K. The Eu site in the large cage is modeled by splitting it into four fractionally occupied positions, displaced 0.4 A from the cage center. Fourier maps of this Eu site support this view in showing residual nuclear density with distinct peaks in the directions of the split-atom positions, and lesser density at the cage center position. The Ga and Ge atoms appear to be fully disordered on the three distinct framework sites. In comparing the isostructural X8Ga16Ge30 (X=Eu, Sr, Ba) compounds, the positional disorder around the large cage site increases progressively with decreasing size of the caged atom. Below 35 K, Eu8Ga16Ge30 orders ferromagnetically, with the preferred direction of the Eu moments along (100). The projected saturation value of the Eu moment, 7 μB, equals the free ion value.

Three-dimensional magnetic correlations in multiferroic LuFe2O4.

We present single crystal neutron diffraction measurements on multiferroic LuFe(2)O(4). Magnetic reflections are observed below transitions at 240 and 175 K indicating that the magnetic interactions in LuFe(2)O(4) are three-dimensional in character. The magnetic structure is refined as a ferrimagnetic spin configuration below the 240 K transition. Below 175 K a significant broadening of the magnetic peaks is observed along with the buildup of a diffuse component to the magnetic scattering.

Magnetic Excitations in the S=1/2 Alternating Chain Compound (VO) {sub 2}P{sub 2}O{sub 7}

Magnetic excitations in an array of (VO)_2P_2O_7 single crystals have been measured using inelastic neutron scattering. Until now, (VO)_2P_2O_7 has been thought of as a two-leg antiferromagnetic Heisenberg spin ladder with chains running in the a-direction. The present results show unequivocally that (VO)_2P_2O_7 is best described as an alternating spin-chain directed along the crystallographic b-direction. In addition to the expected magnon with magnetic zone-center energy gap \Delta = 3.1 meV, a second excitation is observed at an energy just below 2\Delta. The higher mode may be a triplet two-magnon bound state.

Static and dynamic magnetism in underdoped superconductor BaFe1.92Co0.08As2.

We report neutron scattering measurements on single crystals of BaFe1.92Co0.08As2. The magnetic Bragg peak intensity is reduced by 6% upon cooling through TC. The spin dynamics exhibit a gap of 8 meV with anisotropic three-dimensional interactions. Below TC additional intensity appears at an energy of approximately 4.5(0.5) meV, similar to previous observations of a spin resonance in other Fe-based superconductors. No further gapping of the spin excitations is observed below TC for energies down to 2 meV. These observations suggest the redistribution of spectral weight from the magnetic Bragg position to a spin resonance, demonstrating the direct competition between static magnetic order and superconductivity.

Superconducting magnesium diboride films with Tc≈24 K grown by pulsed laser deposition with in situ anneal

Abstract Thin superconducting films of magnesium diboride (MgB 2 ) with T c ≈24 K were prepared on various oxide substrates by pulsed laser deposition followed by an in situ anneal. A systematic study of the influence of various in situ annealing parameters shows an optimum temperature of about 600°C in a background of 0.7 atm of Ar/4%H 2 for layers consisting of a mixture of magnesium and boron. Contrary to ex situ approaches (e.g. reacting boron films with magnesium vapor at ≈900°C), these films are processed at a temperature at which MgB 2 does not decompose rapidly even in vacuum. This may prove enabling in the formation of multilayers, junctions, and epitaxial films in future work. Issues related to the improvement of these films and to the possible in situ growth of MgB 2 at elevated temperature are discussed.

Evidence for strong itinerant spin fluctuations in the normal state of CeFeAsO0.89F0.11 iron-oxypnictide superconductors.

The electronic structure in the normal state of CeFeAsO0.89F0.11 oxypnictide superconductors has been investigated with x-ray absorption and photoemission spectroscopy. All of the data exhibit signatures of Fe d-electron itinerancy. Exchange multiplets appearing in the Fe 3s core level indicate the presence of itinerant spin fluctuations. These findings suggest that the underlying physics and the origin of superconductivity in these materials are likely to be quite different from those of the cuprate high-temperature superconductors. These materials provide opportunities for elucidating the role of magnetic fluctuations in high-temperature superconductivity.