Role of vibrational entropy in the stabilization of the high-temperature phases of iron

The phonon dispersions of the bcc and fcc phases of pure iron ({\alpha}-Fe, {\gamma}-Fe and {\delta}-Fe) at ambient pressure were investigated close to the respective phase transition temperatures. In the open bcc structure the transverse phonons along T1 [{\xi}{\xi}0] and T1 [{\xi}{\xi}2{\xi}] are of particularly low energy. The eigenvectors of these phonons correspond to displacements needed for the transformation to the fcc {\gamma}-phase. Especially these phonons, but also all other phonons soften considerably with increasing temperature. Comparing thermodynamic properties of the fcc and the two bcc phases it is shown that the high temperature bcc phase is stabilized predominantly by vibrational entropy, whereas for the stabilization of the fcc phase electronic entropy provides an equal contribution.Comment: to appear in Physical Review

Magnetic Order in YBa₂Cu₃O₆₊ₓ Superconductors

Polarized and unpolarized neutron diffraction has been used to search for magnetic order in YBa2Cu3O6+x superconductors. Most of the measurements were made on a high quality crystal of YBa2CU3O6.6. It is shown that this crystal has highly ordered ortho-II chain order, and a sharp superconducting transition. Inelastic scattering measurements display a very clean spin-gap and pseudogap with any intensity at 10 meV being 50 times smaller than the resonance intensity. The crystal shows a complicated magnetic order that appears to have three components. A magnetic phase is found at high temperatures that seems to stem from an impurity with a moment that is in the a-b plane, but disordered on the crystal lattice. A second ordering occurs near the pseudogap temperature that has a shorter correlation length than the high temperature phase and a moment direction that is at least partly along the c axis of the crystal. Its moment direction, temperature dependence, and Bragg intensities suggest that it may stem from orbital ordering of the d-density wave type. An additional intensity increase occurs below the superconducting transition. The magnetic intensity in these phases does not change noticeably in a 7 T magnetic field aligned approximately along the c axis. Searches for magnetic order in YBa2Cu3O7 show no signal while a small magnetic intensity is found in YBa2Cu3O6.45 that is consistent with the c axis directed magnetic order. The results are contrasted with other recent neutron measurements

Symbiosis of Magnetism and Superconductivity Studied by Neutron Three-Axis Spectroscopy.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe

Evidence for Three Fluctuation Channels in the Spin Resonance of the Unconventional Superconductor CeCoIn5

Polarized inelastic neutron scattering under a magnetic field is used to get a microscopic insight into the spin resonance of the heavy fermion superconductor CeCoIn5. The resonance line shape is found to depend on the neutron polarization: Some of the spectral weight is common to the two polarization channels while the remaining part is distributed equally between them. This is evidence for the spin resonance being a degenerate mode with three fluctuation channels: A Zeeman split contribution and an additional longitudinal mode

Magnetisation Dynamics in the Normal and Condensate Phases of UPd2Al3. II: Inferences.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe

Magnetic Response Function in URhAl.

URhAl is a ferromagnet with a moment of 1 mB per mole directed parallel to the unique hexagonal axis of the structure. The material is known to exhibit an enormous anisotropy, which is thought, in part, to arise from the hybridization of the U 5f with the 4d electrons. We have performed measurements on a large polycrystalline sample at the HET spectrometer at the ISIS spallation source (UK) to characterize the magnetic response function as a function of energy. Evidence for the interaction with the conduction electrons and for the spin gap is presented.JRC.E-Institute for Transuranium Elements (Karlsruhe

Neutron Diffraction Study of the Magnetic Structure of U0.1Np0.9Ru2Si2.

The magnetic structure of the U0.1Np0.9Ru2Si2 solution has been studied using neutron diffraction techniques on a single crystal. At Tn=25K this compound orders magnetically into an incommensurate longitudinally modulated structure which is very similar to the one reported for pure NpRu2Si2. At Tcom = 13 K, however, an incommensurate transition takes place and the low temperature magnetic structure ism the same as the one observed in URu2Si2. The calculated value of the magnetic moment is consisted with the published neutron diffraction and Mossbauer results.JRC.E-Institute for Transuranium Elements (Karlsruhe

Antiferromagnetic Fluctuations Observed in the Itinerant Ferromagnetic Ce Fe2

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe