RESUMO
SrRuO3 is a ferromagnetic metal with several unusual physical properties such as zero thermal expansion below Tc, so-called Invar behavior. Another anomalous feature is that the a-axis lattice constant is larger than the b-axis lattice constant, a clear deviation from the predictions of the Glazer structural description with rigid RuO6 octahedron motion. Using high resolution neutron diffraction techniques, we show how these two structural anomalies arise from the irregular in-plane deformation, i.e. plastic behavior of the RuO6 octahedron, a weak band Jahn-Teller distortion. We further demonstrate that the ferromagnetic instability of SrRuO3 is related to the temperature-induced localization of Ru 4d bands.
RESUMO
We report inelastic neutron scattering measurements and random phase approximation calculations of the dispersive crystal field excitations of UPd(3). The measured spectra at lower energies agree with those calculated using quadrupolar interaction parameters deduced from bulk and x-ray scattering measurements. The more intense excitations arising from the hexagonal sites were used to obtain exchange parameters which proved to be anisotropic.
RESUMO
We present the results of x-ray and ultraviolet photoelectron spectroscopy of NpPd(3) and PuPd(3). The spectra indicate that for both compounds, the 5f electrons are well localized on the actinide sites. Comparison with bulk measurements indicates that for NpPd(3) the electrons have a valence of Np(3+) and thus a ground state 5f(4) with a Hund's rules (5)I(4) configuration. Similarly for PuPd(3), we find a Pu(3+) valence, 5f(5) ground state and a Hund's rules (6)H(5/2) configuration.
RESUMO
By combining accurate heat capacity and x-ray resonant scattering results we have resolved the long standing question regarding the nature of the quadrupolar ordered phases in UPd(3). The order parameter of the highest temperature quadrupolar phase has been uniquely determined to be antiphase Q{zx} in contrast to the previous conjecture of Q{x{2}-y{2}}. The azimuthal dependence of the x-ray scattering intensity from the quadrupolar superlattice reflections indicates that the lower temperature phases are described by a superposition of order parameters. The heat capacity features associated with each of the phase transitions characterize their order, which imposes restrictions on the matrix elements of the quadrupolar operators.
RESUMO
Dimensionality is one of the most important parameters of physical phenomena. Only two things determine the universality class of a phase transition: the dimensionality of a given system and the symmetry of the order parameter. In most cases, the dimensionality of a substance is predetermined by its crystal structure. Examples in which the effective dimensionality is reduced are quite rare. Here we show that the three-dimensional cubic system of Tl(2)Ru(2)O(7) most probably evolves into a one-dimensional spin-one Haldane system with a spin gap below 120 K, accompanied by anomalies in the structure, resistivity and susceptibility. We argue that these anomalies are due to an orbital ordering of Ru 4d electrons, with a strong coupling among three degrees of freedom: orbital, spin and lattice. Our work provides a unique example of the spontaneous formation of Haldane system with an insight into the intriguing interplay of different degrees of freedom.
RESUMO
The magnetic structure of the localized-5f uranium intermetallic compound U3Pd20Si6 has been determined by means of a neutron diffraction experiment. Our data demonstrate that this compound has a collinear coupling of the sublattice ordering of the uranium spins on the 4a and 8c sites. We conclude that higher-order exchange and/or quadrupole interactions are necessary to stabilize this unique collinear structure. We discovered a new type of spin-flop transition against the uniaxial anisotropy induced by this collinear coupling.
RESUMO
Quadrupolar ordering in a 5f electron system has been observed directly for the first time, using x-ray scattering techniques. In UPd (3) at low temperatures satellite peaks appear at (1,0,l) (orthorhombic notation) with l odd and even. Both sets of peaks show a resonant enhancement of the scattering at the M(IV) edge of U. At resonance, the dominant scattering of the l odd peaks occurs in the unrotated polarization channel, whereas for l even a significant rotated component is found. These results are discussed in terms of possible structures of the antiferroquadrupolar phases.