We have performed high-resolution neutron diffraction and inelastic neutron scattering experiments in the frustrated multiferroic hexagonal compounds RMnO_{3} (R = Ho,Yb,Sc,Y), which provide evidence of a strong magnetoelastic coupling in the whole family. We can correlate the atomic positions, the type of magnetic structure, and the nature of the spin waves whatever the R ion and temperature. The key parameter is the position of the Mn ions in the unit cell with respect to a critical threshold of 1/3, which determines the sign of the coupling between Mn triangular planes.
We have refined the crystal structures of a Pr(0.60)Ca(0.40)MnO(3) single crystal from neutron diffraction data. The result at low temperature gives a superstructure that cannot be interpreted as Mn(3+)/Mn(4+) charge ordering. The pattern of atom displacements suggests the trapping of electrons within pairs of Mn sites, involving both a local double exchange and a polaronic-like distortion. The two mechanisms act together to form vibronic localized electronic states: Zener polarons. We have confirmed this picture by showing how it elucidates the unconventional paramagnetic behavior of half-doped manganites.
The origin of the well-defined collective excitations found in liquid para-H 2 by recent experiments is investigated. The persistence of their relatively long lifetimes down to microscopic scales is well accounted for by calculations carried out by means of path-integral-centroid molecular dynamics. In contrast only overdamped excitations are found in calculations carried within the classical limit. The results provide fully quantitative evidence of quantum effects on the dynamics of a simple liquid. PACS numbers: 67.20. + k, 62.10. + s, 62.60. + v In spite of being composed of the simplest stable molecules, the transport and kinetic properties of liquid hydrogen still resist a quantitative understanding [1]. Such peculiar behaviors are usually thought to arise because of the light masses of the particles forming the liquid (M ഠ 2 amu) and the relatively low temperatures where it exists under its saturated vapor pressure. Quantum effects are thus expected to be noticeable although their relevance for understanding quantitatively most of the transport properties awaits to be established in full. These effects are first manifested by the appearance of a discrete spectrum of transitions between molecular rotational levels. The quantum nature of such motions imposes some symmetry constraints to the total molecular wave function. In particular, the rotational and nuclear spin states of the two protons forming the H 2 molecule are coupled, leading to two distinguishable species, para͑p͒-H 2 and ortho͑o͒-H 2 corresponding to molecules having antiparallel (I 0), and parallel (I 1) spin states, respectively. This has an immediate consequence on the symmetry of the interaction potential between H 2 molecules which is isotropic for p-H 2 molecules but angular dependent for o-H 2 . It is so because the orientational distribution of the internuclear bond in the laboratory frame is spherically symmetric (s-like) for the former and axially symmetric (p-like) for the latter. A noticeable manifestation of quantum behavior is expected concerning the spatial smearing of the molecular wave function. This comes because of the low molecular mass and the low temperatures in question (about 10-20 K) which makes the thermal (de Broglie ) wavelength l ͑2ph 2 ͞Mk B T ͒ to reach values larger than the molecular dimensions. In fact, close to triple point, l ഠ 3.3 Å which becomes comparable to the equilibrium separation between two p-H 2 molecules, r 0 ഠ 3.5 Å. In addition to the spatial spread of the wave function a delocalization time t d can be defined in terms of the fluid number density r and the particle mass through [2] t d M͞2ph͑r͞2.612͒ 2͞3 , which for the triple point density yields an estimate for t d ഠ 1.2 ps. Finally, exchange effects arising because of the indistinguishability of the molecules are deemed to play a very minor role since the temperatures in question are well above the quantum degeneracy temperature T 0 h͞k B t d 6.5 K and therefore most particles will be accommodated in excited levels [3].Two sets of recent in...
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