Lattice vibrations of alpha'-NaV_2O_5 in the low-temperature phase. Magnetic bound states?

We report high resolution polarized infrared studies of the quarter-filled spin ladder compound alpha'-NaV_2O_5 as a function of temperature (5K <= T <= 300K). Numerous new modes were detected below the temperature T_c=34K of the phase transition into a charge ordered nonmagnetic state accompanied by a lattice dimerization. We analyse the Brillouin zone (BZ) folding due to lattice dimerization at T_c and show that some peculiarities of the low-temperature vibrational spectrum come from quadruplets folded from the BZ point (1/2, 1/2, 1/4). We discuss an earlier interpretation of the 70, 107, and 133cm-1 modes as magnetic bound states and propose the alternative interpretation as folded phonon modes strongly interacting with charge and spin excitations.Comment: 15 pages, 13 Postscript figure

Energetic particle influence on the Earth's atmosphere

This manuscript gives an up-to-date and comprehensive overview of the effects of energetic particle precipitation (EPP) onto the whole atmosphere, from the lower thermosphere/mesosphere through the stratosphere and troposphere, to the surface. The paper summarizes the different sources and energies of particles, principally galactic cosmic rays (GCRs), solar energetic particles (SEPs) and energetic electron precipitation (EEP). All the proposed mechanisms by which EPP can affect the atmosphere are discussed, including chemical changes in the upper atmosphere and lower thermosphere, chemistry-dynamics feedbacks, the global electric circuit and cloud formation. The role of energetic particles in Earth’s atmosphere is a multi-disciplinary problem that requires expertise from a range of scientific backgrounds. To assist with this synergy, summary tables are provided, which are intended to evaluate the level of current knowledge of the effects of energetic particles on processes in the entire atmosphere

Structural phase transition in the two-dimensional triangular lattice antiferromagnet RbFe(MoO4)2

We report temperature-dependent x-ray powder diffraction patterns and single-crystal electron spin resonance (ESR) and Raman spectra of the two-dimensional triangular-lattice antiferromagnet RbFe(MoO4)2. A structural phase transition is found to occur in this compound at Tc≈190 K. By analyzing our data and the literature data on similar compounds, we conclude that the 190 K transition comprises rotations of MoO4 tetrahedra and is, most likely, a transformation from a highly symmetric P3¯m1 room-temperature structure into a very similar but less symmetric P3¯c1 low-temperature one. The shift of the ESR frequency and the ESR linewidth were found to be determined by the crystal field influence and are described well considering the single-ion anisotropy with the constant D/kB=0.25 K. The change of the crystal field at the iron site below the phase-transition point Tc is estimated.