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

Toward an internally consistent astronomical distance scale

Accurate astronomical distance determination is crucial for all fields in astrophysics, from Galactic to cosmological scales. Despite, or perhaps because of, significant efforts to determine accurate distances, using a wide range of methods, tracers, and techniques, an internally consistent astronomical distance framework has not yet been established. We review current efforts to homogenize the Local Group's distance framework, with particular emphasis on the potential of RR Lyrae stars as distance indicators, and attempt to extend this in an internally consistent manner to cosmological distances. Calibration based on Type Ia supernovae and distance determinations based on gravitational lensing represent particularly promising approaches. We provide a positive outlook to improvements to the status quo expected from future surveys, missions, and facilities. Astronomical distance determination has clearly reached maturity and near-consistency.Comment: Review article, 59 pages (4 figures); Space Science Reviews, in press (chapter 8 of a special collection resulting from the May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space Age

Sensitization of Er3+ emission at 1.5m (mu)m by Yb3+ in KYb(WO4)2 single crystals

We present our recent achievements in the growth and spectroscopic characterization of KYb(WO4)2 crystals doped with erbium ions ~hereafter KYbW:Er!. We grew single crystals of KYbW:Er at several erbium concentrations with optimal crystalline quality by the top-seeded-solution growth ~TSSG! slow-cooling method. We carried out spectroscopic measurements related to the polarized optical absorption and optical emission at room temperature ~RT! and low temperature ~6 K!. The splitting of the excited energy levels and the ground energy level of erbium in KYbW were determined, derived from the absorption and emission measurements at 6 K, respectively. We determined the near infrared, around 1.5 mm (6667 cm21), emission channels from the emission spectrum, and used the reciprocity method to calculate a maximum emission cross section of 2.7310220 cm2 for the polarization parallel to the Nm principal optical direction for the 1.534 mm (6519 cm21) infrared emission. We measured the lifetime of the 2F5/2→2F7/2 transition of ytterbium and the 4I13/2→4I15/2 transition of erbium at RT for several erbium concentrations. Finally, we present the Judd-Ofelt calculations for the KYbW:Er system

The three wave resonant interaction equations: spectral and numerical methods

The spectral theory of the integrable partial differential equations which model the resonant interaction of three waves is considered with the purpose of numerically solving the direct spectral problem for both vanishing and non vanishing boundary values. Methods of computing both the continuum spectrum data and the discrete spectrum eigenvalues are given together with examples of such computations. The explicit spectral representation of the Manley-Rowe invariants is also displayed