Far-infrared absorption of PbSe nanorods.

Measurements of the far-infrared absorption spectra of PbSe nanocrystals and nanorods are presented. As the aspect ratio of the nanorods increases, the Fröhlich sphere resonance splits into two peaks. We analyze this splitting with a classical electrostatic model, which is based on the dielectric function of bulk PbSe but without any free-carrier contribution. Good agreement between the measured and calculated spectra indicates that resonances in the local field factors underlie the measured spectra.

Symmetry-breaking dynamical pattern and localization observed in the equilibrium vibrational spectrum of NaI.

Intrinsic localized modes (ILMs) - also known as discrete breathers - are localized excitations that form without structural defects in discrete nonlinear lattices. For crystals in thermal equilibrium ILMs were proposed to form randomly, an idea used to interpret temperature activated signatures of ILMs in α-U and NaI. Here, however, we report neutron scattering measurements of lattice vibrations in NaI that provide evidence of an underlying organization: (i) with small temperature changes ILMs move as a unit back-and-forth between [111] and [011] orientations, and (ii) when [011] ILMs lock in at 636 K the transverse optic (TO) mode splits into three modes with symmetry-breaking dynamical structure resembling that of a superlattice, but there are no superlattice Bragg reflections and the pattern itself has crystal momentum. We conclude that this dynamical pattern is not derived from the rearrangement of atoms but from a coherent arrangement of ILMs decorating the crystal lattice in equilibrium.

Holographic Fourier transform spectrometer for terahertz region.

Because of the development of detector arrays stationary interferometric spectrometers now have many applications in the visible and IR; however, these same array sizes make it impractical to design a single Fourier optics system with the necessary large field angles required for the terahertz region. By dividing the Fourier optics into independent components in each arm of an interferometer we show that the aberrations are dramatically reduced, while the same theoretical throughput as in the scanning Michelson interferometer is maintained.

Glasslike two-level systems in minimally disordered mixed crystals.

THz spectroscopy is used to identify a broad distribution of two-level systems, characteristic of glasses, in the substitutional monatomic mixed crystal systems, Ba(1-x)Ca(x)F(2) and Pb(1-x)Ca(x)F(2). In these minimally disordered systems, two-level behavior, which was not previously known to occur, begins at a specific CaF(2) concentration. The concentration dependence, successfully modeled using the statistics of the impurity distribution in the lattice, points to a collective dopant tunneling mechanism.