Z. Schlesinger

Using polarized reflectivity measurements of single-domain crystals, we are able to distinguish chain and plane contributions to the infrared conductivity of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. A substantial chain contribution to \ensuremath{\sigma}(\ensuremath{\omega}) persisting to low frequency and temperature is observed. For the intrinsic conductivity of the ${\mathrm{CuO}}_{2}$ planes a superconducting energy gap of 500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (2\ensuremath{\Delta}/${\mathit{kT}}_{\mathit{c}}$\ensuremath{\simeq}8) is evident in the infrared data, while the normal-state conductivity drops much more slowly with \ensuremath{\omega} than the ordinary Drude form, and can be described in terms of a scattering rate \ensuremath{\Elzxh}/${\mathrm{\ensuremath{\tau}}}^{\mathrm{*}}$\ensuremath{\sim}kT+\ensuremath{\Elzxh}\ensuremath{\omega} at low frequency. The former result (2\ensuremath{\Delta}/${\mathit{kT}}_{\mathit{c}}$\ensuremath{\simeq}8) suggests suppression of ${\mathit{T}}_{\mathit{c}}$; the latter, that ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ is not an ordinary Fermi liquid.

Record high efficiencies for a polymer-based solar cell are reported here for an inorganic/organic composite photovoltaic device containing a single polymer film rather than a polymer/polymer or polymer/nanoparticle blend. The high efficiencies are reported to be due to device construction (see Figure) and the electronic properties of the phenylamino-p-phenylenevinylene-based polymer used.

We report the first infrared and optical measurements of the absolute conductivity of FeSi. Above 200 K the ac conductivity of FeSi resembles that of a dirty metal, while at low temperature most of the conductivity below 60 meV is depleted, consistent with an energy gap of that magnitude. Thus in contrast to what is expected from a picture based on simple thermal activation, the gap disappears at a temperature which is low relative to its size. Moreover, the spectral weight lost below the gap does not reappear just above the gap, but is instead distributed over an energy range of order eV.

Combined magnetotransport and cyclotron-resonance experiments in a two-dimensional hole system at a modulation-doped GaAs-(AlGa)As heterojunction show that the Kramers degeneracy of the lowest subband is lifted for finite $k$ giving rise to two cyclotron masses ${{m}_{1}}^{*}=0.38{m}_{0}$ and ${{m}_{2}}^{*}=0.60{m}_{0}$ at ${E}_{\mathrm{F}}=2.4$ meV. The observation of plateaus in ${\ensuremath{\rho}}_{xy}$ shows that the quantized Hall effect is independent of the details of the host band structure.