Dedicated to Professor Dr. K. W. BOER on the occasion of his 70th birthday Thermal infrared sensors with maximal sensitivity and minimal response time require radiation absorbing layers with small heat capacity and high absorbance. In the near infrared these conditions are well met by black gold layers. In order to examine the question whether black gold layers are suitable absorbers as well in the far infrared, their reflectance is measured in a wave number range from k = 14 to 650 cm-', corresponding to a range in wavelength 1 = l / k between 15 and 710 pm.The samples are prepared at various residual gas pressures. The influence of tempering o m the absorbance and the spectral absorption behavior at low temperatures is discussed. The experimental results may be accounted for by a dielectric function according to the Bergman formalism. The spectral dependence of the absorption coefficient and the real and imaginary parts of the refractive index following from it are presented.

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The mid-infrared spin-wave spectrum of antiferromagnetic YBa 2 Cu 3 O 6.0 was determined by infrared transmission and reflection measurements (k c) at T = 10 K. Excitation of single magnons of the optical branch was observed at E op = 178.0 meV. Two further peaks at 346 meV (≈ 1.94 E op ) and 470 meV (≈ 2.6 E op ) both belong to the two-magnon spectrum. Linear spin wave theory is in good agreement with the measured two-magnon spectrum, and allows to determine the exchange constant J to be about 120 meV, whereas the intrabilayer coupling J 12 is approximately 0.55 J. 74.25.Gz, 74.25.Ha, 75.30.Ds High temperature superconductors are basically layered copper-oxide materials. It is widely accepted that the relevant electronic degrees of freedom are confined to copper-oxide planes. The number of CuO 2 planes per unit cell varies: e.g., La 2−x Sr x CuO 4 exists in a single plane form with a large spacing between planes of ≈ 13.2Å, and YBa 2 Cu 3 O 6+x has a double layer structure with intra-and interbilayer spacings of ≈ 3.3Å and 8.5Å, respectively. Electronic correlations, and hence spin dynamics [1], may depend on the type of stacking of the planes. More specifically, a sizable coupling J 12 between spins on adjacent planes of a bilayer will influence the spin excitation spectrum as well as the nature of the ground state. This may have been seen already in doped compounds: the normal state spin susceptibility of La 2−x Sr x CuO 4 extrapolates to a finite value at zero temperature, whereas it extrapolates to zero for YBa 2 Cu 3 O 6.6 [2]. This may be interpreted as a signature for the opening of a spin excitation gap in YBa 2 Cu 3 O 6.6 at low temperatures [3]-a behavior certainly not encountered in Fermi liquids. Further, a spin density wave ordering for La 2−x Sr x CuO 4 has been proposed, but for YBa 2 Cu 3 O 6.6 a singlet pairing of spins in adjacent CuO 2 planes with strong antiferromagnetic fluctuations within a plane [4,2,5]. Such a scenario seems to require an unrealistically large J 12 2.5J [6], where J is the in-plane exchange coupling of the Heisenberg Hamiltonian supposed to describe the low energy spin dynamics of a single bilayer for zero doping (x = 0). However it was argued that, for finite doping, the itinerant carriers destroy the antiferromagnetism of the insulating phase and, therefore, much smaller values of J 12 will produce a singlet interplane pairing in the conducting phase of YBa 2 Cu 3 O 6.6 . 74Up to now, no experimental evidence has been given of a sizable bilayer coupling (J 12 ∼ J). In neutron-scattering experiments on YBa 2 Cu 3 O 6+x , the in-plane coupling was determined from the dispersion of acoustic spin-waves and was found to be extremely large (J = 120 ± 20 meV [7], J = 150 meV [8], both for x = 0.15). Yet, no optical modes have been found for energies up to 60 meV [7,9], suggesting a bilayer coupling of J 12 8 meV. In Raman-1

Far-infrared reflection measurements on Si:P both on the metallic and insulating side of the metal-insulator transition (MIT) were performed at low temperatures. In the metallic regime freecarrier absorption and, additionally, absorption peaks due to interband transitions from the impurity band to the conduction band and to transitions between the broadened valley-orbit split Is states are observed. This gives clear evidence that the impurity band is formed by the overlap of the ls(Ai) ground states and is well separated from the conduction band when the MIT occurs. Only far beyond the metallic limit the impurity band merges completely with the conduction band.PACS numbers: 71.30.+h, 71.55.Cn, 78.50.GeThe metal-insulator transition (MIT) in doped semiconductors continues to be a major field in solid state physics. The nature of the electronic states close to the critical doping concentration N c is not completely understood. In particular, the question whether the MIT occurs in an impurity band well separated from the conduction band or inside the conduction band itself is a problem still under discussion [1]. Another open question is the influence of the valley-orbit splitting in manyvalley semiconductors like Si and Ge on to the electronic density of states in the metallic regime. It has been suggested that the many-valley nature of tetrahedral semiconductors leads to the absence of a Mott-Hubbard gap in the single-particle excitation spectrum [2].First evidence that the MIT in doped silicon and germanium occurs in an impurity band which merges with the conduction band only when the doping concentration N exceeds approximately SN C came from Knight shift measurements [3,4]. Later, this result was supported by calculations of the Knight shift and the specific heat in Si:P on the basis of a tight-binding model for the impurity band, which led to good agreement with experimental data [5]. More recently Rosenbaum et al.[6] observed a sharp feature in the low-temperature magnetoconductivity in Ge:Sb for a doping concentration above the MIT at a field corresponding to the central-cell splitting. This suggests that central-cell effects in the metallic phase persist and that the electronic states responsible for transport are derived from an impurity band.Optical spectroscopy is a well-established method to probe the nature of these electronic states. The high energy resolution of absorption and reflection measurements provides information on the band structure over a wide energy range. The conductivity function cr{fiw) reflects the joint density of states. Thus, from farinfrared spectroscopy on uncompensated Si:P with the well-known conduction band, detailed information of impurity states and the impurity band should be obtained. In fact, infrared absorption measurements on the single-valley system GaAs:Si near a magnetic-field induced MIT have shown evidence for an impurity band [7,8]. However, the many-valley nature of Si:P might strongly affect the electronic band structure near the MIT as mentioned above.Several i...

We have determined the longitudinal optical conductivity of (TaSe4),I in the energy range from 50 meV to 2 eV at different temperatures between 15 K and 420 K. We find a clear evidence that the free carriers are condensed into a charge density wave ground state not only below the transition temperature of 263 K but also at higher temperature up to the limits of the chemical stability of this compound.