D. L. Huber

Order-disorder transformations in chemisorbed layers: Oxygen on W(110)

We have investigated the order-disorder transformation in oxygen adsorbed on W(110). An analysis of the ordering at T = 0 using the lattice gas formalism shows that there must be significant three-particle interactions to break the particle-hole symmetry. This is necessary since there is a p(2 × 2) phase at three-quarter coverage which is not present at one-quarter coverage. Monte Carlo techniques are used to obtain estimates of the strength of the two and three-particle interactions by matching calculated and measured LEED intensity curves. The qualitative characteristics of the phase diagram are discussed with emphasis on the multicritical points which must be present if the transition at half coverage is second order. Evidence in support of a second order transition is reviewed.

Analysis of a stochastic model for the optical and photon-echo decays of impurities in glasses

Abstract We analyze a stochastic model for the homogeneous optical linewidths and photon-echo decays of impurities in glasses. The dephasing arises from the modulation of the impurity with a random array of tunneling systems. Explicit results are obtained in the high-temperature limit for the optical dipole moment correlation function and the two-pulse photon echo decay with an interaction of the general form A / r n . Results are presented for the contribution from nearby systems, for which A / r n > R , and from distant systems, where A / r n R , R being the relaxation rate for the tunneling system. The effects of randomness in the spatial distribution of tunneling systems and the average over tunneling system parameters are discussed.

Equation of motion of a spin vortex in a two‐dimensional planar magnet

An equation of motion for a spin vortex in a two‐dimensional easy plane magnet is presented and discussed. The theory is based on earlier work on the dynamics of magnetic domains and includes the effects of vortex‐vortex interactions, (weak) applied fields, and dissipation. A possible approach to detecting vortices is also discussed.

Studies of the three-dimensional frustrated antiferromagnetic ZnCr2O4

Results of studies of the susceptibility, magnetic specific heat, and electron paramagnetic resonance spectrum of the geometrically frustrated antiferromagnetic ZnCr2O4 are presented. The temperature dependence of the susceptibility and the specific heat are in good agreement with the predictions of the quantum tetrahedral mean field model for exchange-coupled spin-3/2 ions on a pyrochlore lattice. The origin of the anomalous behavior of the resonance intensity below 90 K is discussed.

T-matrix analysis of donor fluorescence☆

Abstract The time dependence of fluorescence in the presence of a small concentration of traps is studied starting from a set of coupled rate equations for the donors. It is assumed that the traps are substitutional impurities distributed at random on the donor lattice. Transfer to the traps can only take place from nearest-neighbor sites. A general expression for the Laplace transfer of the normalized intensity is derived using results based on the average-t-matrix approximation. The time dependence of the decay of fluorescence, at long times, is obtained for simple cubic lattices as well as for lower dimensional systems. A brief comparison is made with results obtained by other techniques.

Electron-Phonon Coupling in Crystalline Pentacene Films

The electron-phonon (e-p) interaction in pentacene (Pn) films grown on Bi(001) was investigated using photoemission spectroscopy. The spectra reveal thermal broadening from which we determine an e-p mass enhancement factor of lambda=0.36+/-0.05 and an effective Einstein energy of omega{E}=11+/-4 meV. From omega{E} it is inferred that dominant contributions to the e-p effects observed in angle-resolved photoemission spectroscopy come from intermolecular vibrations. Based on the experimental data for lambda we extract an effective Peierls coupling value of g{eff}=0.55. The e-p coupling narrows the highest occupied molecular orbital bandwidth by 15+/-8% between 75 and 300 K.

Inhomogeneous broadening in excitonic systems

Abstract The effect of a random distribution of single-center transition frequencies on the spectrum of a system whose optically active states are Frenkel excitons is addressed from a theoretical perspective. In the weak disorder limit (distribution width/exciton bandwidth ⪡1), the lineshape is adequately described by a theory based on the coherent potential approximation (CPA) in all three dimensions. The theory is shown to account quantitatively for recent numerical results for the linewidth in one dimensional arrays reported by Kohler et al. In the strong disorder limit (distribution width/exciton bandwidth ⪢1), a theory is outlined in which the inter-center coupling is treated as a perturbation. The resulting lineshape has a Voigt profile. The need for a theory interpolating between the weak disorder and strong disorder limits is pointed out.

Raman contribution to the homogeneous optical linewidth of impurity ions in vitreous silica

Abstract We determine the temperature dependence of the Raman contribution to the homogeneous optical linewidth of impurity ions in vitreous silica. Using experimental results for the distribution of non-acoustic low frequency phonon modes obtained from inelastic neutron scattering data, we calculate the linewidth over the interval 10K⩽ T ⩽ 300K. We obtain approximate quadratic variation down to 40 K and near-quadratic behavior at lower temperatures. The persistence of the quadratic variation is a consequence of the presence of a low-lying (100 K) peak in the density of states. The implications of this work for the interpretation of impurity linewidths in other inorganic glasses are discussed.

Diffusion of optical excitation at finite temperatures

We study diffusion of optical excitation as a function of the ratio of the temperature to the inhomogeneous linewidth Δ. When T≪Δ the diffusion constant associated with one‐phonon‐assisted dipolar transfer over a Gaussian line is suppressed by the factor exp[−aΔ2/T2]. The effect is attributed to a decrease in the effective density of ions participating in the transfer.

HOMO band structure and anisotropic effective hole mass in thin crystalline pentacene films

The band dispersion of the two highest occupied molecular orbital (HOMO)-derived bands in thin crystalline pentacene films grown on Bi(001) was determined by photoemission spectroscopy. Compared to first-principles calculations, our data show a significantly smaller bandwidth and a much larger band separation, indicating that the molecular interactions are weaker than predicted by theory\char22{}a direct contradiction to previous reports by H. Kakuta et al. [Phys. Rev. Lett. 98, 247601 (2007)]. The effective hole mass