We consider strongly correlated electrons coupled to low-energy acoustic phonons in one dimension. Using a Luttinger liquid description we calculate the exponents of various response functions and discuss their remarkable sensitivity to the Wentzel-Bardeen singularity induced by the presence of phonons. For the Hubbard model plus phonons the equivalent of a phase diagram is established. By increasing the filling factor towards half filling the Wentzel-Bardeen singularity is approached. This in turn triggers a simple and efficient mechanism to suppress antiferromagnetic fluctuations and to drive the system via a normal metallic state towards a superconducting phase.

The visible emission and excitation spectra of AgBr microcrystals containing single and parallel double twin planes have been obtained. A band at 462.28 nm (2.6813 eV) is observed in the excitation spectrum. This feature is about 2.5 meV below the indirect exciton band edge. Coincident emission features are observed. The band in the excitation spectrum of microcrystals with two parallel twin planes is often split into a doublet with splittings between 1 and 2 meV. The new band is assigned to an exciton created at a stacking fault and the splittings are thought to be due to tunneling of the exciton between proximate twin planes. The small binding energy of the exciton to the twin plane is in accord with the very small strain field about a twin plane.

We study Auger recombination (AR) in the presence of alloy scattering. It is shown that alloy-assisted AR is an important process, which gives a considerable contribution to AR in ternary III-V compounds.

The optical properties of InAs monolayer and submonolayer coverage formed in a GaAs matrix by molecular-beam epitaxy have been studied. Prolonged growth interruptions were introduced after the InAs layer was deposited. In samples with intentionally deposited submonolayer InAs coverage, the luminescence efficiency was maintained and no emission line broadening was observed up to very long interruption times, suggesting this arrangement is stable. On the other hand, one InAs ML was found to be unstable with a tendency towards transformation into two surface arrangements: InAs anisotropic multilayer islands and uniform submonolayer distribution of InAs molecules over the surface. For InAs coverage larger than 1 ML, it was found that long growth-interruption times favor the formation of thicker three-dimensional InAs islands and may result in a local strain relief. It was found that in the case of monolayer coverage, the luminescence line shape broadens and the intensity decreases as the growth interruption time increases. Our results explain how the marked improvement of the optical quality of structures grown by submonolayer epitaxy may arise.

The binding and transition energies of ${\mathit{D}}^0$ and ${\mathit{D}}^{\mathrm{-}}$ off center in GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As quantum wells (QW’s) in a magnetic field are reported. It is found that a magnetic field can make the binding energy of ${\mathit{D}}^{\mathrm{-}}$ off center in QW’s not only increase but also decrease and a very strong magnetic field can dissociate a ${\mathit{D}}^{\mathrm{-}}$ into an electron and a ${\mathit{D}}^0$. On the basis of analysis of the variation of ${\mathit{D}}^0$ and ${\mathit{D}}^{\mathrm{-}}$ transition energies with magnetic field, a method is proposed for experimentalists to confirm the ${\mathit{D}}^{\mathrm{-}}$ states in QW’s.

Photocurrent-spectroscopy studies were performed in the 10<T<300 K temperature range on p-i-n heterostructures grown by molecular-beam epitaxy and incorporating ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Cd}}_{\mathit{x}}$Se/ZnSe multiple quantum wells (with x=0.10 and 0.25) in the undoped region. The extremely well-resolved excitonic features and corresponding continuum edges allowed us to obtain directly the exciton binding energies and compare the spectra with excitonic transition energies calculated taking into account the effect of strain.

Results are presented for the atomic structure and energetics of possible intersections of (necessarily) orthogonal (a/2)〈110〉 edge dislocations in Ge/Si bicrystals, as predicted by classical simulations. The most compact structure is found to be preferred energetically.

We have studied the aperiodic resistance fluctuations at millikelvin temperatures in 90-nm-wide silicon metal-oxide-semiconductor field-effect transistors at high magnetic fields. In the absence of the quantum Hall effect, the correlation length of the fluctuations in magnetic field shows dips commensurate with the Shubnikov–de Haas oscillations in the resistance. These can be explained by the formation of phase-coherent electron trajectories that contain both diffusive parts in the bulk and skipping parts along the boundaries.

We believe that short-wavelength phonons (the phonons whose wave vectors correspond to the edge points of the Brillouin zone) may be emitted by electrons when crossing the interface if it is abrupt. To show this we have obtained the exact solution of Schrödinger’s equation for an electron whose effective mass is a smooth steplike function of distance from the interface, where an electrostatic potential is supposed to be unchanged. We then analyzed the matrix element of the electron-phonon interaction and find it is exponentially small if the interface is smooth, so that the mean width of the step is much greater than the phonon wavelength. This smallness should disappear if the interface becomes abrupt.

The electron tunneling relaxation rate ν caused by scattering from interface roughness in double quantum wells is calculated as a function of the interlevel splitting Δ and the coupling level numbers. The dependence ν(Δ) near the resonances is asymmetric with respect to Δ=0. This asymmetry appears because the roughnesses cause not only nonuniform changes to the electron quantization energies, but also inhomogeneities of the tunneling matrix element. The results are compared with the experimental data.

Electron tunneling relaxation caused by acoustic phonon scattering in semiconductor double quantum wells subject to a perpendicular magnetic field is calculated. Due to the interwell interference contribution in the matrix element of the electron-phonon interaction, the relaxation rate shows oscillations as a function of the magnetic field and level splitting. Significant (more than two orders of magnitude) suppressing of the relaxation rate near the minimums of the oscillations is found.

The electronic structure of graphitic tubules with general chirality is thoroughly studied by means of a recursion tight-binding method. A detailed investigation of densities of states around the Fermi level for various types of tubules confirms the previous theoretical prediction that tubules become metals (M), narrow-gap semiconductors (NS’s), or moderate-gap semiconductors (MS’s), depending on an integer index k specifying the geometrical structure of the tubules. We point out that the MS’s are systematically classified into two types, MS1 and MS2, according to this index. Namely, tubules correspond to M for k=0; otherwise they belong to semiconductors, being NS’s, MS1, or MS2, respectively, for k modulo 3=0, 1, or 2. Our result is compared with an approximate expression for energy gaps of MS’s, derived by White et al. and we discuss the deviation of our results from this expression. An empirical expression for the energy gap, which elucidates such a deviation, is suggested.

We calculate the equilibrium structure of spherical multishell fullerenes and the low-frequency rattling modes of the nested shells. The pairwise nature of the intershell interactions allows us to topologically map these zero-dimensional systems onto a linear chain of fullerene shells. Even though the masses of the inidividual shells and the intershell interactions depend strongly on the shell number, the vibrational density of states of very large structures resembles closely that of an infinite linear chain with site-independent masses and force constants and a very low Debye frequency of 80.2 ${\mathrm{cm}}^{\mathrm{-}1}$.

The Mn 3d contribution to the electronic density of states (DOS) has been evaluated in ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te single crystals cleaved in situ and alloys produced through the Mn-CdTe(110) interface reaction. Resonant photoemission studies show that the Mn 3d partial DOS is relatively independent of the Mn concentration (x) for 0.1≤x≤0.7. This result supports the contention that ${\mathit{d}}^4$ final states mostly account for the satellite observed in the Mn 3d partial DOS, in contrast with one of the conclusions of a recent paper [A. Wall, R. Raisanen, G. Haugstad, L. Vanzetti, and A. Franciosi, Phys. Rev. B 44, 8185 (1991)].

In a recent paper [Phys. Rev. B 48, 15 457 (1993)] Ruckman et al. characterized ultrathin ${\mathrm{C}}_{60}$ films on a Ta(110) surface by photoemission and C K-edge studies. They concluded that some fraction of the fullerene overlayer decomposes on the clean metal surface resulting in a thin layer of carbon between the assembled film and the metal substrate. In this Comment we argue that there is not enough experimental evidence to sustain this interpretation. Their experimental results can be interpreted by a chemisorbed first monolayer without inferring the formation of a thin carbon layer at the interface. Furthermore, their proposed alignment of the ${\mathrm{C}}_{60}$ π and ${\mathrm{\pi }}^{\mathrm{*}}$ energy levels is not correct.

The broad-band noise (BBN) and the frequency fluctuations of the narrow-band noise (NBN) were simultaneously measured in small ${\mathrm{NbSe}}_3$ samples. The two variables were highly correlated. Some discrete switching events show up in both noises. When multiple NBN peaks were present, some large fluctuations appearing in the BBN did not appear in the frequency of the biggest NBN peak, indicating that parallel uncorrelated charge-density-wave conduction paths were present.

We report momentum-resolved inverse-photoemission measurements from a quasi-one-dimensional metal, (${\mathrm{TaSe}}_4$${)}_2$I. Measurements were performed above (300 K) and below (100 K) the Peierls transition temperature (263 K). No change in the spectra is observed that could be associated with the phase transition. The mainly Ta-5d-derived conduction bands display, on the whole, little dispersion. In line with direct photoemission studies, we do not observe a Fermi edge when the material is measured in its metallic phase. We do, however, observe a state just above ${\mathit{E}}_{\mathit{F}}$ that exhibits significant dispersion along the direction of the conducting chains. This dispersing band is compared with previously published direct photoemission data. The comparison supports the existence of a fluctuating charge-density wave, and we suggest that this is the cause for the lack of a metallic Fermi edge from this material.

We report the crossover for the microwave dielectric constant of camphor-sulfonic-acid-doped polyaniline prepared in m-cresol from very large room-temperature negative values to large positive ones with decreasing temperature below ${\mathit{T}}_{\mathit{s}}$∼20 K. We attribute this change to the transition from a phonon-controlled metallic state to a nonmetallic state with localized carriers and hopping transport below ${\mathit{T}}_{\mathit{s}}$. Electronically, one-dimensional chains that couple the three-dimensional crystalline regions are likely central for understanding the origin of this transition as weak localization. At T<${\mathit{T}}_{\mathit{s}}$ we also observed an increase in the thermoelectric power and a linear decrease in the reduced activation energy for dc conductivity.

Multiple-peak structures in photoluminescence spectra of porous silicon are observed in the temperature range of 10–300 K. The peak energies are found to coincide well with the photoluminescence peak pinning energies previously reported. Thus the fine structures of luminescence spectra could be attributed to the size quantization of Si nanostructures formed during the sample preparation. After the sample was stored in atmosphere for six months, the multiple-peak structures in photoluminescence spectra still remain but the line shapes change significantly. The blueshift of the spectra is believed to be due to the variation of size distribution by natural oxidation. The size distribution for the sample showing multiple photoluminescence peaks is much more nonuniform when compared with ordinary samples and it varies from spot to spot. The measured temperature dependencies of the intensities of short-wavelength components show maxima at the temperature around 175 K, while the intensities of long-wavelength components are fairly small and dominate only at low temperatures.

We perform ab initio molecular-dynamics simulations using the higher-order finite-difference-pseudopotential (FDP) method to calculate the interatomic forces. Our approach is compared to molecular-dynamics-simulation calculations where the forces are calculated using a plane-wave basis. The ground-state structures of small silicon clusters obtained from the FDP simulation are in excellent agreement with those of the plane-wave method. The FDP method is performed completely in real space, and is easier to implement than methods based on a plane-wave expansion.