We have measured low-temperature (T=10 K) photoluminescence spectra of InAs quantum dots embedded in a GaAs crystalline matrix under hydrostatic pressures up to 7 GPa. Below 4.2 GPa the spectra are dominated by the emission related to the Γ-like electron–heavy-hole exciton in the InAs dots. Above 4.2 GPa the spectra show two luminescence bands that shift to lower energies with increasing pressure. These bands are attributed to the type-I transition between ${\mathit{X}}_{\mathit{x}\mathit{y}}$ and heavy-hole states in the dots and the type-II transition from X states in GaAs to InAs heavy-hole states, respectively. In the Γ-X crossover regime we find evidence for a pronounced anticrossing behavior due to mixing between InAs Γ-like and GaAs X-like states. The corresponding interaction potential is estimated to be 9 meV, which is an order of magnitude larger compared to mixing interactions reported for semiconductor superlattices.

A number of experimental works provide evidence for the existence of high-field domains in superlattices when the applied voltage exceeds some critical value. A theoretical description of the structure of such a domain is developed. We confine ourselves to the case of narrow-band superlattices, where electrons are strongly localized in the wells. We find that the minimum length of the high-field domain can be larger than one superlattice period. The maximum current in the oscillating part of the I-V characteristic can be significantly smaller than the value of the current at the voltage where the first instability comes aobut. The oscillation period can be considerably smaller than the value corresponding to the energy separation between the first and the second level in a well. For the case of the domain formation at some distance from the anode, we study the field distribution in the low-field region downstream of the domain.

Coulomb-blockade phenomena and quantum fluctuations are studied in mesoscopic metallic tunnel junctions with high charging energies. If the resistance of the barriers is large compared to the quantum resistance, transport can be described by sequential tunneling. Here we study the influence of quantum fluctuations. They are strong when the resistance is small or the temperature is very low. A real-time approach is developed that allows the diagrammatic classification of resonant tunneling processes where different electrons tunnel coherently back and forth between the leads and the metallic island. With the help of a nonperturbative resummation technique we evaluate the spectral density, which describes the charge excitations of the system. From it, physical quantities of interest such as current and average charge can be deduced. Our main conclusions are as follows: An energy renormalization leads to a logarithmic temperature dependence of the renormalized system parameters. A finite lifetime broadening can change the classical picture drastically. It gives rise to a strong flattening of the Coulomb oscillations for low resistances, but in the Coulomb-blockade regime inelastic electron cotunneling persists. The effects become important at temperatures that are accessible in experiments.

The near-surface structure of low-energy- (0.5–1.5 keV) Ar-bombarded Si(100) has been studied using high-resolution x-ray-absorption near-edge structure spectroscopy and extended x-ray-absorption fine-structure spectroscopy with synchrotron radiation, medium-energy ion scattering, variable-energy positron-annihilation spectroscopy, and angle-resolved x-ray photoemission spectroscopy. The ion-induced defect structure for silicon, and the structure and distribution of incorporated Ar and silicon carbide formed during the dynamic mixing process are directly and nondestructively measured and depth profiled in the subnanometer scale. The results indicated that no Ar bubble was formed at moderate bombardment fluences (∼${10}^{16}$/${\mathrm{cm}}^2$) and that epitaxial recrystallization could be achieved by postbombardment vacuum annealing at 700 °C for 30 min.

A strained ${\mathrm{In}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{y}}$As layer is incorporated adjacent to the emitter barrier of an ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As double-barrier resonant-tunneling structure (DBRTS), so that it forms a prewell for the electrons that accumulate prior to tunneling into the GaAs quantum well (QW). Observation of photoluminescence (PL) and photoluminescence excitation (PLE) from the prewell then enables a direct optical determination of the charging behavior in the emitter-accumulation region to be achieved. We show in addition that an optical probe of the prewell can, by consideration of the electrostatics, provide a reliable determination of the charge distribution in the whole DBRTS at the peak of the tunneling resonance. These results are shown to be in agreement with separate determinations of the charge in the GaAs QW by direct PL measurements, and of the charge in the emitter-accumulation layer from magnetotransport studies. The electron density in the prewell can be varied continuously over a wide range, from 0 to 9×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$. The second subband is populated at high applied bias, with the density varying from 0 to 1×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$. This structure is then very well suited to the study of the many-body nature of the excitonic enhancement near the absorption threshold, over a much wider range of electron densities than previously studied on an individual sample. Utilizing temperature-dependent PLE measurements, we have monitored the variation from an atomic exciton in n=1, to a Fermi-energy edge singularity (FEES) in n=1, through to a FEES in n=2, in the same sample.

The Cd and Te desorption from (001), (110), and (111)B CdTe surfaces have been studied as a function of temperature by measuring the electron beam current reflected from these surfaces. For Te, activation energies of 0.96, 1.23, and 2.04±0.10 eV have been found for (001), (110), and (111)B surface orientation, respectively. For Cd, activation energies of 1.9±0.1 and 1.96±0.10 eV are measured for the (111)B and the (001) CdTe surfaces, respectively, similar to the measured values of activation energy for the sublimation of these surfaces. A detailed analysis of the desorption process gives some enlightenment concerning two remarkable phenomena, namely, (i) a reversible transformation between a c(2×2)+(2×1) and a (2×1) reconstruction for (001) CdTe and (ii) a hysteresis cycle between a (1×1) and a c(8×4) surface for the (111)B CdTe.

Neutralization and electronic excitation during low-energy proton-surface scattering have been investigated from a combination of experiments and discrete variational Xα calculations of the adiabatic molecular-orbital-energy diagram. The charge exchange in ${\mathrm{D}}^{+}$ scattering seems to have a more local character than expected from the conventional band picture of resonance neutralization, especially for the ionic-compound surfaces or surfaces with positively or negatively charged adatoms. Some of these local features are shown to arise from the formation of the surface molecule or the promotion of the H 1s orbital during the violent collision, which results in electronic excitation, such as reionization or electron-hole pair formation, and suppression of ion neutralization to some extent. The experimentally observed target-element dependencies of these processes are elucidated qualitatively in line with this scheme.

We have employed Brillouin scattering to investigate elastic-wave velocities as a function of temperature in Dy thin films and in bulk Dy. The single-crystal films were synthesized by molecular-beam epitaxy to ensure high crystalline and interfacial quality. Surface (Rayleigh) and guided-wave (Lamb) modes were probed by the Brillouin technique through the surface ripple-coupling mechanism. Results taken near room temperature in the paramagnetic phase agree very well with a layered-elastic model in which the materials of the samples are represented by elastic behavior using bulk single-crystal elastic data from the literature. At lower temperatures anomalies were found for samples with the thinner Dy layers: Measured wave velocities differed by up to 2% from the model predictions, and the ratio of Lamb-to-Rayleigh intensities was twice the predicted value. The anomalies coincide with the ferromagnetic transitions of the films, and they are believed to originate from coupling between the phonons and the magnetic system in the near-surface region probed by the Brillouin scattering.

The temperature dependence of the structure of the clean, (1×2) reconstructed Pt(110) surface has been investigated by thermal-energy helium-atom scattering and low-energy electron diffraction. Two distinct phase transitions were observed: At about 1025 K a two-dimensional Isinglike transition from the well-ordered (1×2) missing-row reconstructed surface to a disordered but flat surface takes place, followed at about 1095 K by a Kosterlitz-Thouless transition into a rough phase. The measurements provide strong evidence for the phase in the intermediate-temperature regime being a flat phase like the disordered flat phase recently proposed by den Nijs. The precise microscopic structure of this intermediate phase, however, could not be quantified in detail.

We present a macroscopic, phenomenological theory for the heat flow between two material half-spaces of differing temperatures whose surfaces are separated by a gap of width l. Our calculation parallels Liftshitz’s calculation of the van der Waals force between two dielectric slabs. For l sufficiently small, the heat flow is enhanced by a contribution from evanescent waves, and in the limit of a very small gap varies as ${\mathit{l}}^{\mathrm{-}2}$.

Ne projectile 2${\mathit{p}}^4$3${\mathit{s}}^2$ autoionization electron emission has been studied for 700-eV ${\mathrm{Ne}}^{+}$ impact on Mg, Al, and Si surfaces. The Ne 2${\mathit{p}}^4$ ${}_{}{}^1{}_{}{}^{}$D to 2${\mathit{p}}^4$ ${}_{}{}^3{}_{}{}^{}$P conversion rate η has been found to decrease with the atomic number of the target element, to remain constant upon Cs deposition, and to undergo a nonmonotonic change with Na adatom coverage. We interpret the independence of η on the averaged surface work function within the framework of the recently proposed Auger rearrangement model and discuss the possibility of extracting the valence-charge distribution at the vacuum side of the alkali adatom site from the coverage dependence of η.

We study a reversible phase transition of 1 monolayer (ML) of Pb on a Ge(111) surface using a scanning tunneling microscope (STM) over a temperature range from below to above the transition (∼175 °C). Below the transition temperature, a periodic array of Pb trimers centered on ${\mathrm{H}}_3$ sites is resolved. The controversial high-temperature phase is actually a simple Pb-terminated 1×1 structure. Just above the transition temperature, we observe a strongly fluctuating structure. Between 1/3 and 1 ML of Pb coverage, the transition is found to start in the vicinity of the phase boundary with a 1/3-ML √3 phase. At 1 ML we also observe the transition beginning at the step edges on the upper terraces. A sharp increase in the transition temperature occurs near 1 ML. The transition temperature is also found to depend on the domain size. The role of surface-strain fields on the phase transformation is discussed.

Using femtosecond time-resolved photoinduced absorption spectroscopy, we have studied the dynamics of photoinduced electron transfer from semiconducting polymers to ${\mathrm{C}}_{60}$. We find that with ${\mathrm{C}}_{60}$ in poly[2-methoxy-5-(2’-ethyl-hexyloxy)-p-phenylene vinylene)], poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene), and poly(3-octylthiophene) (P3OT), ultrafast photoinduced electron transfer occurs from the ${\mathrm{\pi }}^{\mathrm{*}}$ band of the semiconducting polymer to the lowest unoccupied molecular-orbital level of ${\mathrm{C}}_{60}$. Time-resolved measurements of the dichroic ratio place a 300-fs upper limit on the electron transfer time. The charge-separated state is rendered metastable due to the spatial separation of the electron and hole and to polaron formation associated with the electron on the ${\mathrm{C}}_{60}$ and the hole on the semiconducting polymer. Increasing the ${\mathrm{C}}_{60}$ concentration in P3OT increases the lifetime of the charge-separated state. The two p-phenylene vinylene derivatives also displayed metastable charge separation.

The kinetics of formation of the Cu(100)-c(2×2)Pd surface alloy is studied by adsorption of 1/2 ML of Pd on Cu(100) at 100 K followed by rapid heating up to selected anneal temperatures between 248 and 276 K. The domain-growth process is monitored via low-energy electron diffraction (LEED), Auger-electron spectroscopy, work-function measurements, and hydrogen thermal desorption. The initial stage of alloy formation, during which a considerable fraction of adsorbed Pd atoms replaces Cu atoms located in the first substrate layer, is found to be rapid and short. The subsequent late stages are slow. In the latter case, beam-profile measurements are consistent with self-similar domain growth. The corresponding analysis of the LEED peak intensity indicates that the growth law for the average domain radius has a power-law form with an exponent of 1/8. The scaling arguments presented show that such a low value of the growth exponent can really occur if the Pd-Cu ordering of the Allen-Cahn type is limited by dissolution of Pd islands occurring via the Lifshits-Slyozov scenario. The activation energy for diffusion in the domain-growth regime is estimated to be ≊0.4 eV.

First-principles results are presented for the effective cluster interactions at the surface of a random Ni–10 at. % Al alloy. The derivation is based on an extension of the generalized perturbation method to semi-infinite inhomogeneous binary alloys, using a layer version of the Korringa-Kohn-Rostocker multiple-scattering approach in conjunction with the single-site coherent potential approximation to compute the self-consistent electronic structure of the system. When applied to the bulk, the method yields effective pair interactions that have the full point-group symmetry of the lattice to a very high level of numerical accuracy, despite the fact that intra- and interlayer couplings (scattering-path operators) are treated differently, and which are in perfect agreement with those of a recent three-dimensional treatment. Besides the pair terms, a selected class of triplet and quadruplet interactions are calculated, as well as the point interactions induced by the presence of the surface. The value of the latter in the first lattice plane is strongly exaggerated in our approach, leading to a complete segregation of the minority species to the surface. Using a value corresponding to the difference in the surface energies of the pure components for this term leads to the observed Al concentration of ≊25% at the surface. Possible reasons for the shortcomings of the theory are analyzed, and test calculations for the well studied Cu-Ni system show that the free energy of the semi-infinite alloy cannot be approximated by the sum over the single-particle band energies, once charge self-consistency is enforced at the surface.

We report Raman spectroscopy studies for ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ films, which have been grown and measured, in situ, in an ultrahigh-vacuum chamber. We find interesting dependences of some of the vibrational modes on the excitation energy, and compare this with the corresponding optical transitions as measured by spectroscopic ellipsometry. Modes that show a strong interference with continuum excitations are enhanced for excitations corresponding to transitions involving the partially filled metallic band. Raman detection of external intermolecular modes are reported for both pure ${\mathrm{C}}_{60}$ and for ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$. Some temperature dependence of these modes is observed, but no clear anomaly at the superconducting critical temperature could be determined.

An attempt has been made to clarify the effect of interfacial mixing on the x-ray diffraction (XRD) and ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$ NMR of magnetron-sputtered Co/Cu superlattices. Systematic and quantitative modification of interfaces have been achieved by codeposition of Co and Cu at interfaces. We show that high and low angle XRD patterns are not affected by the existence of compositionally mixed regions thinner than 0.3 nm, and that significant change in the distribution of the hyperfine field of ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$ is found. The interfacial modification by codeposition is a powerful method to investigate the interfacial effects on the properties of superlattices, since only the interfacial structures are modified, while the other structures remain unchanged.

Heat capacities of intercalation compounds ${\mathrm{Ag}}_{\mathit{x}}$${\mathrm{TiS}}_2$ (x=0.361, 0.181,0.0902,and 0.0225) were measured with an adiabatic calorimeter for the temperature of 13–400 K. The intralayer order-disorder phase transition of intercalated Ag atoms was observed at around 280 K in all of the samples. The same √3 a× √3 a superlattice intralayer structure of Ag atoms as in ${\mathrm{Ag}}_{0.361}$${\mathrm{TiS}}_2$ of the stage-1 structure was found by a low-temperature x-ray-diffraction analysis in the sample of x=0.0902, which is only a quarter of x=0.361. An interpretation of the positional ordering of Ag atoms as functions of temperature and composition is given on the basis of the above observations, and a stage-$1_{\mathrm{RI}}$ (where RI means random island) structure is proposed to be composed of Ag islands distributed at random between the host ${\mathrm{TiS}}_2$ layers.

The temperature-dependent conductivity, magnetoresistance, magnetic susceptibility, and room-temperature Raman scattering of carbon aerogel samples with different morphologies and various grain sizes are studied. In particular, carbon aerogels with a polymeric morphology are studied and compared with colloidal carbon aerogels. The conductivity exhibits an exp[- √${\mathit{T}}_0$/T] dependence for all samples at low temperature. This strong localization behavior can be explained by a Coulomb-gap variable-range hopping mechanism, and this identification has been further corroborated by the low-temperature magnetoresistance data obtained in a magnetic field up to 15 T. Magnetic susceptibility measurements suggest that carbon aerogels with a smaller grain size are more disordered, in agreement with the conductivity data. The results suggest that the grains themselves, and not the defects within, act as carrier localization sites in these materials.

Magnetoconductance experiments on metallic nanobridges were performed in a mesoscopic two-probe configuration. In our samples a short wire of length L=100 nm to L=1 μm and cross section 30×30 ${\mathrm{nm}}^2$ connected two funnel-shaped contacts. The contacts served as both voltage and current leads. The dependence of conductance fluctuations (CF’s) on the length of the mesoscopic wires was measured at temperatures T≤100 mK. In this temperature range the rms amplitude δG of the CF equals its low-temperature value for all investigated devices. Magnetoconductance fluctuations are caused by the interference of electrons within a phase coherent area ${\mathit{F}}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}$. In short bridges ${\mathit{F}}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}$ encloses the nm wire and parts of the funnel-shaped contacts. Quantum diffusion of conduction electrons into the contacts leads to high-frequency magneto-CF. The diffusion in the wide contact regions become more and more important with decreasing wire length. This results in a decreasing correlation field ${\mathit{B}}_{\mathit{c}}$ with decreasing wire length. We observed a weak dependence of the rms amplitude δG of the CF on the wire length, which we attribute to a dimensional crossover from a one-dimensional (1D) to a 2D system. Besides the saturation value of δG we studied the temperature dpeendence of CF’s in the range 280 mK to 10 K for a L=200 nm wire. The decrease of the magnitude of the CF starts only at T>500 mK. For higher temperatures it follows a ${\mathit{T}}^{\mathrm{-}1/2}$ law.