Atomistic simulations of hole self-trapping in a pure corundum crystal have been undertaken using three different approaches. The inward Jahn-Teller displacement of two O atoms (on which 80% of the hole density is concentrated), accompanied by the outward displacement of the two nearest Al atoms, gives the energetically most favorable configuration, thus suggesting a self-trapped-hole (STH) model analogous to that in alkali halides (${\mathit{V}}_{\mathit{K}}$ center). The optical absorption of STH and three possible kinds of thermally activated hops are discussed in relation to experimental data.

The neutron diffraction patterns of molten ${\mathrm{K}}_{\mathit{x}}$${\mathrm{Te}}_{1\mathrm{-}\mathit{x}}$ alloys for x=0.12 and x-0.50 demonstrate the persistence of covalently bonded tellurium in the liquid. In the case of liquid ${\mathrm{K}}_{0.12}$${\mathrm{Te}}_{0.88}$, the measured structure is domianted by the Te-Te contribution, and is remarkably similar to that of pure liquid tellurium. The equiatomic alloy ${\mathrm{K}}_{0.50}$${\mathrm{Te}}_{0.50}$ is shown to contain mostly Te pairs which are identified with Zintl ions, ${\mathrm{Te}}_2^{2\mathrm{-}}$. The presence of these ions explains the semiconducting behavior of these alloys deduced from recent electrical transport measurements.

Na-induced 3×1 reconstruction on the Si(111) surface was studied using a scanning tunneling microscope and other tools. Atomic images showed that this surface consisted of Na zigzag chains separated by a missing row, which covered the bulk-terminated Si(111) 1×1 substrate at the coverage of 2/3. Upon further deposition, a Na multilayer was grown following the Stranski-Krastanov mode. Tunneling I-V curves showed that the first layer of Na was insulating but the insulator-metal transition occurred in the second layer. We propose, based on the structural analysis, that this transition represents a two-dimensional Mott-Hubbard system.

The breakdown current ${\mathit{I}}_{\mathit{c}}$ of a percolating system is measured on real materials. We find that ${\mathit{I}}_{\mathit{c}}$∝${\mathit{B}}^{\mathrm{-}\mathit{x}}$, B is the weakly nonlinear response determined by third-harmonic generation. A new criterion for ${\mathit{I}}_{\mathit{c}}$ is suggested, defined as the current at which a hot spot reaches the melting temperature of the metallic grains, ${\mathit{T}}_{\mathit{m}}$. This criterion remains valid in the presence of nonlinear effects. It is also consistent with the experimental observations: At ${\mathit{I}}_{\mathit{c}}$ the resistance either increases to infinity or decreases, i.e., the local geometry is changed at ${\mathit{T}}_{\mathit{m}}$. Modeling the breakdown by hot spots yields the above power law with 0.5≥x≥0.36, in excellent agreement with the measured data: x=0.48 and 0.41 for Ag and Au films.

The steady-state resistance of small metal wires subject to an electric field is found to exhibit periodic resonances that scale with the width of the wire. The energies of the resonances have been determined using the low-temperature corrections to the resistance of the wires, and are shown to correspond to acoustic modes of the wire. We conclude that the resonances originate from increased energy relaxation that occurs when the electron system gains enough energy from the electric field to emit phonons in a higher acoustic subband of the wire.

We have measured the anisotropic thermal conductivity of an untwinned single crystal of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ in the most conducting a-b plane from 14 to 200 K. Unlike previous analyses, ours attributes the observed rapid rise in thermal conductivity in the superconducting state to the electronic contribution of the Cu-O plane. We propose that strong suppression of the quasiparticle scattering rate with decreasing temperature is responsible for the large enhancement of the superconducting-state thermal conductivity.

The well-known normal-core model including the pinning effects is employed to investigate the Nernst and Seebeck effects of flux flow in high-${\mathit{T}}_{\mathit{c}}$ superconductors. We show that the angle between the flux flow and the thermal force can be quite different from the Hall angle, particularly in the Bardeen-Stephen model even with the pinning force ${\mathbf{F}}_{\mathit{p}}$→0. As a result, the Bridgeman and Thomson relations, which have so far been regarded as basic thermomagnetic relations in the mixed state, are significantly modified. Under the adiabatic condition, the Ettingshausen effect and its influence on the transverse voltage are studied. All these effects are also discussed in the flux-creep region.

We have repeated and improved upon the experiment of Lyons et al., in which the observation of circular dichroism in high-${\mathit{T}}_{\mathit{c}}$ superconductors was reported. We found a simple extension to the previous technique which makes the apparatus exclusively sensitive to T-violating circular dichroism. We find no evidence for a temperature-dependent signal in all ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ samples studied. We establish sensitivity of the apparatus to surface roughness and ascribe the previously observed signal to possible surface contamination.

We report on dc magnetization measurements of high-quality single crystals of the title organic super-conductor. The temperature dependence of the magnetic penetration depth is estimated from the slope of the linear $M\left(\ln H\right)$ dependence in the London regime. An excellent agreement with the BCS theory for a weak-coupling superconductor in the clean limit is found. An attempt is made to relate the mixed-state-derived penetration depth, $\gamma (T\to 0)=535\pm 20$ mm, to normal-state Fermi-surface parameters. The strongly correlated heavy carriers appear to be of crucial importance to explain superconductivity in this material.

We calculate the gauge-invariant response of an unconventional ${\mathit{E}}_1$ state superconductor to electromagnetic fields in the collisionless limit using a matrix kinetic equation approach. We show that, despite broadening by nodal quasiparticles and screening currents, excitonic modes with collective frequencies of order the superconducting gap frequency should be observable as large, well-defined peaks in the power absorption. Such an observation would be a definitive signal for unconventional superconductivity. We further argue that the effect of collision broadening in real samples should be small at low temperatures.

We report the observation of coherent optical phonons at 120 and 150 ${\mathrm{cm}}^{\mathrm{-}1}$ in superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (δ<0.1) thin films. These phonons are excited by femtosecond laser pulses and observed by distinct oscillations of the reflectivity signal in real time. Below the transition temperature ${\mathit{T}}_{\mathit{c}}$ the amplitude of the Ba mode at 120 ${\mathrm{cm}}^{\mathrm{-}1}$ increases drastically in close correlation to the density of optically excited quasiparticles. The dephasing times of the coherent Ba and Cu(2) vibrations exhibit sharp increases below ${\mathit{T}}_{\mathit{c}}$ excluding a ‘‘second’’ smaller gap below 150 ${\mathrm{cm}}^{\mathrm{-}1}$.

Infrared reflectivity measurements on high-quality single crystals of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ were performed with polarization perpendicular to the ${\mathrm{CuO}}_2$ planes (E∥c). The c-axis spectrum shows a dramatic change at ${\mathit{T}}_{\mathit{c}}$, from a featureless spectrum above ${\mathit{T}}_{\mathit{c}}$ to the one with a sharp reflectivity edge below ${\mathit{T}}_{\mathit{c}}$. We find that the spectrum below ${\mathit{T}}_{\mathit{c}}$ is dominated by the plasma edge arising from the carriers condensed in the superconducting state, not by a superconductong gap excitation, and that the superconducting transition is accompanied by the onset of coherent charge transport along the c axis.

We present observations and an explanation of a photorefractive effect in strained centrosymmetric KTN and KLTN crystals in the absence of an externally applied electric field. Centrosymmetric crystals are forbidden to display the classical photorefractive effect without application of an applied field. Nevertheless, in diffraction experiments, centrosymmetric KTN:Cu and KLTN:Cu crystals show index changes of p to 1.7×${10}^{\mathrm{-}5}$ and photorefractive response at more than 120 °C above the phase transition. Experiments described here allow us to conclude that a Jahn-Teller relaxation of the spatially modulated ${\mathrm{Cu}}^{2+}$ dopant concentration is responsible.

The depth profiles of big cluster impacts on amorphous targets have been investigated by using the time-evolution Monte Carlo simulation code DYACAT, where (Ag${)}_{\mathit{n}}$ clusters of 200 eV/atom are bombarded on an Al target (n being 1 to 500). It is found that the recoil Al atom clears the way for the aftercoming cluster and that the projected range of (Ag${)}_{500}$ cluster is about four times larger than that of the monoatomic ion.

We provide the first report of a reaction process occurring during the collision between a cluster and a solid surface. Scattering of alkali fluoride clusters composed of >60 atoms from silicon at hyperthermal energies (0.1–0.5 eV/atom) leads to efficient ${\mathrm{F}}^{\mathrm{-}}$ transfer reactions. The reaction probability approaches one-half for large clusters and newly etched Si(111)-H, following heating to desorb hydrogen. The abrupt size threshold implicates a collective mechanism, such as a shock wave in the impacting cluster.

A simple theory is developed for rough seas in which the energy cascade towards small scales is too much for a surface tension dominated wrinkling of the surface to handle. We suggest that a new phase, consisting of an air-water foam, is created and becomes the principal medium for energy dissipation. The foam depth and droplet size are calculated and we suggest how the predicted dependence on energy flux and surface tension could be verified experimentally.

A treatment of Bose-Einstein condensation in a static spacetime with a possible spatial boundary is given. An interpretation in terms of symmetry breaking to give a nonconstant scalar-field vacuum expectation value is provided. Applications to condensation in cavities in flat spacetime, and to curved spacetime, are discussed.

We show that color transparency produces either strong nuclear shadowing or antishadowing, depending on the specific (semi)exclusive photoproduction reaction off nuclei. Neither coherent nor quasielastic diffractive photoproduction of charmonium on nuclei measures the charmonium-nucleon total cross section and the vector-dominance-model interpretation of nuclear photoproduction data cannot be applied. We relate the A dependence of the coherent and quasielastic photoproduction cross sections and derive the energy dependence of shadowing. The results are in good agreement with the New Muon Collaboration data.

It is shown that identical rotational energy spacings in pairs of even-even nuclei do not set these nuclei qualitatively apart from others. Rather, they represent the terminus or limiting case of a continuous range of spacings whose behavior is controlled by the balance between residual p-n pairing interactions and whose phenomenology can be simply described.

We calculate the average helicity dependence of the low-energy neutron-nucleus cross section in terms of a phenomenological optical model. Comparison with recent measurements on ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ by the TRIPLE Collaboration shows that the effective parity-nonconserving interaction is two orders of magnitude larger than estimates based on standard meson-exchange models. The helicity dependence is predicted to oscillate with target mass due to shape resonances.