X-ray diffraction experiments have been carried out on monolayers and submonolayers of the polar molecule C${\mathrm{F}}_3$Cl physisorbed on the (001) planes of exfoliated graphite. The coverage-temperature phase diagram shows three crystalline phases: an incommensurate phase, a commensurate 2×2 phase, and a higher-order commensurate phase. For the latter phase an arrangement of ferroelectric chains with a stacking sequence $p1\mathrm{}$ is proposed which belongs to the oblique plane group $\mathrm{ABCDA}\dots$. Surprisingly the lattice parameters exhibit hexagonal relations ($\sqrt{73}\times \sqrt{73}R5.8\mathrm{}°$).

Several defects are known to have metastable configurations that can be accessed by charge-state change, optical excitation, or heating followed by rapid cooling. Typically, each configuration is stable over a broad temperature range and can be studied by spectroscopic techniques. In this Letter, we report the observation of a novel metastability: A configuration change occurs spontaneously and abruptly at a critical temperature, giving rise to a discontinuous, deep-level transient spectrum. We propose that this phenomenon is a manifestation of entropy variations in the configurational space.

A new method of calculating total energies of solids which uses nonlocal pseudopotentials in conjunction with the variational quantum Monte Carlo approach is presented. By use of pseudopotentials, the large fluctuations of the energies in the core region of the atoms which occur in quantum Monte Carlo all-electron calculations are avoided. The method is applied to calculate the binding energy and structural properties of diamond. The results are in excellent agreement with experiment.

The energy spectra of atoms ejected as a result of the electronic excitation of rare-gas solids by keV electrons and ${\mathrm{He}}^{+}$ ions are presented. These spectra are found to have a well-defined structure with two distinct features indicative of nonradiative electronic relaxation processes. The higher-energy peak is due to radiative decay of vibrationally relaxed dimers that are either desorbed or weakly bound to the surface. The lower-energy peak exhibits a trend from Ar to Xe which is shown to be determined by subsurface dimer decays and is consistent with the trend in the cohesive energy.

When an electric field $F$ is applied parallel to the growth axis of a semiconductor superlattice, the eigenstates are quantized and strongly localized. This Wannier-Stark quantization manifests itself as an effective blue shift of the optical-absorption edge and is accompanied by a new type of electro-optical oscillation of the absorption coefficient which is periodic in $F^{-1\mathrm{}}$. We report experimental evidences for these theoretical predictions from the study of the low-temperature electroreflectance of a GaAs-AlGaAs superlattice.

Experimental as well as theoretical data for the optical properties of ultrathin GaAs-AlAs (001) superlattices are presented and analyzed. The dielectric functions ${\epsilon }_1\left(\omega \right)$ and ${\epsilon }_2\left(\omega \right)$ are calculated by means of the linear muffin-tin-orbital method, whereas the experimental data are obtained by means of ellipsometry. Theory and experiments agree with respect to the structures in ${\epsilon }_2$ and their variation in spectral position with superlattice period. Two structures specific to the superlattice are identified and explained in terms of the reduced symmetry as compared to the bulk materials. Variation of the polarization of the light is predicted to influence the optical properties only slightly.

We report the first observation of femtosecond photon echoes from the band-to-band transitions in a bulk semiconductor. The time decay of the echo, found to vary from 3.5 to 11 fs, has allowed us to determine the polarization dephasing rate in GaAs. This rate was found to depend on the carrier density in the experimental range covered, 1.5×${10}^{17}$ to 7×${10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$, indicating the dominance of carrier-carrier scattering as the principal dephasing mechanism. The observed functional dependence of the dephasing rate on the carrier density has yielded previously unavailable information on Coulomb screening in a nonequilibrium carrier distribution.

The experimental investigation of the electron spin resonance in the two-dimensional electron gas of $\mathrm{GaAs}-{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ heterostructures via the ESR-induced change of magnetoresistivity reveals hysteresis and long-persisting memory effects. We have been able to prove the nuclear origin of the observed effects by performing nuclear magnetic resonance and observing NMR in the magnetoresistivity of the heterostructures.

We observe resonant tunneling through single electronic states in a novel semiconductor device. At low temperatures, Lorentzian peaks of order but less than $\frac{e^2}{h}$ in magnitude are observed in the conductance versus Fermi energy of the device. As the temperature is increased, the peaks change in amplitude and width consistent with thermal broadening of the energy distribution of the electrons. An external magnetic field monotonically decreases the amplitude and reduces the widths of the peaks in accordance with a model based on isolated resonant wells.

The model proposed recently by Yeshurun and Malozemoff to explain the "irreversibility line" in high-temperature superconductors is extended to account for the experimentally measured width and shape of the resistive transition in a magnetic field, without invoking material inhomogeneity. It is argues that high $T_c$ and $H_{c2\mathrm{}}$ are necessary, but not sufficient, conditions for such materials to show zero resistance at room temperature in substantial magnetic fields.

A new dissipation behavior is reported in superconducting ${\mathrm{Bi}}_{2.2}{\mathrm{Sr}}_2{\mathrm{Ca}}_{0.8}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ for all temperatures below $T_c$ and all magnetic fields exceeding $H_{c1\mathrm{}}$. The current-independent electrical resistivity is thermally activated and can be described by an Arrhenius law with a single prefactor and a magnetic-field- and orientation-dependent activation energy $U_0(H,\phi )$. This behavior is markedly different from past observations and will be discussed in terms of flux creep and flux flow. This thermally activated behavior implies a finite resistance at all temperatures and all fields exceeding $H_{c1\mathrm{}}$ determined by the activation energy as the only parameter.

We have studied flux-lattice melting in single crystals of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{Bi}}_{2.2}$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{0.8}$${\mathrm{Cu}}_2$${\mathrm{O}}_8$ using a high-$Q$ mechanical oscillator. In Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ with an applied magnetic field $\mathbf{H}\perp \hat{\mathbf{c}}$, flux-lattice melting occurs at $H_{c2\mathrm{}}$ as in a conventional three-dimensional superconductor. However, for $\mathbf{H}\parallel \hat{\mathbf{c}}$ flux-lattice melting occurs 3.2 K below $H_{c2\mathrm{}}$. We believe that this is evidence for a transition into a vortex-liquid state similar to that seen in two-dimensional superconducting films. For ${\mathrm{Bi}}_{2.2}$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{0.8}$${\mathrm{Cu}}_2$${\mathrm{O}}_8$ the effect is even more pronounced; with a bulk superconducting transition of 75 K, the flux-lattice melting in this material occurs in both orientations near 30 K.

We report measurements of the physical properties of the 30-K superconductor ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$Bi${\mathrm{O}}_3$, including upper and lower critical fields and normal-state susceptibility. From the results and from comparison with Ba${\mathrm{Pb}}_{0.75}$${\mathrm{Bi}}_{0.25}$${\mathrm{O}}_3$ and band-structure calculations it is concluded that the BiO superconductors are in the weak-coupling limit, implying that the anomalously high $T_c\text{'}\mathrm{s}$ are not due to conventional, strong electron-phonon coupling. A sizable oxygen isotope effect of very similar magnitude is found for both compounds ($\alpha =0.2\mathrm{}-0.25$). We speculate that "phononic" effects in these materials are actually only indicative of dressed electronic excitations mediating superconducting pairing.

We have measured the counterion species and concentration dependence of the low-frequency Raman spectrum, speed of sound, and the $A-B$ transition in solid DNA. We find that both the frequency of the lowest Raman active mode, and the water content at the $A-B$ phase transition depend strongly on ion size and concentration. Longitudinal elastic constants show only a small dependence. These results indicate that ion-mediated interhelical forces control several important properties of solid DNA.

We consider polymers (self-avoiding walks) on a randomly triangulated planar surface. The partition function of $L$ polymer lines tied by their extremities, in a fluctuating metric, is calculated exactly. The exact infinite conformal spectra so derived are in complete agreement with the exponents found recently for 2D quantum gravity by Knizhnik, Polyakov, and Zamolodchikov.

We show that strong spatial correlations in a random resistor network can dramatically alter its transport properties. We calculate the average logarithmic resistance of a topologically one-dimensional model characterized by a random multiplicative process. We find a transport exponent that depends explicitly on the form of the spatial correlations; we also find that this problem is related to diffusion in the presence of correlated random fields.

The phenomenological relations between thermodynamic fluxes and forces are normally assumed to be invariant with respect to arbitrary motion of the frame of reference. We describe a breakdown of this invariance strong enough to be observable. It is shown that the diffusivity in a rotating fluid is anisotropic and also smaller in magnitude than in a fluid at rest in an inertial frame, giving rise to a diffusion analog of the Hall effect. For large Brownian particles (e.g., biological macromolecules) the diffusivity may decrease by 50% at the rotation speeds achievable in ultracentrifuges.

It is argued that, if the laws of physics permit an advanced civilization to create and maintain a wormhole in space for interstellar travel, then that wormhole can be converted into a time machine with which causality might be violatable. Whether wormholes can be created and maintained entails deep, ill-understood issues about cosmic censorship, quantum gravity, and quantum field theory, including the question of whether field theory enforces an averaged version of the weak energy condition.

The classical reparametrization-invariant SU(2) self-interactions of a test string are analyzed. Dynamically noncollapsing loop configurations in real space and on the intrinsic $S_2$ manifold are shown to exist, reflecting a combined effect of non-Abelian charges and currents. The dynamics leads naturally to a double-well "Weierstrass" potential whose minima specify two discrete configurations in the internal space.

We study the one-loop corrections in chiral perturbation theory to the $\gamma \gamma$ and $\gamma {\gamma }^{*}$ decays of the pseudoscalar mesons. The latter are divergent, thus requiring the existence of dimension-six terms contributing to the anomalous Lagrangian. Some examples of such terms are given. We also discuss the experimental consequences of the next-to-leading terms.