Using a Langevin algorithm to incorporate low-mass dynamical quarks, we simulate QCD with three light flavors on an $8^3$×4 lattice. We find clear metastabilities in lattice measurables at a quark mass of 0.025 in lattice units, whereas earlier analyses at 4 times the mass above did not prove their presence. Our results suggest a first-order chiral phase transition in three-flavor QCD. We also confirm the presence of a stronger first-order transition with four flavors and indicate the possibility of a weaker first-order transition with two flavors.

The spin alignment of 2.2×${10}^6$ J/ψ particles produced in 252-GeV πN interactions has been measured via their decay to ${\mu }^{+}$${\mu }^{\mathrm{-}}$ in an experiment at Fermilab. Whereas J/ψ’s are produced unpolarized over most of the kinematic range, they are found to become longitudinally polarized as $x_F$→1.

Triple coincidences between intermediate-mass fragments and angle-correlated binary fission events were measured in the at E/A=35 MeV. It was found that fragments emitted at backward angles appear to be associated with reactions in which nearly all ofthe projectile momentum is carried away by the heavy reaction residue and the intermediate-mass fragment. In contrast, fragments emitted at forward angles are accompanied by 20%–30% missing momentum.

We study the Floquet states which determine the response of Rydberg atoms to an external rf field. We discuss the recently observed transition from narrow to broad states and show that the expansion amplitudes of the bound-space–projected Floquet states on high-n bound states of the unperturbed atom decay as a power of n. Therefore, even the broad states are marginally localizable. We also derive an expression for the critical value of n where the transition between narrow and broad states occurs and use this relation to interpret recent measurements of rf ionization of high-n states in hydrogen.

A new type of magnetic shift has been observed in high-precision measurements on the hyperfine structure of ground-state rubidium atoms. In the range of magnetic fields used, 4.6–7.8 T, the measurements in both ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$ and ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ are consistent with a fractional shift of -1.68(15)×${10}^{\mathrm{-}9}$ ${\mathrm{T}}^{\mathrm{-}2}$ in the nuclear-to-electronic g-factor ratio, ${\mathrm{g}}_{\mathrm{I}}$/${\mathrm{g}}_{\mathrm{J}}$. This effect is much larger than predicted by a second-order perturbation-theory analysis which gives other magnetic shifts in agreement with the measurements.

The fluorescent spectrum for a two-level atom which is damped by a squeezed vacuum shows striking differences from the spectrum for ordinary resonance fluorescence. For strong coherent driving fields the Mollow triplet depends on the relative phase of the driving field and the squeezed vacuum field. The central peak may have either subnatural linewidth or supernatural linewidth depending on this phase. The mean atomic polarization also shows a phase sensitivity.

The stability of modulated Couette flow is studied by modulation of the angular velocity, Ω, of the inner cylinder according to Ω=Ω¯[1+ε cos(ωt)], where Ω¯ is the mean angular velocity of rotation. ω is varied for several radius ratios, and ε is varied for one radius ratio. The results are compared to earlier experimental and theoretical results.

Detailed agreement with the experiments of Walsh, Villeneuve, and Baldis is obtained from a theory of the coupling of stimulated Raman scattering and stimulated Brillouin scattering which incorporates the nonlinear physics of the Zakharov model.

Using an electromigration-relaxation method we determined the diffusion coefficient D of interstitial hydrogen in homogeneously strained monocrystalline and polycrystalline vanadium samples. In sharp contrast to Suzuki et al., who claimed to have observed superdiffusion due to stress-induced delocalization of hydrogen, we found that within experimental error D remains constant for uniaxial stresses in the [111] direction up to 115 MPa. The ‘‘superdiffusion’’ of Suzuki et al. is probably due to an inadequate analysis of their experimental data.

We report the observation of both commensurate-commensurate and commensurate-incommensurate transitions of the surface of a covalent material (InAs). Its 2D phase diagram is determined for independent changes of temperature, chemical potential of As surface atoms, and bulk lattice parameter. In agreement with models describing the surface as a frustrated system, all transitions are shown to be controlled by the relationship between the bulk lattice parameter and the mean surface one and are obtained for the first time upon changes of either parameter.

It is shown that the β-incommensurate transitions which take place in quartz at ${\mathrm{T}}_{\mathrm{S}}$≃846 K and in berlinite at ${\mathrm{T}}_{\mathrm{S}}$≃870 K are of a novel type which has recently been discussed theoretically. This transition has features in common with both instability and nucleation types. The wave vector q of the incommensurate phase rotates continuously away from the symmetry axis as the temperature is lowered below ${\mathrm{T}}_{\mathrm{S}}$. Renormalization-group and scaling arguments are presented, suggesting that the perpendicular component of the wave vector should vary as ${\mathrm{q}}_{\mathrm{\perp }}$∼${\mathrm{t}}^{\nu }$, where t=(${\mathrm{T}}_{\mathrm{S}}$-T)/${\mathrm{T}}_{\mathrm{S}}$ and ν is the critical exponent associated with the correlation length.

The structure of a vicinal Si(111) surface, misoriented by 6° toward the [11¯0] direction, has been measured as a function of temperature. At T≳810°C the surface steps have short-range order with an average step-step spacing of 28.4±1.6 Å. Upon cooling through the temperature where the 7×7 reconstruction appears, the spacing begins to decrease although the step height remains the same. This corresponds to clustering of the steps to form a new face whose misorientation changes continuously and reversibly from 6.3°±0.4° at high temperature to 17.0°±3.0° at low temperature. The transition can be described in terms of changes in the equilibrium crystal shape of Si induced by the formation of the 7×7 reconstruction.

GaInAs-InP and GaInAsP-InP multilayers have been investigated by transmission electron microscopy. Electron diffraction studies on 〈1¯10〉 edge-on samples indicate that atomic ordering produces a phase with R3¯m symmetry. This is of different symmetry from the one observed previously in GAlnAs, GaAsSb, and GalnAs. Only one or two variants of the ordered phase are observed in our layered structure.

Dee et al. have advanced the idea that the natural velocity of fronts propagating into an unstable state is related to the stability of these fronts through ‘‘marginal stability.’’ It is shown that this is indeed the case if front solutions lose stability through one particular mechanism. Marginal stability is derived for front propagation in the Swift-Hohenberg equation, but does not hold in a certain range of parameters of an extension of the Fisher-Kolmogorov equation.

Scanning tunneling microscope images of the √19 reconstruction of Ni/Si(111) are shown to rule out existing models for this surface. A new model, based on Si adatom stabilization and sixfold-coordinated Ni sites, is shown to give excellent agreement with the images and to explain the origin of the unit-cell geometry and some surface defects. The results demonstrate that nickel atoms, through Si adatom bonding, the strong effects on the bonding of first-, second-, and third-nearest-neighbor Si surface atoms. This realization provides important insight into the mechanism whereby very small quantities of nickel on the Si(111) surface completely alter the surface structure.

Transition to a superconducting zero-resistance state at 155 K is observed for the first time in bulk material. A new five-element compound has been synthesized with nominal composition ${\mathrm{Y}}_1$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{F}}_2$${\mathrm{O}}_{\mathrm{y}}$@B. Fluorine plays a critical role in achieving this effect. X-ray diffraction and electron microprobe analysis indicate that the samples are multiphasic. Evidence is presented that the samples contain superconducting phases with onset temperatures considerably above 155 K. Magnetic measurements suggest a flux-trapping effect below 260 K, and diamagnetic deviations from Curie-Weiss behavior in the range 250 K≤T≤100 K indicate a Meissner effect in a small superconducting volume fraction.

The perpendicular transport of photoexcited electrons and holes in graded-gap GaAs-AlGaAs superlattices is directly determined by subpicosecond luminescence spectroscopy. We show that the electron and and hole transport is through extended, Bloch-type states for superlattice periods <40 Å, and determine the mobility for such transport. Increasing the superlattice period to 60 Å reduces the mobility by a factor of 20, resulting from an order of magnitude reduction in the hole miniband width. Our results also provide the first evidence of diffusion enhanced by potential steps.

With infrared spectroscopy we study the electronic excitations in laterally periodic GaAs-AlGaAs heterojunctions and metal-oxide-semiconductor devices on InSb in which narrow inversion channels are created and controlled by field effect. Quantization into quasione-dimensional subbands is demonstrated by direct observation of resonance transitions between those subbands.

The valley quantum number of electrons in the conduction band of doped semiconductors is shown to affect the electron dynamics in a manner analogous to the spin quantum number. Recent theoretical and experimental studies have demonstrated the strong effects of disorder-enhanced electron-electron interactions upon the spin susceptibility and electron spin resonance spectrum. We show that similar effects should also exist in the valley fluctuations, leading to measurable consequences in the ultrasonic attenuation at low temperatures.

We propose a method to determine a natural valence-band offset (NVBO), i.e., the change in the valence-band maximum energy which is intrinsic to the bulk band structure of semiconductors. We use the Hg-Te-CdTe system as an example in which we find that the valence-band maximum of HgTe lies 0.35±0.06 eV above that of CdTe. The NVBO of 0.35 eV is in good agreement with the x-ray photoemission spectroscopy measurement of the heterojunction offset. The procedure to determine the NVBO between semiconductors, and its implication of the heterojunction band line-up and the electronic structures of semiconductor alloys, are discussed.