We describe a novel magnetotunneling spectroscopy technique for probing the complicated dispersion curves of hole states in the quantum well of p-type double-barrier resonant-tunneling structures. Strong mixing between light- and heavy-hole states is observed. Some of the states clearly exhibit a negative hole effective mass for motion in the plane of the quantum well.

A ground-state search of a generalized, many-body Ising Hamiltonian whose interaction energies are determined from first-principles local-density calculations reveals that PtX intermetallics for X=Ni, Cu, Rh, and Pd will form stable ordered structures at low temperatures. In contrast, d-band tight-binding models universally predict phase separation in all late-transition-metal alloys. It is shown that the previously neglected s-electron cohesion is responsible for this phase stability.

Relaxation of a nonequilibrium distribution of electrons and holes in GaAs following femtosecond photoexcitation is investigated via spectrally and time-resolved luminescence. A rapid onset of luminescence over a broad spectral range shows that both electrons and holes are redistributed over a wide energy range within 100 fs, even at excitation densities as low as ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$. The data demonstrate carrier-carrier scattering rates higher than predicted by calculations with a statically screened interaction potential. Monte Carlo simulations using dynamical screening account for the experimental results.

We study the electrical conductance and Hall resistance of a two-dimensional quantum system with reflecting walls threaded by a magnetic flux tube of small radius (${\mathit{k}}_{\mathit{F}}$R≪1) through which electrons are allowed to pass. Because of the Zeeman interaction, the observable transmission deviates (as a function of flux) from perfect Aharonov-Bohm periodicity .ul2 in the limit where the radius of the flux tube tends to zero and the Lorentz force is absent. It is explained why this result does not violate gauge invariance. An experimental arrangment where this effect might be observed is the deviation from zero of the Hall resistance of a half-integer flux tube.

We present an alternative derivation for the macroscopic theory of edge excitations of the fractional-quantum-Hall liquid. The predictions of the macroscopic theory are compared with numerical results for finite systems.

We study the problem of anyons with statistics ν in a strong magnetic field by means of a similarity transformation. The ground state of this problem exhibits an ‘‘integer’’ quantum Hall effect at filling factor 1/ν with quasiparticle excitations of charge q/ν and statistics -1/ν. Certain electron fractional quantum Hall states can be considered as a realization of this, for example, the 2/5, 3/7,. . . hierarchy of the 1/3 state. This may explain the observed quasiparticle-quasihole asymmetry in the fractional quantum hall hierarchy.

A large-N expansion technique based on symplectic [Sp(N)] symmetry for frustrated magnetic systems is proposed and applied to the square-lattice quantum antiferromagnet with first-, second-, and third-neighbor antiferromangetic coupling. In addition to disordered states similar to those in unfrustrated systems, phases with incommensurate coplanar spin correlations and unconfined bosonic spinons are found. The occurrence of ‘‘order from disorder’’ is discussed. Neither chirally ordered nor spin-nematic states are found.

For fine multifilamentary ${\mathrm{Nb}}_3$Sn and Nb-Ti wires, it was found that there exists a surprisingly large temperature region (e.g., ∼1 and 0.5 K at 2 T for a ${\mathrm{Nb}}_3$Sn and Nb-Ti wire, respectively) below the mean-field critical-magnetic-field line ${\mathit{H}}_{\mathit{c}2}$(T) where the magnetization is reversible during a cycle of warming and cooling. The magnetic-field dependence of the lower boundary of this temperature region, i.e., the irreversibility temperature ${\mathit{T}}_{\mathit{r}}$(H), was shown to follow theoretical predictions for flux-lattice melting based on an analysis of thermally induced flux-line fluctuations calculated with nonlocal lattice elasticity and the Lindemann melting criterion.

A general expression for the elastic energy of the flux-line lattice (FLL) in anisotropic superconductors is given. From this we derive three tilt moduli ${\mathit{c}}_{44}$(k) for the FLL in uniaxial superconductors with induction B∥ and ⊥ to the basal plane. The discreteness of the FLL leads to a logarithmically dispersive isolated-vortex term, which at B≪${\mathit{B}}_{\mathit{c}2}$ in a large part of the Brillouin-zone area exceeds the usual Lorentzian-dispersive ${\mathit{c}}_{44}$(k) originating from the overlapping vortex fields. The difference between vortex self-energy, line tension, and tilt modulus in an anisotropic superconductor is discussed.

Narrow bridge structures were precisely fabricated (using novel ion-beam-milling techniques) on pulsed-laser-deposited ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ films. The bridges have a length scale down to L≃500 Å. Supercurrents were observed to flow across the bridge structures with critical current densities up to ${\mathit{J}}_{\mathit{c}}$≃1.3×${10}^9$ A/${\mathrm{cm}}^2$, the highest critical current densities yet reported. The current-density magnitudes are consistent with the Onsager-Feynman vortex-ring-creation mechanism (for producing voltages across narrow constrictions) in the length scale region of ξ≪L≤Λ, with ξ the coherence length and Λ the London penetration depth. The highest ${\mathit{J}}_{\mathit{c}}$ is in good agreement with the critical depairing mechanism.

We have calculated the effect of pair breaking on the isotope-effect coefficient (β) of a superconductor. We find that, as the pair-breaking scattering rate is increased, β also increases in absolute value. Values of β much larger than the canonical value of 1/2 can easily be achieved even in models where the electron-phonon interaction contributes only a very small amount to the value of the intrinsic critical temperature.

A simple local learning rule is presented for neural-network models of N two-state neurons. This rule introduces a free parameter k∈(1,4]. We prove that the synaptic matrix converges exponentially to a projection one onto the subspace spanned by the prototypes. This learning mechanism allows embedding without error sets of arbitrary correlated patterns (linearly independent or not), provided that each pattern is presented at most n times, where n is the smallest integer greater than ${\log }_{\mathit{k}}$N.

A pure state of a quantum-mechanical system, like a completely polarized state of light, is specified by the results of a few well-chosen experiments. We present a method for selecting such experiments, with applications to low-j angular momentum states.

In Debye-Hückel theory, the free energy of an electric double layer near a colloidal (or any other) surface can be related to the statistics of random walks near that surface. We present a numerical method based on this correspondence for the calculation of the double-layer free energy for an arbitrary charged or conducting surface. For self-similar surfaces, we propose a scaling law for the behavior of the free energy as a function of the screening length and the surface dimension. This scaling law is verified by numerical computation. Capacitance measurements on rough surfaces of, e.g., colloids can test these predictions.

We calculate the effects of surface singularities, e.g., wedges, on the thermodynamics of electric double layers near a rough colloid or charged membrane. Each wedge yields an additive contribution to the free energy scrF that is a function of the angle. We give a geometrical analysis of scrF entirely similar to that of vibration eigenmodes given by Kac in ‘‘Can one hear the shape of a drum?,’’ with a probabilistic Brownian representation of scrF, our analysis yields a universal scaling law for the free energy of a fractal colloid with its Minkowski dimension. The case of a colloidal random walk is derived from conformal invariance.

A new scenario for a successful inflationary-universe model is examined, with special emphasis on generation of the baryon asymmetry. The condensate of a massive Brans-Dicke field produces too much entropy when it decays, leading to dilution of the asymmetry. An efficient mechanism of baryon production at the time of condensate decay is suggested as a way out of this difficulty.

We have studied the decay chain Υ″→${\mathrm{\chi }}_{\mathrm{b}}^{\prime }$(${\mathrm{\chi }}_{\mathrm{b}}$)γ→Υ′(Υ)γγ→μμ(ee)γγwith the CUSB II detector at the Cornell Electron Storage Ring. For a sample of 1.33×${10}^6$ Υ″'s we find ≈400 events. We measure branching ratios for${\mathrm{\chi }}_{\mathrm{bJ}}^{\prime }$→Υ′(Υ)γ and, using calculated E1 rates, we derive total and hadronic widths of the${\mathrm{\chi }}_{\mathrm{b}}^{\prime }$ states. From these widths we obtain values of ${\mathrm{a}}_{\mathrm{s}}$ in the range between 0.13 and 0.21, in agreement with other determinations. We observe the suppressed decay Υ″→${\mathrm{\chi }}_{\mathrm{b}}$γ. The measured branching ratio suggests that relativistic effects are important. We also determine the branching ratios for Υ″→Υ′${\mathrm{\pi }}^0$${\mathrm{\pi }}^0$ to be (1.3±0.4±0.2)% andΥ″→Υ${\mathrm{\pi }}^0$${\mathrm{\pi }}^0$ to be (1.8±0.3±0.2)%. © 1991 The American Phsyical Society

The production of ${\mathrm{\pi }}^{\pm }$,${\mathit{K}}^{\pm }$,p has been measured in p+Be and p+Au collisions for comparison with central Si+Au collisions. The inverse slope parameters ${\mathit{T}}_0$ obtained by an exponential fit to the invariant cross sections in transverse mass are found to be, ${\mathit{T}}_0^{\mathit{p},}$${\mathit{K}}^{+}$,${\mathrm{\pi }}^{\mathrm{\pi }}$∼140–160 MeV in p+A collisions, whereas in central Si+Au collisions, ${\mathit{T}}_0^{\mathit{p},}$${\mathit{K}}^{+}$∼200–220 MeV >${\mathit{T}}_0^{\mathrm{\pi }\mathrm{\pi }}$∼140–160 MeV at midrapidity. The ${\mathrm{\pi }}^{\pm }$ and ${\mathit{K}}^{+}$ distributions are shifted backwards in p+Au compared with p+Be. A gradual increase of (dn/dy${)}_{\mathit{K}}^{+}$ per projectile nucleon is observed from p+Be to p+Au to central Si+Au collisions, while pions show no significant increase.

An E/A=65 MeV ${}_{}{}^{78}{}_{}{}^{}\mathrm{Kr}$ beam has been used to produce six new isotopes near the proton-drip line. The newly commissioned A1200 beam-analysis device was used to observe the astrophysically interesting isotope ${}_{}{}^{65}{}_{}{}^{}\mathrm{As}$, as well as ${}_{}{}^{61}{}_{}{}^{}\mathrm{Ga}$, ${}_{}{}^{62}{}_{}{}^{}\mathrm{Ge}$, ${}_{}{}^{63}{}_{}{}^{}\mathrm{Ge}$, ${}_{}{}^{69}{}_{}{}^{}\mathrm{Br}$, and ${}_{}{}^{75}{}_{}{}^{}\mathrm{Sr}$. Implications of the observation of these nuclei are discussed in terms of the astrophysical rp process.

We present experimental and theoretical results for Taylor-vortex flow subjected to a Coriolis force. The measurements were done in an apparatus consisting of two concentric horizontal cylinders with the inner one rotating, and mounted on a table rotating about a vertical axis with dimensionless angular velocity Ω. For Ω≲8.5, the first bifurcation upon increasing the inner-cylinder rotation rate was to tilted vortices. For larger Ω, there was a Hopf bifurcation to traveling tilted vortices. Measurements of the critical Reynolds number, wave number, and tilt angle are in quantitative agreement with the theory.