Infrared transmission measurements on the S=1 antiferromagnetic chain compound NENP [Ni(${\mathrm{C}}_2$${\mathrm{H}}_8$${\mathrm{N}}_2$${)}_2$${\mathrm{NO}}_2$${\mathrm{ClO}}_4$] in applied magnetic fields show a sharp absorption line at the field-shifted Haldane gap. This violates a wave-vector selection rule of the Hamiltonian normally used for NENP, as the gap excitations occur at the Brillouin zone boundary. We argue that the crystal structure admits terms which can explain the absorption lines. The observation of a finite gap at all applied fields can be explained by the presence of an induced staggered magnetic field, originally proposed by Chiba et al.

We consider the existence of ‘‘anomalous fixed point behavior’’ for the Kondo two-impurity Hamiltonian. This ‘‘anomalous’’ behavior is predicted by calculations which use the ‘‘energy-independent coupling constants’’ (ECC) approximation. Using well-controlled quantum Monte Carlo methods without the ECC approximation, we find no evidence for ‘‘anomalous’’ behavior. We then show that the ECC approximation is, in general, either inconsistent or uninteresting. These results together strongly suggest that the predicted ‘‘anomalous’’ behavior of two Kondo impurities is simply the result of an unphysical approximation rather than an intrinsic property of the model itself.

We report experimental and theoretical studies of the magnetization curves of Fe/Cr(211) superlattices with antiferromagnetic interlayer coupling and uniaxial in-plane anisotropy. There are substantial differences between structures with an even and an odd number of Fe layers, when the magnetic field is applied along the easy axis. For even layered superlattices, the inequivalence of the terminal Fe layers gives rise to a surface spin-flop transition that evolves into a bulk spin-flop arrangement with increasing magnetic field, as originally envisioned by Keffer and Chow [Phys. Rev. Lett. 31, 1061 (1973)].

The radiative decay of one-dimensional (1D) excitons is studied in novel GaAs quantum wires (QWI’s) prepared by molecular beam epitaxy. The temperature dependence of the radiative lifetime τ(T) is measured, and is compared with that of 2D excitons in quantum wells (QW’s): τ(T) of 1D excitons grows much more slowly with T than that of 2D excitons, resulting in shorter lifetime for T>20 K. This is ascribed to thermalization in the concentrated 1D density of states in QWIs. Analysis based on a theory has revealed that τ(T) is expressed as 92√T ps ${\mathrm{K}}^{\mathrm{-}1/2}$ in QWIs and as 24T ps ${\mathrm{K}}^{\mathrm{-}1}$ in QWs.

Ballistic-electron-emission spectroscopy on Si(111)-(7×7) patches in pinholes of thin ${\mathrm{NiSi}}_2$ films on n-type Si(111) reveals collector currents as high as 210% of the tunneling current at a tip bias of 10 V. This electron multiplication is assigned to impact ionization in Si. Its quantum yield up to 7 eV kinetic energy is extracted from the spectra and compared to recent thoeretical results.

The Si(111)-(7×7) surface can be locally hydrized at the nanometer scale by depositing H atoms from the tip of a scanning tunneling microscope. The H atoms to be deposited are continuously supplied to the tip made of Pt (20% Ir) from an ambient ${\mathrm{H}}_2$ molecule gas through dissociative adsorption (${\mathrm{H}}_2$→2H) and are deposited onto the Si(111)-(7×7) sample surface one by one by field evaporation; although the ambient ${\mathrm{H}}_2$ molecules are not adsorbed on the Si(111)-(7×7) surface, the H atoms from the tip are adsorbed on the surface.

We compute phase diagrams for diblock copolymers in the strong-segregation regime as a function of volume fraction φ. We include the ordered bicontinuous double-diamond (OBDD) phase, which is found to be stable between cylinders and lamellae, consistent with experiment, for φ (or 1-φ) between 0.19 and 0.27. Our phase diagram relies on taking into account the hexagonal shape of the unit cell for the cylindrical phase, and the unique geometry of the OBDD phase.

A new dynamical model for the interpretation of nuclear magnetic resonance relaxation data is presented. It is based on a normal mode description, treating the low frequencies associated with collective motions as adjustable parameters to optimize agreement between calculated and experimental relaxation order parameters. This model provides a compact representation of many aspects of internal dynamics and characterizes motions affecting different spin pairs in a correlated way. Furthermore, it links together dynamical characteristics of different types of NMR observables and allows one to assess vibrational thermodynamic properties. Sample applications are given for a 25-residue zinc-finger peptide.

A new 1D discrete nonlinear Schrödinger (NLS) Hamiltonian is introduced which includes the integrable Ablowitz-Ladik system as a limit. The symmetry properties of the system are studied. The relationship between intrinsic localized states and the soliton of the Ablowitz-Ladik NLS is discussed, including the role of discretization as a mechanism controlling collapse. It is pointed out that a staggered localized state can be viewed as a particle of a negative effective mass. It is shown that staggered localized states can exist in the discrete dark NLS. The motion of localized states and Peierls-Nabarro pinning are also studied.

Let A and B be two atoms or, more generally, a ‘‘source’’ and a ‘‘detector’’ separated by some distance R. At t=0A is in an excited state, B in its ground state, and no photons are present. A theorem is proved that in contrast to Einstein causality and finite signal velocity the excitation probability of B is nonzero immediately after t=0. Implications are discussed.

The statistical mechanics of an anyon gas in a magnetic field is addressed. A harmonic regulator is used to define a proper thermodynamic limit. When the magnetic field is sufficiently strong, only exact N-anyon ground states, where anyons occupy the lowest Landau level, contribute to the equation of state. Particular attention is paid to the interval of definition of the statistical parameter α∈[-1,0] where a gap exists. Interestingly enough, one finds that at the critical filling ν=-1/α where the pressure diverges, the external magnetic field is entirely screened by the flux tubes carried by the anyons.

By numerical and analytic study of the exact solution to the six-vertex model in applied fields, we explore in detail an unusual coexistence point which occurs in the low-temperature ferroelectric regime. This point, whose existence had been previously conjectured, corresponds to additional singularities in the free energy which were overlooked in the discussion of the original solution of the model. Furthermore, we find interesting scaling properties in the vicinity of this point. Implications for crystal shapes and the phase separation of unstable crystal surfaces are also discussed.

A search for slowly moving magnetic monopoles in the cosmic radiation was conducted from October 1989 to November 1991 using the large liquid scintillator detector subsystem of the first supermodule of the MACRO detector at the Gran Sasso underground laboratory. The absence of candidates established an upper limit on the monopole flux of 5.6×${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^{\mathrm{-}2}$ ${\mathrm{sr}}^{\mathrm{-}1}$ ${\mathrm{s}}^{\mathrm{-}1}$ at 90% confidence level in the velocity range of ${10}^{\mathrm{-}4}$≲β<4×${10}^{\mathrm{-}3}$. This result places a new constraint on the abundance of monopoles trapped in our solar system.

We present a practical solution to the ‘‘sign problem’’ in the auxiliary field Monte Carlo approach to the nuclear shell model. The method is based on extrapolation from a continuous family of problem-free Hamiltonians. To demonstrate the resultant ability to treat large shell-model problems, we present results for the ${}_{}{}^{54}{}_{}{}^{}\mathrm{Fe}$ in the full fp-shell basis using the Brown-Richter interaction. We find the Gamow-Teller ${\mathrm{\beta }}^{+}$ strength to be quenched by 58% relative to the single-particle estimate, in better agreement with experiment than previous estimates based on truncated bases.

The first measurement of the reaction ${}_{}{}^3{}_{}{}^{}\mathrm{He}$(γ→,p)X using linearly polarized photons is reported. Cross sections and beam-polarization asymmetries for ${\mathrm{\theta }}_{\mathit{p}}^{\mathrm{lab}}$=60°–100° and ${\mathit{E}}_{\gamma }$=195–304 MeV are compared with a microscopic calculation which includes one-, two-, and three-nucleon absorption mechanisms. One- and two-nucleon absorption alone fails to describe the data at low proton momenta. The inclusion of three-nucleon absorption significantly improves the comparison with the measured cross sections. However, some features of the asymmetry distributions are not explained.

High resolution resonance Auger spectra of HBr are recorded with the photon energy tuned to the HBr Br 3d→5pπ resonance which is broadened by the molecular ligand-field splitting. The spectra show the characteristics of the Auger resonance Raman effect: The resonance Auger peak positions move linearly with the photon energy, and the lifetime broadening does not contribute to the widths of the resonance Auger peaks. Moreover, the ligand-field splitting effects are also absent from the resonance Auger spectra. The prospect for high resolution studies on molecular inner shell decays, using resonance Auger spectroscopy to eliminate both lifetime and ligand-field effects, is discussed.

We have observed novel resonances occuring at subharmonic frequencies in probe transmission spectra of ultracold atoms confined in the antinodes of a 3D optical standing wave by means of dipole forces. We interpret these resonances in terms of multiphoton Raman transitions between vibrational states of the trapped atoms.

We demonstrate the suppression and enhancement of the spontaneous emission of entangled photon pairs in parametric down-conversion using suitably placed external mirrors. The mirrors permit interference between two possible ways of creating the photon pair. In principle these mirrors can be placed at an arbitrarily large distance from the crystal. The effect is of first order in the intensity of the down-conversion radiation.

We have measured the optical transmission of quasiperiodic dielectric multilayer stacks of ${\mathrm{SiO}}_2$ (A) and ${\mathrm{TiO}}_2$ (b) thin films which are ordered according to a Fibonacci sequence ${\mathit{S}}_{\mathit{j}+1\mathrm{}}$={${\mathit{S}}_{\mathit{j}\mathrm{-}1}$}, with ${\mathit{S}}_0$={B} and ${\mathit{S}}_1$={A} up to the sequence ${\mathit{S}}_9$ which consists of 55 layers. We observe a scaling of the transmission coefficient with increasing Fibonacci sequences at quarter-wavelength optical thicknesses. This behavior is in good agreement with theory and can be considered as experimental evidence for the localization of the light waves. The persistence of strong suppression of the transmission (gaps) in the presence of variations in the refractive indices among the layers is surprising.

Time-reversal mirror (TRM) utilizes time-reversal invariance of wave equation. Acoustic TRM are made of large transducer arrays allowing the incident acoustic field to be sampled, time reversed, and reemitted. The ability of TRM to time reverse separately each scattered wave permits us to isolate spatially and temporally the equivalent sources of extended scatterer illuminated by broadband pulse. We investigate more particulary the problem of a cylindrical scatterer: We show that TRM allows us to determine the location of the mode conversions of incident waves into circumferential waves.