Fröhlich and Peierls showed that a one-dimensional system with a half-filled band can lower its ground-state energy by a dimerization from period 1 to period 2. It was an open question whether or not this dimerization was exact, i.e., whether additional symmetry breaking would further lower the energy. We prove that the dimerization is exact for a periodic chain of infinitely massive, harmonically bound atoms with nearest-neighbor electron hopping matrix elements that vary linearly with the nearest-neighbor distance.

We report the first observation of a resonant mixing of the heavy hole (hh) and light hole (lh) plus one optical vibrational modes in a quantum well structure. A series of 27 peaks are observed between the hh and lh regions in the photoluminescence excitation spectra of the GaAs/${\mathrm{Al}}_{0.32}$${\mathrm{Ga}}_{0.68}$As(100) single quantum well studied and correspond closely to sixteen vibrational modes belonging to the classes of confined in the well, confined in the barrier layers, confined at the interface, and unconfined. This remarkable first observation is made possible only in this quantum well with parameters such as to achieve double resonance conditions.

It has been previously proposed that the excess low-temperature specific heat of the orientational glass KCN-KBr is due to dipolar (π) reorientations. To test this assertion, we have prepared related orientational glasses ${\mathrm{N}}_2$-Ar-CO in which the relative concentrations of asymmetric CO and symmetric ${\mathrm{N}}_2$ may be varied. The low-temperature specific-heat measurements on ${\mathrm{N}}_2$-Ar-CO reported here for temperatures between 0.15 and 4 K demonstrate that the asymmetric CO molecules are not important to the excess heat capacity.

The near-edge x-ray-absorption fine-structure technique is used to determine the orientation of cadmium- and calcium-arachidate chains on Si(111) at monolayer coverage. In our films the Cd-arachidate chain is found to be aligned along the surface normal and Ca arachidate is tilted by 33°±5° relative to Cd arachidate. The results are explained in terms of the surface bonding geometry of the terminal carboxylate group and the lateral interlocking of the zig-zag chains.

A fundamental argument for the smectic-A ordering of parallel hard rods is presented, along with a mean-field calculation that agrees well with the main features of numerical simulations of this system. Smectic-A ordering is attributed to more efficient packing of rods compared with nematic ordering. Above a critical volume fraction, the increased entropy per particle in the smectic-A layers more than compensates for the loss of entropy due to the forming of layers.

Using a flux technique we have grown sizable single crystals of ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_4$. With x rays and neutrons we have studied both the static and dynamic aspects of the tetragonal to orthorhombic structural phase transition; classic soft-phonon behavior is observed at the ((1/2, 1) / 2 , 0) zone boundary involving rotations of ${\mathrm{CuO}}_6$ octahedra. The pure and lightly-doped single crystals show hopping conductivity, lnρ∼($T_0$/T${)}^{1/4}$, indicating that the electronic states at the Fermi energy are localized.

On the basis of the calculated electronic structure, an excitonic-enhanced superconducting mechanism is used to explain the high $T_c$ superconductivity in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ system.

Resistivity was measured from $T_c$ to 1100 K for ${\mathrm{La}}_{1.825}$${\mathrm{Sr}}_{0.175}$${\mathrm{CuO}}_4$, where it is linear to 1100 K, and for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, where it is linear to 600 K, increases faster than linear at T>600 K because of loss of oxygen, and changes slope at the 950-K structural transition. The observed absence of saturation in the linear parts of the data implies weak electron-phonon coupling in both superconductors, thus excluding the possibility of phonon-mediated superconductivity. Values of the plasma energy are derived in combination with other experimental data.

Magneto-oscillations have, for the first time, been observed in the thermal conductance of GaAs/AlGaAs heterostructures containing two-dimensional electron systems. The oscillations result from a modulation of the thermal-phonon lifetime via coupling to the Landau-quantized electrons confined in quantum wells near the sample surface. These thermal-conductance measurements provide a new avenue for study of both the high-field density of states and the electron-phonon interaction in these semiconductor systems.

Direct intersubband excitations in narrow, modulation doped GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As quantum well structures are studied by picosecond infrared spectroscopy. The bleaching of the intersubband absorption induced by a high-intensity picosecond pump pulse, is studied by a delayed probe pulse. Typical intersubband relaxation times are of the order of 10 ps at 300 K for a subband splitting of about 150 meV. Polar LO-phonon scattering is discussed as the most relevant relaxation mechanism.

GaAs/AlGaAs heterostructures systematically doped with additional Be or Si impurities near the two-dimensional electron gas show distinct shifts of the quantum Hall plateaus and of the minima in the longitudinal resistivity ${\mathrm{\rho }}_{\mathrm{xx}}$ relative to the expected values of the Landau-level filling factor. A quantum Hall plateau ${\mathrm{\rho }}_{\mathrm{xy}}$=h/${\mathrm{e}}^2$ which does not cross the classical straight line ${\mathrm{\rho }}_{\mathrm{xy}}^0$=B/${\mathrm{n}}_{\mathrm{s}}$e is observed. The results are explained by a microscopic transport calculation emphasizing non-Born scattering of electrons by individual impurities.

In high-mobility GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As heterostructures at magnetic fields below well developed integral quantum Hall effect plateaus, we have observed that the magnetoresistance is independent of the device channel width in sufficiently small channels. Additionally, the current distribution within the channel depends on the total current magnitude and direction. We interpret our observations with a model in which current flows primarily along the edges of the sample when small currents are applied.

We perform differential absorption spectroscopy of a single modulation-doped InGaAs quantum well as its electron density is varied continuously from full (N≊8×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$) to empty (N≊0) by a gate electrode, allowing direct measurement of the density-dependent changes of transition energies and strengths. We observe a progressive quenching of the optical transition at the first subband and red shifts and broadening at higher subbands. We show that the dominant mechanisms are phase-space filling and the quantum-confined Stark effect. This technique also allows direct in situ measurements of the density and temperature of the 2D electron gas in such field-effect–transistor structures.

Measuring the inelastic scattering of synchrotron x rays by Be metal we found a pronounced double peak in the electronic dynamic structure factor S(q,ω) for q parallel [110]. We attribute the double peak to the existence of an excitation gap for final states on the (110) Bragg plane. For q=0.77 a.u. a zone-boundary collective state could be identified by means of a Kramers-Kronig–deduced zero of Reε(q,ω). Applications to band analysis are discussed.

By pseudopotential methods the electronic properties of quasicrystalline metal are examined from the nearly-free-electron viewpoint. The procedure results in the appearance of band gaps associated with each quasicrystal reciprocal-lattice vector, and these lead to singularities in both the density of states and the joint optical density of states. If observable by reflectivity, soft x-ray emission, or tunneling measurements, these quantities will give information on several properties of the quasicrystalline state.

The formation of cosmic strings causes a change in the metric of space-time. This change in the gravitational field creates particles, including photons, neutrinos, and gravitons. An estimate is made of the power and total energy output per unit length of string formed. For a grand-unification-scale string the total energy output is about ${10}^{30}$ ergs ${\mathrm{cm}}^{\mathrm{-}1}$, with a power output during the time of formation of about ${10}^{68}$ ergs ${\sec }^{\mathrm{-}1}$ ${\mathrm{cm}}^{\mathrm{-}1}$.

For a slowly time-dependent Hamiltonian system exhibiting motion which ergodically covers the energy surface, the phase-space volume enclosed inside this surface is an adiabatic invariant. In this paper the scaling of the error in the adiabatic approximation is investigated for this situation via numerical experiments on chaotic billiard systems. It is found that the scaling depends on the long-time behavior of correlations in the chaotic system.

Modification of the transverse intensity profile of a single electromagnetic waveguide mode during the free-electron laser (FEL) interaction has been measured. This effect is the waveguide analog to free-space optical guiding. The studies were carried out at microwave frequencies (8–12 GHz) in a FEL with use of a mildly relativistic electron beam of ≃200 kV energy and ≃1.0 A current. The probing of the rf fields was accomplished by small electric dipole antennas inserted in the interaction region. The observed intensity profiles are compared with the measured FEL-induced phase shifts (wave refractive index).

A new scheme of enhancing the efficiency and gain of a Doppler-shifted electron cyclotron autoresonance maser, with use of a transverse magnetic wiggler to inhibit stimulated absorption, is proposed. Computer simulations using a one and two-halves dimensional, electromagnetic, relativistic particle code show significant improvement in the efficiency and gain may be attained with this scheme.

Soft x-ray 3p→3s lasing in neonlike germanium (${\mathrm{Ge}}^{22+}$) and copper (${\mathrm{Cu}}^{19+}$) in the wavelength interval of 195 to 285 Å is observed for the first time, with gain coefficients ranging from 1.7 to 4.1 ${\mathrm{cm}}^{\mathrm{-}1}$, the higher gain with germanium. The lasing plasmas are produced by focusing a driving laser beam (λ=1.05 μm, 2-ns FWHM) into an 18-mm-long line onto thin films and slab targets. The measured J=0 to 1 gain coefficients are comparable to those of the J=2 to 1 transitions. The measured wavelengths of the six lasing lines compared favorably with recent calculations.