Direct measurements of the pressure dependence of the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ for single-phase ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ provide for the first time substantial evidence that ${\mathit{T}}_{\mathit{c}}$ in ${\mathit{A}}_3$${\mathrm{C}}_{60}$ compounds may be a universal function of the lattice constant (inter-${\mathrm{C}}_{60}$ spacing). Determination of the lower (${\mathit{H}}_{\mathit{c}1}$) and upper (${\mathit{H}}_{\mathit{c}2}$) critical fields, and consequently the superconducting penetration depth and coherence length, is also reported. Compared to ${\mathrm{K}}_3$${\mathrm{C}}_{60}$, the measured quantities are consistent with a 15% larger electronic-state density in ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ arising from a greater inter-${\mathrm{C}}_{60}$ spacing in this compound.

Photoemission and inverse photoemission spectroscopy has been used on ${\mathrm{C}}_{60}$ and ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ to study the change of the electronic structure upon alkali doping and its relation to the occurrence of superconductivity. The experimental results show that alkali doping does not lead to a rigid filling of the lowest unoccupied molecular-orbital band by the doped electrons but causes a transfer of electronic states from that band to an additional band produced in the band gap. A comparison of the photoemission and inverse photoemission spectra indicates that a pseudo-gap of about 0.5 eV opens at the Fermi level in ${\mathrm{K}}_3$${\mathrm{C}}_{60}$. Possible origins for the pseudo-gap and its relation to superconductivity are discussed.

${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR spectra in an oriented sample of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.92}$ for magnetic field parallel and perpendicular to the c axis are presented. An anisotropic behavior of the Knight shift (${\mathit{K}}_{\mathit{s}}$) above ${\mathit{T}}_{\mathit{c}}$ is observed for the first time. A new phenomenon of motional line narrowing around 40 K is detected and interpreted in terms of a partial flux melting with characteristic correlation times of tens of microseconds. Comparisons with muon-spin-rotation results are also given.

The hole-density dependence of the normal-state resistivity and transition temperature ${\mathit{T}}_{\mathit{c}}$ of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ was studied using the electric field effect in ultrathin films of 1 to 8 unit cells thickness. The change in resistivity was found to be equal to the field-induced variation in the areal carrier density, leading to a relation that the conductivity is proportional to the hole density. A similar linear dependence was also found in ${\mathit{T}}_{\mathit{c}}$. However, a saturation occurred in both cases but at different hole densities. The result reveals different effects of hole filling in influencing normal-state and superconducting transport of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$.

We introduce a new nonconservative self-organized critical model. This model is equivalent to a quasistatic two-dimensional version of the Burridge-Knopoff spring-block model of earthquakes. Our model displays a robust power-law behavior. The exponent is not universal; rather it depends on the level of conservation. A dynamical phase transition from localized to nonlocalized behavior is seen as the level of conservation is increased. The model gives a good prediction of the Gutenberg-Richter law and an explanation to the variances in the observed b values.

We report the first observation of a confined-to-propagating transition of LO phonons in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As superlattices (SL’s). Picosecond Raman scattering was used to investigate the generation rate of nonequilibrium GaAs LO phonons in two series of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As SL’s with constant GaAs layer widths (${\mathit{L}}_{\mathit{z}}$) and x, while the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As layer widths (${\mathit{L}}_{\mathit{b}}$’s) varied. In both series we have observed a sudden increase in the generation rate of the Raman-active hot phonons as ${\mathit{L}}_{\mathit{b}}$ decreased over a narrow range. We demonstrate that this abrupt increase is due to the sharp transition from the so-called confined to propagating LO phonons. From our results we have determined the GaAs/LO phonon penetration depth into the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As layers.

We report a strong (300:1) excitonic resonance in time-resolved (100 fs) four-wave mixing in GaAs quantum wells as a function of the incident photon energy and show that it results from the large difference in the dephasing rates of excitons and free carriers. We conclude that excitons dominate even when they make up only a small fraction of the excited states. These results, in conjuction with new results in exciton-phonon scattering, provide new insights into previous studies performed with spectrally broad ultrashort pulses.

A new coupled-quantum-well semiconductor with triply resonant third-order nonlinear susceptibility ${\mathrm{\chi }}_{3\mathrm{\omega }}^{\mathrm{}\left(3\right)\mathrm{}}$ has been designed and demonstrated using AlInAs/GaInAs heterostructures grown by molecular-beam epitaxy. In order to maximize ‖${\mathrm{\chi }}_{3\mathrm{\omega }}^{\mathrm{}\left(3\right)\mathrm{}}$‖ our structure has been tailored in such a way to maximize the product of the dipole matrix elements of the relevant intersubband transitions while maintaining nearly equal spacing between the electronic bound states of the quantum wells. Experimental values of ‖${\mathrm{\chi }}_{3\mathrm{\omega }}^{\mathrm{}\left(3\right)\mathrm{}}$‖ as high as ≃1×${10}^{\mathrm{-}14}$ (m/V${)}^2$ at 10.7-μm pump wavelength have been obtained, in agreement with theoretical estimates.

Cleaved GaAs surfaces decompose under weak pulsed-laser irradiation, leading to the formation of metallic Ga islands. The reduction of band bending nonuniformities in photoemission spectra acquired during the laser pulses, in concert with new core-level fitting techniques which treat broadened spectra, permits a detailed analysis. A decomposition rate consistent with a two-step photochemical excitation process is measured. The surface remains largely intact on an atomic scale, but is increasingly unable to support excited surface states as decomposition proceeds.

A simple ansatz is proposed for the statistical mechanics governing nonlinear steady-state transport in mesoscopic systems. It is shown that unexpected, novel phenomena arise as a natural prediction of the ansatz. In particular, we discuss the situation of resonant tunneling where the nonlinear effect is expected to be strongest.

A ring in a magnetic field whose direction varies in space is considered. It is shown that the Berry phase accumulated by the spins of electrons encircling the ring affects the conductance of the ring in a way similar to the Aharonov-Bohm effect. A time-dependent Berry phase is shown to induce a motive force in the ring. This motive force couples to the electron’s spin, similar to the way Faraday’s law couples an electromotive force to the electric charge.

We study the low-temperature behavior of quantum antiferromagnets in two space dimensions. Our model is the quantum nonlinear σ model (NLσM), which is the long-wavelength limit of the two-dimensional quantum Heisenberg model. Introducing fermion fields in NLσM, we propose a model of doped antiferromagnets. We show that renormalization-group theory predicts an order-disorder transition at finite hole concentration ${\mathrm{\delta }}_{\mathit{c}}$. Very small doping changes the temperature dependence of the correlation length ξ from exponential to power law, ξ∝1/T, for hole concentration δ<${\mathrm{\delta }}_{\mathit{c}}$. We also discuss a crossover between classical and quantum regions with varying coupling constant g.

It is proved that the ground state of the symmetric periodic Anderson model forms a total spin singlet at half filling. This result can also be applied to the Kondo lattice model, at least for the weak and the very strong antiferromagnetic-coupling limit. For the strong coupling limit of the Kondo lattice it is additionally proved that the singlet ground state exists even when two holes are doped to the half-filled band.

For x≤0.3 in the alloy system ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{U}}_{\mathit{x}}$${\mathrm{Pd}}_3$, transport data display signatures of Kondo behavior with the Kondo temperature increasing as x decreases. For the same range of x, the inverse photoemission spectrum shows a growth of U 5f spectral weight near the Fermi energy. We interpret this Fermi-energy weight as the first identification of the Kondo resonance in a uranium material.

Large 4d spin magnetic moments with small spin fluctuation rates have been measured for isolated Rh impurities in paramagnetic dilute PdFe alloys using the time-differential perturbed-angular-correlation method. The Curie-Weiss–like local susceptibility reflects enhanced Rh moment stability with the small addition of Fe into the Pd host, indicated by a drastic reduction of the spin fluctuation temperature. We suggest that magnetism and moment stability of Rh in the PdFe system are mainly governed by an interatomic ferromagnetic exchange interaction between the Rh 4d and the Pd d-band electrons.

A theory of magnetic excitations formulated within the framework of the Lagrange equations of motion is used to study spin waves in stacked triangular antiferromagnets. As a consequence of geometrical frustration, a longitudinal mode parasitically coupled to transverse excitations is predicted. The model demonstrates good agreement with inelastic neutron scattering data for spin-1 ${\mathrm{CsNiCl}}_3$ and ${\mathrm{RbNiCl}}_3$, as well as spin-5/2 ${\mathrm{CsMnI}}_3$, in contrast with results based on the Haldane conjecture and with standard spin-wave theory. This formalism has general relevance to all frustrated spin systems.

The two-impurity Kondo Hamiltonian is known to possess a multicritical point separating stable Fermi-liquid phases in which the two impurities either form a singlet or else are screened by the conduction electrons. We propose an exact critical theory for this non-Fermi-liquid multicritical point, using conformal field theory methods, which predicts all critical properties including the finite-size spectrum, specific heat, susceptibilities, renormalization-group behavior, residual entropy, scattering matrix, and local pair Green’s function.

Orientational disorder appears to persist in solid ${\mathrm{C}}_{60}$ even at low temperature, and similar static disorders is likely to exist in ${\mathit{A}}_3$${\mathrm{C}}_{60}$. The effects of such disorder on their electronic structures are studied by three-band tight-binding models for the ${\mathrm{C}}_{60}$ and ${\mathit{A}}_3$${\mathrm{C}}_{60}$ conduction bands. The densities of states for the disordered systems are quite different from those in the ordered systems. Participation-ratio analyses suggest that most of the states are delocalized, but there should nonetheless be a host of experimental consequences associated with short mean free paths.

We report the longitudinal resistivity and Hall-effect data of thin films of ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ in the normal and superconducting states in magnetic fields up to 12.5 T. The resistivity is 2.5 mΩ cm at room temperature and near the metal-to-insulator transition. The Hall coefficient is small as expected for a half-filled conduction band, and changes sign at 220 K. The resistivity is interpreted in terms of the granularity of the film which leads to zero-dimensional superconductivity in these systems with a length scale of 70 Å. We find a superconducting coherence length of ∼26 Å, which we interpret as a single-grain property. The Pippard coherence length, i.e., in the absence of granularity, is estimated to be ∼150 Å.

Measurements of the superconducting ${\mathit{T}}_{\mathit{c}}$ and Raman spectra in isotopically modified ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ are reported. A decrease in ${\mathit{T}}_{\mathit{c}}$ of 0.65±0.10 K in samples with (75±5)% substitution of ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ yields an exponent of α=0.37±0.05. This interpreted as evidence for phonon-mediated pairing.