We show that the generation and evolution of split shock waves resulting from a dissociative polymorphic phase transition can be modeled using molecular-dynamics (MD) simulations. A model 2D semi-infinite molecular solid, driven with a piston, exhibits both single and split shock waves depending on the piston velocity. The results obtained from these MD simulations are in excellent agreement with continuum theory. These results—explicitly treating fewer than 5000 atoms—demonstrate that MD simulations provide a promising tool for studying the interplay between shock waves and polymorphic phase transitions.

Pressure studies of the molecular orientational ordering in solid ${\mathrm{C}}_{60}$ have exhibited three principal effects: (1) The simple-cubic (sc) to face-centered-cubic (fcc) ordering transition temperature (${\mathit{T}}_{\mathit{c}}$=249 K) increases with pressure at a rate of 10.4±0.2 K/kbar corresponding to ln${\mathit{T}}_{\mathit{c}}$/d ln${\mathit{a}}_0$=-22.8, where ${\mathit{a}}_0$ is the lattice parameter; (2) pressure greatly reduces orientational fluctuations of the ${\mathrm{C}}_{60}$ molecules in the ordered low-temperature sc phase; and (3) the results suggest the presence of two ordering transitions instead of one near 249 K. The implications of these results are discussed.

${\mathit{T}}_{\mathit{c}}$ and ${\mathit{T}}_{\mathit{N}}$ (Néel) have been measured for a series of ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{0.96}$${\mathrm{Zn}}_{0.04}$${)}_3$${\mathrm{O}}_{6+\mathit{x}}$ samples. The T variations of the homogeneous susceptibility ${\mathrm{\chi }}_{\mathit{s}}$ of the ${\mathrm{CuO}}_2$ planes, given by the shift of the ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR line, are found to be nearly unchanged with respect to pure samples for x>0.5, which implies that the charge transfer is negligibly modified by Zn, and that the magentic pseudo-gap is not associated with superconducting pairing. Detection of an unusual Curie contribution to the ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR width for x≥0.5 provides evidence that Zn induces magnetic moments in the ${\mathrm{CuO}}_2$ planes, which play a role in the depression of ${\mathit{T}}_{\mathit{c}}$.

We have carried out high-resolution photoemission experiments on two quasi-one-dimensional compounds: ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_3$ and (${\mathrm{TaSe}}_4$${)}_2$I. In both systems, a metal-insulator transition associated with a lattice distortion is reported. We show that the Fermi-step characteristic of a metallic phase is not observed above the Peierls temperature. As this Fermi step is always observed in higher-dimensional metals, we propose that this behavior results from the singular properties of one-dimensional systems and we suggest different mechanisms in order to explain this striking result.

Coulomb-blockade effects greatly enhance the resistance of low-capacitance tunnel-junction circuits at low bias and temperature. Experiments which involve the charging of small capacitances through such circuits are used to determine semiquantitatively the degree of this enhancement. Resistances of ≳${10}^{17}$ Ω have been observed for four-series-junction circuits whose individual junctions have resistances ∼${10}^6$ Ω. Two-series-junction circuits show lesser enhancements. For the higher resistances, one can observe directly the charging proceeding in discrete steps as individual electrons tunnel through the circuits.

The muon-spin-relaxation technique has been used to study vortex dynamics in single-phase superconducting single crystals of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ (${\mathit{T}}_{\mathit{c}}$≊90 K). The data indicate motional narrowing of the internal field distribution due to vortex motion (on a time scale comparable to the muon lifetime). A field-dependent lattice transition is also observed at ${\mathit{T}}_{\mathit{x}}$∼30 K, as evidenced by the onset of an asymmetric line shape below ${\mathit{T}}_{\mathit{x}}$. Narrowing arising from disordering of the vortices along [001] is also discussed with reference to its effect on the measured penetration depth.

We report a new transition in the magnetic (H-T) phase diagram of the high-temperature superconductor ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. A sharp (ΔT≃0.020 K) step in the irreversibility line is observed within about 1 K of ${\mathit{T}}_{\mathit{c}}$ at fields considerably above the lower critical field. A contour map probing nonlinear region of the H-T plane reveals a ‘‘backflow’’ and reentrant behavior below the irreversibility line, reflecting the collapse step. These observations are consistent with the existence of a thermal softening boundary which crosses the irreversibility line in H-T plane, and at which vortex cores are delocalized on the scale of the coherence length ξ.

Exact 3D space- and time-dependent Green’s functions are computed for the multichannel Kondo effect in the universal large-distance, long-time regime, using conformal field theory. They exhibit crossover from Fermi-liquid form far away, to singular non-Fermi-liquid behavior, near the impurity, where charge, spin, and ‘‘flavor’’ degrees of freedom become ‘‘deconfined.’’ Explicit expressions are given for the resistivity, ρ, for all k and for the spin-density and pair-density Green’s functions, for k=2. For k=2, ρ(T)-ρ(0)∝${\mathit{T}}^{1/2}$ and the pair susceptibility ∝lnT.

Short-period α-Sn/Ge strained-layer superlattices have been prepared on [001] Ge substrates by low-temperature molecular-beam epitaxy. We have achieved almost-defect-free and thermally stable single-crystalline structures. Photocurrent measurements in a series of ${\mathrm{Sn}}_1$${\mathrm{Ge}}_{\mathit{m}}$ (m>10) superlattices reveal a shift of the fundamental energy gap to smaller energies with decreasing Ge layer thickness m, in good agreement with band-structure calculations. A direct fundamental energy gap is predicted for a slightly increased lateral lattice constant in α-Sn/Ge superlattices.

We present a nonperturbative calculation of the ground-state energy of a normal tunnel junction. The junction with a large conductance shows Coulomb blockade of tunneling provided the external charge ${\mathit{Q}}_{\mathit{x}}$ is less than e/4. For ${\mathit{Q}}_{\mathit{x}}$>e/4, the band is flat and the junction behaves like an Ohmic resistor. We predict a phase transition between insulating and conducting behavior of the junction controlled by an external Ohmic resistance. We plot the corresponding zero-temperature phase diagram and calculate the junction effective resistance.

We show how spatial fluctuations of chromium thickness amounting to one atomic monolayer account phenomenologically for the biquadratic magnetic coupling recently reported in epitaxial Fe/Cr/Fe trilayers. Corresponding fluctuations of the short-period oscillatory term of conventional exchange coupling induce static waves of magnetization whose energy has the observed biquadratic form. An existing calculation of conventional exchange and its recent direct measurements in highly ordered trilayers support our interpretation.

The energy barrier for mutual cutting of a pair of twisted flux lines (prepared in a simple entangled configuration) is computed for isotropic and anisotropic superconductors by minimizing their total energy. The cutting barrier for the correctly curved flux lines is much smaller than the analytically calculated barrier for rigid, parallel flux lines and may even be negative, indicating instability of the entangled configuration. Vortex cutting is thus an effective mode of disentanglement of flux lines.

The superconducting contribution to the magnetization and resistivity of twinned and untwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ crystals which are presented here, as well as to the specific heat (Inderhees et al.) and Ettinghausen coefficient (Palstra et al.), displays scaling behavior in the variable [T-${\mathit{T}}_{\mathit{c}}$(H)]/(TH${)}^{2/3}$. This is consistent with Ginzburg-Landau fluctuation theory for a 3D system in a high magnetic field. The scaling property allows for a consistent way of determining the mean-field transition temperature ${\mathit{T}}_{\mathit{c}}$(H).

The temperature dependence of the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ resonance in ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{0.98}$${\mathrm{Fe}}_{0.02}$${)}_4$${\mathrm{O}}_8$ samples shows evidence of a discontinuity in the observed linewidth Γ(T), isomer shift, and quadrupole splitting at ${\mathit{T}}_{\mathit{c}}$. Γ(T) is found to increase precipitously at T<${\mathit{T}}_{\mathit{c}}$ with the excess width ΔΓ(T) thermally activated. This behavior is a signature of a fluctuating electric-field gradient in the planes with the fluctuations slowing down in the superconducting phase.

Using molecular-dynamics simulations, we have mapped out the potential-energy surface of a Si adatom over the ${\mathit{S}}_{\mathit{A}}$ and ${\mathit{S}}_{\mathit{B}}$ Si(001) steps. This identifies the likely binding sites for step growth and provides estimates for the diffusion barriers over the step edges. We find that the barriers at the step edge do not differ substantially from those on the flat terraces. Surprisingly, the ${\mathit{S}}_{\mathit{A}}$ and rebonded ${\mathit{S}}_{\mathit{B}}$ steps are only weak sinks for adatoms. In contrast, the nonbonded ${\mathit{S}}_{\mathit{B}}$ step is a good sink.

A quantity related to the elastic enhancement in nuclear reactions is introduced to analyze how the transition from integrable to chaotic behavior is a classical system is reflected in the eigenstates of the corresponding quantum system. Its behavior is first studied for random matrix models believed to represent such a transition, namely, a band matrix ensemble as well as a Poisson ensemble perturbed by a Gaussian orthogonal ensemble. Both of these display similar behavior. We then show that the same characteristics may be found in a more realistic system showing such a transition, namely, molecular Rydberg states calculated for high angular momenta.

If the dark matter in galaxies and clusters is nonbaryonic, it can interact with additional long-range fields that are invisible to experimental tests of the equivalence principle. We discuss the astrophysical and cosmological implications of a long-range force coupled only to the dark matter and find rather tight constraints on its strength. If the force is repulsive (attractive), the masses of galaxy groups and clusters (and the mean density of the universe inferred from them) have been systematically underestimated (overestimated). Such an interaction also has unusual implications for the growth of large-scale structure.

Exact solutions of ten-dimensional superstring theory corresponding to black five-branes [(5+1)-dimensional extended objects surrounded by event horizons] are constructed. The solutions are represented as products of conformal field theories.

We run all the couplings of the minimal supersymmetric (SUSY) extension of the standard model, taking account of the Yukawa sector. After identifying the scale at which the gauge couplings unify, we place bounds on the top-quark mass by requiring equality of the bottom-quark and τ Yukawa couplings at that scale. For ${\mathit{M}}_{\mathrm{SUSY}}$=1 TeV, ${\mathit{M}}_{\mathit{b}}$=4.6 GeV, we find 139≤${\mathit{M}}_{\mathit{t}}$≤194 GeV, which remarkably satisfy the ρ-parameter bound. Furthermore, using the minimal SUSY boundary condition on the scalar quartic coupling, we obtain bounds for the mass of the Higgs boson, 44≤${\mathit{M}}_{\mathrm{Higgs}}$≤120 GeV.

We have measured the Z-boson production differential cross section as a function of transverse momentum using Z→ee and Z→μμ decays in p¯p collision at √s =1.8 TeV with the Collider Detector at Fermilab. Comparison with standard-model predictions shows good agreement over the range 0<${\mathit{p}}_{\mathit{T}}$<160 GeV/c available from this data sample.