The effects of quantum fluctuations on the ‘‘2-in’’ herringbone ordering in a realistic model of 900 ${\mathrm{N}}_2$ molecules adsorbed in the (√3 × √3 )R30° structure on graphite are studied via path-integral Monte Carlo (PIMC) simulations. Quasiclassical and quasiharmonic calculations agree for high and low temperatures, respectively, but only PIMC gives satisfactory results over the entire temperature range. We can quantify the lowering of the transition temperature and the depression of the ground state order to 10% as compared to classical modeling.

Equilibrium shapes of a smectic-A phase grown from an isotropic phase have been investigated. An integral equation which describes the equilibrium shapes is derived by taking account of the difference in Gibbs free energy between the smectic-A and the isotropic phase, a curvature elastic energy of the smectic-A phase, and an interface energy. It is shown that a popular family of solutions of the equation are given by some Weingarten surfaces. The equilibrium cylinder structures with and without beaded configurations observed in the growth of smectic-A phase can be well explained in terms of such surfaces.

The core structure of a disclination line defect in a nematic liquid crystal is simulated by a Monte Carlo technique in which molecules are represented as hard spherocylinders, with aspect ratios ranging from 5 to 15. For small aspect ratios (5 and 8), we confirm the mean-field Landau theory that the core is biaxial with negative order parameter at the center. The order parameter coherence length is small, of the order of the molecular diameter. For large aspect ratios, we discover a new microscopically ‘‘escaped’’ structure for -1/2 wedge disclinations, in which the molecules point predominantly along the disclination line. Free energy calculations demonstrate that the ‘‘escaped’’ structure is of lower energy than the unescaped ‘‘flat’’ structure, which is metastable. We expect the ‘‘escaped’’ structure to retard disclination mobility.

We propose a new class of structures consisting of compact carbon clusters with surface threefold coordination and bulk fourfold coordination. These clusters are the diamond analogs of fullerenes, maintaining icosahedral symmetry and almost perfectly tetrahedral bonding. Two potentially stable forms are investigated: molecules with hydrogenated surfaces and solid close-packed arrangements of the carbon clusters. The latter form may account for several recent experimental observations of collapsed fullerite, a new form of carbon produced by compression of ${\mathrm{C}}_{60}$ crystals.

An equilibrium uniaxial dodecagonal quasicrystal is reported to be formed by a freezing simple monatomic liquid in a molecular dynamics simulation. Its diffraction pattern closely resembles that of the dodecagonal quasicrystalline phase formed in ${\mathrm{V}}_3$${\mathrm{Ni}}_2$ and ${\mathrm{V}}_{15}$${\mathrm{Ni}}_{10}$Si alloys, and the high-resolution electron micrograph obtained for the latter is found to be consistent with the corresponding pattern of atomic layer in the simulated quasicrystal. The simulated structure is discussed in the context of existing models of quasicrystalline order.

We have calculated the free energy of diffusion for lithium, sodium, and potassium ions in crystalline silicon by combining a thermodynamical integration method with ab initio molecular dynamics simulations. The entropy of diffusion is found to be negative, and its magnitude increases with increasing atomic size of the diffusing species.

We consider the Anderson metal-insulator transition in a network of randomly coupled metallic wires, which is suggested to describe transport properties of highly conducting fibril-form polymers. The absence of closed paths in the model enables us to study the transition exactly. The critical concentration of the cross-links determining the transition depends on the localization length of a single wire and on the interwire coupling. Application of a magnetic field extends the area of the metallic phase.

The Wigner ensemble of band random matrices describes the statistical properties of strongly chaotic Hamiltonians; it may also be viewed as a disordered tight-binding model with an electric field. We investigate the scaling properties of the localization of eigenstates and that of the distribution of level spacings, P(s), for finite matrices. We show that both quantities are uniquely determined by two scaling parameters.

We study the Josephson current through a superconducting grain coupled by two tunnel junctions to macroscopic superconducting leads. As a result of the Coulomb blockade we predict the critical Josephson current to have a threshold behavior when the superconducting gap is suppressed below the charging energy by a magnetic field. This effect is a direct consequence of a discrimination by the superconducting ground state between odd and even number of electrons on the grain. The critical current is periodically modulated by the voltage of a gate electrostatically coupled to the grain. We find a characteristic change in this modulation at the threshold.

We have determined the energy gaps of both prominent sequences of fractional quantum Hall effect of (FQHE) states at filling factor ν=p/(2p±1) around ν=1/2. The gaps increase linearly with ΔB, the deviation of the magnetic field from half filling. Therefore, the data are indistinguishable from Shubnikov–de Haas oscillations of particles of charge e, effective mass ${\mathit{m}}^{\mathrm{*}}$, and lifetime broadening Γ exposed to an effective magnetic field ΔB. Our findings are remarkably consistent with recent proposals of composite particles in the FQHE and the existence of a Fermi surface at even-denominator fillings.

Within the London approximation, we show that in anisotropic, strongly type II superconducting slabs two ‘‘species’’ of vortices can coexist simultaneously when the external magnetic field is applied at an angle with respect to the crystal principal axes. Simple thermodynamic arguments are used to prove that the appropriate thermodynamic potential is minimum when some of the vortices lie parallel to the c axis while others lie at an angle with respect to the c axis.

Magnetic lines of force penetrating a superconducting Nb thin film have been directly observed by electron holography. The tilted specimen is examined in the ‘‘transmission’’ mode that allows the 2D magnetic flux distribution to be discerned in an interference micrograph. The phase distribution of the electron wave transmitted through the specimen has been quantitatively measured. It was found that a 2D array of tiny regions, where the phase distribution rapidly changed, coincided spatially with the spots observed by Lorentz microscopy, and furthermore, these regions were identified to be quantized vortices each having a flux of h/2e by comparing with theoretical calculations.

The authors report ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ NMR spin-lattice relaxation rates, ${\mathit{T}}_1^{\mathrm{-}1}$, in the quasi-2D organic superconductor κ-(ET${)}_2$Cu[N(CN${)}_2$]Br (${\mathit{T}}_{\mathit{c}}$=11.6 K), for an aligned single crystal. The relaxation in the normal-state obeys the Korringa law (${\mathit{T}}_1$T=const). In the superconducting state, for weak fields (${\mathit{H}}_0$=0.59 T), ${\mathit{T}}_1^{\mathrm{-}1}$ is greatly enhanced and displays strong orientation dependence for field directions nearly parallel to the superconducting layers. This behavior indicates that motion of the fluxoid system is the cause of the extra relaxation, and is evidence for a ‘‘lock-in’’ transition of the flux lattice.

Using Majorana fermions to represent spins we reexamine the Kondo lattice model for heavy fermions. The simplest decoupling procedure provides a realization of odd-frequency superconductivity, with resonant pairing and surfaces of gap zeros. Spin and charge coherence factors vanish linearly with the energy on the Fermi surface, predicting a linear specific heat, but a ${\mathit{T}}^3$ NMR relaxation rate. Possible application to heavy fermions is suggested.

Phase separation within the fluid membrane of a vesicle can lead to domain-induced budding. This shape transformation should occur both for the spinodal decomposition and for the nucleation regime and should be observable in quench experiments on phospholipid-cholesterol vesicles. The budding process leads to limit shapes for which the bud is connected by an infinitesimal neck to the original vesicle. This neck can be characterized by a general relation for the principal curvatures of the adjacent membrane segments. The effect of the Gaussian curvature on the shape is also discussed.

The correspondence between error-correcting convolution codes and gauge invariant spin-glass models is used to show that the optimal way to recover the original message is by decoding at a finite temperature ${\mathit{T}}_{\mathit{N}}$(p)>0, where p is the strength of the channel noise and ${\mathit{T}}_{\mathit{N}}$(p) the Nishimori temperature. This improves upon the retrieval performance of the T=0 maximal likelihood Viterbi decoding algorithm without increasing its computational complexity. Numerical simulations support the theory.

Spectral distortions to the cosmic background radiation due to massive unstable relic particles which decayed during the thermalization epoch are calculated. We impose stringent constraints on such particles from recent observations. Early energy injection of this type generates characteristic spectral distortions which could potentially serve to measure the baryon density of the Universe.

We construct an explicit class of dynamic Lorentzian wormholes connecting Friedman-Robertson-Walker spactimes. These wormholes can allow two-way transmission of signals between spatially separated regions of spacetime and could permit such regions to come into thermal contact. The cosmology of a network of early Universe wormholes is discussed.

In the semiclassical limit, the two-dimensional space-time which emerges from string theory is a black hole similar to the Schwarzschild solution. However, we find that in the exact space-time, the singularity is replaced by a regular surface of time-reflection symmetry. The maximally extended space-time thus consists of an infinite sequence of asymptotically flat regions linked by wormholes, rather analogous to the Reissner-Nordström space-time except that there are no singularities in this case. We also calculate the mass and temperature associated with this space-time.

The prediction for ${\mathrm{\alpha }}_{\mathit{s}}$ in the minimal supersymmetric SU(5) grand unified theory is generically subject to uncertainties arising from a gravitationally induced dimension-five operator. Unless the coefficient of this operator is small, the correlation between ${\mathrm{\alpha }}_{\mathit{s}}$ and the mass scale which governs proton decay to Kν is destroyed. Furthermore, a reduction of the experimental uncertainty in ${\mathrm{\alpha }}_{\mathit{s}}$ would not provide a significant test of the theory.