We study the resistance to sliding of liquid films 1-6 molecules thick. The apparent dynamic viscosity at 1 Hz of nonpolar liquids confined between parallel plates of atomically smooth mica is considerably enhanced over that of the isotropic fluids and shows extreme dependence on pressure. Rapid, reversible switching between liquidlike and noncompliant responses as a function of small changes in normal pressure is also found, suggesting a phase transition to a solidlike structure.

In a directional solidification experiment on a moving nematic-isotropic interface, we observe solitary modes propagating across the interface. We also observe the Eckhaus instability. In the presence of solitary modes, the pattern-selection mechanism is dynamic.

We report the first measurements of the electron-stimulated-desorption ion-angular-distributions of negative ions from surfaces. The angular distributions of ${\mathrm{F}}^{-}$ ions for electron-stimulated desorption of P${\mathrm{F}}_3$, N${\mathrm{F}}_3$, and ${\left(\mathrm{C}{\mathrm{F}}_3\right)}_2$CO on Ru(0001) depend on the molecular geometry and the state of the adsorbed species. The structural information obtained from these negative-ion studies complements that from similar positive-ion studies.

The quantum $\mathrm{XY}$ model of interacting spins on a hypercubic lattice has long-range order in the ground state for all values of the spin and all dimensions greater than one. We also show that in the limit of high dimension the spontaneous magnetization converges to the spontaneous magnetization of the Néel state.

The method of Kennedy, Lieb, and Shastry is applied to the antiferromagnetic $\mathrm{XXZ}$ model to examine the existence of long-range order (LRO). On the 3D simple-cubic lattice LRO is proved to exist for any $S\geqq \frac{1}{2}$ and $\Delta \geqq 0$ at low temperature. On the 2D square lattice LRO exists in the ground state for any $\Delta \leqq 0$ if $S\geqq 1$ and for $0\leqq \Delta <0.13\mathrm{}$ and $\Delta =0\mathrm{}$ if $S=\frac{1}{2}$. The existence of LRO in the ground state of the $S=\frac{1}{2} \mathrm{XY}$ model ($\Delta =0\mathrm{}$), therefore, has been confirmed, in agreement with the conjecture by Nishimori et al. The ground state of the $S=\frac{1}{2}$ antiferromagnetic Heisenberg model in two dimensions still remains to be clarified.

In using pulsed laser excitation of surfaces to induce desorption or reaction of adsorbed molecules, it has generally been assumed that the absorbed energy is rapidly randomized, and a thermal model can be used to calculate the surface-temperature change. In this work, the transient temperature jump on a Ag(110) surface induced by an 8-nsec laser pulse is directly monitored with a psec probe pulse. The probe is based on a temperature-dependent second-harmonic-generation effect. The experiment provides the first direct evidence that the heat-diffusion model can correctly predict the magnitude and the time evolution of the temperature on the surface.

We compare for the first time gas-phase $3s$ and $3p$ spectra from atomic Mn with corresponding spectra from the solid compounds Mn${\mathrm{F}}_2$ and MnO. These spectra are found to be strikingly similar, indicating that the solid-state spectra have a dominant origin in unscreened intra-atomic multiplet splittings, perhaps with correlation-induced satellites. Such outer-core hole states are thus not describable in terms of a simple superposition of fully screened and poorly screened spectra as recently proposed.

We report the first time-resolved studies of image-potential states on Ag(100). Femtosecond ultraviolet-pump and visible-probe techniques are combined with two-photon photoemission spectroscopu to measure transient photoemission spectra with femtosecond resolution. The lifetime of the $n=1\mathrm{}$ image-potential state on Ag(100) is observed to be several tens of femtoseconds.

The nonlinear behavior of the current-voltage characteristics observed at low temperatures in $p$-Geagrees well with the predictions of the Landau theory of critical and multicritical phase transitions.

Scanning tunneling microscopy has been used to investigate the effects of titanium impurities on the charge-density-wave (CDW) phase in $1T-\mathrm{Ta}{\mathrm{S}}_2$. The average CDW wavelength has been directly measured and found to increase with increasing titanium concentration. However, the local CDW structure distorts significantly in response to the random lattice potential associated with the distorted titanium sites.

The atomic positional correlations in superconducting ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$Ca${\mathrm{Cu}}_2$${\mathrm{O}}_8$ were studied by pair-distribution-function analysis of the pulsed-neutron scattering data. The results show for the first time strongly correlated local displacements of both thallium and oxygen atoms in the Tl-O plane from the high-symmetry crystallographic sites. As a consequence of the displacements Tl atoms form chains or pairs accompanied by displacement of nearby oxygen atoms, resulting in locally orthorhombic order. The ordering, however, remains very much short range and does not alter the average symmetry. The implications of these results with regard to the superconductivity of these oxides are discussed.

The first exact calculation of ${\left(\frac{\partial T_c}{\partial c}\right)}_{c=0\mathrm{}}$, the initial depression of the superconducting $T_c$, due to a small concentration, $c$, of magnetic impurities is presented. ${\left(\frac{\partial T_c}{\partial c}\right)}_{c=0\mathrm{}}$ has been calculated from the pair-breaking regime ($\frac{T_K}{T_{c^0}}\ll 1$) where the impurities are magnetic, to the pair-weakening regime ($\frac{T_K}{T_{c^0}}\gg 1$). I find that ${\left(\frac{\partial T_c}{\partial c}\right)}_{c=0\mathrm{}}$ can be expressed as the product of two universal functions $f\left({\lambda }_0\right)$ and $g\left(\frac{T_K}{T_{c^0}}\right)$, and that, in contrast to previous results, the maximum $\left|{\left(\frac{\partial T_c}{\partial c}\right)}_{c=0\mathrm{}}\right|$ occurs when $T_{c^0}=T_K$, thus resolving a long-standing experimental puzzle.

Spin polarization of $1s$ electrons has been observed for single-electron F ions on emergence from magnetized thin Fe layers into vacuum by use of the perturbed-angular-correlation technique on the isomeric ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$(5/$2^{+}$) state as a probe. The mean degree of polarization observed, $p_{1s}=0.10\mathrm{}\left(3\right)\mathrm{}$, is consistent with values relevant to measurements of transient magnetic fields.

Viscoelastic measurements on gels near the gel point show power-law frequency and time dependences. Using a percolation model, we find that viscoelastic properties are described by the universal exponent $\Delta =\frac{d\nu }{(d\nu +k)}$ where $k$ is viscosity exponent, $\nu$ is the correlation-length exponent, and $d$ is the dimension of space. This expression leads to a theory for the critical growth of the equilibrium shear modulus beyond the gel point, and the steady-state creep compliance beneath the gel point. Viscoelastic data are given for epoxy resins.

Using the spinor approach, we calculate exactly the complete spectrum of the transfer matrix for the finite-size, planar Ising model with adjustable boundary conditions. The mass gap is found to vanish exponentially or algebraically, depending on the boundary conditions used. These results are compared with recent numerical simulations, and with random-walk and capillary-wave arguments.

We obtain rigorous bounds on the long-time behavior of the densities ${\rho }_A\mathrm{}\left(t\right)\mathrm{}$ and ${\rho }_B\mathrm{}\left(t\right)\mathrm{}$ of species $A$ and $B$, which diffuse and annihilate upon meeting, i.e., $A+B\to \mathrm{inert}$. For equal initial densities ${\rho }_A\mathrm{}\left(0\right)={\rho }_B\mathrm{}\left(0\right)\mathrm{}$, the density goes to zero asymptotically as $t^{\frac{-d}{4}}$ for dimensions $d\le 4$ and as $t^{-1\mathrm{}}$ for $d\ge 4$. When ${\rho }_A\mathrm{}\left(0\right)<{\rho }_B\mathrm{}\left(0\right)\mathrm{}$, ${\rho }_A\mathrm{}\left(t\right)\mathrm{}\sim \exp \left({-\lambda }_1\sqrt{t}\right)$, ${\rho }_A\mathrm{}\left(t\right)\mathrm{}\sim \exp \left(\frac{{-\lambda }_{2}t}{\mathrm{In}t}\right)$ for $d=1,2, \mathrm{r}\mathrm{e}\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{l}\mathrm{y},\mathrm{a}\mathrm{n}\mathrm{d}{\rho }_A\mathrm{}\left(t\right)\mathrm{}\sim \exp \left({-\lambda }_{d}t\right)$ for $d\ge 3$.

A particle moving in two dimensions in a repulsive $r^{-2\mathrm{}}$ potential, and in the presence of a magnetic flux line, is examined as an example of a system for which the Schrödinger Hamiltonian is not essentially self-adjoint. The path-integral propagator is, nevertheless, shown to exist, and to define a unique self-adjoint extension of the Hamiltonian, corresponding to specific boundary conditions at the origin. Physical implications are discussed.

An analytical solution for one-dimensional diffusion in a Gaussian random potential is presented. In the long-time limit, the logarithm of the average population at the center, $\mathrm{In}\left(〈P〉\right)$, grows as fast as $t^{\frac{3}{2}}$, disproving some former estimates that $\mathrm{In}\left(〈P〉\right)$ increases at the rate of $t^2$. Numerical simulations have confirmed the theoretical solution.

Existing data from our Eöt-Wash torsion-balance experiment constrain Goldman, Hughes, and Nieto's quantum-gravity prediction that ordinary matter should feel, in addition to Newtonian gravity, composition-dependent attractive and repulsive Yukawa interactions. The Eöt-Wash null result tightly restricts the difference between the ranges of the proposed attractive and repulsive interactions. In particular, we show that the attractive and repulsive forces deduced from studies of the Earth's gravity do not exhibit the expected composition dependence.

We study the fermionic charge fractionization in two dimensions on a lattice using the Monte Carlo method. In order to avoid the fermion-doubling problem we use the bosonized procedure, and the results show that the fractional charge remains when the quantum dynamics of the soliton is included. We find that the fractional charge shows up at different values of the coupling constant from those at which the symmetry breaking of the initial bosonic field appears, and then the theory looks as if there were two phase transitions.