We study the effects of doping on spin-Peierls (SP) systems using the unimodular mean-field theory. The impurity spins affect the singlet valence bond field and renormalize the magnetic excitations. The SP transition temperature and the energy gap of magnetic excitations are reduced by factors ∼${\mathit{n}}_{\mathit{i}}$ and ∼${\mathit{n}}_{\mathit{i}}^{2/3}$, respectively, with ${\mathit{n}}_{\mathit{i}}$ as the impurity density. At certain values of ${\mathit{n}}_{\mathit{i}}$, a gapless SP phase occurs, and the interaction between impurities becomes RKKY-like. The recently observed reduction of SP transition temperature upon doping and occurrence of a spin glass phase is interpreted using the proposed theory.

In this paper, we show rigorously that there exists the ferrimagnetic long-range order in the ground state of the positive-U Hubbard model at half filling on some bipartite lattices. When ${\mathit{N}}_{\mathit{A}}$>${\mathit{N}}_{\mathit{B}}$(${\mathit{N}}_{\mathit{A}}$ and ${\mathit{N}}_{\mathit{B}}$ are the total site numbers of two sublattices A and B), except for the ferromagnetism which was found by Lieb [Phys. Rev. Lett. 62, 1201 (1989)], there also exists the antiferromagnetic long-range order in the ground state. This result only requires U>0 and is independent of the dimension of the lattices.

We present the results of low frequency dielectric measurements and a detailed kinetic investigation of the specific heat anomaly in the spin-density wave phase of (TMTSF${)}_2$ ${\mathrm{PF}}_6$ in the temperature range between 2 and 4 K. The dielectric relaxation shows a critical slowing down towards a ‘‘static’’glass transition around 2 K. The jump in the specific heat in different controlled kinetic conditions shows all the characteristics of freezing in supercooled liquids. Both effects give direct evidence of a glass transition in the spin-density wave ground state.

Domainlike antiferromagnetic correlations are detected by SQUID measurements of the low-frequency susceptibility in the metamagnet ${\mathrm{FeCl}}_2$ when exposed to an axial magnetic field H at temperatures ${\mathit{T}}_{\mathit{c}}$(H)≤T≤${\mathit{T}}_{\mathit{N}}$. A distinct change of curvature of χ’’ vs T appearing at ${\mathit{T}}_{\mathit{N}}$ is explained in terms of fluctuating distributions of demagnetizing fields and, hence, transition temperatures. The field-induced dynamic domain phase—an analog of the Griffiths phase predicted for a ferromagnet with dilution x within ${\mathit{T}}_{\mathit{c}}$(x)≤T≤${\mathit{T}}_{\mathit{c}}$(0)—is confirmed by elastic light scattering experiments.

Muon spin depolarization has been studied in moderately concentrated AgMn and AuFe alloys from the freezing temperature ${\mathit{T}}_{\mathit{g}}$ up to 300 K. The muon depolarization function can be analyzed to show that the temperature dependence of the strongly nonexponential form of the local spin autocorrelation function in these canonical alloys is similar to that observed in numerical simulations on Ising spin glasses. The dynamical behavior above ${\mathit{T}}_{\mathit{g}}$ appears to be an intrinsic precursor to spin glass freezing.

A simple impulsive model for rotational excitation in photodesorption, combined with a two-level model Hamiltonian to describe hybridization of spin-orbit states, is found to explain the final state rotational and spin-orbit distributions of most photodesorption systems studied.

Magnetic-dipole two-photon absorption (MD-TPA) is introduced as a new spectroscopic technique for the study of odd-parity states. Contrary to the classical electric-dipole two-photon absorption (ED-TPA), where only spin-allowed even-parity transitions can be excited, MD-TPA allows us to excite spin-forbidden odd-parity states. This new technique is therefore very well suited to study paraexcitons in semiconductors and insulators. As examples MD-TPA measurements of RbI, NaI, and NaBr are presented.

Geometric phases of real wave functions in nonintegrable quantum billiards are measured using microwave resonators. They appear as a sign change of the wave function after a cyclic excursion around a diabolic point in the space of shapes of the resonator. For a path encircling a pointlike triple degeneracy of an integrable system, an unexpected sign change for two out of the three wave functions is found, which must be attributed to a hidden symmetry, and not to a Berry phase.

In a random Gaussian wave field there is on average one vortex (phase singularity) for every two coherence areas. These vortices exhibit an unexpected, highly surprising correlation–nearest neighbors are 90% anticorrelated in sign. We show that on the zero crossings of the real or imaginary parts of the field adjacent vortices must be of opposite sign. This principle, which is unaffected by boundaries, accounts for the observed sign anticorrelations. We also show how the sign of any single vortex in a random wave field determines the sign of all other vortices in the wave field.

A generalization of self-guided beams (spatial solitons) is discussed. Each beam is characterized by an axially uniform intensity profile whose polarization state changes with propagation. The soliton therefore has internal dynamics. It is composed of two orthogonally polarized modes of the linear waveguide induced by the dynamic soliton which, in general, do not exist as solitons independently. Our insight and exact analytical results for the threshold nonlinearity are found from the elementary physics of linear optical waveguides via a nonlinear self-consistency procedure.

Solenoidal advection in d dimensions may induce inertial-range behavior 〈‖T(x,t)-T(x+r,t)${\mathrm{\Vert }}^{\mathit{n}}$〉∝${\mathit{r}}^{\mathrm{\zeta }\mathit{n}\left(\mathit{T}\right)}$ of a passive scalar field T(x,t). If the velocity field changes very rapidly in time, the ${\mathrm{\zeta }}_{\mathit{n}}$(T) are determined by the two-particle eddy-diffusion coefficient η(r)∝${\mathit{r}}^{\mathrm{\zeta }\left(\mathrm{\eta }\right)}$. An approximation to the molecular-diffusion terms yields ${\mathrm{\zeta }}_{\mathit{n}}$(T)=1/2 √2nd[2-ζ(η)]+${\mathrm{\alpha }}^2$ -1/2α where α=d+ζ(η)-2. As n→∞, ${\mathrm{\zeta }}_{\mathit{n}}$(T)∝ √nd .

The nonlinearly saturated fluctuation level scaling with linear growth rate is presented for two instabilities: the ion temperature gradient instability and the rotationally driven E×B instability. In some instances, the plasma self-organizes from plasma turbulence and evolves into a quasicoherent state. In the turbulent state, spectral broadening is observed. The emergence of the semicoherent state is reflected by a narrowing spectral width of the mode with higher linear growth rates and fluctuation levels. The fine structure in the spectral width is resolved and indicates the presence of radial harmonics. The increasing coherence is also supported by a bispectral analysis.

The evolution of an optical-field-ionized channel created by a 3.5 ps KrF laser pulse in a low density nitrogen gas jet target (${10}^{17}$ ions ${\mathrm{cm}}^{\mathrm{-}3}$) has been studied using Moiré deflectometry. This allowed the density profiles in the channel and its lateral expansion to be quantitatively measured for the first time. The results were reproduced well by hydrocode simulations and analytical models. In addition, structured Raman backscatter signals indicated very low longitudinal electron velocities in the rising part of the pulse, as confirmed by 2D PIC code simulations.

The local electron energy flux produced by magnetic fluctuations has been measured directly in the MST reversed field pinch (over the radial range r/a>0.75).The flux, produced by electrons traveling parallel to a fluctuating magentic field, is obtained from correlation between the fluctuations in the parallel heat flux and the radial magnetic field. The fluctuation induced flux is large (100 kW/${\mathrm{cm}}^2$) in the ‘‘core’’ (r/a<0.85) and small (<10–30 kW/${\mathrm{cm}}^2$) in the edge.

A solution of equations of resonance scattering of high energy electrons is found which is alternative to that given by McRae [Rev. Mod. Phys. 51, 541 (1979)] and which does not assume the existence of a surface state. It is shown that the angular dependence of the reflectivity does not follow the Breit-Wigner law, and this conclusion agrees well with the result of numerical computation.

A new cooperative self-assembly process where ${\mathrm{C}}_{60}$ molecules adsorbed on Au(110)-(1×2) surfaces induce mass transport of Au surface atoms to form a (1×5) interfacial reconstruction is presented. Scanning tunneling microscopy shows that the underlying Au atomic arrangement is modified, maximizing the number of ${\mathrm{C}}_{60}$ molecules bonded to the Au ridges in a distorted (6×5) hexagonal overlayer.

Deep inelastic neutron scattering measurements have been carried out to measure the kinetic energy and condensate fraction in liquid ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^4{}_{}{}^{}$He mixtures. Samples with ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentration on the order of 10% were studied at 1.4 K and 2.3–2.5 K. The condensate fraction was found to be 0.18±0.03, much larger than theoretical predictions. The measured kinetic energies of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in the mixture remain the same as their values in the pure phase, in disagreement with current theoretical predictions.

We present first principle calculations of ultrathin silicon (111) layers embedded in ${\mathrm{CaF}}_2$, a lattice matched insulator. Our all electron calculation allows a check of the quantum confinement hypothesis for the Si band gap opening as a function of thickness. We find that the gap opening is mostly due to the valence band while the lowest conduction band states shift very modestly due to their pronounced interface character. The latter states are very sensitive to the sample design. We suggest that a quasidirect band gap can be achieved by stacking Si layers of different thickness.

The model of a Kondo chain with an M-fold degenerate band of conduction electrons of spin 1/2 at half filling interacting with localized spins S is studied. It is shown that in the continuous limit the spectrum of spin excitations is described by the O(3) nonlinear sigma model with the topological term with θ=π(2S-M). Thus for ‖M-2S‖ = (even) the system is an insulator and single electron excitations at low energies are massive spin polarons. Otherwise the density of states has a pseudogap and vanishes only at the Fermi level.

Self-trapped carriers of large bipolarons are redistributed among sites of their molecular orbitals in response to atomic motions. This effect lowers the phonon frequencies. The dependence of the zero-point energy on the spatial distribution of large bipolarons produces a phonon-mediated attraction between them. This dynamic quantum-mechanical effect fosters the condensation of large bipolarons into a liquid at sufficiently low temperatures.