A type of intrinsic luminescence in crystals has been revealed. The excitation threshold of well-known fast intrinsic luminescence of CsI (300 nm band) has been obtained from the excitation spectrum measured in the photon-energy range 5–125 eV at 300 K using synchrotron radiation. The excitation starts from an energy of about 20 eV and exponentially increases up to the 4d ${\mathrm{I}}^{\mathrm{-}}$ ionization energy (∼50 eV). Such high-energy broad threshold is not typical for any conventional mechanisms of luminescence excitation. We attribute this type of luminescence to the emission center, which is created by the interaction of several elementary excitations produced in the process of relaxation of a primary excitation. The possible role of 4d ${\mathrm{I}}^{\mathrm{-}}$ core levels is discussed. The nanosecond decay kinetics of this luminescence is nonexponential for all energies of excitation.

The London analytical interpolation equation between zero and packing densities for the ground-state energy of a many-boson hard-sphere system is corrected for the reduced mass of a pair of particles in a ‘‘sphere-of-influence’’ picture. It is thus brought into good agreement with computer simulations and with experimental results extrapolated out to close packing.

The temperature T dependence of the resistivity of decagonal single quasicrystals available at present has been carefully examined from 77 to 340 K. All the samples show a precisely linear T dependence of resistivity along the periodic direction, while in the quasicrystalline plane a quadratic term was added to fit the data. The linear T dependence in the large temperature range implies a very low transport Debye temperature, and can be easily understood by the nearly filled band model used to explain Hall-effect anisotropy. The T dependence of the resistivity in the quasicrystalline plane can be satisfactorily explained by the generalized Faber-Ziman theory, proving an enhanced electron-phonon interaction and an enhanced phonon-phonon coupling in the quasicrystalline plane.

The molecular-dynamics technique is applied to evaluate conductivity fluctuations in an off-lattice resistor-network model. It is shown that the resulting Fourier spectrum displays a 1/f-like behavior which persists for a broad range of physical conditions and a variety of interparticle interactions. These results are then compared to current models for the origin of correlated noise.

Measurements of polarization fluctuations at time scales longer than ${10}^{\mathrm{-}6}$ s have not been feasible via electron spin resonance techniques. Using partially deuterated ${\mathrm{KH}}_2$${\mathrm{PO}}_4$ (KDP) as an example, this study shows that the double-modulation electron spin resonance technique enables the detection of polarization fluctuations on the millisecond time scale. The measured correlation times for 73% deuterated KDP exhibit Vogel-Fulcher behavior, indicating that this lattice may exhibit structural glass behavior, with a glass formation temperature of 138±10 K.

The thermoelectric power (TEP) or Seebeck coefficient is calculated along the lines of Ziman variational method taking into account explicitly the inelastic scattering of electrons or holes (occupying a single parabolic band) by the (acoustic) phonon spectrum. The metallic limit is used to show the relative signs of terms appearing in the TEP expression. It is undoubtedly and quantitatively indicated that inelastic scattering can account for the usual bump seen in TEP vs temperature data at moderate temperatures. Also, TEP does not have a uniform sign even for electronic conduction in a single band. The low-temperature expansion is also given.

A two-site two-electron system is studied in the presence of an electron-phonon (e-ph) interaction and intersite hopping of the electrons to determine the stability of different polaronic states. Exact energies and eigenfunctions for the triplet and Anderson bipolaronic states are found. For the singlet bipolaronic state, the role of different phonon states in lowering the energy is discussed. The nature of polarons with the variation in e-ph coupling strength is studied.

We consider a two-parameter (c¯,c̃) family of quantum integrable isotropic Hamiltonians for a chain of alternating spins of spin s=1/2 and s=1. We determine the thermodynamics for low-temperature T and small external magnetic field H, with T≪H. In the antiferromagnetic (c¯>0,c̃>0) case, the model has two gapless excitations. In particular, for c¯=c̃, the model is conformally invariant and has central charge ${\mathit{c}}_{\mathrm{vir}}$=2. When one of these parameters is zero, the Bethe ansatz equations admit an infinite number of solutions with lowest energy.

Experimental results corresponding to the saturation magnetization and coercive field during the decomposition, upon annealing, of bcc and fcc ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$, obtained by mechanical alloying are reported. The overall behavior points out that the decomposition takes place in two steps: (i) at low temperatures a decrease of the saturation magnetic moment as well as an anomalous thermal dependence of coercive field are observed, however, no phase transformation is detected, and (ii) for further annealing temperatures a new phase appears; the magnetization tends to increase and the coercive field abruptly increases. The analysis of the results leads us to conclude that the first step corresponds to a spinodal decomposition. Fluctuations in the local composition give rise to coexistence of adjacent regions with Curie temperature varying continuously in a range of 1000 K across distances of a few nanometers, thus allowing the tailoring of the magnetic nanostructures.

A formalism developed previously to study the interlayer exchange coupling between ferromagnetic layers separated by a nonmagnetic metal spacer is applied to the case of an insulating spacer. It allows a unified treatment of both cases (metal and nonmetal spacer), provided one introduces the concept of a complex Fermi surface. In contrast to the metal-spacer case, where the exchange coupling decreases with increasing temperature, the coupling across an insulating spacer is found to increase with temperature. This finding is in agreement with recent experimental observations.

The structure function S(k) for the s=1, Haldane-gap antiferromagnetic chain is measured accurately using the recent density-matrix renormalization-group method, with a chain length of 100. Excellent agreement with the nonlinear-σ-model prediction is obtained, both at k≊π, where a single-magnon process dominates, and at k≊0, where a two-magnon process dominates. We repeat our calculation with crystal-field anisotropy chosen to model Ni(${\mathrm{C}}_2$${\mathrm{H}}_8$${\mathrm{N}}_2$${)}_2$${\mathrm{NO}}_2$(${\mathrm{ClO}}_4$), obtaining good agreement with both field-theory predictions and recent experiments. Correlation lengths, gaps, and velocities are determined for both polarizations.

The XY-to-Ising crossover is studied in an anisotropic quantum XY spin chain where a modulating transverse magnetic field introduces a competing periodicity in the system. In the fermion representation, the model describes the perturbed Harper model, exhibiting a critical phase in addition to the extended and the localized phases. The XY-to-Ising crossover results in quadrupling of the butterfly spectrum, which lacks the full symmetry of the Harper butterfly spectrum. However, for infinitesimal field, the Ising butterfly spectrum asymptotically approaches the Harper butterfly spectrum. A duality transformation provides some insight into the various phases of the butterfly spectrum.

We report measurements of the magnetic-field dependence of the resistive transition in ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_{6+\mathrm{\delta }}$ for several crystals with transition temperatures in the range 15 K≤${\mathit{T}}_{\mathit{c}}$≤62 K. Transition of the most overdoped sample (with the lowest transition temperature) shifts monotonically to lower temperature with increasing field with no appreciable change in shape. As reported previously [A. P. Mackenzie et al., Phys. Rev. Lett. 71, 1238 (1993)], the behavior of the upper critical field (${\mathit{H}}_{\mathit{c}2}$) inferred from these measurements does not, however, have the classical BCS form. The samples with higher transition temperatures show a gradual and systematic crossover to field-broadened transitions, with a kneelike structure developing in sufficiently high fields. Some possible interpretations of these observations are discussed.

A technique for determining the electron heat capacity constant γ for metals and superconductors is suggested that also allows one to determine the second and the fourth moments of the phonon density of states g(ω) with high accuracy. The method is based on a high-temperature expansion of a lattice heat capacity. All terms of the expansion are taken into account. From experimental heat-capacity results both the lattice and the electron characteristics of ${\mathrm{TmBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ high-temperature superconducting ceramics have been extracted.

The two- and three-particle contributions to the configurational entropy (${\mathit{S}}_2$ and ${\mathit{S}}_3$, respectively) were calculated by the method of Baranyai and Evans [Phys. Rev. A 42, 849 (1990)] for two different models of a-Si, obtained by reverse Monte Carlo simulations. The first model was the result of an unconstrained calculation, while the second one was obtained by using a constraint requiring 100% fourfold coordination. The pair-correlation functions of the two models were essentially the same, but cosine distributions of bond angles (a projection of the three-body correlation function) were remarkably different. Two-particle contributions to the configurational entropy do not differ for the two models. However, the difference in the three-particle contributions to the configurational entropy is large and the ${\mathit{S}}_3$ of the 100% fourfold coordinated model is much more negative than that of the unconstrained model. This work is an attempt to make a quantitative distinction between structural models with identical pair-correlation functions.

Femtosecond time-resolved spectroscopic studies have been carried out to reveal the relaxation of free electron-hole pairs in KBr. We find that a hole is relaxed to a self-trapped hole with a delay of 10 ps. Two relaxation channels of an electron-hole pair are found: one lasts for 40 ps and leads competitively to a self-trapped exciton (STE) or an F-H pair; the other terminates within 6 ps, bypasses the STE, and leads to an F-H pair. The former channel is ascribed to the interplay between an electron and a self-trapped hole, while the latter demonstrates the dynamic interplay of an electron and a hole during their lattice relaxation.

Geometrical relaxation of the main chain configuration in polydiacetylene (PDA) has been investigated with time-resolved resonance Raman gain spectroscopy with 300-fs resolution. The Raman signal due to a self-trapped exciton in PDA has been observed at 1200 ${\mathrm{cm}}^{\mathrm{-}1}$. The observed Raman signals offer direct evidence of the structure of a self-trapped exciton being a butatrienelike structure after geometrical relaxation from the acetylenelike structure in the free exciton state.

Single (spinor) particle motions in randomly fluctuating U(1) and SU(2) magnetic fluxes in two dimensions are compared. While the problems are similar in the weak-fluctuation limit, strikingly different physical behavior is found in the strong-fluctuation limit. The orbital susceptibility with respect to an external magnetic field vanishes in the strong-coupling U(1) limit, while in the SU(2) case a finite paramagnetic response exists, with an effective coupling constant 2e. Potential applications of the results to spin liquids and other problems are discussed.

A self-consistent theory of both spin and charge fluctuations in the Hubbard model is presented. It is in quantitative agreement with Monte Carlo data at least up to intermediate coupling (U∼8t). It includes both short-wavelength quantum renormalization effects, and long-wavelength thermal fluctuations, which can destroy long-range order in two dimensions. The last effect leads to a small energy scale, as often observed in high-temperature superconductors. The theory is conserving, satisfies the Pauli principle, and includes three-particle correlations necessary to account for the incipient Mott transition.

The magneto-optical activity of superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ observed by Karraï et al. is not present in many commonly employed models of vortex dynamics. Here we propose a simple, unifying picture for the frequency-dependent magneto-optic response of type-II superconductors at low temperatures. We bring together Kohn’s theorem, vortex core excitations, and vortex pinning and damping into a simple expression for the conductivity tensor. The theory describes magneto-optical activity observed in infrared transmission measurements of thin films of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$.