The ground-state properties of electrons in the ${\mathrm{C}}_{60}$ geometry is studied based on the t-J-like Hamiltonian. By using a variational Monte Carlo method, it is found that the Gutzwiller projected Fermi-sea state and the pure RVB state have the lowest variational energies in the undoped case as compared to other spin singlet states. For a doped ${\mathrm{C}}_{60}$ molecule, the electron pairing and itinerant ferromagnetism are discussed. A comparison with related works is also presented.

The temperature and field dependence of the magnetic susceptibility of well-characterized ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ show that the zero-field-cooled (ZFC) diamagnetism below ${\mathit{T}}_{\mathit{c}}$ is irreversible for both systems, while the field-cooled (FC) curve is reversible, and that the ZFC diamagnetism is more than four times larger than the FC. This behavior is attributed to a vortex-glass state below ${\mathit{T}}_{\mathit{c}}$, similar to that observed in high-${\mathit{T}}_{\mathit{c}}$ cuprates as well as disordered conventional type-II superconductors. The field dependence of the vortex glass to vortex fluid transition temperature ${\mathit{T}}^{\mathrm{*}}$ is found to follow the de Almeida–Thouless relation H=${\mathit{H}}_0$[1-${\mathit{T}}^{\mathrm{*}}$(H)/${\mathit{T}}^{\mathrm{*}}$(0)${]}^{\gamma }$, where γ=3/2 for both compounds.

We have studied the half width of the intramolecular vibrational spectrum of the nonadiabatic electron intramolecular vibration (e-MV) coupling origin in doped ${\mathrm{C}}_{60}$. We have also estimated the half width of the adiabatic e-MV coupling origin. We found that along with the two e-MV couplings with ${\mathit{H}}_{\mathit{g}}$ modes, there are some modes with ${\mathit{H}}_{\mathit{u}}$ and ${\mathit{T}}_{1\mathit{u}}$ symmetries that couple with conduction electrons and may have non-negligible phonon widths. This mainly comes from a local symmetry reduction due to the Jahn-Teller distortion. The theoretical results agree with previous experimental results with ${\mathit{H}}_{\mathit{g}}$ modes and may agree qualitatively with recent experimental findings for the ${\mathit{H}}_{\mathit{u}}$ and ${\mathit{T}}_{1\mathit{u}}$ modes. Our e-MV theory based on Jahn-Teller effects explains most of the experimental observations related to the half width of the phonon spectrum in doped ${\mathrm{C}}_{60}$. We predict a doping-induced structure for the highest frequency ${\mathit{H}}_{\mathit{g}}$ mode, which comes from the large coupling constants of both the adiabatic and the nonadiabatic e-MV mode coupling.

It is shown that the linear decrease with increasing temperature of the fractional variation of the velocity of sound, or velocity shift, δv/${\mathit{v}}_0$ observed at intermediate low temperatures in glasses and disordered solids (Bellessa effect) can be reproduced by assuming anharmonic coupling between the sound waves and thermally excited low-energy vibrations. The values of the effective Grüneisen parameters required to fit the published velocity-shift data of polystyrene, PMMA, Se, NiP, and a sodium silicate glass are found to be physically reasonable.

The room-temperature reflectance (100–1000 ${\mathrm{cm}}^{\mathrm{-}1}$) of the rare-earth titanites (R${\mathrm{TiO}}_3$ R=La,Ce,Pr,Nd,Sm,Gd) has been measured. The spectrum for ${\mathrm{LaTiO}}_3$, ${\mathrm{CeTiO}}_3$, and ${\mathrm{PrTiO}}_3$ consists of the three main perovskite vibrational bands. The central band, near 340 ${\mathrm{cm}}^{\mathrm{-}1}$, consists of several barely resolved modes which begin to separate in ${\mathrm{NdTiO}}_3$ and ${\mathrm{SmTiO}}_3$ and which split into two clusters of modes in ${\mathrm{GdTiO}}_3$. These features are correlated with an increasing tetragonal distortion of the ${\mathrm{TiO}}_6$ octahedron as the rare-earth ionic size decreases. Using the LO-TO splitting, the effective ionic charges are estimated and the R${\mathrm{TiO}}_3$ perovskites are shown to be less ionic than ${\mathrm{SrTiO}}_3$ or ${\mathrm{BaTiO}}_3$.

We analyze here the magnetic susceptibility of hopping electrons in an amorphous semiconductor in the presence of a large electric field. We find that the susceptibility should be Curie-like and scale as 1/E in the large electric field, low-temperature limit. The pure Pauli susceptibility is shown to be unchanged by the application of an electric field. Our approach is based on the effective temperature treatment of hopping conduction in amorphous semiconductors by Marianer and Shklovskii. Experimental realizations of this effect are discussed.

Lee [Phys. Rev. B 47, 8293 (1993)] has shown that the odd-numbered derivatives of the Kubo autocorrelation function vanish at t=0. We show that this condition is based on a more general property of nondiagonal Kubo correlation functions. This general property provides that certain functional forms (e.g., simple exponential decay) are not admissible for any symmetric or antisymmetric Kubo correlation function in a Hermitian many-body system. Lee’s result emerges as a special case of this result. Applications to translationally invariant systems and systems with rotational symmetries are also demonstrated.

A neutron-diffraction experiment on a partially ordered ${\mathrm{Ni}}_{78}$${\mathrm{Mn}}_{22}$ sample shows a temperature dependence of the average magnetic moment of the Mn atoms in the Ni site reminiscent of a critical phenomenon. This effect has been attributed to the change with temperature of the number of Mn atoms antiferromagnetically aligned with the bulk magnetization. The possibility of a correlation between the observed phenomenon and the ferromagnetic ordering transition in fully disordered samples is also proposed.

We calculate the order parameter ${\mathit{m}}_{\mathit{e}}$ and the root-mean-square order parameter ${\mathit{m}}_{\mathrm{rms}}$ of an exactly soluble model of a structural phase transition. In this model classical and quantum critical fluctuations depending on the temperature T and the dimension of the space d are present. For the ratio r(d,T)=${\mathit{m}}_{\mathit{e}}$/${\mathit{m}}_{\mathrm{rms}}$, we obtain r(d,T)= √n . This result does not depend on the quantum and/or classical fluctuations.

The critical behavior of a ferromagnet with dominant exchange interaction, dipolar interaction, and uniaxial anisotropy is studied to one-loop order of renormalized field theory. The analytical calculation of the effective exponent ${\gamma }_{\mathit{e}\mathit{f}\mathit{f}}$ for the susceptibility makes it possible to characterize quantitatively the crossover phenomena for arbitrary magnitudes of the competing effects. A type of logarithmic correction to the asymptotic γ depending on the ratio of dipolar versus anisotropy strength is presented. Recent experiments on Gd are discussed in view of the obtained results.

Results of positron lifetime measurements as a function of temperature from 80 to 300 K for the F-doped Bi(Pb)-2:2:2:3 superconductor (${\mathit{T}}_{\mathit{c}}$=118 K) are presented. The lifetime ${\mathrm{\tau }}_1$ is about 200 ps and nearly temperature independent. The lifetime ${\mathrm{\tau }}_2$ fluctuates around 310 ps but its intensity ${\mathit{I}}_2$ increases slightly and the mean lifetime ${\mathrm{\tau }}_{\mathit{m}}$ increases continuously from 212 to 217 ps as the temperature is increased from 80 to 300 K. We discussed the possible effect of fluorine doping on the structural properties in this system and interpreted the temperature dependence of ${\mathrm{\tau }}_{\mathit{m}}$ in terms of a simple thermal expansion. An average linear expansion coefficient α∼35×${10}^{\mathrm{-}6}$ for this system is obtained.

The dynamics of solitons on saturated films of superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is studied under the condition of an overtaking collision of the solitons. It is shown that under second order in the nonlinearity, the effect of the collision is elastic. An explicit expression for the solitary solution with appropriate correction for the amplitude dependence of wave speed is obtained.

Based on extensions of the grand-canonical quantum Monte Carlo algorithm to incorporate magnetic fields, we provide numerical data confirming the existence of a Kosterlitz-Thouless transition in the attractive Hubbard model. Here, we calculate the temperature derivative of the superfluid density, [∂β${\mathit{D}}_{\mathit{s}}$(β)]/∂β, to pin down the transition. The latter quantity is obtained from the difference in internal energy of systems which differ by a phase twist π/2 in the boundary condition along one lattice direction. Away from half-band filling, [∂β${\mathit{D}}_{\mathit{s}}$(β)]/∂β shows a response which increases with lattice size at the transition temperature. In contrast, such a signal is not observed at half-band filling.

The surface impedance of type-II superconductors in the mixed state has been measured as a function of the magnetic field in the low-frequency limit. At the low magnetic fields close to the lower critical field the surface impedance was observed to be proportional to the vortex density, whereas at higher fields a crossover to the square-root dependence on the vortex density was observed. This behavior agrees with the two-mode electrodynamics in which the penetration of electromagnetic fields into the superconductor in the mixed state is governed by two space scales: (i) the skin-layer width, and (ii) the screening length of the order of the intervortex distance arising from the elasticity of the vortex array.

We present a static pair-interaction potential in a layered system with two-dimensional electron sheets. The potential is derived for arbitrary locations of the interacting charges in such a lattice. The local-field factor for a layered electron liquid is evaluated. It is shown that electron correlations in this anisotropic structure lead to ‘‘overscreening’’ of the Coulomb interaction and hence to a negative potential between two like charges within a certain range of intercharge distances.

The observed magnetic spin susceptibility of high-temperature superconductors such as ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ increases when x increases from zero, i.e., as one dopes away from half-filling. Recent Monte Carlo simulations of A. Moreo [Phys. Rev. B 48, 3380 (1993)] suggest that this behavior can be reproduced by the two-dimensional Hubbard model only at large coupling, namely, U/t of order 10. Using longer runs, our Monte Carlo simulations show that the same behavior as for U/t=10 is obtained even in the intermediate coupling regime (U/t=4), as long as the temperature is low enough (T=t/6) that strong antiferromagnetic correlations are building up at half-filling. These results are consistent with the fact that in two dimensions, the generalized random phase approximation should fail in the parameter range where it predicts a magnetic phase transition.

Chaotic electron dynamics for the Azbel’-Kaner effect are considered. It is shown that electron dynamics can be described by a standard map and, hence, chaotic motion appears for certain conditions. The dependence of the surface impedance on the magnetic field is obtained analytically in the limiting case where the stochasticity parameter is large. It is shown that the dependence is not resonant.

The emission spectra of highly excited one-dimensional Frenkel excitons are obtained from the correlation functions of a quantum spin chain. The system-size dependence of the emission intensity reflects critical exponents of the spin system that also characterize statistical properties of Frenkel excitons intermediate between bosons and fermions. Exact expressions for these spectra show blue shifts of emission peaks and fine structure due to finite-size effects.

We present results of self-consistent linear-muffin-tin-orbital calculations with the atomic-sphere approximation for hcp bulk Gd, using the local spin-density approximation (LSDA) and gradient correction (GC) for the description of exchange and correlation. In the LSDA calculation antiferromagnetic order is favored over the ferromagnetic, and experimentally observed, structure. The GC weakens the bonding, leading to a higher equilibrium lattice parameter. At the new equilibrium volume the ground state is ferromagnetic. Our results point towards a magnetic phase transition under pressure.

Raman spectra of ${\mathrm{NiPS}}_3$ (spin S=1) reveal an anomalously broad two-magnon feature whose relative width is comparable to that shown by ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ (S=1/2) and other antiferromagnetic copper oxides. The magnetic continuum is strongly coupled to a nearby phonon, whose scattering amplitude interferes with it. These results question the view that the width enhancement in the cuprates stems from large quantum fluctuations intrinsic to S=1/2 systems. It is proposed that the anomaly is due instead to magnon decay involving strong magnetostrictive coupling to phonons.