From the experimentally obtained H(T) dependence of the transition boundary between short-range and long-range vortex order for ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ superconductors, the temperature dependences of the effective potential barrier for pinning of a single vortex on a pinning center and the effective pinning force are calculated. It is shown that the effective barrier falls almost linearly with the increase of temperature and that the harmonic thermal fluctuations strongly decrease the effective pinning force as ${\mathit{T}}^{3/2}$.

We prove numerically that the saturated ferromagnetic state in the Hubbard model with infinitely large Coulomb repulsion and one hole (Nagaoka state) is unstable for a fullerene cluster if the hopping integral is negative, and possibly stable if possible. We analyze the stability versus the variation of the ratio between the hopping values for the two types of carbon-carbon bonds. The results suggest the existence of ferromagnetism in doped fullerene when the electronic correlation leads to a large effective electron-electron interaction.

The collective pinning of vortices in very thin amorphous superconducting films is experimentally investigated by a systematic study of the nonlinear electrical response of ${\mathrm{Mo}}_{\mathit{x}}$${\mathrm{Ge}}_{1\mathrm{-}\mathit{x}}$ (x=79%) alloy films of various thickness measured in perpendicular magnetic field. In the thickest of these films, the standard collective pinning analysis yields an unusual field dependence of the pinning force, suggesting that more than one type of pinning center may play a substantial role. We then examine the field dependence of the pinning forces in progressively thinner samples. In the thinnest films the influence of the additional pinning centers in this system appears to be less pronounced than in thicker films, although the interplay between changes in the microscopic pinning and the breakdown of the standard collective pinning picture remains to be clarified.

Raman-scattering studies of a high-quality ultrathin-layer (${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$${)}_4$/(${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$${)}_1$ superconducting superlattice resolve, in addition to the combined ${\mathit{B}}_{1\mathit{u}}$ mode at 310 ${\mathrm{cm}}^{\mathrm{-}1}$ previously reported for a 2:1 sample, a peak at 350 ${\mathrm{cm}}^{\mathrm{-}1}$ at low temperature. This peak and also others have been interpreted by a lattice-dynamical calculation for the superlattice. The results are compared with confined or extended phonon pictures. The superlattice mode is not renormalized below the superconducting transition temperature.

We investigate the decay of a phonon into single-electron excitations in the cuprate superconductors. In a clean crystal the screening of the longitudinal phonon field cancels singularity of the Landau damping threshold and makes it experimentally unobservable. In a dirty metal the phase volume of the electrons involved in damping is small, which reduces the probability of the phonon decay. However, in this case the observation of dependence between the phonon linewidth and light penetration depth is more favorable than in the clean metal because the damping has no threshold.

The thermopower and resistivity of bulk superconducting ${\mathrm{HgBa}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ (Hg-1223) have been measured for 15 samples (prepared from two batches) subjected to various annealing treatments in oxygen or nitrogen; all but two samples had values of zero-resistance ${\mathit{T}}_{\mathit{c}}$ of approximately 134 K. In the normal state, the thermopower of the annealed samples shows a similar shape as a function of temperature: it decreases as temperature increases, with the rate of decrease slowing down at higher temperatures. The thermopower remains positive up to room temperature in all of our samples despite the high ${\mathit{T}}_{\mathit{c}}$ values and annealing in oxygen, suggesting that the samples remain slightly underdoped.

The 1/f resistance noise in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals has been measured in both the a-b plane and the c direction from room temperature to the superconducting transition. Evidence that 1/f noise in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is thermally activated was obtained for both directions. We propose oxygen sites as possible fluctuation centers.

The k-space structure and symmetry of the superconducting gap arising from an interlayer tunneling mechanism are discussed. The details of the k dependence of the amplitude of the gap is essentially determined by the k dependence of the interlayer coupling. The symmetry of the gap is, however, completely determined by the symmetry structure of the intralayer part of the pairing kernel.

Persistent photoconductivity, characterized by a long-term increase in the conductivity due to illumination of oxygen-deficient high-temperature superconductors, is greatly enhanced in R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ (R=Y or Gd) films as x decreases to 6.1. These results are in qualitative agreement with a model similar to that used to explain the existence of DX centers in GaAs. However, they cast doubts over several models commonly used to explain the mechanism behind this phenomenon and that of photoinduced superconductivity.

We have measured the Raman scattering from three single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ at high, x→0, intermediate, x≊0.5, and low doping level, x≊0.15. Both fully symmetrical diagonal and off-diagonal phonons without any disorder induced modes have been identified for x→0, whereas only diagonal scattering showed substantial strength in the other two crystals. The high frequency of the apical oxygen phonon, reaching 517 ${\mathrm{cm}}^{\mathrm{-}1}$ at room temperature for x→0, has been found to be a hallmark of the crystals grown by a pulling technique.

A wide variety of experimental conductance spectra in high-${\mathit{T}}_{\mathit{c}}$ superconductors can be explained by a theory of proximity-electron-tunneling spectroscopy (PETS) for d-wave superconductors. From a solution of the Bogoliubov–de Gennes equation, the conductance spectra of the PETS are proved to show gap or zero-bias peak structures depending on the axis orientation of the superconductor. The features of the calculated spectra are consistent with the experimental results of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ observed by low-temperature scanning tunneling spectroscopy.

The ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ nuclear-quadrupole-resonance (NQR) spin-lattice relaxation rate in incommensurate bis(4-chlorophenyl)sulfone (BCPS) is determined by Raman processes below 60 K and by direct one-phonon processes above 60 K. The variation of the ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ spin-lattice relaxation rate over the incommensurate NQR frequency distribution in BCPS has been also measured at various temperatures. The results are explained in terms of phason and amplitudon contributions to the effective spin-lattice relaxation rate. In contrast to ${\mathrm{Rb}}_2$${\mathrm{ZnCl}}_4$-type crystals the changes in the Cl electric-field-gradient tensor produced by modulation wave fluctuations contain both first- and second-derivative terms.

We study the orientational ordering of the fullerene molecules in solid ${\mathrm{C}}_{60}$ with a phenomenological model proposed by Lu, Li, and Martin [Phys. Rev. Lett. 68, 1551 (1992)]. It is shown that the energy of a particular configuration is sensitive to the bond length of the fullerene molecule and there is an orientation that might explain the orientational disorders in solid ${\mathrm{C}}_{60}$. Also, the experimentally observed superstructure is investigated. Numerical calculations show that the orientational ordering of the fullerene molecules will induce structural deformation in the superstructure.

The energy gap due to anion ordering introduces essential differences for electrons of two particular subbands in a magnetic field. The magnetic breakdown restores the behavior of the prototype ${\mathrm{PF}}_6$ in an irregular manner as the field increases. The fast oscillations (periodic in 1/H) appear to be due to the fact that the electron trajectories in ${\mathrm{ClO}}_4$ coincide with those in ${\mathrm{PF}}_6$ only at specific values of H. The fast oscillations superimpose on the cyclotron-resonance-type singularities.

We have measured the electrodynamic response of TDAE-${\mathrm{C}}_{60}$ [where TDAE is tetrakis(dimethylamino) ethylene], which undergoes a ferromagnetic phase transition at ${\mathit{T}}_{\mathit{c}}$=16 K. The optical conductivity does not display a clear metallic component, which indicates that TDAE-${\mathrm{C}}_{60}$ is not an itinerant ferromagnet. Moreover, the optical response is temperature independent and thus unaffected by the ferromagnetic phase transition at ${\mathit{T}}_{\mathit{c}}$. We also found several absorption peaks in the far-infrared spectral range, which are associated with the molecular phonon-mode spectrum of ${\mathrm{C}}_{60}$ and TDAE.

Using our previously developed method for calculating parameter-free potential-energy surfaces for ionic molecular crystals, specifically sulfates, we study phase transitions in ${\mathrm{RbLiSO}}_4$ by means of lattice and molecular dynamics. We found that the high-temperature phase I (Pnam) is highly unstable and transforms to the observed lower temperature phase VI (P${112}_1$/n) at about 475–525 K. Compared with isomorphous ${\mathrm{CsLiSO}}_4$, there are more branches of unstable modes in the Pnam phase for ${\mathrm{RbLiSO}}_4$. The maximum instability of these modes occurs away from the zone center, q=(0.118${\mathbf{a}}^{\mathrm{*}}$,0,0), which implies that a high-order incommensurate phase could form during the phase transformation. The driving mechanism of these phase transitions is directly related to the rotational ordering of tetrahedral ${\mathrm{SO}}_4^{2\mathrm{-}}$ groups. The rms values of the deviations of the S-O bonds from their orientations in the Pnam phase to those in the P${112}_1$/n phase were found to be approximately ±23–25°.

High-resolution x-ray fluorescence spectroscopy has been performed on magnetically oriented polycrystalline iron samples. Circularly polarized synchrotron radiation was used to excite the Fe ${\mathit{L}}_{2,3}$ x-ray emission, either directly from a bending magnet (white radiation), or from an asymmetric wiggler followed by a monochromator tuned to the ${\mathit{L}}_2$ edge. Large differences in the magnetic circular dichroism (MCD) were observed. Spin-polarized relativistic linear muffin-tin orbital calculations and a simple qualitative model demonstrate that the ${\mathit{L}}_3$ x-ray emission dichroism is partially canceled by the ${\mathit{L}}_2$-${\mathit{L}}_3$${\mathit{M}}_{4,5}$ Coster-Kronig process. The Coster-Kronig process is large enough to make MCD x-ray absorption experiments, performed in the fluorescence mode, unreliable.

Interference effects are confirmed on the critical current of a gate-fitted superconductor–normal-metal–superconductor junction in the clean limit. As the normal metal, the junction uses a two-dimensional electron gas (2DEG) with a high mobility of 7.38 ${\mathrm{m}}^2$/Vs and a high carrier density of 1.98×${10}^{12}$ ${\mathrm{cm}}^{\mathrm{-}2}$ at 4.2 K. The superconducting critical current that flows through the 2DEG is measured as a function of the gate voltage and shows oscillations as a function of the 2DEG carrier concentration. This oscillation is explained by the interference effects predicted by Chrestin et al. The typical period of the oscillation agrees well with the theoretical prediction.

We report measurements of the critical fields of ultrathin granular Al films as a function of temperature and magnetic-field orientation. Due to the spin-paramagnetic effect, the parallel field (θ=0°) transition becomes first-order and strongly hysteretic below a tricritical point, ${\mathit{T}}_{\mathrm{tri}}$∼270 mK. Well below ${\mathit{T}}_{\mathrm{tri}}$, the width of the hysteresis collapses according to ΔH(θ)=ΔH(0°)exp(-θ/2.4°) with increasing θ. Our data suggest that the zero-temperature transition goes from first order to second order at θ≊6.5°.

Some characteristic anomalies above the gap reported by tunneling and photoemission spectroscopies on ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ are reproduced numerically and analyzed in the context of conventional strong-coupling theory of superconductivity. The anomalous diplike structure above the gap is evidence of the breakdown of the Landau Fermi-liquid picture because of the strong electron-boson coupling. The second peak or broad band at higher energies is due to the existence of almost localized pairs, indicating that the system is at the beginning of a crossover from BCS superconductivity to Bose condensation.