Tunable ultraviolet laser light has been used to probe the electronic band structure of the charge-transfer insulator ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$. A 1.56-eV Raman-active excitation is observed for incident photon energy >3.5 eV. The excitation does not present clear selection rules in the low-energy wing of this resonance from 3.4 to 3.6 eV. We interpret the resonance and excitation as originating from in-plane Cu-O transitions. The excitation may be a crystal-field-like transition within the Cu d shell, a singlet two-hole state, or an excitonic state localized to a filled Cu 3${\mathit{d}}^{10}$ site.

We report a study of implantation and optical spectroscopy of foreign atoms in a solid-helium matrix. With a sample of more than ${10}^9$ Ba atoms in 1 ${\mathrm{cm}}^3$, a storage time in excess of 5 hours has been demonstrated. Laser-induced fluorescence studies favor the assumption that the isotropy of the trapping site for the implanted atoms is preserved during the liquid-to-solid phase transition.

By ${\mathit{D}}_1$-line optical pumping of alkali-metal atoms (Cs and Rb) in superfluid helium (He II), we could achieve large spin polarization of the atoms in the ground states, more than 50% for both electron and nuclear spins, especially for the Cs atoms. The alkali-metal atoms were implanted into He II by laser sputtering. We detected optically the magnetic resonances between the ground-state Zeeman sublevels, and found that the g values and hyperfine constants in He II were the same as in vacuum within the experimental error. We discuss possible applications of these spin-polarized atoms in He II.

An unconventional microscopic mechanism relating ${\mathit{T}}_{\mathit{c}}$ and the isotope substitution for the doped superconductors such as the high-${\mathit{T}}_{\mathit{c}}$ oxides is proposed. Strong nonadiabaticity, when it is impossible, strictly speaking, to separate fully the nuclear and electronic degrees of freedom, leads to a peculiar dependence of the carrier concentration n on the ionic mass M. This case corresponds, for example, to the isotopic substitution of the axial oxygen in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$. Because of the dependence of ${\mathit{T}}_{\mathit{c}}$ on n, this leads to the dependence of ${\mathit{T}}_{\mathit{c}}$ on M, that is to the isotope effect. The minimum value of the isotope coefficient corresponds to ${\mathit{T}}_{\mathit{c}}$=${\mathit{T}}_{\mathit{c}}^{\max }$.

Recently, several kinds of photoinduced effects on superconducting properties of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ at low temperatures have been reported. Particularly, in the sample at the boundary of a superconducting-semiconducting transition, a large decrease in the resistivity after laser irradiation has been observed at low temperatures. The decreased resistivity was recovered by room-temperature annealing. To clarify the mechanism of these effects, we have investigated the hysteretic behavior of resistivity near room temperature and the photoirradiation effect on the x-ray-diffraction pattern at low temperatures. We explain this effect as a photostructural change involving oxygen rearrangement in the Cu-O basal planes induced by irradiation at low temperatures. This is similar to the mechanism whereby a recombination-enhanced defect reaction is observed in ionic crystals and semiconductors.

The differential resistance in an InAs-inserted-channel ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As/${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As heterostructure-coupled superconducting junction is enhanced within the Nb superconducting gap energy by applying a magnetic field. However, the enhancement saturates under a magnetic field of several mT. This behavior suggests the reduction of Andreev-reflection (AR) probabilities by the magnetic field. The AR probabilities are calculated as a function of the pair-potential penetration length into the InAs channel. The enhanced AR probabilities decrease with decreasing pair-potential penetration length. The magnetic field reduces the penetration of the pair potential in the channel and therefore reduces the AR probability. This reduction of AR probability causes an enhancement of dV/dI.

A square-lattice gauge-theory model for spin-charge-separated superconductivity is introduced. The phases of the spinon and holon order parameters couple gauge invariantly via a statistical gauge field, which arises from the implicit strong interaction physics. We find that physical electronic phase correlations show a superconductor–normal-metal phase transition of the Berezinskii-Kosterlitz-Thouless type, while statistical gauge-field excitations are strictly gapless. These results are discussed in the context of both the superconducting and the normal-state properties of the oxide superconductors.

We present data on the Hall conductivity ${\mathrm{\sigma }}_{\mathit{x}\mathit{y}}$ in ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ thin films. It has been found that the Hall conductivity consists of two terms. The first one is inversely proportional to the magnetic field strength, in agreement with theories for motion of a single vortex. This term changes sign from negative near ${\mathit{T}}_{\mathit{c}}$ to a positive value at low temperatures. The second term is field independent and has been attributed to be due to the normal Hall effect in the vortex cores.

Coexisting electronic correlations and electron-phonon interactions have been proposed to lead to dynamic polaron-tunneling behavior for the axial oxygen in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. Emphasizing nonlinear and nonadiabatic effects, we show that this description leads to distinctive predictions consistent with structural, optical, and inelastic neutron-scattering data in the axial direction.

We report on the photoinduced enhancement of the critical temperature δ${\mathit{T}}_{\mathit{c}}$ and the in-plane conductivity δ${\mathrm{\sigma }}_{\mathit{a}\mathit{b}}$ measured in a c-axis-oriented ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ film as a function of oxygen concentration (6.4≤x≤7). Both δ${\mathit{T}}_{\mathit{c}}$ and δ${\mathrm{\sigma }}_{\mathit{a}\mathit{b}}$ decrease with the oxygen content, display a plateaulike behavior for 6.6≤x≤6.75, and become zero at x=7. This dependence of δ${\mathit{T}}_{\mathit{c}}$ on x is similar to that observed for the ${\mathit{T}}_{\mathit{c}}$ enhancement due to ordering effects in oxygen-deficient ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ single crystals. We propose that the persistent photoinduced changes in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ films are due to photoassisted oxygen ordering.

We have measured 1/f resistance fluctuations at room temperature in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ films as a function of oxygen content, x. The films were single phase, c-axis oriented and grown by pulsed organometallic-beam epitaxy on ${\mathrm{LaAlO}}_3$ substrates. In contrast to the monotonic increase of the film resistance with decreasing x, the normalized noise power was found to exhibit a sharp minimum at x=6.5. We attribute this behavior to vacancy creation in the Cu-O chains and discuss a simple model based on parallel conduction paths through both chains and planes.

We report on point-contact tunneling into polycrystalline ${\mathrm{HgBa}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$ superconductors with a ${\mathit{T}}_{\mathit{c}}$ onset of 97 K. Tunneling data follow the general trend of the best curves seen in cuprate high-temperature superconductors, but these show significantly lower and flatter subgap conductance values than any other cuprate and they can be reproducibly obtained. By fitting the data to a smeared BCS density of states, the energy gap has been determined and the smearing parameter is so small that it is comparable to thermal smearing.

It is pointed out that broadening of the Raman-active ${\mathit{H}}_{\mathit{g}}$-derived phonons in the fulleride superconductors is due to interband electron-phonon interaction and that orientational disorder can render these phonons infrared active. The long-wavelength ${\mathit{A}}_{\mathit{g}}$ phonons are left completely unrenormalized by the electron-phonon interaction.

The x-ray-absorption near-edge structure (XANES) spectra of single-crystal alkali halide salts have been measured at low temperature (T≊80 K) with high resolution in energy. By employing the electron partial-yield detection technique, spectra of NaF, NaCl, and NaBr were obtained near the sodium K edge and spectra of LiF, NaF, and KF were obtained near the fluorine K edge. All spectra showed sharp absorption features at the absorption threshold and broader features extending 50–80 eV above threshold. Some narrow features close to threshold are attributed to core-level excitons. Contributions from atomic multiple-electron excitations, estimated by a comparison to the K-edge photoabsorption spectrum of Ne in the gas phase, are found to be very small. Thus, the intense broader peaks observed further above threshold arise from single-electron scattering resonances. These highly structured spectra of well-characterized ordered crystals provide a set of standard data which is used to test the multiple-scattering x-ray-absorption code feff v. For some spectra, at photon energies more than ≊10 eV above threshold, the calculations achieve very good agreement with experiment. The dependence of the calculation on cluster size, order of scattering, and choice of self-energy is demonstrated.

L-edge x-ray-absorption near-edge structure (XANES) has been used to investigate the charge distribution in the intermetallic compound AuZn. It is found that the Au ${\mathit{L}}_{3,2}$-edge XANES of AuZn exhibits a noticeable white line at the absorption threshold indicating 5d electron depletion at the Au site in AuZn. Analysis of the area under the white line yields a depletion of 0.052 5d electron at the Au site in AuZn relative to Au metal, in qualitative agreement with the charge compensation model analysis using x-ray photoelectron spectroscopy and Mössbauer data. The latter shows a 5d depletion count of 0.14 electron. The implication of these analyses is discussed.

This paper presents various closed-form analytic formulas that relate the group velocity of elastic pulses propagating in an arbitrary direction of the symmetry planes of elastic media with orthorhombic or higher symmetry to their elastic constants. Simple equations relating the direction of a group velocity to that of the corresponding wave normal are derived for both quasilongitudinal and quasitransverse modes. A forward solution to obtaining the elastic constants from the group-velocity data on the symmetry planes by using these relations is illustrated with examples of transversely isotropic zinc and cubic silicon crystals. Both numerically simulated and experimental data are used to check the derived relations and to demonstrate their usefulness.

Optical-reflectivity and x-ray-diffraction techniques have been used to determine the electronic properties and structure of iodine to 181 and 276 GPa, respectively, in the diamond-anvil cell. X-ray measurements were performed by energy dispersive diffraction with synchrotron radiation. Monatomic metallic iodine transforms to an fcc structure at 53 GPa that remains stable to the maximum pressure. At 276 GPa, the volume compression V/${\mathit{V}}_0$ is 0.324. Reflectivity data are consistent with a strong metallic character in iodine from 19 to 181 GPa. The linear muffin-tin orbital method has been used to calculate the equation of state to 500 GPa and to model the reflectivity of iodine at high pressures.

The infrared-active (${\mathit{A}}_{2\mathit{u}}$) O-H vibration of Co(OH${)}_2$ decreases in frequency under hydrostatic compression to 51 GPa at 290 K. Similarly, the bond anharmonicity, determined from the ${\nu }_1$→${\nu }_2$ absorption-band difference, increases by more than a factor of 2 between 0 and 20 GPa. Both changes are attributed to an increase in the O-H bond length due to enhanced hydrogen bonding under pressure. The full width at half maximum (FWHM) of the fundamental absorption band increases abruptly by ∼100 ${\mathrm{cm}}^{\mathrm{-}1}$ at 11.2 (±0.3) GPa, and continues to increase at a rate of ∼3.3 ${\mathrm{cm}}^{\mathrm{-}1}$/GPa up to 36 GPa. Above 36 (±2) GPa and below the onset of amorphization, the FWHM changes at a slower rate, 0.8 (±0.1) ${\mathrm{cm}}^{\mathrm{-}1}$/GPa. The abrupt change in FWHM is reversible on decompression, and is interpreted in terms of a pressure-induced crystal-to-glass transition exhibiting a small hysteresis compared to similar compounds. The rapid variation in FWHM above the transition pressure suggests that the amorphous structure is continuously modified between 11.3 and 36 GPa.

A neutron elastic- and inelastic-scattering study has been performed on single crystals of metallic sodium over a temperature range of 10 to 300 K. On cooling the virgin sample, the incubation time to transform from the bcc structure to the low-temperature structure was found to be more than 2 h at 38 K. The full width at half maximum (FWHM) of the (110${)}_{\mathrm{bcc}}$ Bragg reflection suddenly increased from 0.3° to 0.73°, due to some deformation introduced by the nucleation of the low-temperature structure. In relation to the deformation, strong extra-diffuse scattering was observed around the (110${)}_{\mathrm{bcc}}$ Bragg reflection in addition to thermal diffuse scattering. On warming through the transformation, the FWHM gradually recovered but was larger than that of the virgin state. Apparently some deformation still remained in the bcc structure even though the low-temperature structure disappeared. It was estimated that the cluster size of the low-temperature phase is more than 3.2 nm along the c axis from analyzing the Huang-type diffuse scattering. On cooling again, the phase transition did not take place until 10 K and a waiting time of at least 20 min at that temperature was needed before the transformation took place. No drastic change in the ${\mathrm{TA}}_1$[110] phonon-dispersion branch down to 38 K was observed, though the phonon energies for q≥0.25 decreased slightly with decreasing temperature.