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In this work, we examine the lattice behavior of the economically interesting superhard material, tungsten tetraboride (${\mathrm{WB}}_{4}$), in a diamond anvil cell under nonhydrostatic compression up to 48.5 GPa. From the measurements of lattice-supported differential stress, significant strength anisotropy is observed in ${\mathrm{WB}}_{4}$. The (002) planes are found to support the highest differential stress of 19.7 GPa within the applied pressure range. This result is in contrast to ${\mathrm{ReB}}_{2}$, one of the hardest transition metal borides known to date, where the same planes support the least differential stress. A discontinuous change in the slope of c/a ratio is seen at 15 GPa, suggesting a structural phase transition that has also been observed under hydrostatic compression. Speculations on the possible relationship between the observed structural changes, the strength anisotropy, and the orientation of boron-boron bonds along the $c$ direction within the ${\mathrm{WB}}_{4}$ structure are included.

The low-temperature dc conductivity and magnetoconductivity of ion-implanted (${\mathrm{Ar}}^{+}$) and chemically doped (${\mathrm{H}}_{2}$${\mathrm{SO}}_{4}$) polyaniline films have been studied. The metal-insulator transition has been observed for ion-implanted polyaniline films on increasing the irradiation dose to 3\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ions ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The maximum values of the room-temperature conductivity reached 800 S ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for ion-implanted and 8 S ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for chemically doped polyaniline films. In both cases, for samples on the insulator side of the metal-insulator transition, \ensuremath{\sigma}(T)=\ensuremath{\sigma}(0)exp[-(${\mathit{T}}_{0}$/T${)}^{\mathit{m}}$], where m\ensuremath{\sim}0.5, whereas for the most heavily ion-implanted polyaniline films \ensuremath{\sigma}(T)\ensuremath{\sim}T at Tg20 K; the minimum in the \ensuremath{\sigma}(T) occurs at T\ensuremath{\sim}20 K and a negative magnetoconductance \ensuremath{\Delta}\ensuremath{\sigma}(H,T)\ensuremath{\sim}${\mathit{H}}^{2}$ has been observed. It is shown that electron-electron Coulomb interactions play an important role in charge-carrier transport in ion-implanted polyaniline films near the metal-insulator transition. \textcopyright{} 1996 The American Physical Society.

In this work, we examine the high-pressure behavior of superhard material candidate WB${}_{4}$ using high-pressure synchrotron x-ray diffraction in a diamond anvil cell up to 58.4 GPa. The zero-pressure bulk modulus, ${K}_{0}$, obtained from fitting the pressure-volume data using the second-order Birch-Murnaghan equation of state is 326 $\ifmmode\pm\else\textpm\fi{}$ 3 GPa. A reversible, discontinuous change in slope in the $c/a$ ratio is further observed at \ensuremath{\sim}42 GPa, suggesting that lattice softening occurs in the $c$ direction above this pressure. This softening is not observed in other superhard transition metal borides such as ReB${}_{2}$ compressed to similar pressures. Speculation on the possible relationship between this softening and the orientation of boron-boron bonds in the $c$ direction in the WB${}_{4}$ structure is included. Finally, the shear and Young's modulus values are calculated using an isotropic model based on the measured bulk modulus and an estimated Poisson's ratio for WB${}_{4}$.

We have measured the optical reflectivity over a broad spectral range of single-phase ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ compounds, and have evaluated the optical conductivity both below and above the superconducting transition temperature. We identify various features in the excitation spectrum of the normal state: a Drude contribution at low frequency, a midinfrared absorption, and several interband transitions. In the superconducting state our results are in full agreement with a BCS singlet ground state, with the measured gap values consistent with the weak-coupling limit. We also evaluate several intrinsic parameters characterizing both the normal and superconducting state, such as the plasma frequency, the relaxation scattering rate, and the mean free path, besides the superconducting gaps and the penetration depth. We compare these quantities with similar ones from other experiments and find satisfactory agreement. Moreover, we present our calculation of the electrodynamic response above and below ${\mathit{T}}_{\mathit{c}}$ with the standard Eliashberg electron-phonon theory of superconductivity, which strongly supports a pairing mechanism mediated by high-frequency intramolecular phonon modes.

The onset of the upper critical field ${\mathit{B}}_{\mathit{c}2}$(T) measured up to 60 T in powdered single-phase ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ yields ${\mathit{T}}_{\mathit{c}}$=19.5 and 28.4 K, ${\mathit{B}}_{\mathit{c}2}$(4.2 K)=30 and 55 T, (${\mathit{dB}}_{\mathit{c}2}$/dT${)}_{\mathit{T}=\mathit{T}\mathit{c}}$=-2.80 and -3.85 T/K, and Ginzburg-Landau coherence lengths at 0 K of 31 and 24 \AA{}, respectively. ${\mathit{B}}_{\mathit{c}2}$(T) appears to be that of a ``conventional'' superconductor showing paramagnetic limiting. These measurements are compared with earlier data and qualitative and quantitative differences are discussed.

We have measured the optical reflectivity of single phase K 3 C 60 and Rb 3 C 60 compounds, and have evaluated the optical conductivity both below and above the superconducting transition temperature. Our results are in full agreement with a BCS singlet ground state, with the measured gap values consistent with the weak-coupling limit

The electrodynamic response of ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ was measured over a broad spectral range. We identify various features in the excitation spectrum of the normal state: a Drude contribution, a mid-infrared absorption, and several interband transitions.

Formation of alkali-${\mathrm{C}}_{60}$ compounds ${\mathit{A}}_{\mathit{n}}$${\mathrm{C}}_{60}$(A=K, Rb) induces very large enhancements in the optical strength of two intra-${\mathrm{C}}_{60}$ vibrational resonances and the emergence of a strong electronic transition centered at 1.2 eV. The vibrational spectra are particularly well defined for the ${\mathit{A}}_{6}$${\mathrm{C}}_{60}$ semiconductor compounds, and it is proposed that the ca. ninetyfold enhancement of the 1428-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ mode and its large shift (to 1342 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) are generated by coupling to electronic excitations located on the molecule, consistent with recent theoretical descriptions of ${\mathrm{C}}_{60}^{6\mathrm{\ensuremath{-}}}$ and the ionic solid bcc-${\mathit{A}}_{6}^{+}$${\mathrm{C}}_{60}^{6\mathrm{\ensuremath{-}}}$.

Direct measurements of the pressure dependence of the superconducting transition temperature {ital T}{sub {ital c}} for single-phase Rb{sub 3}C{sub 60} provide for the first time substantial evidence that {ital T}{sub {ital c}} in {ital A}{sub 3}C{sub 60} compounds may be a universal function of the lattice constant (inter-C{sub 60} spacing). Determination of the lower ({ital H}{sub {ital c}1}) and upper ({ital H}{sub {ital c}2}) critical fields, and consequently the superconducting penetration depth and coherence length, is also reported. Compared to K{sub 3}C{sub 60}, the measured quantities are consistent with a 15% larger electronic-state density in Rb{sub 3}C{sub 60} arising from a greater inter-C{sub 60} spacing in this compound.

We have measured the electrical resistivity p of K 3 C 60 prepared in a pressed pellet form, by employing a contactless technique operating at microwave frequency (60 GHz). The temperature dependence of p can be fit by the functional form ρ(T)=a+bT 2 and the magnitude of the resistivity at T c , ρ(20 K), is on the order of 0.5 mΩ cm. We also find that the resistivity drops by almost an order of magnitude between 300 and 20 K

The temperature dependences of the upper critical field H c2 (T) were measured over a broad range for both K 3 C 60 and Rv 3 C 60 superconductors. We found H c2 (0) to be 28 T for K 3 C 60 and 38 T for Rb 3 C 60 corresponding to the Ginzburg-Landau coherence length of 34 and 30 A. The H-T phase diagrams do not show qualitative difference from the behavior of a conventional type-II superconductor