A continuous transition of the vortex ensemble into a Bose-glass state in epitaxial ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ films with columnar defects along the c axis, induced by irradiation with 2.7 GeV ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ ions was observed. The temperature variation of the resistivity, measured with the magnetic field B=${\mathrm{\mu }}_0$H applied parallel to the c axis, fits the expression ρ(T)∼(T-${\mathit{T}}_{\mathrm{BG}}$${)}^{\nu \left(\mathit{z}\mathrm{-}2\right)}$ at low levels, with a field-independent exponent up to B≊2 T. At very low fields, the magnetic-field dependence of the Bose-glass transition temperature ${\mathit{T}}_{\mathrm{BG}}$ is in good agreement with the predictions of theory of boson localization in the presence of correlated disorder, 1/${\mathit{T}}_{\mathrm{BG}}$-1/${\mathit{T}}_{\mathit{c}0}$∼${\mathit{B}}^{1/4}$. However, for applied field values significantly larger than the dose-equivalent field ${\mathit{B}}_{\mathrm{\phi }}$=1 T, the vortex delocalization in our samples seems to proceed via melting, the magnetic-field exponent becoming very close to 1/2. The occurrence of the Bose-glass transition in the case of intrinsically very anisotropic superconductors, as ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$, can be understood through an increase of the tilt modulus in the vortex ensemble in the presence of correlated disorder, which promotes vortex localization.

Giant, strongly nonlogarithmic, magnetic relaxation was observed in BSCCO-2212 crystals irradiated with 5.8-GeV Pb ions due to flux pinning in columnar defects. The underlying flux-creep mechanism was identified as a vortex-loop nucleation process proposed by Nelson and Vinokur [Phys. Rev. Lett. 68, 2398 (1992)]. The crossover in the creep regime from depinning from a single rod to variable-range hopping was observed as a function of rod density. Another type of crossover, from combined pinning involving preexisting defects to purely columnar defect pinning, was observed on crossing the irreversibility line of the pristine material.

Previously we predicted a strong wave-vector (q) dependence of the phonon linewidth for near-zone-center Raman phonons in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ [C. O. Rodriguez, A. I. Liechtenstein, I. I. Mazin, O. Jepsen, O. K. Andersen, and M. Methfessel, Phys. Rev. B 42, 2692 (1990)]. This prediction has now been qualitatively confirmed experimentally, and in order to make a quantitative comparison with experiments we have calculated the linewidths with higher accuracy on an expanded q scale. We find that the thresholds for the Landau damping of the Raman phonons agree very well with the experiments which indicates that the renormalization of the electron velocities at the Fermi level is small. The size of the linewidths, which depend on the electron masses, are, however, too small in the calculations. This discrepancy is possibly due to renormalization of the electron masses.

It has been shown that in the above-mentioned paper the author starts from the orientational Hamiltonian of solid hydrogen containing an unphysical term leading, in general, to the nonvanishing average dipolar momentum of hydrogen molecules.

A metastable orthorhombic form of ${\mathrm{KC}}_{60}$ is identified from synchrotron x-ray powder-diffraction measurements. Structure analysis reveals ${\mathrm{C}}_{60}$ dimers with a 9.34 Å center-to-center separation. This structure competes with equilibrium phase separation (α-${\mathrm{C}}_{60}$ plus ${\mathrm{K}}_3$${\mathrm{C}}_{60}$) and with polymer formation; it is obtained by quenching from above 420 K to 298 K and immediately cooling below 270 K. If the polymer is indeed a one-dimensional metal, the dimer structure should be a Peierls insulator.

By using Nozières’ Fermi-liquid formulation for magnetic impurities as scattering centers at low temperatures (T<${\mathit{T}}_{\mathit{K}}$), we calculate the ${\mathit{x}}^2$ correction terms (in the impurity concentration) to the thermodynamic potential. We interpret the difference in the low-temperature properties between some cerium- and uranium-based alloys in terms of how close the scattering phase shift δ for an isolated impurity center is to the unitary limit (δ=π/2). We apply a picture of independent impurity centers for a large range of x in some Ce-based alloys, but only for x≪1 in U-based alloys. This is due to the Ce-based alloys being much closer to the unitary limit than the U-based alloys, suggesting that in the formation of a collective ground state the change in the density of states is the dominant effect in a dense Kondo lattice where the single-center phase shift is close to the unitary limit, while the magnetic interaction between local-impurity centers becomes important in a system where the phase shift differs from π/2.

The transverse autocorrelation function at an arbitrary temperature is calculated rigorously for a system with an arbitrary number of quantum spins, each of which is coupled to all the other spins with equal exchange. It is shown that the cluster approximation, formed by a model for a real magnet, explains short- and intermediate-time (up to the time to reach the spin-diffusion regime) experimental measurements in the paramagnetic phase.

The dc-magnetization measurements were performed between 50 G and 40 kG using a superconducting quantum interference device magnetometer on powder samples of ${\mathrm{Rb}}_4$${\mathrm{C}}_{60}$. The magnetic susceptibility reveals thermally activated behavior with an activation energy of 1200±100 K and a diamagnetic temperature-independent part ${\mathrm{\chi }}_{\mathit{d}}$=-3×${10}^{\mathrm{-}4}$ emu/mol. By calculation of the related magnetic moments and by comparison with the NMR and EPR data we conclude that the additional electrons are localized on the ${\mathrm{C}}_{60}$ molecule and form a spin-singlet ground state. The temperature-dependent paramagnetism is due to thermally excited electrons. Strong electron correlations and lattice distortions (crystal field or/and Jahn-Teller effect) are discussed as a possible origin for the lowest unoccupied molecular orbital (LUMO) level splitting and for gap formation. The experimental results are in good agreement with the existence of Jahn-Teller or crystal-field distortions, providing the ${\mathit{t}}_{1\mathit{u}}$ LUMO level degeneracy is lifted.

The maximum value Δ of the negative Hall resistivity ${\mathrm{\rho }}_{\mathit{xy}}$ of ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (0≤x≲0.13) and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ (6.73≲y≤6.84) single crystals scales with T/${\mathit{T}}_{\mathit{c}}$, exhibiting a maximum ${\mathrm{\Delta }}_{\max }$ for T/${\mathit{T}}_{\mathit{c}}$≊0.96 and vanishing for T/${\mathit{T}}_{\mathit{c}}$≲0.87. ${\mathrm{\Delta }}_{\max }$ scales with ${\mathit{T}}_{\mathit{c}}$ and decreases monotonically with decreasing ${\mathit{T}}_{\mathit{c}}$ to zero for ${\mathit{T}}_{\mathit{c}}$≲70 K. The Hall conductivity ${\mathrm{\sigma }}_{\mathit{xy}}$ vs B data have been analyzed using the expression ${\mathrm{\sigma }}_{\mathit{xy}}$=${\mathit{C}}_1$/B+${\mathit{C}}_2$B, where the two terms are associated with motion of the magnetic vortices and the quasiparticles in the vortex cores, respectively, yielding ${\mathit{C}}_1$ and ${\mathit{C}}_2$ as functions of x, y, and T.

Pulsed NMR studies of ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ have been carried out in the recently discovered quaternary borocarbide superconductor ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C (${\mathit{T}}_{\mathit{c}}$=15.5 K). These results are as follows: (a) Superconducting state: Though there is only one crystallographic B site in this structure, we have observed two ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ resonances in the superconducting state. One signal comes from a superconducting region, and the other from a normal-metal region. As for the former signal, the nuclear relaxation rate, ${\mathit{T}}_1^{\mathrm{-}1}$ decreases drastically with further decrease of temperature with no appreciable enhancement just below ${\mathit{T}}_{\mathit{c}}$, and it starts to saturate below 6 K. (b) Normal state: (${\mathit{T}}_1$T${)}^{\mathrm{-}1}$ increases as temperature decreases. We interpret this temperature dependence as arising due to two-dimensional or three-dimensional antiferromagnetic spin fluctuations (structure of ${\mathrm{YNi}}_2$${\mathrm{B}}_2$C is tetragonal and highly anisotropic).

A two-dimensional superconducting film in a magnetic field is considered in the adiabatic limit when vortex dynamics is dominated by the Magnus force. By mapping a vortex liquid state into that of electrons in an effective magnetic field, I find that the Hall conductance is quantized as ${\mathrm{\sigma }}_{\mathit{xy}}$=(4${\mathit{e}}^2$/h)[1+n(2m-1)]/n with n,m≥1 integers, predicting a fractional quantum Hall effect in a boson system. This phenomenon can also account for observed steps in the magnetization curve of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. The feasibility of a vortex liquid, i.e., melting of a flux lattice at T=0 by quantum fluctuations, is studied.

We report the synthesis of single crystals of ${\mathrm{Bi}}_{3.9}$${\mathrm{Sr}}_{3.3}$${\mathrm{Cu}}_{1.3}$(${\mathrm{Cu}}_{0.961}$${\mathrm{Fe}}_{0.039}$${)}_3$${\mathrm{O}}_{\mathit{x}}$, which can be regarded as an intergrowth of one half of the ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ and one half of the ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$ unit cell and is stabilized by the substitution of Fe for Cu. The compound possesses the strongest two-dimensional character of the superconducting properties among the bulk high-${\mathit{T}}_{\mathit{c}}$ structures known until now. From the scaling behavior of the reversible magnetization we derive an anisotropy parameter γ≊${10}^3$ for the ratio of the penetration depths ${\lambda }_{\mathit{c}}$/${\lambda }_{\mathit{ab}}$. We observe a magnetic irreversibility line with a maximum irreversibility temperature about 10 K below ${\mathit{T}}_{\mathit{c}}$ and a reentrance behavior at low fields.

We present a detailed derivation of the coupled equations of motion for the many-electron state amplitudes, relevant for Auger surface-ion neutralization. Cases of an ion with initially vacant or singly occupied valence orbitals are considered separately and the differences in the theory are discussed. We critically examine various ways of solving the coupled equations, including using the Fermi golden rule and various forms of wide-band approximations, where at least one of the bands is wide compared to the speed of the ion. We conclude that, although all of these lead to a simple rate equation, in each case it is theoretically necessary to invoke a wide-band approximation in some form. The Auger band will usually be a wide band, so the case of particular interest is that with a wide Auger band and a relatively narrow valence band, which still leads to a standard form for the rate equation.

The presence of a minimum observed in the variation of conductivity of bismuth with impurity concentrations at a constant temperature (4.2 K) has remained unexplained for a long time. An attempt to explain this anomalous behavior is reported here. In order to do so, a calculation has been made to find the change in the number of free carriers in bismuth with the addition of impurities (donors or acceptors). The calculation has been made using simple parabolic bands. It is known that when tin or tellurium atoms are added as impurities to bismuth all of the atoms are ionized. It has been found here that the number of free carriers initially shows a slow rate of decrease (for donors) or a slow rate of increase (for acceptors) as the impurity concentration is increased, as long as the impurity concentration is small, i.e., as long as the shift of the Fermi level is small. For a higher impurity concentration the number of carriers increases at a rate equal to that of the impurity concentration. This finding, combined with the scattering by impurity ions, could explain the anomalous behavior satisfactorily.

We present a theory of second-harmonic generation from thin films of chiral materials. The formalism makes distinct the contributions of electric- and magnetic-dipole transitions to the radiative process. Three susceptibility tensors form a minimal description of the optical nonlinearity, two for the nonlinear surface polarization and one for the nonlinear surface magnetization. The influence of optical interfaces is made explicit by using Fresnel coefficients to describe the reflection and transmission of the fundamental and second-harmonic radiation. Hence, the characterization and metrology of chiral materials by surface second-harmonic generation are given a firm theoretical foundation. In agreement with recent experimental observations, second-harmonic signals from chiral surfaces are predicted to depend sensitively upon whether the fundamental light is right- or left-hand circularly polarized. We show that this second-harmonic-generation circular-dichroism effect is a key signature of chirality and originates fundamentally from contributions of magnetic-dipole transitions to the nonlinear polarization and magnetization of the surface.

It is shown that for a definite cluster in the two-dimensional U=∞ Hubbard model, there are states in any ${\mathit{S}}^{\mathit{z}}$ sector, which are degenerate with the lowest state in the ${\mathit{S}}_{\max }^{\mathit{z}}$ sector. It is shown further that these states become the lowest ones in their sectors as the cluster becomes large enough, keeping the hole concentration constant. Then degenerate ground states of various ${\mathit{S}}^{\mathit{z}}$ are obtained. The properties of the ground state of any ${\mathit{S}}^{\mathit{z}}$<${\mathit{S}}_{\max }^{\mathit{z}}$ can be found through those of the ground state of ${\mathit{S}}_{\max }^{\mathit{z}}$.

We use ab initio pseudopotential electronic-structure methods to describe successfully both the detailed static structures and the structural transformation in ${\mathrm{B}}_2$${\mathrm{O}}_3$ crystals. Employing a reduced cell volume, with full relaxation of all internal coordinates, our calculations model the structural transformation from the polymorph containing the ${\mathrm{BO}}_3$ triangular unit into that containing the ${\mathrm{BO}}_4$ tetrahedral unit. In order to interpret the mechanism, individual energy contributions to the total energy are analyzed.

We describe a method for performing electronic-structure calculations of the total energy and interatomic forces which scales linearly with system size. An energy functional is introduced which possesses a global minimum for which (1) electronic wave functions are orthonormal and (2) the correct electronic ground-state energy is obtained. Linear scaling is then obtained by introducing a spatially truncated Wannier-like representation for the electronic states. The effects of this representation are studied in detail. Molecular-dynamics simulations using an orthogonal tight-binding basis and ab initio local-orbital density-functional methods are presented. We study both Car-Parrinello and conjugate-gradient molecular-dynamics schemes and discuss practical methods for dynamical simulation. A detailed connection between our method and the density matrix approach of Daw [Phys. Rev. B 47, 10 895 (1993)] and Li, Nunes, and Vanderbilt, [Phys. Rev. B 47, 10 891 (1993)] is also provided.

The electronic structures of transition-metal mono-oxides are calculated using the self-interaction-corrected (SIC) local-spin-density approximation (LSDA). The LSDA-SIC has many solutions and various possible solutions are examined. It is shown that the total energies of these solutions are strongly affected by the choice of exchange-correlation energy functionals. Alternatively, if the solutions are chosen so that all orbitals are localized as Wannier functions, the energy gaps are overestimated by 1.5–3 eV. However, in these solutions, the relative positions of occupied transition-metal d bands and oxygen p bands are consistent with the analysis of photoemission spectroscopy by the cluster configuration interaction theory.

The optical properties of trivalent chromium ions in ${\mathrm{LaSr}}_2$${\mathrm{Ga}}_{11}$${\mathrm{O}}_{20}$ crystals have been investigated by means of absorption, emission, excitation, and lifetime measurements for various temperatures of the sample. The data so obtained allow us to determine the values of the crystal-field parameters in the framework of the Tanabe-Sugano model for the transition-metal ion in insulating crystals and glasses. In particular, the relatively narrow emission lines indicate a high value of the crystal field. Polarized absorption and excitation measurements have allowed the assignment of the electric dipole character to the two lowest-energy transitions. From polarized excitation measurements it was possible to infer the ${\mathrm{Cr}}^{3+}$-ion site distortion visible in a splitting of the otherwise degenerate ${}_{}{}^4{}_{}{}^{}$${\mathit{T}}_1$ level. Moreover, the effect of tempeature on the emission bands’ intensities and lifetimes reveals the simultaneous presence of different competitive phenomena during the deexcitation of ${\mathrm{Cr}}^{3+}$ ion.