A local-density electronic-band-structure calculation was performed for a recently discovered non-copper-layered perovskite superconductor, ${\mathrm{Sr}}_2$${\mathrm{RuO}}_4$. It was found that the electronic structure near the Fermi energy is essentially described by antibonding bands of the Ru dɛ and O pπ states. Although two holes in the bands are predominantly situated in a dɛ(xy)-pπ state in the ab plane, the hole occupations in the other dɛ-pπ states vertical to the plane are not negligibly small, possibly in conjunction with the smallness of tetragonal distortion of the ${\mathrm{RuO}}_6$ octahedron. Associated with the antibonding dɛ-pπ bands, the density of states at the Fermi energy is relatively high (4.36 states/eVcell) but not enough to account for the observed specific-heat constant ${\gamma }_{\exp }$ and temperature-independent magnetic susceptibility ${\mathrm{\chi }}_{\exp }$. We found a large Stoner factor, which may explain most of the mass enhancement involved in ${\mathrm{\chi }}_{\exp }$. Certain similarities and dissimilarities in the electronic properties to the cuprate superconductors are discussed.

We have studied the magnetic hysteresis of twinned and detwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ single crystals. Our results show that channeling of vortices occurs and that it decreases vortex pinning in the field region near the peak in the hysteresis width. We demonstrate that the effect of the twin planes cannot simply be described in terms of enhanced pinning within the twin planes. By varying the angle of the twin planes with the magnetic field we have found that vortices lock in to the twin planes and an upper bound on the lock-in angle of 3.5° at 60 K has been obtained.

Measurements of the resistance versus temperature R(T) were made on samples of ${\mathrm{Pb}}_{0.2}$${\mathrm{Hg}}_{0.8}$${\mathrm{Ba}}_2$${\mathrm{Ca}}_{1.75}$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ above their superconductive transition temperatures. Current-voltage (I-V) characteristics were also studied at 77, 27, and 4.2 K in applied magnetic fields varying from 0 to 4.5 T. The R(T) data confirmed the visual observation that the samples were granular. The I-V curves were generally fit best by a modified Ambegaokar-Halperin model from which were obtained the resistance of the samples, the critical current density, and the pinning potential. The first two properties were dominated by the granular nature of the samples, whereas the pinning potential represented an intrinsic property of the material.

We report photoemission measurements on highly textured polycrystalline samples of ${\mathrm{Bi}}_{1.8}$${\mathrm{Pb}}_{0.4}$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10+\mathrm{\delta }}$ (Bi-2223, ${\mathit{T}}_{\mathit{c}}$=104 K). High-resolution data show a superconducting gap opening below ${\mathit{T}}_{\mathit{c}}$. The gap-spectrum line shape suggests an anisotropic gap structure in the Brillouin zone. The maximum gap in Bi-2223 is estimated to be 29 meV so that 2Δ/${\mathit{kT}}_{\mathit{c}}$∼6.5, somewhat larger than the reduced gap in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ (Bi-2212). A spectral dip feature is observed at 72 meV in the superconducting state. The results are compared with angle-resolved experiments on Bi-2212 single crystals.

We measured the surface impedance of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals at 10 GHz in two kinds of magnetic-field configurations (${\mathit{H}}_{\mathrm{\omega }}$∥c,${\mathit{H}}_{\mathrm{\omega }}$ ⊥c). We investigated the temperature dependence of the c-axis microwave conductivity ${\mathrm{\sigma }}_1^{\mathit{c}}$ in the superconducting state for a fully oxygenated sample (${\mathit{T}}_{\mathit{c}}$=93 K) and two oxygen-reduced samples (${\mathit{T}}_{\mathit{c}}$=65 K, 63 K). In the fully oxygenated sample, ${\mathrm{\sigma }}_1^{\mathit{c}}$(T) shows a broad peak similar to that of ${\mathrm{\sigma }}_1^{\mathit{ab}}$(T), while in the oxygen-reduced samples ${\mathrm{\sigma }}_1^{\mathit{c}}$(T) shows quite different dependence on temperature from ${\mathrm{\sigma }}_1^{\mathit{ab}}$(T). Namely, ${\mathrm{\sigma }}_1^{\mathit{c}}$(T) in the superconducting state strongly depends on the oxygen content in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. The result suggests that the dynamics of the quasiparticles in the oxygen-reduced ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ keeps a strong two-dimensional nature in the superconducting state.

A first-principles study of phase transitions in the mixed crystals ${\mathrm{Cs}}_{\mathit{x}}$${\mathrm{Rb}}_{1\mathrm{-}\mathit{x}}$${\mathrm{LiSO}}_4$ was performed by lattice-dynamics and molecular-dynamics simulations. A fractional substitution of x=0.25, 0.5, and 0.75 of Rb by Cs results in an increase of the stability range of the high-temperature disordered phase and in a reduction of the number of intermediate phases involving transient polar states. Conversely, this is true for the substitution of Cs by Rb. The transition temperature from disordered orthorhombic, Pnam, to the monoclinic ferroelastic phase, P${112}_1$/n (${\mathit{T}}_{\mathit{c}}$=202 K), increases as the percentage of impurity Rb in the ${\mathrm{CsLiSO}}_4$ is increased.

We report measurements of the thermal conductivity λ(T) of icosahedral ${\mathrm{Al}}_{70}$${\mathrm{Mn}}_9$${\mathrm{Pd}}_{21}$ in the temperature range 0.06–110 K. The quasilattice thermal conductivity ${\lambda }_{\mathrm{ph}}$ increases monotonically with T in the temperature range between 0.06 and 25 K and above 55 K. A plateau is observed for ${\lambda }_{\mathrm{ph}}$(T) between 25 and 55 K, i.e., at substantially higher temperatures than in amorphous solids. Moreover, the thermal conductivity of icosahedral ${\mathrm{Al}}_{70}$${\mathrm{Mn}}_9$${\mathrm{Pd}}_{21}$ in the λ plateau region is substantially higher than that of insulating and metallic glasses. At lower temperatures, between 0.35 and 1.6 K, ${\lambda }_{\mathrm{ph}}$ varies as ${\mathit{T}}^{2.06}$. This ${\lambda }_{\mathrm{ph}}$(T) variation is compatible with a scattering of phonons by tunneling states. Below 0.35 K the thermal conductivity decreases with increasing slope suggestive of a crossover to a regime of excessive phonon scattering characterized by a temperature-independent mean free path.

Comprehensive x-ray-diffraction data and structure factor calculations are presented for oriented poly(3-octylthiophene) films intercalated by iodine vapor to ${\mathrm{I}}_3^{\mathrm{-}}$ concentrations of 0, 4, 14, 26, and 45 mol wt. % per octylthiophene monomer. These data reconfirm previous reports that this polymer-iodine complex exhibits a continuous and reversible structural evolution. [M. J. Winokur, P. Wamsley, J. Moulton, P. Smith, and A. J. Heeger, Macromolecules 24, 3812 (1991); T. J. Prosa, M. J. Winokur, J. Moulton, P. Smith, and A. J. Heeger, Synth. Met. 55, 370 (1993); J. Moulton and P. Smith, ibid. 40, 13 (1991)]. Structure factor modeling calculations are used to fully specify the overall changes in the three-dimensional host polymer structure. This molecular restructuring involves both translational and rotational rearrangements of the host polymer. The fundamental lamellar base structure of the pristine host polymer is found to remain fully intact throughout the entire structural transformation. The large-scale changes in the observed interlayer repeat distances result from continuous variations in the angular orientation of the polymer backbone and in the average position of the alkyl side chains. Translational displacements of the polymer chains parallel to the main chain axes are found to occur within individual lamellae and this motion creates essentially one-dimensional intercalant channels perpendicular to the main chain axis. This latter process facilitates iodine intercalation and is, in part, responsible for the ability to achieve the very high iodine concentrations which have been measured experimentally. Scattering features are seen at the highest iodine concentrations which are indicative of additional structural ordering by the intercalant within the newly formed one-dimensional columns.

We present a theory on the structural stability of aluminum-rich transition-metal (TM) compounds. Our approach incorporates electronic-structure-based potential-energy functions for the aluminides. It is shown that an argument based on the short-range Al-TM interactions can explain the transition from close-packed structures for early TM aluminides to open and complex structures for middle and late TM aluminides. On the other hand, the medium- and long-range interactions between the TM atoms are responsible for the particular choice of a close-packed (or complex) structure for a given aluminide.

Incoherent transport of excitations in one-dimensional disordered lattices with pairs of traps placed at random is studied by numerically solving the corresponding master equation. Results are compared to the case of lattices with the same concentration of unpaired traps, and it is found that pairing of traps causes a slowdown of the decay rate of both the mean square displacement and the survival probability of excitations. We suggest that this result is due to the presence of larger trap-free segments in the lattices with paired disorder, which implies that pairing of traps causes less disruption on the dynamics of excitations. In the conclusion we discuss the implications of our work, placing it in a more general context.

Spectra of a tight-binding model on square approximants of the quadiperiodic octagonal tiling are calculated for both perfect and phason-randomized tilings. Level statistics for the randomized tilings are given by random matrix theory, while for the perfect tilings a new type of level statistics is found. Their level-spacing distributions are lognormal with power-law tails for large spacings. The spectral measure is seen to be multifractal.

The solid-state phase transformation from an amorphous solid into a nanocrystalline (NC) phase is studied from a thermodynamic point of view. The thermodynamic quantities of the interfaces (including the excess volume, excess energy, enthalpy, entropy, and the Gibbs free energy), which constitute a significant component in the NC materials, were calculated based on a quasiharmonic Debye approximation. By means of thermodynamic equilibrium conditions and quantitative calculations, we found that the structural characteristics of the interfaces are closely correlated with the grain-size limits crystallized from the amorphous phase. With a decrease in grain size, the excess volume as well as the excess energy of the interfaces formed during the crystallization will be reduced or, in other words, with the NC samples crystallized from the amorphous solids, a smaller grain size might be always associated with interfaces containing a smaller excess volume. This conclusion is in good agreement with experimental data of various systems for elements and alloys.

Results of a large-scale computational project for the calculation of the dispersion relations of eigenmodes (surface plasmons) and optical-absorption spectra of disordered clusters (fractal and uncorrelated) are reported. Fractals (cluster-cluster aggregates and the random-walk clusters, both original and diluted) and random-gas clusters consisting of 100–300 monomers are studied. High-accuracy results of Monte Carlo simulations are obtained. Transition of the eigenmodes from extremely localized to fully delocalized is found. Scaling of the dispersion relation of the eigenmodes, i.e., their localization radius or coherence length as a function of the spectral variable X, predicted earlier is quantitatively confirmed for diluted clusters. In contrast to the dispersion relations, the absorption spectra as functions of X do not show pronounced scaling in the intermediate region, but scale in the binary (spectral-wing) region. We suggest a new plot for the absorption profiles, namely absorption as a function of the coherence length of excitations. In such plots for most clusters, scaling is pronounced, but the indices differ dramatically from the predictions of the strong-localization theory. Possible reasons for the observed behavior are discussed.

The Raman spectrum of graphitic amorphous carbon (g-C) is modeled with simple classical methods, using rings as medium-range structural units. g-C provides a simple [nearly two dimensional (2D) with a single type of atom and one dominant bond type] prototypic example of a large class of continuous random network (CRN) solids where ring or cluster vibrations cannot be decoupled from the network in which they are embedded. We determined the in-plane vibrational modes of n=4, 5, 6, 7, and 8-membered symmetric, planar carbon rings using bond-stretching and bond-angle bending force constants; an additional force constant couples each ring node to the surrounding network. Our results support current models of g-C as small rafts of nearly 2D CRN’s comprised of ≊20% n=5, 60% n=6, and 20% n=7 rings, with very few n=4 and 8 rings. Extensions of the embedded-ring approach to more complex polyatomic and 3D CRN systems and to other probes of vibrational dynamics are discussed.

The temperature dependence of Raman spectra of cubic yttria-stabilized zirconia (YSZ) for three yttria concentrations (10, 15, and 20 wt %) is investigated in the range 40–1470 K. The variation of the spectra with temperature implies that first-order scattering (one-phonon processes) is dominant in this material, providing further evidence that the broad features observed in the spectra are caused mainly by partial lattice disorder rather than by combination (higher-order) processes. The high-temperature phase of YSZ is entirely stable in this range and remains cubic throughout. The temperature-induced frequency shifts of the band at ∼600 ${\mathrm{cm}}^{\mathrm{-}1}$ (attributed to the ${\mathit{F}}_{2\mathit{g}}$ zone-center optic mode) are determined for the three different yttria concentrations. Furthermore, using uniaxial stress data as well, the volume contribution to such shifts is calculated and compared to the lattice anharmonicity contribution. It is found that the volume contribution is larger than the total effect (by a factor of about 2) for all three compositions at 300 K, implying that the bonding in cubic YSZ has ionic character.

We present results of an ab initio calculation of the static and dynamical properties of the chlorine molecular crystal. The equilibrium ground-state properties are evaluated employing density-functional theory within the local-density approximation and the plane-wave pseudopotential method. Phonon frequencies and eigenvectors at the Γ and Y point of the Brillouin zone are calculated using density-functional perturbation theory within the linear-response approach. are accordingly too low. As a result, the intermolecular distances are in good agreement with experiment and so are most of the calculated external phonon frequencies. On the other hand, the intramolecular distance turns out too large and the internal phonon frequencies are accordingly too low.

The crossover from mixed-valent to magnetically ordered Kondo or heavy-fermion behavior in Ce(NiSi${)}_{2\mathrm{-}\mathit{x}}$(CuGe${)}_{\mathit{x}}$ has been studied. Results of Ce ${\mathit{L}}_3$-edge, structural, magnetic, and transport measurements are presented. It is suggested that the abnormal increase of the lattice parameter c in the region 1.6≤x≤2.0 be irrelevant to the Ce 4f-ligand orbital dehybridizations and the associated heavy-fermion state. Our results show that the substitution for Cu (and Ge) for Ni (and Si) drives this series from a strong mixed-valent to an antiferromagnetically ordered trivalent Kondo-lattice system. Particularly, it is found that the physical properties of the x=1.4 compound are very similar to that of the heavy-fermion superconductor ${\mathrm{CeCu}}_2$${\mathrm{Si}}_2$. The resistivity data also indicate that there is an active interplay between the crystalline electric field, coherence effects, and Kondo scattering. Moreover, this study supports the proposal that the Kondo-type spin fluctuation can be effectively quenched by the internal 3d-host magnetic field in some ${\mathrm{CeMn}}_2$${\mathrm{Si}}_2$-based materials. Magnetic susceptibility data show that this compound series is antiferromagnetically ordered for x>1.6 and the spin-fluctuation temperature ${\mathit{T}}_{\mathrm{sf}}$ decreases with increasing x in this region. A phase diagram is proposed to illustrate the evolution of the series from a single-impurity Kondo to a coherent Kondo and then to an antiferromagnetically ordered system.

In the functional-integral technique an auxiliary field, coupled to appropriate operators such as spins, linearizes the interaction term in a quantum many-body system. The partition function is then averaged over this time-dependent stochastic field. Quantum Monte Carlo methods evaluate this integral numerically, but suffer from the sign (or phase) problem: the integrand may not be positive definite (or not real). It is shown that, in certain cases that include the many-band Hubbard model and the Heisenberg model, the sign problem is inevitable on fundamental grounds. Here, Monte Carlo simulations generate a distribution of incompatible operators—a Wigner function—from which expectation values and correlation functions are to be calculated; in general no positive-definite distribution of this form exists. The distribution of time-averaged auxiliary fields is the convolution of this operator distribution with a Gaussian of variance proportional to temperature, and is interpreted as a Boltzmann distribution exp(-β${\mathit{V}}_{\mathrm{eff}}$) in classical configuration space. At high temperatures and large degeneracies this classical effective Hamiltonian ${\mathit{V}}_{\mathrm{eff}}$ tends to the static approximation as a classical limit. In the low-temperature limit the field distribution becomes a Wigner function, the sign problem occurs, and ${\mathit{V}}_{\mathrm{eff}}$ is complex. Interpretations of the distributions, and a criterion for their positivity, are discussed. The theory is illustrated by an exact evaluation of the Wigner function for spin s and the effective classical Hamiltonian for the spin-1/2 van der Waals model. The field distribution can be negative here, more noticeably if the number of spins is odd.

An exact representation of the large-U Hubbard model is developed in terms of fermionic holon, fermionic doublon, and local spin operators which requires no constraint between the charge and spin operators. The Hamiltonian in the new representation explicitly expresses the Nagaoka theorem, and there is an exact one to one correspondence between the new representation and the large-U Lieb-Wu wave function for the case of one dimension. Using the new representation, we further show that the signature of spin-charge separation, in any dimension, is the characteristic wave vector 2${\mathbf{k}}_{\mathit{F}}^{\mathrm{SF}}$ in the static density-density correlation function (${\mathbf{k}}_{\mathit{F}}^{\mathrm{SF}}$ encloses a Fermi volume filled by 1-x spinless fermions per site, where x is the doping concentration). In addition, we derive the corresponding representation for the t-J-J’ model, where the J’ term describes the indirect hopping of the electrons, and the holon (doublon) propagator which explicitly shows the dependence of charge propagation on the spin state.

Angle-dependent core-level photoemission excited with polarized x rays can be used to determine the magnetic properties of metallic systems by separating the geometrical dependence from the physical information that is contained in the spectra. There are four different geometries which can be distinguished: magnetic circular dichroism, linear dichroism, circular dichroism in the angular dependence, and magnetic linear dichroism in the angular dependence. The shape of the dichroic spectra in a one-electron model is determined by the core spin-orbit interaction and the spin field. A good agreement between experiment and theory is obtained for the Fe 3p photoemission of iron when the spin and orbit dependence of the lifetime broadening is taken into account. The influence of the orbit field and the spin filtering in the solid can be estimated from the first spectral moment.