Polarized Cu ${\mathit{L}}_3$ absorption experiments have been made on ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8\pm \mathrm{\delta }}$ well-oriented thin films by x-ray spectroscopy. Using a fitting procedure, the doping hole density per copper atom has been measured to be 16%. The results are in agreement with a doping occurring only with the (x,y) symmetry in the [${\mathrm{CuO}}_2$${]}_{\mathrm{\infty }}$ planes of these layered structures. Unpolarized XAS experiments have also been conducted on Tl(2:2:0:1), Tl(2:2:1:2), Tl(2:2:2:3), and Tl(1:2:2:3) ceramics with different doping and superconducting properties. The results show that, in the copper monolayer compound, the change in the doping hole density is accompanied by a drastic change of ${\mathit{T}}_{\mathit{c}}$ while this effect appears less important in the copper bilayer compounds and even uneffective in the three-layer one. These results show that the doping hole density is a key factor to monitor the superconducting properties and that the number of copper layers also plays an important role.

We perform the exact diagonalization of two, four, and six vibrating molecules strongly coupled with one electron to show that a small polaron describes both adiabatic and nonadiabatic regimes. Electron spectra are derived and compared with the adiabatic Holstein approximation and with the Lang-Firsov canonical transformation in a wide range of the adiabatic parameter and the coupling strength. We show that the self-consistent Migdal approximation is violated at an intermediate value of the coupling constant λ≃1 and the adiabatic or nonadiabatic small polaron is the ground state for the intermediate and strong coupling λ>1. The phonon frequencies are calculated for a wide range of the coupling, including the intermediate region.

We have improved a microwave self-resonant technique to measure surface resistance ${\mathit{R}}_{\mathit{s}}$ directly and surface inductance ${\mathit{L}}_{\mathit{s}}$ indirectly. For films prepared by the laser ablation technique we observed that ${\mathit{R}}_{\mathit{s}}$ at 21 GHz decreased by about three orders of magnitude as the temperature decreased from 90 to 80 K reaching a low value of 4.9×${10}^{\mathrm{-}4}$ Ω. We measured the London penetration depth λ and coherence length ξ, and found that both λ and ξ are anisotropic. Their values depended on the direction of the microwave electric field relative to the c axis. We deduced the value of ${\lambda }_{\mathrm{\parallel }}$(0) to be about 1800 Å, ${\lambda }_{\mathrm{\parallel }}$(86.5) about 8000 Å, and ${\lambda }_{\mathrm{\perp }}$(86.5) about 26 000 Å, where ${\lambda }_{\mathrm{\parallel }}$(0) is the penetration depth (as T→0 K) for the electromagnetic electric field parallel and ${\lambda }_{\mathrm{\perp }}$ perpendicular to the film plane. In addition ${\xi }_{\mathrm{\parallel }}$ was determined to be 129 Å and ${\xi }_{\mathrm{\perp }}$ equal to 40 Å at 86.5 K. The anisotropic factor γ is about 3.

The results of group-theory analysis, lattice dynamical calculations, and experimental data on the vibrational spectra of perovskitelike superconductors have been considered together. The layer description of phonon subsystems in these materials has been shown to be valid. The quasi-two-dimensional nature of the phonon subsystems has been established. The genesis of the bulk modes has been studied. It has been shown that optical vibrations in these crystals can be divided into 3(${\mathit{N}}_{\mathit{L}}$-1) interlayer and 3(${\mathit{N}}_{\mathit{A}}$-${\mathit{N}}_{\mathit{L}}$) intralayer modes where ${\mathit{N}}_{\mathit{L}}$ and ${\mathit{N}}_{\mathit{A}}$ are the numbers of layers and atoms per primitive unit cell, respectively. The interlayer bulk vibrations originate from acoustic layer modes whereas the intralayer modes are induced by optical vibrations of isolated layers. A sandwich conception of the high-${\mathit{T}}_{\mathit{c}}$ superconductivity mechanism is discussed based on the different roles of different layers.

Neutron-diffraction experiments have been performed to study the magnetic ordering of Cu ions in a single crystal of ${\mathrm{Eu}}_2$${\mathrm{CuO}}_4$. Magnetic reflections corresponding to the wave vector k=(1/2,1/2,0) develop below the Néel temperature ${\mathit{T}}_{\mathit{N}}$=265(5) K, showing a long-range antiferromagnetic ordering of Cu moments in ${\mathrm{Eu}}_2$${\mathrm{CuO}}_4$. The low-temperature saturated moment was determined to be (0.4±0.1)${\mathrm{\mu }}_{\mathit{B}}$, with the spin direction restricted to the a-b plane. Magnetic-field-dependent studies show no hysteretic behavior at intermediate temperatures, which strongly suggests that the antiferromagnetic spin structure is of the noncollinear double-k type previously observed in ${\mathrm{Sm}}_2$${\mathrm{CuO}}_4$ and also in ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$ at the intermediate temperature. We do not find any evidence of spin reorientation below the Néel temperature down to 1.4 K, the lowest temperature investigated.

The Josephson photoresponse of a current biased granular ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ thin film has been measured at 4.2 K for radiation frequencies ω in the range 20–111 ${\mathrm{cm}}^{\mathrm{-}1}$. The resulting photosignal s depends nonlinearly on the incident power P satisfying the relationship s=K(ω)${\mathit{P}}^{\mathrm{\alpha }\left(\mathrm{\omega }\right)}$, where K is a frequency-dependent constant. The value of α increases monotonically from 0.5 at 20.2 ${\mathrm{cm}}^{\mathrm{-}1}$ to 0.9 at 111 ${\mathrm{cm}}^{\mathrm{-}1}$. At 20.2 ${\mathrm{cm}}^{\mathrm{-}1}$ this power law is valid for incident powers between 5 mW and 1 kW, a range of more than ${10}^5$. These limits are imposed by experimental constraints and not by deviations from the power-law behavior. As the frequency increases, the responsivity of the film drops strongly, K(ω) decreasing approximately as ${\mathrm{\omega }}^{\mathrm{-}3}$, so that the power-law validity is established over three orders of magnitude at 111 ${\mathrm{cm}}^{\mathrm{-}1}$. Such a behavior of the Josephson photoresponse has been previously observed, albeit over reduced dynamic ranges, in granular superconducting films of other materials. Thus we believe it to be an inherent characteristic of the random arrays of Josephson junctions contained in such films.

Momentum-transfer cross sections at the Fermi momentum are calculated by the phase-shift method using the Kohn-Sham density-functional theory applied to the helium-extended electron-gas system. Results obtained for electron–neutral-atom scattering in vacuum below the first ionization threshold are compared. Approximations in both theoretical frameworks are tested. A remarkable similarity between the values of the best cross sections is found. The role of the Friedel sum rule for the phase shifts and the Levinson theorem in this problem are discussed.

The phase transitions of ${\mathrm{KH}}_2$(${\mathrm{P}}_{1\mathrm{-}\mathit{x}}$${\mathrm{As}}_{\mathit{x}}$)${\mathrm{O}}_4$ solid solutions between ferroelectric ${\mathrm{KH}}_2$${\mathrm{PO}}_4$ and its isomorphous ferroelectric ${\mathrm{KH}}_2$${\mathrm{AsO}}_4$ have been studied by dielectric measurements over the whole concentration range 0≤x≤1. The transition temperature versus x, ${\mathit{T}}_{\mathit{c}}$(x), showed a large bowing downwards. The ferroelectricity below ${\mathit{T}}_{\mathit{c}}$ was confirmed by a D-E hysteresis loop, which became weakened near x=0.68.

Damaging fullerene ${\mathrm{C}}_{60}$ upon energetic irradiation has been modeled with molecular-dynamics simulations. The angular dependence of the threshold energy of the primary knock-on atom (PKA) escaping from the cage is investigated in the case of the initial PKA direction within a plane through the cage center and 6-6 ring fusions. The average threshold value is estimated to be about 29 eV. The simulations provided a detailed picture of the damaging processes, in which three mechanisms were revealed. Those included ${\mathrm{C}}_{60}$-C→${\mathrm{C}}_{59}$, ${\mathrm{C}}_{60}$→C${\mathit{C}}_{59}$, and ${\mathrm{C}}_{60}$-2C→${\mathrm{C}}_{58}$. The interactions between carbon atoms are described with the Tersoff mode modified to match a screened Coulomb potential at short range.

The Mössbauer recoilless fraction is calculated from the lowest-order anharmonic-perturbation theory, the Green’s-function method, which sums the lowest-order anharmonic contributions to all orders, and the Monte Carlo method. In all cases we have employed a nearest-neighbor-interaction Lennard-Jones and Morse potential. Excellent agreement is shown to exist between the theory and the Mössbauer and x-ray experimental results.

We have measured the adiabatic elastic constants of two Cu-Al-Ni martensitic alloys using ultrasonic methods and we have compared the results to recent neutron-scattering experiments. It is shown that the elastic behavior of Cu-Al-Ni alloys follows the same trends exhibited by other Cu-based alloys; in particular, the TA2 long-wavelength acoustic modes are softer than all other modes.

Based on the model of quadrupolar glass (QG) of mixed alkali halide cyanide crystals, recently proposed by the authors, the temperature dependence of the QG Edwards-Anderson parameter ${\mathit{q}}_{\mathrm{EA}}$ and the elastic constant ${\mathit{C}}_{44}$ near the QG transition point has been obtained in the absence and presence of a quenched random-strain field. The free energy is a starting point of the calculation and has been expanded into the parameter ${\mathit{q}}_{\mathrm{EA}}$ shifted by Δ${\mathrm{¯}}^2$=(Δ/J${)}^2$ up to the fifth order. It is assumed that Δ¯≪1. Here Δ denotes the variance of the strain field and the parameter J is related to the variance of the orientational coupling between cyanide ions. It has been shown that in the absence of the random-strain field the system undergoes a sharp continuous QG transition. The presence of this field leads to the smearing out of the transition. This effect becomes more apparent with an increase of the parameter Δ. The obtained results appears to be in good qualitative agreement with the experimental data.

Minimal surfaces are considered as alternatives to tilings in the search for simple mathematical objects that have the properties of a quasicrystal. The construction of such objects, it seems, has not been addressed before and we propose a solution in the form of a limiting process. The existence of the desired limit has not been established but is investigated numerically for the case of a quasicrystalline minimal surface with icosahedral symmetry. We find that our approximate minimal surface exhibits jumps in topology analogous to the atomic phason jumps in physical quasicrystals.

In a recently proposed spring-block model of fracture, it was found that growth of domains of positive and negative components of the stress field before cracking sets in, was crucial for the pattern formation of the cracks. In this paper a mean-field theory is proposed to describe the dynamic behavior of the stress field in the spring-block model. Mean-field site and pair approximations are made for the system before cracking sets in. The single-site mean-field approximation gives steady-state densities of positive, negative, and zero components of the stress field, in quantitative agreement with the spring-block model. The pair approximation gives densities of the components of the stress field which are in close agreement with the simulation of the spring model, and predicts domain growth of the positive and negative components of the stress field as seen in the spring-block model.

The dynamics of coherent phonon generation by femtosecond optical pulses in thin gold and silver films is studied using a pump and probe scheme. Detection is achieved by monitoring ultrafast surface vibrations in real time using laser-beam deflection. The phonon strain pulse shapes can be explained through the nonequilibrium coupling of the electron and phonon distributions, suggesting a new method for measuring the electron-phonon coupling constant.

The enhancement in the effective response due to clustering of a nonlinear material in a host medium is demonstrated in an exactly solvable fractal model. We calculate the effective nonlinear response of a fractal cluster embedded in a hierarchical lattice. The results show that a large enhancement in the nonlinear response is found for the case of a nonlinear fractal embedded in a linear host and when the linear host is the better conductor.

The nonlinear Boltzmannn equation in which electron-electron interactions are taken into account and in the presence of an electric field is solved by an iterative strongly convergent procedure starting from the linearized solution. The numerical calculations are interpolated so as to have an explicit analytical Fokker-Planck equation whose steady-state solution is of the kind of a Chapman-Cowling-Davydov expression. The analytical solution is useful to treat the long time tails of the distribution function which would demand ${10}^7$ years of calculation by a Monte Carlo method.

Using the fermion-spin transformation to implement spin-charge separation of constrained electrons, a model of two t-J chains with interchain single-electron hopping is studied by Abelian bosonization. After spin-charge decoupling the charge dynamics can be trivially solved, while the spin dynamics is determined by a strong-coupling fixed point where the correlation functions can be calculated explicitly. This is a generalization of the Luther-Emery line for two coupled t-J chains. The interchain single-electron hopping changes the asymptotic behavior of the interchain spin-spin correlation functions and the electron Green function, but their exponents are independent of the coupling strength.

First-principles electronic-structure calculations were used to determine the electron-transport properties of ${\mathrm{RuO}}_2$ in the rutile structure. Our calculations were performed within the local-density approximation employing soft-core ab initio pseudopotentials and a plane-wave basis. The two independent components of the tetragonal plasma-frequency tensor, ${\mathrm{\Omega }}_{\mathit{p},}$xx, and ${\mathrm{\Omega }}_{\mathit{p},}$zz, were found to be identical within the calculational errors. The calculated isotropy of the plasma-frequency tensor agrees with optical and transport measurements. Our theoretical value for the plasma frequency of 3.3 eV is within 5–10 % of the experimental value as determined from optical and resistivity measurements. We find that the experimental resistivity can be fit accurately to the standard Bloch-Grüneisen model with additional terms representing optical-mode coupling and electron-electron interactions. Previously used models did not conform to the correct Fermi-surface topology. Our ab initio results demonstrate that the transport properties of ${\mathrm{RuO}}_2$ exhibit ‘‘normal’’ behavior as described by the Boltzmann equation.

The electronic structure of the wurtzite-type phase of aluminum nitride has been investigated by means of periodic ab initio Hartree-Fock calculations. The binding energy, lattice parameters (a,c), and the internal coordinate (u) have been calculated. All structural parameters are in excellent agreement with the experimental data. The electronic structure and bonding in AlN are analyzed by means of density-of-states projections and electron-density maps. The calculated values of the bulk modulus, its pressure derivative, the optical-phonon frequencies at the center of the Brillouin zone, and the full set of elastic constants are in good agreement with the experimental data.