We determine the intensity of the nuclear multiphoton sidebands to the forbidden components of the 14.4-keV Mössbauer transition in ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$, generated by the application of radio-frequency magnetic fields to ferromagnetic samples, in the presence of static magnetic fields. Further we determine the intensity of the second-order sidebands to the 14.4-keV transition in the case known as the radio-frequency collapse, when the entire hyperfine magnetic field acting on the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ nuclei follows the oscillations of the applied radio-frequency field. Then we discuss a number of characteristic asymmetries due to quantum interference, appearing in the multiphoton radio-frequency sideband spectra.

We study a model for a pair of copper-oxide planes, described by the t-J model with a small interplane exchange term ${\mathit{J}}_{\mathrm{\perp }}$${\mathbf{S}}_{\mathit{i}}^{\mathrm{}\left(1\right)\mathrm{}}$⋅${\mathbf{S}}_{\mathit{i}}^{\mathrm{}\left(2\right)\mathrm{}}$. We show that this interplane interaction can be responsible for some of the unusual properties of multilayer high-${\mathit{T}}_{\mathit{c}}$ cuprates. An important role is played by antiferromagnetic spin correlations, causing strong peaks in the susceptibility ${\mathrm{\chi }}^{\mathrm{RPA}}$(q,${\mathit{q}}_{\mathit{z}}$) at the incommensurate nesting vectors ${\mathbf{Q}}_{\mathrm{AF}}$≃(π,π±2x). Consequently the effective coupling constants ${\mathit{J}}_{\mathrm{\parallel }}^{\mathrm{eff}}$(q) and ${\mathit{J}}_{\mathrm{\perp }}^{\mathrm{eff}}$(q) are also peaked at q=${\mathbf{Q}}_{\mathrm{AF}}$. We show that this leads to strongly enhanced interplane pairing between fermions on adjacent ${\mathrm{CuO}}_2$ planes, characterized by a pairing order parameter ${\mathrm{\Delta }}_{\mathrm{\perp }}$(${\mathbf{r}}_{\mathit{i}\mathit{j}}$)=〈${\mathit{f}}_{\mathit{i}\mathrm{\uparrow }}^{\mathrm{}\left(1\right)\mathrm{}}$${\mathit{f}}_{\mathit{j}\mathrm{\downarrow }}^{\mathrm{}\left(2\right)\mathrm{}}$-${\mathit{f}}_{\mathit{i}}$ ${}_{\mathrm{\downarrow }}{}^{\mathrm{}\left(1\right)\mathrm{}}{}_{}{}^{}$${\mathit{f}}_{\mathit{j}\mathrm{\uparrow }}^{\mathrm{}\left(2\right)\mathrm{}}$〉 which extends over several lattice spacings. We find that ${\mathrm{\Delta }}_{\mathrm{\perp }}$(k) has an (extended) s-wave symmetry and is peaked at the corners of the Fermi surface. We give qualitative arguments why the gauge field, which at low doping is very effective in destroying the in-plane pairing ${\mathrm{\Delta }}_{\mathrm{\parallel }}$, is less effective in destroying the interplane pairing.

This leads to the conclusion that ${\mathrm{\Delta }}_{\mathrm{\perp }}$(k) can be responsible for the observed spin-gap phase in bilayer cuprates. We use this model to calculate the NMR-relaxation rate, the echo-decay rate, and the Knight shift. Our numerical results are in qualitative agreement with the experimental data on ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.6}$. We also suggest that the superconducting state in bilayer materials is a combination of d-wave pairing in the plane and s-wave pairing between planes. In heavily underdoped materials where the spin gap is large, the interplane pairing may dominate and a nodeless gap structure is predicted.

The optical conductivity, σ(ω), of the two-dimensional one-band Hubbard model is calculated at finite temperature using exact diagonalization techniques on finite clusters. The in-plane dc resistivity, ${\mathrm{\rho }}_{\mathit{a}\mathit{b}}$, is also evalauted. We find that at large U/t and temperature T, ${\mathrm{\rho }}_{\mathit{a}\mathit{b}}$ is approximately linear with temperature. We also observed that σ(ω) displays charge excitations, a midinfrared (MIR) band, and a Drude peak. The combination of the Drude peak and the MIR oscillator strengths leads to a conductivity that decays slower than 1/${\mathrm{\omega }}^2$ at energies smaller than the insulator gap near half filling.

The measurements on strong quasiparticle injection through a tunnel barrier into a high-${\mathit{T}}_{\mathit{c}}$ ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ (YBCO) bicrystal film/junction or a YBCO film are presented. With an increase of tunnel injection current, both the Josephson and film critical currents decreased and were driven to zero to yield the normal state at a certain critical injection current density which ranged 20–140 A/${\mathrm{cm}}^2$ for 70 to a few 100 mV voltage bias. The results are qualitatively different from simple heating phenomenon and reflect nonthermal nature of high-${\mathit{T}}_{\mathit{c}}$ superconductors under injection.

We report here a systematic study of the resistivity, ac magnetic susceptibility, and specific-heat (${\mathit{C}}_{\mathit{p}}$) anomalies in Er-1-2-3 systems for both the pure phase and when Cu is partly (0.5%) replaced by Ni, Zn, Fe, Co, and Ga. These substitutions are aimed at studying the role of site-dependent in-plane and out-of-plane disorder, in conjunction with fluctuation effects. Analysis of the results shows that, as compared to in-plane, the out-of-plane substitutions exhibit a domination of the fluctuation effect. This has been explained from the fact that the out-of-plane disorder substantially decouples the interlayer links between ${\mathrm{CuO}}_2$ planes across the adjoining unit cells. This transforms the system to be more two dimensional, thereby enhancing the contribution of fluctuations.

We have used the parallel-plate resonator technique to study the microwave surface resistance ${\mathit{R}}_{\mathit{s}}$ and the penetration depth λ of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ thin epitaxial films at 5.5 GHz in the presence of magnetic field H≤0.8 T and at 30 K<T<80 K. The magnetic-field dependence of ${\mathit{R}}_{\mathit{s}}$ and λ allows determination of the vortex viscosity η, pinning constant α, and depinning frequency ${\mathrm{\omega }}_0$ for different field orientations. To the best of our knowledge, this is among the first measurements of the angular dependence of η, α, and ${\mathrm{\omega }}_0$. We find that the angular dependences of η, α, and ${\mathrm{\omega }}_0$ are fairly well described by the scaling model of Blatter, Geshkenbein, and Larkin. We demonstrate that (i) the temperature dependence of η is well described by the Bardeen-Stephen model with reduced normal-state resistivity, (ii) the pinning constant is close to its upper limit at lowest temperatures and decreases exponentially with increasing temperature, and (iii) the depinning frequency is of the order of 10 GHz, almost orientation independent and weakly depends on temperature. We analyze the values of the depinning frequency for different type-II superconductors and demonstrate that it is directly related to the quasiparticle scattering rate.

We have measured the current-voltage (I-V) characteristics of several high-temperature-superconducting materials with widely different morphologies (bulk Ag/Pb-Bi-Sr-Ca-Cu-O tapes, thin films of Y-Ba-Cu-O, and melt-textured, bulk Y-Ba-Cu-O samples). The I-V curves were taken at several magnetic fields ranging from 0 to 8 T. The measurements were carried out at three temperatures (4.2, 27, and 77 K) where the samples were immersed in liquid cryogens to ensure good thermal equilibrium. We compared our experimental results to the predictions of dissipation in superconductors made by the following physical models: modified Ambegaokar-Halperin, flux creep, vortex glass, collective flux creep, and a power law. The fits were extremely good for the first model and were not nearly as good for the others. Using the modified Ambegaokar-Halperin model, the critical current ${\mathit{I}}_{\mathit{c}}$, the normal-state resistance ${\mathit{R}}_{\mathit{n}}$, and γ, which is proportional to the pinning potential U(H,T), were obtained for each material. Since the Ambegaokar-Halperin model is the only one which uniquely defines ${\mathit{I}}_{\mathit{c}}$, we conclude that its use puts this parameter on a solid physical basis.

Microwave penetration and losses are derived for the anisotropic normal and superconducting states of single crystals in the shape of thin platelets oriented parallel and perpendicular to the oscillating electromagnetic field. For platelet crystals with the microwave field parallel to the major flat faces, the large anisotropy in the normal state can result in dissipation dominated by microwave field penetration through the thin edges rather than across the main faces. The influence of the extreme anisotropy is also considered for the superconducting state and can account for an anomalous peak in microwave loss below ${\mathit{T}}_{\mathit{c}}$ sometimes observed in Bi-Sr-Ca-Cu-O crystals. When crystals are mounted with their flat faces perpendicular to the microwave field, the losses in both the normal and superconducting states are shown to be strongly peaked towards the outer perimeter of the crystals. This makes critical demands on the degree of perfection of such regions, if the microwave measurements are not to be complicated by nonintrinsic effects associated with local imperfections.

We report measurements of the anisotropic resistivity of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CuO}}_{\mathit{x}}$ crystals using the Montgomery method. We find that the ab-plane resistivity is nearly linear with temperature down to 10 K, but deviates slightly from linearity below 40 K. The out-of-plane resistivity is metallic above 105 K, but nonmetallic below 105 K. The anisotropy ratio ${\mathrm{\rho }}_{\mathit{c}}$/${\mathrm{\rho }}_{\mathit{a}\mathit{b}}$ is temperature dependent and the maximum value is 9×${10}^4$ at low temperature. We estimate the effective mass, the Fermi energy, and Fermi velocity, etc., using Kresin and Wolf’s model. The plasma frequency and the transport electron-phonon coupling constant ${\lambda }_{\mathrm{tr}}$ are also calculated in a self-consistent way in combination with our resistivity and Hall coefficient data. We discuss the resistivity in the context of existing theories.

We report an improved version of a recently developed method that allows the calculation of effective elastic constants of polycrystalline materials. The method is predicated on the idea that the system can be described by a homogeneous substitute material which obeys Hooke’s law with elastic constants that define the macroscopic elastic properties of the original system. In order to determine these constants, one cuts out of a parallelepiped section and replaces it with a cluster of 100–500 grains of the original material. For simplicity we have provisionally assumed these grains to have a shape of rectangular columns. The cluster is connected to the embedding system by invoking the usual boundary conditions. The entire system is subject then to a set of linear independent homogeneous deformations whose number has to be chosen large enough to determine the sought-for independent elastic constants of the substitute system. The method yields apart from these constants also the entire displacement field within the cluster, and hence provides interesting additional information.

Surface steps of induction were observed at the sample edges in superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ crystals on magneto-optically measured induction profiles. A sharp upturn was found for the temperature dependence of the step value near 35 K. Analysis of the changes of the step value and the flux distribution near the sample edge with variation of external field shows that this upturn is caused by an increase of the effect of the Bean-Livingston surface barrier at low temperatures.

Large fluctuation-induced magnetization has been observed recently in the quasi-two-dimensional high-${\mathit{T}}_{\mathit{c}}$ materials. A satisfactory theoretical description of this phenomenon is absent at the moment. We present an extensive study of the fluctuation magnetization ${\mathit{M}}_1$ of two-dimensional superconductors within the Ginzburg-Landau theory in different regions of the phase diagram. In the normal state one has to distinguish the regions of weak/Gaussian and strong/critical fluctuations. Different kinds of scaling dependencies for ${\mathit{M}}_1$ are predicted for these regions. An important feature of the field dependence of ${\mathit{M}}_1$ in the weak-fluctuation regime is saturation of the magnetization at high fields. Taking into account both regular and critical contributions to ${\mathit{M}}_1$ we obtain an expression that is valid in both regions. In the mixed state at high fields the fluctuation magnetization is completely determined by the thermal elastic fluctuations in the lattice state. Due to suppression of the elastic modulii, ${\mathit{M}}_1$ increases mainly as 1/(${\mathit{H}}_{\mathit{c}2}$-B) as the field approaches the critical region near the upper critical field ${\mathit{H}}_{\mathit{c}2}$. At small fields, B≪${\mathit{H}}_{\mathit{c}2}$, there are two independent contributions to ${\mathit{M}}_1$ which can be treated separately. One comes from the Gaussian excitations in the vortex cores and the other from the thermal motion of the vortices. We demonstrate that the vortex system in this limit is equivalent to the model of the one-component two-dimensional Coulomb plasma. Using Monte Carlo results for this model, we estimate ${\mathit{M}}_1$ in the vortex-lattice as well as in the vortex-liquid state.

We report the anisotropy of magnetic properties of c-axis-aligned ${\mathrm{HgBa}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ powders embedded in epoxy. The alignment was done by applying a 7-tesla magnetic field at room temperature. The temperature dependence of the anisotropic ratio, ${\mathrm{\chi }}_{\mathrm{\parallel }\mathit{c}}$(T)/${\mathrm{\chi }}_{\mathrm{\perp }\mathit{c}}$(T), in the superconducting state was derived from data measured with zero-field cooling and field cooling for both parallel and perpendicular to the c axis at 30 G. The anisotropic ratio of about 3.05 was obtained at 5 K. The magnetization hysteresis loops M(H) were measured up to ±5 tesla at various temperatures. The magnetic flux irreversibility, calculated from M(H) curves, decreases exponentially with increasing magnetic field, and the critical current density was estimated using the Bean model (i.e., ${\mathit{J}}_{\mathit{c}}$=32ΔM/d, where d was taken to be the average particle diameter of 2.5 μm). ${\mathit{J}}_{\mathit{c}}$(H∥c) at 5 K in 0.5 tesla was found to be approximately ${10}^7$ A/${\mathrm{cm}}^2$. The anisotropic ratios of the magnetic flux irreversibilities decrease rapidly with increasing magnetic field up to 0.5 tesla but decrease slowly in the field region higher than 0.5 tesla; the decrease of the ratios indicates that more significant activation of fluxoid in the c direction reduces ΔM(H∥c) rapidly. The data of Δ${\mathit{M}}_{\mathrm{\parallel }\mathit{c}}$ and Δ${\mathit{M}}_{\mathrm{\perp }\mathit{c}}$ in the low-temperature region below 30 K can be fitted by the exponential function ΔM(T)=a exp(-T/${\mathit{T}}_0$) with ${\mathit{T}}_0$=14.2 K, a=3.75 emu/g for H∥c and ${\mathit{T}}_0$=18.2 K, a=2.50 emu/g for H⊥c at 4 tesla, and in the high-temperature region above 30 K, the simple power law ΔM(T)=b(1-T/${\mathit{T}}_{\mathit{c}}$${)}^{\mathit{n}}$ was observed with n=6.42, b=32.9 emu/g for H∥c and n=4.65, b=11.2 emu/g for H⊥c at 0.5 tesla.

This work studies the thermodynamics of phase transitions of the first kind in current-carrying conductors when these transitions are accompanied by sharp change of the electric conductivity. The expression for the work of formation of a nucleus of a new phase in a current-carrying conductor as a function of the geometrical parameters of the problem is derived. The dynamics of the evolution of a macroscopic interphase boundary when both phases form the coaxial cylindrical domains is investigated. It is shown that the character of equilibrium of an interphase boundary depends upon the mutual location of phases with lower and higher conductivity. If a low-conductivity phase occupies the external cylinder the position of the interphase boundary is stable. In the opposite case, namely when the low-conductivity phase occupies the internal cylinder, the position of the interphase boundary is unstable. It is shown that the current-carrying conductor melts from the surface at the temperature lower than the melting temperature. However, during propagation of the liquid (low-conductivity) phase from the surface to the axis of a conductor, the nuclei of a solid phase (high conductivity) may form inside it. The possible scenario of melting of the current-carrying conductors is suggested.

We have measured the surface impedance of ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_4$ (NCCO) thin films and single crystals, ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO) single crystals, and Nb single crystals. We find that the electrodynamic properties of Nb are in good agreement with s-wave BCS theory. YBCO single crystals show behavior which has been seen only in very high quality materials and which is clearly not consistent with s-wave BCS theory. Both thin films and single crystals of NCCO have been grown, thoroughly characterized, and measured at microwave frequencies. We find an unusual degree of consistency in the results between thin films and crystals of NCCO. We also find a surprisingly good agreement between the temperature dependence of the surface impedance, and single-gap BCS s-wave calculations over the entire temperature range. These results are puzzling in the light of the fact that NCCO is a cuprate superconductor, and the crystals were grown in a manner very similar to those of YBCO. We discuss these results and consider a variety of ideas currently in the literature which may explain these striking differences.

We propose an extension of the Landau theory of phase transitions to apply to potassium-dihydrogen-phosphate-type ferroelectrics. The polarization at point x can interact with the polarization at a distant point y through a temperature-dependent kernel G(x,y,T). The essence of the proposal is the assumed form for G. The form chosen is such that the dipolar interaction can be described by a free energy with a long-range kernel decreasing exponentially. We also study the relation between the various possible critical exponents and the determination of the variables that describe the model. Finally, of special interest is the problem of crossover from fluctuating polarized clusters to static domains as temperature decreases below ${\mathit{T}}_{\mathit{c}}$.

The short-range structure of amorphous germanium in the as-evaporated state and after Sb-ion implantation was investigated by x-ray diffraction. Significant changes were detected in the structure factors of the two specimens. Reverse Monte Carlo analyses of both data were carried out. An increase of 12–15 % in the atomic number density due to implantation was established. The diffraction data are consistent both with an atomic arrangement where the originally defective structure becomes nearly perfect as a consequence of the ion bombardment, or with the formation of a new phase of a-Ge with an average first coordination number higher than 4.

Thermal conductivity of crystalline natural nitrogen was investigated over the temperature range 1.2–38 K. The temperature dependence of thermal conductivity of α-nitrogen has been determined in temperatures below 18 K. The thermal conductivity coefficient reaches the maximum value 250 mW/cm K at 3.6 K. The results of the experiment below 14 K can be well described by integral equation obtained by the method of Callaway. Relaxation times for various mechanisms of phonon scattering have been determined. The results of our experiments are in accord with earlier literature data for temperatures above 18 K.

Measurements of the time dependence of the remanent magnetization for the strong random magnetic anisotropy amorphous alloy ${\mathrm{Tb}}_2$Fe show a temperature-independent time-logarithmic relaxation rate below the crossover temperature ${\mathit{T}}_{\mathit{Q}}$=8 K. Our results are consistent with the occurrence of quantum tunneling of magnetization, ${\mathit{T}}_{\mathit{Q}}$ being one of the highest crossover temperatures found to date. The estimated mesoscopic tunneling volumes, including around 100 spins, compare with the short-range ferromagnetically correlated volumes existing in the sample.

We report Raman spectra of single-crystalline ${\mathrm{Gd}}_2$${\mathrm{BaNiO}}_5$ and ${\mathrm{Y}}_2$${\mathrm{BaNiO}}_5$, concentrating on identification and assignment of the phonons at room temperature. All nine of the expected Raman-active phonons (3${\mathit{A}}_{\mathit{g}}$+1${\mathit{B}}_{1\mathit{g}}$+2${\mathit{B}}_{2\mathit{g}}$+3${\mathit{B}}_{3\mathit{g}}$) are observed. The three Gd(Y) related modes of ${\mathit{B}}_{2\mathit{g}}$, ${\mathit{A}}_{\mathit{g}}$, and ${\mathit{B}}_{3\mathit{g}}$ symmetry are observed at 123(160), 175(233), and 204(269) ${\mathrm{cm}}^{\mathrm{-}1}$, respectively. The six O(1) vibrational modes of the Gd(Y) compound occur at 183(184), 312(328), 335(340), 422(440), 518(540), and 562(584) ${\mathrm{cm}}^{\mathrm{-}1}$, having ${\mathit{B}}_{3\mathit{g}}$, ${\mathit{B}}_{2\mathit{g}}$, ${\mathit{B}}_{1\mathit{g}}$, ${\mathit{B}}_{3\mathit{g}}$, ${\mathit{A}}_{\mathit{g}}$, and ${\mathit{A}}_{\mathit{g}}$ symmetries, respectively. A strong mode polarized along the Ni-O(2) chain axis at ∼726(755) ${\mathrm{cm}}^{\mathrm{-}1}$ in the Gd(Y) compound may arise from magnetic excitations along the spin chains or from a Raman-forbidden, infrared-allowed vibrational mode along the Ni-O(2) chains.