Raman-scattering measurements involving the ${\mathrm{Nd}}^3$+-ion crystal field Kramers doublets in ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$ are reported. The observed splitting is attributed to the lifting of the Kramers degeneracy by the Nd-Cu exchange interaction. Below 150 K the splitting of these crystal-field excitations is nearly temperature independent. There are also small discontinuities in the amplitude ratio of the doublet components at 30 and 75 K. These discontinuities can be intepreted as arising from the direction reversal of the Nd magnetic moment following the direction reversal of the copper exchange field at these phase transitions.

We present first-principles coherent-potential-approximation calculations for the fcc FeNi alloy. We have found that this system is characterized by a competition between a low-spin (LS) and a high-spin (HS) state, and we have calculated this energy difference as a function of alloy concentration. For fcc Fe we find that several magnetic states are locally stable whereas for the FeNi alloys only one magnetic state is stable. In agremeent with previous theories for explaining the Invar effect in the ${\mathrm{Fe}}_{65}$${\mathrm{Ni}}_{35}$ alloy, we find that at this concentration the LS and HS states are very similar in energy. The calculated magnetic moment, equilibrium volume, and bulk modulus agree well with experimental data, and we predict that the Grüneisen constant exhibits an anomaly at a Ni concentration around 25%.

The modification of the electronic structures of ${\mathrm{Sm}}_2$${\mathrm{Fe}}_{17\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$${\mathrm{N}}_{\mathit{y}}$, ${\mathrm{NdFe}}_{11}$${\mathrm{TiN}}_{\mathit{y}}$, and ${\mathrm{YFe}}_{12\mathrm{-}\mathit{x}}$${\mathrm{Mo}}_{\mathit{x}}$ upon alloying and nitriding are examined with self-consistent spin-polarized calculations and soft-x-ray photoemission measurements between 18 and 135 eV. The changes in the Curie temperature ${\mathit{T}}_{\mathit{c}}$ with substitutional modifications and nitrogen addition are modeled with self-consistent spin-polarized electronic structure calculations and the spin-fluctuation theory of Mohn and Wohlfarth which relates the electronic structure to ${\mathit{T}}_{\mathit{c}}$. The calculations show that the spin-summed density of states at the Fermi energy is related to ${\mathit{T}}_{\mathit{c}}$. The photoemission spectra are dominated by the Fe 3d electrons within 3 eV of the Fermi energy in agreement with calculations. Changes in the density of states at the Fermi energy for interstitial and and substitutional modification compare well with calculations. Using photoemission results with experimental magnetic moments for the substitutional modification of the compound ${\mathrm{Sm}}_2$${\mathrm{Fe}}_{17\mathrm{-}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$, the spin-fluctuation theory predicts a change in ${\mathit{T}}_{\mathit{c}}$ in agreement with the measured change in ${\mathit{T}}_{\mathit{c}}$. Spin-resolved photoemission spectra for c-axis oriented ${\mathrm{Sm}}_2$${\mathrm{Fe}}_{17}$${\mathrm{N}}_{2.6}$ with magnetization perpendicular to the surface are presented and compared to theoretical calculations.

The calculation of the ionization fraction of the ion as a function of the ion atomic number, ion velocity and target Fermi velocity is the first step in effective-charge theory for heavy-ion stopping. We show results using the energy stripping criterion for the ionization fraction and give reasons for its superiority as against the velocity stripping criterion. The second computational step is the transformation of the ionization fraction into an effective-charge fraction assuming a linear dynamic dielectric response of the target material. On the basis of the Kaneko shell theory of solid targets we explain, to what extent the simple free-electron gas model still may work, if the heavy-ion stopping calculation makes use of the heavy-ion scaling rule and experimental proton stopping data.

A microscopic model of the proximity effect in superconductor-insulator-superconductor (SS’IS’’S) Josephson tunnel junctions has been developed for the general case of the finite critical temperature of the S’ (S’’) metal, arbitrary SS’ (SS’’) boundary transparency and the strength of the proximity effect between S and S’ (respectively S and S’’). The metals are assumed to be in the dirty limit and the thickness of the proximity layer is assumed to be small compared to its coherence length. The electrical properties of the SS’IS’’S junction are calculated as a function of the strength of the proximity effect, boundary transparency, critical temperature ratio, and temperature. The experimentally determined electrical characteristics of a series of Nb/${\mathrm{Al}}_1$, Al oxide, ${\mathrm{Al}}_2$/Nb junctions with varying thickness ${\mathit{d}}_1$ of the ${\mathrm{Al}}_1$ layer were interpreted with this model. The current-voltage characteristics and the temperature dependence of the critical current and sum-gap voltage could be described quantitatively well without any other correction than the non-BCS ratio ${\mathrm{\Delta }}_0$/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$≊1.93 of Nb. Deviations from the model for the junctions with the largest ${\mathit{d}}_1$ are attributed to the fact that the Nb and Al are not fully in the dirty limit and ${\mathit{d}}_1$ is not small compared to the coherence length.

We present a study of finite-size effects in a small Josephson junction. Explicit expressions for the total magnetic field have been found for the case of the Meissner state and for that of an ordered array of Abrikosov vortices penetrating the electrodes. We have determined how the behavior of the critical current ${\mathit{I}}_{\mathit{c}}$ as a function of the strength of an external magnetic field depends on the ratio L/${\lambda }_{\mathit{x}}$ of the width of the junction to the appropriate London penetration length. For small L/${\lambda }_{\mathit{x}}$ in particular, significant deviations from the familiar Fraunhofer pattern are found in a number of situations. We find that, in general, for the same field strength, because of an interesting cancellation property of the Josephson phase gradient, vortex penetration tends to increase the critical current. Moreover, for suitable vortex configurations ${\mathit{I}}_{\mathit{c}}$ is found to be determined by the magnetization and to actually increase with the field. The relevance of our results for junctions made out of high-temperature superconductors is discussed.

The LUMO-derived band of K-doped ${\mathrm{C}}_{60}$ was investigated by comparing the results of three measurements: yield spectroscopy and photoemission excited with hν=6.2 and 21.2 eV. Homogeneous single-phase near-surface layers were used in the investigation. The room-temperature line shape of the LUMO-derived band for the ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ phase is established. The insulating character of the ${\mathrm{K}}_4$${\mathrm{C}}_{60}$ phase is confirmed. The presence of an appreciable spectral weight close to the Fermi level in the insulating phases is detected. Evidence against a simple one-electron description of the LUMO-derived band is discussed.

We consider a single vortex line in a tight-binding model on a square lattice with an s-wave pairing interaction. The model is exactly solved within the framework of the self-consistent Bogoliubov–de Gennes formalism for a finite-size system. The energy spectrum, quasiparticle amplitudes, the gap function, and the current distribution are calculated. The tunneling conductance images are found to show a fourfold star shape, whose orientation depends on the bias voltage.

We report unusual behavior of the low-temperature electrical resistivity $\rho$ of nonequilibrium substitutional solid solutions of the form ${\mathrm{Al}}_{1-x}{\mathrm{Si}}_x$. This behavior is thought to be similar to anomalies in $\frac{d^{2}I}{d{V}^2}$ observed in the tunneling or point-contact spectra of metals in the presence of disorder, and to result from strong electron-phonon interactions. The resistivity data, along with low-temperature heatcapacity measurements and Al $L_{\mathrm{I}\mathrm{I}-\mathrm{I}\mathrm{I}\mathrm{I}}$ x-ray-emission spectroscopy and NMR Knight shift experiments, strongly suggest that an enhancement of the electron-phonon interactions due to lattice instabilities is the main reason for the dramatic increase in the superconducting transition temperature $T_c$ to ∼11 K in ${\mathrm{Al}}_{1-x}{\mathrm{Si}}_x$ when $x=0.2\mathrm{}$.

The standard perturbation theory for repulsive bosons breaks down at finite temperature in two dimensions because of the vanishing of both the Bose-Einstein condensate (at finite temperature) and the t matrix at long wavelengths and zero particle density. By adopting polar coordinates and using a running coupling constant, we circumvent the above two difficulties. The resulting theory is more straightforward than an earlier one of Popov which depends on an artificial division of momentum space. It can also be applied to three dimensions.

Experimentally, magnetically modulated resistance (MMR) techniques are effective in ascertaining the properties of granular superconductors, which can be modeled as a network of Josephson junctions. Each junction is parametrized by a critical current, with a capacitance and resistance in parallel to account for the nonsupercurrent components. The overall current-voltage behavior of the network, in the presence of an external magnetic field, is given by a set of coupled nonlinear ordinary differential equations, which are studied numerically. Features of interest in the simulated experiments arise from effects of the applied field on couplings, and the transitions between multiple solution classes of the differential equations. Through these simulations of the phase dynamics, details of the weak link MMR signal can be understood, the typical signal being reproduced with large enough networks.

We have shown that the charges in two-dimensional arrays of small Josephson junctions form Laughlin incompressible states in a magnetic field, so that the charge density is fixed in finite intervals of external parameters. Thus the system resembles a two-dimensional electron gas in the quantum Hall regime. In contrast to the latter one, the system is shown to be equivalent to a Bose gas with a weak contact repulsive interaction. We present numerical results for the main plateau in the charge density and also a toy model that enables us to develop the picture of all plateaus.

Thermoelectric potentials, currents, and charge distributions are continuous functions under a superconducting transition in metals. The experimental power-law rate (-3/2) of the thermoelectric flux near ${\mathit{T}}_{\mathit{c}}$ can be obtained theoretically.

Bubble formation in solutions of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is studied within a density-functional approach. In particular, the temperature dependence of the cavitation pressure for different ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentrations is calculated at low temperatures and compared to that of pure ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. The presence of Andreev states lowers the surface tension and, consequently, nucleation barriers are drastically reduced. This fact means that even at low ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentrations the cavitation process takes place at higher pressures than the spinodal pressure, which is not the case for pure ${}_{}{}^4{}_{}{}^{}\mathrm{He}$.

The equations of motion of pairlike excitations in the superconducting state are studied for various types of pairing using the random-phase approximation. The collective modes are computed of a layered electron gas described by a t-t’ tight-binding band, where the electrons experience besides the long-range Coulomb repulsion an on-site Hubbard U repulsion and a nearest-neighbor attractive interaction. From numerical calculations we see that the collective-mode spectrum now becomes particularly rich. Several branches can occur below the continuum of quasiparticle excitations, corresponding to order-parameter fluctuations of various symmetries of pairing, and collective spin-density fluctuations. From the collective-mode softening near the nesting vectors it is concluded that in the d-wave paired state an instability occurs toward the formation of a spin-density wave.

We calculate the density of states of a layered superconductor in which there are two layers per unit cell. One of the layers contains a d-wave pairing interaction while the other is a normal metal. The goal of this article is to understand how the d-wave behavior of the system is modified by the coupling between the layer types. This coupling takes the form of coherent, single-particle tunneling along the c axis. We find that there are two physically different limits of behavior, which depend on the relative locations of the Fermi surfaces of the two layer types. We also discuss the interference between the interlayer coupling and pairing interaction and we find that this interference leads to features in the density of states.

Thermodynamic integration methods are applied to obtain the classical free energy of solids via first-principles (or ab initio$)— molecular-dynamics simulations based upon the Car-Parrinello strategy. The methods introduced are particularly well suited to the calculation of the formation free energy of defects and of the absolute (classical) free energy of crystals. We find that specific techniques have to be used to ensure canonical sampling, particularly for the case of the perfect-crystal calculations. The method is illustrated with calculations of the free energy of formation of the sodium vacancy and of the free energy of the sodium crystal.

We investigate the one-dimensional (1D) d-p model, simulating a Cu-O linear chain with strong Coulomb repulsion, by using the numerical diagonalization method. Using the Luttinger liquid theory, we obtained phase diagrams of the ground state on ${\mathit{U}}_{\mathit{d}}$-${\mathit{U}}_{\mathit{p}\mathit{d}}$ plane, where ${\mathit{U}}_{\mathit{d}}$ and ${\mathit{U}}_{\mathit{p}\mathit{d}}$ represent on-site interaction at d sites and the nearest-neighbor interaction between p and d sites, respectively. In the weak-coupling region, they agree with the g-ology; a superconducting phase [SC(I)] is restricted to attractive interaction ${\mathit{U}}_{\mathit{p}\mathit{d}}$<0. On the other hand, in the strong-coupling region, we found a superconducting phase [SC(II)] for repulsive interaction ${\mathit{U}}_{\mathit{p}\mathit{d}}$>‖${\mathit{U}}_{\mathit{d}}$‖ and an insulating state with a charge gap for ${\mathit{U}}_{\mathit{d}}$>${\mathit{U}}_{\mathit{d}}^{\mathit{c}}$ and ${\mathit{U}}_{\mathit{p}\mathit{d}}$>${\mathit{U}}_{\mathit{p}\mathit{d}}^{\mathit{c}}$ with critical values ${\mathit{U}}_{\mathit{d}}^{\mathit{c}}$ and ${\mathit{U}}_{\mathit{p}\mathit{d}}^{\mathit{c}}$ at half-filling. Away from half-filling, another superconducting phase [SC(III)] appears for ${\mathit{U}}_{\mathit{d}}$≫${\mathit{U}}_{\mathit{p}\mathit{d}}$>0; which has been found for ${\mathit{U}}_{\mathit{d}}$→∞ in the previous paper [Physica C 205, 170 (1993)]. An analysis of the spin gap suggests that the SC(I) and SC(II) include the Luther-Emery region (with spin gap) in part, while the SC(III) belongs to the Tomonaga-Luttinger region (without spin gap) in whole.

The depinning of a flux line from a columnar defect is studied within the path-integral approach. Instantons of the quantum field theory in 1+1 dimensions are computed for the flux line whose dynamics is dominated by the Magnus force. The universal temperature dependence of the decay rate in the proximity of the critical current is obtained. This problem provides an example of macroscopic quantum tunneling, which is accessible to the direct comparison between theory and experiment.

Low-temperature Raman scattering of crystal-field (CF) excitations within the ${\mathrm{Pr}}^{3+}$ ground-state ${}_{}{}^3{}_{}{}^{}\mathrm{H}_4^{}{}_{}{}^{}$ multiplet of the compound ${\mathrm{Pr}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_4$ suggests the presence of more than one CF environment for the ${\mathrm{Pr}}^{3+}$ ions. The Raman spectra were similar to spectra obtained in inelastic neutron-scattering experiments. The analysis of the Raman spectra for ${\mathrm{Pr}}_{1.85}$${\mathrm{R}}_{0.15}$${\mathrm{CuO}}_4$ samples with R=Pr, La, Y, Ce, and Th indicates three different environments for ${\mathrm{Pr}}^{3+}$. One is associated with charge transfer caused by the replacement of ${\mathrm{Pr}}^{3+}$ by the tetravalent ions ${\mathrm{Ce}}^{4+}$ or ${\mathrm{Th}}^{4+}$. A second is almost identical to that observed in ${\mathrm{Pr}}_2$${\mathrm{CuO}}_4$. The third is attributed to either an orthorhombic distortion or to sites of smaller lattice volume. The Nd- and Nd(Ce)-based compounds are also discussed.