Transfer-matrix solution of the d=2 infinite Ising strip with fixed boundary conditions [D. B. Abraham and A. Martin-Löf, Commun. Math. Phys. 39, 245 (1973)] is adapted to the particular case of the (+/-) boundary. The soft mode of the fluctuating interface is studied at all temperatures T<${\mathit{T}}_{\mathit{c}}$ and beyond ${\mathit{T}}_{\mathit{c}}$ thus generalizing the capillary-wave and solid-on-solid (SOS) results to a system with bulk density fluctuations. In this paper the density or magnetization profile m(z) and its width W are calculated for a wide range of the strip length M perpendicular to the interface. The transition of the density profile from the low-temperature SOS regime through the critical region to the homogeneous one-phase system is described in detail and the scaling of m(z,T,M) and of W(T,M) is found.

${\mathrm{RbMnBr}}_3$ is an XY-like antiferromagnet on a distorted stacked triangular lattice. At low temperatures and fields we show by neutron scattering that there is an incommensurate ordered structure with magnetic Bragg peaks at (h/8±δ,h/8±δ,l) where h and l are odd integers and δ=0.0183±0.0004. At low temperatures, a field of 3 T applied along the a axis produces a first-order phase transition to a commensurate structure corresponding to δ=0, so that the lattice parameter of the magnetic cell is eight times that of the nuclear cell. In addition to these two ordered structures, there is another incommensurate ordered phase found at high temperatures. The magnetic phase diagram shows a tetracritical point at zero field and T=8.5±0.1 K and a bicritical point at H=2.55±0.05 T and T=7.8±0.1 K. Inelastic neutron scattering was used to measure spin waves along the c axis and in the easy plane. The dispersion relations confirm that ${\mathrm{RbMnBr}}_3$ is quasi-one-dimensional, with much stronger exchange along c than in the ab plane.

Ab initio electronic-structure calculations based on the linearized muffin-tin orbital method are reported for the ferromagnetic intermetallic alloys Gd(${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${)}_2$ with various nonmagnetic dopants M and x=0, x=0.25. The results of the theory are compared to x-ray photoemission spectroscopy data and magnetic measurements. We find good agreement between experiment and theory for ground-state properties and for the shape of the spectra near the Fermi level. The measured lowering of the paramagnetic Curie temperature in Gd(${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${)}_2$ for increasing dopant concentration x can be explained by a corresponding decrease of the density of states N(${\mathit{E}}_{\mathit{F}}$) at the Fermi level.

In order to determine if the effects of an overlap length are visible in an Ising Edwards-Anderson spin-glass model, we have performed Monte Carlo simulations that mimic experimental procedures. The spin system was quenched from a random initial condition to a temperature ${\mathit{T}}_1$, equilibrated in zero field for a time ${\mathit{t}}_{\mathit{w}1}$, whereupon the system was subjected to a perturbation in field ΔH, temperature ΔT, or bond ΔJ with a duration ${\mathit{t}}_{\mathit{w}2}$. After the perturbation of the original field, the temperature or bonds were restored. After waiting for a time ${\mathit{t}}_{\mathit{w}3}$, a weak magnetic field H was applied. The susceptibility χ(t)=[${\mathit{M}}_{+}$(t)-${\mathit{M}}_{\mathrm{-}}$(t)]/2H, where ${\mathit{M}}_{\pm }$(t) is the magnetization due to positive or negative field, and the spin autocorrelation function q(t) were studied as functions of time. For small perturbations, ΔH<Δ${\mathit{H}}_0$, ΔT<Δ${\mathit{T}}_0$, or ΔJ<Δ${\mathit{J}}_0$, respectively, the system is undisturbed. For larger perturbations, the correlations built up during ${\mathit{t}}_{\mathit{w}1}$ are gradually destroyed. Similar results are found in all three types of simulations and the qualitative features are in agreement with experimental results. These features are found for both two-dimensional and three-dimensional spin-glass systems. The results can be interpreted in the domain models for spin glasses. Δ${\mathit{H}}_0$ is found to be strongly dependent on the duration ${\mathit{t}}_{\mathit{w}2}$ of the perturbation. If ${\mathit{t}}_{\mathit{w}1}$ is constant and ${\mathit{t}}_{\mathit{w}2}$ increases, Δ${\mathit{H}}_0$ decreases. If ${\mathit{t}}_{\mathit{w}1}$ is set equal to ${\mathit{t}}_{\mathit{w}2}$ and ${\mathit{t}}_{\mathit{w}1}$=${\mathit{t}}_{\mathit{w}2}$ is increased, Δ${\mathit{H}}_0$ decreases, in agreement with the expected behavior. Δ${\mathit{H}}_0$ was found to increase with increasing temperature T, which is in agreement with experimental results, but contrary to the theoretical expectations.

We report a detailed study of magnetoresistance in the intermetallic compounds ${\mathrm{UCu}}_2$${\mathrm{Ge}}_2$ and (${\mathrm{U}}_{0.95}$${\mathrm{Ce}}_{0.05}$)${\mathrm{Cu}}_2$${\mathrm{Ge}}_2$ in the temperature range of 4.5 to 140 K and in magnetic fields up to 50 kOe. The present measurements support the earlier conjecture drawn from the magnetic studies that the ferromagnetic to antiferromagnetic transition takes place gradually over a wide temperature range. The magnetic-field dependence of magnetoresistance shows that the low-temperature antiferromagnetic state is more fragile in ${\mathrm{UCu}}_2$${\mathrm{Ge}}_2$ than in (${\mathrm{U}}_{0.95}$${\mathrm{Ce}}_{0.05}$)${\mathrm{Cu}}_2$${\mathrm{Ge}}_2$.

The quantum XXZ model is studied with the help of the Schwinger-boson formalism. The technique is particularly suited to describe the disordered state of low-dimensional quantum magnets. The Hubbard-Stratonovich transformation is employed to construct the saddle-point equations of the theory. The solution of these equations in one dimension yields the dependence of the Haldane gap as a function of the XY anisotropy.

Experimental data near the critical point for the spontaneous magnetization, the zero-field susceptibility, and the specific heat of the (S=1/2) insulating Heisenberg ferromagnet Cu (${\mathrm{NH}}_4$${)}_2$${\mathrm{Br}}_4$${2\mathrm{H}}_2$O are compared with two extensions of asymptotic scaling. One is a previously described phenomenology which is equivalent to the predictions of the renormalization group (RG) when all irrelevant variables are neglected. The second results if one adds to the asymptotic scaling formulas the confluent singularities due to the leading irrelevant variable. We find that both sets of formulas are consistent with the data for all three quantities: the spontaneous magnetization, the zero-field susceptibility, and the specific heat, but that overall, the phenomenology requires fewer parameters. A discrepancy betwen the experimental and theoretical values for the susceptibility exponent is discussed.

We present a method capable of performing molecular-dynamics simulations on complex systems using ab initio pseudopotentials. Here, we apply this method to examine the properties of liquid silicon. Our simulation method does not use fictitious electron dynamics; at each time step the interatomic forces are computed quantum mechanically. The liquid is prepared by melting the crystal and thermalizing the resulting liquid via Langevin dynamics. Once the desired temperature has been achieved, the Langevin ‘‘viscosity’’ term is turned off and dynamical properties may be examined. Our procedure has a number of advantages over other techniques. For example, large time steps can be used with this method. Metallic systems such as liquid silicon can be handled in a straightforward fashion. No ad hoc dynamics or Nosé dynamics need be employed to equilibrate the system to the desired temperature. Results for liquid silicon including the pair correlation function, density of states, and self-diffusion parameters are presented and compared to previous calculations and to experiment.

We propose a general treatment for solving the case of ferrimagnetic chains made up of two sublattices (S,1/2), characterized by Z-Z exchange couplings between nearest neighbors and a z-uniaxial anisotropy on the classical spins (d=2 point group ${\mathit{D}}_{2\mathit{h}}$ or d=3 point group ${\mathit{D}}_{\mathrm{\infty }\mathit{h}}$). The transfer-matrix method is particularly convenient for obtaining closed-form expressions of the partition function and its derivatives with respect to the applied field, when this last one is parallel to the z axis of quantization. Consequently, we respectively focus on the conjugated effects of anisotropy and classical-spin dimensionality on the low-temperature behaviors of the parallel magnetization and susceptibility. The presence of classical spins alternating with quantum ones allows us to focus on the case where the field is normally applied to the z axis of quantization. We show that it is only possible to derive a closed-form expression of the normal susceptibility ${\mathrm{\chi }}_{\mathrm{\perp }}$ and then study its low-temperature behavior. This work is a good model for testing a previous experimental work achieved on bimetallic chains which are encountered in the compound MnCo(EDTA)⋅${6\mathrm{H}}_2$O.

We propose a general treatment for solving the case of ferrimagnetic chains made up of two sublattices (S,s) and characterized by Z-Z exchange couplings between nearest neighbors. In the present work, we take into account a finite z-uniaxial anisotropy on quantum spins. In order to illustrate this study we have chosen to focus on the case of a (S,1${)}_{\mathit{N}}$ chain. The transfer-matrix method is particularly convenient for obtaining closed-form expressions of the partition function and its derivatives with respect to the applied field when this last one is parallel to the z axis of quantization. Consequently, we respectively focus on the low-temperature behaviors of the parallel magnetization and susceptibility. The presence of classical spins alternating with quantum ones allows one to focus on the case where the field is normally applied to the z axis of quantization. We show that it is only possible to obtain an analytical solution for the normal susceptibility ${\mathrm{\chi }}_{\mathrm{\perp }}$ and then we study its lower-temperature behavior.

The spin-wave spectrum of the ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$ exchange-noncollinear four-sublattice antiferromagnet has been investigated theoretically. It is shown that there exists a quasi-two-dimensionality of the spin-wave spectrum when the wave vector is oriented along a certain direction. This quasi-two-dimensionality originates from a specific crystal and magnetic symmetry of ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$ and is in no way connected with the small values of some exchange parameters in comparison with others. Further, the inelastic neutron scattering by magnons in ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$ has been considered and the effects of the exchange enhancement of the scattering intensities for some particular Bragg peaks are analyzed.

Solving the Bogoliubov–de Gennes equations for a one-dimensional mesoscopic superconductor-normal-metal-superconductor junction at zero temperature, we show that the interference of particles within the normal-metal region drastically alters the behavior of the critical current through the junction. As a function of the normal-metal carrier density, this interference yields striking resonances in the critical current corresponding to the Fabry-Perot interference criteria. The origin and nature of these resonances are analyzed in detail. Their functional form differs markedly from those found in nonsuperconducting mesoscopic structures. We note that the resultant behavior has the potential for interesting device properties.

The superconductor ${\mathrm{U}}_2$${\mathrm{PtC}}_2$ has an electron effective mass midway between that of a normal metal and that of a heavy-fermion system. Via doping of Os or Ir on the Pt sites, we have discovered (a) increasingly strong magnetic correlations which correlate with 5d valence-electron concentration, showing a peak in the 5d valence-electron number between 8 and 8.3, (b) a metamagnetic transition at 3 T in ${\mathrm{U}}_2$${\mathrm{Pt}}_{0.25}$${\mathrm{Ir}}_{0.75}$${\mathrm{C}}_2$ which has 8.6 5d electrons, and (c) strong indications of non-Fermi-liquid behavior (i.e., C/T∝lnT) for ${\mathrm{U}}_2$${\mathrm{Pt}}_{0.78}$${\mathrm{Ir}}_{0.22}$${\mathrm{C}}_2$ and ${\mathrm{U}}_2$${\mathrm{Pt}}_{0.5}$${\mathrm{Ir}}_{0.5}$${\mathrm{C}}_2$.

Bean’s original model successfully explained the phenomenon of irreversible magnetization in type-II superconductors and revealed the threshold behavior of flux motion. The latter has similarities to the sliding of sand grains in a pile of sand, an example of self-organized criticality (SOC) initiated by P. Bak et al. However, the effect of thermal relaxation was not considered in the SOC context. We have generalized Kim and Anderson’s derivation of flux creep by incorporating the effects of avalanches on the relaxation of magnetization close to the self-organized critical state using a time-delayed diffusion equation approach. We predict qualitative changes in the nature of flux creep when measured close to the critical state.

The probability density of large tensions in a random spring network is shown to behave asymptotically as ρ(T)∼exp(-${\mathit{AT}}^{1/\mathrm{\alpha }}$). This result follows from the assumption that large tensions are found at the center of funnel-shaped defects. The probability of finding a defect of a given size then controls the distribution of large tensions. The exponent α (0<α≤1) is determined analytically by solving the biharmonic equation for a critical defect. Numerical simulations of random elastic networks are consistent with the analytic results and show explicitly that the largest tensions are found in funnel-shaped defects.

An ab initio determination of the structural properties and electronic structure of tetragonal ${\mathrm{HgBa}}_2$${\mathrm{CuO}}_4$ has been made, within the local-density approximation using the full-potential linear-muffin-tin-orbital method. We find good agreement with the experimental lattice constant and c/a ratio. The band structure exhibits overall features similar to those of other cuprate superconductors with a marked peculiarity, that of an antibonding band coming from the hybridization of Hg p and d and apical oxygen O(2) p states, which lies slightly on top of the Fermi level (${\mathit{E}}_{\mathit{F}}$). This feature has some similarity to what happens in ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_6$. Within this framework a comprehensive description of the character of the states close to and at the Fermi surface is presented.

In this paper we expand the microscopic Hamiltonian of a self-interacting boson system in terms of the density and phase operators, which are canonical conjugates to each other. When higher-order terms are neglected, the Hamiltonian can be diagonalized. This method is applied to superfluid helium and the fractional quantum Hall effect. We shall first derive the expression of superfluid density for a two-dimensional (2D) system where canonical treatment has been lacking. We then reproduce the 3D results obtained by the Bogoliubov transformation. Finally, we shall show that the basic phenomenologies of the fractional quantum Hall effect at exact filling numbers can be derived without the introduction of the Chern-Simons term.

The three-band Hubbard Hamiltonian with an attractive oxygen-oxygen interaction ${\mathit{U}}_{\mathit{p}\mathit{p}}$ is investigated using the Hartree-Fock approximation. Such a Hamiltonian is assumed to be a generic model for ${\mathrm{CuO}}_2$ planes of high-${\mathit{T}}_{\mathit{c}}$ superconductors. Both superconducting and antiferromagnetic phases are found in the separate regions of the numerically obtained (${\mathit{U}}_{\mathit{p}\mathit{p}}$,δ) phase diagrams, where δ is the hole doping. It is found that the region of stability of the superconducting phase is enhanced with decreasing values of the charge-transfer gap, in good qualitative agreement with the experimental findings.

The free energy of a vortex-antivortex pair in a Josephson-coupled layered superconductor is derived within the framework of the Lawrence-Doniach theory. We find that the free energy of large pairs depends linearly on the pair separation and that there is no logarithmic term in the free energy for large pairs. We also derive the free energy of a pair of vortex lines, i.e., a vortex-antivortex pair in every layer. It is found that the free energy of this system corresponds to the usual result for conventional superconductors. In conclusion we discuss that the vortex-antivortex interaction cannot be described by a simple modification of the Kosterlitz-Thouless theory.

Systematic measurements were made of the dc magnetization in multilayered ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$/ ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ (YBCO/PBCO) films. The films were fabricated using a molecular-beam-epitaxy technique and were oriented with their c axis perpendicular to the substrate. A magnetic field was applied that was both perpendicular to the films and parallel to the film’s c axis. The irreversibility lines of the films were determined to be at the temperature and the magnetic field when the remanent magnetization falls below the detection limits of the instrumentation. We observed that the irreversibility line of a film strongly depends on the thickness of the YBCO layers. On the other hand, for the same thickness of YBCO layer, the irreversibility lines did not change (except for a slight change in the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$) for YBCO/PBCO multilayered films having PBCO layers that were from three to eight unit cells thick. The YBCO layers appeared to completely decouple when the PBCO layer was thicker than three unit cells. As observed in other high-${\mathit{T}}_{\mathit{c}}$ materials, the temperature dependence of the irreversibility field for these films exhibited a power-law behavior predicted by a thermally activated flux-flow (TAFF) model near ${\mathit{T}}_{\mathit{c}}$, and deviated from this behavior at temperatures below T/${\mathit{T}}_{\mathit{c}}$∼0.6, where T is temperature.

Irreversibility lines of YBCO/PBCO multilayered films are considered to be analogous to those of Bi systems, which are more anisotropic than YBCO systems. Relaxation of the magnetization for the films were also measured and the electric field vs current density (E-J) characteristics were estimated. These estimates suggest the existence of a vortex glass state at relatively low temperature. In the temperature dependence of the remanent magnetization in the films that had very thin YBCO layers, another transition was observed below the irreversibility line. The E-J characteristics, scaled using this transition temperature, suggest that this transition is the vortex glass transition and that there are two kinds of vortex states below the irreversibility line, i.e., vortex liquid and vortex glass. From these experimental results, the vortex states and anisotropy of high-${\mathit{T}}_{\mathit{c}}$ superconductors were then discussed from the standpoint of both the TAFF model and the vortex glass model.