We report on femtosecond pump-probe absorption measurements in CdTe/${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Te quantum wells with an unusual distribution of band offset: almost all the band-gap difference between the two constituent materials is found as a conduction-band offset. We have observed a blueshift of the exciton energy due to hard-core repulsion between excitons and an intriguing redshift in the presence of free electron-hole pairs. We evaluate the redshift due to different mechanisms and conclude that the redshift observed is due to hole delocalization throughout the structure resulting from the very shallow valence-band potential wells.

Electroreflectance measurements are made over a temperature range 4 K to 470 K to determine the reverse bias at which zero electric field occurs in a [111] growth-axis ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As strained-layer quantum well (QW) embedded in the intrinsic region of a p-i-n diode. A depletion model of the diode, with embedded ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As QW, is used to extract the dielectric polarization in the QW layer, at zero electric field in the well. We find that the polarization in the QW layer has a significant temperature dependence. This pyroelectric effect is qualitatively similar for ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As wells with x=0.10 and x=0.15, but is more pronounced in the latter case. We also find, over the entire temperature range, that the polarization is smaller than the value predicted at room temperature based on the GaAs-${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As lattice constant mismatch and bulk-crystal piezoelectric theory.

The Mn-induced Cu(100)-c(2×2) structure has been investigated using scanning tunneling microscopy (STM). Mn atoms adsorb as isolated atoms or small clusters at low coverages. Adsorbed Mn atoms replace the substrate Cu atoms to form an ordered surface alloy with a c(2×2) structure at ∼0.5 ML. Large buckling of Mn atoms in the c(2×2) structure, which was proposed to explain a surface ferromagnetism, is not observed by STM.

Inverse photoemission with variable photon energy reveals the presence of an empty surface state on Gd(0001)Γ¯, which shifts towards the Fermi energy (${\mathit{E}}_{\mathit{F}}$) with increasing temperature. Hereby, the surface state leads to a significant increase in the d-like density of states at ${\mathit{E}}_{\mathit{F}}$, suggesting its involvement in the enhanced magnetic coupling in the topmost surface layer of Gd(0001). A temperature-dependent exchange splitting is also observed for the empty 5d band that seems to vanish only well above the bulk Curie temperature.

The structure and electronic properties of B and S impurities in an Fe Σ3[11¯0](111) grain boundary are studied using the DMol molecular cluster method. A large cluster containing 91 atoms is used to simulate the local environment of the boundary. Optimized atomic geometries are obtained by atomic force calculations. The results show that impurities induce large relaxations in the grain boundary, and that the electronic structure of the hybridized B/Fe system is very different from that of the nonhybridized S/Fe system.

We present a rigorous, analytical approach to the Camley-Barnas model of the magnetoresistance (MR) in magnetic multilayers with an arbitrary number of bilayers. Using a path-integral method, we solve the semiclassical Boltzmann-transport equation subject to boundary conditions at the surfaces and interfaces, and obtain analytical results for the MR of magnetic layered systems. Particular attention is given to the multilayer effect for the enhancement of the change in magnetoresistance, and good agreement with experimental data is found for the epitaxial (Fe/Cr${)}_{\mathit{n}}$/Fe samples.

An approximation is presented to analyze the evolution of magnetization with temperature in Heisenberg systems. This approach is based on the decoupling of spin-wave and spin-flip thermal contributions, in analogy with phonon and point-defect decoupling in crystals. Results for bidimensional and tridimensional classical Heisenberg model (with and without anisotropy) as well as quasi-two-dimensional quantum antiferromagnetic systems are discussed, showing, in general, excellent agreement with recent experimental data and Monte Carlo simulations.

We present the results of a theoretical study for the interpretation of the optical and magneto-optical spectra of PrSb. In particular, we address the question of the assignment of the unexpectedly low optical and magneto-optical signals to the ${\mathit{f}}^2$→${\mathit{f}}^1$d transition. We use two different approaches: the crystal-field approach, suitable for the Pr 4f states assumed to be localized, and the band approach for the conduction electrons polarized by the 4f states. The two pictures are merged by the technique of a fixed 4f spin moment used to perform the spin-polarized calculations for the conduction electrons. We calculate the 4f spin moment ${\mathrm{\mu }}_{4\mathit{f}}^{\mathit{s}}$ in the crystal-field ground state of ${\mathrm{Pr}}^{3+}$ including the molecular-field correction λM. From the local spin density approximation calculations we evaluate the molecular-field constant λ, which agrees with the previous value, the interband conductivity, reflectivity, and the Kerr rotation angle for the conduction electrons. On the basis of our model we find that the weak intensity of the optical and magneto-optical signal is ascribable essentially to electric dipole selection rules among crystal-field states. Finally, we interpret the structure at ≃1 eV in the off-diagonal conductivity spectrum in terms of the ${\mathit{f}}^2$→${\mathit{f}}^1$d transition. This is in agreement with two different reflectivity spectra.

A general Lagrangian-based long-wavelength theory of ordered magnetic dielectric crystals (ferromagnetic, antiferromagnetic, and ferrimagnetic) is formulated. Our classical treatment of intrinsic spin uses the anticommuting Grassmann algebra ${\mathit{G}}_3$ developed by Berezin and Marinov and by Casalbuoni. The Grassmann formulation of classical spin gives by the Dirac quantization procedure the usual nonrelativistic spin-1/2 quantum theory. The treatment begins at the microscopic level before a long-wavelength limit is performed to obtain a macroscopic theory so as to incorporate all long-wavelength modes of motion (acoustic, optic, electromagnetic, and spin) and their interaction to all orders of nonlinearity. The crystals can have any symmetry, anisotropy, and structural complexity. The equations of motion for all the modes are obtained and the energy, momentum, pseudomomentum, angular momentum, and spin conservation laws are found. The magnetizations arising from spin and from the motion of bound charge are found to enter the energy conservation law distinguishably. This theory should be particularly useful for the study of magneto-optical phenomena.

From molecular-dynamics simulations, it is demonstrated that (1) a rapidly quenched Ga liquid contains many 1201- and 1311-types atomic bonded pairs and (2) the 1311-type atomic bonded pairs decrease along with a corresponding increase in the 1201 atomic bonded pairs in the crystallization, so that the β-type crystal transition occurs from the supercooled Ga liquid on cooling, or from the pure Ga glass on heating.

The size and geometric structure dependence of the magnetic properties of small ${\mathit{V}}_{\mathit{N}}$ clusters is studied by using a d-band Hubbard-like Hamiltonian in the unrestricted Hartree-Fock approximation. We determine the average magnetic moment μ${\mathrm{¯}}_{\mathit{N}}$ and local magnetic moments ${\mathrm{\mu }}_{\mathit{i}}$ as a function of the interaction parameters. Different structures are considered and charge transfer effects are discussed. For the considered clusters, usually two different magnetic configurations are obtained. The role of the sp electrons and spd hybridization effects on the magnetic properties is calculated for bcc-like clusters. Results are given for the average magnetic moment, the spin-polarized charge distribution within the clusters, and the sp and d electronic density of states. Small local sp magnetic moments ${\mathrm{\mu }}_{\mathit{s}\mathit{p}}$ are obtained, which in most cases are opposite to the d magnetic moment ${\mathrm{\mu }}_{\mathit{d}}$.

The finite-size spectrum in the Kondo problem is obtained from the Bethe-ansatz solution of the exactly solved models. We investigate the Anderson model, the highly correlated SU(ν) Anderson model, and the s-d exchange model. For all these models we find that the spectra exhibit the properties characteristic of the Fermi-liquid fixed point, and hence our microscopic calculations are in accordance with the results obtained by boundary conformal field theory with current algebra symmetry.

We present calculations of the autocorrelation times for the N-fold-way Monte Carlo algorithm applied to the two-dimensional (2D) Ising model. We present a proof of microreversibility which points out an error in the configuration weight factor used in the earlier work. We test the corrected N-fold-way algorithm on the 2D Ising model and, while still suffering from critical slowing, we find significant improvement in the autocorrelation times compared with the standard Metropolis approach.

We investigate a 19-vertex version of the two-dimensional fully frustrated XY (FFXY) model. We construct Yang-Baxter equations for this model and show that there is no solution. Therefore we have chosen a numerical approach based on the transfer matrix. The results show that a coupled XY Ising model is in the same universality class as the FFXY model. We find that the phase coupling over an Ising wall is irrelevant at criticality. This leads to a correction of earlier determinations of the dimension ${\mathit{x}}_{\mathit{h},\mathrm{I}\mathrm{s}}^{\mathrm{*}}$ of the Ising disorder operator. We find ${\mathit{x}}_{\mathit{h},\mathrm{I}\mathrm{s}}^{\mathrm{*}}$=0.123(5) and a conformal anomaly c=1.55(5). These results are consistent with the hypothesis that the FFXY model behaves as a superposition of an Ising model and an XY model. However, the dimensions associated with the energy, ${\mathit{x}}_{\mathit{t}}$=0.77(3), and with the XY magnetization ${\mathit{x}}_{\mathit{h},}$XY≊0.17, refute this hypothesis.

In this paper we use a Schwinger-boson mean-field approach to calculate the neutron-scattering cross section from the S=1/2 antiferromagnet with nearest-neighbor isotropic Heisenberg interaction on a two-dimensional triangular lattice. We investigate two solutions for T=0: (i) a state with long-range order resembling the Néel state and (ii) a resonating valence bond or ‘‘spin liquid’’ state with an energy gap, ${\mathit{E}}_{\mathit{g}}$≊0.17J, for the elementary excitations (spinons). For solution (ii) the neutron cross section shows Bragg rods at κ=K=(1/3, 1) / 3 ), whereas solution (ii) shows no elastic, but a set of broader dispersive spin excitations around κ≊(1/2,0) and around κ≊(1/3, 1) / 3 ) for ω/${\mathit{E}}_{\mathit{g}}$=2.5–4. It should thus be possible to distinguish these two states in a neutron-scattering experiment.

Molecular-dynamics simulations on the structures of 500 Ga atoms in liquid states are carried out at constant pressure for different high temperatures. Thereby, it is demonstrated that such anomalies as those present in the observed structure factors for the Ga liquids at high temperatures are due primarily to the effect of the occurrence of particular clusters (or local-ordering units) on the states of pure Ga liquid.

The linear and nonlinear ac susceptibilities of the canted-spin system Ce(${\mathrm{Fe}}_{0.96}$${\mathrm{Al}}_{0.04}$${)}_2$ have been measured using the mutual-inductance-bridge method as a function of temperature (50–100 K), frequency (19–1370 Hz) with different static biasing fields (12–36 Oe) as well as ac fields in the range of 4 to 24 Oe. An attempt has been made to compare the ‘‘canted-spin’’ system with the so-called ‘‘reentrant spin-glass system’’ from the ac-susceptibility point of view, since the ‘‘canting’’ plays a significant role in the reentrant spin-glass system, below the Curie temperature.

In this paper we develop a microscopic foundation for the Murata-Doniach model of spin fluctuations which has been widely used in connection with band-structure calculations. The main result of the paper is the formulation of the partition function of an itinerant system as a functional integral over magnetization modes, and an explicit formula for the energy functional appearing in the exponent of the Boltzmann factor. Such a derivation is made since former theoretical investigations focus on the interaction part of the parition function Z/${\mathit{Z}}_0$, whereas the Murata-Doniach model is formulated in terms of a functional integral for the complete partition function Z. We start with an approximate description of magnetic excitations of noninteracting fermions with collective modes, and derive a bosonlike partition function for these magnetization modes. This is combined with the well-known result for the interaction part of the partition function in the Hubbard model obtained by functional-integral theory. The leading term of the energy functional appearing in the exponent of the parition function agrees with that of the Ginzburg-Landau expansion for the energy of a classical magnetization field. In the course of the transformation to a bosonlike system we predict that the cutoff wave vector ${\mathit{q}}_{\mathit{c}}$ which must be introduced in the classical model is temperature dependent with ${\mathit{q}}_{\mathit{c}}$∼${\mathit{T}}^{1/3}$. It is shown that the frequencies of the collective modes are reduced by the Stoner enhancement factor compared with the one-particle excitation energies of Stoner theory.

We find the exact ground-state wave function of an open S=1/2 spin chain with exchange that scales in inverse proportion to the square of the sum and the difference of the coordinates of the spins. For isotropic exchange interaction, numerical study reveals that in common with the periodic manifestation of the spin model, the states are grouped into highly degenerate multiplets.

We report the experimental observation of the Josephson critical current across layers, ${\mathit{I}}_{\mathit{c}}^{\mathrm{\perp }}$, for Nb/Cu multilayers. Unique samples with a small cross section (20 μm in diameter) consisting of ten Nb/Cu junctions were fabricated for such measurements. A strong influence of the dimensional 3D-2D crossover on the ${\mathit{I}}_{\mathit{c}}^{\mathrm{\perp }}$ was observed. Thus, as the temperature becomes smaller than ${\mathit{T}}_{2\mathrm{D}}$, hysteresis in the current-voltage characteristic appears and the behavior of the temperature dependence of the ${\mathit{I}}_{\mathit{c}}^{\mathrm{\perp }}$ changes. For T>${\mathit{T}}_{2\mathrm{D}}$ the diminishing of the hysteresis is caused by a sharp decrease of the junction capacitance in the 3D regime when the sample becomes uniform across layers. Calculation of the critical-current temperature dependence ${\mathit{I}}_{\mathit{c}}^{\mathrm{\perp }}$(T) for our multilayers was made. An agreement between experimental and theoretical dependencies ${\mathit{I}}_{\mathit{c}}^{\mathrm{\perp }}$(T) was found. From the theoretical simulations, we have obtained the dependence of the crossover and the critical temperatures of multilayers on the layer thicknesses, the boundary transparency, and layer conductivity.