Table of contents

The equilibrium impurity-ligand distance, R_e, in six fluoroperovskites of the AMF₃ series (A: K, Rb, Cs; M: Mg, Zn, Cd, Ca) doped with Mn²⁺ has been explored through density functional calculations for MnF₆A₈M₆¹⁶⁺ clusters where the effects of the electrostatic potential due to the rest of the lattice are included. The R_e-values obtained coincide, within the experimental uncertainties, with those derived from the analysis of the experimental isotropic superhyperfine constant, A_s, and the 10Dq-parameter and also with available extended x-ray absorption fine-structure results. This important result is shown to be practically independent of the quality of the basis set employed. From the analysis of recent electron paramagnetic resonance data for AlF₃:Mn²⁺ it is concluded that, for this system, R_e197 pm which is 7% smaller than the average distance for pure compounds involving MnF₆^4- complexes. The latter figure is shown to be consistent with the substitution of Mn²⁺ for a trivalent cation in AlF₃:Mn²⁺. The present theoretical scheme has also been applied to Tl⁺ impurities in NaI. First results indicate an outwards relaxation of ligands which is equal to, at least, 5% and thus is much higher than the figure (1%) calculated by Aguado et al (Aguado A, Ayuela A, Lopez J M and Alonso J A 1998 Phys. Rev. B 58 11 964).

We calculate the value of the Fröhlich electron-phonon interaction in manganites, cuprates, and some other charge-transfer insulators and show that this interaction is much stronger than any relevant magnetic interaction. A polaron shift due to the Fröhlich interaction, which is about 1 eV, suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels, in agreement with the oxygen-isotope effect and other data. An opposite conclusion, recently suggested in the literature, is shown to be incorrect. The frequency and temperature dependence of the optical conductivity of ferromagnetic manganites is explained within the framework of the bipolaron theory.

A very large oxygen-isotope shift of the charge-stripe ordering temperature in the high-temperature superconductors La_2-xSr_xCuO₄ has been observed using x-ray absorption spectroscopy, a fast (~10^-15 seconds) and local probe (~5 Å). Upon replacing ¹⁶O with ¹⁸O, the charge-stripe ordering temperature in La_1.94Sr_0.06CuO₄ increases from about 110 K to 180 K. The present results provide compelling evidence for a strong electron-phonon interaction in this compound, and thus place important constraints on the microscopic origin of the stripe formation.

The frequency of photoluminescence is determined by the gap between the conduction and valence band. The width of the band gap depends on the crystal field and the chemical reaction, because the crystal field defines the band structure and the reaction repopulates with valence electrons. It is derived that the surface bond contraction and the rise in surface-to-volume ratio enhance the crystal field and determine the trend of frequency change, while oxidation enhances this behaviour by adding a constant.

Wandering of an array of steps in phase on Si(111) vicinal surfaces (5° off toward the [11] direction) formed by DC heating at temperatures 1000<T<1180 °C was studied. Periods of the step wandering formed on step-down current regions depended on the temperature and had a maximum at around 1100 °C. This also supports the view that the wandering is due to the DC heating effect. The periods were also measured for the step wandering of anti-bands on terraces between step bands formed by DC heating with a step-up current and we found that the periods were appreciably smaller than that in the step-down current regions.

The topic of this special section, `Magnetism at surfaces, interfaces and thin films', covers an area where surface science has led to important technical applications, and is well known through the use of the giant magnetic resistance phenomenon in magnetic reading heads. The interplay between basic research and technology still continues. Accordingly, recent years have seen intense activity and fast progress in the field of basic research on surface magnetism. Possibilities today of manipulating surfaces, interfaces and thin films, almost at will, opens a vast potential for designing magnetic structures involving both new physics and new applications.

This special section, which is timely in view of the progress achieved, reviews recent important developments in the field of surface, interface and thin films magnetism. Both theoretical and experimental points of view are considered. The topics presented include film growth and film magnetism, correlations between magnetism and crystallographic structure as well as between magnetic order and electronic structure, the theory of surface magnetism and of magnetic multilayers, magnetism of nanostructures and nanocrystalline materials and, last but not least, high resolution imaging of magnetic structures by scanning probe techniques and photolectron microscopy. Most articles use special materials systems to describe the issues under consideration, yet the reader will certainly get a general view of the field's present status.

K Heinz

The spin density wave (SDW) magnetism of thin epitaxial Cr films has recently become the focus of interest because of its mediating role in exchange coupled superlattices. While the incommensurate SDW magnetism and the Néel temperature are well established for bulk Cr, the question arises of how these properties are altered in thin films and superlattices either due to dimensionality effects or due to proximity with the ferromagnetic or paramagnetic boundary layers. After a brief introduction to the basic properties of bulk Cr, this review provides an overview of the SDW magnetism in thin Cr films, starting with surface properties and continuing with the discussion of Cr films of various thickness. The emphasis is more on SDW order in different confined environments than on exchange coupling. The scaling of the Néel temperature with thickness, the critical thickness for the onset of SDW order, the orientation of the SDW wave vector in different environments and the enhancement of SDW order due to proximity effects are extensively discussed. Most important is the role of the interface roughness in case of contact with a ferromagnetic layer. Conflicting results obtained with different experimental techniques are critically reviewed and an interpretation of the SDW order depending on interface quality is proposed.

We report on a set of systematic first-principles electronic structure investigations of the magnetic spin moments, the magnetic spin configurations, and the magnetic coupling of ultrathin magnetic films on (001)- and (111)-oriented noble-metal substrates and on the Fe(001) substrate. Magnetism is found for 3d-, 4d-, and 5d-transition-metal monolayers on noble-metal substrates. For V, Cr, and Mn on (001) substrates a c(2 × 2) antiferromagnetic superstructure has the lowest energy, and Fe, Co, Ni are ferromagnetic. On (111) substrates, for Cr the energy minimum is found for a 120° non-collinear magnetic configuration in a ( × )R30° unit cell, and for Mn a row-wise antiferromagnetic structure is found. On Fe(001), V and Cr monolayers prefer the layered antiferromagnetic coupling, and Fe, Co, and Ni monolayers favour the ferromagnetic coupling to Fe(001). The magnetic structure of Mn on Fe(001) is a difficult case: at least two competing magnetic states are found within an energy of 7 meV. The Cr/Fe(001) system is discussed in more detail as the surface-alloy formation is investigated, and this system is used as a test case to compare theoretical and experimental scanning tunnelling spectroscopy (STS) results. The possibility of resolving magnetic structures by STS is explored. The results are based on the local spin-density approximation and the generalized gradient approximation to the density functional theory. The calculations are carried out with the full-potential linearized augmented-plane-wave method in film geometry.

The results of epitaxial growth studies of ferromagnetic metals for a selected class of nonmagnetic substrates is reviewed. The reverse sequence is also discussed for some systems of importance in double layer and sandwich studies. The interrelation between film structure and magnetic properties is pointed out for several examples.

We have applied magnetic force microscopy (MFM) and spin-polarized scanning tunnelling spectroscopy (SP-STS) in ultrahigh vacuum (UHV) to study the magnetic domain structure of Co/Au(111) and Gd/W(110), respectively. A new measurement mode will be presented which minimizes possible accidental interactions between the probe tip and the sample in UHV-MFM. This allows the investigation of the well known spin reorientation transition of Co films grown on Au(111) at nanometre-scale spatial resolution. Details of domain walls and the domain structure as a function of CO coverage are studied. Spin-polarized tunnelling is realized by using an Fe-coated W tip and a sample which exhibits an exchange-split surface state. Magnetic domain imaging with a spatial resolution below 20 nm is demonstrated for Gd/W(110). The observed bias dependent spin polarization of Gd(0001) is in good agreement with spin-resolved inverse photoemission spectroscopy results.

This paper presents a review of the phenomenon of interlayer exchange coupling in magnetic multilayers. The emphasis is put on a pedagogical presentation of the mechanism of the phenomenon, which has been successfully explained in terms of a spin-dependent quantum confinement effect. The theoretical predictions are discussed in connection with corresponding experimental investigations.

The magnetic order in ultrathin films depends critically on a variety of film parameters. Therefore, to understand the magnetic properties of thin films on the basis of the spin-dependent electronic structure is a challenging task for both experimentalists and theoreticians. Experimentally, spin-resolved electron spectroscopies probe directly either the spin-dependent density of states or specific energy states, often at a defined wave vector. In this paper, two surface-sensitive techniques, that provide complementary information about the unoccupied electronic states, are described. Spin-resolved appearance potential spectroscopy gives element-specific access to the spin-dependent local density of states. Spin-resolved inverse photoemission permits detailed investigations of electron states characteristic of the surface and the layers underneath as a function of the wave vector. Both techniques are sensitive also to the film structure. A variety of magnetic film properties is discussed in the light of the electronic structure. The examples described in this paper include Fe films on W(110) and Cu(001) as well as Gd films on W(110).

Though correlations between surface structure and magnetism are evident, surface structure determinations of ultrathin magnetic films in the crystallographic sense are available only for a few cases. We investigate the three examples Ni/Cu(100), Co/Cu(111) and Fe/Cu(100) which each exhibit only little film-substrate lattice misfit but which are rather different as regards the native lattice symmetry of the materials involved. Correspondingly, the structures of the films are rather different. They are described for various film thicknesses in each case and a comparison with the magnetic properties is made.

The more interesting features in magnetism of systems of nanoparticles are reviewed. Tailoring of soft and hard magnetic materials as well as basic studies on magnetic interactions are discussed. Particular emphasis is given to the magnetic properties of the particle shell and grain boundaries, generally different to these of the core, and responsible for phenomena such as interphase exchange penetration, Curie temperature enhancement and magnetic coupling. The magnetic behaviour of different nanocrystalline systems has been described. Spin disorder has been found to be a general trend for the magnetic ground state of the outer shell of magnetic particles. Disorder at the surface can be due to competing interactions with different signs originating from broken bonds or topological disorder (grain boundaries), random surface anisotropy, surface magnetostriction, compositional gradients and in general to the enhanced gradient of different properties at the surface. The spin-glass-like ground state of the surface only affects the macroscopic properties in nanocrystalline samples for which the ratio between the number of atoms at the interface and the number of atoms in the core can be enormous, actually as large as 30%.

Magnetic properties of materials can be tailored in nanostructures, such as thin films, wires and dots. This article gives an overview of what is known about the electronic states that are relevant for magnetic phenomena, such as oscillatory coupling, giant magnetoresistance (GMR) and spin-polarized tunnelling. These states are probed by high-resolution photoemission and inverse photoemission. Methods for fabricating one- and zero-dimensional structures are explored, such as stripe and dot arrays obtained by step decoration.

In the last decade enormous effort has been made in research and investment to study magnetic properties of thin films because of their obvious practical applications. Coincidentally, it happens that theory has made enormous progress. Ab initio calculations and microscopic theories allow us for the first time in the history of magnetism to study and manipulate the magnetism on an atomic scale. In contrast to bulk magnetic materials, ultrathin films allow us to manipulate magnetism via the thickness and, by use of artificial structure growth, to produce structures which do not appear in nature. Here we discuss the fundamental magnetic observables, i.e. magnetization, Curie temperature, magnetic moment per atom, susceptibility and magnetic anisotropy, for idealized prototype thin films like Fe, Co, Ni on metal substrates such as Cu, W, Re. Finally, we present studies on trilayers, i.e. magnetic thin films separated by a spacer, like Cu. These trilayers present prototypes of interlayer coupling relevant for practical use of multilayer structures.

Photoemission electron microscopy (PEEM) has proven to be a powerful analytical tool in surface science. In this contribution, a status report is given on the application of the PEEM technique in the investigation of surface and interface magnetism. Owing to its fast parallel image acquisition and its wide zoom range, allowing fields of view from almost 1 mm down to a few micrometres, combined with a high base resolution of the order of 20 nm, the method offers a unique access to many aspects in surface and thin-film magnetism on the mesoscopic length scale. There are three basically different modes of magnetic imaging using PEEM. The first one exploits the magnetic x-ray circular dichroism (MXCD) for contrast formation. It offers the important advantage of selecting the magnetic contrast of a certain element via the corresponding x-ray absorption edges using a tuneable x-ray source such as synchrotron radiation. This mode gives access to magnetic structures and coupling phenomena with a sensitivity in the submonolayer range and the capability to image the signal of buried layers with an information depth up to more than 5 nm. The two other modes work with simple UV light sources and are therefore highly attractive for standard laboratory applications. The magnetic stray-field-induced changes of the electron trajectories close to the sample surface lead to a Lorentz-type contrast. A third type of contrast arises as a consequence of the Kerr rotation of the dielectric vector inside a magnetic material, a phenomenon which is also responsible for the well known magneto-optical Kerr effect. Examples and typical applications of magnetic imaging using PEEM are discussed.

High-resolution scanning tunnelling microscopy images show that low-coverage room temperature coadsorption of potassium on the Ni(100)(2 × 2)p4g-N surface occurs at step edges aligned preferentially along the 011 directions. The data indicate that the potassium atoms sit at the step edges a distance of 5.0±0.05 Å apart. A model is presented for the coadsorbed step structure.

For spin-polarized low-energy electrons impinging on non-magnetic crystalline surfaces, the collision with a valence electron and the ensuing emission of an electron pair are treated in a distorted-wave Born approximation formalism with exchange, in which the four relevant quasi-one-electron states are solutions of the Dirac equation. Numerical calculations for W(001), which were carried out in two coplanar geometries with normal and grazing incidence of a primary beam polarized normal to the reaction plane, show that the (e, 2e) cross section changes significantly upon reversal of the polarization. Originating mainly from spin-orbit coupling in the valence electron state, asymmetries of up to 30% occur in conjunction with sizable intensity. The calculated spectra respond sensitively to changes in the surface structure.

Angle-resolved energy distributions for photoelectrons emitted from the Bi(111) single crystal face are presented at 16.85 eV photon energy. The variation of peak-energy positions as a function of the electron emission angle shows four main dispersing features located between the Fermi level and 5 eV. The bulk electronic structure is found in qualitative agreement with the pseudopotential calculation performed by Golin along the TMU and TQW symmetry lines of the Brillouin zone. The experimental features located at around 0.3 and 3 eV below the Fermi level are sampled in the two-dimensional Brillouin zone along the and symmetry directions (dispersion about 1 eV) and are assigned to electronic surface states.

Unoccupied density of states (DOS) for the polar (0001) Zn, (000) O and non-polar (100) ZnO surfaces and for a MgO(001)-c(2 × 2) thin film were studied using total current spectroscopy (TCS). TC spectra of the ZnO surfaces reveal the Zn 4s-O 2p band (6.2 eV) and two DOS maxima in the Zn 4p-O 2p band which exhibited different energy positions on the Zn-terminated (8.1 eV) and O-terminated (11.0 eV) surfaces. In the TC spectra of the MgO thin film, the energy location of Mg-derived (8.0, 14.8 and 21.0 eV) and O-derived (11.6, 14.2 and 18.5 eV) maxima of DOS were determined. Previously reported data on DOS of these materials is discussed, and it is shown that additional detailed information on the energy structure of unoccupied DOS in ZnO and MgO in the 5-25 eV range above E_F has been found.

We present a model calculation for the temperature-dependent behaviour of surface states on a ferromagnetic local-moment film. The film is described within the s-f model featuring local magnetic moments being exchange coupled to the itinerant conduction electrons. The surface states are generated by modifying the hopping in the vicinity of the surface of the film. In the calculation for the temperature-dependent behaviour of the surface states we are able to reproduce both Stoner-like and spin-mixing behaviour in agreement with recent (inverse) photoemission data on the temperature-dependent behaviour of a Gd(0001) surface state.

The electronic structures and energies of Co/Cu and Co/Pd multilayers with abrupt or mixed interfaces have been calculated using the linear muffin-tin orbital method in the atomic sphere approximation. The mixed interface is modelled by exchanging atoms between Co layers and Cu (Pd) layers. The calculated total energies of Co/Cu multilayers with mixed interfaces are higher than those of Co/Cu multilayers with abrupt interfaces. The calculated total energies of Co/Pd multilayers with mixed interfaces are lower than those of Co/Pd multilayers with abrupt interfaces. Stable interface structure appears to be abrupt in Co/Cu multilayers and mixed in Co/Pd multilayers, as revealed by experimental observations. Total-energy differences between the multilayer with abrupt interfaces and the multilayer with mixed interfaces are caused by Co 3d-band narrowing for Co/Cu multilayers and Pd spin polarizations for Co/Pd multilayers.

Strontium barium niobate (SBN) thin films of good quality were deposited on Si and Pt/Si substrates using a polymeric resin containing metallic ions. Using x-ray diffraction, the presence of SBN phase for films annealed at 600 °C and 700 °C for 1 hour was identified on both substrates. Films were also crystallized by rapid thermal annealing (RTA) at different temperatures and for different times, presenting good crystallization. Undesired phases such as SrNb₂O₆ and BaNb₂O₆ appear in films deposited on Si and Pt/Si substrates for films crystallized using a conventional furnace. However, using RTA these phases were eliminated for films annealed at 700 °C for 60 and 120 seconds.

The effect of doping on a-Si:H film optical constants was studied. Evaporated films were doped with Al for making p-type material. The optical energy gap E_g decreased with increase in doping concentration. The Urbach parameter increased with doping. The refractive index decreased with increasing Al concentration showing a sharp rise in dispersion curve for p⁺ samples. The absorption coefficient increased with doping and the absorption edge shifted to lower energies.

Starting from the -electron Pariser-Parr-Pople (PPP) Hamiltonian which includes both strong electron-phonon and electron-electron interactions, we propose some strongly correlated wave functions of increasing quality for the ground states of conjugated polymers. These wave functions are built by combining different finite sets of local configurations extended over at most two nearest-neighbour monomers. With this picture, the doped case with one additional particle is expressed in terms of quasi-particles. Thus, the polaron formation problem reverts to the study of a Holstein-like model.

The Raman spectra of nine 216-atom models of amorphous silicon (a-Si) are calculated using the bond polarizability approximation of Raman scattering. These a-Si models, generated by the activation relaxation technique, have different concentrations of coordination defects, ring statistics and local strain distributions, which cause changes in the vibrational density of states and the Raman scattering. Analysis of the vibrational modes indicates that an increase in the number of coordination defects leads to an increase in the high-frequency localization and to mixing of the TA modes with other high-frequency modes. Calculation of partial Raman spectra indicates that five-coordinated Si atoms enhance the high-frequency part of the LO Raman peak at about 400 cm^-1 and lead to characteristic band at about 600 cm^-1 on the high-frequency side of the TO Raman peak. For their part, the three-coordinated Si atoms contribute to the low-frequency part of the LO peak. A weak correlation between the number of four-membered rings and the intensity of the LO Raman peak is also established although there is no correlation between the number of three- and four-membered rings and the total strain energy.

The internal friction for Ti-Ni-Cu shape memory alloys (SMAs) was measured in the Hz and kHz range. The peak temperature in the Hz range is independent of the measuring frequencies. Only in the tens of kHz range does the peak temperature shift with the frequencies, showing a thermally activated progress with = ₀e^{-B/T_c - T}, characteristic of viscous motion of domain walls, instead of the Arrhenius relation. Taking into account the change of the density of the domain walls during the phase transformation, we modified the Q^-1-formula by using a model of viscous motion of domain walls, obtaining a good result in agreement with the experimental data. Additionally, corresponding parameters, which play a key role in the shape memory effect, such as the viscosity coefficient and the effective pinning force constant, were obtained.

The structure and lattice dynamics of CuInSe₂ were studied using ab initio calculations. The phonon dispersion relations and phonon density of states were calculated using the direct method. The results are in good agreement with recently obtained inelastic neutron scattering data.

Density functional theory with the local density approximation has been used to calculate Hellmann-Feynman forces for two chalcopyrite crystals: AgGaSe₂ and AgGaTe₂. Applying the direct method, phonons at all high-symmetry points of the chalcopyrite structure have been obtained. The results are in very good agreement with experimental data from Raman, infrared, and inelastic neutron scattering.

Interatomic potentials are determined in the framework of a shell model used to simulate the structural instabilities, dynamical properties, and phase transition sequence of BaTiO₃. The model is developed from first-principles calculations by mapping the potential energy surface for various ferroelectric distortions. The parameters are obtained by performing a fit of interatomic potentials to this energy surface. Several zero-temperature properties of BaTiO₃, which are of central importance, are correctly simulated in the framework of our model. The phase diagram as a function of temperature is obtained through constant-pressure molecular dynamics simulations, showing that the non-trivial phase transition sequence of BaTiO₃ is correctly reproduced. The lattice parameters and expansion coefficients for the different phases are in good agreement with experimental data, while the theoretically determined transition temperatures tend to be too small.

The UCu_5-xPd_x heavy-fermion alloy system is known to exhibit non-Fermi-liquid (NFL) behaviour for compositions with x1. The magnetic properties associated with NFL behaviour of this system are the focus of this study. Results are presented on the temperature (1.7T300 K) and magnetic field (0B5 T) dependences of the magnetization M and susceptibility for polycrystalline UCu_5-xPd_x samples with x = 0.8, 0.9, 1.0 and 1.2. We emphasize the observed field dependence of at low temperatures and illustrate the instability of NFL properties against magnetic field. Our (T) data present a systematic power-law divergence for NFL UCu_5-xPd_x at low temperatures in the limit of small measuring fields. This is interpreted in terms of Griffiths-phase singularities near a quantum critical point which are treated in a model given by Castro Neto and co-workers that includes the effects of structural disorder and a generic RKKY-Kondo interplay (where RKKY Ruderman-Kittel-Kasuya-Yosida). For the extended range 1.7T300 K, we find that (T) scales reasonably well with a theoretical expectation of Souletie and co-workers of NFL behaviour of Kondo systems. Finally, our M(T,B) data for magnetic fields up to 5 T and 1.7T21 K are shown to conform to a NFL scaling relation that applies to T = 0 quantum critical systems. The above analyses support the role that magnetic interactions are thought to play in NFL UCu_5-xPd_x.

Cu-doped BaTiO₃ polycrystals with the nominal composition of (x = 0.1 - 1) were synthesized. The samples are multiphase. The metallic conducting behaviour was seen for the sample with x = 0.5, while semiconducting behaviour appears for the other compositions. For x = 0.4, variable-range-hopping conduction was observed. The results are briefly discussed. It is proposed that the BaTiO₃ phase modified by Cu doping might be responsible for the metallic behaviour observed.

At a sufficiently low temperature electrons on the surface of liquid helium form a classical Wigner crystal. As long as the electron drift velocity is not too large the electron lattice distorts the helium surface to form a lattice of dimples, which leads to an enhanced electron effective mass. This system is observed to exhibit a complicated non-linear magnetoconductivity, which we discuss in terms of a simple classical model. The model displays in a unified way the effects of two phenomena that have been treated separately by other authors: Bragg-Çerenkov scattering of capillary waves, and decoupling of the electrons from the dimples at high electric fields (sliding). Our results differ in important ways from those of the other authors.

An array of 3 × 10⁷ Ge self-assembled quantum dots is embedded into the active channel of a Si metal-oxide field-effect transistor. Conductance oscillations with the gate voltage resulting from successive loading of holes into the dots are observed. On the basis of measurements of the temperature dependence of the conductance maxima, the charge-transfer mechanism in the channel is identified as being due to variable-range hopping between the dots, with the typical hopping energy determined by inter-dot Coulomb interaction. The characteristic spatial dimension of the hole wavefunctions as well as the charging energies of the dots are determined from the conductance data.

We have calculated binding and transition energies of the ground and some excited states of a shallow donor impurity in a disc-shaped GaAs quantum dot (QD), under the action of a magnetic field applied in the axial direction. The binding energies were obtained using the effective-mass approximation within a variational scheme, assuming an infinite confinement potential at all surfaces. Our results were obtained for several dot radii, the impurity position along the z-direction, and as a function of the applied magnetic field. We found that some excited states are not bounded for some values of the radius of the dot and of the applied magnetic field. We have shown how the applied magnetic field split the degeneracy of some excited states. Also, we have compared our results with those found in GaAs-(Ga, Al) As quantum wells (QWs) and quantum-well wires (QWWs).

We present the results of measurements of the penetration depth anisotropy in pulverized, ceramic La_2-xSr_xCuO₄. The measurements were carried out for x = 0.08, 0.1, 0.125, 0.15 and 0.2. The powdered samples, immersed in wax, were magnetically oriented in a static magnetic field of 10 T. The penetration depth in the a-b plane, _ab, and perpendicular to it, , were derived from alternating-current susceptibility measurements. For underdoped samples they both vary linearly with temperature (for the low-temperature region), while for the samples from the overdoped region the measured points can be fitted by an exponential function. These results support Uemura's picture (Uemura Y J 1997 Physica C 282-287 194) of crossover from Bose-Einstein condensation to a Bardeen-Cooper-Schrieffer mechanism of superconductivity. The penetration depth values extrapolated to T = 0 may be described by a quadratic function of the strontium concentration (for both _ab and ). The anisotropy of the penetration depth as a function of the substitution shows a similar dependence to the critical temperature T_c(x).

We investigate numerically the temperature dependence of the London penetration depth within the mean-field treatment of the interlayer pair tunnelling model for the copper oxide superconductors. It is found that the assumption that the pair tunnelling is the dominant pairing mechanism in YBCO (yttrium-barium-copper oxide) is not consistent with the experimental results on this material. We also consider the Knight shift and the dynamic spin susceptibility at a low temperature within the model. We find that the experimental results for these quantities are consistent with a relatively small contribution of the interlayer pair tunnelling to the pairing channel provided that, at least in the case of the dynamic susceptibility, the in-plane pairing produces a gap of d(x²-y²)-wave symmetry which is non-zero within at most a few tens of meV off the Fermi line.

The interplay between spin and orbital degrees of freedom in the Mott-Hubbard insulator is studied by considering an orbitally degenerate superexchange model. We argue that orbital order and the orbital excitation gap in this model are generated through the order-from-disorder mechanism known previously from frustrated spin models. We propose that the orbital gap should show up indirectly in the dynamical spin structure factor; it can therefore be measured using the conventional inelastic neutron scattering method.

We study the breakdown of the magnetization plateau at the magnetization M = M_S/3 (M_S is the saturation magnetization) of the S = 1/2 anisotropic spin chain with ferromagnetic-ferromagnetic-antiferromagnetic interactions. We consider the model with the isotropic ferromagnetic (trimer) coupling J_F, and anisotropic antiferromagnetic coupling (J_x = J_y = J_AF and J_z = J_AF). For the limit of large J_F/J_AF, the model is equivalent to the S = 3/2 XXZ chain with the exchange anisotropy . There is a phase transition between the plateau (small-) and the no-plateau (large-) regions. This phase transition is of the Berezinskii-Kosterlitz-Thouless type, and we determine the phase boundary from the numerical diagonalization data. For = 1, in particular, the phase transition between the plateau and the no-plateau regions occurs at the point _c = 15.4.

Electrical resistivity (T), magnetoresistance MR, magnetization M(T,B) and magnetic susceptibility (T,B) measurements at temperatures down to 25 mK and applied magnetic fields up to 14 T are presented for Th-diluted U_1-xTh_xPd₂Al₃ alloys. For U-rich alloys, antiferromagnetic order is observed and T_N varies slowly with the Th concentration in the region 0x0.2. From (T) data we calculate a value of the energy gap = 12 K in the spin-wave spectrum. In the U-rich concentration region a coherent Kondo description applies, but for alloys with 0.4x0.93 a non-Fermi-liquid (NFL) phase prevails. An approximately linear temperature dependence of (T) is observed at low temperatures for about a decade in temperature, but deviation from this dependence occurs at the lowest temperatures. We show that the MR of the NFL alloys may be described in terms of the Coqblin-Schrieffer Hamiltonian, from which values of the single-ion characteristic Kondo temperature T_K40 K are calculated. An applied magnetic field is shown to recover Fermi-liquid dynamics in both (T) and (T). We give a description of the low-temperature magnetization M(T,B) in terms of a two-channel Kondo model.

Novel ternary intermetallic compounds La₂Co_17-xM_x (M = Nb,Mo,Mn) with Th₂Zn₁₇-type structure were synthesized and their homogeneous ranges were determined: 0.3x0.6 for Nb, 0.6x1.2 for Mo and 1.0x4.0 for Mn. The lattice parameters and the unit-cell volume increase with increasing x in all of the systems. The preferential occupancy of 6c crystallographic positions (dumb-bell sites) and the stabilization of these compounds by M atoms are attributed to the atomic-size and enthalpy effects. These compounds exhibit favourable uniaxial anisotropy with M content x0.5. The Curie temperature decreases monotonically with increasing x for all of the systems. The saturation moment µ_s decreases rapidly with increasing x for M = Mo and Nb. In the La₂Co_17-xMn_x system, µ_s increases slightly with x for x2.5 and then decreases for x>2.5.

We present a microscopic theory of ferroelectric phase transitions and off-centre displacements in perovskites. We show that the inclusion of strong intraband electron-phonon interaction (i.e. the existence of small polarons) into the framework of the interband theory of displacive-like ferroelectrics leads to local spin-like structural distortions in the paraelectric phase. Their interaction with the soft mode induces spin-spin coupling through the soft phonon leading to a spin-ordering phase transition. The resulting theory is shown to quantitatively explain both displacive-like and order-disorder-like features of two representative perovskites: KNbO₃ and PbTiO₃.

Static current-current correlation leads to a zero-frequency divergence (ZFD) in the definition of optical susceptibilities. Previous computations have shown non-equivalent results for two gauges ( p·A and E·r) for exactly the same unperturbed wavefunctions. We reveal that these problems are caused by the incorrect treatment of the time-dependent gauge phase factor in optical response theory. The gauge phase factor, which is conventionally ignored by the theory, is important in resolving the ZFD problem and obtaining equivalent results for these two gauges. The Hamiltonians with these two gauges are not necessarily equivalent unless the gauge phase factor is properly considered in the wavefunctions. Both Su-Shrieffer-Heeger (SSH) and Takayama-Lin-Liu-Maki (TLM) models of trans-polyacetylene serve as illustrative examples in studying the linear susceptibility ⁽¹⁾ through both current-current and dipole-dipole correlations. Previous improper results of ⁽¹⁾-calculations and for distribution functions obtained with both gauges are discussed. The importance of the gauge phase factor in solving the ZFD problem is emphasized on the basis of the SSH and TLM models. As a conclusion, the reason for dipole-dipole correlation being preferable to current-current correlation in practical computations is explained.