Table of contents

The fundamental measure density functional theory for hard spheres is generalized to binary mixtures of arbitrary positive and moderate negative non-additivity between unlike components. In bulk the theory predicts fluid–fluid phase separation into phases with different chemical compositions. The location of the accompanying critical point agrees well with previous results from simulations over a broad range of non-additivities and both for symmetric and highly asymmetric size ratios. Results for partial pair correlation functions show good agreement with simulation data.

Temperature dependent high-resolution angle-resolved photoelectron spectroscopy has been performed on the quasi-two-dimensional compound ZrTe₅, a metal at low temperatures ( K) that exhibits a maximum resistivity at a temperature (T_c), concomitant with a sign change of the thermopower. A semiconducting gap has been observed in the photoemission spectra, where the valence band maximum shifts upward from 82 meV (75 K) to 40 meV (170 K) as a function of temperature. The band shifts are accompanied by small band distortions. Based on the photoemission experiments, in conjunction with the metallic character of ZrTe₅ at low temperatures, we have modelled the thermopower of ZrTe₅ by treating it as a metal at low temperatures and a semiconductor at elevated temperatures.

We present an analysis of previously published measurements of the London penetration depth of layered organic superconductors. The predictions of the BCS theory of superconductivity are shown to disagree with the measured zero temperature, in plane, London penetration depth by up to two orders of magnitude. We find that fluctuations in the phase of the superconducting order parameter do not determine the superconducting critical temperature as the critical temperature predicted for a Kosterlitz–Thouless transition is more than an order of magnitude greater than is found experimentally for some materials. This places constraints on theories of superconductivity in these materials.

The ultrafast response of metals and semiconductors to electronic excitations is analysed. Time resolved two-photon emission and non-linear optics allow us to study the relaxation of systems at non-equilibrium. Representative results for the spin dependent relaxation of excited electrons in transition metals, in ferromagnetic metals such as Ni, Co, Fe and in high T_c superconductors are given. Also ultrafast dynamics of semiconductors such as Si, Ge and C is analysed. In particular, in covalently bonded solids, electronic excitations from the valence to the conduction band cause a change of the bond character and thus strong changes of various properties and induce coherent phonons, ablation, ultrafast melting and phase transitions at non-equilibrium. The ultrafast relaxation processes are controlled by energy and angular momentum conservation, of course, and this is particularly important for magnetism at non-equilibrium. The time dependence of the magnetic relaxation in nanostructures reflects in a characteristic way the atomic structure. The ultrafast Coulomb explosion of clusters in intense electric fields may serve as a demonstration of non-equilibrium dynamics resulting from electric field induced solid–plasma transition. Dynamics in small clusters can test the validity of concepts used in thermodynamics. Optically controlled electronic population dynamics may be expected to be relevant for ultrafast switching devices and recording.

This review emphasizes the physical principles underlying the dephasing of nonstationary vibrational states of molecules with multiple vibrational degrees of freedom. The motion of atoms within molecules can be described to a very good approximation by a many-level system of coupled anharmonic quantized oscillators. The separation of electronic and nuclear timescales, combined with weak symmetry breaking inherent in molecular structures, guarantees that the Hamiltonian can always be cast in a form that is local in the quantum state space of the molecular vibrations. The resulting nonexponential dephasing dynamics can be controlled with external fields to stabilize nonstationary quantum states, and both quantum–classical and fully quantum control formalisms are described. Coupled molecular vibrations interacting with external fields also offer prospects for quantum computing because vibrational level spacings can be made very large compared to thermal noise, and relaxation mechanisms such as infrared fluorescence are many orders of magnitude slower than the timescales required for coherence transfer. Finally, a toy model that provides insights into the interaction of vibrational degrees of freedom with 'solvent' modes is also discussed, and exhibits nonexponential dynamics in certain regimes.

We review recent experiments on optical detection and manipulation of spin states of impurity centres in a solid. This research topic is of particular interest because of possible applications of the single-spin read-out technique in solid state quantum computing. The experimental background of optical detection of single quantum systems in solids is introduced. The interaction of an impurity centre with an excitation field is analysed in terms of optical Bloch equations. Recent experiments on the electron spin resonance of single organic molecules and paramagnetic defect centres in diamonds are presented. We show how the spin state of the nitrogen–vacancy paramagnetic defect in diamond can be read out optically. Pulsed electron spin resonance of the single paramagnetic defect centre in diamond is discussed.

Detailed strain relaxation studies of epitaxial magnetite, Fe₃O₄, films on MgO(100) substrates grown by magnetron sputtering reveal the accommodation of strain up to 600 nm thickness, a thickness far above the critical thickness (t_c) predicted by theoretical models. The results are in agreement with the suggestion that the excess strain in Fe₃O₄/MgO (100) heteroepitaxy is accommodated by the presence of antiphase boundaries. The compressive strain generated by the antiphase boundaries compensates for the tensile strain within the growth islands, allowing the film to remain fully coherent with the substrate. Contrary to earlier findings, magnetization decreases with an increase in the film thickness. This vindicates the view that the structure of the antiphase boundaries depends on the growth conditions.

The surface geometric and electronic structure of the FeAl(110) intermetallic alloy has been investigated by scanning tunnelling microscopy and angle-resolved photoemission spectroscopy (ARPES). Preferential sputtering results in depletion of Al in the surface region and subsequent annealing promotes surface segregation of Al and gives rise to new reconstructed phases. A bulk terminated surface structure is obtained after annealing the surface to 400 °C. However, an incommensurate phase develops above 800 °C with a stoichiometry consistent with an FeAl₂ structure in the topmost layer. The ARPES measurements confirm the Al segregation with increased density of states (DOS) near the Fermi level. The increased DOS is believed to be due to hybridization between the Fe d and Al sp states. The increased intensity of the Al 2p core level for the incommensurate phase also confirms the higher Al surface concentration for this phase.

Intermetallic compounds of NdMn_6−xFe_xSn₆ () were studied by means of x-ray and neutron diffraction techniques and SQUID magnetic measurements in the temperature range of 30–400 K. The substitution of iron for manganese leads to a phase transition whereby NdMn₆Sn₆ with the HoFe₆Sn₆ structure (space group Immm) changes to TbFe₆Sn₆ (space group Cmcm) for NdMn_6−xFe_xSn₆ with . The iron atoms prefer to occupy the 8g sites at iron content x<2.0 due to the longest Mn/Fe–Sn bond distance. The Curie temperature (T_C) increases from x = 0 to 1.5 and then decreases for the larger iron content. The magnetic moment of the iron sublattice couples ferromagnetically with the manganese and neodymium moments for the x<2.0 samples. Spin reorientation is observed in samples with iron content up to 1.5, and the spin reorientation temperature (T_s) increases with increasing iron content. Except for NdMn₄Fe₂Sn₆, the easy direction of magnetization for all samples is parallel and perpendicular to the (bc) plane of the unit cell at 300 and 30 K, respectively. The easy direction of magnetization for NdMn₄Fe₂Sn₆ is parallel to the a-axis in the entire temperature range mentioned above, as a result of the anisotropic contraction of the unit cell along the (bc) plane.

Piezoresistance properties of quasicrystals due to both phonon and phason stresses are investigated. The classical formulae of the piezoresistance effect in crystals are generalized to the case of quasicrystals. The number of independent components of the piezoresistivity tensor and their matrix forms are determined for three-dimensional icosahedral quasicrystals and all two-dimensional quasicrystals with fivefold, eightfold, tenfold and twelvefold symmetries. Our results show that the piezoresistance effect may be related only to phonon stress in the case of dodecagonal quasicrystals or to both phonon and phason stresses in the other case.

We have grown single crystals of ThCoGa₄ by the self-flux method. X-ray examination showed that it crystallizes in an orthorhombic structure of the Y NiAl₄-type. Electron transport properties examination between 2 and 300 K allowed us to determine its phonon components to the electrical resistivities/thermoelectric powers, which at 300 K are equal (in µΩ cm /µV K⁻¹) to 17.3/11.7, 40.6/1.9 and 35.0/2.7, along the a-, b-, and c-axes, respectively. Specific heat examination allowed us to estimate the Sommerfeld coefficient and Debye temperature, which are equal to 6.9 (± 0.3) mJ K⁻² mol⁻¹ and 288 K, respectively. ThCoGa₄ was found to be a paramagnet with a nearly temperature independent susceptibility above 30 K, equal to about 6.7 × 10⁻⁵ emu mol⁻¹.

A thermodynamically complete ab initio equation of state (EOS) for MgO was obtained using electron density functional theory and the quasiharmonic phonon approximation, and adjusted to match the ambient density. This EOS was demonstrated to be consistent with isotherm, thermal expansivity, heat capacity and melting curve measured in static experiments, and reproduced density and temperature measurements under shock wave loading of bulk and porous periclase. The Grüneisen parameter of periclase at a given density was shown to be weakly dependent on temperature. The B1–B2 phase change was calculated to occur near 320 GPa on the principal Hugoniot. The melting locus of periclase, relevant to the Earth's lower mantle pressures, was predicted to be accessible by shock wave loading of porous periclase, which could also put pressure and temperature bounds on B1–B2 transitions.

Temperature induced structural phase transitions in the lead doped Aurivillius oxides Ba_1−xPb_xBi₂Nb₂O₉ (x = 0.375,0.625) are explored using powder neutron diffraction methods in the context of the pure parent compounds, PbBi₂Nb₂O₉ and BaBi₂Nb₂O₉. At both lead concentrations the system is found to exhibit behaviour similar to that found in PbBi₂Nb₂O₉. That is, at room temperature the oxides are orthorhombic and the Pb rich compound transforms to a tetragonal structure via a second orthorhombic form. The presence of Ba lowers the phase transition temperatures relative to the undoped lead compound but is not sufficient to induce a change in the general mode of structural transition.

Systematic implementations of density functional calculations of magnetic materials, based on atomic orbitals basis sets, are scarce. We have implemented in one such code the ability to compute non-collinear arrangements of the spin moments in the GGA approximation, including spiral structures. We have also made a thorough study of the degree of accuracy of the energy with the size of the basis and the extent of the orbitals. We have tested our results for the different phases of bulk iron as well as for small clusters of this element. We specifically show how the relative stability of the different competing states changes with the degree of completeness of the basis, and present the minimal set which provides reliable results.

Spontaneous ferromagnetic domains in lightly Ca-doped La_1−xCa_xMnO₃ single crystals have been visualized and investigated by means of the magneto-optical technique. In marked difference to the magnetic contrast structures associated with magneto-crystalline anisotropy, which appear only in applied magnetic field, spontaneous ferromagnetic domains show up at low temperatures below the Curie temperature in zero applied field and are characterized by oppositely oriented spontaneous magnetization in adjacent domains. Ferromagnetic domains seen in zero field cooled samples take the form of almost periodic, corrugated stripe-like structures. Application of even a very weak magnetic field during cooling through the magnetic ordering transition changes the stripe domain structures into a bubble domain system.

The two-dimensional ferromagnetic Kondo model with classical core spins is studied via unbiased Monte Carlo simulations for a hole doping up to x = 12.5%. A canonical algorithm for finite temperatures is developed. We show that, with realistic parameters for the manganites and at low temperatures, the double-exchange mechanism does not lead to phase separation on a two-dimensional lattice but rather stabilizes individual ferromagnetic polarons for this doping range. A detailed analysis of unbiased Monte Carlo results reveals that the polarons can be treated as independent particles for these hole concentrations. It is found that a simple polaron model describes the physics of the ferromagnetic Kondo model amazingly well. The ferromagnetic polaron picture provides an obvious explanation for the pseudogap in the one-particle spectral function A_k(ω) observed in the ferromagnetic Kondo model.

This work examines the magnetic order and spin dynamics of a double-exchange model with competing ferromagnetic and antiferromagnetic Heisenberg interactions between the local moments. The Heisenberg interactions are periodically arranged in a Villain configuration in two dimensions with nearest-neighbour, ferromagnetic coupling J and antiferromagnetic coupling −ηJ. This model is solved at zero temperature by performing a expansion in the rotated reference frame of each local moment. When η exceeds a critical value, the ground state is a magnetically frustrated, canted antiferromagnet. With increasing hopping energy t or magnetic field B, the local moments become aligned and the ferromagnetic phase is stabilized above critical values of t or B. In the canted phase, a charge-density wave forms because the electrons prefer to sit on lines of sites that are coupled ferromagnetically. Due to a change in the topology of the Fermi surface from closed to open, phase separation occurs in a narrow range of parameters in the canted phase. In zero field, the long-wavelength spin waves are isotropic in the region of phase separation. Whereas the average spin-wave stiffness in the canted phase increases with t or η, it exhibits a more complicated dependence on field. This work strongly suggests that the jump in the spin-wave stiffness observed in Pr_1−xCa_xMnO₃ with at a field of 3 T is caused by the delocalization of the electrons rather than by the alignment of the antiferromagnetic regions.

Novel quaternary rare-earth copper sulfides EuRCuS₃ (R = Y, Gd–Lu excluding Ho, Er) have been investigated by means of x-ray diffraction, magnetic susceptibility, magnetization, and specific heat measurements. These compounds crystallize in the Eu₂CuS₃-type structure for R = Y, Gd–Dy and in the KZrCuS₃-type structure for R = Tm–Lu. Both EuY CuS₃ and EuLuCuS₃ show a ferromagnetic transition and the other EuRCuS₃ compounds show a ferrimagnetic transition at around 5.0 K.

The sensitivity of the magneto-optical Kerr response to electronic thermalization processes in ultrafast pump–probe experiments is studied by evaluating the complex conductivity tensor of Ni for non-equilibrium electron distributions. The electronic structure and optical matrix elements are calculated within density functional theory. To account for the electronic redistributions generated by the intense pump-laser pulse during the initial stages of electronic thermalization, two kinds of model electron distributions are considered which mimic the so-called dichroic bleaching or state-blocking effect. Thus, certain optical transitions which are allowed under equilibrium conditions are not accessible to the probe laser. It is shown that the conductivity tensor and the complex Kerr angle can be modified substantially by the non-equilibrium electron distributions. Moreover, in striking contrast to the case for ordinary equilibrium conditions, the Kerr rotation and ellipticity are no longer proportional to the magnetization of the sample. The Kerr response at ultrashort times can therefore not be taken as a measure of demagnetization.

A detailed comparative study of the magnetic properties of LiFePO₄ and LiMnPO₄ samples is presented. Magnetic susceptibility, electron paramagnetic resonance and ⁷Li NMR experiments were performed on samples as prepared for electrochemical studies. The ground state of LiFePO₄ seems to be that of a collinear antiferromagnet and very robust against crystal imperfections. On the other hand, our LiMnPO₄ samples possess a weak ferromagnetic ground state with a transition temperature T_N = 42 K. We suggest that solitons may be very important magnetic excitations in these systems and that pinning of solitons below T_N together with frustration plays a decisive role in the formation of the weak ferromagnetic state in LiMnPO₄. The differences between the magnetic properties of these two samples reflect also the differences between their electronic structures and may thus be important for the electrochemistry of LiFePO₄ and LiMnPO₄.

We report on ferromagnetic Co-doped TiO₂ films grown by laser ablation on LaAlO₃ and SrTiO₃ substrates. Films on both types of substrate are highly crystallized, very well epitaxial, single-phased anatase and show ferromagnetism far beyond room temperature. Due to the smaller lattice mismatch in the case of LaAlO₃ substrates, films on LaAlO₃ substrates are more oriented than films on SrTiO₃ substrates, and therefore have better quality with a larger saturation magnetic moment. Besides x-ray diffraction data, our magnetic force microscopy results also suggest that the ferromagnetism in Co-doped TiO₂ films probably does not stem from either Co-enriched TiO₂ clusters or Co segregations.

The crystallization process in Fe_73.5Cu₁RE₃Si_13.5B₉ ribbons with RE = Pr, Nd, Gd is investigated using differential scanning calorimetry, x-ray diffraction and Mössbauer spectrometry. The crystallization steps are characterized after both short time (1 h) isothermal annealing in the 350–650 °C range of temperature and long time (24 h) isothermal annealing at 650 and 700 °C. It is found that the crystallization process is the same for the Pr- and Nd-containing ribbons and the crystallization products are α-Fe(Si), Fe₂B and (Fe,Si)₂₃B₆. For the Gd-containing sample, the crystallization occurs at a higher temperature, the same crystallization products are formed and Gd-containing phases are detected after 24 h, 700 °C annealing. The nature of the rare earth influences the crystallization kinetics, as the crystallization process for both Pr and Nd ribbons starts at a lower temperature and has a lower activation energy than that of the Gd ribbon, which is comparable to that of the Finemet alloy.

Fe–C granular films with different Fe volume fraction x_v were fabricated using a DC facing-target sputtering system at room temperature and subsequently annealed at different temperatures. X-ray diffraction and selected area electron diffraction analyses indicate that as-deposited and low-temperature annealed () samples are composed of amorphous Fe and C, and higher temperature annealing makes the amorphous Fe transform to α-Fe, which is also confirmed by high-resolution transmission electron microscopy. Magnetic measurements indicate that at room temperature the as-deposited Fe–C (x_v = 58) granular films are superparamagnetic and annealed ones are ferromagnetic. The coercivity of 100 nm thick Fe–C (x_v = 58) granular films increases with annealing temperature (for 1 h) and time (at 450 °C). The coercivity of the 100 nm thick Fe–C (x_v = 58) samples annealed at temperatures ranging from 400 to 500 °C decreases linearly with measuring temperature T, signalling a domain wall motion mechanism. For the samples annealed at 550 °C, the change of in-plane coercivity with T satisfies the relation , reflecting that this system behaves better as a set of Stoner–Wohlfarth particles. It was also found that there exists a critical thickness ( nm) for the 450 °C annealed (for 1 h) Fe–C (x_v = 58) granular films with thickness in the range 100–200 nm, below and above which the magnetization reversal is dominated by domain wall motion and by Stoner–Wohlfarth rotation, respectively.

We present the electrodynamic response of TiOBr, which undergoes a transition to a spin-singlet ground state for temperatures below T_C1 = 28 K. The temperature evolution of the phonon anomalies indicates, like in TiOCl, an extended fluctuation regime, extending well above T_C1. At low frequencies, the spectral weight is progressively suppressed by decreasing the temperature, suggesting the formation of a spin-gap and a considerable electron–phonon coupling. A comparison of the two oxyhalides shows a weaker interplane coupling and reduced dimensionality for TiOBr that enhances fluctuations and shifts the ordering temperatures to lower values. In the fluctuation regime the temperature dependences of some phonon parameters track the temperature evolution of the magnetic susceptibility.

The optical absorption spectra in the region of the transition energies of epitaxial layers of EuTe and Pb_0.1Eu_0.9Te, grown by molecular beam epitaxy, were studied using circularly polarized light, in the Faraday configuration. Under σ⁺ polarization a sharp symmetric absorption line (full width at half-maximum 0.041 eV) emerges on the low energy side of the band-edge absorption, for magnetic field intensities greater than 6 T. The absorption line shows a huge red shift (35 meV T⁻¹) with increasing magnetic field. The peak position of the absorption line as a function of magnetic field is dominated by the d–f exchange interaction of the excited electron and the Eu²⁺ spins in the lattice. The d–f exchange interaction energy was estimated to be J_dfS = 0.15 ± 0.01 eV. In Pb_0.1Eu_0.9Te the same absorption line is detected, but it is broader, due to alloy disorder, indicating that the excitation is localized within a finite radius. From a comparison of the absorption spectra in EuTe and Pb_0.1Eu_0.9Te the characteristic radius of the excitation is estimated to be  Å.

As a novel function of ferromagnet (FM)/spacer/FM junctions, we theoretically investigate a multiple-valued (or multi-level) cell property which is in principle realized by sensing conductances of four states recorded with magnetization configurations of two FMs: (up, up), (up, down), (down, up), and (down, down). In order to sense all the states, four-valued conductances corresponding to the respective states are necessary. We previously proposed that four-valued conductances are obtained in FM1/spin-polarized spacer (SPS)/FM2 junctions, where FM1 and FM2 have different spin polarizations, and the spacer depends on spin (Kokado and Harigaya 2003 J. Phys.: Condens. Matter 15 8797). In this paper, an ideal SPS is considered as a single-wall armchair carbon nanotube encapsulating magnetic atoms, where the nanotube shows on-resonance or off-resonance at the Fermi level according to its length. The magnitude of the obtained four-valued conductances has an opposite order between the on-resonant nanotube and the off-resonant one, and this property can be understood by considering electronic states of the nanotube. Also, the magnetoresistance ratio between (up, up) and (down, down) can be larger than the conventional one between parallel and anti-parallel configurations.

In this work, we focus on the ordered structures of V–X systems, where X = Ru, Rh, Pd, and relate the variation in the difference of the numbers of valence electrons of the alloy constituents to the information contained in the constitution phase diagrams, and the electronic and stability properties. The electronic properties deduced from the low-temperature specific heat studies are presented for the V–Ru and V–Rh systems and compared with those of the V–Pd alloys for which new experimental results are also included. The theoretical analysis based on first-principles electronic structure calculations confirms the measured variation of the electronic specific heat coefficients with alloy composition, and predicts specific ordering trends in the V–X systems. The superconducting properties are described for the V–X disordered alloys, the ordered V_1−xRh_x and V_1−xRu_x systems, and are related to their structural instability.

Allegations that our theoretical description of neutron scattering by entangled spin and spatial degrees of freedom is incorrect are shown to be misplaced for the allegations are derived from a naively incomplete, quantum mechanical calculation of neutron scattering by identical nuclei.

Karlsson and Lovesey's explanation of the intensity loss in EVS experiments on H as a consequence of quantum entanglement is flawed as, in particular, their results do not satisfy the first moment sum rule. The reason for the loss of intensity is probably either the failure of the Born–Oppenheimer approximation or experimental error.

Allegations in a Comment (Fillaux and Cousson 2004 J. Phys.: Condens. Matter16 1007) that our calculation of the intensity of neutrons diffracted by potassium bicarbonate is incorrect are shown to be ill-founded. The downfall of the Comment is not to apply quantum mechanics to the calculation of scattering by indistinguishable nuclei with overlapping degrees of freedom (protons or deutrons).