Table of contents

Nonlinear transport is studied in the limit of weak and strong intra-dot Coulomb interaction. The nonequilibrium self-consistent mean-field equations for one-electron transition energies of an open dot and their spectral weights are derived at the strong Coulomb interaction. In this limit populations of states involved in tunnelling equalize upon the increase of the bias-voltage window even at low temperature. This results in a simple analytical relation between the heights of the current steps and the degeneracy of a spectrum in a two-dimensional parabolic dot in a magnetic field.

We report systematic measurements using the ⁵⁹Co nuclear quadrupole resonance (NQR) technique on the cobalt oxide superconductors Na_xCoO₂·1.3H₂O over a wide Na content range x = 0.25–0.34. We find that T_c increases with decreasing x but reaches a plateau for x≤0.28. In the sample with x∼0.26, the spin–lattice relaxation rate 1/T₁ shows a T³ variation below T_c and down to T∼T_c/6, which unambiguously indicates the presence of line nodes in the superconducting (SC) gap function. However, for larger or smaller x,1/T₁ deviates from the T³ variation below T∼2 K even though the T_c (∼4.7 K) is similar, which suggests an unusual evolution of the SC state. In the normal state, the spin correlations at a finite wavevector become stronger upon decreasing x, and the density of states at the Fermi level increases with decreasing x, which can be understood in terms of a single-orbital picture suggested on the basis of LDA calculation.

A skyrmion model of nano-domain ejection from large domain walls in ferroelectrics is presented, together with data on lead germanate Pb₅Ge₃O₁₁ and comparison data on ferromagnetic iron garnet. Notable is the occurrence of a short-wavelength transverse wall instability (wiggles) prior to the nano-domain emission. This nonlinear, defect-free model is qualitatively different from all known models of ferroelectric nucleation and propagation; nucleation in ferroelectrics has almost always been viewed as inhomogeneous, initiated at static impurity or defect sites that are fixed in space; the present model is also inhomogeneous but involves nucleation at existing domain walls, which are dynamic and not fixed in space. This defect-free ferroelectric nucleation model contrasts with the frequently invoked mechanism in thin-film switching of nucleation at electrode–dielectric interfaces and thus has significant implications for the ultimate switching speed in thin-film memory devices.

In this paper, the full-potential linearized augmented plane-wave (LAPW) method within the generalized gradient approximation (GGA) was used to calculate the effect of hydrostatic pressure at zero temperature on the 4d transition metal zirconium. For the hexagonal close-packed (hcp), omega (ω), and body-centred cubic (bcc) structures the enthalpy, H = E+pV, was calculated as a function of pressure p. We obtained an ω to bcc transition pressure of 28.2 GPa. Temperature-dependent contributions were obtained from tight-binding calculations of the phonons in the quasiharmonic approximation and were used to calculate the Gibbs free energy as a function of both temperature T and pressure for these three structures. From the comparison of these free energies the phase boundaries were calculated in the T–p phase diagram, and compared to the experimentally determined boundaries. We discuss the importance of anharmonicity for understanding the material properties of zirconium, and limitations of quasiharmonic phonon theories for predicting phase transformations.

Electronic structure, formation energies, transition levels, and concentration of intrinsic defects in wurtzite ZnO are investigated by the projector augmented wave method in the generalized gradient approximation. Interstitials, vacancies, and antisites at different charge states are considered. Convergence of the formation energies of various intrinsic point defects is carefully checked, and comparison with earlier results is made and discussed. Even though there exists a difference for the calculated formation energies of certain defects, our calculations also show that oxygen and zinc vacancies are the dominant intrinsic donor and acceptor defects in ZnO, indicating a consistency among results by different methods. The oxygen vacancy is not expected to be the main source of strong n-type conductivity in the unintentionally doped ZnO, due to its deep level in the bandgap, but it must be the origin of the experimentally observed visible photoluminescence band centred between 2.3 and 2.5 eV.

We have studied the ferromagnetic and antiferromagnetic fluctuations in the charge-ordered Pr_0.65Ca_0.35MnO₃ antiferromagnet by triple-axis neutron spectrometry. Whereas ferromagnetic fluctuations are observed above and below the charge-ordering transition (T_CO), the antiferromagnetic fluctuations develop only below T_CO. The dynamical exponent z of both ferromagnetic and antiferromagnetic fluctuations are determined. The ferromagnetic fluctuations are not completely suppressed below T_CO and their correlation lengths are short-ranged at all temperatures. The results are discussed with respect to the Zener polaron model recently introduced to describe the charge-ordered state of Pr_0.6Ca_0.40MnO₃.

The local structure and optical and vibrational properties associated with Mn²⁺-doped cubic AMF₃ (A = K, Rb; M = Mg, Zn, Cd) fluoroperovskites are studied by means of embedding calculations using Kohn–Sham equations with constrained electron density. It is shown that while an electronic parameter like 10Dq essentially depends on the Mn²⁺–F⁻ distance, the local vibration frequencies ω_i (i = a_1g, e_g modes) are dominated by the interaction between F⁻ ligands and nearest M²⁺ ions lying along bonding directions. The high ω_a values observed for KMgF₃:Mn²⁺ and KZnF₃:Mn²⁺, the huge variations of ω_e and ω_a frequencies when the host lattice is changed, as well as the increase of Huang–Rhys factors and the Stokes shift following the host lattice parameter, are shown to be related to this elastic coupling of the MnF₆⁴⁻ complex to the rest of the host lattice. The present results support the conclusion that the Stokes shift is determined by the interaction of the excited ⁴T_1g state with a_1g and e_g local modes while the coupling with the t_2g shear mode is not relevant. The variations of local vibrational frequencies and the Stokes shift induced by a hydrostatic pressure on a given system are shown to be rather different to those produced by the chemical pressure associated with distinct host lattices.

We demonstrate theoretically that spin dynamics of electrons injected into a GaAs semiconductor structure through a Schottky barrier possesses strong non-equilibrium features. The electrons injected are redistributed quickly among several valleys. Spin relaxation driven by the spin–orbital coupling in the semiconductor is very rapid. At T = 4.2 K, injected spin polarization decays at a distance of the order of 50–100 nm from the interface. This spin penetration depth reduces approximately by half at room temperature. The spin scattering length is different for different valleys.

Quasiclassical boundary conditions for electrochemical potentials at the interface between diffusive ferromagnetic and non-magnetic metals are derived for the first time. An expression for the boundary resistance accurately accounts for the law of conservation of momentum as well as essential gradients of the chemical potentials. Conditions are established at which spin asymmetry of the boundary resistance has a positive or negative sign. Dependence of the spin asymmetry and the absolute value of the boundary resistance on the exchange splitting of the conduction band opens up new possibilities for estimating spin polarization of the conduction band of ferromagnetic metals. Consistency of the theory is checked on existing experimental data.

The two-band theory of superconductivity of MgB₂ with additional six-fermion interaction is considered. It is shown that a bound state of a spin-singlet Cooper pair of fermions from one of the bands and an individual fermion from the other one is possible. The spin- triples with zero angular momentum are made up of three spin- fermions with charge e. They are gapped Fermi excitations with the gap induced by the gaps of the single fermions. Their contribution to the thermodynamics of the MgB₂ superconductors is considered.

Magnetic interactions in (Ga,Mn)N are studied on the microscopic inter-cluster and intra-cluster scales using first-principles calculations. Ferromagnetic transition temperatures are calculated using Monte Carlo methods. Randomness in Mn substitution is found to reduce Curie temperatures by about 10–20% from those of the corresponding regular (Ga,Mn)N lattice due to clustering. Nevertheless, high Curie temperatures reaching above room temperature are obtained even for completely random Mn distribution in (Ga,Mn)N for the Mn concentration of x≥13.5%. Increasing clustering is always found to decrease the Curie temperature—especially when tetramer clusters are formed.

The spin transfer torque model applied in the context of a Fokker–Planck analysis (Li and Zhang 2004 Phys. Rev. B 69 134416) is shown to account for a complete set of statistical data for switching times obtained with pseudo spin-valves (Fábián et al 2003 Phys. Rev. Lett.91 257209). Current densities of the order of 10⁷ A cm⁻² injected in Co/Cu/Co bilayers electrodeposited in nanoporous membranes gave rise to magnetization switching. Statistics could be accumulated on one single nanowire at a time: the field at which the average residence times in parallel and antiparallel configurations were equal, these times as a function of current, and the ratio of the times as a function of current and field.

The structure of an oxygen-related luminescence centre in manganese-doped LiBaF₃ was investigated by means of photoluminescence (PL) and PL-detected electron paramagnetic resonance. At 20 K an oxygen-related complex shows two luminescence bands peaking at about 430 and 475 nm, when excited at 220 nm. These bands can be attributed to an excited triplet state (S = 1) of an oxygen–vacancy complex with the z axis of the fine structure tensor parallel to the direction. This complex is believed to be next to a Mn²⁺ impurity on a Ba²⁺ site and can be described as an oxygen on a fluorine lattice site with a nearest fluorine vacancy along the direction.

We have grown single crystals of the type-VIII intermetallic clathrate Ba₈Ga₁₆Sn₃₀ from both Sn and Ga fluxes, evaluated their compositions through electron microprobe analysis and studied their transport properties through measurements on the temperature dependent resistivity, thermopower and Hall coefficient. Crystals grown in Sn flux show n-type carriers and those from Ga flux show p-type carriers, whereas all measured compositions remain very close to the stoichiometric 8:16:30 proportion of Ba:Ga:Sn, expected from charge-balance principles. Our results indicate a very high sensitivity of the charge carrier nature and density with respect to the growth conditions, leading to relevant differences in transport properties which point to the importance of tuning this material for optimal thermoelectric performance.

Powder samples of YH₃ and YD₃ have been studied by neutron diffraction (ND) with a much higher statistical accuracy than obtained previously. The profile analysis of the obtained ND patterns confirmed the high-symmetry HoH₃-type structure of YH₃ and ruled out the 'broken symmetry' structures proposed recently to explain the insulating properties and lattice dynamics of this compound. At the same time, it was demonstrated that the HoH₃-type structure is only the structure of the mean lattice of YH₃. Large static displacements of H atoms from the symmetrical positions in this structure do occur, and ordering of these displacements on a short-range scale can settle the controversies between the crystal structure and physical properties of YH₃.

The electrical, magnetic and transport properties of Zn doped polycrystalline samples of Sr₂Fe_1−xZn_xMoO₆ (x = 0,0.05,0.15 and 0.25) with the double perovskite structure have been investigated. The subtle replacement of Fe³⁺ ions by Zn²⁺ ions facilitates the formation of a more ordered structure, while further substitution leads to disordered structure because of the presence of a striped phase. Analysis of the x-ray powder diffraction patterns based on Rietveld analysis indicates that the replacement of Fe³⁺ by Zn²⁺ ions favours the formation of Mo⁶⁺ ions. The spin-glass behaviour can be explained on the basis of the competition between the antiferromagnetic superexchange and the ferromagnetic double-exchange interaction. The low-field magnetoresistance was moderately enhanced at x = 0.05, and its origin was found to be the competition between the decrease of the concentration of the itinerant electrons and the weaker antiferromagnetic superexchange in the antiphase boundaries. An almost linear negative magnetoresistance in moderate field has been observed for x = 0.25. A possible double-exchange mechanism is proposed for elucidating the observations; it also suggests a coexistence of (Fe³⁺,Mo⁵⁺) and (Zn²⁺,Mo⁶⁺) valence pairs.

We propose a new efficient method to achieve terahertz (THz) radiation with multi-mode and equal spectral power in multi-periodically poled dielectric materials. The optimal structure consists of a uniform grating and multiple phase-reversal sequences. Single-, dual-, triple-, quartic-, and even multi-mode THz radiations can be generated flexibly using different pre-designed structures. The dependence of THz radiation on the optical pulse duration is studied and analysed; this turns out to be an important parameter for optimum THz generation.

In this paper we offer an interpretation of the previously observed trend of the electric field gradient (EFG) values measured in a group of semiconducting delafossites CuBO₂ (B = Al, Fe, Cr, Nd) at Cd impurities which substitute either Cu or B atoms. Our theoretical study indicates that this EFG trend reveals one of the most subtle details in the electronic spectrum of the compounds, namely whether the impurity states are formed within or out of the band gap. When Cd substitutes the Cu, it exhibits a larger EFG value in CuAlO₂ and CuFeO₂ than in CuCrO₂ and CuNdO₂. This occurs because the Cd states form in the first two compounds a shallow band within the gap, but in the second two compounds they do not. When Cd substitutes the B atom it exhibits almost the same EFG in all delafossites. In this case, Cd states are not formed within the gap in any of the compounds. The same interpretation can be applied to the whole family of CuBO₂ delafossites, whatever the B atom is. To arrive at these conclusions we analysed and calculated various systems (Cd-doped CuAlO₂ and CuCrO₂ compounds, and molecular clusters) using the full-potential linear augmented plane wave method.

A mesoscopic theory for the primitive model of ionic systems is developed for arbitrary size, λ = σ₊/σ₋, and charge, Z = e₊/|e₋|, asymmetry. Our theory is an extension of the theory that we developed earlier for the restricted primitive model. The case of extreme asymmetries and is studied in some detail in a mean field approximation. The phase diagram and correlation functions are obtained in the asymptotic regime and , and for infinite dilution of the larger ions (volume fraction n_p∼1/Z or less). We find a coexistence between a very dilute 'gas' phase and a crystalline phase in which the macroions form a bcc structure with the lattice constant ≈3.6σ₊. Such coexistence was observed experimentally in deionized aqueous solutions of highly charged colloidal particles.

Adiabatic calorimetry measurements have been performed on Cs₂CdBr₄ from 90 to 270 K. Phase transitions at 252.03, 237.02, and 154.88 K were found. The thermal expansion coefficients and the elastic constant values were used to separate the harmonic and anharmonic contributions to the specific heat, which permitted us to establish a baseline for the precise determination of the thermodynamic functions associated with the phase transitions. Anharmonic quantities, such as the Grüneisen parameter and the isothermal compressibility, were also calculated. Finally, no experimental evidence of the suggested transitions at 208 and 130 K was found.

The interplay between the Fano effect and electronic correlations in a double-quantum-dot (DQD) structure is investigated by the finite-U slave-boson mean-field method, and the focus is put on the influence of the Aharonov–Bohm (AB) effect on the transport properties. In the singly occupied regime, with weak antiferromagnetic correlation, the Fano–Kondo effect greatly suppresses the conductance G, whereas the strong parity splitting blocks the path through the DQD, which is similar to the time-reversal symmetric case. At the particle–hole symmetric point the peak usually still exists in the would-be peak-zero structure, but with the magnetic flux increased the zero is enhanced and eventually removed at quarter of a flux quantum. Results similar to the frequency doubling of AB oscillation and the 'pinning' of the AB maximum can be found, which account for the appearance of a new transmission zero in the variation of G with the energy levels on dots.

A micro-Raman study of high quality Nd_0.5Ca_0.5MnO₃ single crystals is presented, with temperature variation. Twelve A_g and eight B_2g Raman active modes (simplified symmetry: Pmma space group) have been observed in resonance conditions and compared to the parent and charge ordered compound phonons.

The detected Nd_0.5Ca_0.5MnO₃ phonons retrace the magnetic and structural temperature evolutions in the paramagnetic, charge ordered (T_co∼250 K) and antiferromagnetic orbital ordered (T_N∼160 K) regimes. In particular, at T<T_N, phase separation and also rotational and coherent Jahn–Teller MnO₆ distortions prevail.

Magnetic and spectroscopic techniques were used to study the intermetallic antiperovskite Mn₃GaC. An antiferromagnetic–ferromagnetic magnetostructural transition at 160 K underlies a remarkable magnetocaloric effect; these phenomena are suppressed in the substoichiometric composition Mn₃GaC_1−δ. X-ray absorption spectroscopy (XAS) data reported for three compositions Mn₃GaC_1−δ, δ = 0, 0.10, 0.22, are the basis for drawing inferences concerning the mechanism controlling magnetic order as a function of carbon stoichiometry. While the temperature dependence of the Mn₃GaC carbon K edge reveals no observable change across the first-order magnetic transition, a clear splitting of the carbon absorption bands is observed that increases with increasing carbon deficiency. The room temperature Mn and Ga K edges indicate no significant variation with C content. FEFF 8.2 code calculations are in good qualitative agreement with data for the stoichiometric sample, but do not predict the changes in XAS observed in C-deficient samples. These results and the Goodenough–Anderson–Kanamori rules are the basis for a phenomenological model that attributes the carbon content dependence of the low temperature transition to the promotion of weak near-neighbour 90° Mn–Mn pairs in the carbon-deficient compound over the stronger 180° Mn–C–Mn interaction, locking in dominant ferromagnetism at low temperatures.

The photoluminescence spectrum of band edge transitions in Al_xGa_1−xAs is studied as a function of temperature and electron concentration. The parameters that describe the temperature dependence redshift of the band edge transition energy are evaluated using different models. We find that a semi-empirical relation based on a phonon dispersion related spectral function leads to an excellent fit to the experimental data.

We have investigated the role of transition metal impurities and oxygen vacancies in the formation of ferromagnetism in Co-doped TiO₂ using the LSDA +U approach which takes into account strong on-site Coulomb correlations for electronic structure calculations. Several model supercells of rutile TiO₂, with a Co²⁺ ion in high-spin state substituted into the titanium site and with oxygen vacancies, were considered. We found that the exchange interaction of Co magnetic ions is ferromagnetic, but very weak due to the large average impurity–impurity distance. However, it becomes three times stronger when there is a magnetic vacancy nearby. The magnetic moment values are 3 μ_B for Co²⁺ and −1 μ_B for vacancy. Our investigation showed that the exchange interaction energy of Co and vacancy moments varies from 330 to 40 meV depending on the distance between them in the supercell. We assume that the strong interaction between Co and vacancy moments should be taken into account to explain the high Curie temperature value in Co-doped TiO₂.

The complete energy matrix including all the spin states is constructed for d⁴ ions in tetragonal symmetry within a strong-field representation. The excitation levels, fine-structure splitting, and EPR parameters in Rb₂CrCl₄ and CrF₂ are studied with the energy matrix. The contributions of the spin singlets to zero-field splitting (ZFS) are investigated for the first time. The results show that the spin-singlet contribution to D is negligible, but the contributions to a and F cannot be neglected.

We investigate the linear transport properties of quantum point contacts (QPCs) whose symmetry is deliberately broken in a controlled manner. The devices that we study consist of a conventional split-gate QPC, which is modified by the inclusion of an additional perturbing gate that is used to modulate the electron density on one side of the device. As the voltage applied to this 'finger gate' is varied, we observe several reproducible features below the last integer plateau, as well as strong modifications of the integer-plateau staircase. Self-consistent calculations, performed for the exact device structure utilized in experiment, suggest that these features are related to the ability of the finger gate to strongly disrupt the symmetry of the QPC, reducing the electron density significantly on one side of the device. We discuss these results within the context of recent models for many-body electron transport in QPCs.

Fluorine nuclear magnetic resonance (NMR) lineshape data were collected for a pure LaF₃ single crystal and La_1−xSr_xF_3−x single crystals, doped with admixtures concentrations x up to 16%, from room temperature up to 775 K. The experimental data are analysed within the framework of a theoretical model linking the shape of the spectrum to parameters describing motional processes in the system. The elaborated model includes various types of the fluorine dynamics: diffusion processes occurring inside two distinct fluorine sublattices and inter-lattice exchange motion. This theoretical approach provides a satisfactory and consistent interpretation of the NMR data, without assuming any distribution of the characteristic time constants. The obtained results are discussed in a context of Arrhenius temperature dependences of the correlation times and effects caused by higher concentrations of the admixtures.

The process of hydrogenation of the antiferromagnetic compounds CePdIn and CePdSn has been studied. Investigation of the new hydrides CePdInH and CePdSnH by means of x-ray powder diffraction reveals that they adopt the same crystal symmetry as the original intermetallic but the unit cell volume increases during the hydrogenation. Magnetization, electrical resistivity, thermoelectric power and ¹¹⁹Sn Mössbauer spectroscopy measurements reveal that CePdInH and CePdSnH order antiferromagnetically below T_N = 3.0(2) and 5.0(2) K respectively. Moreover, the physical properties of the hydrides are less influenced by the Kondo effect. The changes of the Néel temperature induced by H insertion are explained on the basis of Doniach's diagram considering the competition between Kondo and Ruderman–Kittel–Kasuya–Yosida interactions.

The electronic structure of TlCo₂Se₂ has been investigated by means of band structure calculations and x-ray photoelectron and emission spectroscopy (XPS and XES). The formation of the valence band is described in connection with calculations of partial densities of states of the valence band and Co L_α,β x-ray emission () aligned to the binding energy scale. The experimental results are in agreement with the calculated distribution of Co 3d states, which lie close to the Fermi level. The effect of Co–Se bonding is seen as satellite structures in the XPS Co 2p signals. Thermoelectric and Hall effect data as well as resistivity measurements show that TlCo₂Se₂ is metallic with electron holes as charge carriers with broadband mobility characteristics. The observed anisotropy of the resistivity in the ab plane and along c remains constant as a function of temperature, i.e. without signs of the magnetic ordering that occurs (T_N∼85 K). The results of local spin density functional calculations show ferromagnetic coupling within the cobalt plane as a result of direct magnetic interactions between the magnetic moments.

Within the Hartree Fock RPA analysis, we derive the spin wave spectrum for the weak ferromagnetic phase of the Hubbard model on a honeycomb lattice. Assuming a uniform magnetization, the polar (optical) and acoustic branches of the spin wave excitations are determined. The bipartite lattice geometry produces a q-dependent phase difference between the spin wave amplitudes on the two sub-lattices. We also find an instability of the uniform weakly magnetized configuration towards a weak antiferromagnetic spiraling spin structure, in the lattice plane, with wavevector Q along the Γ–K direction, for electron densities n>0.6. We discuss the effect of diagonal disorder on both the creation of electron bound states and the enhancement of the density of states, and the possible relevance of these effects to disorder-induced ferromagnetism, as observed in proton-irradiated graphite.

We present a theoretical study of the damping phenomenon in a device of coupled quantum dots, in the context of quantum computation. We derive the one-phonon contribution to the damping rate, based on Fermi's golden rule, for the case where the device lies in the super-Ohmic regime. The result is in agreement with that of Leggett et al derived within the noninteracting blip approximation (NIBA). Moreover, we consider the multi-phonon contribution to damping according to Würger, and estimate the temperature range for the coupled dots to operate as a qubit device. A threshold temperature T^*, 18 K, is determined, above which the tunnelling time becomes longer than the damping time.

The cubic perovskite related material CaCu₃Ti₄O₁₂ has attracted a great deal of attention due to the high values of the static dielectric constant, of order 10⁴, approximately constant in the temperature range 100–600 K. The substitution of Ca by Cd results in a similar temperature dependence but a static dielectric constant more than one order of magnitude lower. A theoretical electronic structure study is performed on CaCu₃Ti₄O₁₂ (CCTO) and CdCu₃Ti₄O₁₂ (CdCTO) using a tight binding with overlap method. Although the calculations are performed in a paramagnetic configuration, excellent agreement with experiment was found for the calculated band gap of CCTO. In spite of the fact that the band structures of both systems look practically the same, a significant difference is found in the calculated bond strength of Ca–O and Cd–O pairs, driven by the presence of Ti, with Ca–O interaction in CCTO loosened with respect to Cd–O interaction in the cadmium compound. It is suggested that O vacancies are more easily formed in CCTO, this being related to the lower electronegativity of Ca as compared to Cd. The formation of oxygen vacancies could be the origin of the difference in the static dielectric constant of the two compounds.

For a two-dimensional Heisenberg antiferromagnetic -depleted square lattice, it has been analytically proved that its ground state possesses simultaneously ferromagnetic and antiferromagnetic long-range orders, and exhibits ferrimagnetism. Numerical simulations of finite lattices using exact diagonalization show that the depletion strengthens the short-range spin–spin correlations whereas it weakens the long-range ones. The -depleted square lattice has a smaller susceptibility. This means that the geometric structure of the usual two-dimensional square lattice is more advantageous for establishing magnetic long-range order. By comparison of these two kinds of lattices, it is quite reliably inferred that Néel order exists in the ground state of the usual two-dimensional spin- Heisenberg antiferromagnetic square lattice.

We calculate the spin and charge dynamical susceptibilities of a strongly correlated impurity model in a renormalized perturbation theory. The irreducible vertices for the quasiparticle scattering are deduced from the renormalized parameters, which have been calculated by fitting the low-lying levels of a numerical renormalization group (NRG) calculation to those of an effective Anderson model. The susceptibilities are asymptotically exact in the low frequency limit and satisfy the Korringa–Shiba relation. By comparing the results with those calculated from a direct NRG calculation, we show that the renormalized perturbation theory (RPT) description gives a very good description of the spin dynamics for all values of the local interaction U, not only at low frequencies but over the whole relevant frequency range.

In the presence of a magnetic field the approach can be generalized using field dependent renormalized parameters to calculate the transverse and parallel spin dynamic susceptibilities. The RPT gives accurate results for the spin dynamics over the whole frequency range and for all values of the magnetic field.

A simple explanation for the logarithmic ageing of the photoconductivity in YH_3−δ is proposed. We show that the scaling ('simple' ageing) of the relaxation response with the illumination time t_w is consistent with the superposition of independently relaxing excitations with time offsets distributed over a window of width t_w.

In figure 3 on page 5891 the solid circles denote the data at 4 K and open circles denote the data at 300 K.