Table of contents

We investigate the frustrated quantum three-spin model (S₁,S₂,S₃) of spin = 1/2 on a triangle, in which spins are coupled with lattice-vibrational modes through the antiferromagnetic exchange interaction depending on distances between spin sites. The present model corresponds to the dynamic Jahn–Teller system proposed by Longuet-Higgins et al (1958 Proc. R. Soc. A 244 1). This correspondence is revealed by using the transformation to Nakamura–Bishop's bases used in Phys. Rev. Lett.54 861 (1985). Furthermore, we elucidate the relationship between a chiral order parameter in the spin system and the electronic orbital angular momentum in vibronic model: the regular oscillatory behaviour of the expectation value with increasing energy can be found as in the case of for vibronic structures. The increase of the additional anharmonicity(chaoticity) is found to yield a rapidly decaying irregular oscillation of .

The properties of nanoscopic superconducting structures fabricated with a scanning tunnelling microscope are reviewed, with emphasis on the effects of high magnetic fields. These systems include the smallest superconducting junctions which can be fabricated, and they are a unique laboratory in which to study superconductivity under extreme conditions. The review covers a variety of recent experimental results on these systems, highlighting their unusual transport properties, and theoretical models developed for their understanding.

The dispersion curves of the surface polaritons of a cylinder which is made of a material with dispersive permittivity and permeability, and which is left-handed over a frequency band, are calculated. It is found that there exist three surface mode bands. With increasing wavevector the three bands converge to three limiting frequencies, which are equal to the three corresponding frequencies obtained for the slab geometry. The extinction properties of the cylinder are derived, and the dependence of the extinction spectrum on the cylinder radius is investigated.

A generalized derivation of the equations governing surface carrier diffusion in the surface region of an insulator is presented, based on the Mott–Gurney model of ionic diffusion as first proposed in Liesegang et al (1995 J. Appl. Phys.77 5782; 1996 J. Appl. Phys.80 6336). The resulting non-linear equations are decoupled for the case of one-dimensional diffusion and we show that the decay of the electric field is described by the inviscid Burgers equation. Imposing initial and boundary conditions reflecting the experimental configuration for a Cartesian system as discussed in Liesegang et al, a general solution for the carrier density in the surface of an insulating sample is derived for the case of one-dimensional charge motion.

Polycrystalline europium monoxide (EuO) films doped with and without Gd were prepared using a reactive deposition method in a MBE system. Two different electron doping methods, Gd substitution and oxygen deficiency, enhance the Curie temperature (T_C) and the temperature dependence of magnetization differently. Gd doping increases T_C up to 125 K, while oxygen deficient samples enhance it above 150 K. The enhanced T_C is attributed to the increase in the indirect exchange interaction between Eu²⁺ spins via conduction electrons. The resistivities as a function of temperature for both quasi-stoichiometric (QS) EuO and QS-Gd_xEu_1−xO show maxima near 70 and 110 K, respectively. The conduction band magnetic splitting explains the resistivity behaviours. Both the temperature dependences of the large magneto-resistance and the absorption edge are also interpreted by the conduction band magnetic splitting mechanism. The unusual blue shift observed in Gd-doped samples is not yet understood.

Ni/Cu(100) films are known to exhibit an 'inverse' spin reorientation transition (SRT). The magnetization rotates from in-plane to out-of-plane upon increasing the thickness. The driving force of this phenomenon is a volume uniaxial anisotropy due to the tetragonal film structure, which favours an out-of-plane orientation. We investigated the structure and magnetism of Ni_xMn_1−x alloys on Cu(100). We find an improved growth of Ni_xMn_1−x alloy films compared to Ni/Cu(100). The SRT of Ni_xMn_1−x alloy films is studied using the magneto-optical Kerr effect (MOKE). We find that a Ni-type reorientation can be observed for Mn contents up to 13%. For two concentrations, we were able to determine the volume anisotropy K_V and surface anisotropy K_S values using hard axis magnetization curves, which we discuss in the context of experimentally determined Ni/Cu(100) anisotropy values. We find that the driving force of the reorientation is a positive K_V due to a tetragonal distortion. The positive value of K_V can be related to the magneto-elastic properties although the variation of the reorientation thickness d_c as a function of Mn concentration fails within this model.

The case of two oppositely charged planar surfaces interacting across a salt solution is reconsidered to explain why the release of counterions from the slit between the plates can lead to an attractive contribution to the effective interaction between the plates. We estimate the order of magnitude of the effective inter-plate potential by first considering a Gouy–Chapman layer in front of a single charged plate. The Gouy–Chapman problem is fully characterized by just one parameter, the dimensionless surface charge parameter s. Considering the number of ions in the double layer as a function of s, we can identify two regimes: one in which the electro-neutrality of the double layer is ensured through the exchange of co- and counterions, and another, at large s, where counterions are to be recruited from the reservoir for the system to remain electro-neutral. The counterion-release force plays a major role only for plates with surface parameters that belong to the second regime. Having at hand the results from the single-plate case we are then able to give a simple interpretation of the counterion-release interaction. The counterion-release force is dominant for systems with large s and at large distances. At small distances, when all extra ions have left the slit between the plates, the release mechanism ceases to work and the two plates attract each other due to an trivial electrostatic attraction.

NH--N hydrogen bonded ferroelectric dabcoHBF₄, [C₆H₁₃N₂]⁺·BF₄⁻, has been studied by dielectric spectroscopy, differential thermal analysis and pyroelectric charge measurement investigations in the low-temperature range between 12 and 300 K, and at elevated hydrostatic pressures up to 1 GPa. The p–T phase diagram has been determined and described. In addition to the known first-order phase transitions at 378 and 153 K, two new structural transitions of continuous type have been revealed, i.e., the transition at 37 K and the pressure-induced one occurring above the triple point situated at 80 MPa and 165 K. Both of these transitions have essentially displacive character and a mechanism related to distortions of the hydrogen bonded polycationic chains, preserving polar properties of the crystal. The possible order–disorder contribution to the transition at 153 K between the ferroelectric phases II and III is discussed in relation to the conformational properties of the dabco cation.

The first ab initio calculations were carried out for the electronic and structural properties of the wide band gap semiconductor alloy Mg_3xBe_3−3xN₂ employing the full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). We used the Perdew et al generalized gradient approximation (GGA96), which is based on exchange–correlation energy optimization, to optimize the internal parameters by relaxing the atomic positions in the force directions and to calculate the total energy. For band structure calculations, we utilized both the Engel–Vosko's generalized gradient approximation (EVGGA), which optimizes the exchange–correlation potential, and also GGA96. We investigated the effect of composition on a variety of different structural and electronic parameters such as lattice constant, bond length, internal parameter, bulk modulus and band gap. We found out that the linear concentration dependence (LCD) is inadequate to explain the results, hence we fitted our data with a quadratic expression and were able to obtain the bowing parameter for each case. Our results for the band gap, lattice parameter, cohesive energy and bulk modulus indicate that each of them can be explained by a constant bowing parameter.

By properly generalizing Nozières' Fermi liquid theory, we construct a Hamiltonian approach to the scattering of conduction electrons off a spin-1/2 impurity in the overscreened Kondo regime as . We derive the S-matrix at the interacting fixed point, and the corresponding phase shifts, together with leading energy corrections to the unitary limit. We apply our results to obtain the low-temperature dependence of the two-channel Kondo conductance, and we relate it to possible transport experiments in a quantum dot.

Taking simultaneously into account the electron-injected current from one normal-metal (N) electrode and the hole-injected current from the other N electrode, we study the coherent tunnelling conductance and quantum interference effects in N/d-wave superconductor (S)/N double tunnel junctions. It is found that oscillations of all quasiparticle transport coefficients and the conductance spectrum with quasiparticle energy and thickness of the d-wave S depend to a great extent on the crystal orientation of the d-wave S. The zero-bias conductance peak is gradually lowered with increasing barrier strength and/or temperature, its magnitude exhibiting damped oscillatory behaviour with thickness of S.

We compare magnetotransport measurements under pressure on the organic superconductor κ-(BEDT-TTF)₂Cu(SCN)₂ (BEDT- (ethylene-dithio)tetrathiafulvalene) with different pressure media and discover that the results are pressure media dependent. This pressure medium dependence is thought to originate from the difference in thermal contraction between the very soft and highly anisotropic sample and the isotropically contracting, but solid pressure medium, thus resulting in non-hydrostatic pressure on the sample. However, comparison of pressure measurements with different media reveals a pressure medium independent correlation between the superconducting transition temperature, T_c, and the size of the quasi-two-dimensional Fermi surface pocket and thus the quasi-two-dimensional carrier density in κ-(BEDT-TTF)₂Cu(SCN)₂. The observed pressure-induced increase in the quasi-two-dimensional carrier density can be interpreted as a transfer of carriers from quasi-one-dimensional Fermi surface sections, reminiscent of a mechanism in cuprate superconductors, where pressure is known to transfer carriers from the insulating charge reservoir layers into the conducting cuprate sheets.

High quality single crystals of CeNiGe₂ have been investigated by means of magnetic susceptibility, magnetization, electrical resistivity, magnetoresistivity and thermoelectric power measurements, carried out along all three principal crystallographic directions. The compound is an antiferromagnetic Kondo system that orders magnetically at T_N = 3.9 K and undergoes a spin structure rearrangement at T₁ = 3.2 K. The magnetic behaviour is strongly anisotropic with the easy magnetic direction parallel to the crystallographic a-axis. The Kondo temperature and the total crystal field splitting are of the order of 20 and 100 K, respectively.

¹⁷⁰Y b Mössbauer spectroscopy has been used to study Y bMn₂Si_2−xGe_x for . Y bMn₂Si₂ contains only trivalent Yb. A Y b²⁺ component appears at x = 1.15(5) and the Yb in Y bMn₂Ge₂ is fully divalent. Transferred hyperfine fields at the Yb site for x<1.2 show that a canting of the Mn moments occurs between 35 and 65 K, while a second event is seen at for which probably reflects a further reorganization of the Mn order rather than ordering of the Y b³⁺ moments.

We present a comparative study on the magnetic and magnetotransport properties of (Ga, Mn)N epitaxial films grown on undoped GaN and n-type GaN templates by plasma-enhanced molecular beam epitaxy. Regardless of the kind of substrate, the (Ga, Mn)N epitaxial films were found to obviously exhibit ferromagnetic ordering with a Curie temperature exceeding room temperature. However, the negative magnetoresistances of the (Ga, Mn)N films grown on undoped GaN and n-type GaN templates show a remarkable difference. Negative magnetoresistance for the (Ga, Mn)N films grown on undoped GaN templates was observed up to room temperature, while negative magnetoresistance for the (Ga, Mn)N grown on n-type GaN templates was observed only below 100 K. Considering that the Mn concentrations in the (Ga, Mn)N films are the same, it is believed that the content of Mn spins in the (Ga, Mn)N films grown on n-type GaN templates is not sufficient to order all the carriers in the n-type GaN and the film itself, since the Mn spins in (Ga, Mn)N interact with the carriers in both.

The low-temperature proton spin–lattice relaxation is analysed for lanthanum hydrides LaH_x intentionally doped with Gd or Ce. These paramagnetic impurities were also characterized by static magnetic susceptibility and electron spin resonance measurements. A quantitative description of the proton relaxation rate maxima, as well as of the electron spin relaxation data, is presented. These analyses indicate that the density of states at the rare earth site disappears only for .

¹⁵N and ¹³C NMR chemical shifts for three nitroimidazoles have been calculated and compared with experimental data. The solvent effects on NMR spectra were simulated with the polarizable continuum model (PCM) and an alternative sequential molecular dynamics/quantum mechanics methodology (S-MD/QM). The sampling of the structures for the quantum mechanical calculations is made by using the interval of statistical correlation obtained from the auto-correlation function of the energy. Magnetic shielding tensors were evaluated at the GIAO-B3LYP level using basis set. It has been shown that it is essential to incorporate the dynamics and solvent effects in NMR calculations in the condensed phase.

The visible luminescence of Pr³⁺ and Tm³⁺ ions in lead borate glasses has been investigated as a function of activator concentration. The Judd–Ofelt analysis and the Inokuti–Hirayama model for energy transfer between activator ions have been applied for investigations of the radiative and non-radiative relaxation of the Pr and Tm excited states. Based on the luminescence decay curve analysis, the concentration quenching of the ¹D₂ emission of Pr³⁺ and ¹G₄ emission of Tm³⁺ ions has been attributed to cross-relaxation processes. The infrared spectroscopic measurements provide information on structural changes in the borate network initiated by optically active (Pr or Tm) ions. Contrary to the praseodymium ions, the thulium ions play an additional role as a glass-modifier in the PbO–B₂O–Al₂O₃–WO₃ composition.

We present results on both the intensity and phase dynamics of the transient non-linear optical response of a single quantum dot (SQD). The time evolution of the four wave mixing (FWM) signal on a subpicosecond timescale is dominated by biexciton effects. In particular, for the cross-polarized excitation case a biexciton bound state is found. In this latter case, mean-field results are shown to give a poor description of the non-linear optical signal at small times. By properly treating exciton–exciton effects in an SQD, coherent oscillations in the FWM signal are clearly demonstrated. These oscillations, with a period corresponding to the inverse of the biexciton binding energy, are correlated with the phase dynamics of the system's polarization, giving clear signatures of non-Markovian effects in the ultrafast regime.

Localization driven by disorder has a strong influence on the conducting properties of conducting polymers. Some authors hold the opinion that disorder in the material is homogeneous and that the conducting polymer is a disordered metal close to the Anderson–Mott metal–insulator (MI) transition, while others treat the disorder as inhomogeneous and have the opinion that conducting polymers are a composite of ordered metallic regions and disordered insulating regions. The morphology of conducting polymers is an important factor that has an influence on the type and extent of disorder. Different protonic acids used as dopants and moisture have influence on the polymer chain arrangement and interchain interactions. We performed optical reflectance measurements on several PANI-CSA/PANI-DBSA composite films with different dopant ratios and moisture contents. Optical conductivity and the real part of the dielectric function are calculated by Kramers–Kronig (KK) relations. σ₁(ω) and ε₁(ω) deviate from the simple Drude model in the low frequency range and the tendencies of the three sample are different and non-monotonic. The localization modified Drude model (LMD) in the framework of the Anderson–Mott theory cannot give a good fit to the experimental data. By introducing the distribution of relaxation time into the LMD, reasonable fits for all three samples are obtained. This result supports the inhomogeneous picture.

Electron irradiated undoped liquid encapsulated Czochralski (LEC) grown GaSb samples were studied by positron lifetime spectroscopy (PLS) and photoluminescence (PL). In addition to the 315 ps component reported in the previous studies, another defect with a lifetime of 280 ps was also identified in the present electron irradiated samples. The bulk lifetime of the GaSb material was found to be 258 ps. The V_Ga,280 ps and the V_Ga,315 ps defects were associated with two independent Ga vacancy related defects having different microstructures. The well known 777 meV PL signal (usually band A) was also observed in the electron irradiated undoped GaSb samples. The band A intensity decreases with increasing electron irradiation dosage and it disappears after the 300 °C annealing regardless of the irradiation dosage. The origin of the band A signal is also discussed.

We have been able to synthesize Lu³⁺ substituted La_0.67Ca_0.33MnO₃ (LCMO) by an auto-combustion method. Synthesis of this compound is not successful by conventional ceramic or other chemical methods. The magnetic and electrical transport properties of the Lu substituted LCMO [(La_1−xLu_x)_0.67Ca_0.33MnO₃ ()] system have been investigated and compared with those of the Y³⁺, Pr³⁺, Dy³⁺ and Tb³⁺ substituted LCMO systems. All the compounds show a ferromagnetic metal to paramagnetic insulator transition at T_C. The tolerance factor reduces from 0.917 for x = 0 to 0.909 for x = 0.12, and for this range all are ferromagnetic metals, indicating the dominance of the coupling between spins due to double exchange over the antiferromagnetic superexchange interaction. The transition temperatures and magnetization decrease as the Lu concentration increases. This is satisfactorily accounted for on the basis of a transition from ferromagnetic at x = 0 to canted spin order for x>0. All the samples show a higher magnitude of MR compared to that in pure LCMO at 80 kOe field in the temperature range 5–320 K. A fairly high value of low field magnetoresistance (LFMR) of about 30% is obtained in all the samples at a field less than 5 kOe.

The effect of the pore structure on the low frequency (0.01–100 kHz) dielectric response of sintered ZrO₂–8 mol% Y₂O₃ ceramic compacts has been investigated. Pore characteristics such as the pore size distribution, specific surface area and pore morphology have been estimated by means of small angle neutron scattering (SANS). It has been observed that both the real and the imaginary parts of the complex dielectric permittivity for the specimens depend not only on the porosity but also on the pore characteristics, significantly. Unlike in the normal Debye relaxation process, the imaginary part of the dielectric constant increases in the lower frequency region. The variation in the dielectric response is explained by a pore structure dependent interfacial polarization, ion hopping and conduction.

The resonantly enhanced 266 nm excited electronic Raman spectrum has been recorded for polycrystalline Cs₂NaPrCl₆ at 10 K. Significant differences are observed in comparison with the 514.5 nm spectrum. These are rationalized by intensity calculations which, for the intermediate virtual excited (4f5d) states, utilize (I) the direct product of 4f¹ core and 5d¹ electronic states; (II) 4f5d coupled states obtained by using the Reid f² and fd programs; (III) or for the initial and final (4f²) states, using the configuration-mixed wavefunctions (4f np) obtained by a configuration interaction assisted crystal field calculation. In general, the calculated trends in intensities are fairly similar (except where the calculated corresponding energy levels differ appreciably), showing that not only the interaction between the 5d electron and the (4f)¹ core in the 4f5d configuration, but also the inclusion of the 4f np configuration, do not provide the dominant contributions to the electronic Raman transition intensity of the Pr³⁺ ion. The calculated enhancements to the intensity under 266 nm, compared with 514.5 nm, excitation differ for different transitions within a given terminal multiplet term, and can vary by several orders of magnitude. The calculations are successful in pinpointing the strongest observed electronic Raman transitions, and provide an insight into the mechanistic pathways involving intermediate states.

Electron energy loss spectroscopy has been used to probe the coverage-dependent electronic excitations from a C₆₀ covered graphite surface. In the energy region corresponding to the optical band-gap of the bulk C₆₀ solid, the dipole-scattering contribution to the loss features is simulated by a double dielectric layer model. We find that a good agreement between the simulated spectra and the experimental results can be achieved for a C₆₀ multilayer. However, an additional energy loss channel is required in order to describe the electronic excitations for a C₆₀ monolayer. The possible significances of this finding are discussed.

The influence of the electronic structure and the lattice constant on hydrogen absorption in bulk Pd₃M₁ (M = Cd, Ag, Au, Pd, Cu, Ni, Pt, Pb, Sn, Fe, Rh, Ru) has been studied by density-functional calculations. We have assumed face-centred cubic structure for all the alloys, and hydrogen has been placed in the octahedral site surrounded by six Pd atoms. We have calculated the absorption energy of hydrogen in the alloys, and found that the influence of the electronic structure is much more important than that of the lattice constant. The results demonstrate that Miedema's empirical rule is also satisfied in this system, i.e., the higher the binding energy of the host alloy, the less stable the hydride. We have also calculated the detailed electronic structures of the alloys and their hydrides. We found that more stable hydrogen absorption is correlated with the hydrogen 1s electrons, palladium s electrons, palladium s-like electrons and the palladium d electrons moving higher in energy towards the Fermi level. The two latter relations have previously been described for bulk systems and surfaces respectively, while this study is apparently the first to point out the correlation between the position of the hydrogen band and the stability of the hydride, i.e., the deeper the hydrogen band, the less stable the hydride.

Magnetic and magnetotransport properties of glass-covered amorphous microwires of nominal composition Fe₈₉B₁Si₃C₃Mn₄ have been studied. Samples of two families with the same composition but different metallic core diameters were annealed in a furnace or by Joule heating. The electrical resistance was measured as a function of time during annealing to study the effects on the samples' microstructures (evolution from the amorphous to the crystalline phase). Static hysteresis loops were measured on all samples by means of a vibrating sample magnetometer. Giant magneto-impedance measurements (GMI) performed on a coaxial line using a vectorial network analyser at frequencies up to 6 GHz showed that furnace-annealed samples display larger anisotropies with respect to Joule-heated ones; however, Joule heating is more effective in improving the magneto-impedance response of the alloy. The maximum GMI ratio is ≈ 175% at 4 GHz. The influence of the different metallic core/total wire diameter on the induced anisotropies has been studied for the two families of samples through hysteresis loops and high-frequency GMI.

Westudied the electron-paramagnetic resonance linewidth of colossal magnetoresistance manganites. Starting from the quantum Langevin equation, we derived the transverse susceptibility of the double-exchange interaction systems and identified the damping function to be the inverse of the relaxation time T₂. We argued that the anisotropic energy caused by the crystal field was the most probable relaxation mechanism. The spin correlation caused by the double-exchange interaction was examined with the Schwinger boson approach. Taking into account the constraints of Schwinger bosons, we found that the linewidth is approximately proportional to the temperature. At the low-temperature end (near the Curie temperature T_C), there is a minimum, below which the linewidth increases sharply. At the high-temperature limit, the linewidth is proportional to . Most importantly, the linewidth is a universal function of T/T_C.

To establish a correlation between dislocations and deep levels in GaN, a deep-level transient spectroscopy study has been carried out on GaN samples grown by metalorganic chemical vapour deposition. In addition to typical undoped and Si-doped GaN samples, high-quality crack-free undoped GaN film grown intentionally on heavily doped cracked Si-doped GaN and cracked AlGaN templates are also chosen for this study. The purpose of growth of such continuous GaN layers on top of the cracked templates is to reduce the screw dislocation density by an order of magnitude. Deep levels in these layers have been characterized and compared with emphasis on their thermal stabilities and capture kinetics. Three electron traps at E_c–E_T∼0.10–0.11, 0.24–0.27 and 0.59–0.63 eV are detected common to all the samples while additional levels at E_c–E_T∼0.18 and 0.37–0.40 eV are also observed in the Si-doped GaN. The trap levels exhibit considerably different stabilities under rapid thermal annealing. Based on the observations, the trap levels at E_c–E_T∼0.18 and 0.24–0.27 eV can be associated with screw dislocations, whereas the level at E_c–E_T∼0.59–0.63 eV can be associated with edge dislocations. This is also in agreement with the transmission electron microscopy measurements conducted on the GaN samples.

We theoretically demonstrate flat photonic bands in two-dimensional photonic crystals with kagome lattices. In such photonic crystals composed of circular rods, electromagnetic waves localizing at the rods form flat photonic bands. The flat photonic bands are important for the omnidirectional lasing and the enhancement of the omnidirectional electromagnetic interaction of materials. Dependences of flat photonic bands on the dielectric constant and radii of rods are discussed.

Effects of Co substitution for Fe on the Curie temperature (T_c), glass-forming ability (GFA) and thermal stability of amorphous Fe_61−xCo_xZr₅B₃₀Nb₄ (FCZBN) alloys were studied for Co content ranging from 0 to 15 at. %. The T_c shows a sinusoid-like behaviour with increasing Co content, revealing two maxima at 3 and 12.5 at. % Co and a minimum at 7.5 at. % Co. Co content dependences of glass transition (T_g), crystallization (T_x) and reduced glass transition temperatures (T_rg) of the amorphous alloys are almost completely opposite to that of the T_c. The T_c decreases with increasing T_g and T_rg, but increases with increasing Co content. The Co content dependence of the T_c is suggested to relate to both Co content and high GFA of the amorphous alloys.