Table of contents

Quasiparticle relaxation and non-local effects on the surface impedance Z_s of pure metals are analysed for arbitrary relaxation times τ and mean free paths l. The analysis predicts a non-monotonic variation of the differential loss tangent (∂R_s/∂τ)/(∂X_s/∂τ) in the normal state, which distinguishes it from the classical limits. In the superconducting state, Z_s can be described by a two-fluid model incorporating a Drude-type conductivity and an effective relaxation time τ_e. The τ-values derived from theory and data measured on Sr₂RuO₄-crystals using a hollow dielectric resonator technique at 10 GHz are consistent, and in accordance with DC resistivity data.

It is found that an icosahedral quasicrystalline phase is formed in the Zr-Al-Ni-Cu glassy alloy by addition of Nb, Ta or V elements. The icosahedral phase was confirmed as a primary precipitation phase in the melt-spun Zr₆₅Al_7.5Ni₁₀Cu_12.5X₅ (X=Nb, Ta or V) glassy alloys with a two-stage crystallization process after the distinct glass transition. The onset temperature of the transformation from glass to icosahedral phase is 705 K for Nb, 710 K for Ta and 702 K for V at the heating rate of 0.67 Ks^-1. The size of the icosahedral particles is in the range of 10 to 50 nm. The second crystallization reaction results in the formation of Zr₂Cu + Zr₂Ni + Zr₃Al₂ phases through a sharp exothermic reaction. The formation of a nano scale icosahedral phase in the Zr-Al-Ni-Cu glassy alloy by addition of Nb, Ta or V, as well as noble metals, indicates that the increase of the nucleation rate contributes to the precipitation of the icosahedral phase, leading to the concept that the icosahedral short-range order exists in the glassy state in the Zr-Al-Ni-Cu alloy.

We report, on the basis of Cu nuclear quadrupole resonance measurements, that superconductivity (SC) and antiferromagnetism (AF) coexist on a microscopic level in CeCu₂(Si_1-xGe_x)₂, once a tiny amount of 1% Ge (x = 0.01) is substituted for Si. This coexistence arises because Ge substitution expands the unit-cell volume in nearly homogeneous CeCu₂Si₂ where the SC coexists with slowly fluctuating magnetic waves. We propose that the underlying exotic phases of SC and AF in nearly homogeneous and in slightly Ge-substituted CeCu₂Si₂ can be accounted for on the basis of the SO(5) theory that unifies the SC and AF. We suggest that the SC and AF in CeCu₂Si₂ have a common mechanism.

The nature of the attractive electron-electron interaction, leading to the formation of Cooper pairs in unconventional superconductors, has still to be fully understood and is subject to intensive research. Here we show that the sequence spin-Peierls, antiferromagnetism, superconductivity observed in (TMTTF)₂PF₆ under pressure makes the (TM)₂X phase diagram universal. We argue that the suppression of the spin-Peierls transition under pressure, the close vicinity of antiferromagnetic and superconducting phases at high pressure, as well as the existence of critical antiferromagnetic fluctuations above T_c strongly support the intriguing possibility that the interchain exchange of antiferromagnetic fluctuations provides the pairing mechanism required for bound charge carriers.

Effects of misfit stresses on the high-transition-temperature superconducting properties of thin-film cuprates are predicted and theoretically examined. A substantial enhancement of the critical transition temperature is predicted for YBa₂Cu₃O_7-y and Bi₂Sr₂CaCu₂O_x superconducting films, which is induced by misfit stresses generated at interphase boundaries with special crystallography and large misfit. The influence of misfit stresses on the structure and the transport properties of low-angle tilt boundaries in superconducting thin-film cuprates is theoretically analysed.

We report dc and ac magnetization of the newly discovered Cd-Mg-RE quasicrystals (RE = Gd, Tb, Dy, Ho, Er, Tm and Yb). For all the RE atoms except Yb, the temperature dependence of the dc magnetization obeys the Curie-Weiss law at higher temperatures, T>50 K. Estimated effective moments indicate that these atoms are in the trivalent states. In contrast, the Cd-Mg-Yb quasicrystal shows very small magnetization, suggesting that Yb is in the non-magnetic divalent state. At lower temperatures, spin-glass-like freezing was observed for the RE atoms except Tm and Yb. In particular, the freezing proceeds in two successive steps for RE = Gd, Tb, Dy and Ho, being quite different from canonical spin-glasses.

Icosahedral Al-Cu-TM (TM: transition metals = V, Cr, Mn) alloys were obtained by single-roller melt-spinning in a protective inert-gas atmosphere. The high-pressure properties up to 20 GPa were investigated by in situ energy-dispersive x-ray diffraction at a synchrotron source using a gas-membrane diamond-anvil cell. No phase transitions could be observed. The bulk modulus of the Al-Cu-TM quasicrystalline phases was found to increase with the Hume-Rothery factor. Moreover, the bulk modulus of the quasicrystalline phase is imposed by the bulk modulus of the transition metal element. The derived values for the first pressure derivative B₀' indicate strong anharmonic contributions in the lattice potentials.

The compressional behaviour of a new dense form of silicon nitride with the cubic spinel structure is studied by energy dispersive x-ray diffraction, following in situ synthesis from the low pressure form by laser heating in the diamond anvil cell, combined with theoretical density functional calculations (LDA and GGA). The unit cell dimension and the ambient temperature bulk modulus and its pressure derivative are determined to be V₀ = 8.29(±0.03) Å³/atom, K₀ = 308(±5) GPa and K'₀ = 4±(0.2), in excellent agreement with theoretical calculations within the LDA and GGA. The calculated shear modulus is two to three times those of corresponding oxide spinels, and there is a substantial Cauchy violation, indicating a material with strong covalent bonding that is likely to be extremely hard.

In terms of a model for size-dependent melting and the Lindemann criterion, a model to interpret thermodynamic superheating of all low-dimensional metallic crystals is developed. A superheating is present when the metallic nanocrystals have coherent or semi-coherent interfaces with the surrounding matrix and when the atomic diameter of the nanocrystals is larger than that of the matrix. The superheating temperatures of the nanocrystals are lower than the stability temperatures of the coherent interfaces. The model prediction shows good agreement with the experimental evidence.

The magnetism of relaxed and nonrelaxed vanadium-molybdenum (V_n/Mo) systems is investigated for n = 1-3 layers in the (001) and (111) orientations. This study is carried out using the the real-space self-consistent tight-binding recursion method in the Hartree-Fock approximation with Hubbard Hamiltonian. The magnetic moment of a V monolayer epitaxially grown on a semi-infinite Mo substrate is found to be larger in the (001) orientation than that in the (111). The magnetisms in bilayer and three layer V systems become comparable in the (001) and the (111) orientations in the nonrelaxed case, but are larger in the (001) for the relaxed systems. The magnetic moments are found to decrease in the relaxed systems for both cases.

The direct band gap of the conjugated polymer poly(p-phenylenevinylene) (PPV) is calculated in an effective single-band model by means of a renormalization scheme. This model provides a simple computational method for obtaining the band gap of a many-band conjugated π-electron system of long polymers. The calculation shows that, for the existing data for PPV, the present renormalization scheme works well and its results as regards the band energy gap are comparable to those obtained by other approaches including ab initio calculation.

A model of nonradiative transitions in spin-crossover molecules is developed. The model takes into account linear and quadratic terms of electron-vibrational interaction. The frequency effect is shown to arise from the quadratic term of this interaction. With account of the frequency effect analytical expressions are derived for the multiphonon decay rate of the high-spin state of a spin-crossover molecule. In the framework of the suggested model for different temperatures the probability of the nonradiative decay of the high-spin Fe(II) state in the diluted [Zn_1-xFe_x(ptz)₆](BF₄)₂, x = 0.1 system is calculated. The theoretical results are in agreement with the experimental ones.

We have investigated transport and magnetic properties of a series of double-perovskite alloys in the system Sr₂FeMo_xW_1-xO₆. These compounds exhibit a metal-insulator transition as a function of doping, x. The compounds with x in the range 1.0⩾x⩾0.3 show metallic behaviour in the resistivity while compounds with x in the range 0.2⩾x⩾0 are insulating. All compounds with 1.0⩾x⩾0.2 are ferrimagnetic, while Sr₂FeWO₆ (x = 0) is antiferromagnetic. The magnetization (M) is shown to increase slowly with decreasing x in the range 1.0⩾x⩾0.3 due to an enhanced crystallographic ordering; however, M decreases rapidly with decreasing x thereafter (x<0.3), probably due to composition fluctuations near the critical concentration, x_c. Our data suggest primarily Fe³⁺-(Mo, W)⁵⁺ ordering for x⩾0.3, while compositions with smaller x possibly contain both Fe³⁺ and Fe²⁺ species inhomogeneously due to the presence of the W⁶⁺ state. All samples with x⩾0.3 show a significant amount of negative magnetoresistance, as has been observed earlier for Sr₂FeMoO₆ (x = 1.0).

We investigate a polyacetylene ring in an axially symmetric, static electric field using a modification of the Su-Schrieffer-Heeger (SSH) Hamiltonian for a polyacetylene chain. An effective gauge potential of the single-electron Hamiltonian due to spin-field interaction is obtained and it results in a Fröhlich-type superconductivity equivalent to the effect of a travelling lattice wave. The total energy as well as the persistent current density are shown to be periodic functions of the flux of the gauge field embraced by the polyacetylene ring.

We apply a quantum-mechanical approach to study critical current suppression in a superconductor (SC) due to injection of spin-polarized carriers from a ferromagnet (FM) through a FM/SC tunnel junction. It is found that for a given bias voltage, the superconductivity suppression depends strongly on the polarization of the injection current, on the spin-diffusion length in the SC, and on the insulating barrier strength at the FM/SC interface.

Metastable magnetism is observed in two important compounds (those with x = 0.35 and x = 0.5) of the Li_xNi_1-xO series, with similar experimental features found in bulk susceptibility measurements. For each of these two compositions, a frequency dependence in the first-order alternating-current (ac) susceptibility (χ₁), a negative peak in the third-order ac susceptibility (χ₃), and a field-cooled/zero-field-cooled bifurcation in the direct-current magnetization were seen. We have used χ₃ to discern the nature of the metastability for these samples. We find that for x = 0.35, χ₃ becomes critical as a function of field (h), frequency (f), and temperature (T), confirming the existence of a spin-glass-like phase. However, χ₃ for x = 0.5 does not become critical as a function of h and f, and shows a -T^-3-dependence as predicted by Wohlfarth's model of superparamagnetism. Thus Li_xNi_1-xO shows magnetic phases of different types in two important composition ranges which are identified using χ₃. The results also indicate the importance of non-linear susceptibility for distinguishing the two metastable phases by means of bulk susceptibility measurements.

Core-level Mn and Co 2p x-ray photoelectron spectroscopic studies give evidence for different spin states of Mn and Co in the two different phases of the ferromagnetic compound LaMn_0.5Co_0.5O₃. Ferromagnetism in the high-T_c (T_c≈230 K) phase of the compound arises from Mn³⁺-O-Mn³⁺ superexchange interactions, whereas in the low-T_c (T_c≈150 K) phase Mn⁴⁺-O-Co²⁺ superexchange interactions are responsible for the ferromagnetism.

We report the results of ab initio energy-band calculations for the ternary intermetallic compound GdMn₆Ge₆. The ferrimagnetic arrangement of Gd and Mn magnetic sublattices is reproduced as well as the average moment per Mn atom. The comparatively low strength of the Gd-Mn effective exchange coupling and the occurrence of a weak orbital polarization (3%) at the manganese sites are further results of this analysis. The numerical results are compared with prior magnetization and hyperfine-field analyses.

Structural and magnetic properties of nanocrystalline Fe₂Co and (Fe₂Co)_0.30 Cu_0.70 alloys prepared by high energy ball milling have been studied basically by x-ray, Mössbauer spectroscopy and magnetization measurements. For the Fe₂Co alloy case, the Mössbauer measurements indicate that the sample with 160 hours of milling has two magnetic components with the same average hyperfine parameters: one magnetic crystalline component associated with the bcc Fe₂Co phase and another component attributed to the small particles of the same bcc Fe₂Co phase (SP-Fe₂Co). (Fe₂Co)_0.30Cu_0.70 alloys have been prepared by milling in two different ways: (1) starting from the mixture of Fe₂Co milled alloy and pure Cu powders (sample I) and (2) milling of the elemental powder mixture of Fe, Co and Cu (sample II). The x-ray diffraction and bulk magnetization results of samples I and II indicate the formation of a (Fe₂Co)_0.30Cu_0.70 supersaturated solid solution, with features of a ferromagnetic material and T_c at about (420±1) K. High temperature magnetization measurements of the (Fe/Co)Cu milled materials show particle precipitation effects. Heat treatment at 675 and 875 K of the final milled materials leads to different results: in the sample I case to the precipitation of single magnetic Fe/Co particles into the Cu matrix, and in the case of sample II the precipitation of single magnetic particles of Fe and of Co into the Cu matrix.

This work is concerned with the behaviour of a regular 2D rectangular lattice of Ni₃Fe nanoparticles, determined by the dipole interaction between them. The samples under study, prepared by electron-beam lithography, consisted of about 10⁵ particles approximately 50 nm in size. The magnetization curves were studied by Hall magnetometry for different external magnetic field orientations at 4.2 K and 77 K. The results indicate a collective behaviour of the system. The magnetization curves depend on the external magnetic field direction and temperature; the system exhibits multistability. A model system of interacting 3D magnetic dipoles forming a rectangular lattice was numerically simulated by solving a system of stochastic Landau-Lifshitz equations. The multistability of the system and steps in the magnetization curves were obtained. It is shown analytically that the shape of the magnetization curves depends on the character of the interaction in the system.

In situ ellipsometry and Kerr polarimetry have been used to follow the continuous evolution of the optical and magneto-optical properties of multiple layers of Co and Pd during their growth. Films were sputter deposited onto a Pd buffer layer on glass substrates up to a maximum of N = 10 bi-layer periods according to the scheme glass/Pd(10)N×(0.3Co/3Pd) (nm). Magnetic hysteresis measurements taken during the deposition consistently showed strong perpendicular anisotropy at all stages of film growth following the deposition of a single monolayer of Co. Magneto-optic signals associated with the normal-incidence polar Kerr effect indicated strong polarization of Pd atoms at both Co-Pd and Pd-Co interfaces and that the magnitude of the complex magneto-optic Voigt parameter and the magnetic moment of the Pd decrease exponentially with distance from the interface with a decay constant of 1.1 nm^-1. Theoretical simulations have provided an understanding of the observations and allow the determination of the ultrathin-film values of the elements of the skew-symmetric permittivity tensor that describe the optical and magneto-optical properties for both Co and Pd. Detailed structure in the observed Kerr ellipticity shows distinct Pd-thickness-dependent oscillations with a spatial period of about 1.6 nm that are believed to be associated with quantum well levels in the growing Pd layer.

The low temperature (6 K) EPR spectrum of Cu²⁺ in K₂Zn(SO₄)₂·6H₂O is characteristic for the ground state a|x²-y²⟩-b|z²⟩ with essentially the |x²-y²⟩ state and 4.5% admixture of the |z²⟩ state due to the zero-point motions. Temperature variations of the g and A parameters are characteristic for two-well vibronic dynamics and the vibronic averaging of the g-factor is well described with the simple Silver-Getz model above 60 K with energy difference δ₁₂ = 68 cm^-1 between the wells. However, this model does not reproduce the A(T) dependence, which seems to be influenced by temperature induced changes in the unpaired electron delocalization onto ligands. Electron spin relaxation was measured at low temperatures up to 55 K where the electron spin echo signal was detectable. At low temperatures, in the static Jahn-Teller limit, the Cu(H₂O)₆ complexes are strongly localized in the deepest potential well and the slow vibronic dynamics is overdominated by two-phonon Raman processes in electron spin-lattice relaxation. The relaxation rate is described by 1/T₁ = aT⁹I₈(Θ_D/T) with transport integral I₈ and the Debye temperature Θ_D = 170 K. Electron spin echo decay is strongly modulated by dipolar coupling with ¹H and ³⁹K nuclei and the decay function is V(2τ) = V₀exp (-aτ-mτ²). The quadratic term dominates in the static Jahn-Teller limit (below 18 K) and describes the decay produced by the nuclear spectral diffusion. For higher temperatures the exp (-aτ) term dominates and describes an effect of the intrawell excitations with phase relaxation rate 1/T_M = a + bexp (-Δ/kT) with Δ = 67 cm^-1 being the energy of the first excited vibronic level. The excitations produce a clear broadening of the Fourier transform electron spin echo (FT-ESE) spectra (at 18 K), where peaks from potassium and hydrogen nuclei have been identified.

The complex dielectric permittivity of K_1-xLi_xTaO₃ (KTL) single crystals with x = 0.006 has been experimentally studied in detail in the temperature interval from 5 to 300 K and at frequencies from 100 Hz to 1 MHz. In agreement with previous studies, a very large effect of the Li impurities on the dielectric response, even for such a small Li concentration, is found. It consists in the appearance of a pronounced low-temperature dielectric dispersion with giant magnitude. This unusually large dielectric response cannot be caused only by the relaxation of the Li⁺-impurity off-centres themselves, because the Li concentration is too small. Also, the host lattice response itself cannot give such a large dispersion, as evidenced. We present a theoretical model, which considers the coupling of the Li⁺-related relaxators to the TO soft mode, giving a good description of the experimental data obtained.

The dielectric constant of TlInS₂ crystal was measured in the temperature range including incommensurate (IC1 and IC2) and commensurate (C1 and C2) phase transitions under external bias electric field. It has been revealed that maxima in the temperature dependence of dielectric constant of TlInS₂ at the C1 and C2 phase transition points shift to higher temperatures. The anomaly at IC2 transition point shifts to lower temperatures on applying the external electric fields. A new complete theoretical model including the presence of two order parameters and two polar sublattices in TlInS₂ has been suggested.

We used coherent anti-Stokes Raman scattering (CARS) spectroscopy to study the symmetry of the low-lying levels of the Sm²⁺ ion doped in sites of cubic (O_h) symmetry in the SrF₂ and CaF₂ matrices. Taking advantage of the polarization sensitivity of the Raman spectroscopic techniques, we confirm that the line at ≈652 cm^-1 (that we could only observe in the SrF₂ case because of wavelength limitations) corresponds to excitation from the ground (4f^6 7F₀) A_1g level to the Stark T_2g level originating from the splitting of the (4f^6 7F₂) multiplet. In the same SrF₂ sample, the (4f⁶) ⁷F₀→ ⁷F₁ transition was observed to be weaker. In CaF₂ matrices in which it is more easily detected, we checked its T_1g symmetry.

We have presented absorption spectra related to octahedrally coordinated Co²⁺ in the Sr²⁺ position in SrLaGa₃O₇. Quantitative analysis of the line shape of the ⁴T₁→⁴T₂ transition shows the existence of the strong T⊗τ Jahn-Teller effect in the ⁴T₁ ground state of the system. We have estimated the Jahn-Teller stabilization energy to be equal to 507 cm^-1. The procedure of estimation of the crystal-field strength parameter 10Dq for the system that is affected by the strong Jahn-Teller coupling has been discussed.

CaNaYF₆ (CNYF) single crystals doped with Ce³⁺ have the CaF₂-type cubic-fluorite structure where Ca²⁺, Na⁺, and Y³⁺ randomly occupy the common cation sites. The optical absorption and luminescence spectra of Ce³⁺ in the CNYF crystal, being broadened by a strong electron-phonon interaction in the 5d excited state, have a large Stokes shift. Substitutional disorder in the CNYF crystal produces inhomogeneous broadening of the optical spectra and a distribution of the lifetime of the Ce³⁺ luminescence. The linewidth and lifetime are discussed in terms of a distribution of the crystal-field splitting of the 5d excited state of Ce³⁺ in the CNYF crystal.

Excited state absorption spectra from the 4f² levels of Pr³⁺ in YPO₄ to 4f5d were measured at 293 and 77 K using a pulsed pump-probe technique. The spectra from the ¹D₂ manifold were recorded by employing a continuous wave probe, whilst those from the ³P_J-¹I₆ states were recorded by using for the first time a pulsed probe. These experimental results are compared with the numerical predictions of a full calculation of the 4f5d levels and the agreement is found to be good. A detailed study of the UV luminescence properties of the Pr³⁺ and the Ce³⁺ ions in the YPO₄ host crystal, including the energy transfer between these two ions, is also reported. Fluorescence spectra and decay curves from the lower excited-state levels of the 4f5d and 5d electronic configurations of the Pr³⁺ and Ce³⁺ ions, respectively, were measured both at 293 and 77 K. The Pr³⁺→Ce³⁺ energy transfer efficiency was evaluated. Further considerations about the possibility to obtain laser emission based on the 5d→4f optical transition seem to indicate that this material is probably not a very good candidate to build a UV tunable laser because of a relatively short lifetime of the emitting level corresponding to a reduced quantum efficiency.

The energy shifts of optical interband transition edges, E^'₀, E₁, E₁+Δ₁ and E₂, of relaxed Si_1-xGe_x alloys grown epitaxially on Si(001) substrates by molecular beam epitaxy have been studied as a function of Ge composition using their complex dielectric functions measured by spectroscopic ellipsometry at room temperature. The interband transition edges were resolved by a line shape fitting on the numerical second derivative spectra of the dielectric functions. The E^'₀, E₁, E₁+Δ₁ and E₂(Σ) edges are found to shift to lower energies with increasing Ge composition while the E₂(X) edge shifts to higher energies. Also it is found for E₁ and E₁+Δ₁ energies that downward bowing exists and for Δ₁ energy that upward bowing exists. These behaviours of the transition edges are understood by comparing the band structure of Si with that of Ge and interpreted as due to the effect of the random potential originated by alloying disorder.