Table of contents

The reorientation of one small paramagnetic molecule (spin probe) in glassy polystyrene (PS) is studied by high-field electron spin resonance spectroscopy at two different Larmor frequencies (190 and 285 GHz). The exponential distribution of the energy barriers for the rotational motion of the spin probe is unambiguously evidenced at both 240 and 270 K. The same shape for the distribution of the energy barriers of PS was evidenced by the master curves provided by previous mechanical and light scattering studies. The breadth of the energy barrier distribution of the spin probe is in the range of the estimates of the breadth of the PS energy barrier distribution. The evidence that the deep structure of the energy landscape of PS exhibits the exponential shape of the energy barrier distribution agrees with the results from extreme-value statistics (Bouchaud and Mezard 1997 J. Phys. A: Math. Gen. 30 7997) and the trap model by Bouchaud and co-workers (1996 J. Phys. A: Math. Gen. 29 3847, 2001 Phys. Rev. B 64 104417).

The present review article discusses the physics of the non-oxide perovskite superconductor MgCNi₃ on the basis of theoretical and experimental results available on the material up to July 2004. It was discovered following on from the breakthrough of the finding of the MgB₂ superconductor at the beginning of 2001, which has subsequently been intensively studied; however, less attention has been paid to it due to its much lower superconducting transition temperature, T_c ( K), as compared to that of MgB₂ ( K). But it has many interesting properties which need to be focused on to obtain an understanding of its complicated physics. Energy band calculations show that the density of states (DOS) at the Fermi level, N(E_F), is dominated by Ni d states and there is a von Hove singularity in the DOS just below E_F (<50–120 meV). It is surprising that the conduction electrons in it are derived from partially filled Ni d states, which typically lead to ferromagnetism in metallic Ni and many Ni-based binary alloys. MgC_xNi₃ has a simple cubic perovskite structure with space group and the lattice parameter a is for at ambient temperature and pressure. However, the Ni₆(O₆) octahedron is locally distorted from those expected in the perfect cubic form. The carbon atom of MgCNi₃ at the body centre is surrounded by six Ni atoms at the face centred positions and eight Mg atoms at the cube corners. The carriers in it are of electron type in the normal state, although theoretically they were predicted to be of hole type. T_c increases with increase of x in MgC_xNi₃, but generally decreases with Ni site doping with Co, Fe, Mn, Cu etc. Theoretically, the DOS peak should be greatly reduced by doping at the Mg or Ni site, which accounts for the reduced T_c. The T_c is found to increase with increase of the external pressure (P) at a rate of , which is the same as that for the intermetallic RNi₂B₂C (R = rare earth) superconductors but about one order lower than that for MgB₂. The T_c(P) result focuses our attention on the feature that N(E_F) should increase with pressure due to the broadening of the energy level. Also, a controversial magnetoresistance is reported. It has been observed that the electronic contribution is slightly higher than the lattice one in the normal state thermal conductivity. Specific heat and tunnelling spectroscopic studies indicate that this is an s-wave BCS-type weak/moderate coupling type-II superconductor, but this needs further confirmation as the penetration depth distinctly exhibits a non-s-wave BCS low temperature behaviour which theoretically suggests a d-wave superconductor.

We present a detailed overview of numerical Monte Carlo studies of the dipolar spin ice model, which has been shown to be an excellent quantitative descriptor of the Ising pyrochlore materials Dy₂Ti₂O₇ and Ho₂Ti₂O₇. We show that the dipolar spin ice model can reproduce an effective quasi-macroscopically degenerate ground state and spin ice behaviour of these materials when the long range nature of dipole–dipole interaction is handled carefully using Ewald summation techniques. This degeneracy is, however, ultimately lifted at low temperature. The long range ordered state is identified via Monte Carlo simulation techniques. Finally, we investigate the behaviour of the dipolar spin ice model in an applied magnetic field and compare our predictions to experimental results. We find that a number of different long range ordered ground states are favoured by the model, depending on field direction.

The fabrication of thin films of colloidal semiconductor nanocrystals is attracting much attention due to their exceptional optoelectronic properties. This requires the development of new methods for depositing nanocrystals under well-controlled conditions. Here, we report the use of the pulse injection method to deposit CdSe nanocrystals under ultrahigh vacuum (UHV) on clean and well-ordered surfaces. The deposition of nanocrystals has been tested by x-ray photoelectron spectroscopy (XPS) and near edge x-ray absorption fine structure spectroscopy. Special attention has been paid to the preparation of very pure solutions of CdSe nanocrystals using cadmium stearate, trioctylphosphine oxide (TOPO) and the TOP/Se adduct for the nanocrystals synthesis followed by dissolution in pentane. It has been found that CdSe nanocrystals adsorb with similar sticking coefficients on graphite, hydrogenated silicon (100) and hydrogenated diamond (100) surfaces. Furthermore, the XPS analysis has revealed that the surface of the CdSe nanocrystal is Cd rich, which has important consequences for the optical and chemical properties. This ability to deposit semiconductor nanocrystals under UHV conditions on clean and well-ordered surfaces opens up new perspectives for studying in a reliable manner all their chemical, electronic and optical properties.

The spin–orbit-split L-gap surface states on Au(111) are investigated by means of spin- and angle-resolved photoelectron spectroscopy and relativistic first-principles calculations, the latter including both electronic structure and photoemission. The dispersion, momentum distribution and spin polarization of the surface states are consistent with those of a two-dimensional electron gas with Rashba–Bychkov spin–orbit coupling (SOC). The surface symmetry manifests itself in the spin-integrated photoemission intensities, thereby providing details of the orbital composition of the surface states. For the spin polarization, modulations which show up in theory are below the experimental detection limit of 5%. The dependence of both dispersion and spin polarization on the SOC strength are addressed theoretically. The combination of experiment and theory establishes a consistent picture of the L-gap surface states.

It was shown recently that ferroelectricity and switching of the spontaneous polarization exist even in one monolayer (5–17 Å) of the ferroelectric copolymer P[VDF-TrFE]. Ferroelectric properties were found in the ultrathin perovskite-like PZT films with a thickness of 12 Å (three unit-cell parameters). Moreover, the simulation model, based on first principles, has shown that the critical size of perovskite ferroelectrics is three elementary cells. On the other hand the Ginzburg soft mode conception for displacive ferroelectrics supposed the existence of the low frequency vibration mode ω₀ (soft mode) and the corresponding long vibration wavelength λ₀. Therefore the mechanism of the ferroelectric phase transition, connected with the soft mode, leads to the existence of a critical thickness of the film . At l_cr<λ₀ the mechanism of the ferroelectricity is not caused by the soft mode. We show that in the ultrathin ferroelectric films the soft mode does not exist.

InAs self-organized nanostructures were grown with variant deposition thickness and growth rate on closely matched InAlAs/InP by molecular-beam epitaxy. The structural properties of InAs and InAlAs layer were studied. It is found that the InAs morphology is insensitive to the growth conditions. Transmission electron microscopy and reflectance difference spectroscopy measurements show that the InAlAs matrix presents lateral composition modulation which gives birth to surface anisotropy. Based on the dependence of the InAs morphology on the anisotropy of the InAlAs layer, a modified Stranski–Krastanow growth mode is presented to describe the growth of the nanostructure on a composition-modulated surface.

Neutron powder diffraction has been used to study the two title compounds, Sr₂HoRuO₆ and Sr₂TbRuO₆, in order to determine the crystal and magnetic structures of the materials. Both materials have a distorted double perovskite structure and at temperatures below 30–40 K long-range magnetic order is observed. The magnetic structure for each compound consists of two interpenetrating Type I arrangements, one for the ruthenium sublattice and one for the rare earth sublattice, which are antiferromagnetically coupled to each other and order at the same temperature. From variable temperature measurements the combination of magnetic interactions and their relative strengths was deduced. The antiferromagnetic Ru–O–O–Ru interaction is approximately constant in the compounds, but it is a factor of 2.5 and 0.7 times stronger than the antiferromagnetic Ru–O–Ho and Ru–O–Tb interactions respectively.

Two series of mixed Na/K and Na/Li aluminoborosilicate glasses have been irradiated with electrons of 1.8 MeV at doses close to 10⁹ Gy. Si, B and Al glass former environment changes under irradiation have been probed by MAS NMR spectroscopy. It was shown that the mixed alkali effect acts on the middle range compositions by operating with a selective blockage in irradiated glasses, depending on the alkali nature and concerning either modifier or charge compensator alkalis.

The paraelectric–ferroelectric phase transition of a single crystal of triglycine selenate TGSe, whose first- or second-order character is controversial, has been studied using a high sensitivity calorimetric technique. The specific heat of a highly pure TGSe has been measured and no evidence of latent heat has been found. The anomalous part of the specific heat shows Landau tricritical behaviour. Experimental data have been fitted to a 2–6 Landau potential, whose coefficients have also been obtained. A weak uniaxial stress applied along the ferroelectric axis (10 bar) decreases the anomalous part of the specific heat. This effect is different for cooling and heating runs. The equilibrium data are those obtained on heating that fit to a 2–4–6 Landau potential, which indicates that the transition shifts to second order with the effect of uniaxial stress.

The hydrogen uptake and distribution in wedged Mg–Ni films, with composition ranging from Mg_0.85Ni_0.15 to Mg_0.55Ni_0.45, were investigated. Upon hydrogen loading at 298 K these films undergo a transition from a mirror-like metallic to a semiconducting transparent state. After exposure to a hydrogen pressure of 1 bar, the samples exhibit large variation in optical appearance, ranging from a pale yellowish (Mg rich side) to a brownish shade (Ni rich side). The change in the effective optical band gap E_g^eff as a function of sample composition and hydrogen concentration was determined; it showed changes from 3.6 eV in the Ni poor domain to 2.4 eV in the Ni rich domain. Composition analysis using the ¹⁵N nuclear resonance method showed close to homogeneous hydrogen distribution throughout the film and close to linear increase in the hydrogen uptake with increasing Mg content. The thermal stability of the films is limited; annealing above 393 K results in significant redistribution of the constituents. Mg is enriched at the surface, reacting with Pd and thereby degrading the capping layer through the formation of Mg₆Pd and MgO, as determined by x-ray diffraction, x-ray photoelectron spectroscopy and Rutherford backscattering studies. This redistribution results in a severe decrease of the hydrogen uptake rate, as monitored by in situ resistivity measurements.

A transmission experiment utilizing thin foil targets has been conducted in order to establish the stopping powers of the cobalt-base alloy, havar, for 0.6–5.9 MeV protons and 2.6–24 MeV alpha particles. The basic technique of the novel experimental method used was to record both the projectile energy and the time of flight while alternating measurements with and without the target in place. The uncertainties of the proton and alpha particle data sets ranged from 1.4 to 2.3% and 1.1 to 1.5%, respectively. Modified Bethe–Bloch theory was applied to the measurements in order to ascertain values of the target mean excitation energy (I) and Barkas-effect parameter (b) for each projectile. The extracted values were I = 304.3 ± 2.4 eV and b = 1.37 ± 0.04 for the case of protons, and I = 306.3 ± 2.3 eV and b = 1.47 ± 0.03 for the case of alpha particles. The I-values are somewhat higher than the additivity-based expectation of 295.7 eV, whereas the b-values are clearly consistent with the expected range of 1.4 ± 0.1. The parameter values extracted from the measurements are appraised for compatibility with recently observed trends in values of I and of b with increasing projectile atomic number.

We study the 3D integer transport coefficients of the electron gas in a tilted magnetic field with the model of Bloch electrons. Calculations without any approximation of the relevant basis functions and matrix elements yield the topologically invariant conductances and their Diophantine equation.

The magnetic field dependences of the frequencies of standing spin-wave modes in a tangentially magnetized array of thin rectangular permalloy dots (800 × 550 nm) were measured experimentally by a Brillouin light scattering technique and calculated theoretically using an approximate size-dependent quantization of the spin-wavevector components in the dipole-exchange dispersion equation for spin waves propagating in a continuous magnetic film. It was found that the inhomogeneous internal bias magnetic field of the dot has a strong influence on the profiles of the lowest spin-wave standing modes. In addition, the dynamic magnetization distributions found for both longitudinally and transversely magnetized long magnetic stripes gives a good approximation for mode distributions in a rectangular dot magnetized along one of its in-plane sides. An approximate analytic theory of exchange-dominated spin-wave modes, strongly localized along the dot edge that is perpendicular to the bias magnetic field, is developed. A good quantitative agreement with the results of the BLS experiment is found.

It is common knowledge that the magnetic structure of fcc Fe (γ-Fe) can be described by means of an incommensurate spin-density wave (SDW). Previous experimental data, however, were mostly obtained for γ-FeCo alloy precipitates in Cu in order to avoid the complexity of the structural phase transition. Since Fe is a fundamental element, reliable data on the SDW state for pure γ-Fe are highly desired. We present here neutron scattering data for the SDW in pure γ-Fe precipitates in Cu and discuss the particle size effect of the SDW.

We study the equilibrium properties of a system of particles in two dimensions, interacting via pair and three-body potentials. This system undergoes a structural transition from a square to a rhombic lattice and thus constitutes a simple model for a generic tetragonal to orthorhombic transition. We aim at an intermediate level of description lying in between that of coarse grained elastic strain Hamiltonians and microscopic ab initio approaches. We obtain macroscopic thermodynamic properties and the phase diagram at zero and finite temperatures as a function of the density and the relative strengths of the pair and three-body energies using lattice sums, an approximate 'cell model' theory and molecular dynamics simulations in the NVT ensemble. In addition, we study the dynamics of nucleation following a quench from the square to the triangular phase (Rao and Sengupta 2003 Phys. Rev. Lett. 91 045502). As in real solids, the final microstructure depends sensitively on the depth of the quench—a shallow quench results in an equilibrium ferrite while a deep quench gives rise to a metastable twinned martensite. We find, in accordance with experiments, that the twinned martensite is associated with a diffusionless transformation. We propose that this model solid may be used as a test bed for studies of the statics and dynamics of structural transitions.

We consider a strongly correlated spin model with a positive uniaxial anisotropy. We show that in 2+1 dimensions this model is equivalent to spinon and holon excitations coupled to a Z₂ Ising gauge field. The Z₂ gauge field represents the vortex monopole excitations of the X–Y model. As a function of the hole concentration the X–Y model has two phases: a spin wave phase and a free vortex monopole phase. In the spin wave phase the spinons and the holons are weakly interacting, giving rise to a spin–charge separated phase which at zero temperature is superconducting. When the hole concentration increases the Z₂ excitations (the monopole currents) become free, giving rise to holon–spinon binding. The destruction of superconductivity at zero temperature coincides with the appearance of the free monopole currents.

The electrical conductivity of the AgI+BN composite system was measured both at 298 and at 453 K. The electrical conductivity increases with the increase of the volume fraction of AgI at both temperatures. The behaviour of ionic conductivity was analysed based on the generalized effective medium (GEM) theory. The threshold values of the volume fraction for the percolation of ionic conduction were 16% at 298 K and 17% at 453 K respectively. These threshold values as well as the determined power indexes agree with those given by computer simulations for the percolation.

An in situ single-crystal high-pressure x-ray diffraction study of decagonal Al–Co–Cu up to 19.1 GPa has been performed using diamond anvil cells and synchrotron radiation. Quantitative reciprocal space reconstruction from image plate data was used for data analysis. No significant variations with pressure were observed in Bragg as well as diffuse scattering, indicating the high stability of the quasiperiodic structure in the investigated pressure range. The bulk modulus at zero pressure was determined from fitting a second-order Birch–Murnaghan equation of state as K₀ = 131(8) GPa. The influence of different crystal orientations on the accessible amount of reciprocal space data is discussed.

The high-pressure behaviour of LiGdF₄ scheelite (I4₁/a, Z = 4) was studied by measuring its angle-dispersive x-ray powder diffraction patterns as a function of pressure and temperature in a diamond anvil cell and a large-volume Paris–Edinburgh cell using a synchrotron radiation source. Upon compression to about 11 GPa at room temperature, the stable structure is of the scheelite type. At higher pressures and T = 298 K, new reflections occur that cannot be explained with the fergusonite structural model previously observed for LiY F₄. Associated with this is the growth of an amorphous component. All the transformations are largely irreversible upon decompression. Annealing of the sample at 13.1 GPa led to a nucleation of a solid solution series Li_yGd_1−yF_3−2y (P6₃/mmc, Z = 2) and traces of LiF. The new material Li_yGd_1−yF_3−2y (P6₃/mmc, Z = 2) was recovered to ambient conditions but back-transformed to a Y F₃-type phase (Pnma, Z = 4) after regrinding at room temperature for several hours. These observations are discussed in relation to the high-pressure high-temperature systematics of the AMX₄-type compounds.

The temperature dependence of the structural parameters of orthorhombic silicide GdSi₂ was studied by the in situ powder x-ray diffraction method. A structural change from orthorhombic to α-ThSi₂-type tetragonal was found at around 818 K. The change in structure with temperature is continuous and reversible without a discontinuous change in volume. The thermal expansion coefficient was calculated from the refined cell parameters. The compound began to react with the residual oxygen in the argon environment and form oxide above 973 K.

Visible and infrared absorption, Raman scattering, x-ray powder diffraction and scanning electron microscope (SEM) have been used to characterize Er/Yb (0.6 mol%/0.3 mol%) codoped and singly Yb-doped (1.2 mol%) Z-cut LiNbO₃ crystals that were vapour-transport-equilibrated (VTE) at 1110 and 1120 °C for 100, 120 and 220 h in a Li-rich atmosphere. Optical absorption edge and 153 cm⁻¹ E(TO) phonon linewidth have been used to roughly evaluate the composition of the crystals. The results obtained show that the VTE treatment has brought all crystals towards stoichiometric composition. As a relatively strong VTE condition was adopted, the treatment results in the formation of submicron-sized ErNbO₄ (+YbNbO₄) and YbNbO₄ flat polyhedron-like precipitates in the Er/Yb:LiNbO₃ and Yb:LiNbO₃ crystals, respectively. On the other hand, a relatively weak VTE procedure cannot induce the precipitation of both the singly and doubly doped crystals. The characteristic optical absorptions, characteristic x-ray diffractions, crystalline structure and crystallographic morphology of the ErNbO₄ and YbNbO₄ precipitates are discussed, summarized and compared with those of singly Er-doped LiNbO₃ crystals reported previously. The absorption characteristics of the unprecipitated Er/Yb codoped and singly Yb-doped crystals are also summarized and compared with the case of singly Er doping. An atomic force microscope has been used to further verify the SEM results and to evaluate the roughness of the surface of precipitated crystals. Optical absorption measurement was also carried out on a precipitated Er/Yb codoped crystal with polished surfaces to verify that the precipitates grow not only on the surface of the crystal but also in the bulk.

The atomic structure of Fe nanoclusters embedded in a range of matrix materials has been studied using synchrotron radiation. In particular, the effect of embedding the clusters in Ag, amorphous carbon (a-C) and a porous C₆₀ matrix is investigated. The embedded cluster samples were prepared by co-deposition using a gas aggregation cluster source. Samples with both dilute and high-volume-filling fraction of clusters, at 4 and 40% respectively, were prepared. Fe K edge EXAFS measurements were used to probe the structure within the clusters. In a Ag matrix, the Fe clusters retain the b.c.c. structure of bulk Fe while in a-C there is evidence for both b.c.c. and f.c.c. structures in the clusters. These results are independent of cluster volume-filling fraction over the range investigated. When embedded in a porous C₆₀ matrix, the Fe clusters oxidize to Fe₂O₃.

The effect of stress due to the lattice misfit in the Cu/Au interface has been studied by carrying out molecular dynamics simulations. Results show that the stress can induce stacking faults to relieve the stress.

The zinc-blende boron compound BP is one of the promising III–V semiconductors. The density functional linear response approach is used to explore the lattice dynamics and equilibrium properties of it. The complete phonon dispersions and corresponding phonon density of states (DOS) are computed. Simultaneously, we also make a systematic research on the thermodynamical quantities, bulk modulus, shear modulus, elastic constants, dielectric constant, piezoelectric constant, internal strain, and electron band eigenvalues of it. Our results are in reasonably good agreement with numerous experimental and theoretical investigations where available, and provide predictions where they are not.

The possibilities of two-dimensional (2D) short-range magnetic correlations and frustration effects in the mineral tapiolite are investigated using bulk-property measurements and neutron Laue diffraction. In this study of the magnetic properties of synthetic single-crystals of tapiolite, we find that single crystals of FeTa₂O₆ order antiferromagnetically at T_N = 7.95 ± 0.05 K, with extensive two-dimensional correlations existing up to at least 40 K. Although we find no evidence that FeTa₂O₆ is magnetically frustrated, hallmarks of two-dimensional magnetism observed in our single-crystal data include: (i) broadening of the susceptibility maximum due to short-range correlations, (ii) a spin-flop transition and (iii) lambda anomalies in the heat capacity and d(χT)/dT. Complementary neutron Laue diffraction measurements reveal 1D magnetic diffuse scattering extending along the c^* direction perpendicular to the magnetic planes. This magnetic diffuse scattering, observed for the first time using the neutron Laue technique by VIVALDI, arises directly as a result of 2D short-range spin correlations.

Using synchrotron radiation, the x-ray diffraction of P- and Sb-based skutterudite compounds has systematically been studied at room temperature and high pressures. Bulk moduli of binary skutterudites CoX₃ (X = P and Sb), filled skutterudites CeT₄X₁₂ (T = Fe, Ru and Os), LaRu₄X₁₂and PrRu₄X₁₂ are obtained from the volume versus pressure curves fitted by a Birch equation of state. Bulk moduli are 150–225 GPa for P-based skutterudites and 80–115 GPa for Sb-based skutterudites. The bulk modulus of the phosphides is about two times that of the corresponding antimonides. The antimonides are very compressible compared with the phosphides. The Grüneisen constants (γ) for skutterudite compounds are estimated from the compressibility and its volume derivative in the Debye approximation. The bulk modulus of CoX₃ and CeT₄X₁₂ increases with increasing lattice constant, attaining a maximum for the Ru compounds. Bulk moduli for the Ru-based skutterudites are larger than those of the corresponding Fe and Os compounds. This may be closely related to the interesting physical properties in the Ru-based skutterudites. The bulk modulus of the binary skutterudites is smaller than that of the filled skutterudites. The cell volume for CeT₄X₁₂ (T = Fe, Ru and Os; X = P and Sb) decreases monotonically with increasing pressure. The valence state of Ce ion in these compounds does not change at high pressures.

V₆O₁₃ with a metal–insulator transition at T_c ≃ 150 K and an antiferromagnetic order at T_N ≃ 50 K has a dd_yz-type single trellis layer and a d_xy-type double trellis layer, where x and y correspond to the a- and b-axis, respectively. Magnetic properties above T_c are consistent with theoretical results for a one-dimensional Hubbard model with the uniform electron concentration and a bandwidth comparable to the effective on-site Coulomb energy. Below T_c, in the light of properties of δ-phase vanadium bronzes, the spin-singlet state is considered to appear in the double trellis layer and the one-dimensional Heisenberg chain-like state is formed in the single layer, accompanied with valence order; and at T_N, the antiferromagnetic order takes place in the single layer. The temperature dependence of the electrical resistivity for the one-dimensional chain above T_c is mainly due to antiferromagnetic spin fluctuations, and those for the normal directions nearly follow 1/T which is likely due to the renormalization effect of the Fermi surface by the fluctuations. The Hall coefficient above T_c is found to be roughly linear in T.

1,2,3-Trichloropropane was investigated by means of nuclear quadrupole resonance (NQR) and differential thermal analysis (DTA). One amorphous state and two crystalline monotropic phases (forms I and II) were found depending on the thermal history of the sample and only one is stable up to the melting point. The different phases were characterized by ³⁵Cl NQR. It was concluded that form I is an ordered crystal with only one molecular conformation present in the unit cell while form II is a disordered state with at least two molecules present in the unit cell.

In the two crystalline phases, the presence of group reorientations (−CH₂Cl) and molecule reorientations as a whole can be inferred from ³⁵Cl spin–lattice relaxation times (T₁) with activation energies of 33 and 42 kJ mol⁻¹, respectively, in form I. In form II, these activation energies are in the range 20–29 kJ mol⁻¹ indicating that this polymorph is less compactly crystalline packed in comparison to form I.