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Polarized Raman spectra of the calcium vanadium oxide bronze β-Ca_0.33V₂O₅ are measured in the temperature range between 300 and 20 K. The charge ordering phase transition at about 150 K is characterized by the appearance of new Raman-active modes in the spectra, and by anomalies in the electronic background scattering. The high-temperature Raman scattering spectra of β-Ca_0.33V₂O₅ show an apparent resemblance to those of α'-NaV₂O₅, which suggests that the charge–phonon dynamics of the two compounds are similar. A study of the dynamical properties and a symmetry analysis of the Raman modes show that in the mixed-valence state of β-Ca_0.33V₂O₅ the electrons are delocalized into V₁ – O₅ – V₃ orbitals. We propose that in the charge ordered state below 150 K the d electrons localize within V₁ – V₃ ladders, either in a 'zigzag' fashion as in α'-NaV₂O₅ or in the form of double chains as in γ-LiV₂O₅.

We examine the out-of-equilibrium dynamical evolution of density profiles of ultrasoft particles under time-varying external confining potentials in three spatial dimensions. The theoretical formalism employed is the dynamical density functional theory (DDFT) of Marini, Bettolo, Marconi and Tarazona (1999 J. Chem. Phys.110 8032), supplied by an equilibrium excess free energy functional that is essentially exact. We complement our theoretical analysis by carrying out extensive Brownian dynamics simulations. We find excellent agreement between theory and simulations for the whole time evolution of density profiles, demonstrating thereby the validity of the DDFT when an accurate equilibrium free energy functional is employed.

Lithography is a key technology enabling progress both in fundamental research and in widespread applications. Besides standard technologies new alternative approaches have emerged over the last few years. Here we summarize the status of the field of atom lithography where light is used to focus matter on the nanometre scale. Using the special features of the atom–light interaction a variety of structures can be produced with a spatial period limited to half the wavelength of the light. Further reduction of feature separation can be obtained utilizing the substructure of atomic matter and light polarization. Also we show how, besides various lateral structures, three-dimensional patterns can be generated in a single-step process utilizing the selectivity of the atom–light interaction.

Molecular dynamics simulations have been performed for a large-scale system consisting of 400 000 atoms of liquid metal Al. To describe the complex microstructural evolutions in the liquid system during the rapid cooling processes, the tracing atom method and cluster bond-type index method have been used. It is demonstrated that the number of (12 0 12 0) icosahedral clusters, consisting of the 1551 bond type, with a higher degree of ordering, increases continuously and plays a critical and leading role in the solidifying transition. Various cluster configurations, formed by icosahedral clusters and Frank–Kasper, Bernal and defective polyhedra, produce the short-range-order regions in this amorphous system, while the atoms not taking part in forming clusters give the sparse regions possessing disorder characteristics. Large cluster configurations consisting of more than 150 atoms have been found and are shown to be formed by combining smaller clusters and to be different from those obtained by gaseous deposition and ionic spray methods.

Glasses containing mixtures of cations and anions of nominal compositions [Sb₂O₃]_x – [ZnCl₂]_1−x where x = 0.25, 0.50, 0.75, and 1.00, have been studied by means of neutron diffraction and Raman and Mössbauer spectroscopy. There is preferential bonding within the system with the absence of Sb–Cl bonds. Antimony is found to be threefold coordinated to oxygen, and zinc fourfold coordinated. The main contributing species are of the form [Sb(OSb)₂(OZn)] and [Zn(ClZn)₂(OSb)₂].

The temperature dependence of the saturation magnetization of Fe₃O₄surfacted nanoparticles does not follow the law in T² that corresponds to bulk ferrite: M_s (T) = M_s (0)[1 − BT² ] (where M_s (T) is the saturation magnetization at temperature T, M_s (0) is the saturation magnetization at 0 K and B is a constant that depends on the exchange integral). This abnormal behaviour was studied for a ferrofluid that contains magnetite particles covered in oleic acid (surfactant) in a carrier fluid (kerosene); the anomaly is attributed to modification of the superexchange interaction between the iron ions from the surface layer of the nanoparticles, a layer that is formed due to the presence of the surfactant. Taking into consideration the size distribution of the particles, according to magneto-granulometric measurements and transmission electron microscopy, we have shown that the thickness of the surface layer at a temperature of 300 K is ⟨η⟩ ∼ = 0.9 nm. By adopting the 'core–shell' model we have shown that the layer at the particles' surface is paramagnetic at room temperature and it gradually becomes ferrimagnetically ordered as the temperature decreases. The consequence of this change is the increase of the mean volume of the nanoparticles' magnetic core where the spins are aligned due to the superexchange interaction from 1280 to 1910 nm³ when the temperature decreases from 300 to 77 K.

The densities of liquid Bi, Sn, Pb and Sb have been precisely measured from the melting point up to about 1100 K using an improved Archimedean method. The densities at the melting point for liquid Bi, Sn, Pb and Sb are 10.042 × 10³, 6.983 × 10³, 10.635 × 10³ and 6.454 × 10³ kg m⁻³, respectively. Comparisons between our data and those from the literature have been made and they show the present results to be more reliable. Rather than a linear fit for the temperature dependence of the density, a slight deviation from linearity in the temperature dependence of the densities has been observed.

We have demonstrated the appearance of photonic minibands within the photonic bandgaps of a disordered system represented by randomly distributed 'vacancies' of air cylinders. The positions of the photonic minibands are defined by the energies of the localized photonic states of the single defect, and their width increases with increase in the concentration of the defects. The appearance of the minibands makes possible the construction of spectral filters with thin transmission bands.

The crystal and magnetic structures of a new ternary phase in the Th–Co–B system have been investigated by high resolution neutron diffraction. The spontaneous magnetization and Curie temperature were determined by magnetic measurements. The ThCo₄B phase crystallizes in a hexagonal crystal structure, space group P6/mmm with a = 5.088 Å and c = 7.003 Å. ThCo₄B orders ferromagnetically at 303 K, a temperature much smaller than that of the isotypic RCo₄B where R is a rare-earth element or yttrium. The Co magnetic moments are found to be aligned along the c-axis of the hexagonal structure in the whole ordered temperature region. The two inequivalent Co crystal sites are found to exhibit different magnetic behaviours. At 2 K a substantial magnetic moment of 1.8 ± 0.1μ_B is observed on the Co 2c site whereas a nearly zero magnetic moment is found on the Co 6i site. A similar behaviour was also found at room temperature, 1.2 ± 0.2μ_B on the Co 2c site and zero μ_B on the Co 6i site. The influence of the local environment on the magnitude of the Co moments in ThCo₄B is discussed. The magnetic properties of ThCo₄B are compared to that of isotypic YCo₄B.

Neutron diffraction studies of polycrystalline R₃Mn₄Sn₄ (R = La, Pr, Nd) intermetallic compounds with the orthorhombic Gd₃Cu₄Ge₄-type crystal structure indicate the existence of different magnetic structures. The magnetic structures were studied with the support of group theoretical considerations. For R = La a sine wave modulated structure described by the propagation vector k = (k_x, 0, 0) is observed below T_N = 300 K. This ordering is stable down to 1.5 K. For the Pr₃Mn₄Sn₄ and Nd₃Mn₄Sn₄ compounds the Mn moments form a non-collinear antiferromagnetic structure below the Néel temperatures of 230 and 210 K, respectively. Below T_t = 25 and 60 K for R = Pr and Nd, respectively, the rare earth Pr and Nd moments also order and form collinear magnetic structures while the magnetic order of the Mn moments remains unchanged.

The cohesive properties of cubic ionic crystals are, in principle, affected by the response of anions to symmetry-preserving changes of their environment away from the equilibrium nuclear geometry. The importance of such a response is investigated by performing non-empirical computations with the Relativistic Integrals program. For each crystal, the properties computed using anion wavefunctions optimal for each nuclear geometry are compared with those predicted using 'frozen' potentials in which the same anion wavefunction, one optimal for a near-equilibrium nuclear geometry, is used to compute all the inter-ionic interactions. Use of such 'frozen' potentials leaves essentially unchanged the excellent predictions of the fully optimal computations for both the lattice energy and the closest equilibrium cation–anion separation. The bulk compressibility is significantly overestimated by using 'frozen' potentials, often by 50% or even 80%, thus destroying the agreement between experiment and the predictions of the fully optimal computations.

We investigated the quantum states of a free electron with time-dependent effective mass subjected to a time-dependent magnetic field by solving the Schrödinger equation under the choice of Landau and symmetric gauges. Using the invariant operator and unitary transformation methods together, we derived exact wavefunctions of the system. The wavefunctions rely on the solution of associated classical dynamical systems. We confirmed that the quantum analysis of the system under the two gauges coincides mutually.

The optical properties of the antiperovskite superconductor MgCNi₃ have been calculated using the full-potential linearized augmented-plane-wave method within the generalized gradient approximation scheme for the exchange–correlation potential. In order to fully elucidate the optical properties of MgCNi₃, the dielectric function ε(ω), the reflectivity R(ω), the optical absorption coefficient I(ω), the optical conductivity σ(ω), the energy-loss function L(ω), the refractive index n(ω) as well as the extinction coefficient k(ω) were calculated. The prominent features in the spectra of the optical parameters are discussed.

Recent studies have shown evidence for the aggregation of self-interstitials in ion-implanted Si, resulting in nanoscopic damage structures. Similarly, self-interstitial atoms are expected to play an important role for defect clustering in ion-implanted GaAs. Accurate first-principles total-energy calculations are reported for different As and Ga self-interstitial configurations. These results are used to study by first-principles total-energy calculations the structural and electronic properties of small complexes involving self-interstitials and/or antisites.

Using a mixed basis pseudopotential method, total energy calculations were performed to obtain the enthalpy of vacancy formation in Ta as a function of pressure, which is important for understanding the effects of pressure on mechanical properties. The vacancy formation enthalpy is found to increase from 2.95 eV at ambient pressures to 12.86 eV at 300 GPa, and the vacancy formation volume decreases from being 53 ± 5% of the bulk volume per atom at ambient pressure to 20 ± 2% at 300 GPa, for a 54-atom supercell. We also show that there is a strong correspondence between the vacancy formation enthalpy and the melting temperature in Ta.

In the cubic C-type rare-earth (R) sesquioxides, C-R₂ O₃, the trivalent R ions (R³⁺) occupy two different crystallographic sites with S₆ and C₂ symmetries. The R ions in the C₂ lattice site have been studied intensively whereas the properties of the R³⁺ ions in the S₆ site are largely unknown or the data are contradictory. Based on the spectroscopic data reinterpreted by a phenomenological crystal field (cf) analysis a new interpretation was obtained for the energy level scheme of the Eu³⁺ ions in the S₆ site of C-R₂ O₃. The cf parameters obtained were then used to predict the energy level scheme of the Yb³⁺ ion in the same host lattices. In the prediction, the evolution of the cf parameters along the R series studied earlier in the R oxide and garnet systems was used. The relationship between the cf strength parameter and the overall splitting of the ^2S+1L_J levels as well as the relationship between the barycentres of the free ion levels were used, also, to reinterpret the energy level scheme of the Yb³⁺ ion in the C₂ site of C-Y₂ O₃.

The thermal and electrical transport properties of the ordered intermetallic FeAl₂ have been measured as a function of temperature between 10 and 300 K. The electrical resistivity (ρ) is about 400μΩ cm over a broad temperature range. In character, it resembles those of semimetals, in spite of its anomalous low-temperature upturn. The thermal conductivity (κ) at room temperature is high, approximately 7 W m⁻¹ K⁻¹. The measured thermoelectric power is negative at low temperatures where there is also a positive phonon-drag effect, but changes to positive values above 220 K. This observation is ascribed to the positive carriers thermally excited across the pseudogap at higher temperatures. The electronic characteristics of FeAl₂ are compared to those of other semimetallic materials.

We studied the effect of disorder on the superconducting properties of polycrystalline MgB₂ by specific-heat measurements. In the pristine state, these measurements give a bulk confirmation of the presence of two superconducting gaps with 2Δ₀/k_BT_c = 1.3 and 3.9 with nearly equal weights. The scattering introduced by irradiation suppresses T_c and tends to average the two gaps although less than predicted by theory. We also found that by a suitable irradiation process by fast neutrons, a substantial bulk increase of dH_c2/dT at T_c can be obtained without sacrificing more than a few degrees in T_c. The upper critical field of the sample after irradiation exceeds 28 T at T → 0.

Electrical and magnetic properties of four series of manganates Ln_xCa_1−xMnO₃ (Ln = La, Nd, Gd and Y) have been studied in the electron-doped regime (x = 0.02 – 0.25) in order to investigate the various inter-dependent phenomena such as ferromagnetism, phase separation and charge ordering. The general behaviour of all four series of manganates is similar, with some of the properties showing a dependence on the average radius of the A-site cations, ⟨r_A ⟩ and cation size disorder. Thus, all the compositions show an increase in magnetization at 100–120 K (T_M) for x < x_max, the magnetization increasing with increasing x. The value of x_max increases with decreasing ⟨r_A ⟩, probably due to the increased phase separation induced by site disorder. This is also reflected in the larger width of the hysteresis loops at T < T_M for small x or ⟨r_A ⟩. In this regime, the electrical resistivity decreases with increasing x, but remains low and nearly constant, T > T_M. The percolative nature of the conduction mechanism at T < T_M is substantiated by the fit of the conductivity data to the scaling law, σ ∝ |x_c − x|^p, where p is in the 2–4 range. When x > x_max, the materials become antiferromagnetic (AFM) and charge-ordered at a temperature T_CA, accompanied by a marked increase in resistivity. The value of T_CA increases with increasing ⟨r_A ⟩ and x (up to x = 0.3). Thus, all four series of manganates are characterized by a phase-separated regime between x = 0.02 and 0.1–0.15 and an AFM charge-ordered regime at x > 0.1– 0.15.

The spin-1/2 Cu²⁺ ions in β-Cu₃V₂O₈ occupy the sites of a Kagomé-staircase lattice, an anisotropic variant of the Kagomé net: buckled layers and imbedded plaquettes of three edge-shared CuO₄ squares break the ideal Kagomé symmetry. Susceptibility and heat capacity measurements show the onset of short-range ordering at approximately 75 K, and a magnetic phase transition with the characteristics of antiferromagnetism at ∼ 29 K. Comparison to the Curie–Weiss theta (θ_CW = −135 K) indicates that the geometric frustration is largely relieved by the anisotropy.

We study the low-lying excitations and thermodynamic properties of a ferrimagnetic Heisenberg two-leg ladder, which is composed of alternating-spin (1/2, 1) double chains with antiferromagnetic coupling between the chains. The spin excitation spectra as well as the corresponding energy gaps are calculated by means of the Schwinger-boson mean-field theory. The two interactions between the chains and along the chains play an important role in determining the two branches of the spectrum, not only regarding the size of the low-lying excitation energy gap but also the bandgap between the optical spectrum and the acoustic spectrum. In addition, the two energy gaps and the specific heat have also been calculated at low temperatures.

Single crystals of La_1−xBa_xMn_1−yCo_yO₃ (0.26 ≤ x ≤ 0.37; 0 ≤ y ≤ 0.31) perovskites were grown by the flux melt technique using the BaO–B₂O₃–BaF₂ ternary system as a solvent. It was found that the cobalt content in the crystals is determined by the average oxidized state of the manganese ions and depends nonlinearly on the molar ratio of Co/Mn in the initial mixture. All the crystals exhibit ferromagnetic behaviour at low temperatures as well as a jump of resistivity and a magnetoresistance peak around the Curie temperature, gradually decreasing as the cobalt content increases. The low-temperature ferromagnetic phase transforms from the metallic to the semiconducting state with increase in cobalt content.

Magnetization recoveries for nuclear spin relaxation of like spins due to magnetic dipolar coupling and diffusion on inequivalent sites involve a sum of exponentials. The theory is applied to diffusion on octahedral and tetrahedral interstitial sites in the face-centred cubic structure. Monte Carlo simulations have been used to generate relaxation data for parameters typical for H in metals. It is found that only a single exponential would be observable in the high- and low-temperature limits, but that two-exponential recoveries could be observable in the vicinity of the maximum in the relaxation rate as a function of temperature. The Monte Carlo relaxation data has been fitted using a Bloembergen–Pound–Purcell (BPP) model to assess the accuracy of the BPP model.

Phase separation and crystallization in the bulk amorphous alloys Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 and Zr_46.7Ti_8.3Cu_7.5Ni₁₀Be_27.5 have been studied by Mössbauer spectroscopy and x-ray diffraction (XRD) investigations. Mössbauer spectra of Zr–Ti–Cu–Ni–Be with ⁵⁷Fe additions were measured during the course of isothermal annealing near their glass transition temperature. The spectra show a broad asymmetric quadrupole doublet which reveals the presence of different local environments. The values of the quadrupole splitting, centre shift and area ratios undergo characteristic changes in Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 due to a decomposition process. An abrupt change of the parameters is observed when the sample begins to crystallize at 633 K after an annealing time of 600 min. This is in agreement with the results of XRD investigations that show sharpening of the diffraction maximum as a result of the formation of nanocrystals. The alloy Zr_46.7Ti_8.3Cu_7.5Ni₁₀Be_27.5 does not show any indications of decomposition and crystallization in the Mössbauer spectra. The processes are discussed from the viewpoint of the local atomic short-range order in the immediate neighbourhood of the Mössbauer spy atom.

The topology and the trace of ferroelastic domains, namely W walls, of Ca-doped lead orthophosphate (Pb_1−x, Ca_x)₃ (PO₄)₂ with a Ca content of 2.7% mol were studied on the monoclinic cleavage plane (100) using contact mode atomic force microscopy. Furthermore, conducting atomic force microscopy was applied using a bias voltage across the cantilever and the sample inducing a tunnelling current. As a reference a pure lead phosphate crystal was used. Only the Ca-doped lead phosphate crystals showed a considerable difference in conductivity between walls and the bulk. The conductivity in the bulk was found to be approximately 7% higher than in the domain walls. The experimental results show that ferroelastic domain walls of atomistic width can work as barriers to dielectric transport.

(Pb, Nb)(Zr, Sn, Ti)O₃ antiferroelectric (AFE) thin films have been fabricated on LaNiO₃/Pt/Ti/SiO₂/Si wafers using a sol–gel process. The electric field-induced antiferroelectric-to-ferroelectric (AFE–FE) phase transformation behaviour and its dependence on the temperature were examined by investigating the dielectric constant and dielectric loss versus temperature and electrical field. The AFE–FE phase transformation temperature can be adjusted as a function of the DC bias field and the thickness of the thin film. With increasing DC bias field, the FE phase region was enlarged, the AFE–FE transformation temperature shifted to lower temperature, and the ferroelectric-to-paraelectric transformation temperature shifted to higher temperature. With increasing film thickness, the modulation effect of the DC bias field on the AFE–FE phase transformation temperature is increased.

We have investigated annealing behaviours of quench-deposited films of the binary CsI–PbI₂ system by in situ optical absorption spectroscopy. Various films composed of multiple crystalline phases of CsPbI₃ and/or Cs₄PbI₆ as well as of CsI and/or PbI₂ are obtained, depending on the mixing ratio of the CsI and PbI₂. It is difficult to prepare films purely composed of a single CsPbI₃ or Cs₄PbI₆ phase alone. However, it is possible to obtain films where crystallites of either CsPbI₃ or Cs₄PbI₆ coexist with the CsI phase. Using such films, we measure the fundamental optical absorption spectrum of CsPbI₃ and Cs₄PbI₆ for the first time. Cs₄PbI₆ exhibits stronger oscillator-like absorption compared to CsPbI₃, due to the localized nature of both the Pb 6s and 6p states.

Density-functional studies of structural and electronic properties of transition-metal sulfides formed by 3d transition metals, based on the local spin-density approximation and including non-local corrections to the exchange–correlation functional (generalized gradient approximation), have demonstrated the importance of magneto-volume effects and magneto-structural effects, but could not achieve full agreement with experiment. A further improvement is to consider electronic correlation effects due to tightly bound and localized d-states on the transition metal atoms. With the DFT + U method used in this work, these correlation effects are taken in account and yield improved predictions for volume, magnetic moment, exchange splitting and bandgap. For MnS the semiconducting gap is correctly predicted, and for MnS₂ the high-spin AFM type-III state can be stabilized over the low-spin state. For FeS even weak correlation effects lead to better predictions for the semiconducting gap, volume and magnetic moment.

A theoretical study of the indirect coupling of nuclear spins (qubits) embedded in a mesoscopic ring and in a finite-length quantum wire in a magnetic field is presented. It is found that the hyperfine interaction, via the conduction electrons, between nuclear spins exhibits sharp maxima as a function of the magnetic field and nuclear spin positions. This phenomenon can be used for manipulation of qubits with almost atomic precision. Experimental feasibility and implications for quantum logic devices is discussed.

The convolution method for the calculation of local densities of states is presented more thoroughly along with its expression in terms of Green functions. This constructive approach allows us to produce results for a higher dimensionality from lower-dimensional parts. Its applications and different aspects are discussed for some simple cases.

The following important reference was omitted from this article.

[4] Long G J, Hautot D, Grandjean F, Isnard O and Miraglia S 1999 J. Magn. Magn. Matter.202 100