Heteroepitaxial growth of oxides on sapphire induced by laser radiation in the solid–liquid interface

2 O₃, Fe₂O₃ and MnO₂ on sapphire from an aqueous solutions of either CrO₃, FeCl₃, or KMnO₄, respectively, under laser irradiation of the interface sapphire/liquid. The interface is exposed through the sapphire substrate to the radiation of a copper vapor laser (wavelength of 510 nm). The etching of sapphire is accompanied by the deposition of oxide films, which are shown to grow epitaxially on the sapphire substrate, while the deposition of the polycrystalline oxide film occurs on a glass substrate under the same experimental conditions. Similarly, the epitaxial growth of cubic Fe₂O₃ and orthorhombic MnO₂ is observed, though their crystallographic structure is different from the hexagonal structure of sapphire. Received: 26 June 1997/Accepted: 7 July 1997     

Effective magnetic anisotropy of nanocrystalline Nd-Fe-Ti-N hard magnetic alloys

The intermetallic compound Nd-Fe-Ti-N has been successfully synthesized by a mechanical alloying process. The structure and magnetic properties of the sample have been studied using X-ray diffraction and magnetic measurements. It is found that alloy exhibits a nanocrystalline ThMn₁₂-type tetragonal structure with lattice parameters of a=0.8723 nm and c=0.4896 nm. The saturation magnetization M_S and effective magnetic anisotropy K_eff of the compound have been determined by investigating magnetization processes. The calculated results based on the law of approach to magnetic saturation have been successfully used to determine the constant K_eff. The difference between observed and calculated values in magnetization is lower than 3%. Of all terms in the law of approach to saturation, it is the 1/H ² term, which is attributed more to non-compensated anisotropy energy, that has the prevailing effect for the compound. The absorption of nitrogen is found to increase unit cell volume, M_S and K_eff. Received: 28 October 1996 / Revised: 14 March 1997 and 4 August 1997 / Accepted: 8 August 1997     

Positron lifetime study of vacancy-type defects in amorphous and polycrystalline nanometer-sized alumina

2 O₃ and nanocrystalline Al₂O₃ specimens. The short-lifetime (170±20 ps), intermediate-lifetime (410±20 ps) and long-lifetime components correspond to three different kinds of defects: monovacancy-like free volumes, microvoids, and larger voids. The appearance of lifetimes in the range 1–5 ns indicates the formation of positronium. The influence of thermal annealing from 873 K to 1373 K on positron lifetime parameters was also analyzed. The components with lifetimes τ₁=170 ps and τ₂=410 ps persisted even after the grains had grown to 100 nm in size, while the long-lifetime component declined significantly when grain sizes exceeded 10 nm. The interface characteristics of polycrystalline nano-Al₂O₃ prepared by the two methods were compared by analyzing the variations of the positron-lifetime parameters with grain growth. Received: 1 April 1997/Accepted: 13 August 1997     

Ferroelectric SrxBa1-xNb2O6 optical waveguiding thin films on SiO2-coated Si(100) substrates

x Ba_1-xNb₂O₆ (x=0.5) films (abbreviated as SBN:0.5) on SiO₂-coated Si substrates are potential components for the application of integrated electro-optics devices. SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates with a very thin MgO diffusion buffer have been successfully prepared by pulsed laser deposition. The as-grown films have a refractive index of 2.28, which is close to that of bulk SBN. X-ray analysis showed that the as-grown films have a single-phase tetragonal tungsten bronze structure. The SBN:0.5 thin films prepared by PLD exhibit favorable ferroelectric and optical waveguiding properties. The composition and the morphology of the films were also examined by XPS and by SEM, respectively. Ferroelectric SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates are expected to be used in integrated electro-optic devices. Received: 27 February 1997/Accepted: 17 October 1997     

Effects of excimer-laser irradiation of LaAlO3(100) single crystals: Influence on superconducting YBa2Cu3O7-x film growth

3 (100) single crystal substrates have been investigated. Poorly absorbed KrF irradiation leads to localized, deep, and unstable damage on the crystal surfaces. By contrast ArF has been found to induce well-localized roughness and microcracks on LaAlO₃(100) surfaces at fluences between 0.2 and 1.8 J/cm². The material emission threshold was estimated at around 0.5 J/cm² and cracks appeared above 1.4 J/cm². The substrate surface state determines YBa₂Cu₃O_7-x film growth, leading to morphological changes that have been related to a decrease in adatom mobility on the substrate. The most striking feature is the drastic reduction in YBa₂Cu₃O_7-x outgrowth density inside the irradiated areas. Irradiation has little effect on film crystalline properties. The main effect on the electrical properties can be restricted to a decrease of critical current density up to a factor of 10⁴, but such an important decrease is only observed in the most intensively irradiated substrates. Received: 18 November 1996/Accepted: 12 June 1997     

First observation of ferromagnetism and ferromagnetic domains in a liquid metal

80 Pd₂₀ alloy below the Curie temperature T_C(l)=1257 K of the liquid state. The magnetization of the undercooled liquid sample has been measured as a function of temperature using a modified Faraday balance. Below T_C(l), the magnetization of the liquid metal shows a plateau in the weak external field of μ₀H_z=5.6 mT. The effect is comparable to the behaviour of the corresponding solid phase, indicating the onset of spontaneous magnetization with ferromagnetic domains. T_C(l) is about 20 K lower than the Curie temperature of the solid phase T_C(s). Received: 24 February 1997/Accepted: 16 May 1997     

Magnetic properties of the low dimensional spin system (VO)PO:ESR and susceptibility

Experimental results on magnetic resonance (ESR) and magnetic susceptibility are given for single crystalline (VO)₂P₂O₇. The crystal growth procedure is briefly discussed. The susceptibility is interpreted numerically using a model with alternating spin chains. We determine J =51 K and . Furthermore we find a spin gap of meV from our ESR measurements. Using elastic constants no indication of a phase transition forcing the dimerization is seen below 300 K. Received: 22 December 1997 / Accepted: 17 March 1998     

Production, isolation, and characterization of group-2 metal-containing endohedral metallofullerenes

80 , Ca/Sr/Ba@C₈₂, Ca/Sr/Ba@C₈₄) are investigated for the first time by UV-Vis-NIR absorption spectroscopy and high-resolution ¹³C NMR. Direct evidence of the endohedral nature and cage structures are revealed by these measurements. Furthermore, we have found that each metallofullerene has 2–4 structural isomers, which have been isolated by multistage high-performance liquid chromatography. These isomers have different cage structures and give characteristic UV-Vis-NIR absorption spectra. Received: 22 September 1997/Accepted: 16 October 1997     

Growth-induced anisotropy in Ba0.6K0.4BiO3 superconducting single crystal

0.6 K_0.4BiO₃ single crystal have been performed to study the nature of the growth-induced anisotropy revealed using chemical etching technique. It was found that this anisotropy is responsible for differences in low-field susceptibility and in hysteresis loops observed for different magnetic field orientations. Received: 9 December 1997/Accepted: 28 May 1998     

Shape anisotropy and magnetic domain structures in striped monolayers

We study the effect of dipolar interactions on a magnetic striped monolayer with a microscopic unit cell of square symmetry, and of size spins. Even if the aspect ratio r=N _x /N _y is very large, an in-plane shape anisotropy is always negligible, except if N_y is fairly small (N _y <40). In-plane domains are not possible, except for values of the dipolar coupling larger than the domain wall energy. Received: 11 July 1997 / Revised: 24 September 1997 / Accepted: 24 October 1997     

Predictions of geometrical structures and ionization potentials for small barium clusters Ba

The geometrical structure of ground state Ba_n clusters (n =2-14) has been predicted from various types of calculations including two ab initio approaches used for the smaller sizes namely HF+MP2( n =2-6), DFT (LSDA)( n =2-6, 9) and one model approach HF+pairwise dispersion used for all sizes investigated here. The lowest energy configurations as well as some isomers have been investigated. The sizes n =4, 7 and 13 are predicted to be the relatively more stable ones and they correspond to the three compact structures: the tetrahedron, the pentagonal bipyramid and the icosahedron. The growth behavior from Ba₇ to Ba₁₃ appears to be characterized by the addition of atoms around a pentagonal bipyramid leading to the icosahedral structure of Ba₁₃ which is consistent with the observed size-distribution of barium clusters. Values for vertical ionization potentials calculated for n =2-5 at the CI level are seen to be in quite good agreement with recent measures. Received: 14 May 1997 / Received in final form: 2 February 1998 / Accepted: 27 February 1998     

On the high-pressure phase transition in

X-ray diffraction (XRD) experiments have been carried out on quartz-like GaPO₄ at high pressure and room temperature. A transition to a high pressure disordered crystalline form occurs at 13.5 GPa. Slight heating using a YAG infrared laser was applied at 17 GPa in order to crystallize the phase in its stability field. The structure of this phase is orthorhombic with space group Cmcm. The cell parameters at the pressure of transition are a =7.306?, b =5.887? and c =5.124?. Received: 7 October 1997 / Received in final form: 17 November 1997 / Accepted: 18 November 1997     

Rare-gas-induced broadening and shift of two-photon transitions to intermediate Rydberg states of atomic thallium*

Results of broadening and shift measurements of Doppler-free two-photon lines for transitions from the ground state to the Rydberg states with intermediate principle quantum numbers of atomic thallium perturbed by rare gases are reported. The rates show a distinct behaviour in this range of principle quantum numbers and significant dependence on the total angular momentum of the upper nP state. The experimental results are compared with calculations using a Van der Waals potential and a superposition of polarization and Fermi potentials. Additionally, broadening and shift rates of the transition Tl 6P _3/2 -9P _3/2 have been measured for quadrupolar as well as for mainly scalar excitation. The rates for both kinds of excitation coincide within the limits of error reflecting the small perturbation of the 6P _3/2 state compared to that of the upper 9P _3/2 state. Received: 6 August 1997 / Revised in final form: 29 November 1997 / Accepted: 11 December 1997     

Kinetic and thermodynamic investigation of cBN formation in the system BN-Mg3N2

3 N₂BN system are presented and discussed. In particular, a temperature region slightly above the lower limit of the formation region characterized by fast transformation processes and submicron cBN grains strongly agglomerated is investigated in more detail concerning thermodynamics and kinetics. The catalyst/solvent active in that temperature interval is Mg₃BN₃ previously formed at about 5.5 G Pa and 1170 K in a solid state reaction. The cBN formation proceeds as precipitation from a eutectic melt formed by the intermediate compound and BN. These results are used to improve the previously proposed phase diagram for the Mg₃N₂BN system. The kinetics is discussed based on the usual Avrami theory. It follows that the processes may be described by nearly instantaneous nucleation at the beginning of the reaction with large rate. Growth processes are soon inhibited due to strong impingement of the formed nuclei and grains so that an Avrami exponent n in the neighbourhood of 1 results. This exponent, as well as the rate constant k summarizing the physical mechanisms of nucleation and growth, show a temperature dependence. Received: 6 January 1997/Accepted: 16 May 1997     

Energy pooling in caesium vapour:

The total rate coefficient k for the energy pooling process has been measured in dependence on the ratio of the number densities of the Cs 6P fine-structure sublevels. The measurements were performed applying the laser fluorescence method. From the set of the obtained data for k, the rate coefficients for the particular collisions were derived. The obtained values for the and collisions at T = 600 K are , and , respectively. The value for k₁ and the estimate for k₃ are consistent with the data previously published by other authors. The rate coefficient k₂ is reported for the first time. Received: 10 June 1997 / Revised: 13 October 1997 / Accepted: 5 January 1998     

Structural manipulation of ultrathin organic films on metal surfaces: the case of decane thiol/Au(111)

3 (CH₂)₉]SH) have been adsorbed on Au(111) single crystals both via vacuum deposition and from an ethanolic solution. The epitaxial structure of the ultrathin organic films has been identified at room temperature via low-energy electron diffraction to be c(4×2)R30° for the solution grown film and rectangular c(23×) for the vacuum deposited film. These structures correspond to molecules adsorbed on the surface with their carbon chains flat on the surface (vacuum deposited) and nearly perpendicular (solution grown). It is demonstrated that this orientation can be changed reversibly in vacuum via either annealing the films or exposing them to additional gas. Received: 7 February 1997/Accepted: 27 May 1997     

Phase diagram for diamond growth in atmospheric oxyacetylene flames

2 /C₂H₂ ratio for diamond synthesis in oxyacetylene flames can be theoretically predicted. When the substrate temperature is between 1000 K and 1250 K, the corresponding O₂/C₂H₂ ratio range is about between 0.8 and 1.1. The suitable range of substrate temperature is widest for O₂/C₂H₂ ratio close to unity and is narrowed rapidly when the flow ratio deviates from unity. Received: 2 March 1997/Accepted: 17 July 1997     

Impurity scattering in d-wave superconductivity. Unitarity limit versus Born limit

As is well known, Zn-substitution of Cu in the Cu-O₂ plane in the hole-doped high T_c cuprates provides a semi-quantitative test of underlying d-wave superconductivity. Here we complement this with a parallel study of Ni-substitution, which gives rise to weak scattering described with the Born approximation. Received: 7 November 1997 / Revised: 14 November 1997 / Accepted: 24 November 1997     

Crystalline structure and optical spectroscopy of Er3+-doped KGd(WO4)2 single crystals

4 )₂ single crystals doped with Er³⁺ have been grown by the flux top-seeded-solution growth method. The crystallographic structure of the lattice has been refined, being the lattice constants a=10.652(4), b=10.374(6), c=7.582(2) Å, β=130.80(2)°. The refractive index dispersion of the host has been measured in the 350–1500 nm range. The optical absorption and photoluminescence properties of Er³⁺ have been characterised in the 5–300 K temperature range. At 5 K, the absorption and emission bands show the (2J+1)/2 multiplet splittings expected for the C₂ symmetry site of Er in the Gd site. The energy positions and halfwidths of the 72 sublevels observed have been tabulated as well as the cross sections of the different multiplets. Six emission band sets have been observed under excitation of the ⁴F_7/2 multiplet. The Judd–Ofelt (JO) parameters of Er³⁺ in KGW have been calculated: Ω₂=8.90×10^-20 cm², Ω₄=0.96×10^-20 cm², Ω₆=0.82×10^-20 cm². Lifetimes of the ⁴S_3/2, ⁴F_9/2, and ⁴I_11/2 multiplets have been measured in the 5–300 K range of temperature and compared with those calculated from the JO theory. A reduction of the ⁴S_3/2 and ⁴I_11/2 measured lifetimes with increasing erbium concentration has been observed, moreover the presence of multiphonon non-radiative processes is inferred from the temperature dependence of the lifetimes. Received: 15 December 1997/Revised version: 10 July 1998     

Infrared properties of etched mercuric iodide surfaces

2 samples and on solution-grown KHgI₃,H₂O and NH₄HgI₃,H₂O crystals. A detailed study of the infrared absorption lines confirms that, taking into account small variations in the HgI₂ substrates, the residues on the surface are actually the same complexes. Received: 29 September 1997/Accepted: 31 October 1997