National Institute Of Materials Physics - Romania

In this paper we investigate the quantum phase transition from magnetic Bose Glass to magnetic Bose-Einstein condensation induced by amagnetic field in NiCl2 center dot 4SC(NH2)(2) (dichloro-tetrakis-thiourea-nickel, or DTN), doped with Br (Br-DTN) or site diluted. Quantum Monte Carlo simulations for the quantum phase transition of the model Hamiltonian for Br-DTN, as well as for site-diluted DTN, are consistent with conventional scaling at the quantum critical point and with a critical exponent z verifying the prediction z = d; moreover the correlation length exponent is found to be nu = 0.75(10), and the order parameter exponent to be beta = 0.95(10). We investigate the low-temperature thermodynamics at the quantum critical field of Br-DTN both numerically and experimentally, and extract the power-law behavior of the magnetization and of the specific heat. Our results for the exponents of the power laws, as well as previous results for the scaling of the critical temperature to magnetic ordering with the applied field, are incompatible with the conventional crossover-scaling Ansatz proposed by Fisher et al. [Phys. Rev. B 40, 546 (1989)]. However they can all be reconciled within a phenomenological Ansatz in the presence of a dangerously irrelevant operator.

Smooth and adherent bioactive coatings with similar to 0.5 mu m thickness were deposited onto Si substrates by the radiofrequency-magnetron sputtering method at 150 degrees C under 0.4 Pa of Ar atmosphere using a bioglass powder as target with a composition in the SiO2-CaO-MgO-P2O5-CaF2-B2O3-Na2O system. The bioactivity of the as-prepared bioglass samples was assessed by immersion in simulated body fluid for different periods of time up to 30 days. Grazing incidence X-ray diffraction, Fourier transform infra-red spectrometry and energy dispersive spectroscopy revealed that important structural and compositional changes took place upon immersing the samples in SBF. Whilst the excellent biomineralisation capability of the BG thin films was demonstrated by the in vitro induction of extensive and homogenous crystalline hydroxyapatite in-growths on their surfaces, a series of bioactivity process kinetics peculiarities (derogations from the classical model) were emphasised and thoroughly discussed. (C) 2012 Elsevier B. V. All rights reserved.

Bi1.5-xZn0.92-yNb1.5O6.92-delta thin films have the potential to be implemented in microwave devices. This work aims to establish the effect of the substrate and of the grain size on the optical and dielectric properties. Bi1.5-xZn0.92-yNb1.5O6.92-delta thin films were grown at 700 degrees C via pulsed-laser deposition on R-plane sapphire and (100)(pc) LaAlO3 substrates at various oxygen pressures (30, 50, and 70 Pa). The structure, morphology, dielectric and optical properties were investigated. Despite bismuth and zinc deficiencies, with respect to the Bi1.5Zn0.92Nb1.5O6.92 stoichiometry, the films show the expected cubic pyrochlore structure with a (100) epitaxial-like growth. Different morphologies and related optical and dielectric properties were achieved, depending on the substrate and the oxygen pressure. In contrast to thin films grown on (100)(pc) LaAlO3, the films deposited on R-plane sapphire are characterized by a graded refractive index along the layer thickness. The refractive index (n) at 630 nm and the relative permittivity (epsilon(t)) measured at 10 GHz increase with the grain size: on sapphire, n varies from 2.29 to 2.39 and epsilon(r) varies from 85 to 135, when the grain size increases from 37 nm to 77 nm. On the basis of this trend, visible ellipsometry can be used to probe the characteristics in the microwave range quickly, nondestructively, and at a low cost.

Samarium is deposited on Si(001) at various temperatures (room temperature to 400 A degrees C), and the surface structure, interface reactivity, electron configuration, and magnetic properties are investigated by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and magneto-optical Kerr effect (MOKE), respectively. It is found that metal Sm is present on samples prepared at room temperature with an interface layer containing mostly Sm2+ and a lower amount of Sm3+. When samples are prepared at high temperature, much less Sm-0 is found with an increasing amount of Sm2+. Freshly prepared Sm-0 and SmSi2 layers react strongly with oxygen from the residual gas, promoting formation of Sm2O3 at the expense of both metal Sm and SmSi2. Room temperature ferromagnetism is observed for all prepared layers with a decrease of the saturation magnetisation when samples are prepared at high temperature. It is found that ferromagnetism implies mostly Sm3+ and Sm metal. In addition to these findings, this work proposes a new assignment of the Sm 3d chemically shifted components. Also, a noticeable variation of the XPS Sm 3d spin-orbit splitting is found as a function of the Sm ionization state.

Bi1.5-xZn0.92-yNb1.5O6.92-delta thin films have the potential to be implemented in microwave devices. This work aims to establish the effect of the substrate and of the grain size on the optical and dielectric properties. Bi1.5-xZn0.92-yNb1.5O6.92-delta thin films were grown at 700 degrees C via pulsed-laser deposition on R-plane sapphire and (100)(pc) LaAlO3 substrates at various oxygen pressures (30, 50, and 70 Pa). The structure, morphology, dielectric and optical properties were investigated. Despite bismuth and zinc deficiencies, with respect to the Bi1.5Zn0.92Nb1.5O6.92 stoichiometry, the films show the expected cubic pyrochlore structure with a (100) epitaxial-like growth. Different morphologies and related optical and dielectric properties were achieved, depending on the substrate and the oxygen pressure. In contrast to thin films grown on (100)(pc) LaAlO3, the films deposited on R-plane sapphire are characterized by a graded refractive index along the layer thickness. The refractive index (n) at 630 nm and the relative permittivity (epsilon(t)) measured at 10 GHz increase with the grain size: on sapphire, n varies from 2.29 to 2.39 and epsilon(r) varies from 85 to 135, when the grain size increases from 37 nm to 77 nm. On the basis of this trend, visible ellipsometry can be used to probe the characteristics in the microwave range quickly, nondestructively, and at a low cost.

Thin films with the composition [(Bi0.5Na0.5)TiO3](0.92)-[BaTiO3](0.08) (hereafter BNT-BT0.08) were deposited on Pt-Si by spin-coating from a stable sol precursor. The BNT-BT0.08 film, crystallized on the Bi0.5Na0.5TiO3 rhombohedral lattice, was obtained after annealing the film-gel at 700 A degrees C. The films have a smooth surface (Rms = 2.76 nm) and grains with ferroelectric domains. The film showed a bandgap of 3.25 eV and a refractive index of 2.20 at a wavelength of 630 nm. The dielectric characteristics of BNT-BT0.08 thin films were measured at room temperature and 10 kHz the dielectric constant (epsilon (r)) was 243 and the loss tangent (tan delta) was 0.38. The remnant polarization (P (r)) was 0.87 mu C/cm(2) and the coercive field (E (c)) was 220 kV/cm at 10 kHz and room temperature. The current density was approximately 2.7 x 10(-5) A/cm(2) at low electric fields (100 kV/cm). BNT-BT0.08 thin films shown piezoelectric properties (d (33eff) = 100 pm/V) comparable to those of PZT thin films.

The low-temperature states of bosonic fluids exhibit fundamental quantum effects at the macroscopic scale: the best-known examples are Bose-Einstein condensation and superfluidity, which have been tested experimentally in a variety of different systems. When bosons interact, disorder can destroy condensation, leading to a 'Bose glass'. This phase has been very elusive in experiments owing to the absence of any broken symmetry and to the simultaneous absence of a finite energy gap in the spectrum. Here we report the observation of a Bose glass of field-induced magnetic quasiparticles in a doped quantum magnet (bromine-doped dichloro-tetrakis-thiourea-nickel, DTN). The physics of DTN in a magnetic field is equivalent to that of a lattice gas of bosons in the grand canonical ensemble; bromine doping introduces disorder into the hopping and interaction strength of the bosons, leading to their localization into a Bose glass down to zero field, where it becomes an incompressible Mott glass. The transition from the Bose glass (corresponding to a gapless spin liquid) to the Bose-Einstein condensate (corresponding to a magnetically ordered phase) is marked by a universal exponent that governs the scaling of the critical temperature with the applied field, in excellent agreement with theoretical predictions. Our study represents a quantitative experimental account of the universal features of disordered bosons in the grand canonical ensemble.

The growth of La5Ca9Cu24O41 thin films on SrLaAlO4 (1 0 0) and Gd3Ga5O12 (1 0 0) substrates by pulsed laser deposition is reported in this paper. The influence of deposition process parameters, such as oxygen pressure, substrate temperature, and laser repetition rate, on the crystallinity, orientation and microstructure of the films has been investigated. X-ray diffraction and atomic force microscopy studies showed a clear dependence of the film crystallographic orientation and morphology (grain size) on the substrate temperature and the oxygen pressure used in the deposition process. (c) 2012 Elsevier B.V. All rights reserved.

The hysteretic behavior of the epitaxial Pb(Zr,Ti)O-3 thin films with different top metal electrodes is studied, with emphasis on the influence of the leakage current and trap generation current on the shape of the loop as well as on the magnitude of the measured polarization. Cu, Pt, and SrRuO3 were used as top contacts and important differences were observed for measurements performed in both dynamic and static modes, although the contacts were deposited on the same epitaxial Pb(Zr,Ti)O-3 film grown on SrRuO3/SrTiO3 substrate. A peculiar behavior was observed especially for the static hysteresis loops where, depending of the top contact, the loop is influenced mainly by the leakage current (Pt) or by the trap generation current (Cu and SrRuO3). The last one can contribute with an additive charge, having a linear dependence on the applied voltage, as suggested by the simple model developed to explain the abnormally high values of the dielectric constant extracted from the linear part of the static hysteresis loop. It is concluded that the properties of the top electrode interface can significantly impact the hysteretic behavior of the ferroelectric films. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754318]

Transparent ZnO films are synthesized by RF-magnetron sputtering (1.78 MHz) onto glass substrates, using a mild-pressed ZnO powder target. The depositions were carried at three inert argon pressures (0.25 Pa, 0.30 Pa, and 0.45 Pa) at two substrate temperatures (100 degrees C and 400 degrees C). The role of the sputtering conditions on ZnO thin films nanostructuring, optical properties and morphology is investigated by Xray diffraction (XRD), X-ray reflectometry (XRR) and Spectroscopic ellipsometry (SE). XRD investigations revealed that ZnO films show a (0 0 l) texture with nanosized crystallites. Right-angle asymmetry of the (0 0 2) diffraction peak is observed. The peak profile analysis using pseudo-Voigt functions unveils a double overlapped peak structure with different coherent zone size values. A double layer structure is evidenced by analyzing the XRR data. Samples prepared at 0.3 Pa at a temperature of 400 degrees C have a similar to 4 nm bottom layer consisting of highly depleted in oxygen ZnO1-x structure, continued by a 53 nm top layer of textured ZnO. Electrical measurements show that the temperature dependence of the conductivity is well described by the Mott variable range hopping (VRH) law. The samples obtained at 400 degrees C have a significantly lower resistivity. (C) 2012 Elsevier B. V. All rights reserved.