Near-field microscopy discloses a peculiar potential to explore novel quantum state of matter at the nanoscale, providing an intriguing playground to investigate, locally, carrier dynamics or propagation of photoexcited modes as plasmons, phonons, plasmon-polaritons or phonon-polaritons. Here, we exploit a combination of hyperspectral time domain spectroscopy nano-imaging and detectorless scattering near-field optical microscopy, at multiple terahertz frequencies, to explore the rich physics of layered topological insulators as Bi2Se3 and Bi2Te2.2Se0.8, hyperbolic materials with topologically protected surface states. By mapping the near-field scattering signal from a set of thin flakes of Bi2Se3 and Bi2Te2.2Se0.8 of various thicknesses, we shed light on the nature of the collective modes dominating their optical response in the 2-3 THz range. We capture snapshots of the activation of transverse and longitudinal optical phonons and reveal the propagation of sub-diffractional hyperbolic phonon-polariton modes influenced by the Dirac plasmons arising from the topological surface states and of bulk plasmons, prospecting new research directions in plasmonics, tailored nanophotonics, spintronics and quantum technologies.

Lead halide perovskites have demonstrated outstanding performance in photovoltaics, photodetectors, radiation detectors and light-emitting diodes. However, the electromechanical properties, which are the main application of inorganic perovskites, have rarely been explored for lead halide perovskites. Here, we report the discovery of a large electrostrictive response in methylammonium lead triiodide (MAPbI(3)) single crystals. Under an electric field of 3.7 mu m(-1), MAPbI(3) shows a large compressive strain of 1%, corresponding to a mechanical energy density of 0.74 J cm(-3), comparable to that of human muscles. The influences of piezoelectricity, thermal expansion, intrinsic electrostrictive effect, Maxwell stress, ferroelectricity, local polar fluctuation and methylammonium cation ordering on this electromechanical response are excluded. We speculate, using density functional theory, that electrostriction of MAPbI(3) probably originates from lattice deformation due to formation of additional defects under applied bias. The discovery of large electrostriction in lead iodide perovskites may lead to new potential applications in actuators, sonar and micro-electromechanical systems and aid the understanding of other field-dependent material properties.

Crystal plasticity occurs by deformation bursts due to the avalanchelike motion of dislocations. Here we perform extensive numerical simulations of a three-dimensional dislocation dynamics model under quasistatic stress-controlled loading. Our results show that avalanches are power-law distributed and display peculiar stress and sample size dependence: The average avalanche size grows exponentially with the applied stress, and the amount of slip increases with the system size. These results suggest that intermittent deformation processes in crystalline materials exhibit an extended critical-like phase in analogy to glassy systems instead of originating from a nonequilibrium phase transition critical point.

Aliovalent doping is an effective way to increase conductivity, both electronic and ionic, in ceramic materials. In this study, the transport properties of Sr2MgSi2O7 are investigated and results show that Na-doping in this system introduces ionic conductivity

We explore the charge transport mechanism in organic semiconductors based on a model that accounts for the thermal intermolecular disorder at work in pure crystalline compounds, as well as extrinsic sources of disorder that are present in current experimental devices. Starting from the Kubo formula, we describe a theoretical framework that relates the time-dependent quantum dynamics of electrons to the frequency-dependent conductivity. The electron mobility is then calculated through a relaxation time approximation that accounts for quantum localization corrections beyond Boltzmann theory, and allows us to efficiently address the interplay between highly conducting states in the band range and localized states induced by disorder in the band tails. The emergence of a "transient localization" phenomenon is shown to be a general feature of organic semiconductors that is compatible with the bandlike temperature dependence of the mobility observed in pure compounds. Carrier trapping by extrinsic disorder causes a crossover to a thermally activated behavior at low temperature, which is progressively suppressed upon increasing the carrier concentration, as is commonly observed in organic field-effect transistors. Our results establish a direct connection between the localization of the electronic states and their conductive properties, formalizing phenomenological considerations that are commonly used in the literature.

The nature of ionic disorder and the effect of structural defects on the photocatalytic function of anatase are revisited in the light of direct experimental evidence retrieved on the molecular scale by cathodoluminescence (CL) spectroscopy. CL spectra, collected on different types of photocatalytically efficient or inefficient anatase films, embodied a composite optical response of electron-compensating majority types of ionic disorder in the anatase lattice. This paper describes the dual experimental output obtained by systematically monitoring optically active off-stoichiometry sites, as follows: (i) quantitative analyses of film stoichiometry including the interactions of different lattice-defect populations; and (ii) stability/evolution of off-stoichiometry sites upon post-fabrication annealing cycles and their effects on the photocatalytic activity of the films. CL experiments provided us with direct access to the structural state of the defective anatase lattice, thus unfolding some missing detail about the complex physicochemical interactions behind its photocatalytic efficiency.

Yttrium aluminium borate single crystals, doped with 1 and 4 mol% of Pr3+, were analyzed in the wave number range 500-25000 cm-1 and temperature range 9-300 K by means of high-resolution Fourier transform spectroscopy. In spite of the complex spectra, exhibiting broad and split lines, the energy level scheme was obtained for several excited manifolds. The careful analysis of the spectra as a function of the temperature allowed us to identify most of the sublevels of the ground manifold. The thermally induced line shift, well described by a single-phonon coupling model, could be exploited to provide information about the energy of the phonons involved. The orientation of the dielectric ellipsoid and of the dipole moments associated to a few transitions was also determined from linear dichroism measurements. The experimental data were fitted in the framework of the crystal-field theory, but the agreement was not satisfactory, as already reported for Pr3+ ion in other matrices. Additional discrepancies came from the dichroic spectra analysis and the line splitting, possibly associated to hyperfine interaction. Some causes which might be responsible for the difficulties encountered in the Pr3+ ion theoretical modelling are discussed.

: In this paper we report a complete optical investigation of Nd3+ doped (Lu (x) + Gd1-x )(3)Ga5O12 crystal including the absorption and emission spectroscopy at room temperature and 10 K, the excitation spectroscopy at 10 K clearly showing the multi-site feature, and the lifetime at temperatures from 10 K to 300 K. The Judd-Ofelt theory has been applied to calculate the spontaneous transition probabilities, the branching ratio, and the radiative lifetime of the F-4(3/2) multiplet; the calculated results are in good agreement with the experimental ones. Finally, we calculated the emission cross sections of the transitions concerned to evaluate the potential of the material in laser field application.

Optochemical sensing properties of thick films of titanium dioxide (titania) in anatase phase have been studied and compared with tin dioxide cassiterite. Anatase titania exhibits a large photoluminescence response to nitrogen dioxide, which acts as a luminescence enhancer. Intrinsic surface phenomena rather than bulk defectivity are proposed to account for the behaviour and the experimental results are fitted with the Langmuir model. Good operational performances working at room temperature are achieved.

Yb doped Lu3Al5O12 single crystal fibers has been grown with various Yb3+ ion concentrations: 2%, 5%, 10%, 15%, and 50% Yb doping levels. The diameter of the fibers is around 3 mm and their length is up to 110 mm. The fibers have been annealed in air at 1400 degrees C for 24 h to remove Yb2+ produced during the growth in the fibers. Structural characterization by x-ray diffraction method has been performed to confirm the monocrystalline phase and the orientation of the fibers. Absorption measurements have been performed on different pieces of the fibers to check the homogeneity of the distribution of Yb3+ along the fiber length. The results indicate that the distribution coefficient is close to 1. The fluorescence lifetime was measured for different Yb3+ concentrations. Results are in agreement with the literature and can be interpreted as a mixture of the radiation trapping and concentration quenching effects. The emission cross section has been calculated both from absorption spectra and from emission data. The two different independent methods give similar results, but emission data suffer from strong reabsorption effects on some of the lines. On the zero phonon line at high doping levels a central dip appears that can be interpreted with a reabsorption model.

"We report on the microwave surface resistance of two polycrystalline (MgB2)-B-11 samples; one consists of pristine material, the other has been irradiated at very high neutron fluence. It has already been reported that in the strongly irradiated sample the two gaps merge into a single value. The mw surface resistance has been measured in the linear regime as a function of the temperature and the DC magnetic field, at increasing and decreasing fields. The results obtained in the strongly irradiated sample are quite well justified in the framework of a generalized Coffey and Clem model, in which we take into account the field distribution inside the sample due to the critical state. The results obtained in the pristine sample show several anomalies, especially at low temperatures, which cannot be justified in the framework of standard models for the fluxon dynamics. Only at temperatures near T-c and for magnetic fields greater than 0.5H(c2)(T) the experimental data can quantitatively be accounted for by the Coffey and Clem model, provided that the upper-critical-field anisotropy is taken into due account."

The present work focuses on the sintering of the Nd1.85Ce0.15CuO4 phase in the form of sputtering targets. The method of manufacture, based on a careful control of the microstructure, is of fundamental importance in ensuring the reliability of Nd1.85Ce0.15CuO4 targets and the subsequent realization of high-quality sputter-deposited thin films. In this study the Nd1.85Ce0.15CuO4 targets were prepared by a standard solid state reaction technique. We investigated the influence of the thermal treatment on the phase formation by employing X-ray diffraction (XRD) technique, scanning electron microscopy (SEM) and energy dispersion spectrometry (EDS) analyses. As the growth temperature increases beyond the eutectic point, the achievement of a liquid phase yields a homogeneous grain growth. The results presented here are expected to be of particular usefulness in tailoring the growth of high quality thin films. (C) 2008 Elsevier B.V. All rights reserved.

We report confocal micro-Raman spectra of the organic semiconductor alpha-sexithiophene (T6) on bulk crystals and on thin films grown on technologically relevant substrates and devices. We show that the two polymorphs, which are clearly identified by their lattice phonon spectra, may coexist as physical impurities of one inside the other in the same crystallite. Spatial distribution of the two phases is monitored by Raman phonon mapping of crystals grown upon different conditions. Raman microscopy has then been extended to T6 thin films grown on silicon oxide wafers. We identify the crystal phase in thin films whose thickness is just 18 run. The most intense total-symmetric Raman vibration is still detectable for a two-monolayer thick film. Comparative analysis between micro-Raman and AFM of T6 thin films grown on field effect transistors shows that electrode-channel steps favour the nucleation and growth of T6 molecules on the substrate, at least below 50 nm.

We have determined the ytterbium valence as a function of temperature in the reported near-zero thermal expansion material YbGaGe using x-ray photoemission at various incident photon energies. The Yb 3d, 4d, and 4f levels, which directly yield the Yb valence, have been measured. Careful analysis enabled the clear separation of surface and bulk contributions. Resonant photoemission at the 4d-4f absorption edge was used to enhance the low contribution of the Yb3+ component. Contrary to the initially proposed Yb valence transition, we find no change in the valence from room temperature down to 115 K.

Under stress, many crystalline materials exhibit irreversible plastic deformation caused by the motion of lattice dislocations. In plastically deformed microcrystals, internal dislocation avalanches lead to jumps in the stress-strain curves (strain bursts), whereas in macroscopic samples plasticity appears as a smooth process. By combining three-dimensional simulations of the dynamics of interacting dislocations with statistical analysis of the corresponding deformation behavior, we determined the distribution of strain changes during dislocation avalanches and established its dependence on microcrystal size. Our results suggest that for sample dimensions on the micrometer and submicrometer scale, large strain fluctuations may make it difficult to control the resulting shape in a plastic-forming process.

We investigated the morphological, structural, electronic, and transport properties of pentacene thin films grown by vacuum thermal evaporation on different inert substrates at room temperature. The results of our atomic force microscopy (AFM), X-ray diffraction and scanning tunnelling microscopy (STM) analysis show a structure in the so called 'thin film phase' with 1-2 gm sized gains. Atomic terraces are clearly evidenced with AFM and give an inter-planar spacing of 1.54 nm corresponding to the (001) distance. The Scanning Tunneling Spectroscopy measurements show an HOMO-LUMO gap of 2.2 eV. After vacuum thermal evaporation on patterned substrates with different inter-electrodes distances, we have performed in situ measurements of the electrical response of such thin films. We found for these films a resistivity of p=4.7 +/- 0.2 center dot 104 Omega in, that is an order of magnitude lower than the value reported to date in literature for single crystals of pentacene. This value is not affected by the presence of grain boundaries. The resistivity is further reduced by a factor 8.9 +/- 0.7 , 14 +/- 1, 2.3 +/- 0.3 upon exposure to oxygen, nitrogen and ambient air, respectively. In addition density functional theory calculations have been performed to investigate the electronic structure of pentacene in this specific phase, focusing on the effects on the relevant electronic properties of the relative orientation of the molecules within the crystalline unit cell, so far experimentally unknown. Our results show that the energy bandwidth and band-gap are crucially affected by the molecular stacking. Furthermore, by comparing our theoretical spectra with the scanning tunneling spectroscopy (STS) measurements, we propose a molecular arrangement that gives a good agreement with experiments as far as the relevant orbitals are concerned. For this polymorph, we find a HOMO and LUMO bandwidth of approximate to: 0.7 eV and approximate to 0.8 eV, respectively, which are significantly larger than those obtained for the pentacene bulk-phase and are consistent with the larger conductivity experimentally observed in pentacene thin films. (C) 2007 Elsevier B.V. All rights reserved.

We have studied the magnetization of a series of spin-charge-ordered La2-xSrxNiO4+delta single crystals with 0 <= x <= 0.5. For fields applied parallel to the ab plane there is a large irreversibility below a temperature T(F1)similar to 50 K and a smaller irreversibility that persists up to near the charge-ordering temperature. We observed memory effects in the thermoremnant magnetization across the entire doping range. We found that these materials retain a memory of the temperature at which an external field was removed and that there is a pronounced increase in the thermoremnant magnetization when the system is warmed through a spin reorientation transition.

We report results of microwave second-harmonic generation in ceramic samples of MgB2, prepared by different methods. The SH signal has been investigated as a function of the temperature and the static magnetic field. The results are discussed in the framework of models reported in the literature. We show that the peculiarities of the SH signal are related to the specific properties of the sample. A comparison with the results obtained in ceramic and crystalline YBa2Cu3O7 shows that the second-harmonic emission in MgB2 is weaker than that observed in ceramic YBa2CuO7. (c) 2006 Wiley Periodicals, Inc.

The structural, electronic, and magnetic properties of a MnxGe1-x alloy prepared through room-temperature ion implantation (100 keV, 2x10(16) ions/cm(2)) and subsequent 400 degrees C annealing have been investigated with several experimental techniques. The as-implanted sample shows a quasi-Gaussian Mn concentration depth profile with a projected range (peak Mn concentration x similar or equal to 12 at./%) at 55 nm and end of range at 140 nm. The structural investigation shows that the overall implanted Ge layer is amorphous. In particular, up to a depth of 60 nm, the implanted layer is also porous and oxidized, whereas the deepest implanted region (60-140 nm) is purely composed of amorphous Ge with Mn atoms diluted in it. This sample manifests magnetic hysteresis up to 20 K and a strong nonlinear S-shaped magnetic response up to 150 K. Upon annealing at 400 degrees C, the top porous layer remains essentially amorphous, whereas partial reconstruction into Ge nanocrystals (similar to 10 nm in diameter) occurs in the 60-140-nm-deep implanted region. Part of the Mn atoms, mainly belonging to the top porous layer, further diffuses toward the surface and forms chemical bonds with O contaminants, becoming magnetically inactive. The other Mn atoms, mainly in the region between 60 and 140 nm from the surface, remain trapped in the residual amorphous matrix or in the Ge nanocrystals, whereas formation of Mn-Ge extrinsic phases (like Mn11Ge8 and Mn5Ge3) is excluded. The magnetic response of the annealed sample originates from the existence of a soft and a harder magnetic component, assigned to the dilution of Mn atoms in residual amorphous Ge and Ge nanocrystals, respectively. The hard component, attributable to a MnxGe1-x diluted magnetic semiconductor in nanocrystalline form, manifests magnetic hysteresis up to above 250 K.

The propagation properties of the emitted light from oligothiophene crystals are discussed. The internal reflectivity at the interface between the crystal and an isotropic ambient medium has been derived, allowing to discuss the effect of self-waveguiding of the emission on the basis of the components of the material dielectric tensor.