J. Arvanitidis

Temperature-induced valence transition and associated lattice collapse in samarium fulleride

The different degrees of freedom of a given system are usually independent of each other but can in some materials be strongly coupled, giving rise to phase equilibria sensitively susceptible to external perturbations. Such systems often exhibit unusual physical properties that are difficult to treat theoretically, as exemplified by strongly correlated electron systems such as intermediate-valence rare-earth heavy fermions and Kondo insulators, colossal magnetoresistive manganites and high-transition temperature (high-Tc) copper oxide superconductors. Metal fulleride salts—metal intercalation compounds of C60—and materials based on rare-earth metals also exhibit strong electronic correlations. Rare-earth fullerides thus constitute a particularly intriguing system—they contain highly correlated cation (rare-earth) and anion (C60) sublattices. Here we show, using high-resolution synchrotron X-ray diffraction and magnetic susceptibility measurements, that cooling the rare-earth fulleride Sm2.75C60 induces an isosymmetric phase transition near 32 K, accompanied by a dramatic isotropic volume increase and a samarium valence transition from (2 + ε) + to nearly 2 + . The negative thermal expansion—heating from 4.2 to 32 K leads to contraction rather than expansion—occurs at a rate about 40 times larger than in ternary metal oxides typically exhibiting such behaviour. We attribute the large negative thermal expansion, unprecedented in fullerene or other molecular systems, to a quasi-continuous valence transition from Sm2+ towards the smaller Sm(2+ε)+, analogous to the valence or configuration transitions encountered in intermediate-valence Kondo insulators like SmS (ref. 3).

Pressure screening in the interior of primary shells in double-wall carbon nanotubes

The pressure response of double-wall carbon nanotubes has been investigated by means of Raman spectroscopy up to

Hydrothermal synthesis of copper based nanoparticles: antimicrobial screening and interaction with DNA.

10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}

High-pressure induced metastable phase in tetragonal 2D polymeric C60.

. The intensity of the radial breathing modes of the outer tubes decreases rapidly but remain observable up to

On the nature of the laser irradiation induced reversible softening of phonon modes in C60 single crystals

9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}

Stability, enhancement of elastic properties and structure of multilayered amorphous carbon films

, exhibiting a behavior similar (but less pronounced) to that of single-wall carbon nanotubes, which undergo a shape distortion at higher pressures. In addition, the tangential band of the external tubes broadens and decreases in amplitude. The corresponding Raman features of the internal tubes appear to be considerably less sensitive to pressure. All findings lead to the conclusion that the outer tubes act as a protection shield for the inner tubes whereas the latter increase the structural stability of the outer tubes upon pressure application.

Structural anisotropic properties of a-plane GaN epilayers grown on r-plane sapphire by molecular beam epitaxy

Copper based nanoparticles (Cu-based NPs) of different compositions and sizes have been hydrothermally synthesized by varying the reaction time in the presence of the biocompatible surfactants polyoxyethylene (20) sorbitan laurate (Tween 20) and polyethylene glycol 8000 (PEG 8000). Effective control of the above synthetic parameters gave rise to Cu, Cu2O and Cu/Cu2O NPs of 10-44 nm. The antibacterial activity of the NPs was screened against Gram-positive (Bacillus subtilis, Bacillus cereus, Staphylococcus aureus) and Gram-negative (Xanthomonas campestris, Escherichia coli) bacteria. The Cu-based NPs induce pDNA degradation in a dose-dependent manner as well as extensive ds CT-DNA degradation. Cu2O NPs of 16 nm and 12 nm exhibit the lowest IC50 values (2.13 μg/mL and 3.7 μg/mL) against B. cereus and B. subtilis, respectively. The agarose gel electrophoresis of ds CT-DNA treated with Cu2O NPs demonstrated degradation at high concentration. In lower concentrations, viscosity measurements indicated groove binding. In regard to the enhanced antibacterial effect and specificity of Cu2O NPs against the Gram-positive strains, the activity pathway was further explored and ROS production and lipid peroxidation verified. The released copper ions 5.15 mg/L in distilled water and 16.32 mg/L in nutrient medium, found below the critical value to inhibit bacterial growth and thus nanosized composition effect is predominant.

μ+SR study of carbon-doped MgB2 superconductors

The structural stability of the tetragonal two-dimensional (2D) polymeric phase of C60 has been studied under pressure up to 24 GPa and room temperature by means of in situ Raman scattering. An irr ...

Structural stability of the rhombohedral 2D polymeric phase of C60 studied by in situ Raman scattering at pressures up to 30 GPa

Abstract The average temperature rise in the laser excitation spot of the C60 single crystal has been determined using the Stokes to anti-Stokes integrated peak intensity ratio for the Hg(1) phonon mode. The reversible softening of the Ag(2) pentagon pinch mode was found to be due to the heating of the sample caused by the laser irradiation, in agreement with experimental results obtained for uniformly heated samples. These findings are in quantitative agreement with results obtained by numerical calculations of the local temperature rise, which indicates the highly non-uniform temperature distribution in the laser excitation spot associated with the small thermal conductivity of solid C60.

Effect of high hydrostatic pressure on the phonon modes of Tb3Al5O12 and Dy3Al5O12 single crystals

Abstract The growth of sputtered amorphous carbon (a-C) films in layer structure with alternating (negative/positive) substrate bias voltage V b , was applied to control their intrinsic stress level and stability. The main benefit of the process was the development of thick, stable, hard and rich in sp 3 sites films proving their usefulness for many practical applications. In order to investigate the structure and the mechanisms of film stability we performed in-situ Spectroscopic Ellipsometry, Stress, Nanoindentation, Raman and Transmission Electron Microscopy (TEM) measurements. The latter provides details about the layered structure of the films. A stress relief was found to occur in films depending on the sequence of layers and their modulation period. Despite the film stability an improvement in film hardness and elastic modulus was also achieved, whereas nanocrystalline carbon phases were detected and identified by Raman spectra.