The morphologic correlation between vortex transformation and upper critical field line in opal-based nanocomposites.

In this study, we investigate metallic nanocomposites to elucidate the properties of nanostructured conventional superconductors. Liquid tin, indium, and mercury are loaded into opal matrices by high pressure up to 10 kbar. The opal templates preserve the 3D dendritic morphology of confined superconducting metals to model a dendritic second phase with particular grain shape in bulk superconductors observed by a DualBeam microscope. We carry out measurements of the dc and ac magnetizations to study the superconducting phase diagrams, vortex dynamics, and impact of grain morphology in the opal composites. Besides, we apply the small-angle neutron scattering (SANS) to deny a regular vortex structure. The phase diagrams reveal an enhanced upper critical field Hc2(0) and curvature crossover in the upper critical field line. We also calculate the vortex activation barriers Ua and observe a transformation in the vortex system. According to the field dependence of Ua, the vortex structure transformation highly correlates with the curvature crossover in the upper critical field line. Our observations suggest that the similarity in the normalized phase diagrams and field dependences of Ua in the three nanocomposites is owing to their particular morphology of confinement.

Polymorphism of Metallic Sodium under Nanoconfinement.

(23)Na NMR studies of sodium nanoparticles confined to porous glass with the 3.5 nm mean pore size were carried out. The emergence of the second component of the NMR line was observed below 240 K that evidences the occurrence of another modification of metallic sodium. The phase transition temperature is much higher than the martensite transformation temperature in bulk sodium.

Atomic mobility in nanostructured liquid Ga-In alloy.

Nuclear spin relaxation and the Knight shift for (71)Ga, (69)Ga, and (115)In isotopes were studied by nuclear magnetic resonance (NMR) in liquid gallium-indium alloy confined to porous glass and alloy surface film and were compared with the bulk counterparts. Drastic spin relaxation acceleration under nanoconfinement was observed for the three isotopes. Quadrupole and magnetic contributions to spin relaxation were separated for gallium and indium isotopes using the experimental data obtained, which allowed, in particular, the evaluation of correlation times of atomic mobility. The strong decrease in the correlation time was found for confined alloy which evidenced a remarkable diffusion slowdown. The effect of changes in atomic mobility on NMR line broadening was also discussed.

The negative phonon confinement effect in nanoscopic sodium nitrite.

A nanocomposite of porous glass and a NaNO(2) ferroelectric (channels of approximately 7 nm diameter) was studied using infrared reflectivity, THz transmission and Raman spectroscopy as a function of temperature in the range of 300-500 K, including the ferroelectric transition. From the infrared and THz response the effective dielectric function was calculated and compared with the dielectric functions calculated from the Bruggeman and Lichtenecker models of the effective medium, using the known data on the polar phonon modes of the NaNO(2) single crystals. The results show good qualitative agreement, indicating that the stiffening of the effective modes is due to local depolarization fields on the glass-ferroelectric interfaces. The nonpolar Raman modes show no substantial modification compared to those of the bulk NaNO(2). Some signatures of the ferroelectric transition were even seen. The results indicate that the intrinsic size effect (phonon confinement) is negligible in this nanocomposite.

Superconductivity and structure of gallium under nanoconfinement.

Superconductivity and crystalline structure were studied for two nanocomposites consisting of gallium loaded porous glasses with different pore sizes. The superconducting transition temperatures were found to differ from those in known bulk gallium modifications. The transition temperatures 7.1 and 6.7 K were ascribed to two new confined gallium structures, ι- and κ-Ga, observed by synchrotron radiation x-ray powder diffraction. The evolution of superconductivity on decreasing the pore filling with gallium was also studied.

Diffusion of benzene confined in the oriented nanochannels of chrysotile asbestos fibers.

We used quasielastic neutron scattering to study the dynamics of benzene that completely fills the nanochannels of chrysotile asbestos fibers with a characteristic diameter of about . The macroscopical alignment of the nanochannels in fibers provided an interesting opportunity to study anisotropy of the dynamics of confined benzene by means of collecting the data with the scattering vector either parallel or perpendicular to the fibers axes. The translational diffusive motion of benzene molecules was found to be isotropic. While bulk benzene freezes at 278.5 K, we observed the translational dynamics of the supercooled confined benzene on the time scale of hundreds of picoseconds even below 200 K, until at about its dynamics becomes too slow for the resolution of the neutron backscattering spectrometer. The residence time between jumps for the benzene molecules measured in the temperature range of 260 K to 320 K demonstrated low activation energy of 2.8 kJ/mol.

Translational dynamics of water in the nanochannels of oriented chrysotile asbestos fibers.

We performed a high-resolution quasielastic neutron scattering study of water dynamics in fully hydrated oriented chrysotile asbestos [chemical formula Mg3Si2O5(OH)4] fibers. The fibers possess sets of macroscopically long, parallel channels with a characteristic diameter of about 5 nm. Freezing of water in the channels was observed at about 237 K. Measurements at 280, 260, and 240 K revealed a translational dynamics of water molecules with the relaxation time slower by more than an order of magnitude compared to bulk water. The Q dependence of the quasielastic broadening was typical of a translational diffusion motion with a distribution of jump lengths, similar to that observed in bulk water. The relaxation time showed no significant anisotropy in the measurements with the scattering vector parallel and perpendicular to the fiber axes.

Structure and properties of confined sodium nitrite.

The temperature evolution of the structure of NaNO(2) nanocomposite ferroelectric material in a porous glass with 7 nm pores was studied by neutron diffraction in temperature region from room temperature up to the melting, i.e. in the ferro- and paraelectric phases. It is demonstrated that in the ferroelectric phase the structure is consistent with the structure of the bulk, but above the ferroelectric phase transition (and up to approximately 513 K) a volume premelted state is formed, manifesting itself in a growth of amplitudes of ion thermal vibrations, a steep increase of elementary cell volume and "softening" of lattice. For the first time the temperature dependence of order parameter eta for confined sodium nitrite is determined. eta (T) follows a power law with T(C)=425.6+/- 2.1 K and beta= 0.31+/- 0.04, which is essentially different from that for bulk NaNO(2). Our obtained data are in a good agreement with the results of earlier dielectric and neutron diffraction measurements.

Temperature evolution of sodium nitrite structure in a restricted geometry.

The NaNO2 nanocomposite ferroelectric material in porous glass was studied by neutron diffraction. For the first time, the details of the crystal structure including positions and anisotropic thermal parameters were determined for the solid material, embedded in a porous matrix, in ferro- and paraelectric phases. It is demonstrated that in the ferroelectric phase the structure is consistent with bulk data, but above transition temperature the giant growth of amplitudes of thermal vibrations is observed, resulting in the formation of a "premelted state." Such a conclusion is in good agreement with the results of dielectric measurements published earlier.

Spin-lattice relaxation enhancement in liquid gallium confined within nanoporous matrices.

Nuclear spin relaxation for liquid gallium embedded into nanoporous matrices was found to accelerate remarkably compared to the bulk melt. NMR measurements on two gallium isotopes showed that the dominant mechanism of relaxation was changed from magnetic to quadrupolar and the relation rate depended on the Larmor frequency. The correlation time of electric field gradient fluctuations was estimated using data for quadrupolar relaxation contribution and was found to increase drastically compared to bulk, which corresponded to slowing down mobility in confined liquid gallium.