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.

13C NMR of DIPAC and DIPAB organic ferroelectrics.

The diisopropylammonium chloride (C6H16ClN, DIPAC) and diisopropylammonium bromide (C6H16BrN, DIPAB) molecular crystals are recently discovered ferroelectrics with sufficiently high spontaneous polarization and Curie temperature. We performed first studies of these crystals by 13C NMR. CP MAS spectra were collected within large temperature ranges covering the Curie points. The reconstructive phase transition from the initial orthorhombic P212121 structure of DIPAB to the monoclinic ferroelectric P21 structure leads to an abrupt alteration in the 13C spectrum. The 13C spectra for DIPAC and DIPAB in the ferroelectric P21 phase are quite similar with four lines at lower frequencies, which correspond to the CH3 groups, and two lines with close chemical shifts, which correspond to two CH groups. The transition into the paraphase leads to gradual reduction of the interline distances in the low-frequency quadruplet and in the doublet. The step-like changes in the interline frequency shifts at this transition indicating its first order. The analysis of the spectrum evolution in the paraphase shows that only a CH group lays in the reflection plane above the P21 → P21/m transition, while the second CH group only moves closer to the reflection plane upon further heating.

Dielectric properties of an organic ferroelectric of bromide diisopropylammonium embedded into the pores of nanosized Al2O3 films.

The study presents experimental results for investigating linear and nonlinear dielectric properties of nanocomposites based on bromide diisopropylammonium (C6H16NBr, DIPAB) and aluminum oxide films (Al2O3) with pore diameter of 330, 100 and 60 nm. It was indicated that the phase transition was blurred and shifted toward lower temperatures. This anomaly became more significant with decreasing pore size. The reduction of phase transition temperature in the nanocomposites, containing DIPAB, was consistent with theoretical models for the influence of size effects on the structural phase transition.

Liquid-liquid transition in supercooled gallium alloys under nanoconfinement.

NMR studies of the thermal evolution of the Ga-In-Sn and Ga-In liquid alloys embedded into opal matrices were carried out. Temperature dependences of the gallium lineshape, shift of the resonance frequency (Knight shift), and intensity were obtained upon cooling down to the alloy freezing and subsequent warming. A second high-frequency 71Ga NMR signal emerged for both alloys upon cooling, the NMR line intensity transferring gradually into this additional signal. The Knight shifts of the signals differed noticeably. The transformations of the gallium line upon warming were continuous and not affected by changes in the alloy compositions induced by melting. 115In NMR measurements were conducted to monitor the alloy compositions at freezing and melting. The findings suggest the occurrence of the liquid-liquid phase transition in the strongly supercooled alloys under nanoconfinement.

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.

Phase transitions in K(1-x)Na(x)NO(3) embedded into molecular sieves.

Dielectric studies of molecular sieves MCM-41 filled with K(1-x)Na(x)NO(3) salts (x = 0, 0.05 and 0.10) were carried out and compared with results obtained for their bulk counterparts. The regular increase upon warming of the temperature of the reconstructive phase transition from phase II to phase I with decreasing pore sizes was observed for all x. A thermodynamic model for shifts of reconstructive phase transitions was suggested. Ferroelectric phase III was shown to occur upon cooling in bulk binary salts and for all x in confined geometry under the thermal conditions when phase III does not arise in bulk KNO(3). Ferroelectricity persisted below room temperature in the bulk binary salt with x = 0.10.

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.

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.

Impurity nuclear spin-lattice relaxation suppression and charge exchange of chromium ions in a gamma-irradiated ruby crystal.

The possibility of directly separating and measuring the impurity and lattice contributions to nuclear spin-lattice relaxation under the condition of a steady-state magnetic saturation is demonstrated in the particular case of 27Al nuclei in a gamma-irradiated ruby crystal. Contributions from trivalent and charge-exchanged chromium ions to impurity nuclear relaxation were separated. The number of charge-exchanged chromium ions was found. The spin relaxation time of Cr4+ centers was estimated.