RESUMO
Niobium carbide (NbC) is a high-field type II superconductor with a critical temperature (TC) of 11.1 K, slightly exceeding that of pure Nb (TC = 9 K). The reduction of NbC to the nanoparticle scale leads to significant changes in its critical field and/or the superconducting temperature. This study presents findings on superconducting NbC nanoparticles with TC ≃ 10 K produced through laser ablation in acetone, where different conditions of laser fluence and centrifugation were studied. Analysis by X-ray diffraction confirmed the cubic NbC phase, while electron microscopy images displayed approximately 8 nm spherical particles, showing no noticeable size variation with laser fluence. Additionally, magnetization curves exhibited both magnetic and superconducting loops for all investigated samples. A decrease in laser fluence resulted in the suppression of diamagnetic behavior below TC. Furthermore, all samples exhibited a weak electron spin resonance (ESR) Curie-like signal at g ≃ 2.0, probably linked to localized defects on the particle's surface. The simultaneous existence of superconductivity and magnetism in nanoparticles has recently garnered significant research attention. This intricate scenario and unique properties arise from the significant enhancement of the surface-to-volume ratio in these superconducting NbC nanoparticles, emphasizing the need for further investigation to unveil novel material properties and shed new light on our comprehension of the superconducting phenomenon in this particular morphology.
RESUMO
Photon upconversion taking place in small rare-earth-doped nanoparticles has been recently observed to be thermally modulated in an anomalous manner, showing thermal enhancement of the emission intensity. This effect was proved to be linked to the role of adsorbed water molecules as surface quenchers. The surface capping of the particles has a direct influence on the thermal dynamics of water adsorption and desorption, and therefore on the optical properties. Here, we show that the upconversion intensity of small-size (<25 nm) nanoparticles co-doped with Yb3+ and Er3+ ions, and functionalized with different capping molecules, presents clear irreversibility patterns upon thermal cycling that strongly depend on the chemical nature of the nanoparticle surface. By performing temperature-controlled luminescence measurements we observed the formation of a thermal hysteresis loop, resembling an optical switching phenomenon, whose shape and trajectory depend on the hydrophilicity of the surface. Additionally, an intensity overshoot takes place immediately after turning off the heating source, affecting each radiative transition differently. We performed numerical modelling to understand this effect considering non-radiative energy transfer from the surface defect states to the Er3+ ions. These findings are relevant for the comprehension of nanoparticle-based luminescence and the interplay between the surface and volume effects, and more generally, for applications involving UCNPs such as nanothermometry and bioimaging, and the development of optical encoding systems.
RESUMO
We report on electronic collective excitations in RMn(2)O(5) (R =Pr, Sm, Gd, Tb) showing condensation starting at and below ~T(N) ~T(C)~ 40-50 K. Their origin is understood as partial delocalized e(g) electron orbitals in the Jahn-Teller distortion of the pyramid dimer with strong hybridized Mn(3+)-O bonds. Our local probes, Raman, infrared, and x-ray absorption, back the conclusion that there is no structural phase transition at T(N)~T(C). Ferroelectricity is magnetically assisted by electron localization triggering lattice polarizability by unscreening. We have also found phonon hardening as the rare earth is sequentially replaced. This is understood as a consequence of lanthanide contraction. It is suggested that partially f-electron screened rare earth nuclei might be introducing a perturbation to e(g) electrons prone to delocalize as the superexchange interaction takes place.