National Institute Of Materials Physics - Romania

Pockels and Kerr effects are linear and nonlinear electro-opticaleffects, respectively, used in many applications. The modulation ofthe refractive index is employed in different photonic circuits. However,the greatest challenge is in photonic elements for quantum computingat room temperature. For this aim, materials with strong Pockels/Kerreffects and & chi;((2))/& chi;((3)) nonlinearsusceptibilities are necessary. Here, we demonstrate composition-modulatedstrong electro-optical response in epitaxial films of (Ba,Ca)(Ti,Zr)O-3 perovskite titanate. These films are grown by pulsed laserdeposition on SrTiO3. Depending on the ratios of Ca/Baand Ti/Zr, films show high Pockels or Kerr optical nonlinearities.We relate the variable electro-optic response to the occurrence ofnanopolar domains with different symmetries in a selected compositionrange. These findings open the route to easily implement nonlinearoptical elements in integrated photonic circuits.

Using the master equation approach, we look for fingerprints of the interaction between the localized spin S of a nanomagnet coupled to spin-polarized leads and its quantized vibrational modes. We find that the stationary and transient currents are sensitive to vibron-assisted transitions of the molecular spin on both sides of the anisotropy barrier. Such transitions are associated with vibron-dressed states and triggered under resonant conditions. Transport calculations are presented for two antiparallel configurations of the spin-polarized electrodes. In the first configuration, and far from a resonance point, a blockade is imposed on both the electronic and molecular spins via their exchange interaction. When sweeping the magnetic field through resonance, the spin-vibron interaction removes this blockade and allows the indirect reading of resonant transitions as the molecular spin climbs the left side of the anisotropy barrier. In the second configuration, the anisotropy barrier is overcome but the vibron-assisted transitions on the right side of the anisotropy barrier "delocalize" the molecular spin and do not allow the complete current-induced magnetic switching -S & RARR; S. In both configurations, the stationary current increases on resonance, due to additional transport channels triggered by the spin-vibron coupling. Therefore, the switching of the spin-vibron coupling could be detected in future transport experiments.

Chiral materials showing Kramers-Weyl fermions represent a suitable platform for quantum technology, i.e., for engineering quantum solenoids, spin-torque devices, polarization-sensitive photodetectors based on quantized circular photogalvanic effect, etc. Accordingly, the stability of this class of materials in oxidative environments, such as the ambient atmosphere, should be carefully investigated to succeed in technology transfer. Here, taking as case-study example the well-recognized topological chiral system cadmium diarsenide (CdAs2), we assess its chemical reactivity towards ambient gases (oxygen and water) and air by density functional theory and experiments. The surface of CdAs2 evolves into an oxide skin, but its thickness remains nanometric even after one year in air, as directly imaged by high-resolution transmission electron microscopy. Accordingly, it is evident that future quantum devices based on Kramers-Weyl fermions could be stable in air, as the oxide layer formed on chiral quantum materials only represents a native oxide, which actually protects bulk features, including Kramers-Weyl fermions (correlated to bulk band structure), from degradation in air.

Recent experiments demonstrated that single-particle quantum walks can reveal the topological properties of single-particle states. Here, we generalize this picture to the many-body realm by focusing on multiparticle quantum walks of strongly interacting fermions. After injecting N particles with multiple flavors in the interacting SU(N) Su-Schrieffer-Heeger chain, their multiparticle continuous-time quantum walk is monitored by a variety of methods. We find that the many-body Berry phase in the N-body part of the spectrum signals a topological transition upon varying the dimerization, similarly to the single-particle case. This topological transition is captured by the single- and many-body mean chiral displacement during the quantum walk and remains present for strong interaction as well as for moderate disorder. Our predictions are well within experimental reach for cold atomic gases and can be used to detect the topological properties of many-body excitations through dynamical probes.

Nanostructured surfaces based on silver nanoparticles decorated ZnO-CuO core-shell nanowire arrays, which can assure protection against various environmental factors such as water and bacteria were developed by combining dry preparation techniques namely thermal oxidation in air, radio frequency (RF) magnetron sputtering and thermal vacuum evaporation. Thus, high-aspect-ratio ZnO nanowire arrays were grown directly on zinc foils by thermal oxidation in air. Further ZnO nanowires were coated with a CuO layer by RF magnetron sputtering, the obtained ZnO-CuO core-shell nanowires being decorated with Ag nanoparticles by thermal vacuum evaporation. The prepared samples were comprehensively assessed from morphological, compositional, structural, optical, surface chemistry, wetting and antibacterial activity point of view. The wettability studies show that native Zn foil and ZnO nanowire arrays grown on it are featured by a high water droplet adhesion while ZnO-CuO core-shell nanowire arrays (before and after decoration with Ag nanoparticles) reveal a low water droplet adhesion. The antibacterial tests carried on Escherichia coli (a Gram-negative bacterium) and Staphylococcus aureus (a Gram-positive bacterium) emphasize that the nanostructured surfaces based on nanowire arrays present excellent antibacterial activity against both type of bacteria. This study proves that functional surfaces obtained by relatively simple and highly reproducible preparation techniques that can be easily scaled to large area are very attractive in the field of water repellent coatings with enhanced antibacterial function.

Dense fine-grained Ba0.6Sr0.4TiO3 ceramics with submicronic grains sizes (GS) have been prepared using nanopowders synthesized via sol-gel route and consolidated by Spark Plasma Sintering (SPS). By changing SPS parameters, the GS was reduced from 214 nm to 74 nm. Diffuse ferroelectric-paraelectric phase transitions and low values of dielectric permittivity (<1000) at the Curie temperature (T-C similar to 280 K) were revealed by Impedance Spectroscopy in all sintered ceramics. The GS reduction from submicron to nanoscale range reflects in a gradually diminishment of dielectric constant, tunability, polarisation and storage energy properties. Raman spectroscopy investigations pointed out the presence of polar nanoclusters above the T-C. The short-range polar order is affected by the GS decrease, but becomes more thermally stable. The observed properties of Ba0.6Sr0.4TiO3 nanostructured ceramics are interpreted by considering the interplay between the GS reduction, the role of low-permittivity grain boundaries and the diffuse character of the ferroelectric-to-paraelectric transformation.