arXiv subjects

Krzysztof Fronc

Publications and source records attributed to Krzysztof Fronc.

8 recordsLinked to original sources

arXiv2024

Up-Converting Luminescent Nanoparticles as Probes of Surface Dynamics in Single Evaporating Microdroplets of Suspension

We have investigated the optically measurable properties of single evaporating microdroplets of suspensions containing up-converting luminescent nanoparticles (Gd2O3:Er3+), levitated in a linear electrodynamic trap. These microdroplets served as spherical optical resonators, with their resonance properties influenced by the distribution and interactions of nanoparticles (NPs) acting as nanoprobes. Using a combination of light scattering theories, we examined the evolution of the microdroplet radius, nanoparticle surface organization, and effective refractive index during evaporation. Up-conversion luminescence, driven by 805 nm excitation, enhanced the resolution of these observations. Key findings reveal that the increasing NP density in the surface layer leads to surface saturation, followed by transitions to a gel-like state through successive layer formation and collapse. Scattering and luminescence signals exhibit complex oscillatory behaviors linked to resonance phenomena and nanoparticle migration driven by photophoretic forces. Theoretical modeling closely aligns with experimental trends, confirming the interplay between NP distribution and optical resonance properties.

physics.optics↗

arXiv2023

Temperature Dependence of Spin Pumping in Ni81Fe19/NbN Bilayer Thin Films

We present a comprehensive study of broadband spin pumping utilizing the inverse spin Hall effect phenomena in bilayer samples comprising Ni81Fe19 (15 nm) and NbN (with NbN thickness varying from 20 nm to 140 nm), conducted over a temperature and frequency range spanning from 300 K to 4 K and 2 GHz to 12 GHz, respectively. Our investigations reveal a systematic shift in ferromagnetic resonance fields, amplitude, and line widths as functions of both frequency and temperature. Notably, we observed a temperature-dependent increase in the spin Hall angle value, surpassing previously reported values. Furthermore, our results demonstrate a pronounced temperature dependence in the inverse spin Hall effect voltage, exhibiting a significant reduction below the Tc. This reduction in inverse spin Hall effect voltage is accompanied by an increase in the linewidth of the ferromagnetic resonance mode.

cond-mat.mes-hall↗

arXiv2023

Structural pathways for ultrafast melting of optically excited thin polycrystalline Palladium films

Due to its extremely short timescale, the non-equilibrium melting of metals is exceptionally difficult to probe experimentally. The knowledge of melting mechanisms is thus based mainly on the results of theoretical predictions. This work reports on the investigation of ultrafast melting of thin polycrystalline Pd films studied by optical laser pump - X-ray free-electron laser probe experiments and molecular-dynamics simulations. By acquiring X-ray diffraction snapshots with sub-picosecond resolution, we capture the sample's atomic structure during its transition from the crystalline to the liquid state. Bridging the timescales of experiments and simulations allows us to formulate a realistic microscopic picture of melting. We demonstrate that the existing models of strongly non-equilibrium melting, developed for systems with relatively weak electron-phonon coupling, remain valid even for ultrafast heating rates achieved in femtosecond laser-excited Pd. Furthermore, we highlight the role of pre-existing and transiently generated crystal defects in the transition to the liquid state.

cond-mat.mtrl-sci↗

arXiv2023

ZnO nanowires grown on Al2O3-ZnAl2O4 nanostructure using solid-vapor mechanism

We present Al2O3-ZnAl2O4-ZnO nanostructure, which could be a prominent candidate for optoelectronics, mechanical and sensing applications. While ZnO and ZnAl2O4 composites are mostly synthesized by sol-gel technique, we propose a solid-vapor growth mechanism. To produce Al2O3-ZnAl2O4-ZnO nanostructure, we conduct ZnO:C powder heating resulting in ZnO nanowires (NWs) growth on sapphire substrate and ZnAl2O4 spinel layer at the interface. The nanostructure was examined with Scanning Electron Microscopy (SEM) method. Focused Ion Beam (FIB) technique enabled us to prepare a lamella for Transmission Electron Microscopy (TEM) imaging. TEM examination revealed high crystallographic quality of both spinel and NW structure. Epitaxial relationships of Al2O3-ZnAl2O4 and ZnAl2O4-ZnO are given.

cond-mat.mtrl-sci↗

arXiv2011

Tuning the inter-shell splitting in self-assembled CdTe quantum dots

We present photoluminescence studies of highly excited single self--assembled CdTe quantum dots under continuous--wave and pulsed excitations. We observe appearance of emission bands related to sequential filling of s--, p-- and d--shells. We analyze the inter-shell splitting for five samples, in which the dots were formed from a strained CdTe layer of different width. We find that with increasing the CdTe layer width the inter-shell splitting increases. In a time resolved measurement, we observe a radiative cascade between transitions involving one, two, and more than two excitons.

cond-mat.mes-hall↗

arXiv2011

Inter-shell exchange interaction in CdTe/ZnTe quantum dots: magneto-photoluminescence of X, X2- and XX-

We present a comprehensive photoluminescence study of exchange interaction in self-assembled CdTe/ZnTe quantum dots. We exploit the presence of multiple charge states in the photoluminescence spectra of single quantum dots to analyze simultaneously fine structure of different excitonic transitions, including recombination of neutral exciton/biexciton, doubly charged negative exciton and negatively charged biexciton. We demonstrate that the fine structure results from electron-hole exchange interaction and that spin Hamiltonians with effective exchange constants $δ_i$ can provide a good description of each transition in magnetic field for Faraday and Voigt field geometry. We determine and discuss values of the effective exchange constants for a large statistics of quantum dots.

cond-mat.mes-hall↗

arXiv2011

Stark Spectroscopy and Radiative Lifetimes in Single Self-Assembled CdTe Quantum Dots

We present studies on Coulomb interactions in single self-assembled CdTe quantum dots. We use a field effect structure to tune the charge state of the dot and investigate the impact of the charge state on carrier wave functions. The analysis of the quantum confined Stark shifts of four excitonic complexes allows us to conclude that the hole wave function is softer than electron wave function, i. e. it is subject to stronger modifications upon changing of the dot charge state. These conclusions are corroborated by time-resolved photoluminescence studies of recombination lifetimes of different excitonic complexes. We find that the lifetimes are notably shorter than expected for strong confinement and result from a relatively shallow potential in the valence band. This weak confinement facilitates strong hole wave function redistributions. We analyze spectroscopic shifts of the observed excitonic complexes and find the same sequence of transitions for all studied dots. We conclude that the universality of spectroscopic shifts is due to the role of Coulomb correlations stemming from strong configuration mixing in the valence band.

cond-mat.mes-hall↗

arXiv2010

Magnetic polaron formation and exciton spin relaxation in single CdMnTe quantum dots

We study the formation dynamics of a spontaneous ferromagnetic order in single self-assembled CdMnTe quantum dots. By measuring time-resolved photoluminescence, we determine the formation times for QDs with Mn ion contents x varying from 0.01 to 0.2. At low x these times are orders of magnitude longer than exciton spin relaxation times evaluated from the decay of photoluminescence circular polarization. This allows us to conclude that the direction of the spontaneous magnetization is determined by a momentary Mn spin fluctuation rather than resulting from an optical orientation. At higher x, the formation times are of the same order of magnitude as found in previous studies on higher dimensional systems. We also find that the exciton spin relaxation accelerates with increasing Mn concentration.

cond-mat.mes-hall↗