Tetiana Nosach

Distinct quasiparticle relaxation dynamics in an electron-doped superconductor, BaFe1.9Ni0.1As2

We report an investigation of the distinct quasiparticle (QP) relaxation dynamics in an electron-doped superconductor, BaFe1.9Ni0.1As2, employing femtosecond pump?probe measurements. Two distinct relaxation components and one sub-nanosecond long-lived component are clearly observed in our transient reflectivity spectra. The slow relaxation component, which is on a picosecond time scale, strongly correlates with the superconducting (SC) transition, and we attribute it to the recombination dynamics of Cooper pairs. We calculated the SC gap, ?(0)?5.7?meV. The fast relaxation component, on a sub-picosecond scale, is ascribed to the QP relaxation from a range of large gap states to the state above the SC gap. We obtained the low end of these bandgaps to be ?G?9?meV. Furthermore, we estimated the electron?phonon (e?ph) coupling constant and the Coulomb pseudopotential from the fast relaxation lifetime. The small values of the e?ph constant and the negative Coulomb pseudopotential suggest that the e?ph interaction is not the dominant contribution of the SC transition.

Angular dependence of spin-wave resonance and relaxation in half-metallic Sr2FeMoO6 films

We investigated the magnetic anisotropic parameters and spin-wave relaxation of thin films of the ferromagnetic half-metallic Sr2FeMoO6 by ferromagnetic resonance technique. The resonance field and linewidth were recorded as a function of relative angle between applied magnetic field and crystallographic axes of the sample. The resonance field varies sinusoidally and considerable linewidth broadening occurs when the applied field is rotated parallel to the sample plane. The results are described using higher order components of anisotropy fields. We obtain the values of the cubic anisotropic field, 2K4∥∕M=0.09611T, the effective demagnetization field, 4πM−2K2⊥∕M=0.1216T, and the planar anisotropic field, 2K4⊥∕M=−0.108T. Moreover, we estimated the spin relaxation time (damping factor) from the analysis of the angular dependence of peak-to-peak linewidth, leading to an intrinsic value of α∼0.00025 (Gilbert damping).

Lithium chloride molten flux approach to Li2MnO3:LiMO2 (M = Mn, Ni, Co) “composite” synthesis for lithium-ion battery cathode applications

In this scientific report, a scalable method for the fabrication of cathodes based on firing pristine compounds Li2MnO3 and LiMn0.33Ni0.33Co0.33O2 with lithium chloride molten flux is explored. This approach offers flexibility in synthesis temperature since the process does not require precursor decomposition. Moreover, the synthesis technique allows for the study of the development from the pristine compounds to the final product (Li1.2Mn0.53Ni0.13Co0.13O2). This could help us to understand if the Li2MnO3:LiMO2 material system is a true solid solution or a phase-separated composite. Cathode materials were prepared and characterized by electrochemical charge and discharge studies, electrochemical impedance spectroscopy, and different characterization techniques, including lithium magic angle spinning-nuclear magnetic resonance. Stability studies were conducted to investigate the effects of synthesis duration and temperature on the cathode material. Optimal performance was achieved by firing the pristine compounds for 6 hours at 1000 °C and for 48 hours at 800 °C, both in the LiCl molten flux, resulting in a powder with a solid solution behaviour and specific discharge capacity near 240 mA h g−1.

Quasiparticle relaxation across the multiple superconducting gaps in the electron-doped BaFe1.85Co0.15As2

We study the quasiparticle relaxation dynamics in the electron-doped superconductor, BaFe1.85 Co0.15As2 (BFCA) by using optical spectroscopy in both time and frequency domains. The estimate of the electron−phonon (el–ph) coupling constant from quasiparticle relaxation rate in time domain gives a value of ∼0.16 that is much too small to give a TC of 25 K. We obtain a negative value of the Coulomb pseudopotential that implies that the el−ph interaction alone is not strong enough to induce a superconducting transition in BFCA. More importantly, the electron-boson spectral function determined from optical scattering rate in frequency domain does not resemble the measured phonon density of states and has a dominant peak (at 53 meV) beyond the phonon frequencies. Our results support that the BFCA is a multi-gap s-wave superconductor and that its electron pairing is based on a spin-mediated pairing mechanism.