arXiv subjects
Publications and source records attributed to J. Fedor.
This article is a collection of contributions from speakers at the 2025 DEAMN [Dynamics of Electrons in Atomic and Molecular Nanoclusters] workshop at the Majorana Centre in Erice. Not ordinary contributions to a conference proceeding, this gives a new and different perspective on the work done by the workshop participants.
We experimentally show that the N-H bond cleavage in pyrrole molecule following resonant electron attachment is allowed and controlled by the motion of the atoms which are not dissociating, namely of the carbon-attached hydrogen atoms. In order to interpret these findings, we have developed a method for locating all resonant and virtual states of an electron-molecule system in the complex plane, based on all-electron R-matrix scattering calculations. Mapping these as a function of molecular geometry allows us to separate two contributing dissociation mechanisms: a $\pi^*$ resonance formation inducing strong bending deformations and a non-resonant $\sigma^*$ mechanism originating in a virtual state. The coupling between the two mechanisms is enabled by the out-of-plane motion of the C-H bonds and we show it must happen on an ultrafast few-fs timescale.
We experimentally probe electron collisions with HC$_3$N in the energy range from 0 to 10 eV with the focus on vibrational excitation and dissociative electron attachment. The vibrational excitation cross sections show a number of resonances which are mode specific: the two dominant $π^*$ resonances are visible in the excitation of all the vibrational modes, however, broad $σ^*$ resonances are visible only in certain bond-stretching vibrational modes. The lower $π^*$ resonance shows a pronounced boomerang structure. Since it overlaps with the threshold peak originating from a long-range electron-molecule interaction, the interference pattern is rather unusual. Somewhat surprisingly, the boomerang structure is visible also in the elastic scattering cross section. The dissociative electron attachment cross sections agree qualitatively with the data of Gilmore and Field [J. Phys. B 48 (2015) 035201], however, approximately a factor of two difference is found in the absolute values.
We have investigated the magnetization properties and flux dynamics of superconducting Cu$_x$TiSe$_2$ single crystals within wide range of copper concentrations. We find that the superconducting anisotropy is low and independent on copper concentration ($γ\sim1.7$), except in the case of strongly underdoped samples ($x\leq0.06$) that show a gradual increase in anisotropy to $γ\sim1.9$. The vortex phase diagram in this material is characterized by broad region of vortex liquid phase that is unusual for such low-$T_c$ superconductor with low anisotropy. Below the irreversibility line the vortex solid state supports relatively low critical current densities as compared to the depairing current limit ($J_c/J_0\sim10^{-7}$). All this points out that local fluctuations in copper concentration have little effect on bulk pinning properties in this system.
Using scanning tunneling microscopy and Ginzburg-Landau simulations we explore vortex configurations in magnetically coupled NbSe$_2$-Permalloy superconductor-ferromagnet bilayer. The Permalloy film with stripe domain structure induces periodic local magnetic induction in the superconductor creating a series of pinning-antipinning channels for externally added magnetic flux quanta. Such laterally confined Abrikosov vortices form quasi-1D arrays (chains). The transitions between multichain states occur through propagation of kinks at the intermediate fields. At high fields we show that the system becomes non-linear due to a change in both the number of vortices and the confining potential. The longitudinal instabilities of the resulting vortex structures lead to vortices `levitating' in the anti-pinning channels.