arXiv subjects

Pawel Wzietek

Publications and source records attributed to Pawel Wzietek.

5 recordsLinked to original sources

arXiv2026

Molecular-to-polymeric crossover in ion diffusion in glyme-based electrolytes: from vehicular to hopping transport

Ion transport in glyme-based electrolytes arises from a complex interplay between solvation structure, ion correlations, and polymer chain length. Here, combining pulsed-field gradient nuclear magnetic resonance (PFG-NMR), ionic conductivity measurements, and molecular dynamics (MD) simulations, we investigate the diffusion of monovalent cations (Li$^+$, Na$^+$, Cs$^+$) and TFSI$^-$ anions across a wide molecular-weight range, from monoglyme to long poly(ethylene oxide) (PEO) chains up to 4000~g/mol, corresponding to $n$ up to 88, where $n$ is the number of ethylene oxide repeat units. We identify a crossover region at $n \approx 8$ separating two transport regimes. For short chains, ion motion is consistent with a vehicular mechanism, accompanied by pronounced ion correlations. For longer chains, ion transport decouples from polymer motion and proceeds via rapid coordination exchanges within a slowly relaxing matrix. This transition is accompanied by reduced ion clustering and enhanced anion mobility, leading to increasingly anion-dominated charge transport. Overall, our results provide a molecular picture of ion transport across the molecular-to-polymeric transition and highlight the central role of solvation shell dynamics and polymer relaxation in governing ion dynamics in glyme-based electrolytes.

cond-mat.soft↗

arXiv2025

The importance of shear on the collective charge transport in CDWs revealed by an XFEL source

Charge transport in materials has an impact on a wide range of devices based on semiconductor, battery or superconductor technology. Charge transport in sliding Charge Density Waves (CDW) differs from all others in that the atomic lattice is directly involved in the transport process. To obtain an overall picture of the structural changes associated to the collective transport, the large coherent X-ray beam generated by an X-ray free-electron laser (XFEL) source was used. The CDW phase can be retrieved over the entire sample from diffracted intensities using a genetic algorithm. For currents below threshold, increasing shear deformation is observed in the central part of the sample while longitudinal deformation appears above threshold when shear relaxes. Shear thus precedes longitudinal deformation, with relaxation of one leading to the appearance of the other. Moreover, strain accumulates on surface steps in the sliding regime, demonstrating the strong pinning character of these surface discontinuities. The sliding process of nanometric CDW is based on an impressive spatial coherence involving the macroscopic sample dimensions.

cond-mat.mtrl-sci↗

arXiv2024

NMR study of the local magnetic order in the kagome Weyl semimetal Co$_3$Sn$_2$S$_2$

A magnetic Weyl semimetal presents the intriguing possibility of controlling topological properties through magnetic order. The kagome compound \CoSnS~has emerged as one of the most thoroughly characterized magnetic Weyl semimetals, yet the potential coexistence of a ferromagnetic state below $T_c$ = 172~K with a non-collinear antiferromagnetic phase or a glassy state remains unresolved. We employ $^{59}$Co NMR to gain a local perspective on the magnetic order. The magnetic and electric field gradient tensors at room temperature are determined by fitting the NMR spectra using evolutionary algorithms. Zero-field NMR measurements reveal that all Co sites are equivalent in the magnetic phase at low temperatures and up to 90~K. The local magnetic field follows in intensity the macroscopic magnetization as a function of temperature and is tilted from the c-axis by a few degrees toward the nearest triangle center. Above 90~K, a shoulder appears on the low-field side, which we attribute to a preferential tilting of the local field in one direction, breaking the equivalence between the three Co sites of the kagome structure. We rule out any coexistence with an in-plane antiferromagnetic phase and suggest instead that in-plane ferromagnetic-like moments appear above 90~K and play an increasing role in the magnetic order up to the magnetic transition.

cond-mat.str-el↗

arXiv2013

NMR investigation of the pressure induced Mott transition to superconductivity in Cs3C60 isomeric compounds

The discovery in 1991 of high temperature superconductivity (SC) in A3C60 compounds, where A is an alkali ion, has been initially ascribed to a BCS mechanism, with a weak incidence of electron correlations. However various experimental evidences taken for compounds with distinct alkali content established the interplay of strong correlations and Jahn Teller distortions of the C60 ball. The importance of electronic correlations even in A3C60 has been highlighted by the recent discovery of two expanded fulleride Cs3C60 isomeric phases that are Mott insulators at ambient pressure. Both phases undergo a pressure induced first order Mott transition to SC with a (p, T) phase diagram displaying a dome shaped SC, a common situation encountered nowadays in correlated electron systems. NMR experiments allowed us to establish that the bipartite A15 phase displays Néel order at 47K, while magnetic freezing only occurs at lower temperature in the fcc phase. NMR data do permit us to conclude that well above the critical pressure, the singlet superconductivity found for light alkalis is recovered. However deviations from BCS expectations linked with electronic correlations are found near the Mott transition. So, although SC involves an electron-phonon mechanism, correlations have a significant incidence on the electronic properties, as had been anticipated from DMFT calculations.

cond-mat.str-el↗

arXiv2010

NMR study of the Mott transitions to superconductivity in the two Cs_3C_60 phases

We report an NMR and magnetometry study on the expanded intercalated fulleride Cs_3C_60 in both its A15 and face centered cubic structures. NMR allowed us to evidence that both exhibit a first-order Mott transition to a superconducting (SC) state, occuring at distinct critical pressures p_c and temperatures T_c. Though the ground state magnetism of the Mott phases differs, their high $T$ paramagnetic and SC properties are found similar, and the phase diagrams versus unit volume per C_60 are superimposed. Thus, as expected for a strongly correlated system, the inter-ball distance is the relvevant parameter driving the electronic behavior and quantum transitions of these systems.

cond-mat.supr-con↗