Quantum spin dynamics

Abstract:

Abstract Successive oxidations, or reductions, of an extended molecular π-system can give rise to significant changes in the electronic structure compared to the neutral or singly charged counterparts. In this work, we investigated highly reduced states of butadiyne-linked porphyrin oligomers with up to nine porphyrin units and one negative charge per porphyrin. CW-EPR and 1H Mims ENDOR spectroscopy were used to probe the proton hyperfine interactions in these polyanions. Both techniques show a continuous narrowing of the spectra with simultaneously increasing charge and oligomer length corresponding to a decrease in the magnitudes of the largest hyperfine interactions. This is consistent with density functional theory calculations that support delocalization of spin density over the entire lengths of the molecules with high degrees of spin polarization and nonuniform spin density distributions. These spin distributions can also be described by a Hubbard model with antiferromagnetic spin coupling between electrons on adjacent sites. Finding systems with coherent wave functions that span the entire length of π-conjugated molecules, up to a length of more than 10 nm, will help identifying suitable candidates capable of coherent electron tunneling through molecular junctions.

Abstract:

Ferroelectric materials are widely used in diverse applications, where their performance is strongly dominated by grain size. Here, dense Er-doped lead titanate-based relaxor ferroelectrics were synthesized via spark plasma sintering, enabling precise grain size control from 0.9 to 11.1 μm. Fine-grained ceramics exhibit high defect concentrations and internal stress, stabilizing the tetragonal phase and resulting in weak, disordered polarization with low domain wall density and constrained mobility. At intermediate grain sizes, dense nanodomain networks with narrow walls (∼150 nm) form, allowing sharp and reversible polarization switching. Coarse-grained ceramics develop hierarchical, web-like domains with thicker walls (∼400 nm), reducing the pinning effect and enhancing wall mobility. Both saturation and remanent polarizations increase with grain size up to 5.4 μm before plateauing, while the piezoelectric coefficient rises by 200%, reaching 723 pC N–1. These results demonstrate grain-size engineering as an effective route to optimize domain wall structure and relaxor ferroelectric performance.