Quantum Phase Transitions Probed by EPR Spectra in Dimeric Spin Arrays with Supramolecular Couplings

CALVO, RAFAEL; SARTORIS, ROSANA P; NASCIMENTO, OTACIRO R; SEDIVÝ, MATÚS; SOJKA, ANTONIN; NEUGEBAUER, PETR; SANTANA, VINICIUS T.

COORDINATION CHEMISTRY REVIEWS

ELSEVIER SCIENCE SA

Lugar: Amsterdam; Año: 2023 vol. 480

0010-8545

Dimeric compounds with nearly isolated molecular units (d-units)having pairs of spins s1 and s2coupled by antiferromagnetic (AFM) exchange are non-trivialquantum spin systems having primary roles in magnetism. Weakly coupled infinitearrays of AFM d-unitsin crystal structures have an appealing spin dynamic arising from their energy-gappedlevel structure and display magnetic properties with important roles inmaterials science. They received great additional attention when it wasdiscovered that the spin entanglement introduced by interdimeric couplings withmagnitude  (with) givesrise to bosonic systems with novel properties and quantum phase transitions athigh temperature (T). In this work,we collect recent advances in the interpretation of EPR spectral changes interms of quantum phase transitions of arrays of d-units in the presenceof weak interdimeric couplings. We review previous investigations of theproblem and focused new experiments on the paradigmatic compound copper acetatemonohydrate (CAH), collecting a detailed set of spectra in single-crystaland powder samples. The spectral features arising from this coupling aremerging and narrowing of the peaks of the spectra of single crystals forspecific magnetic field ()orientations, and an extraordinary “U-peak”in the powder samples associated with the quantum phase transition of thedimeric spin array. Our historical overview collects studies of similarcompounds with the same dimeric feature in which the U-peaks were generally misinterpreted as a double-quantum transition, orignored. We describe procedures to identify and quantify the U-peak andthe merging and narrowing phenomena, with a protocol to extract theinterdimeric coupling magnitude. As a novel contribution, we explain theexperimental results by proposing a spin model with a microscopic flip-flopmechanism involving the absorption and emission of two simultaneous spin-one excitationshaving energy ,connecting singlet and triplet levels of neighbor d-units and giving rise to a quantum phase displaying spin entanglement.This phase is tuned with the orientation of  applied along directions within "magicrings”, the positions of the EPR peaks of the dimeric units intersect, that wepropose as a phase diagram. Our model considers explicitly the role of energyconservation in the process and allows analyzing and simulating the features ofthe EPR spectra arising from the couplings, describing their roles in thespectral behavior and the magnetic phases. In conclusion, we review the historyof dimeric compounds and the possibility of detecting interdimeric couplings inthe EPR spectra experimentally, and we introduce a novel spin model for thedimeric array appropriate to analyze EPR data which allows us to understand thespin dynamics and the phase transitions arising from these couplings.