Collapse of the EPR Fine Structure of a 1D Array of Weakly Interacting Binuclear Units: A Dimensional Quantum Phase Transition

CALVO, RAFAEL; ABUD, JULIAN; SARTORIS, ROSANA P; SANTANA, RICARDO

PHYSICAL REVIEW B

AMER PHYSICAL SOC

Lugar: Washington; Año: 2011 vol. 84 p. 104433 - 104433

1098-0121

Binuclear (also called dimeric) compounds with pairs of antiferromagnetically coupled spins 1/2 , S1 and S2 (Hex = −J0 S1.S2, with J0 < 0 for antiferromagnets), have been around for ∼60 years, providing roots to the field of molecular magnetism. In addition, as reported in recent years, weak interactions between binuclear units in a crystalline network give rise to interesting systems of interacting bosons having an energy gap, which are important in the study of quantum phenomena in many body systems coupled by stochastic distributions of interactions. Binuclear compounds have gained new relevance in the last decade with the observation of Bose-Einstein condensation. In this work, we use electron paramagnetic resonance (EPR) to study the role of weak inter-binuclear exchange couplings J? (|J?| |J0|) in the spectra, elementary excitations, and spin dynamics of a one-dimensional (1-D) array of antiferromagnetic (AFM) binuclear units in the hybrid (organic-inorganic) CuII compound [Cu(CH3COO)(phen)(H2O)]2·(NO3)2·4H2O. In this material, the acetate (CH3COO)− anion supports the intra-binuclear exchange coupling J0, and the stacking of the (phen) = 1,10-phenanthroline rings of neighbor units supports the inter-binuclear interactions J, giving rise to well-isolated chains. This has advantages over other binuclear compounds studied previously because magnetically equal units are arranged in a 1-D spatial arrangement along the direction of their symmetry axis, simplifying the analysisof the data and allowing a simpler treatment. In addition, single crystals of good quality allow detailed EPR experiments. EPR spectra were collected at ∼33.8 and ∼9.4?9.8 GHz in oriented single crystals at room temperature and in powder samples at temperatures (T) between 10 and 300 K. By varying the energy levels of the binuclear units with the magnetic field orientation, or changing the population of the excited triplet state with temperature and, consequently, the effective coupling between units, we observe in single-crystal samples sudden merges of the fine structure peaks, accompanied by a large narrowing, when the inter-binuclear coupling becomes larger than the splitting of the triplet state. In addition, and because of this collapse of the fine structure peaks, the spectra of powder samples display a strong and unexpected central peak that decreases in intensity with decreasing temperature, as it occurs with the binuclear signals. We first discuss the dimensional quantum phase transition indicated by the spectral changes using Anderson-Kubo?s theory of exchange narrowing. The data allow evaluation of the binuclear exchange coupling J0 = (−74 ± 3) cm−1, and the interactions between neighbor binuclear units|J | = (0.04 ± 0.01) cm−1.We also consider the magnetic excitations (triplet excitons or triplons) arising from the inter-binuclear couplings, and introduce a model explaining qualitatively the observed collapse and narrowing of the EPR spectra in terms of these excitons. We analyze the role of temperature in the inter-binuclear interactions and the exchange correlation times of the binuclear systems and compare our results with those in binuclear compounds in which Bose-Einstein condensation occurs.