Isotropic and anisotropic spin-spin interactions and a quantum phase transition in a dinuclear Cu(II) compound

Abstract

We report electron-paramagnetic resonance (EPR) studies at 9.5 GHz (X band) and 34 GHz (Q band) of powder and single-crystal samples of the compound Cu2[TzTs]4 [N-thiazol-2-yltoluenesulfonamidatecopper(II)], C40H36Cu2N8O8S8, having copper(II) ions in dinuclear units. Our data allow determining an antiferromagnetic interaction Jo = (-113 +/- 1) cm-1 (Hex = -JoS1·S2) between Cu(II) ions in the dinuclear unit and the anisotropic contributions to the spin-spin coupling matrix D (Hani = S1·D·S2), a traceless symmetric matrix with principal values D/4 = (0.198 +/- 0.003) cm-1 and E/4 = (0.001 +/- 0.003) cm-1 arising from magnetic dipole-dipole and anisotropic exchange couplings within the units. In addition, the single crystal EPR measurements allow detecting and estimating very weak exchange couplings between neighbor dinuclear units, with an estimated magnitude Jo = (0.060 +/- 0.015) cm-1. The interactions between a dinuclear unit and the “environment” of similar units in the structure of the compound produce a spin dynamics that averages out the intradinuclear dipolar interactions. This coupling with the environment leads to decoherence, a quantum phase transition that collapses the dipolar interaction when the isotropic exchange coupling with neighbor dinuclear units equals the magnitude of the intradinuclear dipolar coupling. Our EPR experiments provide a new procedure to follow the classical exchange-narrowing process as a shift and collapse of the linestructure (not only as a change of the resonance width), which is described with general but otherwise simple theories of magnetic resonance. Using complementary procedures, our EPR measurements in powder and single-crystal samples allow measuring simultaneously three types of interactions differing by more than three orders of magnitude (between 113 cm-1 and 0.060 cm-1).