RAF Model and Solid Oxygen: Infinite Degeneracy, Soft Lines, No LRO in Classical Approximation and Order by Quantum Disorder

Abstract

Exchange coupling competition in a Heisenberg Hamiltonian is the basic origin of non coliinear helical configurations; furthermore, on suitable phase boundaries between different helical phases in the parameter space, infinite degeneracy of the ground state appears ( for S ➔∞) corresponding to infinite unequivalent isoenergetic helices of wave vector \(\vec Q\) belonging to lines \({L_{\vec Q}}\) in the reciprocal space we call “degeneration lines”. In the isotropic model the magnon dispersion curve shows “soft lines” vanishing for all wave vectors falling on \({L_{\vec Q}}\) /1/. This causes a low frequency catastrophe that destroys long-range order (LRO), even in 3D, at any finite temperature leading to an unorthodox phase with possible algebraic decay of the correlation function /2/. Planar anisotropy leaves the infinite degeneracy of the ground state but it lifts the soft lines so restoring LRO. For anisotropy small enough we expect a conventional low-temperature ordered phase followed by a “degenerate helix” (DH)-like phase at intermediate temperature before the paramagnetic high-temperature phase is reached. Further exchange interactions remove the infinite degeneracy of the ground state /2/ and lift the soft lines, but a phase reminiscent of the classical scenario is still expected at intermediate temperature.KeywordsDiffuse ScatteringElastic Cross SectionPlanar AnisotropyGround State ConfigurationElastic NeutronThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.