Direct measurement of the spin gap in a quasi-one dimensional clinopyroxene: NaTiSi2O6

Abstract

True inorganic Spin-Peierls materials are extremely rare, but NaTiSi2O6 was at one time considered an ideal candidate due to it having well separated chains of edge-sharing TiO6 octahedra. At low temperatures, this material undergoes a phase transition from C2/c to P1 symmetry, where Ti3+-Ti3+ dimers begin to form within the chains. However, it was quickly realized with magnetic susceptibility that simple spin fluctuations do not progress to the point of enabling such a transition. Since then, considerable experimental and theoretical endeavours have been taken to find the true ground state of this system and explain how it manifests. Here, we employ the use of x-ray diffraction, neutron spectroscopy, and magnetic susceptibility to directly and simultaneously measure the symmetry loss, spin singlet-triplet gap, and phonon modes. A gap of 53(3) meV was observed, fit to the magnetic susceptibility, and compared to previous theoretical models to unambiguously assign NaTiSi2O6 as having an orbital-assisted Peierls ground state.