The role of 1-D finite size Heisenberg chains in increasing the metal to insulator transition temperature in hole rich VO₂

Abstract

VO₂ samples are grown with different oxygen concentrations leading to different monoclinic, M1, and triclinic, T, insulating phases which undergo a first order metal to insulator transition (MIT) followed by a structural phase transition (SPT) to the rutile tetragonal phase. The metal insulator transition temperature (T_c) was found to be increased with increasing native defects. Vanadium vacancy (V_V) is envisaged to create local strains in the lattice which prevents twisting of the V-V dimers promoting metastable monoclinic, M2 and T phases at intermediate temperatures. It is argued that MIT is driven by strong electronic correlation. The low temperature insulating phase can be considered as a collection of one-dimensional (1-D) half-filled bands, which undergo a Mott transition to 1-D infinitely long Heisenberg spin ½ chains leading to structural distortion due to spin-phonon coupling. The presence of V_V creates localized holes (d⁰) in the nearest neighbor, thereby fragmenting the spin ½ chains at the nanoscale, which in turn increases the T_c value more than that of an infinitely long one. The T_c value scales inversely with the average size of the fragmented Heisenberg spin ½ chains following a critical exponent of ⅔, which is exactly the same as predicted theoretically for the Heisenberg spin ½ chain at the nanoscale undergoing SPT (spin-Peierls transition). Thus, the observation of MIT and SPT at the same time in VO₂ can be explained from our phenomenological model of reduced 1-D Heisenberg spin ½ chains. The reported increase (decrease) in the T_c value of VO₂ by doping with metals having valency less (more) than four can also be understood easily with our unified model, for the first time, considering finite size scaling of Heisenberg chains.