Title: Role of 1-D finite size Heisenberg chain in increasing 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 rutile tetragonal phase. The metal insulator transition
temperature (Tc) was found to be increased with increasing native defects.
Vanadium vacancy (VV) 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 band, which undergoes Mott
transition to 1-D infinitely long Heisenberg spin 1/2 chains leading to
structural distortion due to spin-phonon coupling. Presence of VV creates
localized holes (d0) in the nearest neighbor, thereby fragmenting the spin 1/2
chains at nanoscale, which in turn increase the Tc value more than that of an
infinitely long one. The Tc value scales inversely with the average size of
fragmented Heisenberg spin 1/2 chains following a critical exponent of 2/3,
which is exactly the same predicted theoretically for Heisenberg spin 1/2 chain
at 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 1/2 chains. The reported increase
(decrease) in Tc value of VO2 by doping with metal 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.