Role Of V-V Dimers On Structural, Electronic, Magnetic, And Vibrational Properties Of Vo2 By First-Principles Simulations And Raman Spectroscopic Analysis

We investigate the vibrational properties of VO2, particularly the low-temperature M1 phase by first-principles calculations using the density functional theory as well as Raman spectroscopy. In the theoretical aspect, the phonon calculation requires structure optimization and force calculations on a large supercell to obtain an accurate result, and strong correlation effects in VO2 make this task very challenging. We perform the structural optimization using SCAN meta-GGA functional and obtain the optimized crystal structures with the correct energy hierarchy as well as correct electronic properties for the rutile and M-1 phases of VO2 including both the nonmagnetic (NM) and antiferromagnetic (AFM) spin ordering. We show both the NM-M-1 and AFM-M-1 are insulators, whereas NM-R and AFM-R are metals. However, FM-R and FM-M-1 are found be same as energetically favorable and becomes half-metal. Based on the harmonic approximation around the optimized structures obtained from the NM calculations at zero temperature, we reproduce the phonon softening in the rutile phase as well as the phonon stiffening in the M-1 phase. Our phonon calculations reveal that both the NM-M-1 phase and AFM-M-1 are dynamically stable, whereas FM-M-1 is found to be dynamically unstable. In addition, we perform Raman experiment on VO2 thin films which stimulate enormous interests in the field of thin-film engineering for transition metal oxides. We demonstrate from the comparison between theoretical calculations and Raman measurements that the subtle change of the V-V dimer due to the strain effect can be reliably detected by the Raman spectroscopy, which could be a new framework to determine the subtle change of crystal structure for transition metal oxide thin films.