Probing phase transition in VO2 with the novel observation of low-frequency collective spin excitation

Abstract

is well known for its first order, reversible, metal-to-insulator transition (MIT) along with a simultaneous structural phase transition (SPT) from a high-temperature metallic rutile tetragonal (R) to an insulating low-temperature monoclinic (M1) phase via two other insulating metastable phases of monoclinic M2 and triclinic T. At the same time, VO 2 gains tremendous attention because of the half-a-century-old controversy over its origin, whether electron-electron correlation or electronphonon coupling trigger the phase transition. in this regard, V 1-x Mg x o 2 samples were grown in stable phases of Vo 2 (M1, M2, and T) by controlled doping of Mg. We have observed a new collective mode in the low-frequency Raman spectra of all three insulating M1, M2 and T phases. We identify this mode with the breather (singlet spin excitation) mode about a spin-Pierls dimerized one dimensional spin ½ Heisenberg chain. the measured frequencies of these collective modes are phenomenologically consistent with the superexchange coupling strength between V spin ½ moments in all three phases. The significant deviation of Stokes to anti-Stokes intensity ratio of this low-frequency Raman mode from the usual thermal factor exp(hʋ/K B T) for phonons, and the orthogonal dependency of the phonon and spinon vibration in the polarized Raman study confirm its origin as spin excitations. The shift in the frequency of spin-wave and simultaneous increase in the transition temperature in the absence of any structural change confirms that SPT does not prompt MIT in VO 2. on the other hand, the presence of spin-wave confirms the perturbation due to spin-Peierls dimerization leading to SPT. Thus, the observation of spin-excitations resulting from 1-D Heisenberg spin-½ chain can finally resolve the years-long debate in Vo 2 and can be extended to oxide-based multiferroics, which are useful for various potential device applications.

Basic aspects of the metal–insulator transition in vanadium dioxide VO2: a critical review

“…Note that such defects are different from the S = 1/2 charged decoupled solitons formed in SP disordered systems such as doped CuGeO 3 [141,142]. Finally remark that low-frequency collective spin excitations have been recently detected in Raman spectra recorded in insulating phases of Mg doped VO 2 where S = 0 V 5+ defects are created [143]. This mode has been identified as the breather (singlet spin excitation) mode of the SP chain interrupted by spin defects.…”

Thin Film Stabilization of Different VO2Polymorphs

“…Because of MIT and the associated huge changes in the structural, electronic and optical properties, VO 2 (M 1 ) and VO 2 (R) are attractive for applications in smart windows, switching devices, memory materials and so on [16,17]. Figure 2 shows the structural arrangement of four different phases of VO 2 [64]. In the VO 2 (R) phase, the vanadium atoms are equally spaced along the rutile c axis (c R ), while in the VO 2 (M 1 ) phase, simultaneous dimerization and tilting in equivalent chains occur, leading to a zigzag pattern.…”

“…Red and blue balls denote vanadium and oxygen atoms, respectively. (d) The arrangement of vanadium chains in the four phases without oxygen atoms (a-d reprinted from Ref [64]…”

Probing the role of thermal vibrational disorder in the SPT of VO$$_2$$ by Raman spectroscopy

“…Raman spectroscopy probes the local structural properties of materials based on the characteristic of their vibrational modes (frequencies, widths and intensities) and it has already been successfully used to study the coupling between charge and lattice degrees of freedom in strongly correlated electron systems 22 , 23 and observation of low-fequency spin excitation 24 . Moreover, the evolution of phonon intensities as a function of temperature provides crucial information on the electronic properties as well as the bonding covalence and the local thermal disorder.…”