Metal-insulator transitions (MITs) in correlated electron systems are a complex and actively debated area of materials science. Vanadium dioxide (VO₂), known for its first-order MIT at 340 K, serves as a prime example of this phenomenon. For decades, the relative importance of electron correlation (Mott physics) and structural instability due to vanadium dimerization in driving this transition has been a point of contention. Many studies have attempted to resolve this, with some arguing for the dominance of electron correlation in creating a Mott-Hubbard gap, while others emphasize the role of the structural transition to a monoclinic phase below 340 K, where vanadium dimers form, localizing electrons and leading to insulating behavior. The ability to manipulate the electronic state through external parameters, without significantly altering the intrinsic electron correlations, could provide crucial insights. Applying a high magnetic field offers a unique approach. The Zeeman effect could potentially dissociate the vanadium dimers by aligning electron spins, thus influencing the MIT. If the Mott-Hubbard gap is largely insensitive to spin, as expected, a magnetic-field-induced transition would strongly suggest the structural instability as the dominant factor. This study utilizes ultra-high magnetic fields to investigate the potential for a magnetic-field-induced metallization of W-doped VO₂, where the MIT temperature is reduced to approximately 100 K, making it more accessible to experimental manipulation with current technology. This allows for the investigation of a potential magnetic-field-induced insulator-to-metal transition, offering further insight into the driving force behind VO₂'s MIT.

The literature extensively covers the debate surrounding the driving force behind VO₂'s MIT. Early work by Morin (1959) established the existence of the MIT. Subsequent studies by Zylbersztejn and Mott (1975) and Rice et al. (1994) proposed different models, highlighting the conflict between Mott-Hubbard and Peierls mechanisms. Eyert (2002) provided a detailed band theoretical approach, while Hiroi (2015) focused on the structural instability aspect. The roles of Goodenough's (1960) direct cation-cation interactions and Wentzcovitch et al.'s (1994) calculations also added to the discussion. More recent theoretical and experimental works, like those by Huffman et al. (2017) and Najera et al. (2017), continued to refine the understanding of the interplay between electron correlation and structural changes. Studies on W-doped VO₂ (Shibuya et al., 2010; Lee et al., 2012) explored the influence of doping on the MIT temperature, providing further avenues for investigation into the underlying mechanisms. The practical applications of VO₂ as a switching device (Jia et al., 2018) underscore the importance of fully comprehending its MIT behavior.

The researchers prepared thin films of V₁₋ₓWxO₂ (x = 0, 0.036, 0.06) using pulsed laser deposition on TiO₂ (001) substrates. Detailed characterization of the films, including optical absorption spectra (using a JASCO V570 spectrometer), temperature-dependent electrical resistivity (four-probe method), and magneto-transmission measurements (using an infrared fiber laser and HgCdTe photodiode), was performed. The key experiment involved subjecting the films to ultra-high magnetic fields (up to 540 T) generated using electromagnetic flux compression. The optical transmission at 1.977 µm was measured simultaneously with the magnetic field application. The experiments were conducted at various low temperatures to observe the behavior across different regions of the MIT. Control experiments with varying W-doping levels were performed to understand the influence of doping on the magnetic-field-induced transition. The data analysis involved fitting the optical transmission spectra, accounting for both changes in the energy gap and free-carrier absorption, to better understand the observed changes in transmission upon application of the magnetic field. Careful consideration was given to the potential for sample heating due to eddy currents generated by the pulsed magnetic fields, and analyses were performed to ascertain if temperature stability was maintained throughout the experiments, accounting for thermal relaxation to the substrate. This involved both experimental verification using various pulsed magnetic field waveform methods and theoretical estimations of temperature rise in different scenarios.

The principal finding is the observation of a magnetic-field-induced insulator-metal (MFI-IM) transition in W-doped VO₂ (x=0.06) under an ultra-high magnetic field of 500 T at 14 K. This transition manifested as a significant decrease in optical transmission at 1.977 µm, indicating a change from the insulating to the metallic state. The onset of the transition occurred at approximately 120 T, with a corresponding Zeeman energy close to the MIT temperature at zero field. The transition was broader than expected for a first-order transition, potentially due to the inhomogeneous distribution of W sites. The study found that the threshold magnetic field (Bc) for the MFI-IM transition increased with increasing W-doping concentration (Bc ≈ 200 T for x = 0.036 at 27 K), suggesting that the energy scale governing the stability of the insulating phase plays a significant role. Importantly, no such transition was observed in undoped VO₂ (x = 0) up to 540 T, implying that a significantly higher magnetic field would be required to induce the MFI-IM transition in the undoped material. Analysis of the relative change in the absorption coefficient showed a clear dependence on the W-doping level and the magnetic field strength, supporting the interpretation that the observed effect is directly tied to the disruption of vanadium dimers.

The results strongly support the hypothesis that the structural instability caused by vanadium dimerization is the dominant driving force behind the MIT in VO₂. The magnetic field, via the Zeeman effect, effectively disrupts the vanadium dimers by forcing parallel alignment of the electron spins. This destabilizes the molecular orbital responsible for electron localization in the insulating phase, leading to the observed metallization. The observed dependence of the transition field on the W-doping concentration and MIT temperature further corroborates this explanation. The absence of a transition in undoped VO₂ at fields up to 540 T indicates a significantly higher energy barrier to overcome in the absence of the rutile-like nuclei introduced by W doping. While the Zeeman energy at 500T is significantly smaller than the eV-scale binding energy of a single V-V dimer, many-body interactions likely contribute to the observed effect. The study highlights a phenomenon similar to chemical catastrophe, where chemical bonding is disrupted by a strong magnetic field.

This study demonstrates a magnetic-field-induced insulator-metal transition in W-doped VO₂ at ultra-high magnetic fields. The results provide compelling evidence that the structural instability arising from V-V dimerization is the primary driving force behind the MIT in VO₂. The findings suggest that electron correlation plays a secondary role, modifying the details of the transition but not fundamentally determining its occurrence. Future research should focus on exploring this transition in other strongly correlated insulators with singlet spin states and high MIT temperatures, potentially requiring magnetic fields exceeding 1000 T to observe similar effects.

The main limitation of the study is the use of pulsed magnetic fields, which introduce challenges in maintaining isothermal conditions during measurements. Although efforts were made to mitigate this, subtle temperature fluctuations could influence the results. The inhomogeneous distribution of W sites in the doped samples also contributes to the broadening of the transition, making precise determination of the transition field challenging. Further investigation with more homogeneous samples is warranted. The experiments were performed on thin films, and extrapolating the results to bulk material requires caution.