Positive and Negative Pressure Regimes in Anisotropically Strained V₂O₃ Films

Abstract

The metal-insulator phase transitions in V 2 O 3 are considered archetypal manifestations of Mott physics. Despite decades of research, the effects of doping, pressure, and anisotropic strains on the transitions are still debated. To understand how these parameters control the transitions, anisotropically strained pure V 2 O 3 films are explored with nearly the same contraction along the c-axis, but different degrees of ab-plane expansion. With small ab-plane expansion, the films behave similar to bulk V 2 O 3 under hydrostatic pressure. However, with large ab-plane expansion, the films are driven into the "negative pressure" regime, similar to that of Cr-doped V 2 O 3 , exhibiting clear coexistence of paramagnetic insulator and paramagnetic metal phases between 180-500 K. This shows that c-axis contraction alone, or an increase in c/a ratio is insufficient for inducing "negative pressure" effects. Actually, c-axis contraction alone destabilizes the two insulating phases of V 2 O 3 , whereas a-axis expansion tends to stabilize them. The effects of strain are modeled using density functional theory providing good agreement with experimental results. The findings show that chemical pressure alone cannot account for the phase diagram of (V 1−x Cr x ) 2 O 3 . This work enables to manipulate a Mott transition above room temperature, thereby expanding the opportunities for applications of V 2 O 3 in novel electronics.

Mechanical Bending Strain-Actuated Modulation of Metal–Insulator Transition Temperature in Flexible Epitaxial V2O3 Films

“…On the other hand, for the sample with c / a = 2.82 bent along CO [210], the increase in the T MIT is only ∼5 K under ε b = +0.25%, while the decrease is as high as ∼24 K under ε b = −0.25%. Previous reports indicated that V 2 O 3 exhibits an intricate lattice strain dependence of MIT characteristics due to the strong interplay among multiple DOF. ,, Herein, the substantial and anisotropic dependence of Δ T MIT on ε b further highlights the great potential for application and extensive fundamental physics in the V 2 O 3 material system.…”

“…48 Previous investigations have shown that factors such as doping, defects, offstoichiometry, hydrostatic pressure, and strain are influential in determining the MIT behavior of V 2 O 3 films. 11,14,18,19,49 Although there are numerous factors to consider, the c/a ratio is commonly utilized as an indicator that is closely linked to MIT behavior. 12,18,45 Considering that the use of different growth conditions inevitably leads to distortions, defects, and off-stoichiometry, the various ρ vs T curves in our samples grown under different conditions could be due to the potential existence of nonstoichiometric V 2−y O 3 phases.…”

“…On one hand, the underlying mechanisms behind the unusual MIT, whose understanding is still debated, have aroused the heightened interests of physicists. ,,− The interrelationships among the MIT, SPT, and MPT in V 2 O 3 have been recently investigated by decoupling the multiple transitions from each other via doping, pressure, and epitaxial strain. ,,− Previous experimental results indicated that hydrostatic pressure could suppress the formation of the insulating phase and thereby decrease the T MIT in V 2 O 3 . Additionally, Ti (Cr) doping has been suggested to increase (decrease) the bandwidth of V 2 O 3 , ultimately stabilizing the metallic (insulating) phase. , Furthermore, V 2 O 3 epilayers grown on the substrates with varying lattice constants and orientations exhibit anisotropic strain, leading to distinct ground electrical states. , From these previous investigations, it is concluded that a biaxial compressive (tensile) strain within the c plane would drive the V 2 O 3 film toward the metallic (insulating) phase. ,,− Hence, the MIT characteristics including T MIT can be effectively tuned by a biaxial strain. ,,− On the other hand, the coincidence of the MIT, SPT, and MPT offers opportunities for the realization of various innovative multifunctional devices. ,, A giant nonvolatile resistive switching was previously achieved via applying strain on V 2 O 3 film using a ferroelectric substrate . In another previous report, a transition from metal to insulator was observed in a V 2 O 3 thin film at room temperature by introducing a local stress utilizing an atomic force microscopy …”

“…14 Additionally, Ti (Cr) doping has been suggested to increase (decrease) the bandwidth of V 2 O 3 , ultimately stabilizing the metallic (insulating) phase. 11,19 orientations exhibit anisotropic strain, leading to distinct ground electrical states. 18,19 From these previous investigations, it is concluded that a biaxial compressive (tensile) strain within the c plane would drive the V 2 O 3 film toward the metallic (insulating) phase.…”

“…11,19 orientations exhibit anisotropic strain, leading to distinct ground electrical states. 18,19 From these previous investigations, it is concluded that a biaxial compressive (tensile) strain within the c plane would drive the V 2 O 3 film toward the metallic (insulating) phase. 3,4,11−19 Hence, the MIT characteristics including T MIT can be effectively tuned by a biaxial strain.…”