Modulation-doping a correlated electron insulator.

Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO ₂ ) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO ₂ -especially without inducing structural changes-has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO ₂ -based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard X-ray photoelectron spectroscopy, and structural characterization, we show that the insulating state can be doped to achieve carrier densities greater than 5 × 10 ²¹  cm ^-3 without inducing any measurable structural changes. We find that the MIT temperature (T _MIT ) continuously decreases with increasing carrier concentration. Remarkably, the insulating state is robust even at doping concentrations as high as ~0.2 e ^- /vanadium. Finally, our work reveals modulation-doping as a viable method for electronic control of phase transitions in correlated electron oxides with the potential for use in future devices based on electric-field controlled phase transitions.
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