Research Projects

The overarching objective of this work is to realize and study phase transitions at interfaces in transition metal oxide based heterostructures. 3d transition metal oxides host a rich variety of phases and phase transitions among the various phases owing to electron-electron correlations. However, these transitions are realizable as a function of temperature or doping or pressure in bulk materials. While phase transitions in bulk materials are exciting from a fundamental standpoint, they are not amenable for easy control and are also not utilizable for applications. Furthermore, such bulk phase transitions, in general, involve volumetric changes to the lattice, resulting in transition-induced fatigue and poor endurance in devices. In this work, we propose to design, synthesize, and study phase transitions in nanometer scale thin film heterostructures of transition metal compounds. We propose to induce these phase transitions in modulation-doped heterostructures. In modulation-doping, charge transfer to the active layer is enabled by electronic chemical potential mismatch across the hetero-interface. While modulation-doping is well established for band semiconductors such as GaAs and Si, modulation-doping in correlated electron insulators is not very well established. For example, we are one of the very few groups (https://arxiv.org/abs/2301.02798) to show this in a transition metal compound (apart from Prof. Stemmer’s group at UCSB and Prof. Pryds group at TU Denmark) In a recent work, we established that modulation-doping correlated electron materials can alter phase transition temperature in vanadium dioxide (VO2) by about 30% (from 300 K to 230 K). We showed that this change in the phase transition temperature was achieved without any changes to the lattice parameter in VO2 thin film heterostructures. This purely electronically controlled phase transition has not been reported before. In this work, we intend to build on the proof-of-concept work performed with VO2 and extend this to realizing magnetic and superconducting phase transitions at modulation-doped heterostructure interfaces. Our experiments closely resemble filling-control in a correlated electron insulator, where small carrier density changes from half-filling are expected to lead to electronically-induced phase transitions of the correlated insulating state – which is the ground state of most undoped transition metal oxides. The heterostructure design proposed in this work uses amorphous spacer and dopant layers that can be used for modulation-doping a broad range of materials, without any restriction on lattice matching and time-consuming optimization of epitaxial deposition procedures. Therefore, our work opens a host of possibilities for new device architectures utilizing electronically induced phase transitions in correlated electron materials.