Research Projects

The orbital degrees of freedom (ODF) of correlated 3d transition metal oxides (TMOs) with metal-oxygen building blocks has been a fascinating topic in condensed matter research. The ODF in a solid can be manipulated via the Jahn-Teller (JT) coupling, and strong JT interaction can lead to different magnetic and structural transition temperatures than systems with weak JT distortions. This is due to strong competition between spin interactions and orbital order/disorder phenomena. A recent observation of an unexpected insulator-to-metal (IMT) transition in a Fe-doped R-block Hollandite vanadate, PbV4.25Fe1.75O11, with coexisting magnetic transition and colossal electroresistance response raises several questions. This colossal electroresistance response (CER) has only been reported earlier in the case of manganites and has been connected with the double exchange driven Colossal magnetoresistance (CMR) response. However, this response cannot normally be anticipated for a t2g-active lower transition metal system. The electronic mechanism behind the phase transition, possibly associated with structural anomaly around the same temperature, the distribution of Fe3+/V3+/V4+ ions within the structure, and the role of the Pb lone pair remain open questions. This project proposes studying a series of Fe-doped PbV6O11 and SrV6O11 experimentally and theoretically to shed light on the ODF of t2g-active vanadium, the IMT, CER, and other effects.