Surface Electronic States Induced High Terahertz Conductivity of CoO Microhollow Structure.

Herein, we report the observation of unusual electronic and magnetic phases in traditional antiferromagnetic CoO micromaterials and modulation of their properties on a temperature scale. In particular, we demonstrate a comparative low-energy carrier dynamics of CoO microflower and microhollow flower (MHF) structures of same average size of 2 μm to unravel the ground-state information induced by surface electronics across the insulator-semiconductor transition using terahertz (THz) time domain spectroscopy. Interestingly, the THz optical constants of these structures are found to exhibit remarkably distinct features both as a function of frequency and temperature. Detailed study reveals that the partial metallization through large two-dimensional surface electronic states of MHF structure enables to achieve significantly higher carrier dynamics in contrast to its wide-band-gap solid counterparts and the magnetic measurements reconfirm the presence of these surface states by indicating ferromagnetism in CoO MHF structures. Moreover, the simultaneous existence of insulator-semiconductor and antiferromagnetic-paramagnetic transitions near the Néel temperature points out the significant role of magnetically active Co ions at the tetrahedral site of CoO normal spinel structure in determining the conduction dynamics instead of 3d band related to Co ions at octahedral site. Finally, we demonstrate that the continuous modulation of temperature-controlled charge transport coupled with intrinsic phase transition in CoO microstructures has the potential to design efficient analog-like THz modulator, filter, and sensor. We believe that these outcomes can stimulate new opportunities toward next-generation caloritronics-based ultrafast energy-efficient transition-metal oxide electronics having both economic and environmental significance.