Unveiling the electronic transformations in the semi-metallic correlated-electron transitional oxide Mo8O23

V. Nasretdinova, Ya. A. Gerasimenko, J. Mravlje, G. Gatti, P. Sutar, D. Svetin, A. Meden, V. Kabanov, A. Yu. Kuntsevich, M. Grioni & D. Mihailovic in Scientific Reports Volume 9, November 2019, Article number: 15959 (2019), DOI: doi:10.1038/s41598-019-52231-4

Mo8O23 is a low-dimensional chemically robust transition metal oxide coming from a prospective family of functional materials, MoO3−x, ranging from a wide gap insulator (x = 0) to a metal (x = 1). The large number of stoichometric compounds with intermediate x have widely different properties. In Mo8O23, an unusual charge density wave transition has been suggested to occur above room temperature, but its low temperature behavior is particularly enigmatic. We present a comprehensive experimental study of the electronic structure associated with various ordering phenomena in this compound, complemented by theory. Density-functional theory (DFT) calculations reveal a cross-over from a semi-metal with vanishing band overlap to narrow-gap semiconductor behavior with decreasing temperature. A buried Dirac crossing at the zone boundary is confirmed by angle-resolved photoemission spectroscopy (ARPES). Tunneling spectroscopy (STS) reveals a gradual gap opening corresponding to a metal-to-insulator transition at 343 K in resistivity, consistent with CDW formation and DFT results, but with large non-thermal smearing of the spectra implying strong carrier scattering. At low temperatures, the CDW picture is negated by the observation of a metallic Hall contribution, a non-trivial gap structure in STS below ∼170 K and ARPES spectra that together represent evidence for the onset of the correlated state at 70 K and the rapid increase of gap size below ∼30 K. The intricate interplay between electronic correlations and the presence of multiple narrow bands near the Fermi level set the stage for metastability and suggest suitability for memristor applications.

Mo₈O₂₃ je dvodimenzionalen kemijsko odporen oksid prehodnih kovin. Spada v skupino funkcionalnih materialov MoO_3-x, kateri segajo od izolatorjev z veliko režo (x = 0) do kovin (x = 1). Veliko število stehiometričnih spojin z vmesnim x ima širok spekter različnih lastnosti. V spojini Mo₈O₂₃ je bil predpostavljen neobičajen prehod v stanje z valom gostote naboja, ki se pojavi nad sobno temperature. Hkrati pa je njeno nizkotemperaturno obnašanje velika uganka.
V članku predstavljamo obsežno eksperimentalno študijo elektronske strukture, povezane z različnimi fenomeni urejanja v spojini, dopolnjeno s teorijo. Izračuni na podlagi teorije gostotnih funkcionalov (DFT) pri zniževanju temperature razkrivajo preskok iz polkovine z manjkajočim prekrivanjem pasov, do polprevodnikov z ozkimi režami. Dirac-ovo sedlo na meji Brillouinove cone je potrjeno s pomočjo kotno-ločljivostne fotoemisijske spektroskopije (ARPES). Tunelska spektroskopija (STS) s spremembo upornosti razkriva postopno odpiranje vrzeli, ki ustreza prehodu kovina-izolator pri 343 K, kar je konsistentno s tvorbo vala gostote naboja in DFT rezultati, vendar pa z velikim netermičnim zabrisanjem spektrov kaže na močno sipanje nosilca. Pri nizkih temperaturah se CDW slika izniči z opazovanjem prispevka kovinskega Hallovega efekta, netrivialno strukturo reže v STS pod ∼170 K in ARPES spektri, ki skupaj predstavljajo dokaz za začetek spremenjenega stanja pri 70 K in hitro povečanje velikosti vrzeli pod ∼30 K.
Zapletena medsebojna povezava med elektronskimi korelacijami in prisotnostjo več ozkih pasov blizu Fermijeve ravni je postavila metastabilnost materiala na mesto, kjer se je pokazal kot primeren za uporabo v memristorskih aplikacijah.