Characteristic length scales of the electrically induced insulator-to-metal transition

The oxide growth was supported by the European Research Council under the European Union's Seventh Framework Program (No. FP7/2007-2013)/ERC Grant Agreement No. 319286. T.L. acknowledges support by the Cusanuswerk, Bischöfliche Studienförderung. J.d.V. acknowledges support from the Spanish Ministry of Science through a Ramón y Cajal Fellowship (No. RYC2021-030952-I) and from Asturias FICYT under Grant No. AYUD/2021/51185 with the support of FEDER funds (...)Luibrand, T., Bercher, A., Rocco, R., Tahouni-Bonab, F., Varbaro, L., Rischau, C.W., Domínguez, C., Zhou, Y., Luo, W., Bag, S., Fratino, L., Kleiner, R., Gariglio, S., Koelle, D., Triscone, J.-M., Rozenberg, M.J., Kuzmenko, A.B., Guénon, S., Del Valle, J

Combined optical imaging and electric transport studies on resistive switching and light-induced phase transitions in strongly correlated insulators

The emerging field of neuromorphic computing strongly demands for memristive devices in order to emulate basic functionalities of the human brain. In recent years, there is growing interest in devices based on strongly correlated insulator thin films, which undergo thermally driven metal-to-insulator transitions (MIT) and insulator-to-metal transitions (IMT). These materials show memristive behavior due to resistive switching, i.e. the change of electical resistance induced by an external stimulus. In the three publications of this cumulative PhD thesis, we adress distinct research questions in the scope of the fundamental mechanism behind resisitve switching, spatial switching dynamics, and thermodynamical aspects of laser light-induced phase transitions. In publication 1, we investigate the characteristic length scales of conductive filaments in planar two-terminal thin film devices of the perovskite nickelates NdNiO3 and SmNiO3. For potential applications, it is crucial to have knowledge about the spatial filament dynamics, e.g. for miniaturization, but this fundamental question has been mostly unnoticed so far. In a study with complementary imaging techniques, namely optical widefield and scattering-type scanning near-field optical microscopy and with theoretical support by numerical simulations, we find key parameters, that determine the size of the filament. We identify the resistivity contrast during MIT/IMT to be a key parameter for the filament’s size. Larger resistive drops and sharp switching leads to narrower filaments with higher current density. Besides of that, bias current, temperature and thermal conductivity of the substrate play important roles, too. In publication 2, we present a study on resistive switching in the prototypical Mott insulator V2O3, touching the fundamental question of the origin of resistive switching. By use of optical widefield microscopy, we visualize the thermally driven strain-induced phase separation during MIT/IMT as well as volatile resistive switching in a planar two-terminal thin film device. The current-voltage-characteristics and the shape of the filaments are well reproduced by a numerical model, unveiling clear signs of an electrothermal breakdown, and correlating local heterogeneities like strain directly to the switching dynamics. In publication 3, we study laser irradiation of a V2O3 thin film and gain thermodynamical insights into the first-order nature of the Mott transition. We irradiate the sample by scanning a focused laser beam across the thin film surface and acquire optical widefield micrographs in situ. Laser irradiation modifies the phase configuration depending on the thermal history: during IMT, the laser induces predominantly metallic domains, whereas during MIT, it predominantly induces insulating domains. Most likely, the laser beam drives metastable states into stable ones in a non-thermal way. A numerical model supports this hypothesis

Mechanism of irreversible inhibition of <i>Mycobacterium</i> <i>tuberculosis </i>shikimate kinase by ilimaquinone

Ilimaquinone (IQ), a marine sponge metabolite, has been considered as a potential therapeutic agent for various diseases due to its broad range of biological activities. We show that IQ irreversibly inactivates Mycobacterium tuberculosis shikimate kinase (MtSK) through covalent modification of the protein. Inactivation occurred with an apparent second-order rate constant of about 60 M −1 s −1. Following reaction with IQ, LC-MS analyses of intact MtSK revealed covalent modification of MtSK by IQ, with the concomitant loss of a methoxy group, suggesting a Michael-addition mechanism. Evaluation of tryptic fragments of IQ-derivatized MtSK by MS/MS demonstrated that Ser and Thr residues were most frequently modified with lesser involvement of Lys and Tyr. In or near the MtSK active site, three residues of the P-loop (K15, S16, and T17) as well as S77, T111, and S44 showed evidence of IQ-dependent derivatization. Accordingly, inclusion of ATP in IQ reactions with MtSK partially protected the enzyme from inactivation and limited IQ-based derivatization of K15 and S16. Additionally, molecular docking models for MtSK-IQ were generated for IQ-derivatized S77 and T111. In the latter, ATP was observed to sterically clash with the IQ moiety. Out of three other enzymes evaluated, lactate dehydrogenase was derivatized and inactivated by IQ, but pyruvate kinase and catalase-peroxidase (KatG) were unaffected. Together, these data suggest that IQ is promiscuous (though not entirely indiscriminant) in its reactivity. As such, the potential of IQ as a lead in the development of antitubercular agents directed against MtSK or other targets is questionable. </p