Complex transition-metal oxides exhibit a remarkable variety of ordering phenomena that can be useful for applications, such as (multi-)ferroicity, superconductivity and metal-insulator transitions. Typically, near the ordering temperature, different phases coexist at the nanoscale. Furthermore, some of the important effects appear at the nanometer-thick interfaces between the different oxide layers [1,2]. Infrared spectroscopy probes charge, spin and lattice dynamics and therefore is extremely useful to elucidate the physical origin of various ordered phases. However, conventional infrared measurements are diffraction limited and do not provide a nanoscale resolution. On the other hand, the technique of scanning near-field optical microscopy (SNOM), based on the near-field interaction between an atomic-force microscope (AFM) tip and the sample [3], overcomes the diffraction limit, while keeping all the advantages of conventional spectroscopy. Recently, it has been extended to cryogenic temperatures finally allowing us to optically examine all the mentioned phenomena with nanoscale resolution.

I will start my talk by introducing the principle of cryo-SNOM. Then I will present our infrared nanoscopy studies of two representative oxide systems. First, we perform SNOM mapping of a conducting two-dimensional electron gas (2DEG) formed at the interface between two insulating materials SrTiO3 and LaAlO3 [1,4]. We find that the SNOM signal is highly sensitive to the density and the mobility of the charge carriers in the 2DEG, which is explained by the formation of coupled plasmon-phonon polariton modes at the interface. We also demonstrate that SNOM can be used to map spatially inhomogeneous structures, such as AFM-written conducting wires and domain walls in SrTiO3. Second, we focus on a metal-insulator transition in ultrathin thin NdNiO3 films [5]. Our measurements reveal a complex spatial structure of the phase separation close to the transition. Interestingly, the surface morphology and local strain have a strong influence on the transition temperature.

References:
[1] A. Ohtomo and H.Y. Hwang, Nature, 427, 423 (2004).
[2] J. Hoppler, J. Stahn, Ch. Niedermayer, V. K. Malik, H. Bouyanfif, A. J. Drew, M. Rössle, A. Buzdin, G. Cristiani, H.-U. Habermeier, B. Keimer and C. Bernhard, Nature Materuals 8, 315 (2009).
[3] F. Keilmann and R. Hillenbrand, Phi. Trans. R. Soc. Lond. A 362, 787 (2004).
[4] W. Luo, M. Boselli, J.-M. Poumirol, I. Ardizzone, J. Teyssier, D. van der Marel, S. Gariglio, J.-M. Triscone and A.B. Kuzmenko, Nature Communications 10, 2774 (2019).
[5] W. Luo et al. to be published.