Session D05: Quantum Materials on Nanoscale I (Oxides)

Cryogenic s-SNOM studies of metal-insulator transitions in correlated oxides

Transition-metal oxides show rich phase diagrams due to a complex interplay between the charge, spin and lattice degrees of freedom. In particular, transitions from metallic to insulating phase can occur as a function of temperature or as result of fine structural tuning via atomic substitution, epitaxial strain or heterostructuring. These transitions often evolve through spatially inhomogeneous phase-separated states, where each phase has a distinct optical character. Therefore, an experimental tool combining optical spectroscopy with nanoscale resolution is highly needed in this case. We use cryogenic scattering-type scanning near-field optical microscopy (s-SNOM) to address these phenomena in epitaxially grown multilayer structures made of pseudocubic perovskite materials. Specifically, I will present two studies of such systems: (i) infrared nanoscopy of 2D electron gas formed at the interface between LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ [1,2] and (ii) temperature-driven metal-insulator transition in thin NdNiO$_{\mathrm{3}}$ films on LaAlO$_{\mathrm{3}}$ substrate [3]. References: 1. W.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, \textbf{10, } 8 (2019). 2. M. Boselli, G. Scheerer, M. Filippone, W. Luo, A. Waelchli, A.B. Kuzmenko, S. Gariglio, T. Giamarchi, and J.-M. Triscone, submitted to Phys. Rev. B (2020); arXiv:2009.07867. 3. W.W. Luo et al., in preparation (2020).   

Symmetry crossover in layered MPS3 complexes (M= Mn, Fe, Ni) via near-field infrared spectroscopy

We employ synchrotron-based near-field infrared spectroscopy to reveal the vibrational properties of bulk, few-, and single-sheet members of the $M$PS$_3$ ($M$ = Mn, Fe, Ni) family of materials and compare our findings with complementary lattice dynamics calculations. MnPS$_3$ and the Fe analog %sport significant commonalities are similar in terms of their symmetry crossovers, from $C2/m$ to $P{\bar{3}}1m$, as the monolayer is approached. These states differ as to the presence of a C$_3$ rotation around the metal center. On the other hand, NiPS$_3$ does not show a symmetry crossover, and the lack of a $B_u$ symmetry mode near 450 cm$^{-1}$ suggests that C$_3$ rotational symmetry is already present - even in the bulk material. We discuss these findings in terms of local symmetry and temperature effects as well as the curious relationship between these symmetry transformations and what takes place under pressure. Time permitting, we will compare the $M$PS$_3$ family of materials with complementary work on CrPS$_4$ - a system in which the P-P dimer is absent.   

Electronic Phenomena in Correlated Oxides at the nanoscale

I report current induced metal-insulator-metal stripes and moir\'{e}-type phase separation in correlated transition metal oxides at the nanoscale. Mesoscopic periodic textures in Ca$_{\mathrm{2}}$RuO$_{\mathrm{4\thinspace }}$single crystals and La$_{\mathrm{0.67}}$Sr$_{\mathrm{0.33}}$MnO$_{\mathrm{3\thinspace }}$thin films were revealed by infrared nano-imaging and optical-microscopy measurements. Those textures have orientations tied uniquely to the crystallographic axes of the sample or miscut steps in the substrate, implying a strong coupling of the electronic transition to lattice degrees of freedom. I will also use this opportunity to report the recent advances in the infrared (IR) and terahertz (THz) near-field nanoscopy and spectroscopy technologies and discuss their future applications for quantum materials. This includes the recent advances in data analysis and the cryogenic, ultrafast, and multi-modal imaging capabilities. These new developments set the stage for future spectroscopic investigations to access the low energy electron, phonon, and spin dynamics in complex quantum materials at the nanoscale.   

Partially-metal--coated Tips for Near-field Nanospectroscopy.

Here, we test the possibility of a generic design scheme for wavelength-selective tip enhancement via finite-element numerical modeling. We employ a Si-based tip with various gold coating lengths on the top, yielding a tunable near-field field strength at the tip apex. Calculations show a wavelength-dependent enhancement factor of the metal-coated tip due to the geometrical antenna resonances, which can be precisely tuned throughout a broad spectral range from visible to terahertz frequencies by adjusting the length of the metal coating. By changing the coating pattern into a chiral helical structure on an achiral tip, we also demonstrate the importance of coating-length effect in designing high-performance enantiomeric near-field scanning. Our methods and findings offer interesting perspectives for developing near-field optical probes, pushing the detection and resolution limits of tip-enhanced near-field detections and nanospectroscopies.