Metal-Insulator Transition and Novel Ground States in Epitaxially Strained SmBaMn₂O₆ Thin Films

For the realization of the next generation of fast, energy-efficient nanoelectronics, there is a great need for new materials whose electrical and optical conductivities can be sensitively tuned between high (on) and low (off) states by altering a thermodynamic control parameter such as strain or temperature. Unfortunately, most materials are either metallic or insulating and their conductivities cannot be changed substantially. Materials exhibiting a metal-insulator transition (MIT) above room temperature are quite rare, limiting their applicability in devices.

One noteworthy example of such a material is the A-site layer-ordered double perovskite SmBaMn₂O₆. While its synthesis in bulk form was reported by Yamada et al. [1], the successful growth of SmBaMn₂O₆ thin films remained elusive for over a decade. Here, we demonstrate the growth of untwinned epitaxial thin films of phase-pure SmBaMn₂O₆ on various single-crystalline oxide substrates using molecular-beam epitaxy (MBE), exploring a wide range of tensile to compressive biaxial strains. The latter has been predicted to host a different ground state based on first-principles calculations by Nowadnick et al. [2].

To stabilize the A-site layer-ordered double perovskite phase, we employ a two-step approach as originally described by Millange et al. for LaBaMn₂O₆ bulk crystals [3]. The key elements are, first, a high-temperature synthesis step of a brownmillerite-like, oxygen-deficient precursor, and second, a topotactic oxidation at low temperature. Notably, the former requires temperatures higher than 1100 °C that are unattainable in conventional oxide MBE systems. In this study, we utilize a recently installed, high-power CO₂-laser-based substrate heater at the PARADIM user facility at Cornell University, which allows growth temperatures up to 2000 °C.

Ongoing efforts entail the comprehensive investigation of the structural and spectroscopic properties of epitaxial SmBaMn₂O₆ thin films as a function of temperature and strain. To this end, we are employing an ensemble of X-ray, optical, and electrical transport techniques, alongside scanning transmission electron microscopy and electron energy-loss spectroscopy. Our aim is to elucidate potential hidden ground states and coupled structural, magnetic, and electronic phase transitions in this MIT compound.

References:
[1] Yamada, Maeda, Arima, “Successive Electronic Transitions and Anisotropic Properties in a Double-Perovskite SmBaMn₂O₆ Single Crystal”, J. Phys. Soc. Jpn. 2012, 81, 113711
[2] Nowadnick, He, Fennie, “Coupled structural distortions, domains, and control of phase competition in polar SmBaMn₂O₆”, Phys. Rev. B 2019, 100, 195129
[3] Millange, Caignaert, Domengès, Raveau, Suard, “Order-Disorder Phenomena in New LaBaMn₂O_6-x CMR Perovskites. Crystal and Magnetic Structure”, Chem. Mater 1998, 10, 1974-1983