Abstract
Bulk NdNiO3 exhibits a metal-to-insulator transition (MIT) as the temperature is lowered that is also seen in tensile strained films. In contrast, films that are under a large compressive strain typically remain metallic at all temperatures. To clarify the microscopic origins of this behavior, we use position averaged convergent beam electron diffraction in scanning transmission electron microscopy to characterize strained NdNiO3 films both above and below the MIT temperature. We show that a symmetry lowering structural change takes place in case of the tensile strained film, which undergoes an MIT, but is absent in the compressively strained film. Using space group symmetry arguments, we show that these results support the bond length disproportionation model of the MIT in the rare-earth nickelates. Furthermore, the results provide insights into the non-Fermi liquid phase that is observed in films for which the MIT is absent.
Highlights
Be decoupled from the structural distortion[26]
Resistivity curves as a function of temperature for the films studied here are shown in Fig. 1, along with corresponding scanning transmission electron microscopy (STEM) images of the film grown on NdGaO3 at room and cryo temperatures, respectively
We have shown that LA-position averaged convergent beam electron diffraction (PACBED) allows for the detection of subtle symmetry changes in ultrathin films due to epitaxial film strain and the metal-to-insulator transitions (MITs), which may be missed in other diffraction methods
Summary
Upton et al dismiss charge order or symmetry changes, and propose Ni 3d hybridization with O 2p, as well as Ni charge redistribution to Nd 5d states as underlying mechanisms[27] These results are in contrast to experimental[22,28,29] and theoretical works[30,31,32] that associate the MIT with charge/bond length order on the Ni sites. The resolution of this debate hinges on the ability to detect very subtle symmetry changes in strained RNiO3 films as they undergo the MIT. The results, combined with symmetry arguments, provide a complete and remarkably simple understanding of the MIT and its suppression
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