Understanding and control of the metallic state in epitaxial NdNiO3

This thesis reports a systematic analysis on epitaxial nickelate thin films. As oxides with strong electron-lattice coupling, the electrical properties of nickelate thin films can be tuned by many factors that are often inter-twinned. By separating the contributions of these different factors, we unveil that in pristine nickelate films, the resistivity follows a linear temperature dependence, suggesting Planckian dissipation in a wide temperature range (100- 500 K) with no signatures of bad metal behavior, a characteristic that has been reported in some nickelates. By comparing with other electron correlated metals, we show that the nickelates belong to a class of systems with strength of electron interactions intermediate between those normal and strongly correlated metals, explaining the difficulties in their classification. We also show that, both the increase of strain and disorder exert significant effects on their electrical performance, including the metal-insulator transition and resistivity-temperature scaling. These tunable properties impact greatly their applications. Therefore, the study of the underlying physics of these materials should be accompanied by a detailed characterization of the microstructure

From hidden metal-insulator transition to Planckian-like dissipation by tuning the oxygen content in a nickelate

Abstract Heavily oxygen-deficient NdNiO3 (NNO) films, which are insulating due to electron localization, contain pristine regions that undergo a hidden metal-insulator transition. Increasing oxygen content increases the connectivity of the metallic regions and the metal-insulator transition is first revealed, upon reaching the percolation threshold, by the presence of hysteresis. Only upon further oxygenation is the global metallic state (with a change in the resistivity slope) eventually achieved. It is shown that sufficient oxygenation leads to linear temperature dependence of resistivity in the metallic state, with a scattering rate directly proportional to temperature. Despite the known difficulties to establish the proportionality constant, the experiments are consistent with a relationship 1/τ = α k B T/ℏ, with α not far from unity. These results could provide experimental support for recent theoretical predictions of disorder in a two-fluid model as a possible origin of Planckian dissipation

Tunable resistivity exponents in the metallic phase of epitaxial nickelates

We report a detailed analysis of the electrical resistivity exponent of thin films of NdNiO3 as a function of epitaxial strain. Thin films under low strain conditions show a linear dependence of the resistivity versus temperature, consistent with a classical Fermi gas ruled by electron-phonon interactions. In addition, the apparent temperature exponent, n, can be tuned with the epitaxial strain between n = 1 and n = 3. We discuss the critical role played by quenched random disorder in the value of n. Our work shows that the assignment of Fermi/Non-Fermi liquid behaviour based on experimentally obtained resistivity exponents requires an in-depth analysis of the degree of disorder in the material

Guidelines for the stabilization of a polar rhombohedral phase in epitaxial Hf0.5Zr0.5O2 thin films

The unconventional Si-compatible ferroelectricity in hafnia-based systems, which becomes robust only at nanoscopic sizes, has attracted a lot of interest. While a metastable polar orthorhombic (o-) phase (Pca21) is widely regarded as the responsible phase for ferroelectricity, a higher energy polar rhombohedral (r-) phase is recently reported on epitaxial HfZrO4 (HZO) films grown on (001) SrTiO3 (R3m or R3), (0001) GaN (R3), and Si (111). Armed with results on these systems, here we report a systematic study leading towards identifying comprehensive global trends for stabilizing r-phase polymorphs in epitaxially grown HZO thin films (6 nm) on various substrates (perovskites, hexagonal and Si).Comment: Final version as it appears in the 50th Anniversary issue of Ferroelectric

Nanostructure and thermal power of highly-textured and single-crystal-like Bi2Te3 thin films

Bi2Te3-based alloys are known to have outstanding thermoelectric properties. Although structure-property relations have been studied, still, detailed analysis of the atomic and nano-scale structure of Bi2Te3 thin film in relation to their thermoelectric properties remains poorly explored. Herein, highly-textured (HT) and single-crystal-like (SCL) Bi2Te3 films have been grown using pulsed laser deposition (PLD) on Si wafer covered with (native or thermal) SiOx and mica substrates. All films are highly textured with c-axis out-of-plane, but the in-plane orientation is random for the films grown on oxide and single-crystal-like for the ones grown on mica. The power factor of the film on thermal oxide is about four times higher (56.8 mu W.cm(-1).K-2) than that of the film on mica (12.8 mu W.cm(-1).K-2), which is comparable to the one of the polycrystalline ingot at room temperature (RT). Reduced electron scattering in the textured thin films results in high electrical conductivity, where the SCL film shows the highest conductivity. However, its Seebeck coefficient shows a low value. The measured properties are correlated with the atomic structure details unveiled by scanning transmission electron microscopy. For instance, the high concentration of stacking defects observed in the HT film is considered responsible for the increase of Seebeck coefficient compared to the SCL film. This study demonstrates the influence of nanoscale structural effects on thermoelectric properties, which sheds light on tailoring thermoelectric thin films towards high performance

Phenomenological classification of metals based on resistivity

Efforts to understand metallic behavior have led to important concepts such as those of strange metals, bad metals, or Planckian metals. However, a unified description of metallic resistivity is still missing. An empirical analysis of a large variety of metals shows that the parallel resistor formalism used in the cuprates, which includes T-linear and T-quadratic dependence of the electron scattering rates, can be used to provide a phenomenological description of the electrical resistivity in all metals. Here, we show that the different metallic classes are then determined by the relative magnitude of these two components and the magnitude of the extrapolated residual resistivity. These two parameters allow us to categorize a few systems that are notoriously hard to ascribe to one of the currently accepted metallic classes

Phenomenological classification of metals based on resistivity

Efforts to understand metallic behavior have led to important concepts such as those of strange metals, bad metals, or Planckian metals. However, a unified description of metallic resistivity is still missing. An empirical analysis of a large variety of metals shows that the parallel resistor formalism used in the cuprates, which includes T-linear and T-quadratic dependence of the electron scattering rates, can be used to provide a phenomenological description of the electrical resistivity in all metals, where these two contributions are shown to correspond to the first two terms of a Taylor expansion of the resistivity, detached from their physics origin, and thus valid for any metal. Here, we show that the different metallic classes are then determined by the relative magnitude of these two components and the magnitude of the extrapolated residual resistivity. These two parameters allow us to categorize a few systems that are notoriously hard to ascribe to one of the currently accepted metallic classes. This approach also reveals that the T-linear term has a common origin in all cases, strengthening the arguments that propose the universal character of the Planckian dissipation bound.Q.G. acknowledges financial support from a China Scholarship Council (CSC) grant and Q.G. and B.N. acknowledge financial support from CogniGron and the Ubbo Emmius Funds (University of Groningen).Peer reviewe