Search

We report on the successful fabrication of (111) oriented thin films of pyrochlore iridate Y2Ir2O7 by means of solid phase epitaxy. Essentially, we developed an entirely in situ annealing protocol, which is superior to the conventional ex-situ routine that requires multi-hour annealing to stabilize the proper pyrochlore structure. The morphological quality of the films has been confirmed by x-ray diffraction, reflectivity, and reciprocal space mapping, demonstrating their high crystallinity with a pure pyrochlore phase and an expected epitaxial relation to the substrate. Strikingly, below the magnetic phase transition, the temperature dependence of resistivity shows a power-law behavior with the power exponent of ∼3/2, characteristic of a Weyl semimetal in the presence of impurities and electron–electron correlations. Below 10 K, the magneto-transport measurements reveal the emergence of a non-saturated negative magnetoresistance up to 9 T with a small "valley" around zero field. These findings are further interpreted in relation to the weak anti-localization effect and the non-collinear antiferromagnetic ordering on the Ir sublattice. [ABSTRACT FROM AUTHOR]

High-quality (001)-oriented (pseudo-cubic notation) ferromagnetic YTiO3 thin films were epitaxially synthesized in a layer-by-layer way by pulsed laser deposition. Structural, magnetic, and electronic properties were characterized by reflection-high-energy-electron-diffraction, X-ray diffraction, vibrating sample magnetometry, and element-resolved resonant soft X-ray absorption spectroscopy. To reveal ferromagnetism of the constituent titanium ions, X-ray magnetic circular dichroism spectroscopy was carried out using four detection modes probing complementary spatial scale, which overcomes a challenge of probing ferromagnetic titanium with pure Ti3+(3d¹). Our work provides a pathway to distinguish between the roles of titanium and A-site magnetic rareearth cations in determining the magnetism in rare-earth titanates thin films and heterostructures. [ABSTRACT FROM AUTHOR]

We address the fundamental issue of growth of perovskite ultra-thin films under the condition of a strong polar mismatch at the hetero-interface exemplified by the growth of a correlated metal LaNiO3 on the band insulator SrTiO3 along the pseudo cubic [111] direction. While in general the metallic LaNiO3 film can effectively screen this polarity mismatch, we establish that in the ultra-thin limit, films are insulating in nature and require additional chemical and structural reconstruction to compensate for such mismatch. A combination of in-situ reflection high-energy electron diffraction recorded during the growth, X-ray diffraction, and synchrotron based resonant X-ray spectroscopy reveal the formation of a chemical phase La2Ni2O5 (Ni2+ ) for a few unit-cell thick films. First-principles layer-resolved calculations of the potential energy across the nominal LaNiO3/SrTiO3 interface confirm that the oxygen vacancies can efficiently reduce the electric field at the interface. [ABSTRACT FROM AUTHOR]

Ultra-thin films of the electron doped manganite La0.8Ce0.2MnO3 were grown in a layer-by-layer growth mode on SrTiO3 (001) substrates by pulsed laser interval deposition. High structural quality and surface morphology were confirmed by a combination of synchrotron based x-ray diffraction and atomic force microscopy. Resonant X-ray absorption spectroscopy measurements confirm the presence of Ce4þ and Mn2þ ions. In addition, the electron doping signature was corroborated by Hall effect measurements. All grown films show a ferromagnetic ground state as revealed by both dc magnetization and x-ray magnetic circular dichroism measurements and remain insulating contrary to earlier reports of a metal-insulator transition. Our results hint at the possibility of electron-hole asymmetry in the colossal magnetoresistive manganite phase diagram akin to the high-Tc cuprates. [ABSTRACT FROM AUTHOR]

We have synthesized epitaxial NdNiO3 ultrathin films in a layer-by-layer growth mode under tensile and compressive strain on SrTiO3 (001) and LaAlO3 (001), respectively. A combination of x-ray diffraction, temperature dependent resistivity, and soft x-ray absorption spectroscopy has been applied to elucidate electronic and structural properties of the samples. In contrast to the bulk NdNiO3, the metal-insulator transition under compressive strain is found to be completely quenched, while the transition remains under the tensile strain albeit modified from the bulk behavior. [ABSTRACT FROM AUTHOR]

We have synthesized epitaxial NdNiO3 ultrathin films in a layer-by-layer growth mode under tensile and compressive strain on SrTiO3 (001) and LaAlO3 (001), respectively. A combination of x-ray diffraction, temperature dependent resistivity, and soft x-ray absorption spectroscopy has been applied to elucidate electronic and structural properties of the samples. In contrast to the bulk NdNiO3, the metal-insulator transition under compressive strain is found to be completely quenched, while the transition remains under the tensile strain albeit modified from the bulk behavior. [ABSTRACT FROM AUTHOR]

An optical study of NdNiO3 ultrathin films with insulating and metallic ground states reveals new aspects of the insulator-to-metal transition that point to Mott physics as the driving force. In contrast with the behavior of charge-ordered systems, we find that the emergence of the Drude resonance across the transition is linked to a spectral weight transfer over an energy range of the order of the Coulomb repulsion U, as the energy gap is filled with states instead of closing continuously. [ABSTRACT FROM AUTHOR]

We report on the epitaxial stabilizationeffect of strain on thegrowth of ultrathin heterostructures of a correlated metal LaNiO3(10 unit cells, ∼3.84 nm; u.c. hereafter) grown ona series of perovskite oxide substrates with both tensile and compressivestrain. An unusual polynomial dependence of the activation energyfor surface relaxation processes in terms of the lattice misfit wasobserved. Our experimental investigations further demonstrate theinfluence of strain relaxation on the self-ordering of complex oxidecompounds in the perovskite structure during high supersaturationmonolayer (interrupted layer-by-layer) deposition. [ABSTRACT FROM AUTHOR]

An optical study of the effects of strain and heterostructuring on nickelates is presented. The optical conductivity data obtained for a series of ultrathin films of NdNiO3 with varying amounts of strain reveal that two mechanisms, each associated with a different energy scale, are responsible for the tuning of the Drude-like spectral weight as a function of strain. Additionally, the optical properties of the LaNiO3 layers in a LaNiO3/LaAlO3 superlattice are investigated, showing that the coherent Drude transport associated with strained LaNiO3 is suppressed due to the presence of the LaAlO3 interlayers. [ABSTRACT FROM AUTHOR]

We report on a systematic study of the temperature-dependent Hall coefficient and thermoelectric power in ultrathin metallic LaNiO3 films that reveal a strain-induced, self-doping carrier transition that is inaccessible in the bulk. As the film strain varies from compressive to tensile at fixed composition and stoichiometry, the evolution of the transport coefficients is strikingly similar to those of bulk hole-doped superconducting cuprates with varying doping level. Density functional calculations reveal that the strain-induced changes in transport properties arise from changes in the low-energy electronic band structure that induce self-doping, a transfer of charge between O p and Ni d states. The results suggest that thin-film epitaxy can serve as a means to vary the charge-carrier concentration in other (negative) charge-transfer gap transition-metal oxides without resorting to chemical substitution. [ABSTRACT FROM AUTHOR]

Using resonant x-ray spectroscopies combined with density functional calculations, we find an asymmetric biaxial strain-induced d-orbital response in ultrathin films of the correlated metal LaNiO3 which are not accessible in the bulk. The sign of the misfit strain governs the stability of an octahedral "breathing" distortion, which, in turn, produces an emergent charge-ordered ground state with an altered ligand-hole density and bond covalency. Control of this new mechanism opens a pathway to rational orbital engineering, providing a platform for artificially designed Mott materials. [ABSTRACT FROM AUTHOR]

An optical study of fully strained ultrathin LaNiO3 films is presented and compared with LDA?+?DMFT calculations. LaNiO3 films were grown by pulsed laser deposition on LaAlO3 and SrTiO3 substrates which provide compressive and tensile strain, respectively. Optical conductivity data show a Drude peak with a spectral weight that is significantly reduced compared to that obtained from LDA calculations. The extended Drude analysis reveals the presence of a pseudogap around 80 meV for the film on SrTiO3 and near 40 meV, at low temperature only, for the film on LAO. An unusual temperature dependence of the optical conductivity is observed, with the Drude plasma frequency increasing by up to 20% at low temperature due to spectral weight transfer from bands lying 2-4 eV below the Fermi energy. Such a strong temperature dependence of the Drude spectral weight has previously been reported for correlated electron systems in which a phase transition is present. In LaNiO3, however, no phase transition is observed. [ABSTRACT FROM AUTHOR]