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Register a new userOptical properties of LaNiO3 films tuned from compressive to tensile strain
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Materials with strong electronic correlations host remarkable -- and technologically relevant -- phenomena such as magnetism, superconductivity and metal-insulator transitions. Harnessing and controlling these effects is a major challenge, on which key advances are being made through lattice and strain engineering in thin films and heterostructures, leveraging the complex interplay between electronic and structural degrees of freedom. Here we show that the electronic structure of LaNiO3 can be tuned by means of lattice engineering. We use different substrates to induce compressive and tensile biaxial epitaxial strain in LaNiO3 thin films. Our measurements reveal systematic changes of the optical spectrum as a function of strain and, notably, an increase of the low-frequency free carrier weight as tensile strain is applied. Using density functional theory (DFT) calculations, we show that this apparently counter-intuitive effect is due to a change of orientation of the oxygen octahedra.The calculations also reveal drastic changes of the electronic structure under strain, associated with a Fermi surface Lifshitz transition. We provide an online applet to explore these effects. The experimental value of integrated spectral weight below 2 eV is significantly (up to a factor of 3) smaller than the DFT results, indicating a transfer of spectral weight from the infrared to energies above 2 eV. The suppression of the free carrier weight and the transfer of spectral weight to high energies together indicate a correlation-induced band narrowing and free carrier mass enhancement due to electronic correlations. Our findings provide a promising avenue for the tuning and control of quantum materials employing lattice engineering.
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Strain control is one of the most promising avenues to search for new emergent phenomena in transition-metal-oxide films. Here, we investigate the strain-induced changes of electronic structures in strongly correlated LaNiO3 (LNO) films, using angle-resolved photoemission spectroscopy and the dynamical mean-field theory. The strongly renormalized eg-orbital bands are systematically rearranged by misfit strain to change its fermiology. As tensile strain increases, the hole pocket centered at the A point elongates along the kz-axis and seems to become open, thus changing Fermi-surface (FS) topology from three- to quasi-two-dimensional. Concomitantly, the FS shape becomes flattened to enhance FS nesting. A FS superstructure with Q1 = (1/2,1/2,1/2) appears in all LNO films, while a tensile-strained LNO film has an additional Q2 = (1/4,1/4,1/4) modulation, indicating that some instabilities are present in metallic LNO films. Charge disproportionation and spin-density-wave fluctuations observed in other nickelates might be their most probable origins.
The spin states of Co$^{3+}$ ions in perovskite-type LaCoO$_3$, governed by complex interplay between the electron-lattice interactions and the strong electron correlations, still remain controversial due to the lack of experimental techniques which can detect directly. In this letter, we revealed the tensile-strain dependence of spin states, $i. e.$ the ratio of the high- and low-spin states, in epitaxial thin films and a bulk crystal of LaCoO$_3$ via resonant inelastic soft x-ray scattering. The tensile-strain as small as 1.0% was found to realize different spin states from that in the bulk.
MnO thin films with various thicknesses and strains were grown on MgO substrates by pulsed laser deposition, then characterized using x-ray diffraction and infrared reflectance spectroscopy. Films grown on (001)-oriented MgO substrates exhibit homogenous biaxial compressive strain which increases as the film thickness is reduced. For that reason, the frequency of doubly-degenerate phonon increases with the strain, and splits below Neel temperature TN due to the magnetic-exchange interaction. Films grown on (110)-oriented MgO substrates exhibit a huge phonon splitting already at room temperature due to the anisotropic in-plane compressive strain. Below TN, additional phonon is activated in the IR spectra; this trend is evidence for a spin-order-induced structural phase transition from tetragonal to monoclinic phase. Total phonon splitting is 55 cm-1 in (110)-oriented MnO film, which is more than twice the value in bulk MnO. This result is evidence that the nearest neighbor exchange interaction, which is responsible for the magnetically driven phonon splitting, is greatly increased in compressively strained films.
We have studied the effect of tensile strain on the superconductivity in FeSe films. 50 nm, 100 nm, and 200 nm FeSe films were grown on MgO, SrTiO$_3$, and LaAlO$_3$ substrates by using a pulsed laser deposition technique. X-ray diffraction analysis showed that the tetragonal phase is dominant in all of our FeSe films. The 50 nm FeSe films on MgO and SrTiO$_3$ are under tensile strain, while the 50 nm FeSe film on LaAlO$_3$ and the other thick FeSe films are unstrained. Superconducting transitions have been observed in unstrained FeSe films with T$_{onset}$ $approx$ 8 K, which is close to the bulk value. However, no sign of superconductivity has been observed in FeSe films under tensile strain down to 5 K. There is evidence to show that tensile strain suppresses superconductivity in FeSe films.
In order to analyse the effect of strain on the magnetic properties of narrow-band manganites, the temperature and field dependent susceptibilities of about 8.5 nm thick epitaxial Pr0.7Ca0.3MnO3 films, respectively grown on (001) and (110) SrTiO3 substrates, have been compared. For ultrathin samples grown on (001) SrTiO3, a bulk-like cluster-glass magnetic behaviour is found, indicative of the possible coexistence of antiferromagnetic and ferromagnetic phases. On the contrary, ultrathin films grown on (110) substrates show a robust ferromagnetism, with a strong spontaneous magnetization of about 3.4 mB /Mn atom along the easy axis. On the base of high resolution reciprocal space mapping analyses performed by x-ray diffraction, the different behaviours are discussed in terms of the crystallographic constraints imposed by the epitaxy of Pr0.7Ca0.3MnO3 on SrTiO3. We suggest that for growth on (110) SrTiO3, the tensile strain on the film c-axis, lying within the substrate plane, favours the ferromagnetic phase, possibly by allowing a mixed occupancy and hybridization of both in-plane and out-of-plane eg orbitals. Our data allow to shed some physics of inhomogeneous states in manganites and on the nature of their ferromagnetic insulating state.
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