Origin of the asymmetric orbital--lattice interactions in correlated oxide heterostructures (original) (raw)

Asymmetric Orbital-Lattice Interactions in Ultrathin Correlated Oxide Films

Physical Review Letters, 2011

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 LaNiO 3 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.

Local atomic and electronic structures of epitaxial strained LaCoO3thin films

Physical Review B, 2012

We have examined the atomic and electronic structures of perovskite lanthanum cobaltite (LaCoO3) thin films using Co K-edge x-ray absorption fine structure (XAFS) spectroscopy. Extended XAFS (EXAFS) demonstrates that a large difference between in-plane and out-of-plane CoO bond lengths results from tetragonal distortion in highly strained films. The structural distortions are strongly coupled to the hybridization between atomic orbitals of the Co and O atoms, as shown by x-ray absorption near edge spectroscopy (XANES). Our results indicate that increased hybridization is not the cause of ferromagnetism in strained LaCoO3 films. Instead, we suggest that the strain-induced distortions of the oxygen octahedra increase the population of eg electrons and concurrently depopulate t2g electrons beyond a stabilization threshold for ferromagnetic order. Discoveries of novel properties in transition metal (TM) oxides, such as superconductivity in cuprates 1 and colossal magnetoresistance in manganites, 2 have prompted a flurry of research on these materials and proposals for numerous oxide thin film based electronic devices. 3,4 Fabrication of complex oxides in the form of thin films is crucial to the development of such devices. However, when synthesized as a thin film, the properties of a material can differ substantially from those of their bulk counterpart as a consequence of epitaxial strain. A striking example is the appearance of ferromagnetism in strained LaCoO 3 (LCO), 5-12 a perovskite oxide that lacks long-range magnetic order in bulk form. Below 100 K, LCO exists in a nonmagnetic insulating state, where the six valence electrons of Co 3+ occupy the t 2g levels of the crystal field split 3d states. The t 6 2g e 0 g configuration of Co 3+ is commonly referred to as the low spin (LS) state. Above 100 K, LCO becomes paramagnetic and semiconducting. Upon heating to 500 K, a transition to a paramagnetic and metallic phase occurs. These phase transitions are believed to coincide with spin state transitions of Co 3+ ions to t 5 2g e 1 g intermediate spin (IS) or t 4 2g e 2 g high spin (HS) states. However, the exact nature of the spin states present in LCO is not fully understood. 13 The spin states of the perovskite cobaltites are strongly influenced by competition between the crystal field splitting (∆ CF) of the Co(3d) states into e g and t 2g levels, which favors a LS configuration, and Hund exchange, which favors a HS configuration. The effects of chemical and hydrostatic pressure on the nonmagnetic to paramagnetic transition temperature illustrate the sensitivity of the spin state to the e g-t 2g gap (∆), where ∆ = ∆ CF − W/2, and W is the overlap between Co(3d) derived e g and O(2p) orbitals. 14,15 The dependence of the orbital overlap and crystal field splitting on the CoO distance (r Co−O) and the CoO Co angle (θ) are approximated by the expressions W ∝ r −3.5 Co−O sin(θ/2) and ∆ CF ∝ r −5 Co−O. 6,14 Therefore, ∆ can be reduced by an increase in r Co−O or θ. In the case of chemical pressure, replacing lanthanum with a rare earth ion having a smaller ionic radius reduces θ. This increases ∆ and thereby raises the nonmagnetic to paramagnetic transition temperature. 13-15 On the other hand, application of external pressure can induce a transition to the LS state by reducing r Co−O and therefore increasing ∆. 14 Numerous studies have demonstrated that epitaxial LCO thin films have a ferromagnetic ground state with a Curie temperature near 85 K. 5-12 These works indicate that tetragonal distortion of LCO is critical to the appearance of ferromagnetism, a conclusion supported by theoretical studies. 16 However, the local atomic structure resulting from tetragonal distortion is unclear. Local structure is strongly correlated with magnetic properties, and therefore without knowledge of the local structure the nature of ferromagnetism cannot be well understood. Previous extended x-ray absorption fine structure (EXAFS) studies of LCO thin films deposited on (LaAlO 3) 0.3 (Sr 2 AlTaO 6) 0.7 indicate that all six CoO bonds are of equal length. 17 This result supports the conclusion that ferromagnetic order in LCO is due to strain induced suppression of the Jahn-Teller distortion that has been suggested to occur in bulk LCO. 5 However, existence of a Jahn-Teller distortion in bulk LCO has been called into question by recent studies. 18,19 Furthermore, in other TM perovskite oxide thin films, epitaxial strain causes significant distortion of oxygen octahedra and can also alter octahedral rotations. 20-23 In addition to affecting the spin state as discussed above, distortion and rotation of oxygen octahedra can alter magnetic properties by changing the magnetic exchange energy, which increases with additional overlap between Co(3d) derived e g and O(2p) orbitals, W. In this way, strain induced changes in CoO hybridization can modify the magnetic properties of ferromagnetic perovskite oxides. 20,23 Increased CoO hybridization has also been posited as a requirement for ferromagnetic ordering in LCO. 6 In order to provide insight into the nature of ferromagnetism in epitaxial LCO, we have undertaken a detailed investigation of the local and electronic structures of LCO thin films. Bulk LCO has a rhombohedral structure belonging to space group R3c with a pseudocubic lattice parameter (a 0) of 3.830Å. 24 The present study examines ∼ 20 nm thick LCO films pulsed-laser deposited on SrTiO 3 (STO), a 0 = 3.905Å, and LaAlO 3 (LAO), a 0 = 3.791Å, substrates for direct comparison. 7 The films are capped with two unit cells of STO. Further details of the deposition and properties of these films can be found in Refs. 7 and 8. STO and LAO substrates have respective lattice misfits of 2.0 % and −1.0 % with LCO. However, while the LCO film deposited on STO is coherently strained, having an in-plane lattice parameter (a f) equal to that of the substrate, the film deposited on LAO is not, having a f equal to 3.842Å, presumably due to the increased misfit between LCO and LAO at the growth temperature. 8,25 Thermal stress that occurs upon cooling from the growth temperature has a substantial impact on lattice parameters of LCO thin films. 6 The out-of-plane lattice parameters (c f) of LCO films deposited on STO and LAO substrates are 3.781Å and 3.864Å, respectively. 7,8 Lattice parameters were measured by x-ray diffraction at beamlines 6-ID-C and 33-BM-C of the Advanced Photon Source (APS). The different strain states that result from the two substrates have been shown to strongly affect the magnetic properties of LCO, with films deposited on STO being ferromagnetic and films deposited on LAO showing magnetic behavior consistent with that of a spin glass. 7 Thus, differences in the local and electronic structures of these two films reveal aspects of the material that likely determine the magnetic state of LCO. Co K-edge x-ray absorption fine structure (XAFS) spectroscopy was performed at the National Institute of Standards and Technology (NIST) beamline X23A2 of the National Synchrotron Light Source (NSLS) in order to examine the local atomic and electronic structures of the LCO films. A four-element silicon drift detector was used for collection of EXAFS data, while the near edge portion of the XAFS spectrum was measured using a single-element

Connecting bulk symmetry and orbital polarization in strained RNiO3ultrathin films

Physical Review B, 2013

We examine the structural and electronic properties of LaNiO 3 and NdNiO 3 epitaxial thin films grown on cubic (001) SrTiO 3 from the viewpoint of bulk crystal symmetry and misfit strain. X-ray scattering and polarizationdependent x-ray absorption spectroscopy measurements are performed to determine the crystal symmetry and extract the local Ni 3d orbital response, respectively, to understand the strain-induced distortions of the bulk structure. A strain-induced orbital polarization is found in NdNiO 3 films, but is absent in LaNiO 3 films. The difference in electronic structure is attributed to the bulk thermodynamic phases through group theoretical methods, which reveals that thin film perovskites retain a "memory" of their preferred electronic and structural configurations.

Confinement-induced metal-to-insulator transition in strained LaNiO_{3}/LaAlO_{3} superlattices

Physical Review B, 2011

Using density functional theory calculations including a Hubbard U term we explore the effect of strain and confinement on the electronic ground state of superlattices containing the band insulator LaAlO3 and the correlated metal LaNiO3. Besides a suppression of holes at the apical oxygen, a central feature is the asymmetric response to strain in single unit cell superlattices: For tensile strain a band gap opens due to charge disproportionation at the Ni sites with two distinct magnetic moments of 1.45µB and 0.71µB. Under compressive stain, charge disproportionation is nearly quenched and the band gap collapses due to overlap of d 3z 2 −r 2 bands through a semimetallic state. This asymmetry in the electronic behavior is associated with the difference in octahedral distortions and rotations under tensile and compressive strain. The ligand hole density and the metallic state are quickly restored with increasing thickness of the (LaAlO3)n/(LaNiO3)n superlattice from n = 1 to n = 3.

Orbital control in strained ultra-thin LaNiO 3 /LaAlO 3 superlattices

EPL (Europhysics Letters), 2011

In pursuit of rational control of orbital polarization, we present a combined experimental and theoretical study of single unit cell LaNiO3/LaAlO3 superlattices. Polarized x-ray absorption spectra show a distinct asymmetry in the orbital response under tensile vs. compressive strain. A splitting of orbital energies ∼100 meV with octahedral distortions is found for the case of compressive strain which is much smaller than the 3d bandwidth. In sharp contrast, for tensile strain, no splitting is found although a strong orbital polarization is still present. Density functional theory calculations of the electronic properties reveal that the asymmetry results from a combination of strain effects and altered covalency in the bonding across the interfacial apical oxygen to the Al site, leading to the opening of a pseudogap in the heterostructure for tensile strain.

Electronic structure of La2/3Sr1/3MnO3 : Interplay of oxygen octahedra rotations and epitaxial strain

Physical Review B

Influence of epitaxial strain and oxygen octahedra rotations on electronic structure of La 2/3 Sr 1/3 MnO 3 ultrathin films was systematically studied. The investigated films were grown by pulsed laser deposition on four different substrates: cubic (001)-oriented LaAlO 3 , (001) (LaAlO 3) 1/3 (Sr 2 AlTaO 6) 2/3 , (001) SrTiO 3 , and orthorhombic (110) DyScO 3 , providing a broad range of induced epitaxial strains. Magnetic properties were found to deteriorate with increasing value of the epitaxial strain, as expected due to unit cell distortion increasingly deviating from the bulk and effect of the magnetically inert layer. A combination of spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy was used to determine spectra of the diagonal and offdiagonal elements of permittivity tensor. The off-diagonal elements at room temperature confirmed the presence of two previously reported electronic transitions in the spectra of all films. Moreover they revealed another electronic transition around 4.3 eV only in the spectra of films grown under compressive strain. We proposed classification of this transition as a crystal field paramagnetic Mn t 2g → e g transition. Ab initio calculations were employed to distinguish between the potential influence of oxygen octahedra rotations and distortions. The ab initio calculations indicated a negligible influence of oxygen octahedra rotations on magneto-optical properties of La 2/3 Sr 1/3 MnO 3. They further supported the proposed classification of the additional electronic transition, showing a key role of strain in controlling the electronic structure of ultrathin perovskite films.

Interplay Between Spin-Orbit Coupling and Structural Deformations in Heavy Transition-Metal Oxides with Tetrahedral Coordination

Acta Physica Polonica A, 2018

We analyze the effects of a large spin-orbit coupling on the magnetic state of a d 1 transition-metal ion located in a tetrahedral environment. While in the ideal tetrahedral symmetry the spin-orbit coupling acts only as a perturbation on the atomic energy levels set by the crystal-field splitting, we demonstrate that its effects are strongly enhanced in the case of distorted geometries. In particular, we consider the specific case in which the tetrahedron is compressed along the z direction, and show that, by increasing the degree of flattening, a large spin-orbit interaction (i) can induce a substantial anisotropic, unquenched orbital momentum and (ii) can affect the hierarchy of the lowest energy levels that are involved in the magnetic exchange.