We study how strain affects orbital ordering and magnetism at the interface between SrMnO$_3$ and LaMnO$_3$ from density-functional calculations and interpret the basic results in terms of a three-site Mn-O-Mn model. Magnetic interaction between the Mn atoms is governed by a competition between the antiferromagnetic superexchange of the Mn t$_{2g}$ core spins and the ferromagnetic double exchange of the itinerant e$_g$ electrons. While the core electrons are relatively unaffected by the strain, the orbital character of the itinerant electron is strongly affected, which in turn causes a large change in the strength of the ferromagnetic double exchange. The epitaxial strain produces the tetragonal distortion of the MnO$_6$ octahedron, splitting the Mn-e$_g$ states into x$^2$-y$^2$ and 3z$^2$-1 states, with the former being lower in energy, if the strain is tensile in the plane, and opposite if the strain is compressive. For the case of the tensile strain, the resulting higher occupancy of the x$^2$-y$^2$ orbital enhances the in-plane ferromagnetic double exchange owing to the larger electron hopping in the plane, causing at the same time a reduction of the out-of-plane double exchange. This reduction is large enough to be overcome by antiferromagnetic superexchange, which wins to produce a net antiferromagnetic interaction between the out-of-plane Mn atoms. For the case of the in-plane compressive strain, the reverse happens, viz., that the higher occupancy of the 3z$^2$-1 orbital results in the out-of-plane ferromagnetic interaction, while the in-plane magnetic interaction remains antiferromagnetic. Concrete density-functional results are presented for the (LaMnO$_3$)$_1$/(SrMnO$_3$)$_1$ and (LaMnO$_3$)$_1$/(SrMnO$_3$)$_3$ superlattices for various strain conditions.
In this Letter, we present investigations of thermal transport in (LaMnO$_3$)$_n$/(SrMnO$_3$)$_n$ superlattices (SLs) with SL periods $Λ=2n$ between $2$ and $12$ unit cells. The analysis of the experimental data clearly reveals a minimum in the thermal conductivity $κ$ at $Λ=6$ u.c. Furthermore, we theoretically estimate the phonon mean free path to be about $3Λ$ in our SLs. These findings clearly show that, in a wide temperature range of $\pm 100\,$K around room temperature, thermal transport in our manganite SLs is coherent. In addition, we demonstrate that antiferromagnetic magnetic orderin…
We conducted lateral force microscopy measurements on seven [LaMnO$_{3}$]$_{m}$/[SrMnO$_{3}$]$_{n}$ superlattices with varied layer thicknesses. We observe that the friction forces and the friction coefficients initially increase with increasing LaMnO3 top layer thickness, followed by saturation when the top layer thickness exceeds a few nanometers. These observations clearly demonstrate that sliding friction is affected by sub-surface material properties to a depth of several nanometers and is not just determined by dynamics in the contact interface. We argue that the sub-surface dissipated e…
We study the magnetic structure of the (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ superlattices from density-functional calculations. In agreement with the experiments, we find that the magnetism changes with the layer thickness `n'. The reason for the different magnetic structures is shown to be the varying potential barrier across the interface, which controls the leakage of the Mn-e$_g$ electrons from the LMO side to the SMO side. This in turn affects the interfacial magnetism via the carrier-mediated Zener double exchange. For n=1 superlattice, the Mn-e$_g$ electrons are more or less spread over t…
The modulation of charge density and spin order in (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ ($n$=1-4) superlattices is studied via Monte Carlo simulations of the double-exchange model. G-type antiferromagnetic barriers in the SrMnO$_{3}$ regions with low charge density are found to separate ferromagnetic LaMnO$_{3}$ layers with high charge density. The recently experimentally observed metal-insulator transition with increasing $n$ is reproduced in our studies, and $n=3$ is found to be the critical value.
We have performed ab initio calculations within the LDA+U method in the multilayered system (LaMnO$_3$)$_{2n}$ / (SrMnO$_3$)$_n$. Our results suggest a charge-ordered state that alternates Mn$^{3+}$ and Mn$^{4+}$ cations in a checkerboard in-plane pattern is developed at the interfacial layer, leading to a gap opening. Such an interfacial charge-ordered situation would be the energetically favored reconstruction between LaMnO$_3$ and SrMnO$_3$. This helps understanding the insulating behavior observed experimentally in these multilayers at intermediate values of $n$, whose origin is known to b…
Polarized neutron reflectivity measurements of a ferromagnetic [(LaMnO$_3$)$_{11.8}$/(SrMnO$_3$)$_{4.4}$]$_6$ superlattice reveal a modulated magnetic structure with an enhanced magnetization at the interfaces where LaMnO$_3$ was deposited on SrMnO$_3$ (LMO/SMO). However, the opposite interfaces (SMO/LMO) are found to have a reduced ferromagnetic moment. The magnetic asymmetry arises from the difference in lateral structural roughness of the two interfaces observed via electron microscopy, with strong ferromagnetism present at the interfaces that are atomically smooth over tens of nanometers. …
Photoinduced magnetization dynamics is investigated in chemically ordered $(\mathrm{LaMnO}_3)_{2n}/(\mathrm{SrMnO}_3)_n$ superlattices using the time-resolved magneto-optic Kerr effect. A monotonic frequency-field dependence is observed for the $n=1$ superlattice, indicating a single spin population consistent with a homogeneous hole distribution. In contrast, for $n\geq2$ superlattices, a large precession frequency is observed at low fields indicating the presence of an exchange torque in the dynamic regime. We propose a model that ascribes the emergence of exchange torque to the coupling bet…
Using first-principles calculations based on the density functional theory, we show a strong strain dependence of magnetic order in (LaMnO$_{3}$)$_n$/(SrMnO$_{3}$)$_{2n}$ (001) superlattices with $n=1,2$. The epitaxial strain lifts the degeneracy of Mn $e_{g}$ orbitals, thus inducing an inherent orbital order, which in turn strongly affects the ferromagnetic double exchange of itinerant $e_{g}$ electrons, competing with the antiferromagnetic superexchange of localized $t_{2g}$ electrons. For the case of tensile strain induced by SrTiO$_3$ (001) substrate, we find that the ground state is A-typ…
We have studied the influence of epitaxial strain on magnetic orderings and the couplings between the spin and polar phonons in the 1:1 SrMnO$_3$/LaMnO$_3$ superlattice from first principles. Magnetic phase transitions of the superlattice induced by epitaxial strain are observed, consistent with previous reports. We find that oxygen octahedral rotations lower the ground state energy but do not destroy the magnetic phase transitions induced by strain. We compute zone center phonon frequencies and eigenvectors as functions of epitaxial strain and magnetic ordering. A substantial increase of the …
Electronic reconstruction at the polar interface LaMnO$_3$/SrMnO$_3$ (LMO/SMO) (100) resulting from the polar catastrophe is studied from a model Hamiltonian that includes the double and super exchange interactions, the Madelung potential, and the Jahn-Teller coupling terms relevant for the manganites. We show that the polar catastrophe, originating from the alternately charged LMO layers and neutral SMO layers, is quenched by the accumulation of an extra half electron per cell in the interface region as in the case of the LaAlO$_3$/SrTiO$_3$ interface. In addition, the Mn e$_g$ electrons leak…
In this paper we report studies on the correlation between the presence and distribution of cation vacancies in doped manganites (La,M)MnO3+delta (where M = Na, Ca) and their magnetic properties. Results indicate that cation vacancies are distributed differently for the different crystal structures and dopant ion type. In particular it is shown that knowledge of the total vacancies concentration alone is not enough to fully characterize the physical properties of manganites and that their distribution between the A and B sites of the perovskite structure plays a crucial role which should be ta…
Single crystals of electron-doped SrMnO3 with a cubic perovskite structure have been systematically investigated as the most canonical (orbital-degenerate) double-exchange system, whose ground states have been still theoretically controversial. With only 1-2% electron doping by Ce substitution for Sr, a G-type antiferromagnetic metal with a tiny spin canting in a cubic lattice shows up as the ground state, where the Jahn-Teller polarons with heavy mass are likely to form. Further electron doping above 4%, however, replaces this isotropic metal with an insulator with tetragonal lattice distorti…
We report the synthesis of SrMnO$_{3-δ}$F$_{γ}$ perovskite oxyfluoride thin films using a vapor transport method to fluorinate as-grown SrMnO$_{2.5}$ epitaxial thin films. The influence of the fluoropolymer, which acts as a fluorine vapor source, was investigated by utilizing polyvinyl fluoride (PVF), polyvinylidene difluoride (PVDF) and polytetrafluoroethylene (PTFE) in the reaction. The same process was carried out with polyethylene (PE) to isolate the role of carbon in the vapor transport process. The F distribution was probed by X-ray photoemission spectroscopy, which confirmed the incorpo…
In a recent publication (S. Dong et al., Phys. Rev. Lett.103, 127201 (2009)), two (related) mechanisms were proposed to understand the intrinsic exchange bias present in oxides heterostructures involving G-type antiferromagnetic perovskites. The first mechanism is driven by the Dzyaloshinskii-Moriya interaction, which is a spin-orbit coupling effect. The second is induced by the ferroelectric polarization, and it is only active in heterostructures involving multiferroics. Using the SrRuO$_3$/SrMnO$_3$ superlattice as a model system, density-functional calculations are here performed to verify …
The functional properties of transition metal perovskite oxides are known to result from a complex interplay of magnetism, polarization, strain, and stoichiometry. Here, we show that for materials with a cooperative Jahn-Teller distortion, such as LaMnO$_3$ (LMO), the orbital order can also couple to the defect chemistry and induce novel material properties. At low temperatures, LMO exhibits a strong Jahn-Teller distortion that splits the $e_g$ orbitals of the high-spin Mn$^{3+}$ ions and leads to alternating long, short, and intermediate Mn--O bonds. Our DFT+$U$ calculations show that, as a r…
Defect chemistry, strain, and structural, magnetic and electronic degrees of freedom constitute a rich space for the design of functional properties in transition metal oxides. Here, we show that it is possible to engineer polarity and ferroelectricity in non-polar perovskite oxides via polar defect pairs formed by anion vacancies coupled to substitutional cations. We use a self-consistent site-dependent DFT+$U$ approach that accounts for local structural and chemical changes upon defect creation and which is crucial to reconcile predictions with the available experimental data. Our results fo…
Perovskite structure SrMnO$_3$ is a rare example of a multiferroic material where strain-tuning and/or cation substitution could lead to coinciding magnetic and ferroelectric ordering temperatures, which would then promise strong magnetoelectric coupling effects. Here, we establish the temperature and strain dependent ferroic phase diagram of SrMnO$_3$ using first-principles-based effective Hamiltonians. All parameters of these Hamiltonians are calculated using density functional theory, i.e., no fitting to experimental data is required. Temperature dependent properties are then obtained from …
The understanding of oxygen reduction reaction (ORR) activity on perovskite oxide surfaces is essential for promising future fuel cell applications. We report a comparative study of ORR mechanisms on La$B$O$_3$ ($B$=Mn, Fe, Cr) surfaces by first-principles calculations based on density functional theory (DFT). Results obtained from varied DFT methods such as generalized gradient approximation(GGA), GGA+$U$ and the hybrid Hartree-Fock density functional method are reported for comparative purposes. We find that the results calculated from hybrid-functional method suggest that the order of ORR a…
First-principles density-functional calculations reveal a large spin-phonon coupling in cubic SrMnO$_{3}$, with ferromagnetic ordering producing a polar instability. Through combination of this coupling with the strain-polarization coupling characteristic of perovskites, the bulk antiferromagnetic paraelectric ground state of SrMnO$_3$ is shown to be driven to a previously unreported multiferroic ferroelectric-ferromagnetic state by increasing epitaxial strain, both tensile and compressive. This state has a computed polarization and estimated Curie temperature above 54 $μ$C/cm$^2$ and 92 K. La…
Heterostructures of mixed-valence manganites are still under intense scrutiny, due to the occurrence of exotic quantum phenomena linked to electronic correlation and interfacial composition. For instance, if two anti-ferromagnetic insulators as LaMnO$_3$ and SrMnO$_3$ are grown in a (001)-oriented superlattice, a half-metallic ferromagnet may form, provided that the thickness is sufficiently small to allow tunneling across interfaces. In this article, we employ electronic structure calculations to show that all the layers of a (111)-oriented LaMnO$_3$|SrMnO$_3$ superlattice retain a half-metal…
Modulation-doped oxide two-dimensional electron gas (2DEG) formed at the LaMnO3 (LMO) buffered disorderd-LaAlO3/SrTiO3 (d-LAO/LMO/STO) heterointerface, provides new opportunities for electronics as well as quantum physics. Herein, we studied the dependence of Sr-doping of La1-xSrxMnO3 (LSMO, x=0, 1/8, 1/3, 1/2, and 1) thus the filling of the Mn eg subbands as well as the LSMO polarity on the transport properties of d-LAO/LSMO/STO. Upon increasing the LSMO film thickness from 1 unit cell (uc) to 2 uc, a sharp metal to insulator transition of interface conduction was observed, independent of x. …
By applying Wannier-based extended Kugel-Khomskii model, we carry out first-principles calculations and electronic structure analysis to understand the spin-phonon coupling effect in transition-metal perovskites. We demonstrate the successful application of our approach to SrMnO$_3$ and BiFeO$_3$. We show that both the electron orbitals under crystal field splitting and the electronic configuration should be taken into account in order to understand the large variances of spin-phonon coupling effects among various phonon modes as well as in different materials.
Correlated oxides, such as BiMnO$_3$ and LaMnO$_3$, show complex interplay of electronic correlations and crystal structure exhibiting multiple first order phase transitions, some without a clear order parameter. The quantitative theoretical description of this temperature dependent electronic-structural interplay in the vicinity of a Mott transition is still a challenge. Here we address this issue by simultaneously considering both structural and electronic degrees of freedom, within a self-consistent density functional theory with embedded dynamical mean field theory. Our results show the ex…
In this work, we use the thermodynamically consistent and conserving self-energy embedding theory (SEET) to study the spectra of the prototypical undistorted cubic perovskites SrVO$_3$ and SrMnO$_3$. In the strongly correlated metallic SrVO$_3$ we find that the usual attribution of the satellite peaks at -1.8eV to Hund or Hubbard physics in the $t_{2g}$ orbitals is inconsistent with our calculations. In the strongly correlated insulator SrMnO$_3$ we recover insulating behavior due to a feedback effect between the strongly correlated orbitals and the weakly correlated environment. Our calculati…
Controlling the electronic properties of interfaces has enormous scientific and technological implications and has been recently extended from semiconductors to complex oxides which host emergent ground states not present in the parent materials. These oxide interfaces present a fundamentally new opportunity where, instead of conventional bandgap engineering, the electronic and magnetic properties can be optimized by engineering quantum many-body interactions. We utilize an integrated oxide molecular-beam epitaxy and angle-resolved photoemission spectroscopy system to synthesize and investigat…
The formation of a novel spin-polarized 2D electron gas at the LaMnO$_3$ monolayer embedded in SrMnO$_3$ is predicted from the first-principles density-functional calculations. The La (d) electrons become confined in the direction normal to the interface in the potential well of the La layer, serving as a positively-charged layer of electron donors. These electrons mediate a ferromagnetic alignment of the Mn t$_{2g}$ spins near the interface via the Anderson-Hasegawa double exchange and become, in turn, spin-polarized due to the internal magnetic fields of the Mn moments.
We have studied the interplay between orbital ordering, Jahn-Teller and GdFeO3-type lattice distortions in perovskite-type transition-metal oxides using model Hartree-Fock calculations. It has been found that the covalency between A-site cations and oxygens causes interaction between the Jahn-Teller and GdFeO3-type distortions. The present calculations explain why the d-type Jahn-Teller distortion and orbital ordering compatible with it are realized in LaMnO3, YVO3 and YTiO3
Candidates for correlated topological insulators, originated from the spin-orbit coupling as well as Hubbard type correlation, are expected in the ($111$) bilayer of perovskite-structural transition-metal oxides. Based on the first-principles calculation and tight-binding model, the electronic structure of a LaMnO$_3$ ($111$) bilayer sandwiched in LaScO$_3$ barriers has been investigated. For the ideal undistorted perovskite structure, the Fermi energy of LaMnO$_3$ ($111$) bilayer just stays at the Dirac point, rendering a semi-metal (graphene-like) which is also a half-metal (different from g…
Solar thermochemical hydrogen production (STCH) is a renewable alternative to hydrogen produced using fossil fuels. While serial bulk experimental methods can accurately measure STCH performance, screening chemically complex materials systems for new promising candidates is more challenging. Here we identify double-site Ce-substituted (Ba,Sr)MnO3 oxide perovskites as promising STCH candidates using a combination of bulk synthesis and high-throughput thin film experiments. The Ce substitution on the B-site in 10H-BaMnO3 and on the A-site in 4P-SrMnO3 lead to 2-3x higher hydrogen production comp…
LaMnO$_3$ is considered as a prototypical Jahn-Teller perovskite compound, exhibiting a metal to insulator transition at $T_{JT} = 750K$ related to the joint appearance of an electronic orbital ordering and a large lattice Jahn-Teller distortion. From first-principles, we revisit the behavior of LaMnO$_3$ and show that it is not only prone to orbital ordering but also to charge ordering. Both charge and orbital orderings appear to be enabled by rotations of the oxygen octahedra and the subtle competition between them is monitored by a large tetragonal compressive strain, that is itself a Jahn-…
We present a systematic investigation of the role and importance of excitonic effects on the optical properties of transitions metal oxide perovskites. A representative set of fourteen compounds has been selected, including 3$d$ (SrTiO$_3$, LaScO$_3$, LaTiO$_3$, LaVO$_3$, LaCrO$_3$, LaMnO$_3$, LaFeO$_3$ and SrMnO$_3$), 4$d$ (SrZrO$_3$, SrTcO$_3$ and Ca$_2$RuO$_4$) and 5$d$ (SrHfO$_3$, KTaO$_3$ and NaOsO$_3$) perovskites, covering a band gap ranging from 0.1 eV to 6.1 eV and exhibiting different electronic, structural and magnetic properties. Optical conductivities and optical transitions inclu…
Goldstone modes are massless particles resulting from spontaneous symmetry breaking. Although such modes are found in elementary particle physics as well as in condensed matter systems like superfluid helium, superconductors and magnons - structural Goldstone modes are rare. Epitaxial strain in thin films can induce structures and properties not accessible in bulk and has been intensively studied for (001)-oriented perovskite oxides. Here we predict Goldstone-like phonon modes in (111)-strained SrMnO3 by first-principles calculations. Under compressive strain the coupling between two in-plane …
The equilibrium structure and exchange constants of CaMnO$_3$ and LaMnO$_3$ have been investigated using total energy unrestricted Hartree-Fock (UHF) and localised orbital configuration interaction (CI) calculations on the bulk compounds and Mn$_2$O$_{11}^{14-}$ and Mn$_2$O$_{11}^{16-}$ clusters. The predicted structure and exchange constants for CaMnO$_3$ are in reasonable agreement with estimates based on its Néel temperature. A series of calculations on LaMnO$_3$ in the cubic perovskite structure shows that a Hamiltonian with independent orbital ordering and exchange terms accounts for the …
In this proceeding, recent theoretical investigations by the authors on the multiferroic $R$MnO$_3$ perovskites are briefly reviewed at first. Using the double-exchange model, the realistic spiral spin order in undoped manganites such as TbMnO$_3$ and DyMnO$_3$ is well reproduced by incorporating a weak next-nearest neighbor superexchange ($\sim10%$ of nearest neighbor superexchange) and moderate Jahn-Teller distortion. The phase transitions from the A-type antiferromagnet (as in LaMnO$_3$), to the spiral phase (as in TbMnO$_3$), and finally to the E-type antiferromagnet (as in HoMnO$_3$), wit…
The model developed for LaMnO$_3$ addresses the spin-orbital order by superexchange and Jahn-Teller orbital interactions in the cubic (perovskite) symmetry up to now whereas real crystal structure is strongly deformed. We identify and explain three \textit{a priori} important physical effects arising from tetragonal deformation: (i) the splitting of $e_g$ orbitals $\propto E_z$, (ii) the directional renormalization of $d-p$ hybridization $t_{pd}$, and (iii) the directional renormalization of charge excitation energies. Using the example of LaMnO$_3$ crystal we evaluate their magnitude. It is f…
We present a theoretical study on the relation between the size of the rare earth ions, often known as chemical pressure, and the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites. Using a Keating model expressed in terms of atomic scale symmetry modes, we show that there exists a coupling between the uniform shear distortion and the staggered buckling distortion within the Jahn-Teller energy term. It is found that this coupling provides a mechanism by which the coherent Jahn-Teller distortion is more stabilized by smaller rare earth ions. We analyze the appear…
Oxygen vacancy formation energy is an important quantity for enabling fast oxygen diffusion and oxygen catalysis in technologies like solid oxide fuel cells. Both previous literature in various systems and our calculations in LaMnO3, La0.75Sr0.25MnO3, LaFeO3, and La0.75Sr0.25FeO3, show mixed results for the direction and magnitude of the change in vacancy formation energy with strain. This paper develops a model to make sense of the different trend shapes in vacancy formation energy versus strain. We model strain effects using a set of consistent ab initio calculations, and demonstrate that ou…
Anionic ordering is a promising route to engineer physical properties in functional heteroanionic materials. A central challenge in the study of anion-ordered compounds lies in developing robust synthetic strategies to control anion occupation and in understanding the resultant implications for electronic structure. Here, we show that epitaxial strain induces preferential occupation of F and O on the anion sites in perovskite oxyfluoride SrMnO2.5-dFg films grown on different substrates. Under compressive strain, F tends to take the apical-like sites, which was revealed by F and O K-edge linear…
We report a modulation of the in-plane magnetotransport in artificial manganite superlattice (SL) [(NdMnO3)n /(SrMnO3)n /(LaMnO3)n]m by varying the layer thickness n while keeping the total thickness of the structure constant. Charge transport in these heterostructures is confined to the interfaces and occurs via variable range hopping (VRH). Upon increasing n, the interfacial separation rises, leading to a suppression of the electrostatic screening between carriers of neighboring interfaces and the opening of a Coulomb gap at the Fermi level (EF). The high-field magnetoresistance (MR) is univ…
Perovskite bilayers with (111)-orientation combine a honeycomb lattice as a key feature with the strongly correlated, multiorbital nature of electrons in transition metal oxides. In a systematic DFT+$U$ study of (111)-oriented (La$X$O$_3$)$_2$/(LaAlO$_3$)$_4$ superlattices, we establish trends in the evolution of ground states versus band filling in (111)-oriented (La$X$O$_3$)$_2$/(LaAlO$_3$)$_4$ superlattices, with $X$ spanning the entire $3d$ transition metal series. The competition between local quasi-cubic and global triangular symmetry triggers unanticipated broken symmetry phases, with m…
We introduce an effective model for $e_g$ electrons to describe three-dimensional perovskite (La$_{1-x}$Sr$_{x}$MnO$_3$ and La$_{1-x}$Ca$_{x}$MnO$_3$) manganites and study the magnetic and orbital order on $4\times 4\times 4$ cluster using correlated wave functions. The model includes the kinetic energy, and on-site Coulomb interactions for $e_g$ electrons, antiferromagnetic superexchange interaction between $S=3/2$ core spins, and the coupling between $e_g$ electrons and Jahn-Teller modes. The model reproduces the experimentally observed magnetic order: ($i$) $A$-type antiferromagnetic phase …
The Jahn-Teller (JT) distortion that can remove electronic degeneracies in partially occupied states and results in systematic atomic displacements is a common underlying feature to many of the intriguing phenomena observed in 3d perovskites, encompassing magnetism, superconductivity, orbital ordering and colossal magnetoresistance. Although the seminal Jahn and Teller theorem has been postulated almost a century ago, the origins of this effect in perovskite materials are still debated, including propositions such as super exchange, spin-phonon coupling, sterically induced lattice distortions,…
Modern theory of the orbital magnetization is applied to the series of insulating perovskite transition metal oxides (orthorhombic YTiO$_3$, LaMnO$_3$, and YVO$_3$, as well as monoclinic YVO$_3$), carrying a net ferromagnetic (FM) moment in the ground state. For these purposes, we use an effective Hubbard-type model, derived from the first-principles electronic-structure calculations and describing the behavior of magnetically active states near the Fermi level. The solution of this model in the mean-field Hartree-Fock approximation with the relativistic spin-orbit coupling typically gives us …
