Electronic Structure of Oxide Interfaces: A Comparative Analysis of GdTiO3_3/SrTiO3_3 and LaAlO3_3/SrTiO3_3 Interfaces

Emergent phases in the two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides have attracted great attention in the past decade. We present ab-initio electronic structure calculations for the interface between a Mott insulator GdTiO$_3$ (GTO) and a band insulator SrTiO$_3$ (STO) and compare our results with those for the widely studied LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface between two band insulators. Our GTO/STO results are in excellent agreement with experiments, but qualitatively different from LAO/STO. We find an interface carrier density of 0.5$e^{-}$/Ti, independent of GTO thickness in both superlattice and thin film geometries, in contrast to LAO/STO. The superlattice geometry in LAO/STO offers qualitatively the same result as in GTO/STO. On the other hand, for a thin film geometry, the interface carrier density builds up only beyond a threshold thickness of LAO. The positive charge at the vacuum surface that compensates the 2DEG at the interface also exhibits distinct behaviors in the two systems. The top GTO layer is found to be insulating due to correlation-driven charge disproportionation, while the top LAO layer is metallic within band theory and may become insulating due to surface disorder or surface reconstruction.Comment: 7 figure

Importance of electronic correlations for the magnetic properties of the two-dimensional ferromagnet CoBr2

We investigate the emergence of ferromagnetism in the two-dimensional metal halide CoBr2, with a special focus on the role of electronic correlations. The calculated phonon spectrum shows that the system is thermodynamically stable, unlike other Co halides. We apply two well-known methods for the estimation of the Curie temperature. First, we do density-functional theory +U calculations to calculate exchange couplings, which are subsequently used in a classical Monte Carlo simulation of the resulting Ising spin model. The transition temperature calculated in this way is of the order of 100 K but shows a strong dependence on the choice of interaction parameters. Second, we apply dynamical mean-field theory to calculate the correlated electronic structure and estimate the transition temperature. This results in a similar estimate for a noticeable transition temperature of approximately 100 K, but without the strong dependence on the interaction parameters. The effect of electron-electron interactions are strongly orbital selective, with only moderate correlations in the three low-lying orbitals (one doublet plus one singlet) and strong correlations in the doublet at higher energy. This can be traced back to the electronic occupation in DMFT, with five electrons in the three low-lying orbitals and two electrons in the high-energy doublet, making the latter one half filled. Nevertheless, the overall spectral gap is governed by the small gap originating from the low-lying doublet+singlet orbitals, which changes very weakly with interaction U. In that sense, the system is close to a Mott metal-to-insulator transition, which was shown previously to be a hot spot for strong magnetism

Decoupling the effects of geometry and nature of strain in LaMnO3:Interplay of dynamic correlations and uniaxial strain driving magnetic phase transitions

Recent years have seen tremendous progress in experimental techniques to create uniaxial strain. Motivated by these advances we investigate the effect of uniaxial strain on LaMnO 3 employing ab-initio dynamical mean-field theory, and put it in contrast to biaxial strain that occurs in epitaxial systems. Projecting on the low-energy subspace of Mn 3d states, and solving multi-impurity problems, our approach emphasizes on local dynamic correlations at Mn sites. At ambient pressures, LaMnO 3 crystallizes in an orthorhombic unit cell, with in-plane lattice constants a&lt;b, and shows an A-type antiferromagnetic ground state. If we apply uniaxial compressive strain such that the in-plane lattice becomes square with lattice constant a, we find a ferromagnetic insulating state. This is in sharp contrast to DFT results using various functionals like PBE, PBE+U, and hybrid functionals like HSE, which all predict a half-metallic ferromagnetic behavior. Interestingly, applying uniaxial tensile strain, such that the in-plane lattice becomes square with the longer lattice constant b, an antiferromagnetic insulating state is observed. We trace back these results to the reduction in Jahn–Teller distortion in the case of compressive strain, favoring a ferromagnetic state. This reduction is absent in the tensile case, and the antiferromagnetic state therefore survives. Our study shows that it is the flavor of the strain (compressive or tensile) which is decisive for the magnitude of Jahn–Teller distortions and, hence, the magnetic state.</p

Electronic Structure of Oxide Interfaces:A Comparative Analysis of GdTiO3/SrTiO3 and LaAlO3/SrTiO3 Interfaces

Emergent phases in the two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides have attracted great attention in the past decade. We present ab-initio electronic structure calculations for the interface between a Mott insulator GdTiO3 (GTO) and a band insulator SrTiO3 (STO) and compare our results with those for the widely studied LaAlO3/SrTiO3 (LAO/STO) interface between two band insulators. Our GTO/STO results are in excellent agreement with experiments, but qualitatively different from LAO/STO. We find an interface carrier density of 0.5 e−/Ti, independent of GTO thickness in both superlattice and thin film geometries, in contrast to LAO/STO. The superlattice geometry in LAO/STO offers qualitatively the same result as in GTO/STO. On the other hand, for a thin film geometry, the interface carrier density builds up only beyond a threshold thickness of LAO. The positive charge at the vacuum surface that compensates the 2DEG at the interface also exhibits distinct behaviors in the two systems. The compensating positive charge at the exposed surface of GTO charge disproportionates due to correlation effect making the surface insulating as opposed to that in LAO which remains metallic within band theory and presumably becomes insulating due to surface disorder or surface reconstruction

Tuning Electronic and Optical Properties of 2D/3D Construction based on Hybrid Perovskites through Interfacial Charge Transfer: Towards Higher Efficiency Solar Cells

The 2D/3D construction of hybrid perovskite interfaces is gaining increasing attention due to their enhanced stability towards degradation without compromising the corresponding solar cell efficiency. Much of it is due to the interfacial charge transfer and its consequences on the electronic and optical response of the composite system, which are instrumental in the context of stability and efficiency. In this work, we have considered a case study of an experimentally motivated 2D/3D interface constructed based on Ruddlesden-Popper phases of (A43)$_2$PbI$_4$ and (A43)$_2$MAPb$_2$I$_7$ hybrid perovskites to envisage the unique tuning of electronic and optical properties through the associated charge transfer. The corresponding tuning of the band gap is seen to be related to a unique charge transfer process between the 2D and 3D counterparts of the interface mediated from valence to conduction band edges of the composite. We have found that the optical absorption spectra can also be tuned by the construction of such a hetero-interface and the emergence of a unique two-peak step feature on the absorption edge, which is not present in either the 2D or 3D hybrid perovskites. Formation of the composite is found to increase the spectroscopic limited maximum efficiency for the use of these materials as solar cells from $\approx$ 24\% for individual components to $\approx$ 32\% for the composite hetero-structure

Topological transitions to Weyl states in bulk Bi2Se3:Effect of hydrostatic pressure and doping

Bi2Se3, a layered three-dimensional (3D) material, exhibits topological insulating properties due to the presence of surface states and a bandgap of 0.3 eV in the bulk. We study the effect of hydrostatic pressure P and doping with rare earth elements on the topological aspect of this material in bulk from a first principles perspective. Our study shows that under a moderate pressure of P . 7:9 GPa, the bulkelectronic properties show a transition from an insulating to a Weyl semi-metal state due to band inversion. This electronic topological transition may be correlated to a structural change from a layered van der Waals material to a 3D system observed at P ¼ 7:9 GPa. At large P, the density of states have a significant value at the Fermi energy. Intercalating Gd with a small doping fraction between Bi2Se3 layers drives the system to a metallic anti-ferromagnetic state, with Weyl nodes below the Fermi energy. At the Weyl nodes, time reversal symmetry is broken due to the finite local field induced by large magnetic moments on Gd atoms. However, substituting Bi with Gd induces anti-ferromagnetic order with an increased direct bandgap. Our study provides novel approaches to tune topological transitions, particularly in capturing the elusive Weyl semimetal states, in 3D topological materials

Half-metallic ferromagnetism in layered CdOHCl induced by hole doping

Next-generation spintronic devices will benefit from low-dimensionality, ferromagnetism, and half-metallicity, possibly controlled by electric fields. We find these technologically-appealing features to be combined with an exotic microscopic origin of magnetism in doped CdOHCl, a van der Waals material from which 2D layers may be exfoliated. By means of first principles simulations, we predict homogeneous hole-doping to give rise to $p$-band magnetism in both the bulk and monolayer phases and interpret our findings in terms of Stoner instability: as the Fermi level is tuned via hole-doping through singularities in the 2D-like density of states, ferromagnetism develops with large saturation magnetization of 1 $\mu_B$ per hole, leading to a half-metallic behaviour for layer carrier densities of the order of 10$^{14}$ cm$^{-2}$. Furthermore, we put forward electrostatic doping as an additional handle to induce magnetism in monolayers and bilayers of CdOHCl. Upon application of critical electric fields perpendicular to atomically-thin-films (as low as 0.2 V/$A{\deg}$ and 0.5 V/$A{\deg}$ in the bilayer and monolayer case, respectively), we envisage the emergence of a magnetic half-metallic state. The different behaviour of monolayer vs bilayer systems, as well as an observed asymmetric response to positive and negative electric fields in bilayers, are interpreted in terms of intrinsic polarity of CdOHCl atomic stacks, a distinctive feature of the material. In perspective, given the experimentally accessible magnitude of critical fields in bilayer of CdOHCl, one can envisage $p$ band magnetism to be exploited in miniaturized spintronic devices

The stability and redox mechanisms of Ni-rich NMC cathodes:Insights from first-principles many-body calculations

Ni-rich LiNi_aMn_bCo_cO_2 (NMC) cathodes undergo a series of degradation reactions, a prominent one being oxygen loss from the surface of the NMC particles, this process being more pronounced as Ni content is increased and at high voltages. Our first-principles study examines the redox behavior of transition metals and O in Ni-rich NMC cathodes as a function of (de)lithiation. We use ab initio multiple scattering, density-functional theory (DFT) based core-loss spectroscopy, and dynamical mean-field theory (DMFT) to give a many-body treatment of both dynamic and static correlations. Despite Ni, Mn, and Co K-edges calculated using ab initio multiple scattering based on Green's functions showing an excellent match with experimentally obtained X-ray absorption near-edge spectra (XANES), we demonstrate that the ionic model of ascribing shifts in the XANES spectra to changes in metal oxidation states is inappropriate. We show that in these cases, which are characterised by strong covalency between the transition metal and oxygen, DMFT calculations based on Wannier projections are essential to calculate charges and hence assign oxidation states accurately. Due to the corresponding charge transfer from O p to Ni d, a ligand hole forms on O in Ni-rich regions. The individual Ni charge remains fairly constant throughout the charging/discharging process, particularly in Ni-rich environments in the material. In contrast, O has dual redox behavior, showing greater involvement in redox in Ni-rich regions while showing negligible redox involvement in Ni-poor regions. The Ni-O covalent system starts participating in redox around a state of delithiation of ~17%, which represents, in our system, the beginning of charge. Contrary to previous DFT calculations, we show that Co oxidation does not occur at the very end of charge but rather starts at an earlier state of delithiation of ~67%. The dual behaviour of O in terms of participation in the redox process helps explain the overall higher relative stability of lower Ni content NMCs compared to Ni-rich NMCs or LiNiO_2 in terms of O loss and evolution of singlet oxygen

Tuning Electronic and Optical Properties of 2D/3D Construction based on Hybrid Perovskites through Interfacial Charge Transfer:Towards Higher Efficiency Solar Cells

The 2D/3D construction of hybrid perovskite interfaces is gaining increasing attention due to their enhanced stability towards degradation without compromising the corresponding solar cell efficiency. Much of it is due to the interfacial charge transfer and its consequences on the electronic and optical response of the composite system, which are instrumental in the context of stability and efficiency. In this work, we have considered a case study of an experimentally motivated 2D/3D interface constructed based on Ruddlesden-Popper phases of (A43)$_2$PbI$_4$ and (A43)$_2$MAPb$_2$I$_7$ hybrid perovskites to envisage the unique tuning of electronic and optical properties through the associated charge transfer. The corresponding tuning of the band gap is seen to be related to a unique charge transfer process between the 2D and 3D counterparts of the interface mediated from valence to conduction band edges of the composite. We have found that the optical absorption spectra can also be tuned by the construction of such a hetero-interface and the emergence of a unique two-peak step feature on the absorption edge, which is not present in either the 2D or 3D hybrid perovskites. Formation of the composite is found to increase the spectroscopic limited maximum efficiency for the use of these materials as solar cells from $\approx$ 24\% for individual components to $\approx$ 32\% for the composite hetero-structure

Importance of electronic correlations in exploring the exotic phase diagram of layered LixMnO2

Using ab initio dynamical mean-field theory we explore the electronic and magnetic states of layered LixMnO2 as a function of x, the state-of-charge. Constructing real-space Wannier projections of Kohn-Sham orbitals based on the low-energy subspace of Mn 3d states and solving a multi-impurity problem, our approach focuses on local correlations at Mn sites. The antiferromagnetic insulating state in LiMnO2 has a moderate Néel temperature of TN=296K in agreement with experimental studies. Upon delithiation the system proceeds through a number of states: ferrimagnetic correlated metals at x=0.92, 0.83; multiple charge disproportionated ferromagnetic correlated metals with large quasiparticle peaks at x=0.67, 0.50, 0.33; ferromagnetic metals with small quasiparticle peaks at x=0.17, 0.08 and an antiferromagnetic insulator for the fully delithiated state, x=0.0. At moderate states of charge, x=0.67-0.33, a mix of +3/+4 formal oxidation states of Mn is observed, while the overall nominal oxidation of Mn state changes from +3 in LiMnO2 to +4 in MnO2. In all these cases the high-spin state emerges as the most likely state in our calculations considering the full d manifold of Mn based on the proximity of eg levels in energy to t2g. We observe a crossover from coherent to incoherent behavior on delithiation as function of state-of-charge.</p