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We use first-principles methods to study doped strong ferroelectrics (taking BaTiO 3 as a prototype). Here, we find a strong coupling between itinerant electrons and soft polar phonons in doped BaTiO 3 , contrary to Anderson/Blount’s weakly coupled electron mechanism for \"ferroelectric-like metals”. As a consequence, across a polar-to-centrosymmetric phase transition in doped BaTiO 3 , the total electron-phonon coupling is increased to about 0.6 around the critical concentration, which is sufficient to induce phonon-mediated superconductivity of about 2 K. Lowering the crystal symmetry of doped BaTiO 3 by imposing epitaxial strain can further increase the superconducting temperature via a sizable coupling between itinerant electrons and acoustic phonons. Our work demonstrates a viable approach to modulating electron-phonon coupling and inducing phonon-mediated superconductivity in doped strong ferroelectrics and potentially in polar metals. Our results also show that the weakly coupled electron mechanism for \"ferroelectric-like metals” is not necessarily present in doped strong ferroelectrics. Usually the coupling between polar phonons and itinerant electrons is weak in polar metals. Here, the authors show that in doped ferroelectrics (approximate polar metals), this coupling can be increased across the structural phase transition and as a result, phonon-mediated superconductivity emerges.

The discovery of unconventional superconductivity in hole doped NdNiO 2 , similar to CaCuO 2 , has received enormous attention. However, different from CaCuO 2 , R NiO 2 ( R  = Nd, La) has itinerant electrons in the rare-earth spacer layer. Previous studies show that the hybridization between Ni- d x 2 − y 2 and rare-earth- d orbitals is very weak and thus R NiO 2 is still a promising analog of CaCuO 2 . Here, we perform first-principles calculations to show that the hybridization between Ni- d x 2 − y 2 orbital and itinerant electrons in R NiO 2 is substantially stronger than previously thought. The dominant hybridization comes from an interstitial- s orbital rather than rare-earth- d orbitals, due to a large inter-cell hopping. Because of the hybridization, Ni local moment is screened by itinerant electrons and the critical U Ni for long-range magnetic ordering is increased. Our work shows that the electronic structure of R NiO 2 is distinct from CaCuO 2 , implying that the observed superconductivity in infinite-layer nickelates does not emerge from a doped Mott insulator. The discovery of superconductivity in doped NdNiO 2 has generated excitement due to similarities with cuprates. Here, the authors use first-principles calculations to show that different from cuprates, a hybridization between Ni d -orbitals and itinerant electrons in NdNiO 2 disfavours magnetism by screening Ni moment, as in Kondo systems.

Interaction-driven metal–insulator transitions or Mott transitions are widely observed in condensed matter systems. In multi-orbital systems, many-body physics is richer in which an orbital-selective metal–insulator transition is an intriguing and unique phenomenon. Here we use first-principles calculations to show that a magnetic transition (from paramagnetic to long-range magnetically ordered) can simultaneously induce an orbital-selective insulator–metal transition in rock-salt ordered double perovskite oxides A 2 BB ′O 6 , where B is a non-magnetic ion (Y 3+ and Sc 3+ ) and B ′ a magnetic ion with a d 3 electronic configuration (Ru 5+ and Os 5+ ). The orbital-selectivity originates from geometrical frustration of a face-centered-cubic lattice on which the magnetic ions B ′ reside. Including realistic structural distortions and spin-orbit interaction do not affect the transition. The predicted orbital-selective transition naturally explains the anomaly observed in the electric resistivity of Sr 2 YRuO 6 . Implications of other available experimental data are also discussed. This work shows that by exploiting geometrical frustration on non-bipartite lattices, new electronic/magnetic/orbital-coupled phase transitions can occur in correlated materials that are in the vicinity of metal–insulator phase boundary. Metal–insulator transitions: Magnetically-driven Mott transition in double perovskite oxides First-principle calculations shed new light on orbital-selective Mott transitions in magnetic perovskites, providing new insight and explaining existing data. A Mott transition is a metal–insulator transition whereby electric-field screening causes the potential felt by electrons to become strongly peaked, making the electrons localized. In multi-orbital systems an orbital-selective Mott transition can occur: electrons become localized on some orbitals but remain itinerant on the others. Hanghui Chen from New York University Shanghai in China uses first-principle calculations to show that a magnetic transition can induce an orbital-selective Mott transition in an ordered double perovskite oxide, in which the occurrence of long-range magnetic order makes electrons in one orbital metallic while leaving the others insulating. This is related to geometrical frustration in the magnetic lattice, and structural distortions and spin-orbit interactions do not affect the transition.

Intrinsic polar metals are rare, especially in oxides, because free electrons screen electric fields in a metal and eliminate the internal dipoles that are needed to break inversion symmetry. Here we use first-principles high-throughput structure screening to predict a new polar metal in bulk and thin film forms. After screening more than 1000 different crystal structures, we find that ordered BiPbTi2O6 can crystallize in three polar and metallic structures, which can be transformed between via pressure or strain. In a heterostructure of layered BiPbTi2O6 and PbTiO3, multiple states with different relative orientations of BiPbTi2O6 polar displacements, and PbTiO3 polarization, can be stabilized. At room temperature, the interfacial coupling enables electric fields to first switch PbTiO3 polarization and subsequently drive 180° change of BiPbTi2O6 polar displacements. At low temperatures, the heterostructure provides a tunable tunnelling barrier and might be used in multi-state memory devices.Polar metal oxides are not frequently observed, yet offer attractive properties for functional devices. Now, high-throughput structure screening of a thousand crystal structures reveals that BiPbTi2O6 can form both polar and metallic structures.

A charge order (CO) with a wavevector q ≃ 1 3 , 0 , 0 is observed in infinite-layer nickelates. Here we use first-principles calculations to demonstrate a charge-transfer-driven CO mechanism in infinite-layer nickelates, which leads to a characteristic Ni 1+ -Ni 2+ -Ni 1+ stripe state. For every three Ni atoms, due to the presence of near-Fermi-level conduction bands, Hubbard interaction on Ni- d orbitals transfers electrons on one Ni atom to conduction bands and leaves electrons on the other two Ni atoms to become more localized. We further derive a low-energy effective model to elucidate that the CO state arises from a delicate competition between Hubbard interaction on Ni- d orbitals and charge transfer energy between Ni- d orbitals and conduction bands. With physically reasonable parameters, q = 1 3 , 0 , 0 CO state is more stable than uniform paramagnetic state and usual checkerboard antiferromagnetic state. Our work highlights the multi-band nature of infinite-layer nickelates, which leads to some distinctive correlated properties that are not found in cuprates. Recent experiments reported charge order with a stripe pattern in parent compounds of infinite-layer nickelate superconductors. Chen et al. use first principles and effective model calculations to propose an electronic, charge-transfer-driven mechanism of the charge order.

Optimal materials to induce bulk photovoltaic effects should lack inversion symmetry and have an optical gap matching the energies of visible radiation. Ferroelectric perovskite oxides such as BaTiO 3 and PbTiO 3 exhibit substantial polarization and stability, but have the disadvantage of excessively large band gaps. We use both density functional theory and dynamical mean field theory calculations to design a new class of Mott multiferroics–double perovskite oxides A 2 VFeO 6 ( A  = Ba, Pb, etc). While neither perovskite A VO 3 nor A FeO 3 is ferroelectric, in the double perovskite A 2 VFeO 6 a ‘complete’ charge transfer from V to Fe leads to a non-bulk-like charge configuration–an empty V- d shell and a half-filled Fe- d shell, giving rise to a polarization comparable to that of ferroelectric A TiO 3 . Different from nonmagnetic A TiO 3 , the new double perovskite oxides have an antiferromagnetic ground state and around room temperatures, are paramagnetic Mott insulators. Most importantly, the V d 0 state significantly reduces the band gap of A 2 VFeO 6 , making it smaller than that of A TiO 3 and BiFeO 3 and rendering the new multiferroics a promising candidate to induce bulk photovoltaic effects.

The superior size and power scaling potential of ferroelectric-gated Mott transistors makes them promising building blocks for developing energy-efficient memory and logic applications in the post-Moore’s Law era. The close to metallic carrier density in the Mott channel, however, imposes the bottleneck for achieving substantial field effect modulation via a solid-state gate. Previous studies have focused on optimizing the thickness, charge mobility, and carrier density of single-layer correlated channels, which have only led to moderate resistance switching at room temperature. Here, we report a record high nonvolatile resistance switching ratio of 38,440% at 300 K in a prototype Mott transistor consisting of a ferroelectric PbZr 0.2 Ti 0.8 O 3 gate and an R NiO 3 ( R : rare earth)/La 0.67 Sr 0.33 MnO 3 composite channel. The ultrathin La 0.67 Sr 0.33 MnO 3 buffer layer not only tailors the carrier density profile in R NiO 3 through interfacial charge transfer, as corroborated by first-principles calculations, but also provides an extended screening layer that reduces the depolarization effect in the ferroelectric gate. Our study points to an effective material strategy for the functional design of complex oxide heterointerfaces that harnesses the competing roles of charge in field effect screening and ferroelectric depolarization effects. Ferroelectric transistors are promising building blocks for developing energy-efficient memory and logic applications. Here, the authors report a record high 300 K resistance on-off ratio achieved in ferroelectric-gated Mott transistors by exploiting a charge transfer layer to tailor the channel carrier density and mitigate the ferroelectric depolarization effect.

The discovery of high-temperature superconductivity in La 3 Ni 2 O 7 under pressure has drawn great attention. However, consensus has not been reached on its pairing symmetry in theory. By combining density-functional-theory (DFT), maximally-localized-Wannier-function, and linearized gap equation with random-phase-approximation, we find that the pairing symmetry of La 3 Ni 2 O 7 is d x y , if its DFT band structure is accurately reproduced by a downfolded bilayer two-orbital model. More importantly, we reveal that the pairing symmetry of La 3 Ni 2 O 7 sensitively depends on the crystal field splitting between two Ni- e g orbitals. A slight increase in Ni- e g crystal field splitting alters the pairing symmetry from d x y to s ± . Such a transition is associated with the change in inverse Fermi velocity and susceptibility, while the shape of Fermi surface remains almost unchanged. Our work highlights the sensitive dependence of pairing symmetry on low-energy electronic structures in multi-orbital superconductors, which calls for care in the downfolding procedure when one calculates their pairing symmetry. Recently, a discrepancy arose in predicting the pairing symmetry of high-temperature superconductor La 3 Ni 2 O 7 . Xia et al. find that a slight increase in Ni- e g crystal field splitting sensitively alters the pairing symmetry of La 3 Ni 2 O 7 from d -wave to s -wave.

The origin of insulating ferromagnetism in epitaxial LaCoO3 films under tensile strain remains elusive despite extensive research efforts are devoted. Surprisingly, the spin state of its Co ions, the main parameter of its ferromagnetism, is still to be determined. Here, the spin state in epitaxial LaCoO3 thin films is systematically investigated to clarify the mechanism of strain-induced ferromagnetism using element-specific X-ray absorption spectroscopy and dichroism. Combining with the configuration interaction cluster calculations, it is unambiguously demonstrated that Co3+ in LaCoO3 films under compressive strain (on LaAlO3 substrate) is practically a low-spin state, whereas Co3+ in LaCoO3 films under tensile strain (on SrTiO3 substrate) have mixed high-spin and low-spin states with a ratio close to 1:3. From the identification of this spin state ratio, it is inferred that the dark strips observed by high-resolution scanning transmission electron microscopy indicate the position of Co3+ high-spin state, i.e., an observation of a spin state disproportionation in tensile-strained LaCoO3 films. This consequently explains the nature of ferromagnetism in LaCoO3 films. The study highlights the importance of spin state degrees of freedom, along with thin-film strain engineering, in creating new physical properties that do not exist in bulk materials.