Breathing Distortion Research Articles

Role of the coupling of the electronic transitions on the order of the metal-to-insulator phase transition in nickelates

Rare-earth nickelates (chemical formula RNiO3, R being a rare-earth cation) display a temperature-dependent metal-to-insulator transition (MIT) together with a breathing distortion of the NiO6 octahedra units at a temperature ranging from 0 to 600 K depending on the size of the R cation. Their rich phase diagram is also characterized by a paramagnetic to antiferromagnetic transition that occurs at the same temperature as the MIT for R = Pr, Nd, while it arises at lower temperatures for all the other members of the series. In this work, we have investigated the order of the MIT in a portion of the phase diagram spanning from SmNiO3 to NdNiO3 by means of temperature dependent transport measurements and resonant elastic x-ray scattering performed on high quality epitaxial SmxNd1−xNiO3 solid solution thin films. Our results show that the order of the metal-to-insulator transition does not depend on whether or not the MIT is coupled with the magnetic transition.

Influence of thickness scaling on the electronic structure and optical properties of oxygen deficient BaBiO3-δ thin films grown on SrTiO3-buffered Si(001) substrate

BaBiO3 has attracted a lot of research attention since it was discovered as the parent compound for the high-Tc superconducting BaPbxBi1–xO3 and Ba1–xKxBiO3. In its pure state, BaBiO3 is an insulator due to the presence of a breathing distortion of the BiO6 octahedra. The distortion is attributed to the valency of Bi in the compound being charge-ordered in the form of Bi3+ and Bi5+ along the lattice, resulting in alternating expanded or contracted BiO6 octahedra. The interaction between the electronic properties and the thickness of the thin film is crucial to study. We conducted a thorough study to investigate the effect of the thickness reduction of BaBiO3-δ grown on SrTiO3-buffered Si substrates on the optical properties as well as the Bi electronic structure of the thin films. We conclude that modifications in the valency of Bi in the ultra-thin film regime result in an optically conducting layer.

Designing orbital and charge ordering multiferroics by superlattice strategy and strain engineering

Orbital ordering (OO) or charge ordering (CO) in ${e}_{g}^{1}$ compounds like $R{\mathrm{MnO}}_{3}$ (R = rare-earth element), $R{\mathrm{NiO}}_{3}$, and ${\mathrm{CaFeO}}_{3}$ makes them attractive functional materials. Despite belonging to the ${e}_{g}^{1}$ family, ${\mathrm{SrFeO}}_{3}$ is metallic and free from OO and CO, which may limit its technological potential. Here, through first-principles calculations, we propose a rational design of OO and CO insulating ${\mathrm{SrFeO}}_{3}$ by superlattice strategy and strain engineering. With a systematical investigation of the interplay among Jahn-Teller distortion related to OO, breathing distortion associated with CO, strain, octahedral rotations, and cation ordering, we reveal that cation ordering in ${\mathrm{SrFeO}}_{3}/{\mathrm{ABO}}_{3}$ superlattices not only is capable to stabilize Jahn-Teller and breathing distortions, but is also indispensable in triggering metal-insulator transition by the collaboration with Jahn-Teller or breathing distortion, leading to ferromagnetic insulating ${\mathrm{SrFeO}}_{3}$. Owing to the different sensitivity of Jahn-Teller and breathing distortions to strain, purposeful design of OO or CO phase and even as multiferroics coexisting ferroelectric and ferromagnetic orders can be realized by tuning the relative stability of CO and OO phases by strain engineering. The coupled charge, orbital, spin, and ferroelectricity degrees of freedom make ${\mathrm{SrFeO}}_{3}/{\mathrm{ABO}}_{3}$ superlattices appealing for desired applications.

Cluster quantum Monte Carlo study of two-dimensional weakly coupled frustrated trimer antiferromagnets

We report results from spin trimer-based cluster quantum Monte Carlo simulations for the thermodynamic properties of two-dimensional frustrated quantum antiferromagnets that are composed of weakly-coupled three-spin (trimer) clusters. In particular, we consider the spin-1/2 kagome lattice with a strong breathing distortion, and the triangle-square lattice model proposed previously for the cuprate La${}_4$Cu${}_3$MoO${}_{12}$. For both cases, we demonstrate that an appropriately chosen trimer-based computational basis allows us to significantly reduce the quantum Monte Carlo sign problem down to the low-temperature regime. Besides exploring the thermodynamic behavior for the triangle-square lattice model we also assess a mean field theory-based prediction for the onset of chiral order. For the breathing distorted kagome lattice model, we observe a robust two-peak structure in the specific heat, both in the quantum spin liquid and the lattice-nematic regimes.

Octahedral rotation induced spin state and metal–insulator transition in LaCoO[formula omitted] films

The discussions over the origins of the magnetic and insulating properties in LaCoO3 films has continued unabated even now. We investigate the energy of natural low spin, homogeneous intermediate spin, and mixed spin configurations using first principle calculations, taking into account various distortions in bulk and films. The spin state transition can be controlled by rhombohedral distortion and octahedral rotations in films bulk. The octahedral rotation reserved in the films bulk, and it can also induce the breathing distortion or the Jahn–Teller distortion when increase the mismatch ratio. The breathing distortion or the Jahn–Teller distortion change the electronic structure from a metal to semiconductor. The octahedral rotations can be a efficient regulatory to control electronic structure and spin states. The constrain of mixed spin configuration (IS:LS=1:2) potentially cause a charge disproportionation, resulting in a quasi-breathing distortion similar to magnetoelastic effect.

Structural, magnetic and dielectric properties of Ni doped LiInCr4O8 breathing pyrochlore

Herein, polycrystalline compounds of breathing pyrochlore LiInCr4-xNixO8 (x = 0, 0.1, 0.2, 0.4) were synthesized by the solid state method. Structure, magnetism, and dielectric properties have been investigated with x-ray diffraction, Raman spectroscopy, magnetic susceptibility, isothermal magnetization, AC impedance, and heat capacity measurements. The mean particle size of the Ni substituted samples decreased from 1.95 μm to 1.22 μm, which is larger than the size determined by the Scherrer formula and the Williamson-Hall method. Raman scattering revealed slightly red-shifted modes Eg, F2g(2), F2g(3), and A1g, which were caused by higher effective atomic mass and increased bond strength. At low temperatures, field cooled and zero field cooled susceptibility measurements, as well as frequency dependent χac data, show a spin-glass type freezing. The resulting Mydosh parameter p of x = 0.4 is 0.01, suggesting the spin glass state. Magnetic measurements exhibit a large negative CW temperature (|θCW| > 290 K), initially decreasing and then increasing at high Ni concentration, in support of the relationship between magnetic interactions and the structural parameters of Cr–Cr spins. Despite the large negative θCW in substituted samples, long-range magnetic order was suppressed at high Ni contents. With increasing nickel concentration, the spin gap gradually vanished, indicating a significant reduction in breathing distortion. The isothermal magnetization at 2 K confirms antiferromagnetic characteristics and weak ferromagnetic components in the substituted samples. When the frequency is increased at room temperature, the dielectric data shows a drop in dielectric constant as well as lower dielectric loss. At low frequencies, interfacial polarization mechanisms play a crucial role. In contrast, at high frequencies, the dielectric constant becomes frequency independent up to ∼ 105 Hz, due to reverse electron motion. The low temperature specific heat behavior implies the good insulator characteristic of LiInCr3.6Ni0.4O8 and a residual entropy feature related to spin-glass type freezing.

Global Analysis of Heme Proteins Elucidates the Correlation between Heme Distortion and the Heme-Binding Pocket.

Heme proteins play diverse and important biological roles, from electron transfer and chemical catalysis to oxygen transport and/or storage. Although the distortion of heme porphyrin correlates with the physical properties of heme, such as the redox potential and oxygen affinity, the relationship between heme distortion and the heme protein environment is unclear. Here, we tested the hypothesis that the protein environment of the heme-binding pocket determines heme distortion (conformation). We analyzed the correlations between the amino acid composition of the heme-binding pocket and the magnitude of heme distortion along 12 vibrational modes using machine learning. A correlation was detected in the three lowest vibrational modes. Analysis of heme distortions in nearly the same environments of the heme-binding pocket supported this notion. Our analyses indicate that the heme-binding pocket environment is a major factor impacting the distortion of heme porphyrin along the three lowest vibrational modes. In addition, statistical analysis of the distortion of heme porphyrin revealed that the peaks of distributions of the ruffling and breathing distortions are shifted from 0 (the equilibrium structure). Both the ruffling and breathing distortions are correlated with the redox potential of heme, so that heme molecules with these distortions have a lower redox potential than planar molecules. These findings explain the structure-function relationship of heme.

Strength of the Hubbard potential and its modification by breathing distortion in BaBiO3

${\text{BaBiO}}_{3}$ compound is known as an archetype example of a three-dimensional Holstein model with the realization of the charge-density wave state at half filling and the superconducting state when doped. Although many works are devoted to the study the electron--phonon interaction in ${\text{BaBiO}}_{3}$, the influence of the electron--electron Hubbard interaction on the electronic structure in this system is still under investigation. In our work, we obtain analytical expression for the screened Coulomb potential, and along with the basis of ab initio-computed maximally localized Wannier orbitals, we quantitatively estimate the magnitude of the effective on-site Hubbard potential scrutinizing the effects of distortion of the crystal lattice. We show that a proper inclusion of the electron--electron interactions into the Holstein model significantly lowers the value of the underlying electron--phonon coupling. Finally, we find that the amplitudes of the repulsive electron--electron potential and its attractive counterpart mediated by the electron--phonon coupling are rather comparable. This may open a way for a realization of the intermediate phase of ${\text{BaBiO}}_{3}$ in terms of the Holstein-Hubbard model.

Octahedral rotations trigger electronic and magnetic transitions in strontium manganate under volume expansion

The perovskite structure of manganate yields a series of intriguing physical properties. Based on the results of first-principles calculations, strontium manganate appears to undergo a magnetic phase transition and a metal–insulator transition—from antiferromagnetic insulator to ferromagnetic metal and then to ferromagnetic insulator—under isotropic volume expansion combined with oxygen octahedral distortions. Interestingly, the results show that increasing the Mn–O bond length and adding rotation of the oxygen octahedra can soften the breathing distortion and account for the insulator phase. We further build a simple model to explain such transitions. Due to electron transfer and the favoring of a hole state of ligand p orbitals, the electron state transfer from to and then to . Such rearrangement of charges is responsible for the transitions of its magnetic order and electronic structure. Furthermore, we calculate spin susceptibility under the bare conditions and random phase approximation. The magnetic order of the intermediate metal state of itinerant electrons behaves as a ferromagnetic.

Two-dimensional superconductivity driven by interfacial electron-phonon coupling in a BaPbO3/BaBiO3 bilayer

The recent discovery of 2D superconductivity at the interface of BaPbO$_3$ (BPO) and BaBiO$_3$ (BBO) has motivated us to study in depth the electronic and structural properties and the relation between them in this particular heterostructure, by means of first-principles calculations. Our results indicate that the breathing distortions, the charge ordering and the semiconducting behaviour that characterize the parent compound BBO in its bulk form, are preserved at the innermost layers of the BBO side of the BPO/BBO bilayer. On the other hand, at the interface, there is a partial breaking of the breathing distortions with a concomitant charge transfer between the interfacial Bi ions and the on top BPO layer. We show that two types of carriers coexist at the interface, the delocalized 3D like sp states coming from Pb ions and the quasi 2D s states from the Bi ones. We obtain a substantial electron-phonon coupling between the 2D Bi states with the interfacial stretching phonon mode and a large density of states that can explain the critical temperature experimentally observed bellow 3.5 K. We hope these findings will motivate future research to explore different interfaces with charge ordered semiconductors as BBO in order to trigger this fascinating 2D behavior.

Realization of Electron Antidoping by Modulating the Breathing Distortion in BaBiO3

The recent proposal of antidoping scheme breaks new ground in conceiving conversely functional materials and devices; yet, the few available examples belong to the correlated electron systems. Here, we demonstrate both theoretically and experimentally that the main group oxide BaBiO3 is a model system for antidoping using oxygen vacancies. The first-principles calculations show that the band gap systematically increases due to the strongly enhanced Bi-O breathing distortions away from the vacancies and the annihilation of Bi 6s/O 2p hybridized conduction bands near the vacancies. Our further spectroscopic experiments confirm that the band gap increases systematically with electron doping, with a maximal gap enhancement of ∼75% when the film's stoichiometry is reduced to BaBiO2.75. These results unambiguously demonstrate the remarkable antidoping effect in a material without strong electron correlations and underscores the importance of bond disproportionation in realizing such an effect.

Are Superconductivity Mechanisms a Matter for Chemists?

From a tight-binding approach to the instability of nonbonding electronic states, along a double-well potential, we consider here how the coupling of these states with a phonon mode can open a superconducting gap at the Fermi level. The alternation of broken- and unbroken-symmetry states, along the phonon breathing distortion, induces the mixing of band-edge states on a very short timescale, according to the noncrossing rule of chemists. We show that this mixing may generate cationic and anionic disproportionation. The negative U mechanism is thus justified here, leading to the mixing of occupied and unoccupied pair states, for the opening of a 2Δ superconducting gap. The closeness of broad σ* and narrow π* bands in the vicinity of the Fermi level should favor the superconducting phase over the insulating or metallic state, in agreement with Micnas et al.’s studies. We applied this approach to several families of superconducting materials, i.e., doped strontium titanate, high-TC cuprates and iron selenide.

Fluctuation-frustrated flat band instabilities in NdNiO2

The discovery that Nd$_{1-x}$Sr$_x$NiO$_2$, with the CaCuO$_2$ infinite-layer structure, superconducts up to 15 K around the hole-doping level $x$=0.2 raises the crucial question of its fundamental electronic and magnetic processes. The unexplained basic feature that we address is that, for $x$=0 and as opposed to strongly antiferromagnetic (AFM) CaCuO$_2$, NdNiO$_2$ with the same structure and formal $d^9$ configuration does not undergo AFM order. We study this issue not in the conventional manner, as energetically unfavored or as frustrated magnetic order, but as an instability of the AFM phase itself. We are able to obtain the static AFM ordered state, but find that a flat-band, one-dimensional-like van Hove singularity (vHs) is pinned to the Fermi level. This situation is unusual in a non-half-filled, effectively two-band system. The vHs makes the AFM phase unstable to spin-density disproportionation, breathing and half-breathing lattice distortions, and (innate or parasitic) charge-density disproportionation. These flat-band instabilities, distant relatives of single band cuprate models, thereby inhibit but do not eliminate incipient AFM tendencies at low temperature. The primary feature is that a pair of active bands ($d_{x^2-y^2}$, $d_{z^2})$ eliminate half-filled physics and, due to instabilities, preclude the AFM phase seen in CaCuO$_2$. This strongly AFM correlated, conducting spin-liquid phase with strong participation of the Ni $d_{z^2}$ orbital, forms the platform for superconductivity in NdNiO$_2$.

Interplay between breathing and polar instabilities in transition metal perovskites with active A-sites

This paper identifies the microscopic origin of breathing versus polar distortions exhibited by ABO3 perovskites, containing active A sites like Bi or Pb. The authors show that that the condition by which the system exhibits breathing type distortion manifested as alternating compressed and expanded AO12 polyhedra or polar distortion manifested as off-centric movement of A cations is determined by the energy positioning of O-2p within the 6s-6p energy window of A site.

Unusual electronic state of Sn in AgSnSe2

${\mathrm{AgSnSe}}_{2}$, by formal electron count, should have Sn in a highly unusual $3+$ valence state and was therefore suggested to be a valence-skipping compound with potential for negative-$U$ centers and local electron pairing. It has been proposed that the latter may be the mechanism beyond seemingly conventional superconductivity in this compound. We report NMR measurements and first-principles calculations that agree with each other perfectly, and both indicate that valence skipping does not take place and the highly unusual ${\mathrm{Sn}}^{3+}$ state is realized instead, likely because of geometrical constraint prohibiting a breathing distortion that could screen the on-site Coulomb repulsion.

Structural and stoichiometric modifications in ultrathin epitaxial BaBiO3 films

${\mathrm{BaBiO}}_{3}$ (BBO) is well known as the parent material for the high-${T}_{c}$ superconducting compounds ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{BiO}}_{3}$ and ${\mathrm{BaPb}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}{\mathrm{O}}_{3}$. In its pristine state, BBO is a charge-ordered (CO) insulator, resulting from a static breathing distortion of the ${\mathrm{BiO}}_{6}$ octahedra with alternating long and short bond lengths. Recently, it has been reported that the CO state is suppressed for BBO films grown on ${\mathrm{SrTiO}}_{3}$ (STO) below a thickness of approximately 4 nm, possibly resulting in a metallic phase. While we do confirm structural modifications in our BBO/Nb:STO samples in this thickness range by Raman spectroscopy and electron diffraction, in situ photoemission evidences that these changes are accompanied by a Bi deficit and that the films remain insulating. We hence conclude that, in line with previous findings for the BBO/STO interface, the thickness-controlled suppression of the CO state is not purely driven by the two-dimensional confinement but rather originates from modifications of the composition and structure inherent to the epitaxial growth of BBO on ${\mathrm{SrTiO}}_{3}$(001).

Spin ordering induced by lattice distortions in classical Heisenberg antiferromagnets on the breathing pyrochlore lattice

We theoretically investigate effects of lattice distortions on the spin ordering of classical Heisenberg antiferromagnets on the breathing pyrochlore lattice. In the model, local lattice distortions originating from the site vibration are taken into account to yield effective spin interactions via the spin-lattice coupling (SLC). The breathing lattice-alternation is characterized by the ratio of the nearest-neighbor interaction for large tetrahedra to that for small ones, J'/J. It is found by Monte Carlo simulations that the system exhibits a first-order transition into four different types of collinear magnetic ordered states. In the uniform case (J'/J=1), the state realized at stronger SLC is cubic-symmetric characterized by the (1/2,1/2,1/2) magnetic Bragg peaks, while the one at weaker SLC is tetragonal-symmetric characterized by the (1,1,0) ones. With increasing the breathing distortion (decreasing J'/J), the ordering pattern of the (1/2,1/2,1/2) state becomes non-cubic with its magnetic Bragg reflections almost unchanged, while the (1,1,0) state is robust. The non-cubic state peculiar to the breathing pyrochlores is further categorized into two types. We demonstrate that these two non-cubic orderings result in the massive degeneracy of the ground state and that the associated residual entropy per spin takes characteristic values of k_B ln(4)/16 and k_B ln(6)/16. Experimental implications of the results are discussed.

Strain-engineered Peierls instability in layered perovskite La3Ni2O7 from first principles

Using first-principles calculations, we predict a strong coupling of lattice, charge, and spin degrees of freedom in Ruddlesden-Popper phase ${\mathrm{La}}_{3}{\mathrm{Ni}}_{2}{\mathrm{O}}_{7}$, which enables a phase control with epitaxial strain. While the bulk ground state is metallic, moderate compressive strain is found to trigger a Peierls transition to an insulating state in concurrence with a breathing distortion of ${\mathrm{NiO}}_{6}$ octahedra. The Peierls transition is microscopically interpreted as a band-gap opening arising from lifting and lowering of the Ni $d\ensuremath{-}{e}_{g}$ states due to the octahedral breathing.

Finite-temperature phase diagram of (111) nickelate bilayers

We report a density functional theory plus dynamical mean field theory (DFT+DMFT) study of an oxide heterostructure of LaNiO$_3$ (LNO) bilayers in [111] direction interleaved with four atomic monolayers of LaAlO$_3$. DFT+$U$ optimizations yield two stable solutions: a uniform structure with equivalent NiO$_6$ octahedra, as well as a bond-disproportionated (BD) structure featuring a breathing distortion. For both structures, we construct the low-energy models describing the Ni $e_g$ states by means of Wannier projections supplemented by the Kanamori interaction, and solve them by DMFT. Using the continuous-time quantum Monte Carlo algorithm in the hybridization expansion, we study the temperature range between 145 and 450 K. For the uniform and the BD structure, we find similar phase diagrams that comprise four phases: a ferromagnetic metal (FM), a paramagnetic metal (PM), an antiferro-orbitally-ordered insulator (AOI), as well as a paramagnetic insulator (PI). By calculating momentum-resolved spectral functions on a torus and a cylinder, we demonstrate that the FM phase is not a Dirac metal, while both insulating phases are topologically trivial. By a comparison with available experimental data and model DMFT studies for the two-orbital Hubbard model, we suggest that LNO bilayers are in the AOI phase at room temperature.

Magnetoelastic octahedral breathing mode in the ferrimagnetic La2CoIrO6 double perovskite

Distortion of transition-metal ion-oxygen octahedra in perovskite structure has been a long standing subject for exploring the physics of charge-orbital--spin-lattice couplings. Recent findings in nickelates or $\mathrm{BaBi}{\mathrm{O}}_{3}$ have suggested that a charge disproportionation in the transition-metal ions can evoke certain cooperative breathing distortions (CBD), constituted by the symmetric expansion and shrinkage of the octahedra in contrast to the Jahn-Teller-type asymmetric distortion. A double perovskite ferrimagnet, $\mathrm{L}{\mathrm{a}}_{2}\mathrm{CoIr}{\mathrm{O}}_{6}$ bears inherently the charge disproportionation in the cationic sites with $\mathrm{C}{\mathrm{o}}^{2+}$ and $\mathrm{I}{\mathrm{r}}^{4+}$ sublattices. In light of seeking the clues for the CBD and its coupling to the magnetic order, the crystal structure and magnetic structure of $\mathrm{L}{\mathrm{a}}_{2}\mathrm{CoIr}{\mathrm{O}}_{6}$ were examined at various temperatures by using neutron diffraction. Structure refinements and mode decomposition analyses revealed that, indeed, a CBD mode exists substantially in $\mathrm{L}{\mathrm{a}}_{2}\mathrm{CoIr}{\mathrm{O}}_{6}$ while preserving the symmetry of the lattices ($P{2}_{1}/n$) and the magnetism $[{\mathrm{\ensuremath{\Gamma}}}_{1}({A}_{g})]$. And more importantly, the magnitude of the symmetric distortion is correlated strongly with the size of the magnetic moment at temperatures below the Curie temperature (\ensuremath{\sim}95 K). This strongly suggests a magnetoelastic coupling of the CBD mode. The order parameters for CBD, e.g., the movement of O1 toward Ir ions at low temperatures, are found to be in a linear-quadratic relationship with the value of the magnetic moment ($\ensuremath{\sim}0.002\phantom{\rule{0.16em}{0ex}}\AA{}/{{\ensuremath{\mu}}_{\mathrm{B}}}^{2}{{M}_{\mathrm{Co}}}^{2}$), manifesting the magnetoelastic CBD in double perovskite.