arXiv subjects

George A. Sawatzky

Publications and source records attributed to George A. Sawatzky.

At least 19 recordsLinked to original sources

arXiv2026

Pressure induced redistribution of oxygen hole states in La$_{4}$Ni$_{3}$O$_{10}$

Using density functional calculations and multi-orbital, multi-atom cluster exact diagonalization that includes local exchange and Coulomb interactions, we explored the local low-energy electronic states of trilayer La$_4$Ni$_3$O$_{10}$ via a minimal Ni$_3$O$_{14}$ cluster. We find that, at ambient pressure, starting with all three Ni being nominally 2+ valence, one of the two extra holes is localized in the central NiO$_2$ layer forming a Zhang-Rice singlet (ZRS) with $d_{x^2-y^2}$ orbital. The other hole mainly occupies the antibonding combination of the two interplane apical O $p_z$ orbitals and thereby hybridizes with an out-of-plane three-spin-polaron (3SP) formed by the $d_{z^2}$ orbitals of three NiO$_2$ layers. At high pressure, however, the two extra holes are concentrated on one of two outer layers and the inner layer separately forming the ZRS with $d_{x^2-y^2}$ orbitals. We highlight the similarities between the bilayer La$_3$Ni$_2$O$_7$ and trilayer La$_4$Ni$_3$O$_{10}$ via speculated possible charge and spin configurations as well as the in-plane 3SP on two neighboring clusters suggested by our isolated cluster results.We thereby propose that the hole transfer from apical to in-plane oxygen orbitals of outer layer generates in-plane 3SP-like quasiparticles that act as mobile carriers coupled by interlayer superexchange; while the interplane 3SP-like states may provide the pairing glue. Since the low-pressure phase lacks freely propagating in-plane quasiparticles, this scenario naturally favors SC in the high-pressure phase.

cond-mat.str-el↗

arXiv2026

Disentangling the contributions of individual cations to magnetic order in a spinel high entropy oxide

High entropy oxides (HEOs) can possess long-range ordered magnetic states despite their extreme chemical disorder. Very little is known about how the different chemical constituents in HEOs contribute to the emergence of these magnetic states. In this work, we leverage element-specific magnetometry attained via x-ray magnetic circular dichroism (XMCD) to understand how magnetic order is driven in two ferrimagnetic spinel-structured HEOs with compositions (Cr,Mn,Fe,Co,Ni)$_3$O$_4$ and (Cr,Mn,Fe,Co,Ni)$_{2.4}$Ga$_{0.6}$O$_4$. We find that while the magnetic transition is simultaneous for all chemical species, the rate at which their magnetic moments grow is strongly cation dependent. This behavior is explained by the varying $\textit{3d}$ crystal field level fillings of the magnetic cations, which in turn determine their ability to participate in the different magnetic exchange pathways available in the spinel structure. Dominant $A$-$B$ sublattice exchange enables some species to harden rapidly ($\textit{e.g.}$ tetrahedral Fe$^{3+}$ and octahedral Ni$^{2+}$) while others exhibit a sluggish transition due to frustration from competing interactions ($\textit{e.g.}$ octahedral Fe$^{3+}$ and Cr$^{3+}$). Non-magnetic substitution suppresses these differences, introducing broken magnetic linkages that relieve frustration. Tailoring the magnetism of HEO spinels therefore requires detailed knowledge of both their site selectivities and their exchange pathways.

cond-mat.mtrl-sci↗

arXiv2026

Direct Fabrication of a Superconducting Two-Dimensional Electron Gas on KTaO3(111) via Mg-Induced Surface Reduction

Two-dimensional electron gases (2DEGs) at the surfaces of KTaO3 have become an exciting platform for exploring strong spin-orbit coupling, Rashba physics, and low-carrier-density superconductivity. Yet, a large fraction of reported KTaO3-based 2DEGs has been realized through chemically complex overlayers that both generate carriers and can obscure the native electronic structure, making spectroscopic access to the underlying 2DEG challenging. Here, we demonstrate a simple and direct method to generate a superconducting 2DEG on KTaO3(111) using Mg-induced surface reduction in molecular-beam epitaxy (MBE). Mg has an extremely low sticking coefficient at elevated temperatures, enabling the formation of an ultrathin (less than 1-2 monolayers) MgO layer that is transparent to soft x-ray photoemission spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES). This allows direct measurement of the surface chemistry and low-energy electronic structure of the pristine reduced surface without the need for a several-nanometer-thick capping layer. XPS shows clear reduction of Ta5+ to lower oxidation states, while ARPES reveals a parabolic Ta 5d conduction band with an approximately 150 meV bandwidth and additional subband features arising from quantum confinement. Transport measurements confirm a superconducting transition below 0.7 K. Together, these results demonstrate a chemically straightforward and controllable pathway for fabricating spectroscopically accessible superconducting 2DEGs on KTaO3(111), and provide a powerful new platform for investigating the mechanisms underlying orientation-dependent superconductivity in KTaO3-based oxide interfaces.

cond-mat.supr-con↗

arXiv2025

Charge distribution and magnetism in bilayer La$_3$Ni$_2$O$_7$: a hybrid functional study

An accurate understanding of the ground state electronic properties of La$_3$Ni$_2$O$_7$, a high-temperature superconductor under pressure, is key for unveiling the origin of its superconductivity. In this paper, we conduct a theoretical study of the electronic structure of the bilayer polymorph of La$_3$Ni$_2$O$_7$ using the hybrid functional approach, which is well suited to tackle the non-local correlation effects arising in this system from the molecular orbital splitting of the Ni $3d_{3z^2-r^2}$ states inside Ni-Ni dimers. Our calculations reveal that bilayer La$_3$Ni$_2$O$_7$ is a strongly correlated magnetic system with robust Ni spin moments. Spin moments on individual Ni sites take on unusually small values because of the electron delocalization over molecular orbitals involving multiple Ni and O sites. We further find that the magnetism of bilayer La$_3$Ni$_2$O$_7$ is intimately linked with charge distribution between different Ni and O orbitals. Two distinct regimes are identified in this regard. In one, molecular orbital physics drives the Ni $3d_{x^2-y^2}$ band towards half-filling, which is a well-established condition for unconventional high-temperature superconductivity upon hole doping in cuprates. In the other, the Ni $3d_{x^2-y^2}$ band is quarter-filled favouring spin- and charge-density wave states and Ni-O bond-disproportionation, which is consistent with several recent experimental claims. It is possible that superconductivity in La$_3$Ni$_2$O$_7$ occurs as a result of a pressure-induced transition between these two competing regimes. Since none of the low energy phases discovered in this study are metallic, non-stoichiometry would be required for superconductivity to occur.

cond-mat.supr-con↗

arXiv2025

Checkerboard-type Zhang-Rice States in Overdoped Cuprate Superconductors

Cuprate superconductors remain central to condensed matter physics due to their technological relevance and unconventional, incompletely understood electronic behavior. While the canonical phase diagram and low-energy models have been shaped largely by studies of underdoped and moderately doped cuprates, the overdoped regime has received comparatively limited attention.Here, we track the evolution of the electronic structure from optimal to heavy overdoping in La2-xSrxCuO4(LSCO) using broadband optical spectroscopy across x=0.15-0.60. The measured spectral changes--including the redistribution of Zhang-Rice-related spectral weigh--are in qualitative agreement with determinant quantum Monte Carlo simulations of the three-orbital Emery model, which together indicate a pronounced reconstruction of the electronic structure beyond hole concentrations x>0.2. Guided by these observations, we propose a spontaneous checkerboard-type Zhang-Rice electronic configuration that captures the coexistence of itinerant and localized carriers characteristic of the heavily overdoped state. Our results refine the doping-dependent Zhang-Rice-based framework for cuprates, illuminate how correlations persist deep into the overdoped regime, and provide new constraints on microscopic mechanisms of high-temperature superconductivity, with broader implications for correlated transition-metal oxides.

cond-mat.supr-con↗

arXiv2025

Percolative Pathway to Stripe Order in KTaO3-Based Superconductivity

The sensitivity of low dimensional superconductors to fluctuations gives rise to emergent behaviors beyond the conventional Bardeen Cooper Schrieffer framework. Anisotropy is one such manifestation, often linked to spatially modulated electronic states and unconventional pairing mechanisms. Pronounced in plane anisotropy recently reported at KTaO3 based oxide interfaces points to the emergence of a stripe order in superconducting phase, yet its microscopic origin and formation pathway remain unresolved. Here, we show that controlled interfacial disorder in MgO/KTaO3(111) heterostructures drives a percolative evolution from localized Cooper-pair islands to superconducting puddles and eventually to stripes. The extracted stripe width matches the spin precession length, suggesting a self organized modulation governed by spin orbit coupling and lattice-symmetry breaking. These findings identify disorder as both a tuning parameter and a diagnostic probe for emergent superconductivity in two dimensional quantum materials.

cond-mat.supr-con↗

arXiv2025

Magnetic Ni-N-Ni Centers in N-substituted NiO

To explore how anion substitution modifies the existing magnetism in strongly correlated oxides, we investigate local electronic states and magnetic ordering in nickel oxide (NiO) induced by substituting oxygen (O) with nitrogen (N). Each N introduces an additional N 2p hole and modifies the magnetic moment of a neighboring nickel (Ni) cation site, as the exchange interaction between this hole and the Ni eg electrons exceeds the Ni-O-Ni superexchange interaction. This leads to the formation of Ni-N-Ni centers consisting of five spins, without perturbing the antiferromagnetic NiO lattice. These centers are studied using density functional theory and confirmed through high-resolution spectroscopy on N-substituted NiO thin films grown by molecular beam epitaxy. This type of magnetic design may enable future advances in quantum technologies based on strongly correlated materials, such as quantum sensors and spin-based qubits.

cond-mat.str-el↗

arXiv2025

Spectral signatures of residual electron pairing in the extended-Hubbard-Su-Schrieffer-Heeger model

We study the electron addition spectrum of the one-dimensional extended Hubbard-Su-Schrieffer-Heeger (HSSH) model in the dilute limit using the density matrix renormalization group method. In addition to the expected renormalization to the band structure, we find that the electron-phonon (e-ph) interaction produces an anomalous spectral feature when electrons are added in the singlet channel but which is absent in the triplet channel. By comparing these results with those obtained from perturbation theory in the antiadiabatic limit, we demonstrate that this anomalous feature is a remnant of the strong electron-electron interaction mediated by the SSH coupling previously derived in the two-particle limit. By studying the evolution of this feature as a function of doping, we track the fate of this attraction to higher carrier concentrations and provide predictions for the spectral features to help guide future searches for strong e-ph mediated pairing.

cond-mat.str-el↗

arXiv2025

Intertwined charge and spin instability of La$_3$Ni$_2$O$_7$

Research on nickel-based superconductors has progressed from infinite-layer LaNiO$_2$ to finite-layer La$_{6}$Ni$_{5}$O$_{12}$, and most recently to the Ruddlesden-Popper phase La$_3$Ni$_2$O$_7$, which was found to exhibits onset of superconductivity at $\sim$80\,K under a pressure of $\sim$16\,GPa. Unlike the superconductivity mainly driven by the $d_{x^2-y^2}$ orbital in infinite-layer nickelates, the Ni-$d_{z^2}$ and O-2$p$ orbitals contribute significantly to the low energy states and potentially to the superconducting electron pairing mechanism of La$_3$Ni$_2$O$_7$. Employing density functional calculations and multi-orbital multi-atom cluster exact diagonalization including local exchange and Coulomb interactions, here we analyze the pressure dependent low-energy electronic states of the Ni$_2$O$_9$ cluster, relevant for the bilayer phase of La$_3$Ni$_2$O$_7$. The various possible spin states and the exchange and superexchange mechanisms of the Ni$_2$O$_9$ cluster are quantified via the involvement of the Ni-$3d_{3z^2-r^2}$ orbitals and the atomic Hund's rule exchange, the apical bridging O-$2p_z$ orbitals, and the orbitals involved in the formation of local Zhang-Rice singlet like states. We find that the leading configurations contributiong to the cluster ground-states both for nominal valence and also with local charge fluctuations, do not involve occupation of the apical oxygen, instead they favor formation of in-plane Zhang-Rice singlet like states between an O ligand hole and the Ni $3d_{z^2-y^2}$ orbital. We also highlight two possible charge and spin ordered states suggested by our cluster results, that are nearly degenerate at all relevant pressures within our modelling.

cond-mat.supr-con↗

arXiv2024

Critical Role of Disorder for Superconductivity in the Series of Epitaxial Ti(O,N) Films

Realizing experimental control of superconductivity is of paramount importance to advancing both basic research and technological applications. Disorder, generally existing in most superconductors, intricately interacts with Cooper pairs and also impacts the performance of quantum devices. In this paper, we report the study of a series of Ti(O,N) crystalline films prepared via molecular beam epitaxy (MBE). We discover that substituting nitrogen (N) for oxygen (O) in TiO, namely TiO(N), considerably increases the normal-state conductivity and the superconducting transition temperature Tc. The Tc of TiO(N) falling between those of TiO (about 0.5 K) and TiN (about 6 K) is contrary to their comparable Tc predicted by the Migdal Eliasberg theory. It is found that their resistivity vs temperature obeys the Mooij rule, known as the characteristic of metallic glasses. Density functional theory (DFT) calculations demonstrate that strong disorder severely scatters the Bloch electron waves at nonzero momenta, which consequently weakens electron-phonon coupling in TiO(N).

cond-mat.supr-con↗

arXiv2024

Negative charge transfer energy in correlated compounds

In correlated compounds containing cations in high formal oxidation states (assigned by assuming that anions attain full valence shells), the energy of ligand to cation charge transfer can become small or even negative. This yields compounds with a high degree of covalence and can lead to a self-doping of holes into the ligand states of the valence band. Such compounds are of particular topical interest, as highly studied perovskite oxides containing trivalent nickel or tetravalent iron are negative charge transfer systems, as are nickel-containing lithium ion battery cathode materials. In this report, we review the topic of negative charge transfer energy, with an emphasis on plots and diagrams as analysis tools, in the spirit of the celebrated Tanabe-Sugano diagrams which are the focus of this Special Topics Issue.

cond-mat.str-el↗

arXiv2024

Mixed-valence state in the dilute-impurity regime of La-substituted SmB$_6$

Homogeneous mixed-valence (MV) behaviour is one of the most intriguing phenomena of $f$-electron systems. Despite extensive efforts, a fundamental aspect which remains unsettled is the determination of the limiting cases for which MV emerges. Here we address this question for SmB$_6$, a prototypical MV system characterized by two nearly-degenerate Sm$^{2+}$ and Sm$^{3+}$ configurations. By combining angle resolved photoemission spectroscopy (ARPES) and x-ray absorption spectroscopy (XAS), we track the evolution of the mean Sm valence, $v_{Sm}$, in the Sm$_x$La$_{1-x}$B$_6$ series. Upon substitution of Sm ions with trivalent La, we observe a linear decrease of valence fluctuations to an almost complete suppression at $x$$\,$=$\,$0.2, with $v_{Sm}$$\,$$\sim$$\,$2; surprisingly, by further reducing $x$, a re-entrant increase of $v_{Sm}$ develops, approaching the value of $v_{imp}$$\,$$\sim$$\,$2.35 in the dilute-impurity limit. Such observation departs from a monotonic evolution of $v_{Sm}$ across the whole series, as well as from the expectation of its convergence to an integer value for $x$$\,$$\rightarrow$$\,$0. Our ARPES and XAS results, complemented by a phenomenological model, demonstrate an unconventional evolution of the MV character in the Sm$_x$La$_{1-x}$B$_6$ series, paving the way to further theoretical and experimental considerations on the concept of MV itself, and its influence on the macroscopic properties of rare-earth compounds in the dilute-to-intermediate impurity regime.

cond-mat.str-el↗

arXiv2023

Distinct electridelike nature of infinite-layer nickelates and the resulting theoretical challenges to calculate their electronic structure

We demonstrate in this paper that the recently discovered infinite-layer (IL) nickelates have much in common with a class of materials known as electrides. Oxide based electrides are compounds in which topotactic removal of loosely bound oxygens leaves behind voids with a landscape of attractive potentials for electrons. We show that this is also what happens in the IL nickelates, where one of the two electrons (per formula unit) freed during the topotactic synthesis is to a large degree located in the oxygen vacancy position, occupying partially a local $s$-symmetry interstitial orbital, rather than taking part alongside the other electon in converting Ni from 3+ to a full 1+ oxidation state. We demonstrate that the interstitial orbital in question, referred to by us as the zeronium $s$ or Z $s$ orbital, forms strong covalent bonds with neighboring Ni $3d_{3z^2-r^2}$ orbitals, which in turn facilitates the one-dimensional-like dispersion of the Ni $3d_{3z^2-r^2}$ band along the $c$-axis direction, leading also to a possible large out-of-plane coupling between Ni magnetic moments. This finding, reinforced by our electron localization function analysis, points to a fundamental distinction between the nickelates and the structurally analogous cuprates, may explain the absence of superconductivity in hydrogen-poor samples, and is certainly in agreement with the observed large $z$-polarized component in the Ni $L_3$-edge x-ray absorption spectra. In addition, by using DFT+U calculations as an illustration, we show that the electride-like nature of the IL nickelates is one of the main reasons for the theoretical difficulty in determining the much debated elusive Fermi surface of these novel superconductors and aslo in exploring the possibility of them becoming excitonic insulators at low temperatures.

cond-mat.supr-con↗

arXiv2023

Correlation Induced Magnetic Topological Phases in Mixed-Valence Compound SmB6

SmB6 is a mixed-valence compound with flat f-electron bands that have a propensity to magnetism. Here, using a realistic Gamma8 quartet model, we investigate the dynamical spin susceptibility and describe the in-gap collective mode observed in neutron scattering experiments. We show that as the Sm valence increases with pressure, the magnetic correlations enhance and SmB6 undergoes a first-order phase transition into a metallic antiferromagnetic state, whose symmetry depends on the model parameters. The magnetic orderings give rise to distinct band topologies: while the A-type order leads to an overlap between valence and conduction bands in the form of Dirac nodal lines, the G-type order has a negative indirect gap with weak Z2 indices. We also consider the spin polarized phase under a strong magnetic field, and find that it exhibits Weyl points as well as nodal lines close to the Fermi level. The magnetic phases show markedly different surface states and tunable bulk transport properties, with important implications for experiments. Our theory predicts that a magnetic order can be stabilized also by lifting the Gamma8 cubic symmetry, thus explaining the surface magnetism reported in SmB6.

cond-mat.str-el↗

arXiv2022

Entropy engineering and tunable magnetic order in the spinel high entropy oxide

Spinel oxides are an ideal setting to explore the interplay between configurational entropy, site selectivity, and magnetism in high entropy oxides. In this work we characterize the magnetic properties of the spinel (Cr,Mn,Fe,Co,Ni)$_3$O$_4$ and study the evolution of its magnetism as a function of non-magnetic gallium substitution. Across the range of compositions studied here, from 0% to 40% Ga, magnetic susceptibility and powder neutron diffraction measurements show that ferrimagnetic order is robust in the spinel HEO. However, we also find that the ferrimagnetic order is highly tunable, with the ordering temperature, saturated and sublattice moments, and magnetic hardness all varying significantly as a function of Ga concentration. Through x-ray absorption and magnetic circular dichroism, we are able to correlate this magnetic tunability with strong site selectivity between the various cations and the tetrahedral and octahedral sites in the spinel structure. In particular, we find that while Ni and Cr are largely unaffected by the substitution with Ga, the occupancies of Mn, Co, and Fe are each significantly redistributed. Ga substitution also requires an overall reduction in the transition metal valence, and this is entirely accommodated by Mn. Finally, we show that while site selectivity has an overall suppressing effect on the configurational entropy, over a certain range of compositions, Ga substitution yields a striking increase in the configurational entropy and may confer additional stabilization. Spinel oxides can be tuned seamlessly from the low-entropy to the high-entropy regime, making this an ideal platform for entropy engineering.

cond-mat.mtrl-sci↗

arXiv2022

Stabilization of singlet hole-doped state in infinite-layer nickelate superconductors

Motivated by the recent X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) experiments, we use a detailed impurity model to explore the nature of the parent compound and hole doped states of (La, Nd, Pr)NiO$_2$ by including the crystal field splitting, the Ni-$3d$ multiplet structure, and the hybridization between Ni-$3d$, O-$2p$, and Nd-$5d$ orbitals. For simplicity and stimulated by the recent electronic structure calculations, the latter are formally replaced with symmetric orbitals centered at the missing O sites in the Nd layer, forming a two-dimensional (2D) band strongly hybridizing with the Ni-$3d^9_{z^2}$ state. This hybridization pushes the main part of the $3d^9_{z^2}$ spectral function up in energy by several eV and stabilizes the singlet with considerable $d^9_{z^2}$ and other configurational components. For the parent compound, we find that states of Ni-$3d^9_{z^2}$ character spread over a large energy range in the spectra, and cannot and should not be represented by a single orbital energy, as suggested in other approximations. This is qualitatively consistent with the RIXS measurements showing a broad distribution of the Ni-$3d^9_{z^2}$ hole state, although the shape of the Ni-$3d^9_{z^2}$ related structure is much more complicated requiring reinterpretations of the RIXS data. For the hole-doped systems, we show that adding these additional ingredients can still result in the lowest-energy hole doped state having a singlet character.

cond-mat.str-el↗

arXiv2022

Modulation doping of the FeSe monolayer on SrTiO$_3$

The discovery of higher-temperature superconductivity in FeSe monolayers on SrTiO$_3$ (STO) substrates has sparked a surge of interest in the interface superconductivity. One point of the agreement reached to date is that modulation doping by impurities in the substrate is critical for the enhanced superconductivity. Remarkably, the universal doping of about 0.1 electrons per Fe, \textit{i.e.}, so-called ``magic'' doping, has been observed on a range of Ti oxide substrates, which concludes that there likely is some important interaction limiting the FeSe doping. Our study discovers that the polarization change at the interface Se because of the close proximity to the substrate from that in the free-standing FeSe film significantly amplifies the total potential difference at the interface above and beyond the work function difference for charge transfer. Additionally, the titanate substrate with a large number of free electrons basically serves as an ``infinite'' charge reservoir, which leads to the saturated FeSe doping with the complete removal of interface potential gradient. Our work has developed the theory for modulation doping in the Van der Waals materials/oxides heterostructure, providing a solution to the puzzle of ``magic'' doping in FeSe monolayers on titanates. The information also presents experimental pathways to accommodate a variable carrier density of FeSe monolayers via modulation doping.

cond-mat.supr-con↗

arXiv2022

Dressing due to correlations strongly reduces the effect of electron-phonon coupling

We investigate the difference between the coupling of a bare carrier to phonons versus the coupling of a correlations-dressed quasiparticle to phonons, and show that latter may be weak even if the former is strong. Specifically, we analyze the effect of the hole-phonon coupling on the dispersion of the quasiparticle that forms when a single hole is doped into a cuprate layer. To model this, we start from the three-band Emery model supplemented by the Peierls modulation of the $p$-$d$ and $p$-$p$ hoppings due to the motion of O ions. We then project onto the strongly correlated $U_{dd}\rightarrow \infty$ limit where charge fluctuations are frozen on the Cu site. The resulting effective Hamiltonian describes the motion of a doped hole on the O sublattice, and its interactions with Cu spins and O phonons. We show that even though the hole-phonon coupling is moderate to strong, it leads to only a very minor increase of the quasiparticle's effective mass as compared to its mass in the absence of coupling to phonons, consistent with a weak coupling to phonons of the correlations-dressed quasiparticle. We explain the reasons for this suppression, revealing why it is expected to happen in any systems with strong correlations.

cond-mat.str-el↗