arXiv subjects

George Sawatzky

Publications and source records attributed to George Sawatzky.

6 recordsLinked to original sources

arXiv2025

Atomic and bond polarization causing strong screening of short-range Coulomb interactions and its effect in cuprate superconductors

We present a novel and efficient real space, semiclassical model of electric polarization with general applicability to any system in which screening plays an important role. This model includes the effects of both atomic and bond polarizabilities, the latter originating from the modification of local bond charge transfer energies induced by polarizing charges. The nonlinear interference of multiple polarization clouds and the emergence of local field effects are highlighted as key phenomena highly influencing the short-range screening of the Coulomb interaction. As a representative system to showcase this model, the screened interaction between doped holes in the CuO$_2$ planes of cuprate high-temperature superconductors is investigated. This leads to the emergence of striking direction-dependent short-range minima in their Coulomb repulsion, which can strongly reduce the need for retardation effects and allow for an enhancement of the attractive interaction resulting from the exchange of bosons between two electrons or holes. This in turn enhances T$_C$, shortens the Cooper pair coherence length and supports the materialization of the pseudogap phase anisotropy observed in many high-T$_C$ superconductors.

cond-mat.supr-con↗

arXiv2024

Light-element and purely charge-based topological materials

We examine a class of Hamiltonians characterized by interatomic, interorbital even-odd parity hybridization as a model for a family of topological insulators without the need for spin-orbit coupling. Non-trivial properties of these materials are exemplified by studying the topologically-protected edge states of s-p hybridized alkali and alkaline earth atoms in one and two-dimensional lattices. In 1D the topological features are analogous to the canonical Su-Schrieffer-Heeger model but, remarkably, occur in the absence of dimerization. Alkaline earth chains, with Be standing out due to its gap size and near particle-hole symmetry, are of particular experimental interest since their Fermi energy without doping lies directly at the level of topological edge stats. Similar physics is demonstrated to occur in a 2D honeycomb lattice system of s-p bonded atoms, where dispersive edge states emerge. Lighter elements are predicted using this model to host topological states in contrast to spin-orbit coupling-induced band inversion favoring heaving atoms.

cond-mat.mes-hall↗

arXiv2024

Computationally efficient method for calculating electron-phonon coupling for high-throughput superconductivity search

Using a computationally inexpensive frozen phonon approach we have developed a technique which can be used to screen large unit cell materials and systems for enhanced superconducting critical temperatures. The method requires only density functional theory (DFT) calculated electronic band structures of phonon modes corresponding to atomic displacements for various materials. We have applied this method to well known conventional superconductors including MgB$_2$, H$_{3}$S and other hydrides as examples.

cond-mat.supr-con↗

arXiv2022

Suppression of Peierls-like, nesting-based instabilities in solids

The understanding of lattice instabilities is of vast importance in material science. The famous example is the Peierls instability of one-dimensional metals and for strongly-nested Fermi surfaces in two and three dimensions. Through an analysis of H and Li chains in band theory, we find that the Bloch wave nature of the wavefunctions, if involving strong k-dependent hybridization of oppositeparity atomic states, strongly suppresses susceptibility peaks and associated instabilities and is thus essential to consider in searching for materials with strong responses to external perturbations.

cond-mat.other↗

arXiv2021

Electronic Structure of the Bond Disproportionated Bismuthate Ag$_2$BiO$_3$

We present a comprehensive study on the silver bismuthate Ag$_2$BiO$_3$, synthesized under high-pressure high-temperature conditions, which has been the subject of recent theoretical work on topologically complex electronic states. We present X-ray photoelectron spectroscopy results showing two different bismuth states, and X-ray absorption spectroscopy results on the oxygen $K$-edge showing holes in the oxygen bands. These results support a bond disproportionated state with holes on the oxygen atoms for Ag$_2$BiO$_3$. We estimate a band gap of $\sim$1.25~eV for Ag$_2$BiO$_3$ from optical conductivity measurements, which matches the band gap in density functional calculations of the electronic band structure in the non-symmorphic space group $Pnn2$, which supports two inequivalent Bi sites. In our band structure calculations the disproportionated Ag$_2$BiO$_3$ is expected to host Weyl nodal chains, one of which is located $\sim$0.5~eV below the Fermi level. Furthermore, we highlight similarities between Ag$_2$BiO$_3$ and the well-known disproportionated bismuthate BaBiO$_3$, including breathing phonon modes with similar energy. In both compounds hybridization of Bi-$6s$ and O-$2p$ atomic orbitals is important in shaping the band structure, but in contrast to the Ba-$5p$ in BaBiO$_3$, the Ag-$4d$ bands in Ag$_2$BiO$_3$ extend up to the Fermi level.

cond-mat.mtrl-sci↗

arXiv2016

Ground state oxygen holes and the metal-insulator transition in the negative charge transfer rare-earth nickelates

The metal-insulator transitions and the intriguing physical properties of rare-earth perovskite nickelates have attracted considerable attention in recent years. Nonetheless, a complete understanding of these materials remains elusive. Here, taking a NdNiO3 thin film as a representative example, we utilize a combination of x-ray absorption and resonant inelastic x-ray scattering (RIXS) spectroscopies to resolve important aspects of the complex electronic structure of the rare-earth nickelates. The unusual coexistence of bound and continuum excitations observed in the RIXS spectra provides strong evidence for the abundance of oxygen 2p holes in the ground state of these materials. Using cluster calculations and Anderson impurity model interpretation, we show that these distinct spectral signatures arise from a Ni 3d8 configuration along with holes in the oxygen 2p valence band, confirming suggestions that these materials do not obey a conventional positive charge-transfer picture, but instead exhibit a negative charge-transfer energy, in line with recent models interpreting the metal to insulator transition in terms of bond disproportionation.

cond-mat.str-el↗