Importance of charge self-consistency in first-principles description of strongly correlated systems

First-principles approaches have been successful in solving many-body Hamiltonians for real materials to an extent when correlations are weak or moderate.As the electronic correlations become stronger often embedding methods based on first-principles approaches are used to better treat the correlations by solving a suitably chosen many-body Hamiltonian with a higher level theory.The success of such embedding theories,often referred to as second-principles,is commonly measured by the quality of self-energy E which is either a function of energy or momentum or both.However,E should,in principle,also modify the electronic eigenfunctions and thus change the real space charge distribution.While such practices are not prevalent,some works that use embedding techniques do take into account these effects.In such cases,choice of partitioning,of the parameters defining the correlated Hamiltonian,of double-counting corrections,and the adequacy of low-level Hamiltonian hosting the correlated subspace hinder a systematic and unambiguous understanding of such effects.Further,for a large variety of correlated systems,strong correlations are largely confined to the charge sector.Then an adequate non local low-order theory is important,and the high-order local correlations embedding contributes become redundant.Here we study the impact of charge self-consistency within two example cases,TiSez and CrBrs,and show how real space charge re-distribution due to correlation effects taken into.account within a first-principles Green's function-based many-body perturbative approach is key in driving qualitative changes to the final electronic structure of these materials.

Electron correlation effects on exchange interactions and spin excitations in 2D van der Waals materials

Despite serious effort,the nature of the magnetic interactions and the role of electron-correlation effects in magnetic twodimensional(2D)van der Waals materials remains elusive.Using CrI3 as a model system,we show that the calculated electronic structure including nonlocal electron correlations yields spin excitations consistent with inelastic neutron-scattering measurements.Remarkably,this approach identifies an unreported correlation-enhanced interlayer super-superexchange,which rotates the magnon Dirac lines off,and introduces a gap along the high-symmetryΓ-K-M path.This discovery provides a different perspective on the gap-opening mechanism observed in CrI3,which was previously associated with spin–orbit coupling through the Dzyaloshinskii–Moriya interaction or Kitaev interaction.Our observation elucidates the critical role of electron correlations on the spin ordering and spin dynamics in magnetic van der Waals materials and demonstrates the necessity of explicit treatment of electron correlations in the broad family of 2D magnetic materials.

Dodecagonal bilayer graphene quasicrystal and its approximants

Dodecagonal bilayer graphene quasicrystal has 12-fold rotational order but lacks translational symmetry which prevents the application of band theory.In this paper,we study the electronic and optical properties of graphene quasicrystal with large-scale tight-binding calculations involving more than ten million atoms.We propose a series of periodic approximants which reproduce accurately the properties of quasicrystal within a finite unit cell.By utilizing the band-unfolding method on the smallest approximant with only 2702 atoms,the effective band structure of graphene quasicrystal is derived.The features,such as the emergence of new Dirac points(especially the mirrored ones),the band gap at M point and the Fermi velocity are all in agreement with recent experiments.The properties of quasicrystal states are identified in the Landau level spectrum and optical excitations.Importantly,our results show that the lattice mismatch is the dominant factor determining the accuracy of layered approximants.The proposed approximants can be used directly for other layered materials in honeycomb lattice,and the design principles can be applied for any quasi-periodic incommensurate structures.

Dielectric tunability of magnetic properties in orthorhombic ferromagnetic monolayer CrSBr

Monolayer CrSBr is a recently discovered semiconducting spin-3/2 ferromagnet with a Curie temperature of around 146 K.In contrast to many other known 2D magnets,the orthorhombic lattice of CrSBr gives rise to spatial anisotropy of magnetic excitations within the 2D plane.Triaxial magnetic anisotropy and considerable magnetic dipolar interactions in CrSBr challenge its theoretical description in terms of spin Hamiltonians.Here,we employ a Green’s function formalism combined with first-principles calculations to study the magnetic properties of monolayer CrSBr in different regimes of surrounding dielectric screening.In the free-standing limit,the system is close to an easy-plane magnet,whose long-range ordering is partially suppressed.On the contrary,in the regime of large external screening,monolayer CrSBr behaves like an easy-axis ferromagnet with more stable magnetic ordering.Our findings suggest that anisotropic layered magnets form a potentially promising platform for studying the effects of substrate screening on magnetic ordering in 2D.

Coexisting charge density wave and ferromagnetic instabilities in monolayer InSe

Recently fabricated InSe monolayers exhibit remarkable characteristics that indicate the potential of this material to host a number of many-body phenomena.In this work,we systematically describe collective electronic effects in hole-doped InSe monolayers using advanced many-body techniques.To this end,we derive a realistic electronic-structure model from first principles that takes into account the most important characteristics of this material,including a flat band with prominent van Hove singularities in the electronic spectrum,strong electron–phonon coupling,and weakly screened long-ranged Coulomb interactions.We calculate the temperature-dependent phase diagram as a function of band filling and observe that this system is in a regime with coexisting charge density wave and ferromagnetic instabilities that are driven by strong electronic Coulomb correlations.This regime can be achieved at realistic doping levels and high enough temperatures,and can be verified experimentally.We find that the electron–phonon interaction does not play a crucial role in these effects,effectively suppressing the local Coulomb interaction without changing the qualitative physical picture.