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.

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.

Common microscopic origin of the phase transitions in Ta_(2)NiS_(5) and the excitonic insulator candidate Ta_(2)NiSe_(5)

The structural phase transition in Ta-NiSes has been envisioned as driven by the formation of an excitonic insulating phase.However,the role of structural and electronic instabilities on crystal symmetry breaking has yet to be disentangled.Meanwhile,the phase transition in its complementary material Ta_(2)NiS_(5)does not show any experimental hints of an excitonic insulating phase.We present a microscopic investigation of the electronic and phononic effects involved in the structural phase transition in Ta_(2)NiSe_(5)and Ta-Niss using extensive first-principles calculations.In both materials the crystal symmetries are broken by phonon instabilities,which in tum lead to changes in the electronic bandstructure also observed in the experiment.A total energy landscape analysis shows no tendency towards a purely electronic instability and we find that a sizeable lattice distortion is needed to open a bandgap.We conclude that an excitonic instability is not needed to explain the phase transition in both Ta_(2)NiSe_(5)and Ta_(2)NiS_(5).

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.