Auxetic two-dimensional transition metal selenides and halides

Auxetic two-dimensional(2D)materials provide a promising platform for biomedicine,sensors,and many other applications at the nanoscale.In this work,utilizing a hypothesis-based data-driven approache,we identify multiple materials with remarkable in-plane auxetic behavior in a family of buckled monolayer 2D materials.These materials are transition metal selenides and transition metal halides with the stoichiometry MX(M=V,Cr,Mn,Fe,Co,Cu,Zn,Ag,and X=Se,Cl,Br,I).First-principles calculations reveal that the desirable auxetic behavior of these 2D compounds originates from the interplay between the buckled 2D structure and the weak metal-metal interaction determined by their electronic structures.We observe that the Poisson’s ratio is sensitive to magnetic order and the amount of uniaxial stress applied.A transition from positive Poisson’s ratio(PPR)to negative Poisson’s ratio(NPR)for a subgroup of MX compounds under large uniaxial stress is predicted.The work provides a guideline for the future design of 2D auxetic materials at the nanoscale.

Beyond the RPA and GW methods with adiabatic xc-kernels for accurate ground state and quasiparticle energies

We review the theory and application of adiabatic exchange–correlation(xc)-kernels for ab initio calculations of ground state energies and quasiparticle excitations within the frameworks of the adiabatic connection fluctuation dissipation theorem and Hedin’s equations,respectively.Various different xc-kernels,which are all rooted in the homogeneous electron gas,are introduced but hereafter we focus on the specific class of renormalized adiabatic kernels,in particular the rALDA and rAPBE.The kernels drastically improve the description of short-range correlations as compared to the random phase approximation(RPA),resulting in significantly better correlation energies.This effect greatly reduces the reliance on error cancellations,which is essential in RPA,and systematically improves covalent bond energies while preserving the good performance of the RPA for dispersive interactions.For quasiparticle energies,the xc-kernels account for vertex corrections that are missing in the GW self-energy.In this context,we show that the short-range correlations mainly correct the absolute band positions while the band gap is less affected in agreement with the known good performance of GW for the latter.The renormalized xc-kernels offer a rigorous extension of the RPA and GW methods with clear improvements in terms of accuracy at little extra computational cost.