Two-Layer High-Throughput:Effective Mass Calculations Including Warping

In this paper,we perform two-layer high-throughput calculations.In the first layer,which involves changing the crystal structure and/or chemical composition,we analyze selected Ⅲ-Ⅴ semiconductors,filled and unfilled skutterudites,as well as rock salt and layered chalcogenides.The second layer searches the full Brillouin zone(BZ)for critical points within 1.5 eV(1 eV=1.602176×10^(-19)J)of the Fermi level and characterizes those points by computing the effective masses.We introduce several methods to compute the effective masses from first principles and compare them to each other.Our approach also includes the calculation of the density-of-states effective masses for warped critical points,where traditional approaches fail to give consistent results due to an underlying non-analytic behavior of the critical point.We demonstrate the need to consider the band structure in its full complexity and the value of complementary approaches to compute the effective masses.We also provide computational evidence that warping occurs only in the presence of degeneracies.

An efficient and accurate framework for calculating lattice thermal conductivity of solids:AFLOW-AAPL Automatic Anharmonic Phonon Library

One of the most accurate approaches for calculating lattice thermal conductivity,κ_(l),is solving the Boltzmann transport equation starting from third-order anharmonic force constants.In addition to the underlying approximations of ab-initio parameterization,two main challenges are associated with this path:high computational costs and lack of automation in the frameworks using this methodology,which affect the discovery rate of novel materials with ad-hoc properties.Here,the Automatic Anharmonic Phonon Library(AAPL)is presented.It efficiently computes interatomic force constants by making effective use of crystal symmetry analysis,it solves the Boltzmann transport equation to obtain κ_(l),and allows a fully integrated operation with minimum user intervention,a rational addition to the current high-throughput accelerated materials development framework AFLOW.An“experiment vs.theory”study of the approach is shown,comparing accuracy and speed with respect to other available packages,and for materials characterized by strong electron localization and correlation.Combining AAPL with the pseudo-hybrid functional ACBN0 is possible to improve accuracy without increasing computational requirements.

Coordination corrected ab initio formation enthalpies

The correct calculation of formation enthalpy is one of the enablers of ab-initio computational materials design.For several classes of systems(e.g.oxides)standard density functional theory produces incorrect values.Here we propose the“coordination corrected enthalpies”method(CCE),based on the number of nearest neighbor cation–anion bonds,and also capable of correcting relative stability of polymorphs.CCE uses calculations employing the Perdew,Burke and Ernzerhof(PBE),local density approximation(LDA)and strongly constrained and appropriately normed(SCAN)exchange correlation functionals,in conjunction with a quasiharmonic Debye model to treat zero-point vibrational and thermal effects.The benchmark,performed on binary and ternary oxides(halides),shows very accurate room temperature results for all functionals,with the smallest mean absolute error of 27(24)meV/atom obtained with SCAN.The zero-point vibrational and thermal contributions to the formation enthalpies are small and with different signs—largely canceling each other.

Spin Hall effect in prototype Rashba ferroelectrics GeTe and SnTe

Ferroelectric Rashba semiconductors(FERSCs)have recently emerged as a promising class of spintronics materials.The peculiar coupling between spin and polar degrees of freedom responsible for several exceptional properties,including ferroelectric switching of Rashba spin texture,suggests that the electron’s spin could be controlled by using only electric fields.In this regard,recent experimental studies revealing charge-to-spin interconversion phenomena in two prototypical FERSCs,GeTe and SnTe,appear extremely relevant.Here,by employing density functional theory calculations,we investigate spin Hall effect(SHE)in these materials and show that it can be large either in ferroelectric or paraelectric structure.We further explore the compatibility between doping required for the practical realization of SHE in semiconductors and polar distortions which determine Rashba-related phenomena in FERSCs,but which could be suppressed by free charge carriers.

Discovery of higher-order topological insulators using the spin Hall conductivity as a topology signature

The discovery and realization of topological insulators,a phase of matter which hosts metallic boundary states when the ddimension insulating bulk is confined to(d−1)-dimensions,led to several potential applications.Recently,it was shown that protected topological states can manifest in(d−2)-dimensions,such as hinge and corner states for three-and two-dimensional systems,respectively.These nontrivial materials are named higher-order topological insulators(HOTIs).Here we show a connection between spin Hall effect and HOTIs using a combination of ab initio calculations and tight-binding modeling.The model demonstrates how a non-zero bulk midgap spin Hall conductivity(SHC)emerges within the HOTI phase.Following this,we performed high-throughput density functional theory calculations to find unknown HOTIs,using the SHC as a criterion.We calculated the SHC of 693 insulators resulting in seven stable two-dimensional HOTIs.Our work guides novel experimental and theoretical advances towards higher-order topological insulator realization and applications.

Long-range current-induced spin accumulation in chiral crystals

Chiral materials,similarly to human hands,have distinguishable right-handed and left-handed enantiomers which may behave differently in response to external stimuli.Here,we use for the first time an approach based on the density functional theory(DFT)+PAOFLOW calculations to quantitatively estimate the so-called collinear Rashba–Edelstein effect(REE)that generates spin accumulation parallel to charge current and can manifest as chirality-dependent charge-to-spin conversion in chiral crystals.Importantly,we reveal that the spin accumulation induced in the bulk by an electric current is intrinsically protected by the quasi-persistent spin helix arising from the crystal symmetries present in chiral systems with the Weyl spin–orbit coupling.In contrast to conventional REE,spin transport can be preserved over large distances,in agreement with the recent observations for some chiral materials.This allows,for example,the generation of spin currents from spin accumulation,opening novel routes for the design of solid-state spintronics devices.

Absorption and emission modulation in a MoS2–GaN（0001） heterostructure by interface phonon–exciton coupling

Semiconductor heterostructures based on layered two-dimensional transition metal dichalcogenides(TMDs)interfaced to gallium nitride(Ga N)are excellent material systems to realize broadband light absorbers and emitters due to their close proximity in the lattice constants.The surface properties of a polar semiconductor such as Ga N are dominated by interface phonons,and thus the optical properties of the vertical heterostructure are influenced by the coupling of these carriers with phonons.The activation of different Raman modes in the heterostructure caused by the coupling between interfacial phonons and optically generated carriers in a monolayer MoS_2–Ga N(0001)heterostructure is observed.Different excitonic states in MoS_2 are close to the interband energy state of intraband defect state of Ga N.Density functional theory(DFT)calculations are performed to determine the band alignment of the interface and revealed a type-I heterostructure.The close proximity of the energy levels and the excitonic states in the semiconductors and the coupling of the electronic states with phonons result in the modification of carrier relaxation rates.Modulation of the excitonic absorption states in MoS_2 is measured by transient optical pump-probe spectroscopy and the change in emission properties of both semiconductors is measured by steady-state photoluminescence(PL)emission spectroscopy.There is significant red-shift of the C excitonic band and faster dephasing of carriers in MoS_2.However,optical excitation at energy higher than the bandgap of both semiconductors slows down the dephasing of carriers and energy exchange at the interface.Enhanced and blue-shifted PL emission is observed in MoS_2.Ga N band-edge emission is reduced in intensity at room temperature due to increased phonon-induced scattering of carriers in the Ga N layer.Our results demonstrate the relevance of interface coupling between the semiconductors for the development of optical and electronic applications.