Excitonic Instability in Ta2Pd3Te5 Monolayer

By systematic theoretical calculations,we reveal an excitonic insulator(EI)in the Ta_(2)Pd_(3)Te_(5)monolayer.The bulk Ta_(2)Pd_(3)Te_(5)is a van der Waals(vdW)layered compound,whereas the vdW layer can be obtained through exfoliation or molecular-beam epitaxy.First-principles calculations show that the monolayer is a nearly zero-gap semiconductor with the modified Becke–Johnson functional.Due to the same symmetry of the band-edge states,the two-dimensional polarization 2D would be finite as the band gap goes to zero,allowing for an EI state in the compound.Using the first-principles many-body perturbation theory,the GW plus Bethe–Salpeter equation calculation reveals that the exciton binding energy is larger than the single-particle band gap,indicating the excitonic instability.The computed phonon spectrum suggests that the monolayer is dynamically stable without lattice distortion.Our findings suggest that the Ta_(2)Pd_(3)Te_(5) monolayer is an excitonic insulator without structural distortion.

Intermediate polaronic charge transport in organic crystals from a many-body first-principles approach

Charge transport in organic molecular crystals (OMCs) is conventionally categorized into two limiting regimes − band transport,characterized by weak electron-phonon (e-ph) interactions,and charge hopping due to localized polarons formed by strong e-ph interactions.However,between these two limiting cases there is a less well understood intermediate regime where polarons are present but transport does not occur via hopping.Here we show a many-body first-principles approach that can accurately predict the carrier mobility in this intermediate regime and shed light on its microscopic origin.Our approach combines a finite-temperature cumulant method to describe strong e-ph interactions with Green-Kubo transport calculations.We apply this parameter-free framework to naphthalene crystal,demonstrating electron mobility predictions within a factor of 1.5−2 of experiment between 100 and 300 K.Our analysis reveals the formation of a broad polaron satellite peak in the electron spectral function and the failure of the Boltzmann equation in the intermediate regime.

Ab initio electron-defect interactions using Wannier functions

Computing electron–defect(e–d)interactions from first principles has remained impractical due to computational cost.Here we develop an interpolation scheme based on maximally localized Wannier functions(WFs)to efficiently compute e–d interaction matrix elements.The interpolated matrix elements can accurately reproduce those computed directly without interpolation and the approach can significantly speed up calculations of e–d relaxation times and defect-limited charge transport.We show example calculations of neutral vacancy defects in silicon and copper,for which we compute the e–d relaxation times on fine uniform and random Brillouin zone grids(and for copper,directly on the Fermi surface),as well as the defect-limited resistivity at low temperature.Our interpolation approach opens doors for atomistic calculations of charge carrier dynamics in the presence of defects.