arXiv subjects

Leeor Kronik

Publications and source records attributed to Leeor Kronik.

At least 37 records · Page 2Linked to original sources

arXiv2023

Effects of Quantum and Dielectric Confinement on the Emission of Cs-Pb-Br Composites

The halide perovskite CsPbBr$_3$ belongs to the Cs-Pb-Br material system, which features two additional thermodynamically stable ternary phases, Cs$_4$PbBr$_6$ and CsPb$_2$Br$_5$. The coexistence of these phases and their reportedly similar photoluminescence have resulted in a debate on the nature of the emission in these systems. Here, we combine optical and microscopic characterization with an effective mass, correlated electron-hole model of excitons in confined systems, to investigate the emission properties of the ternary phases in the Cs-Pb-Br system. We find that all Cs-Pb-Br phases exhibit green emission and the non-perovskite phases exhibit photoluminescence quantum yields orders of magnitude larger than CsPbBr$_3$. In particular, we measure blue- and red-shifted emission for the Cs- and Pb-rich phases, respectively, stemming from embedded CsPbBr$_3$ nanocrystals. Our model reveals that the difference in emission shift is caused by the combined effects of nanocrystal size and different band mismatch. Furthermore, we demonstrate the importance of including the dielectric mismatch in the calculation of the emission energy for Cs-Pb-Br composites. Our results explain the reportedly limited blue shift in CsPbBr$_3$@Cs$_4$PbBr$_6$ composites and rationalize some of its differences with CsPb$_2$Br$_5$.

cond-mat.mtrl-sci↗

arXiv2023

Transferable screened range-separated hybrid functionals for electronic and optical properties of van der Waals materials

The accurate description of electronic properties and optical absorption spectra is a long-standing challenge for density functional theory. Recently, the introduction of screened range-separated hybrid (SRSH) functionals for solid-state materials has allowed for the calculation of fundamental band gaps and optical absorption spectra that are in very good agreement with many-body perturbation theory. However, since solid-state SRSH functionals are typically tuned to reproduce the properties of bulk phases, their transferability to low-dimensional structures, which experience substantially different screening than in the bulk, remains an open question. In this work, we explore the transferability of SRSH functionals to several prototypical van der Waals materials, including transition-metal sulfides and selenides, indium selenide, black phosphorus, and hexagonal boron nitride. Considering the bulk and a monolayer of these materials as limiting cases, we show that the parameters of the SRSH functional can be determined systematically, using only the band-edge quasiparticle energies of these extremal structural phases as fitting targets. The resulting SRSH functionals can describe both electronic bandstructures and optical absorption spectra with accuracy comparable to more demanding ab initio many-body perturbation theory (GW and Bethe-Salpeter equation) approaches. Selected examples also demonstrate that the SRSH parameters, obtained from the bulk and monolayer reference structures, display good accuracy for bandstructures and optical spectra of bilayers, indicating a degree of transferability that is independent of the fitting procedure.

cond-mat.mtrl-sci↗

arXiv2023

Spontaneous Electric Polarization in Graphene Polytypes

A crystalline solid is a periodic sequence of identical cells, each containing one or more atoms. If the constituting unit cell is not centrosymmetric, charge may distribute unevenly between the atoms, resulting in internal electric polarization. This effect serves as the basis for numerous ferroelectric, piezoelectric, and pyroelectric phenomena. In nearly all polar materials, including multilayered van der Waals stacks that were recently found to exhibit interfacial polarization, inversion symmetry is broken by having two or more atomic species within the unit cell. Here, we show that even elemental crystals, consisting of one type of atom, and composed of non-polar centrosymmetric layers, exhibit electric polarization if arranged in an appropriate three-dimensional architecture. This concept is demonstrated here for inversion and mirror asymmetric mixed-stacking tetra-layer polytypes of non-polar graphene sheets. Furthermore, we find that the room temperature out-of-plane electric polarization increases with external electrostatic doping, rather than decreases owing to screening. Using first-principles calculations, as well as tight-binding modeling, we unveil the origin of polytype-induced polarization and its dependence on doping. Extension of this idea to graphene multilayers suggests that solely by lateral shifts of constituent monolayers one can obtain multiple meta-stable interlayer stacking sequences that may allow for even larger electrical polarization.

cond-mat.mtrl-sci↗

arXiv2023

Two-dimensional carrier gas at a polar interface without surface band gap states: A first-principles perspective

We present first-principles calculations of the interface between GaN and strained AlN, using a slab model in which polarization is compensated via surface fractional-charge pseudo-hydrogen atoms. We show that an interface two-dimensional carrier electron or hole gas emerges naturally in response to different compensating surface charges, but that this need not involve in-gap surface states.

cond-mat.mtrl-sci↗

arXiv2023

Range-Separated Hybrid Functional Pseudopotentials

Consistency between the exchange-correlation (xc) functional used during pseudopotential construction and planewave-based electronic structure calculations is important for an accurate and reliable description of the structure and properties of condensed-phase systems. In this work, we present a general scheme for constructing pseudopotentials with range-separated hybrid (RSH) xc functionals based on the solution of the all-electron radial integro-differential equation for a spherical atomic configuration. As proof-of-principle, we demonstrate pseudopotential construction with the PBE, PBE0, HSE, and sRSH (based on LC-$ω$PBE0) xc functionals for a select set of atoms, and then investigate the importance of pseudopotential consistency when computing band gaps, equilibrium lattice parameters, bulk moduli, and atomization energies of several solid-state systems. In doing so, we find that pseudopotential consistency errors (PSCE) tend to be systematic and can be as large as $1.4\%$ when computing these properties.

cond-mat.mtrl-sci↗

arXiv2022

Uncovering Multiple Intrinsic Chiral Phases in (PEA)2PbI4 halide Perovskites

Two-dimensional (2D) halide perovskites offer a unique platform to investigate the ground-state of materials possessing significant anharmonicity. In contrast to three-dimensional perovskites, their 2D counterparts offer substantially fewer degrees of freedom, resulting in multiple well-defined crystal structures. In this work, we thoroughly investigate the anharmonic ground-state of the benchmark (PEA)2PbI4 compound, using complementary information from low-temperature x-ray diffraction (XRD) and photoluminescence spectroscopy, supported by density functional theory (DFT) calculations. We extrapolate four crystallographic configurations from the low-temperature XRD. These configurations imply that the ground-state has an intrinsic disorder stemming from two coexisting chiral sub-lattices, each with a bi-oriented organic spacer molecule. We further show evidence that these chiral structures form unevenly populated ground-states, portraying uneven anharmonicity, where the state population may be tuned by surface effects. Our results uncover a disordered ground-state that may induce intrinsic grain boundaries, which cannot be ignored in practical applications.

cond-mat.mtrl-sci↗

arXiv2022

Nuclear cusps and singularities in the non-additive kinetic potential bi-functional from analytical inversion

The non-additive kinetic potential $v^{\text{NAD}}$ is a key quantity in density-functional theory (DFT) embedding methods, such as frozen density embedding theory and partition DFT. $v^{\text{NAD}}$ is a bi-functional of electron densities $ρ_{\rm B}$ and $ρ_{\rm tot} = ρ_{\rm A} + ρ_{\rm B}$. It can be evaluated using approximate kinetic-energy functionals, but accurate approximations are challenging. The behavior of $v^{\text{NAD}}$ in the vicinity of the nuclei has long been questioned, and singularities were seen in some approximate calculations. In this article, the existence of singularities in $v^{\text{NAD}}$ is analyzed analytically for various choices of $ρ_{\rm B}$ and $ρ_{\rm tot}$, using the nuclear cusp conditions for the density and Kohn-Sham potential. It is shown that no singularities arise from smoothly partitioned ground-state Kohn-Sham densities. We confirm this result by numerical calculations on diatomic test systems HeHe, HeLi$^+$, and H$_2$, using analytical inversion to obtain a numerically exact $v^{\rm NAD}$ for the local density approximation. We examine features of $v^{\rm NAD}$ which can be used for development and testing of approximations to $v^{\rm NAD}[ρ_{\rm B},ρ_{\rm tot}]$ and kinetic-energy functionals.

physics.chem-ph↗

arXiv2022

Cumulative Polarization Coexisting with Conductivity at Interfacial Ferroelectrics

Ferroelectricity in atomically thin bilayer structures has been recently predicted1 and measured[2-4] in two-dimensional (2D) materials with hexagonal non-centrosymmetric unit-cells. Interestingly, the crystal symmetry translates lateral shifts between parallel 2D layers to a change of sign in their out-of-plane electric polarization, a mechanism referred to as "Slide-Tronics"[4]. These observations, however, have been restricted to switching between only two polarization states under low charge carrier densities[5-12], strongly limiting the practical application of the revealed phenomena[13]. To overcome these issues, one needs to explore the nature of the polarization that arises in multi-layered van der Waals (vdW) stacks, how it is governed by intra- and inter-layer charge redistribution, and to which extent it survives the introduction of mobile charge carriers, all of which are presently unknown14. To explore these questions, we conduct surface potential measurements of parallel WSe2 and MoS2 multi-layers with aligned and anti-aligned configurations of the polar interfaces. We find evenly spaced, nearly decoupled potential steps, indicating highly confined interfacial electric fields, which provide means to design multi-state "ladder ferroelectrics". Furthermore, we find that the internal polarization remains significant upon electrostatic doping of a mobile charge carrier density as high as 1013 cm-2, with substantial in-plane conductivity. Using first-principles calculations based on density functional theory (DFT), we trace the extra charge redistribution in real and momentum space and identify an eventual doping-induced depolarization mechanism.

cond-mat.mtrl-sci↗

arXiv2022

An Optimally-Tuned Starting Point for Single-Shot $GW$ Calculations of Solids

The dependence of ab initio many-body perturbation theory within the $GW$ approximation on the eigensystem used in calculating quasiparticle corrections limits this method's predictive power. Here, we investigate the accuracy of the recently developed Wannier-localized optimally tuned screened range-separated hybrid (WOT-SRSH) functional as a generalized Kohn-Sham starting point for single-shot $GW$ ($G_0W_0$) calculations for a range of semiconductors and insulators. Comparison to calculations based on well-established functionals, namely PBE, PBE0, and HSE, as well as to self-consistent $GW$ schemes and to experiment, shows that band gaps computed via $G_0W_0$@WOT-SRSH have a level of precision and accuracy that is comparable to that of more advanced methods such as quasiparticle self-consistent $GW$ (QS$GW$) and eigenvalue self-consistent $GW$ (ev$GW$). We also find that $G_0W_0$@WOT-SRSH improves the description of states deeper in the valence band manifold. Finally, we show that $G_0W_0$@WOT-SRSH significantly reduces the sensitivity of computed band gaps to ambiguities in the underlying WOT-SRSH tuning procedure.

cond-mat.mtrl-sci↗

arXiv2021

Band gaps of crystalline solids from Wannier-localization based optimal tuning of a screened range-separated hybrid functional

Accurate prediction of fundamental band gaps of crystalline solid state systems entirely within density functional theory is a long standing challenge. Here, we present a simple and inexpensive method that achieves this by means of non-empirical optimal tuning of the parameters of a screened range-separated hybrid functional. The tuning involves the enforcement of an ansatz that generalizes the ionization potential theorem to the removal of an electron in an occupied state described by a localized Wannier function in a modestly sized supercell calculation. The method is benchmarked against experiment for a set of systems ranging from narrow band gap semiconductors to large band gap insulators, spanning a range of fundamental band gaps from 0.2 to 14.2 eV and is found to yield quantitative accuracy across the board, with a mean absolute error of $\sim$0.1 eV and a maximal error of $\sim$0.2 eV.

cond-mat.mtrl-sci↗

arXiv2021

Theory of Chirality Induced Spin Selectivity: Progress and Challenges

We provide a critical overview of the theory of the chirality-induced spin selectivity (CISS) effect, i.e., phenomena in which the chirality of molecular species imparts significant spin selectivity to various electron processes. Based on discussions in a recently held workshop, and further work published since, we review the status of CISS effects - in electron transmission, electron transport, and chemical reactions. For each, we provide a detailed discussion of the state-of-the-art in theoretical understanding and identify remaining challenges and research opportunities.

cond-mat.mtrl-sci↗

arXiv2019

Mechanical and Tribological Properties of Layered Materials Under High Pressure: Assessing the Importance of Many-Body Dispersion Effects

The importance of many-body dispersion effects in layered materials subjected to high external loads is evaluated. State-of-the-art many-body dispersion density functional theory calculations performed for graphite, hexagonal boron nitride, and their hetero-structures were used to fit the parameters of a classical registry-dependent interlayer potential. Using the latter, we performed extensive equilibrium molecular dynamics simulations and studied the mechanical response of homogeneous and heterogeneous bulk models under hydrostatic pressures up to 30 GPa. Comparison with experimental data demonstrates that the reliability of the many-body dispersion model extends deep into the sub-equilibrium regime. Friction simulations demonstrate the importance of many-body dispersion effects for the accurate description of the tribological properties of layered materials interfaces under high pressure.

cond-mat.mtrl-sci↗

arXiv2019

Breakdown of the static picture of defect energetics in halide perovskites: the case of the Br vacancy in CsPbBr3

We consider the Br vacancy in CsPbBr3 as a prototype for the impact of structural dynamics on defect energetics in halide perovskites (HaPs). Using first-principles molecular dynamics based on density functional theory, we find that the static picture of defect energetics breaks down; the energy of the Br vacancy level is found to be intrinsically dynamic, oscillating by as much as 1 eV on the ps time scale at room temperature. These significant energy fluctuations are correlated with the distance between the neighboring Pb atoms across the vacancy and with the electrostatic potential at these Pb atomic sites. We expect this unusually strong coupling of structural dynamics and defect energetics to bear important implications for both experimental and theoretical analysis of defect characteristics in HaPs. It may also hold significant ramifications for carrier transport and defect tolerance in this class of photovoltaic materials.

cond-mat.mtrl-sci↗

arXiv2018

Prediction of electronic couplings for molecular charge transfer using optimally-tuned range-separated hybrid functionals

Electronic coupling matrix elements are important to the theoretical description of electron transfer processes. However, they are notoriously difficult to obtain accurately from time- dependent density functional theory (TDDFT). Here, we use the HAB11 benchmark dataset of coupling matrix elements to assess whether TDDFT using optimally-tuned range-separated hybrid functionals, already known to be successful for the description of charge transfer excitation energies, also allows for an improved accuracy in the prediction of coupling matrix elements. We find that this approach outperforms all previous TDDFT calculations, based on semi-local, hybrid, or non-tuned range-separated hybrid functionals, with a remaining average deviation as low as ~12%. We discuss potential sources for the remaining error.

physics.chem-ph↗

arXiv2018

Charge transfer excitations from exact and approximate ensemble Kohn-Sham theory

By studying the lowest excitations of an exactly solvable one-dimensional molecular model, we show that components of Kohn-Sham ensembles can be used to describe charge transfers. Furthermore, we compute the approximate excitation energies obtained by using thee exact ensemble densities in the recently formulated ensemble Hartree-exchange theory [Gould and Pittalis, Phys. Rev. Lett. 119, 243001 (2017)]. Remarkably, our results show that triplet excitations are accurately reproduced across a dissociation curve in all cases tested, even in systems where ground state energies are poor due to strong static correlations. Singlet excitations exhibit larger deviations from exact results but are still reproduced semi-quantitatively.

physics.chem-ph↗

arXiv2018

The Effect of Ionic Composition on Acoustic Phonon Speeds in Hybrid Perovskites from Brillouin Spectroscopy and Density Functional Theory

Hybrid organic-inorganic perovskites (HOIPs) have recently emerged as highly promising solution-processable materials for photovoltaic (PV) and other optoelectronic devices. HOIPs represent a broad family of materials with properties highly tuneable by the ions that make up the perovskite structure as well as their multiple combinations. Interestingly, recent high-efficiency PV devices using HOIPs with substantially improved long-term stability have used combinations of different ionic compositions. The structural dynamics of these systems are unique for semiconducting materials and are currently argued to be central to HOIPs stability and charge-transport properties. Here, we studied the impact of ionic composition on phonon speeds of HOIPs from Brillouin spectroscopy experiments and density functional theory calculations for FAPbBr$_3$, MAPbBr$_3$, MAPbCl$_3$, and the mixed halide MAPbBr$_{1.25}$Cl$_{1.75}$. Our results show that the acoustic phonon speeds can be strongly modified by ionic composition, which we explain by analysing the lead-halide sublattice in detail. The vibrational properties of HOIPs are therefore tuneable by using targeted ionic compositions in the perovskite structure. This tuning can be rationalized with non-trivial effects, for example, considering the influence of the shape and dipole moment of organic cations. This has an important implication to further improvements in the stability and charge-transport properties of these systems.

cond-mat.mtrl-sci↗

arXiv2017

Energy Level Alignment at Molecule-Metal Interfaces from an Optimally-Tuned Range-Separated Hybrid Functional

The alignment of the frontier orbital energies of an adsorbed molecule with the substrate Fermi level at metal-organic interfaces is a fundamental observable of significant practical importance in nanoscience and beyond. Typical density functional theory calculations, especially those using local and semi-local functionals, often underestimate level alignment leading to inaccurate electronic structure and charge transport properties. In this work, we develop a new fully self-consistent predictive scheme to accurately compute level alignment at certain classes of complex heterogeneous molecule-metal interfaces based on optimally-tuned range-separated hybrid functionals. Starting from a highly accurate description of the gas-phase electronic structure, our method by construction captures important nonlocal surface polarization effects via tuning of the long-range screened exchange in a range-separated hybrid in a non-empirical and system-specific manner. We implement this functional in a plane-wave code and apply it to several physisorbed and chemisorbed molecule-metal interface systems. Our results are in quantitative agreement with experiments, both the level alignment and work function changes. Our approach constitutes a new practical scheme for accurate and efficient calculations of the electronic structure of molecule-metal interfaces.

cond-mat.mes-hall↗

arXiv2017

Local polar fluctuations in lead halide perovskite crystals

Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in both hybrid (CH$_3$NH$_3$PbBr$_3$) and all-inorganic (CsPbBr$_3$) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations show that head-to-head Cs motion coupled to Br face expansion, on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr$_3$.

cond-mat.mtrl-sci↗