arXiv subjects

Sergei I. Simak

Publications and source records attributed to Sergei I. Simak.

11 recordsLinked to original sources

arXiv2025

Dynamic Screening Effects on Auger Recombination in Metal-Halide Perovskites

The performance of modern light-emitting technologies, from lasers to LEDs, is limited by nonradiative losses, with Auger recombination being the dominant channel at device-relevant carrier densities. Reliable modeling of this process is essential, yet conventional treatments neglect dynamic dielectric effects, limiting the predictive reliability at operating conditions. We develop a general framework that incorporates the frequency-dependent screened Coulomb interaction $W_{00}(\mathbf{q},ω)$, computed from low-scaling \textit{GW}, into both direct and phonon-assisted Auger amplitudes. Demonstrated on orthorhombic $γ$-CsPbI$_3$ (band gap $E_g\approx1.73$ eV) and $γ$-CsSnI$_3$ ($E_g\approx1.30$ eV), the approach shows that dynamic screening enhances the dielectric response, lowering the room-temperature Auger coefficient by $\sim$50-60 %. This renormalization shifts the crossover between radiative and nonradiative recombination by nearly a factor of two in carrier density. Dynamic dielectric screening thus emerges as a quantitative determinant of Auger recombination, offering a transferable framework for predictive modeling across polar semiconductors where frequency-independent screening models are inadequate.

cond-mat.mtrl-sci↗

arXiv2025

Halogen Chains with One-Dimensional Semi-Metallic Electronic Structure and Peierls Physics in Polymorphs of Na4X5 (X = I, Br, Cl) Compounds

Since the pioneering works of Peierls, one-dimensional materials have attracted great attention. Still, the synthesis of truly monoatomic chains remains elusive. In this study, we explore a novel path of experimental synthesis of monoatomic one-dimensional chains by their chemical stabilization in ionic compounds. We demonstrate that in synthesized at high pressure sodium halides Na4X5 (X = I, Br, Cl) with hP18 Ga4Ti5-type structures, transfer of valence electrons from cations to anions leads to the formation of halogen chains connected with other atoms only by ionic interaction and having one-dimensional electronic structure. The Peierls physics in the systems is confirmed by theoretical calculations, newly synthesized incommensurately modulated i-hP18-Na4X5 (X = I, Br, Cl) compounds, as well as by the discovered hP36 phases of Na4Cl5 and Na4Br5.

cond-mat.mtrl-sci↗

arXiv2025

Machine Learning for Improved Density Functional Theory Thermodynamics

The predictive accuracy of density functional theory (DFT) for alloy formation enthalpies is often limited by intrinsic energy resolution errors, particularly in ternary phase stability calculations. In this work, we present a machine learning (ML) approach to systematically correct these errors, improving the reliability of first-principles predictions. A neural network model has been trained to predict the discrepancy between DFT-calculated and experimentally measured enthalpies for binary and ternary alloys and compounds. The model utilizes a structured feature set comprising elemental concentrations, atomic numbers, and interaction terms to capture key chemical and structural effects. By applying supervised learning and rigorous data curation we ensure a robust and physically meaningful correction. The model is implemented as a multi-layer perceptron (MLP) regressor with three hidden layers, optimized through leave-one-out cross-validation (LOOCV) and k-fold cross-validation to prevent overfitting. We illustrate the effectiveness of this method by applying it to the Al-Ni-Pd and Al-Ni-Ti systems, which are of interest for high-temperature applications in aerospace and protective coatings.

cond-mat.mtrl-sci↗

arXiv2023

Giant magnetocaloric effect in the (Mn,Fe)NiSi-system

The search for energy-efficient and environmentally friendly cooling technologies is a key driver for the development of magnetic refrigeration based on the magnetocaloric effect (MCE). This phenomenon arises from the interplay between magnetic and lattice degrees of freedom that is strong in certain materials, leading to a change in temperature upon application or removal of a magnetic field. Here we report on a new material, Mn$_{1-x}$Fe$_x$NiSi$_{0.95}$Al$_{0.05}$, with an exceptionally large isothermal entropy at room temperature. By combining experimental and theoretical methods we outline the microscopic mechanism behind the large MCE in this material. It is demonstrated that the competition between the Ni$_2$In-type hexagonal phase and the MnNiSi-type orthorhombic phase, that coexist in this system, combined with the distinctly different magnetic properties of these phases, is a key parameter for the functionality of this material for magnetic cooling.

cond-mat.mtrl-sci↗

arXiv2023

Exploring Magnetism of Lead-free Halide Double Perovskites: A High-Throughput First-Principles Study

We have performed a comprehensive, first-principles high-throughput study of the magnetic properties of halide double perovskites, $Cs_2BB^\prime Cl_6$, with magnetic ions occupying one or both B and B$^\prime$ sites. Our findings indicate a general tendency for these materials to exhibit antiferromagnetic ordering with low Néel temperatures. At the same time, we reveal a few potential candidates that predicted to be ferromagnetic with relatively high Curie temperatures. Achieving ferromagnetic coupling might be feasible via simultaneously alloying at B and B$^\prime$ sites with magnetic 3d and non-magnetic 5d ions. With this approach, we discover that $Cs_2HgCrCl_6$, $Cs_2AgNiCl_6$ and $Cs_2AuNiCl_6$ have high Curie temperatures relative to their peers, with the latter two exhibiting half metallic behaviour. Further, this study illuminates the underpinning mechanism of magnetic exchange interactions in halide double perovskites, enabling a deeper understanding of their magnetic behaviour. Our findings, especially the discovery of the compounds with robust half-metallic properties and high Curie temperatures holds promise for potential applications in the field of spintronics.

cond-mat.mtrl-sci↗

arXiv2023

Ab initio Determination of Phase Stabilities of Dynamically Disordered Solids: rotational C2 disorder in Li2C2

The temperature-induced orthorhombic to cubic phase transition in Li2C2 is a prototypical example of a solid to solid phase transformation between an ordered phase, which is well described within the phonon theory, and a dynamically disordered phase with rotating molecules, for which the standard phonon theory is not applicable. The transformation in Li2C2 happens from a phase with directionally ordered C2 dimers to a structure, where they are dynamically disorderd. We provide a description of this transition by employing ab initio molecular dynamics (AIMD) based stress-strain thermodynamic integration on a deformation path that connects the ordered and dynamically disordered phases. The free energy difference between the two phases is obtained. The entropy that stabilizes the dynamically disordered cubic phase is captured by the behavior of the stress on the deformation path. We also show that a combination of the stress-strain thermodynamic integration and machine learning force field methodologies appears as a promising path in studies of dynamically disordered materials.

cond-mat.mtrl-sci↗

arXiv2022

Electronic structure of the magnetic halide double perovskites Cs2(Ag,Na)FeCl6 from first-principles

A family of magnetic halide double perovskites (HDPs) have recently attracted attention due to their potential to broaden application areas of halide double perovskites into e.g. spintronics. Up to date the theoretical modelling of these systems have relied on primitive approximations to the density functional theory (DFT). In this paper, we study structural, electronic and magnetic properties of the Fe$^{3+}$-containing HDPs Cs$_2$AgFeCl$_6$ and Cs$_2$NaFeCl$_6$ using a combination of more advanced DFT-based methods, including DFT+U, hybrid-DFT and treatments of various magnetic states. We examine the effect of varying the effective Hubbard parameter, U$_{eff}$, in DFT+U and the mixing-parameter, $α$, in hybrid DFT on the electronic structure and structural properties. Our results reveal a set of localized Fe(d) states that are highly sensitive to these parameters. Cs$_2$AgFeCl$_6$ and Cs$_2$NaFeCl$_6$ are both antiferromagnets with Neél temperatures well below room temperature and are thus in their paramagnetic (PM) state at the external conditions relevant to most applications. Therefore, we have examined the effect of disordered magnetism on the electronic structure of these systems and find that while Cs$_2$NaFeCl$_6$ is largely unaffected, Cs$_2$AgFeCl$_6$ shows significant renormalization of its electronic band structure.

cond-mat.mtrl-sci↗

arXiv2019

Anharmonicity and Ultra-Low Thermal Conductivity in Lead-Free Halide Double Perovskites

The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to toxicity of Pb and long-term stability of the lead-containing halide perovskites. We present a first-principles study of the lattice vibrations in Cs$_2$AgBiBr$_6$ , the prototypical compound in this class, and show that the lattice dynamics of Cs$_2$AgBiBr$_6$ is highly anharmonic, largely in regards to tilting of AgBr$_6$ and BiBr$_6$ octahedra. Using an energy and temperature dependent phonon spectral function, we then show how the experimentally observed cubic-to-tetragonal phase transformation is caused by the collapse of a soft phonon branch. We finally reveal that the softness and anharmonicity of Cs$_2$AgBiBr$_6$ yield an ultra-low thermal conductivity, unexpected of high symmetry cubic structures.

cond-mat.mtrl-sci↗

arXiv2018

A polaron cloud of correlated electron states in ceria

The electronic structure of cerium oxide is investigated here using a combination of ab initio one-electron theory and many-body physics, with emphasis on the nature of the 4f electron shell of cerium ions. We propose to use the hybridization function as a convenient measure for the degree of localization of the 4f shell of this material, and observe that changing the oxidation state is related to distinct changes in the hybridization between the 4f shell and ligand states. The theory reveals that CeO$_2$ has essentially itinerant 4f states, and that in the least oxidized form of ceria, Ce$_2$O$_3$, the 4f states are almost (but not fully) localized. Most importantly, our model points to that diffusion of oxygen vacancies in cerium oxide may be seen as polaron hopping, involving a correlated 4f electron cloud, which is located primarily on Ce ions of atomic shells surrounding the vacancy.

cond-mat.mtrl-sci↗

arXiv2016

Large effects of subtle electronic correlations on the energetics of vacancies in alpha-Fe

We study the effect of electronic Coulomb correlations on the vacancy formation energy in paramagnetic alpha-Fe within ab initio dynamical mean-field theory. The calculated value for the formation energy is substantially lower than in standard density-functional calculations and in excellent agreement with experiment. The reduction is caused by an enhancement of electronic correlations at the nearest neighbors of the vacancy. This effect is explained by subtle changes in the corresponding spectral function of the d-electrons. The local lattice relaxations around the vacancy are substantially increased by many-body effects.

cond-mat.str-el↗

arXiv2013

Temperature dependent effective potential method for accurate free energy calculations of solids

We have developed a thorough and accurate method of determining anharmonic free energies, the temperature dependent effective potential technique (TDEP). It is based on \emph{ab initio} molecular dynamics followed by a mapping onto a model Hamiltonian that describes the lattice dynamics. The formalism and the numerical aspects of the technique are described in details. A number of practical examples are given, and results are presented, which confirm the usefulness of TDEP within \emph{ab initio} and classical molecular dynamics frameworks. In particular, we examine from first-principles the behavior of force constants upon the dynamical stabilization of body centered phase of Zr, and show that they become more localized. We also calculate phase diagram for $^4$He modeled with the Aziz \emph{et al.} potential and obtain results which are in favorable agreement both with respect to experiment and established techniques.

cond-mat.stat-mech↗