arXiv subjects

Ilja Makkonen

Publications and source records attributed to Ilja Makkonen.

13 recordsLinked to original sources

arXiv2026

Challenges in predicting positron annihilation lifetimes in lead halide perovskites: correlation functionals and polymorphism

Halide perovskites have emerged in the last decade as a new important class of semiconductors for a variety of optoelectronic applications. A lot of previous studies were thus devoted to the characterisation of their point defects. Positron annihilation spectroscopy is a well recognized tool for probing vacancies in materials. Recent applications of this technique to APbX$_3$ halide perovskites are sparse, and the rare theoretical predictions of positron lifetimes in these materials, published in association with experiments, do not fully agree with each other. These works suggest that vacancies on the A site are not detected. In our theoretical study we focus on the role of the electron-positron correlation functional. We thoroughly revisit and compare several approximations when applied to methylammonium lead iodide (MAPbI$_3$) with or without vacancies, as well as inorganic perovskites CsPbI$_3$ and CsPbBr$_3$, in various phases. We show also the relationship between the size of the voids, through Voronoi volumes, and the calculated lifetimes. For the cubic phases we investigate in detail the role of polymorphism, including the distribution of vacancy formation energies and positron annihilation lifetimes. In our lifetimes calculations, apart from older and more recent semi-local approximations for the electron-positron correlation potential, we also consider the weighted density approximation (WDA), which is truly non-local and should better describe positron annihilation in regions with strong electronic density variations. We show that for this class of materials, and especially for cations vacancies, the influence of the chosen approximation is crucial, much stronger than in metals, alloys and conventional semiconductors. This influence may induce to reconsider the interpretation of experimentally determined lifetimes.

cond-mat.mtrl-sci↗

arXiv2026

Investigating the Electronic and Magnetic Properties of Na$_x$Fe$_{1/2}$Mn$_{1/2}$O$_2$ Cathode Materials with X-ray Compton Scattering

We discuss electronic and magnetic properties of Na$_x$Fe$_{1/2}$Mn$_{1/2}$O$_2$, a promising Na-ion battery cathode material. Using x-ray Compton scattering, SQUID magnetometry, and density-functional-theory based modeling, we probe how electrons and spins evolve during sodiation. By comparing Compton profiles of sodiated and desodiated samples, we show that oxygen 2$p$ orbitals drive the redox process, while transition-metal 3$d$ electrons become more delocalized, explaining the metallic phase at $x=2/3$. These profile differences define a quantitative descriptor for the sodiation range associated with improved conductivity. Electron holes on oxygen, reflected in oxygen magnetization, confirm the important role of oxygen in the electrochemical activity of the cathode.

cond-mat.mtrl-sci↗

arXiv2025

Defect analysis of the $β$- to $γ$-Ga$_{2}$O$_{3}$ phase transition

In this study, we investigate the ion-irradiation-induced phase transition in gallium oxide (Ga2O3) from the $β$ to the $γ$ phase, the role of defects during the transformation, and the quality of the resulting crystal structure. Using a multi-method analysis approach including X-ray diffraction (XRD), transmission electron microscopy (TEM), Rutherford backscattering spectrometry in channeling mode (RBS/c), Doppler broadening variable energy positron annihilation spectroscopy (DB-VEPAS) and variable energy positron annihilation lifetime spectroscopy (VEPALS) supported by density functional theory (DFT) calculations, we have characterized defects at all the relevant stages before, during, and after the phase transition. Reduction in backscattering yield was observed in RBS/c spectra after the transition to the $γ$ phase. This is corroborated by a significant decrease in the positron trapping center density due to generation of embedded vacancies intrinsic for the $γ$-Ga2O3 but too shallow in order to trap positrons. A comparison of the observed positron lifetime of $γ$-Ga2O3 with different theoretical models shows good agreement with the three-site $γ$ phase approach. A characteristic increase in the effective positron diffusion length and the positron lifetime at the transition point from $β$-Ga2O3 to $γ$-Ga2O3 enables visualization of the phase transition with positrons for the first time. Moreover, a subsequent reduction of these quantities with increasing irradiation fluence was observed, which we attribute to further evolution of the $γ$-Ga2O3 and changes in the gallium vacancy density as well as relative occupation in the crystal lattice.

cond-mat.mtrl-sci↗

arXiv2024

Band Offsets at β/γ-$\mathrm{Ga}_{2}\mathrm{O}_{3}$ Interface

Ultrawide bandgap semiconductor gallium oxide (Ga2O3) and its polymorphs have recently attracted increasing attention across physics, materials science, and electronics communities. In particular, the self-organized formation of the beta/gamma-Ga2O3 double polymorph structures was demonstrated recently [A. Azarov et al., Nat. Commun. 14, 4855 (2023)], paving the way for prospective applications of such structures in electronics. Consequently, determining the conduction band offset in such structures is crucial since it dictates the behavior of conduction electrons at the interface and, consequently, the potential functionality of such interfaces. Thus, in this work, we calculate the band offsets at the beta/gamma-Ga2O3 interface using density functional theory in correlation with the data provided by the experimental atomistic interface analysis. Specifically, to unravel the strain state of the beta/gamma-Ga2O3 interface, nanoscale strain maps were recorded using high-resolution transmission electron microscopy. In its turn, theoretically, lineup potential and vacuum alignment methods were used to analyze the band offsets, with and without strain, at the beta/gamma-Ga2O3 interface. Altogether, the collected results suggest that the band offsets between the beta and gamma phases are likely not exceeding a few hundred meV, remaining highly sensitive to the strain state at the interface. At this end, we conclude that even though the formation of a two-dimensional electron gas (2DEG) at the beta/gamma interface is theoretically possible, the gradual strain relaxation--if it occurs as a function of the distance from the interface--poses a significant challenge, as it may shift the 2DEG localization or even reduce the overall probability of its formation.

cond-mat.mtrl-sci↗

arXiv2024

Compton scattering study of strong orbital delocalization in a LiNiO$_2$ cathode

Cobalt is used in Li-ion batteries, but it is expensive and could be replaced by nickel to deliver better performance at a lower cost. With this motivation, we discuss how the character of redox orbitals of LiNiO$_2$ can be ascertained through x-ray Compton scattering measurements combined with parallel first-principles simulations. Our analysis reveals the nature of hole states in Li-doped NiO resulting from the hybridization of O 2$p$ and Ni 3$d$ orbitals. Our study also gives insight into the ferromagnetic ground state and provides a pathway toward the rational design of next-generation battery materials.

cond-mat.mtrl-sci↗

arXiv2023

Calculation of the energies of the multideterminant states of the nitrogen vacancy center in diamond with quantum Monte Carlo

Certain point defects in solids can efficiently be used as qubits for applications in quantum technology. They have spin states that are initializable, readable, robust, and can be manipulated optically. New theoretical methods are needed to find the best host materials and defect configurations. Most methods proposed so far rely either on cluster models or restrict the many-body treatment of the defects to a subspace of single-particle orbitals. We explore best practices and theory for the use of quantum Monte Carlo to predict the excitation spectra for spin defects, by using the negatively charged nitrogen vacancy (NV$^-$) center in diamond as a test system. Quantum Monte Carlo can be used to explicitly simulate electronic correlations with larger systems and sets of orbitals than previous methods due to favourable scaling with respect to system size and computing power. We consider different trial wave functions for variational and diffusion Monte Carlo methods, explore the nodal surface errors of the ground and excited state wave functions and study whether the variational principle holds for the excited states. We compute the vertical excitation energies in different simulation cell sizes and extrapolate to infinite system size, and include backflow corrections to the extrapolated energies. The final results for vertical excitation energies are found to overestimate the experimental estimates, but the triplet-to-triplet and singlet-to-singlet transitions are accurate against experiment. Finally, we list further developments for QMC needed to address the problem of accurately predicting structural and spin properties of the solid-state defects.

physics.comp-ph↗

arXiv2023

Quantum Monte Carlo study of Doppler broadening of positron annihilation radiation in semiconductors and insulators

Positron annihilation in solid state matter can be utilized to detect and identify open-volume defects. The momentum distribution of the annihilation radiation is an important observable in positron-based measurements, and can reveal information on the chemical surroundings of the defect sites. In this work we present a variational quantum Monte Carlo method for simulation of the momentum densities of annihilating electron-positron pairs in semiconductors and insulators. We study finite-size effects, effects of lattice vibrations, and different levels of trial wave functions. Small simulation cells and simple wave function forms are found to be sufficient for accurate calculations in simulation of pristine lattices, enabling cheap accumulation of results. We compare calculated predictions of the Doppler broadening of the 511-keV 2γ annihilation line of the in aluminium nitride and silicon against experimental data measured from reference samples. Our results achieve better agreement with experiments in the these materials than conventional state-of-the-art methods, and proves that direct modeling of the electron-positron correlations is important for a supporting theory of positron annihilation sprectroscopies

cond-mat.mtrl-sci↗

arXiv2020

Split Ga vacancies and the unusually strong anisotropy of positron annihilation spectra in $\boldsymbolβ$-Ga$_2$O$_3$

We report a systematic first principles study on positron annihilation parameters in the $β$-Ga$_2$O$_3$ lattice and Ga mono-vacancy defects complemented with orientation-dependent experiments of the Doppler broadening of the positron-electron annihilation. We find that both the $β$-Ga$_2$O$_3$ lattice and the considered defects exhibit unusually strong anisotropy in their Doppler broadening signals. This anisotropy is associated with low symmetry of the $β$-Ga$_2$O$_3$ crystal structure that leads to unusual kind of one-dimensional confinement of positrons even in the delocalized state in the lattice. In particular, the split Ga vacancies recently observed by scanning transmission electron microscopy produce unusually anisotropic positron annihilation signals. We show that in experiments, the positron annihilation signals in $β$-Ga$_2$O$_3$ samples seem to be often dominated by split Ga vacancies.

cond-mat.mtrl-sci↗

arXiv2016

Positron surface state as a spectroscopic probe for characterizing surfaces of topological insulator materials

Topological insulators are attracting considerable interest due to their potential for technological applications and as platforms for exploring wide-ranging fundamental science questions. In order to exploit, fine-tune, control and manipulate the topological surface states, spectroscopic tools which can effectively probe their properties are of key importance. Here, we demonstrate that positrons provide a sensitive probe for topological states, and that the associated annihilation spectrum provides a new technique for characterizing these states. Firm experimental evidence for the existence of a positron surface state near Bi$_2$Te$_2$Se with a binding energy of $E_b = 2.7 \pm 0.2 \, \text{eV}$ is presented, and is confirmed by first-principles calculations. Additionally, the simulations predict a significant signal originating from annihilation with the topological surface states and shows the feasibility to detect their spin-texture through the use of spin-polarized positron beams.

cond-mat.mtrl-sci↗

arXiv2013

Enhancement models of momentum densities of annihilating electron-positron pairs: the many-body picture of natural geminals

The correlated motion of a positron surrounded by electrons is a fundamental many-body problem. We approach this by modeling the momentum density of annihilating electron-positron pairs using the framework of reduced density matrices, natural orbitals and natural geminals (electron-positron pair wave functions) of the quantum theory of many-particle systems. We find that an expression based on the natural geminals provides an exact, unique and compact expression for the momentum density. The natural geminals can be used to define and determine enhancement factors for enhancement models going beyond the independent-particle model for a better understanding of results of positron annihilation experiments.

cond-mat.str-el↗

arXiv2013

Scaling in the correlation energies of two-dimensional artificial atoms

We find an unexpected scaling in the correlation energy of artificial atoms, i.e., harmonically confined two-dimensional quantum dots. The scaling relation is found through extensive numerical examinations including Hartree-Fock, variational quantum Monte Carlo, density-functional, and full configuration-interaction calculations. We show that the correlation energy, i.e., the true ground-state total energy subtracted by the Hartree-Fock total energy, follows a simple function of the Coulomb energy, confimenent strength and, the number of electrons. We find an analytic expression for this function, as well as for the correlation energy per particle and for the ratio between the correlation and total energies. Our tests for independent diffusion Monte Carlo and coupled-cluster results for quantum dots -- including open-shell data -- confirm the generality of the obtained scaling. As the scaling is also well applicable to $\gtrsim$ 100 electrons, our results give interesting prospects for the development of correlation functionals within density-functional theory.

cond-mat.mes-hall↗

arXiv2012

Exchange-correlation potentials for inhomogeneous electron systems in two dimensions from exact diagonalization: comparison with the local-spin-density approximation

We consider electronic exchange and correlation effects in density-functional calculations of two-dimensional systems. Starting from wave function calculations of total energies and electron densities of inhomogeneous model systems, we derive corresponding exchange-correlation potentials and energies. We compare these with predictions of the local-spin-density approximation and discuss its accuracy. Our data will be useful as reference data in testing, comparing and parametrizing exchange and correlation functionals for two-dimensional electronic systems.

cond-mat.mes-hall↗

arXiv2011

Identifying vacancy complexes in compound semiconductors with positron annihilation spectroscopy: a case study of InN

We present a comprehensive study of vacancy and vacancy-impurity complexes in InN combining positron annihilation spectroscopy and ab-initio calculations. Positron densities and annihilation characteristics of common vacancy-type defects are calculated using density functional theory and the feasibility of their experimental detection and distinction with positron annihilation methods is discussed. The computational results are compared to positron lifetime and conventional as well as coincidence Doppler broadening measurements of several representative InN samples. The particular dominant vacancy-type positron traps are identified and their characteristic positron lifetimes, Doppler ratio curves and lineshape parameters determined. We find that In vacancies and their complexes with N vacancies or impurities act as efficient positron traps, inducing distinct changes in the annihilation parameters compared to the InN lattice. Neutral or positively charged N vacancies and pure N vacancy complexes on the other hand do not trap positrons. The predominantly introduced positron trap in irradiated InN is identified as the isolated In vacancy, while in as-grown InN layers In vacancies do not occur isolated but complexed with one or more N vacancies. The number of N vacancies per In vacancy in these complexes is found to increase from the near surface region towards the layer-substrate interface.

cond-mat.mtrl-sci↗