arXiv subjects

Fernando P. Sabino

Publications and source records attributed to Fernando P. Sabino.

12 recordsLinked to original sources

arXiv2026

Alloy engineering of Magnetic phases in two-dimensional Chromium Trihalides

Two-dimensional magnetic materials offer unique opportunities for exploring low-dimensional spin phenomena and next-generation spintronic devices. Chromium trihalides CrX3 (X = Cl, Br, I) belong to an important family of these materials, where alloying opens pathways for tailoring their electronic, magnetic, optical properties, and thermodynamic stability. In this work, we present a density functional theory study of CrX3 compounds and their ternary alloys. Our results show that for the pure compounds, the ground state is ferromagnetic (FM), with the antiferromagnetic-zigzag (AFM-Z) and paramagnetic (PM) phases being close in energy. For these pure systems, the band gap variation among different magnetic phases does not exceed 0.16 eV, and the average magnetic moments on Cr atoms increase from Cl to Br to I. For the alloys, the FM state remains the lowest-energy configuration, but the energy difference towards the AFM-Z phase decreases for compounds with lower iodine concentration. The calculated band gaps reveal a pronounced bowing along the compositional edge connecting CrCl3 and CrI3. The Curie temperatures show a smooth variation across compositions, consistent with the nearly linear behavior of the magnetic exchange parameters. Based on the calculated mixing enthalpy and configurational entropy, the approximate Gibbs free energy indicates that alloy formation becomes thermodynamically favorable at finite temperatures, which is important to overcome the intrinsic experimental instability of these compounds.

cond-mat.mtrl-sci↗

arXiv2026

Upward band gap bowing and negative mixing enthalpy in multi-component cubic halide perovskite alloys

Physical properties intermediate between constituents of alloys can be achieved as downward convex positive bowing, upward concave negative bowing, or zero bowing. Such bowing effects are essential for band gap engineering in semiconductor alloys. Upward band gap bowing effects are rather rare, hindering the exploration on half of the available physical property space of alloys. Part of the this being a rare event is related to the need to stabilize an alloy with low mixing enthalpy, so it does not phase separate. In this paper we find via density functional theory that one can satisfy the simultaneous conditions of negative mixing enthalpy and upward band gap bowing in four-component ABX3 halide perovskite alloys in the cubic perovskite structure. Such perovskite alloys have the B-site occupied by a mixture of group IVB and IIB elements that have the IVB-s and IIB-s states in the valence bands and conduction bands, respectively, leaving the delocalized s states strongly repel each other. This s-s repulsion leads to the upward band gap bowing and negative mixing enthalpies simultaneously. Remarkably, we identify a perovskite alloy that has a band gap much larger than all its components. Analogous trends of upward band gap bowing and negative mixing enthalpy also appear in the corresponding three-component and two-component ABX3 halide perovskite alloys. These observations of upward band gap bowing and negative mixing enthalpy will significantly accelerate the design of stable upward band gap bowing alloys in a broad range of material families.

cond-mat.mtrl-sci↗

arXiv2025

Exchange field induced symmetry breaking in quantum hexaborides

Symmetry breaking (SB) has proven to be a powerful approach for describing quantum materials: strong correlation, mass renormalization, and complex phase transitions are among the phenomena that SB can capture, even when coupled to a mean-field-like theory. Traditionally, corrective schemes were required to account for these effects; however, SB has emerged as an alternative that can also successfully describe the intricate physics of quantum materials. Here, we explore spin SB on EuB6 and SmB6 and how its relation to the exchange field can determine onsite properties, depending on the type of symmetry breaking. Using spin-polarized Density Functional Theory (DFT) calculations with the r2SCAN functional, we systematically compare four magnetic configurations, one totally symmetric - non-magnetic (NM) configuration - and three with different types of symmetry breaking: ferromagnetic (FM), antiferromagnetic (AFM) and a paramagnetic (PM) configuration - modeled through a Special Quasirandom Structure (SQS) method - to capture local symmetry-breaking effects. Our results show that the PM configuration produces distinct magnetic environments for the rare-earth atoms, leading to different exchange fields. These, in turn, induce symmetry breaking in the electronic and magnetic properties of Eu and Sm. Those results provide an alternative explanation for the experimental results on both materials, EuB6 and SmB6, where X-ray Absorption Spectroscopy (XAS) and X-ray Absorption Near Edge Structure (XANES) measurements suggest the presence of multiple atomic environments, previously attributed to a mixed-valence configuration.

cond-mat.mtrl-sci↗

arXiv2023

Dramatic enhancement of visible-light absorption in TiO2 by adding Bi

TiO2 is a wide band-gap semiconductor that has been intensively investigated for photocatalysis and water-spiting. However, weak light absorption in the visible region of the spectrum poses stringent limitation to its practical application. Doping of TiO2 with N or transition-metal impurities has been explored to shift the onset of optical absorption to the visible region, yet with limited success. Based on hybrid density functional calculations, we propose adding Bi to TiO2, in the form of dilute Ti_(1-x)Bi_(x)O2 alloys, to efficiently shift the optical absorption to the visible region. Compared to N, Bi introduces an intermediate valence band that is significantly higher in the band gap, and leaves the conduction band almost unchanged, leading to a remarkable redshift in the absorption coefficient to cover almost all the visible-light spectrum. Comparing formation enthalpies, our results show that adding Bi costs significantly less energy than N in oxidizing conditions, and that Ti_(1-x)Bi_(x)O2 might make a much more efficient photocatalyst than TiO_(2-y)N_(y) for water splitting.

cond-mat.mtrl-sci↗

arXiv2022

Hole conductivity through a defect band in $\rm ZnGa_2O_4$

Semiconductors with wide band gap (3.0 eV), high dielectric constant (> 10), good thermal dissipation, and capable of $n$- and $p$-type doping are highly desirable for high-energy power electronic devices. Recent studies indicate that $\rm ZnGa_2O_4$ may be suitable for these applications, standing out as an alternative to $\rm Ga_2O_3$. The simple face centered cubic spinel structure of $\rm ZnGa_2O_4$ results in isotropic electronic and optical properties, in contrast to the large anisotropic properties of the $β$-monoclinic $\rm Ga_2O_3$. In addition, $\rm ZnGa_2O_4$ has shown, on average, better thermal dissipation and potential for $n$- and $p$-type conductivity. Here we use density functional theory and hybrid functional calculations to investigate the electronic, optical, and point defect properties of $\rm ZnGa_2O_4$, focusing on the possibility for $n$- and p-type conductivity. We find that the cation antisite $\rm Ga_{Zn}$ is the lowest energy donor defect that can lead to unintentional $n$-type conductivity. The stability of self-trapped holes (small hole polarons) and the high formation energy of acceptor defects make it difficult to achieve $p$-type conductivity. However, with excess of Zn, forming $\rm Zn_{(1+2x)}Ga_{2(1-x)}O_4$ alloys display an intermediate valence band, facilitating $p$-type conductivity. Due to the localized nature of this intermediate valence band, $p$-type conductivity by polaron hopping is expected, explaining the low mobility and low hole density observed in recent experiments.

cond-mat.mtrl-sci↗

arXiv2021

Intrinsic doping limitations in inorganic lead halide perovskites

Inorganic Halide perovskites (HP's) of the CsPbX3 (X=I, Br, Cl) type have reached prominence in photovoltaic solar cell efficiencies. Peculiarly, they have shown, however, an asymmetry in their ability to be doped by holes rather than by electrons. Indeed, both structural defect-induced doping as well as extrinsic impurity-induced doping strangely result in a unipolar doping (dominantly p-type) with low free carriers concentration. This raises the question whether such doping limitations presents just a temporary setback due to insufficient optimization of the doping process, or perhaps this represents an intrinsic, physically-mandated bottleneck. In this paper we study three fundamental Design Principles (DP's) for ideal doping, applying them via density functional doping theory to these HP's, thus identifying the violated DP that explains the doping limitations and asymmetry in these HP's. Here, the target DP are: (i) requires that the thermodynamic transition level induced by the dopants must ideally be energetically shallow both for donors (n-type) or acceptors (p-type); DP-(ii) requires that the 'Fermi level pinning energies' for electrons and holes (being the limiting values of the Fermi level before a structural defect that compensate the doping forms spontaneously) should ideally be located inside the conduction band for n-type doping and inside the valence band for p-type doping. DP-(iii) requires that the doping-induced equilibrium Fermi energy shifts towards the conduction band for n-type doping (shift towards the valence band, for p-type doping) to be sufficiently large. We find that, even though in HP's based on Br and Cl there are numerous shallow level dopants that satisfy DP-(i), in contrast DP-(ii) is satisfied only for holes and DP-(iii) fail for both holes and electrons, being the ultimate bottleneck for the n-type doping in Iodine HP's.

cond-mat.mtrl-sci↗

arXiv2019

Bismuth-doped Ga2O3 as candidate for p-type transparent conducting material

Gallium oxide (Ga2O3) is a wide-band-gap semiconductor promising for UV sensors and high power transistor applications, with Baliga's figure of merit that far exceeds those of GaN and SiC, second only to diamond. Engineering its band structure through alloying will broaden its range of applications. Using hybrid density functional calculations we study the effects on adding Bi to Ga2O3. While in III-V semiconductors, such as GaAs and InAs, Bi tend to substitute on the pnictide site, we find that in Ga2O3, Bi prefers to substitute on the Ga site, resulting in dilute (Ga_{1-x}Bi_{x})2O3 alloys with unique electronic structure properties. Adding a few percent of Bi reduces the band gap of Ga2O3 by introducing an intermediate valence band that is significantly higher in energy than the valence band of the host material. This intermediate valence band is composed mainly of Bi 6s and O 2p orbitals, and it is sufficiently high in energy to provide opportunity for p-type doping.

cond-mat.mtrl-sci↗

arXiv2019

Weak antilocalization in quasi-two-dimensional electronic states of epitaxial LuSb thin films

Observation of large non-saturating magnetoresistance in rare-earth monopnictides has raised enormous interest in understanding the role of its electronic structure. Here, by a combination of molecular-beam epitaxy, low-temperature transport, angle-resolved photoemssion spectroscopy, and hybrid density functional theory we have unveiled the bandstructure of LuSb, where electron-hole compensation is identified as a mechanism responsible for large magnetoresistance in this topologically trivial compound. In contrast to bulk single crystal analogues, quasi-two-dimensional behavior is observed in our thin films for both electron and holelike carriers, indicative of dimensional confinement of the electronic states. Introduction of defects through growth parameter tuning results in the appearance of quantum interference effects at low temperatures, which has allowed us to identify the dominant inelastic scattering processes and elucidate the role of spin-orbit coupling. Our findings open up new possibilities of band structure engineering and control of transport properties in rare-earth monopnictides via epitaxial synthesis.

cond-mat.mtrl-sci↗

arXiv2018

Enabling visible-light absorption and p-type doping in In2O3 by adding Bi

In2O3 is a prototype wide-band-gap semiconductor that exhibits metallic conductivity when highly doped with Sn while retaining a high degree of transparency to visible light. It is widely used as a transparent window/electrode in solar cells and LEDs. The functionality of In2O3 would be greatly extended if p-type conductivity could also be achieved. Using electronic structure calculations, we show that adding Bi to In2O3, in the form of dilute (In_{1-x}Bi_{x})_2O3} alloys, leads to a new valence band that is sufficiently higher in energy than the original O-2p valence band to allow for p-type doping. Moreover, the raised valence band in the (In_{1-x}Bi_{x})_2O3} dilute alloys leads to strong optical absorption in the visible spectrum, opening up for new applications such as a wide band gap absorber layer in tandem solar cells.

cond-mat.mtrl-sci↗

arXiv2018

Large Disparity Between Optical and Fundamental Band Gaps in Layered In2Se3

In$_2$Se$_3$ is a semiconductor material that can be stabilized in different crystal structures (at least one 3D and several 2D layered structures have been reported) with diverse electrical and optical properties. This feature has plagued its characterization over the years, with reported band gaps varying in an unacceptable range of 1 eV. Using first-principles calculations based on density functional theory and the HSE06 hybrid functional, we investigated the structural and electronic properties of four layered phases of In$_2$Se$_3$, addressing their relative stability and the nature of their fundamental band gaps, i.e., direct {\em versus} indirect. Our results show large disparities between fundamental and optical gaps. The absorption coefficients are found to be as high as that in direct-gap III-V semiconductors. The band alignment with respect to conventional semiconductors indicate a tendency to $n$-type conductivity, explaining recent experimental observations.

cond-mat.mtrl-sci↗

arXiv2018

A Comprehensive Study of $g$-Factors, Elastic, Structural and Electronic Properties of III-V Semiconductors using Hybrid-Density Functional Theory

Despite the large number of theoretical III-V semiconductor studies reported every year, our atomistic understanding is still limited. The limitations of the theoretical approaches to yield accurate structural and electronic properties on an equal footing, due to the unphysical self-interaction problem that affects mainly the band gap and spin-orbit splitting (SOC) in semiconductors and, in particular, III-V systems with similar magnitude of the band gap and SOC. In this work, we will report a consistent study of the structural and electronic properties of the III-V semiconductors employing the screening hybrid-DFT framework, fitting the $α$ parameters for 12 different III-V compounds, namely, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, in order to minimize the deviation between the theoretical and experimental values of the band gap and SOC. Structural relaxation effects were also included. Except for AlP, whose $α= 0.127$, we obtained $α$ values spreading from 0.209 to 0.343, deviating less than 0.1 from the universal value of 0.25. Our results for the lattice parameter and elastic constants indicate that the fitting of the $α$ does not affect those structural parameters when compared with the HSE06 functional, where $α= 0.25$. Our analysis of the band structure based on the $\textbf{k}{\cdot}\textbf{p}$ method shows that the effective masses are in agreement with the experimental values, which can be attributed to the simultaneous fitting of the band gap and SOC. Finally, we estimate the values of $g$-factors, extracted directly from band structure, which are close to experimental results indicating that the obtained band structure produced a realistic set of $\textbf{k}{\cdot}\textbf{p}$ parameters.

cond-mat.mtrl-sci↗

arXiv2016

Stability and accuracy control of $\mathbf{k \cdot p}$ parameters

The $\mathbf{k \cdot p}$ method is a successful approach to obtain band structure, optical and transport properties of semiconductors, and it depends on external parameters that are obtained either from experiments, tight binding or ab initio calculations. Despite the widespread use of the $\mathbf{k \cdot p}$ method, a systematic analysis of the stability and the accuracy of its parameters is not usual in the literature. In this work, we report a theoretical framework to determine the $\mathbf{k \cdot p}$ parameters from state-of-the-art hybrid density functional theory including spin-orbit coupling, providing a calculation where the gap and spin-orbit energy splitting are in agreement with the experimental values. The accuracy of the set of parameters is enhanced by fitting over several directions at once, minimizing the overall deviation from the original data. This strategy allows us to systematically evaluate the stability, preserving the accuracy of the parameters, providing a tool to determine optimal parameters for specific ranges around the $Γ$-point. To prove our concept, we investigate the zinc blende GaAs that shows results in excellent agreement with the most reliable data in the literature.

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