# A.H. RomeroWest Virginia University | WVU · Department of Physics

A.H. Romero

PhD Physics and Chemistry

## About

350

Publications

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Introduction

Additional affiliations

January 2013 - present

April 2012 - December 2012

January 2001 - December 2002

## Publications

Publications (350)

A two-dimensional material – Mg 2 B 4 C 2 , belonging to the family of the conventional superconductor MgB 2 , is theoretically predicted to exhibit superconductivity with critical temperature T c estimated in the 47–48 K range (predicted using the McMillian-Allen-Dynes formula) without any tuning of external parameters such as doping, strain, or s...

This investigation has studied the six most-used macroscopic models to compute hardness from the elastic constants, using an experimental database of 143 materials. "The Hardness Calculator" is proposed as a solution to estimate hardness in an easy, fast and confident manner. This study divides into two stages. The first approach, referred to as "T...

GaGeTe is a layered material composed of germanene and GaTe sublayers that has been recently predicted to be a basic Z2 topological semimetal. To date, only one polytype of GaGeTe is known with trigonal centrosymmetric structure (alpha phase, space group R-3m, No. 166). Here we show that as-grown samples of GaGeTe show traces of at least another po...

Antiferromagnetic manganese-based nitride antiperovskites, such as Mn$_3$NiN, hold a triangular frustrated magnetic ordering over their kagome lattice formed by the Mn atoms along the (111)-plane. As such, frustration imposes a non-trivial interplay between the symmetric and asymmetric magnetic interactions, which can only reach equilibrium in a no...

In recent years, superlattices and layered materials have been highlighted as potential candidates for thermoelectric applications, this thanks to their low thermal conductivity. Moreover, external applied pressure and biaxial strain can be used to enhance their properties by achieving a band engineering and electronic tuning. With this in mind, we...

Multiferroics are a unique class of materials where magnetic and ferroelectric orders coexist. The research on multiferroics contributes significantly to the fundamental understanding of the strong correlations between different material degrees of freedom and provides an energy‐efficient route toward the electrical control of magnetism. While mult...

Portable Raman instruments provide quick, nondestructive analysis of organic and inorganic compounds, making it widely applicable in various disciplines. However, the instrument’s accuracy when analyzing pure, or multiple component mixtures is still an aspect that needs improvement. This study explored machine learning algorithms to classify single...

The density-functional theory is widely used to predict the physical properties of materials. However, it usually fails for strongly correlated materials. A popular solution is to use the Hubbard correction to treat strongly correlated electronic states. Unfortunately, the values of the Hubbard U and J parameters are initially unknown, and they can...

This joint experimental and theoretical study of the structural and vibrational properties of β-In 2 S 3 upon compression shows that this tetragonal defect spinel undergoes two reversible pressure-induced order–disorder transitions up to 20 GPa.

Inorganic metal halide perovskites are promising materials for next-generation technologies due to plethora of unique physical properties many of which cannot be observed in the oxide perovskites. On the other hand, the search for ferroelectricity and multiferroicity in lead-free inorganic halide perovskites remains a challenging research topic. He...

Inorganic metal halide perovskites are promising materials for next-generation technologies due to a plethora of unique physical properties, many of which cannot be observed in the oxide perovskites. On the other hand, the search for ferroelectricity and multiferroicity in lead-free inorganic halide perovskites remains a challenging research topic....

A new two-dimensional material { Mg2B4C2, belonging to the family of the conventional superconductor MgB2, is theoretically predicted to exhibit superconductivity with critical temperature Tc estimated in the 47-48 K range (predicted using the McMillian-Allen-Dynes formula) without any tuning of external parameters such as doping, strain, or substr...

Fluorapatite flotation is influenced by the dissolved lattice metal ions. Al³⁺ and Mg²⁺ from the associating gangue minerals influence the adsorption of collector molecules onto the fluorapatite surface during flotation. Hence, unveiling new insights on such interactions in the context of froth flotation at an atomic level paves the way for improvi...

Density-functional theory is widely used to predict the physical properties of materials. However, it usually fails for strongly correlated materials. A popular solution is to use the Hubbard corrections to treat strongly correlated electronic states. Unfortunately, the exact values of the Hubbard $U$ and $J$ parameters are initially unknown, and t...

Front Cover, Issue 32, Vol 23 (2021) Physical Chemistry Chemical Physics

Multiferroics are a unique class of materials where magnetic and ferroelectric orders coexist. The research on multiferroics contributes significantly to the fundamental understanding of the strong correlations between different material degrees of freedom and provides an energy-efficient route toward the electrical control of magnetism. While mult...

Density functional theory is the most used methodology in the characterization of the electronic structure of materials. Its applications have spread out to almost every STEM field and it is recognized as one of the most successful theories in materials science. In this paper we measure the specific impact of this theory by means of the citation re...

Oxynitride perovskites of the type ABO2N have attracted considerable attention thanks to their potential ferroelectric behavior and tunable bandgap energy, making them ideal candidates for photocatalysis processes. Therefore, in order to shed light on the origin of their ferroelectric response, here we report a complete analysis of the structural a...

The MechElastic Python package evaluates the mechanical and elastic properties of bulk and 2D materials using the elastic coefficient matrix (Cij) obtained from any ab-initio density-functional theory (DFT) code. The current version of this package reads the output of VASP, ABINIT, and Quantum Espresso codes (but it can be easily generalized to any...

We perform first-principles calculations to study the structural, mechanical, thermal, electronic, and magnetic properties of Cr1−xMoxB2 for x = 0.25, 0.33, 0.50, 0.67 and 0.75. Based on structural search methods, we determine the ground-state structure for each concentration. The ternaries are either monoclinic (x = 0.25, 0.75) or trigonal (x = 0....

The structural and electronic properties of KTaO3/KZnF3 and KTaO3/KNiF3 oxyfluoride superlattices are studied from first-principles density functional theory calculations. We highlight, that beyond a critical layer thickness, these systems exhibit an insulator to metal transition that gives rise to the appearance of two-dimensional electron and hol...

The MechElastic Python package evaluates the mechanical and elastic properties of bulk and 2D materials using the elastic coefficient matrix ($C_{ij}$) obtained from any ab-initio density-functional theory (DFT) code. The current version of this package reads the output of VASP, ABINIT, and Quantum Espresso codes (but it can be easily generalized t...

Dynamical Mean Field Theory (DMFT) is a successful method to compute the electronic structure of strongly correlated materials, especially when it is combined with density functional theory (DFT). Here, we present an open-source computational package (and a library) combining DMFT with various DFT codes interfaced through the Wannier90 package. The...

Delafossites are promising candidates for photocatalysis applications because of their chemical stability and absorption in the solar region of the electromagnetic spectrum. For example, CuAlO2 has good chemical stability but has a large indirect bandgap (~3 eV), so that efforts to improve its absorption in the solar region through alloying are inv...

We report the results of an ultrafast, direct structural measurement of optically pumped phonons in a Cr thin film using ultrashort x-ray pulses from a free-electron laser. In addition to measuring and confirming the known long-wavelength dispersion relation of Cr along a particular acoustic branch, we are able to determine the relative phase of th...

Machine learning inspired potentials continue to improve the ability for predicting structures of materials. However, many challenges still exist, particularly when calculating structures of disordered systems. These challenges are primarily due to the rapidly increasing dimensionality of the feature-vector space which in most machine-learning algo...

ABINIT is probably the first electronic-structure package to have been released under an open-source license about 20 years ago. It implements density functional theory, density-functional perturbation theory (DFPT), many-body perturbation theory (GW approximation and Bethe-Salpeter equation), and more specific or advanced formalisms, such as dynam...

Abinit is a material- and nanostructure-oriented package that implements density-functional theory (DFT) and many-body perturbation theory (MBPT) to find, from first principles, numerous properties including total energy, electronic structure, vibrational and thermodynamic properties, different dielectric and non-linear optical properties, and rela...

Dynamical Mean Field Theory (DMFT) is a successful method to compute the electronic structure of strongly correlated materials, especially when it is combined with density functional theory (DFT). Here, we present an open-source computational package (and a library) combining DMFT with various DFT codes interfaced through the Wannier90 package. The...

Body: The calculation of phonon frequencies in dilute anisotropic alloys is not a simple undertaking and the successful methodologies are not well developed. We propose weighted dynamical-matrix (WDM) approach for calculating optical phonon spectra that is applicable to such complex alloys for a large range of alloying elements. Our WDM approach fo...

Here we report a new versatile approach for the calculation of phonon modes which is applicable to anisotropic, dilute alloys with allowance for a large variety of alloying elements. This approach has significant advantages over previously reported methods, especially for the lattice dynamics of such complex alloys. We use this approach to model th...

We employ an ab-initio structure search algorithm to explore the configurational space of bismuth in quasi-two dimensions. A confinement potential is introduced to restrict the movement of atoms within a pre-defined thickness to find the stable and metastable forms of monolayer Bi. In addition to the two known low-energy structures (puckered monocl...

The PyProcar Python package plots the band structure and the Fermi surface as a function of site and/or s,p,d,f - projected wavefunctions obtained for each k-point in the Brillouin zone and band in an electronic structure calculation. This can be performed on top of any electronic structure code, as long as the band and projection information is wr...

We present the use and implementation of the firefly algorithm to help in scanning the multiple metastable minima of orbital occupations in Density Functional Theory (DFT) plus Hubbard $U$ correction and to identify the ground state occupations in strongly correlated materials. We show the application of this implementation with the Abinit code on...

The high pressure lattice dynamics of rhombohedral antimony have been studied by a combination of diffuse scattering and inelastic x-ray scattering. The evolution of the phonon behavior as a function of pressure was analyzed by means of two theoretical approaches: density functional perturbation theory and symmetry-based phenomenological phase tran...

Here, we present the theoretical analysis of the structural and electronic degrees of freedom of two different oxide/fluoride perovskite superlattices, KTaO$_3$/KZnF$_3$ and KTaO$_3$/KNiF$_3$. Using first-principles calculations, we found the appearance of a two-dimensional electron, 2DEG, and hole, 2DHG, gases as a function of the number of layers...

A classical interatomic potential for the iron/iron-fluoride system is developed in the framework of the charge optimized many-body (COMB) potential. This interatomic potential takes into consideration the effects of charge transfer and many-body interactions depending on the chemical environment. The potential is fit to a training set composed of...

The PyProcar Python package plots the band structure and the Fermi surface as a function of site and/or s,p,d,f-projected wavefunctions obtained for each k-point in the Brillouin zone and band in an electronic structure calculation. This can be performed on top of any electronic structure code, as long as the band and projection information is writ...

The PyProcar Python package plots the band structure and the Fermi surface as a function of site and/or s,p,d,f - projected wavefunctions obtained for each $k$-point in the Brillouin zone and band in an electronic structure calculation. This can be performed on top of any electronic structure code, as long as the band and projection information is...

First-principles calculations have been carried out in order to analyze the structural, vibrational, and magnetic properties in the A -type antiferromagnetic (AFM) - M(OH) 2 metal transition hydroxides. Theoretical results demonstrate a quasi layer-by-layer magnetic behavior with strong ferromagnetic (FM) interactions in the intralayer plane and we...

Employing first-principles calculations, we have investigated the possible existence of the ferroelectric instability in the G -type antiferromagnetic NaNiF 3 and LiNiF 3 fluoroperovskites. The behavior of the unstable modes, at the cubic high-symmetry structure, is studied as a function of pressure. This study shows the vibrational landscape and d...

The high pressure lattice dynamics of rhombohedral antimony have been studied by a combination of diffuse scattering and inelastic x-ray scattering. The evolution of the phonon behavior as function of pressure was analyzed by means of two theoretical approaches: density functional perturbation theory and symmetry-based phenomenological phase transi...

The electronic correlations in materials are responsible for a variety of fascinating phenomena including magnetism, superconductivity, colossal magnetoresistance and metal-insulator transitions. As shown in previous studies, the ability to manipulate the oxygen vacancies within these strongly correlated materials gives rise to new degrees of freed...

The B1l and B1h vibrational modes of GaN are silent, i. e. they cannot appear neither in Raman nor in infra-red spectroscopies. However, the B1l mode appears with a small linewidth in the Raman scattering spectra on high quality ultra-narrow GaN nanowires, while the B1h mode does not. The simulltaneous appearance of the A1(LO) and B1l in the ultrav...

Aspects of the optoelectronic performance of thin-film ferromagnetic materials are evaluated for application in ultra-fast devices. Dynamics of photocarriers and their associated spin polarization are measured using transient reflectivity (TR) measurements in cross linear and circular polarization configurations for La0.7Sr0.3MnO3 films with a rang...

We employ an ab-initio structure search algorithm to explore the configurational space of Bi in quasi two dimensions. A confinement potential restricts the movement of atoms within a pre-defined thickness during structure search calculations within the minima hopping method to find the stable and metastable forms of bilayer Bi. In addition to recov...

Transient reflectivity (TR) measurements for La0.7Sr0.3MnO3 thin films indicate enhanced surface recombination of charge carriers for thinner films (d < 20 nm). Wavelet analysis of residual TR shows abrupt oscillatory modes with close energy ranges.

Recently published discoveries of acoustic- and optical-mode inversion in the phonon spectrum of certain metals became the first realistic example of noninteracting topological bosonic excitations in existing materials. However, the observable physical and technological use of such topological phonon phases remained unclear. In this paper, we provi...

We have combined a neural network formalism with metaheuristic structural global search algorithms to systematically screen the Mg-Ca binary system for new (meta)stable alloys. The combination of these methods allows for an efficient exploration of the potential energy surface beyond the possibility of the traditional searches based on ab initio en...

We investigate the hydrostatic pressure dependence of the zone center optical phonons of c-plane and a-plane wurtzite InN epilayers grown on GaN substrates. The longitudinal to transverse mode splitting for the A1 and E1 modes was found to increase with increasing pressure, whereas the associated transverse effective charge decreases for both modes...

Graphene/MoS2 van der Waals (vdW) heterostructures have promising technological applications due to their unique properties and functionalities. Many experimental and theoretical research groups across the globe have made outstanding contributions to benchmark the properties of graphene/MoS2 heterostructures. Even though some research groups have m...

Abstract The Mn valence in thin film La0.7Sr0.3MnO3 was studied as a function of film thickness in the range of 1–16 unit cells with a combination of non-destructive bulk and surface sensitive X-ray absorption spectroscopy techniques. Using a layer-by-layer valence model, it was found that while the bulk averaged valence hovers around its expected...

Magnetic systems represent an important challenge for electronic structure methods, in particular Density Functional Theory (DFT), which uses a single determinant wavefunction. To assess the predictions obtained by DFT in this type of materials, we benchmark different exchange correlation functionals with respect to each other, and with respect to...

Wurtzite semiconductor compounds have two silent modes, B1l and B1h. A silent mode is a vibrational mode that carries neither a dipole moment nor Raman polarizability. Thus, they are forbidden in both infrared reflectivity and Raman spectroscopy. Astonishing, we detected the B1l mode in high-quality, ultra-narrow GaN nanowires using resonant Raman...

In most non-collinear crystal magnets, the number of metastable states is quite large and any calculation that tries to predict the ground state can fall into one of the possible metastable phases. In this paper, we generalize the population based meta-heuristic firefly algorithm to the problem of the non collinear magnetic phase ground state predi...

Recently published discoveries of acoustic and optical mode inversion in the phonon spectrum of certain metals became the first realistic example of non-interacting topological bosonic excitations in existing materials. However, the observable physical and technological use of such topological phonon phases remained unclear. In this work we provide...

Graphene/MoS2 heterostructures are formed by combining the nanosheets of graphene and monolayer MoS2. The electronic features of both constituent mo