Papers by Fernando Reboredo
Physical Review Materials
We present diffusion Monte Carlo (DMC) results for equation of state and quasiparticle gaps of ma... more We present diffusion Monte Carlo (DMC) results for equation of state and quasiparticle gaps of manganese binary oxides MnO and MnO2 and the ternary oxide LaMnO3. Owing to the limited approximations made and the direct treatment of electronic correlations, our DMC-based study correctly describes structural properties such as the lattice constant, bulk moduli, and cohesive energies. It correctly predicts the ground-state phase of these oxides, which have different valences. Our study demonstrates the capability of DMC methods to predict the structural properties of highly correlated systems, which have been identified as a suitable candidates for many applications ranging from catalysis to electronic devices. Our study also serves as a benchmark for both the manganese pseudopotential and other methodological choices to be used in calculations of similar oxides.
Physical Review B
We present a many-body diffusion quantum Monte Carlo (DMC) study on the ground and excited state ... more We present a many-body diffusion quantum Monte Carlo (DMC) study on the ground and excited state properties of crystalline CoO polymorphs. To our knowledge, DMC is the only electronic structure method available to provide correct energetic ordering within experimental error bars between the three CoO polymorphs: Rocksalt, Wurtzite, and Zincblende. We compare these results to density functional theory (DFT) using state-of-the-art functionals such as SCAN. For the structural properties, such as the lattice parameters and bulk moduli, our results are comparable to HSE and SCAN. Using DMC, we calculated the indirect and direct optical gaps as 3.8(2) and 5.2(2) eV. Our indirect optical gap compares well with the conductivity measurements of 3.6(5) eV and GW calculations with 3.4 eV. Similarly, we obtained the DMC indirect and direct quasiparticle gaps as 3.9(2) and 5.5(2) eV. DMC direct quasiparticle gaps compare well with the direct band gap of 5.53 eV obtained from ellipsometry studies.
The Journal of chemical physics, Jan 7, 2018
We have studied the structural stability of monolayer and bilayer arsenene (As) in the buckled (b... more We have studied the structural stability of monolayer and bilayer arsenene (As) in the buckled (b) and washboard (w) phases with diffusion quantum Monte Carlo (DMC) and density functional theory (DFT) calculations. DMC yields cohesive energies of 2.826(2) eV/atom for monolayer b-As and 2.792(3) eV/atom for w-As. In the case of bilayer As, DMC and DFT predict that AA-stacking is the more stable form of b-As, while AB is the most stable form of w-As. The DMC layer-layer binding energies for b-As-AA and w-As-AB are 30(1) and 53(1) meV/atom, respectively. The interlayer separations were estimated with DMC at 3.521(1) Ã… for b-As-AA and 3.145(1) Ã… for w-As-AB. A comparison of DMC and DFT results shows that the van der Waals density functional method yields energetic properties of arsenene close to DMC, while the DFT + D3 method closely reproduced the geometric properties from DMC. The electronic properties of monolayer and bilayer arsenene were explored with various DFT methods. The bandg...
Physical Review B
Electron-hole exchange interactions can lead to spin-forbidden ''dark'' excitons in direct-gap qu... more Electron-hole exchange interactions can lead to spin-forbidden ''dark'' excitons in direct-gap quantum dots. Here, we explore an alternative mechanism for creating optically forbidden excitons. In a large spherical quantum dot made of a diamond-structure semiconductor, the symmetry of the valence band maximum Í‘VBMÍ’ is t 2. The symmetry of the conduction band minimum Í‘CBMÍ’ in direct-gap material is a 1 , but for indirect-gap systems the symmetry could be Í‘depending on sizeÍ’ a 1 , e, or t 2. In the latter cases, the resulting manifold of excitonic states contains several symmetries derived from the symmetries of the VBM and CBM Í‘e.g., t 2 Ï«t 2 ÏA 1 Ï©EÏ©T 1 Ï©T 2 or t 2 Ï«eÏT 1 Ï©T 2). Only the T 2 exciton is optically active or ''bright,'' while the others A 1 , E, and T 1 are ''dark.'' The question is which is lower in energy, the dark or bright. Using pseudopotential calculations of the single-particle states of Si quantum dots and a direct evaluation of the screened electron-hole Coulomb interaction, we find that, when the CBM symmetry is t 2 , the direct electronhole Coulomb interaction lowers the energy of the dark excitons relative to the bright T 2 exciton. Thus, the lowest energy exciton is forbidden, even without an electron-hole exchange interaction. We find that our dark-bright excitonic splitting agrees well with experimental data of Calcott et al., Kovalev et al., and Brongersma et al. Our excitonic transition energies agree well with the recent experiment of Wolkin et al. In addition, and contradicting simplified models, we find that Coulomb correlations are more important for small dots than for intermediate sized ones. We describe the full excitonic spectrum of Si quantum dots by using a many-body expansion that includes both Coulomb and exchange electron hole terms. We present the predicted excitonic spectra.
Scientific reports, Aug 24, 2016
Developing nanostructures with tunable magnetic states is crucial for designing novel data storag... more Developing nanostructures with tunable magnetic states is crucial for designing novel data storage and quantum information devices. Using density functional theory, we investigate the thermodynamic stability and magnetic properties of tungsten adsorbed tri-vacancy fluorinated (TVF) graphene. We demonstrate a strong structure-property relationship and its response to external stimuli via defect engineering in graphene-based materials. Complex interplay between defect states and the chemisorbed atom results in a large magnetic moment of 7 μB along with high in-plane magneto-crystalline anisotropy energy (MAE) of 17 meV. Under the influence of electric field, spin crossover effect accompanied by a change in the MAE is observed. The ascribed change in spin-configuration is caused by the modification of exchange coupling between defect states and a change in the occupation of d-orbitals of the metal complex. Our predictions open a promising way towards controlling the magnetic properties...
Phys Rev Lett, 1999
Implantation and plasma indiffusion of H in crystalline Si are known to induce planar defects (pl... more Implantation and plasma indiffusion of H in crystalline Si are known to induce planar defects (platelets) whose structure has remained elusive. We report extensive first-principles calculations that reveal mechanisms for the nucleation and growth of aggregates of second-neighbor hydrogenated vacancies. These defects exhibit the key features associated with platelets, including a preference for (111) and (100) planes, H release, and trapping of Hâ molecules. {copyright} {ital 1999} {ital The American Physical Society }
Bulletin of the American Physical Society, 2009
Aps Meeting Abstracts, Mar 1, 2005
A C$_3$-symmetric crystal-field potential in the Fe(II)Fe(III) bimetallic oxalates splits the L=2... more A C$_3$-symmetric crystal-field potential in the Fe(II)Fe(III) bimetallic oxalates splits the L=2 Fe(II) multiplet into two doublets and a singlet. In compounds that exhibit magnetic compensation, one of the doublets was predicted to lie lowest in energy and carry a non-quenched orbital angular momentum $\pm \ld $, where $\ld $ exceeds a threshold value. In a range of $\ld $, a Jahn-Teller (JT) distortion increases the energy splitting of the low-lying doublet and breaks the C$_3$ symmetry of the bimetallic planes around the ferrimagnetic transition temperature. At low temperatures, the JT distortion disappears in compounds that display magnetic compensation due to the competition with the spin-orbit coupling. A comparison with recent measurements provides strong evidence for this re-entrant, low-temperature JT transition and a prediction for the normal, high-temperature JT transition. The size of the JT distortion is estimated using first-principles calculations, which suggest that the long-range ordering of smaller, non-C$_3$-symmetric organic cations can eliminate magnetic compensation.
Aps March Meeting Abstracts, Mar 1, 1998
Aps Meeting Abstracts, Mar 1, 2011
Aps Meeting Abstracts, Mar 1, 2011
Bulletin of the American Physical Society, Mar 5, 2015
due to the presence of strong electronic correlations. The complex interplay among correlation an... more due to the presence of strong electronic correlations. The complex interplay among correlation and exchange in d subshells, crystal field effects, p-d hybridization and charge transfer gives rise to a rich variety of structural and electronic phases. NiO is one such challenging d system, where conventional band theory fails. Compared to the experimental value, the cohesive energy of bulk NiO computed within DFT-LDA differs by almost a factor of 18 %. Band gap computed within standard local or semi-local functionals are off by a factor of 80 %. A quasi-particle correction like the G 0 W 0 approach cannot correct the band gap and is still by far too low. In this work we adopt the Diffusion Quantum Monte (DMC) approach to study the structural and electronic properties of NiO. Trial wave-functions were self consistently generated in a Slater-Jastrow form. To test pseudopotentials used, DMC calculations were done on atomic Ni and O and their computed ionization potentials showed excellent agreement with experiments (within 0.04%). The equilibrium bond length and binding energy of the NiO dimer were also computed that were 0.001% and 0.03%, respectively, from experimental values. DMC calculations of equation of state and band gap of bulk NiO will be presented.
Bulletin of the American Physical Society, Mar 5, 2015
ABSTRACT Since Dmitri Mendeleev developed a table in 1869 to illustrate recurring ("peri... more ABSTRACT Since Dmitri Mendeleev developed a table in 1869 to illustrate recurring ("periodic") trends of the elements, it has been understood that most chemical and physical properties can be described by taking into account the outer most electrons of the atoms. These valence electrons are mainly responsible for the chemical bond. In many ab-initio approaches only valence electrons are taken into account and a pseudopotential is used to mimic the response of the core electrons. Typically an all-electron calculation is used to generate a pseudopotential that is used either within density functional theory or quantum chemistry approaches. In this talk we explain and demonstrate a new method to generate pseudopotentials directly from all-electron many-body diffusion Monte Carlo (DMC) calculations and discuss the results of of the transferability of these pseudopotentials. The advantages of incorporating the exchange and correlation directly from DMC into the pseudopotential are also discussed.
Bulletin of the American Physical Society, Mar 4, 2015
Aps Meeting Abstracts, Mar 1, 2008
Solid buckminsterfullerene (C60) is known to be very soft, with a large number of crystalline pha... more Solid buckminsterfullerene (C60) is known to be very soft, with a large number of crystalline phases that can be accessed by temperature or pressure. External pressure reduces the intermolecular distance, which affects the electronic structure in three ways: by increasing the overlap between molecular orbitals on neighbor molecules, by inducing additional chemical bonds between molecules, and by deforming the molecular
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Papers by Fernando Reboredo