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Papers by Pablo Borges

Applied Surface Science, 2008

We present our theoretical results for the structural, electronic, vibrational and optical proper... more We present our theoretical results for the structural, electronic, vibrational and optical properties of MO2 (M=Sn, Zr, Hf and Ti) obtained by first-principles calculations. Relativistic effects are demonstrated to be important for a realistic description of the detailed structure of the electronic frequency-dependent dielectric function, as well as of the carrier effective masses. Based on our results, we found that

In this work, we have performed spin-polarized calculations for the structural and electronic pro... more In this work, we have performed spin-polarized calculations for the structural and electronic properties of vacancies and anti-site defects in the rocksalt PbTe. Our obtained results have shown that both the Pb and Te antisites are the favorable defects in Pb and Te rich conditions, respectively. Moreover, in the perfect stoichiometry condition, the antisites, as well as the Te vacancy, are equally probable to find. Considering the charge injection in the system, all the defects change from the 2+ charge state to the 2one within 14 meV, at around 80 meV from the valence band edge within the bandgap. This feature makes it difficult to experimentally characterize these defects in PbTe.

In this article, using first-principles electronic structure calculations within the spin density... more In this article, using first-principles electronic structure calculations within the spin density functional theory, alternated magnetic and non-magnetic layers of rutile-CrO2 and rutile-SnO2 respectively, in a (CrO2)n(SnO2)n superlattice (SL) configuration, with n being the number of monolayers which are considered equal to 1, 2, ..., 10 are studied. A half-metallic behavior is observed for the (CrO2)n(SnO2)n SLs for all values of n. The ground state is found to be FM with a magnetic moment of 2 μB per chromium atom, and this result does not depend on the number of monolayers n. As the FM rutile-CrO2 is unstable at ambient temperature, and known to be stabilized when on top of SnO2, the authors suggest that (CrO2)n(SnO2)n SLs may be applied to spintronic technologies since they provide efficient spin-polarized carriers. Introduction A variety of heterostructures have been studied for spintronics applications, and they have proved to have a great potential for high-performance spin-...

Journal of Crystal Growth

The European Physical Journal B

Recent theoretical works on NiO have not agreed upon the nature of the first optical transition. ... more Recent theoretical works on NiO have not agreed upon the nature of the first optical transition. By altering band filling – with highly concentrated O vacancies and Fe impurities – here, the orbital density of states is changed near the Fermi energy. The variation in optical properties, relative to the changes in orbital character, along with group theory analysis of hybridized orbitals, provides new insight when evaluating the first optical transition of NiO. Here, based on density functional theory, the first optical transition is found to have two possibilities – either superexchange site-hopping or a transition from the hybridized eg state to the hybridized a1u state, rather than the intra-atomic transitions which are causing disagreement in the recent literature.Graphical abstract

ACS Omega

First-principles calculations done via density functional theory were used to study the structura... more First-principles calculations done via density functional theory were used to study the structural and electronic properties of sodium montmorillonite clay (Mt-Na +) of general formula M x Al 3 Si 8 O 24 H 4 Na•nH 2 O (M x : Mg or Fe). The final position of the interlamellar sodium atom is found to be close to the oxygen atoms located on the upper surface of silica. Following Fe-Mt-Na + system relaxation, with subsequent analysis of magnetic moment and magnetic states, the electroneutrality of the system established that both Fe 2+ and Fe 3+ oxidation states are possible to occur. The Mg 2+-Mt-Na + material shows a band gap energy greater than that of Fe 2+-Mt-Na + when iron is in the octahedral site. It is found that the valence-band maximum and the conduction-band minimum of iron-doped montmorillonite are both at the Γ-point, while it is at V → Γ for magnesium-doped montmorillonite. The calculated band gap from hybrid functional (HSE06) of Fe 2+-Mt-Na + is equal to 4.3 eV, exhibiting good agreement with experimental results obtained from ultraviolet−visible spectroscopy of the natural Mt-Na + (Cloisite-Na +).

Journal of Applied Physics

First principles band structure calculations are employed to perform a comprehensive study of the... more First principles band structure calculations are employed to perform a comprehensive study of the electronic and optical properties of the 2H-phase XTO2 (X=Cu, Ag; T=Al, Cr) delafossites, and the effects on those properties due to Cu, Ag, and O vacancies, as well as p-type doping with magnesium. From the band structures, the frequency-dependent real and imaginary parts of the complex dielectric function, (), are obtained, together with refractive index, extinction coefficient, optical conductivity, reflectivity, absorption coefficient, and energy loss function. Moreover, hole effective masses are calculated for all systems and are shown to be highly anisotropic. Comparisons between the results obtained in this work and previous experimental and other theoretical results are made, when available. X vacancies and Mg doping are predicted to only marginally affect transparent character, but are also predicted to increase the hole effective mass. O vacancies are predicted to lower the hole effective mass, but at the cost of increased absorption in the optical portion of the spectrum, harming transparent character.

Applied Clay Science

The adsorption of textile dyes onto Layered Double Hydroxides (LDH) and their thermally decompose... more The adsorption of textile dyes onto Layered Double Hydroxides (LDH) and their thermally decomposed products is a promising strategy for the treatment of contaminated effluents-combining high removal efficiency with reasonable cost. The main purpose of this paper was to investigate the adsorption of textile azo dye Acid Yellow 42 (AY) onto calcined and uncalcined Mg-Al-CO 3-LDH. A set of analytical techniques was used to characterize the materials, namely X-ray diffraction (XRD), Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), thermogravimetric analyses (TGA), N 2 adsorption-desorption isotherms and Scanning Electron Microscopy (SEM). In the study of azo dye adsorption, the following factors were assessed: kinetics, adsorption capacity, effect of temperature, initial pH value, and recyclability of the adsorbent material. The adsorption capacity of calcined LDH (CLDH) was almost four times greater than that of the LDH precursor: 1266 mg•g −1 (1.669 mmol•g −1) and 330.0 mg•g −1 (0.4350 mmol•g −1), respectively, at pH equal to 7.0 and 25°C. The greater adsorption capacity for CLDH is related to the recovery property of these materials in light of the so called "memory effect", which allows an intercalation process of the anionic dye, as demonstrated by XRD data.

Journal of Solid State Chemistry

ABSTRACT Doping in a manner that introduces anisotropy in order to reduce thermal conductivity is... more ABSTRACT Doping in a manner that introduces anisotropy in order to reduce thermal conductivity is a significant focus in thermoelectric research today. By solving the semiclassical Boltzmann transport equations in the constant scattering time (τ) approximation, in conjunction with ab initio electronic structure calculations, within Density Functional Theory, we compare the Seebeck coefficient (S) and figure of merit (ZT) of bulk PbTe to PbTe/SnTe/PbTe heterostructures and PbTe doping superlattices (SLs) with periodically doped planes. Bismuth and Thallium were used as the n- and p-type impurities, respectively. The effects of carrier concentration are considered via chemical potential variation in a rigid band approximation. The impurity bands near the Fermi level in the electronic structure of PbTe SLs are of Tl s- and Bi p-character, and this feature is independent of the doping concentration or the distance between impurity planes. We observe the impurity bands to have a metallic nature in the directions perpendicular to the doping planes, yet no improvement on the values of ZT is found when compared to bulk PbTe. For the PbTe/SnTe/PbTe heterostructures, the calculated S presents good agreement with recent experimental data, and an anisotropic behavior is observed for low carrier concentrations (n<1018 cm−3). A large value of ZT|| (parallel to the growth direction) of 3.0 is predicted for n=4.7×1018 cm−3 and T=700 K, whereas ZTp (perpendicular to the growth direction) is found to peak at 1.5 for n=1.7×1017 cm−3. Both electrical conductivity enhancement and thermal conductivity reduction are analyzed.

AIP Advances

Antiferromagnetic NiO is a candidate for next generation high-speed and scaled RRAM devices. Here... more Antiferromagnetic NiO is a candidate for next generation high-speed and scaled RRAM devices. Here, electronic and optical properties of antiferromagnetic NiO: Fe 25% in the rock salt structure are studied and compared to intrinsic NiO. From density of states and complex dielectric function analysis, the first optical transition is found to be at lower frequency than intrinsic NiO due to an Fe impurity level being the valence band maximum. The resulting effects on refractive index, reflectivity, absorption, optical conductivity and loss function for Fe-doped NiO are compared to those of intrinsic NiO, and notable differences are analyzed. The electronic component of the static dielectric constant of NiO: Fe 25% is calculated to be about 2% less than that of intrinsic NiO.

AIP Conference Proceedings, 2007

The electronic and optical properties of rutile SnO2 are studied by means of the ab initio full-p... more The electronic and optical properties of rutile SnO2 are studied by means of the ab initio full-potential linear augmented plane-wave method and within a full relativistic approach. Relevant relativistic and spin-orbit coupling effects are observed. We provide values for the dielectric constant and refraction index, as well as for conduction- and valence-band effective masses. These later are shown to be highly anisotropic, and with the holes heavier than the electrons. The values obtained for the electronic part of the dielectric constant are &Vegr;xx(0) = 4.5 and &Vegr;zz(0) = 4.8. For the refraction index nxx and nzz we obtained the values of 2.15 and 2.20, respectively.

Lead chalcogenides, most notably PbTe and PbSe, have become an active area of research due to the... more Lead chalcogenides, most notably PbTe and PbSe, have become an active area of research due to their thermoelectric (TE) properties. The high figure of merit (ZT) of these materials has brought much attention to them, due to their ability to convert waste heat into electricity, with a possible application being in engine exhaust. Here, we examine the effects of altering the lattice parameter on total ground state energy and the band gap using first principles calculations performed within Density Functional Theory and the Projector Augmented Wave approach and the Vienna Ab-initio Simulation Package (VASP-PAW) code. Both PbTe and PbSe, in NaCl, orthorhombic, and CsCl structures are considered. It is found that altering the lattice parameter, which is analogous to applying external pressure on the material experimentally, has notable effects on both ground state energy and the band gap. The implications of this behavior in the TE properties of these materials are analyzed.

Journal of Solid State Chemistry

Doping in a manner that introduces anisotropy in order to reduce thermal conductivity is a signif... more Doping in a manner that introduces anisotropy in order to reduce thermal conductivity is a significant focus in thermoelectric research today. By solving the semiclassical Boltzmann transport equations in the constant scattering time (τ) approximation, in conjunction with ab initio electronic structure calculations, within Density Functional Theory, we compare the Seebeck coefficient (S) and figure of merit (ZT) of bulk PbTe to PbTe/SnTe/PbTe heterostructures and PbTe doping superlattices (SLs) with periodically doped planes. Bismuth and Thallium were used as the n- and p-type impurities, respectively. The effects of carrier concentration are considered via chemical potential variation in a rigid band approximation. The impurity bands near the Fermi level in the electronic structure of PbTe SLs are of Tl s- and Bi p-character, and this feature is independent of the doping concentration or the distance between impurity planes. We observe the impurity bands to have a metallic nature i...

The primary relaxation in glass forming supercooled liquids (SCLs) above the glass transformation... more The primary relaxation in glass forming supercooled liquids (SCLs) above the glass transformation temperature T_g is discussed in terms of the first-order (steepness) and the second-order (curvature) temperature derivatives of the observed primary relaxation timescale. We report new insights into the problem of the domain of the Vogel-Fulcher-Tamman (VFT) equation, raised by Stickel et al. (J. Chem. Phys.,1995, 1996) and

Theoretical Chemistry Accounts, 2010

We report results on the electronic, vibrational, and optical properties of SnO 2 obtained using ... more We report results on the electronic, vibrational, and optical properties of SnO 2 obtained using first-principles calculations performed within the density functional theory. All the calculated phonon frequencies, real and imaginary parts of complex dielectric function, the energyloss spectrum, the refractive index, the extinction, and the absorption coefficients show good agreement with experimental results. Based on our calculations, the SnO 2 electron and hole effective masses were found to be strongly anisotropic. The lattice contribution to the low-frequency region of the SnO 2 dielectric function arising from optical phonons was also determined resulting the values of e 1\ latt (0) = 14.6 and e 1k latt (0) = 10.7 for directions perpendicular and parallel to the tetragonal c-axis, respectively. This is in excellent agreement with the available experimental data. After adding the electronic contribution to the lattice contribution, a total average value of e 1 (0) = 18.2 is predicted for the static permittivity constant of SnO 2 .

Physical Review B, 2013

ABSTRACT The phonon dispersion and the lattice contribution to the dielectric function of PbTe in... more ABSTRACT The phonon dispersion and the lattice contribution to the dielectric function of PbTe in the NaCl structure are calculated ab initio. The results obtained are in agreement with the available experimental data and reproduce the main features observed in this material, such as an anharmonic LA-TO phonon coupling in the �-X direction, as well as high values for the dielectric constants �e1(0) and e�1(∞). Calculations include the pressure dependence and indicate that the anharmonic phonon coupling is very sensitive to applied pressure. The calculations indicate that the higher values for the dielectric constant together with the anharmonic LA-TO coupling reduce the lattice thermal conductivity and increase the internal electric field, thereby enhancing the electronic conductivity in PbTe, key conditions to increase the thermoelectric figure of merit.

Nanoscale Research Letters, 2012

Transition-metal (TM)-doped diluted magnetic oxides (DMOs) have attracted attention from both exp... more Transition-metal (TM)-doped diluted magnetic oxides (DMOs) have attracted attention from both experimental and theoretical points of view due to their potential use in spintronics towards new nanostructured devices and new technologies. In the present work, we study the magnetic properties of Sn0.96TM0.04O2 and Sn0.96TM0.04O1.98(V O)0.02, where TM = Fe and Co, focusing in particular in the role played by the presence of O vacancies nearby the TM. The calculated total energy as a function of the total magnetic moment per cell shows a magnetic metastability, corresponding to a ground state, respectively, with 2 and 1 μB/cell, for Fe and Co. Two metastable states, with 0 and 4 μB/cell were found for Fe, and a single value, 3 μB/cell, for Co. The spin-crossover energies (E S) were calculated. The values are E S 0/2 = 107 meV and E S 4/2 = 25 meV for Fe. For Co, E S 3/1 = 36 meV. By creating O vacancies close to the TM site, we show that the metastablity and E S change. For iron, a new s...

Applied Surface Science, 2008

We present our theoretical results for the structural, electronic, vibrational and optical proper... more We present our theoretical results for the structural, electronic, vibrational and optical properties of MO2 (M=Sn, Zr, Hf and Ti) obtained by first-principles calculations. Relativistic effects are demonstrated to be important for a realistic description of the detailed structure of the electronic frequency-dependent dielectric function, as well as of the carrier effective masses. Based on our results, we found that

In this work, we have performed spin-polarized calculations for the structural and electronic pro... more In this work, we have performed spin-polarized calculations for the structural and electronic properties of vacancies and anti-site defects in the rocksalt PbTe. Our obtained results have shown that both the Pb and Te antisites are the favorable defects in Pb and Te rich conditions, respectively. Moreover, in the perfect stoichiometry condition, the antisites, as well as the Te vacancy, are equally probable to find. Considering the charge injection in the system, all the defects change from the 2+ charge state to the 2one within 14 meV, at around 80 meV from the valence band edge within the bandgap. This feature makes it difficult to experimentally characterize these defects in PbTe.

In this article, using first-principles electronic structure calculations within the spin density... more In this article, using first-principles electronic structure calculations within the spin density functional theory, alternated magnetic and non-magnetic layers of rutile-CrO2 and rutile-SnO2 respectively, in a (CrO2)n(SnO2)n superlattice (SL) configuration, with n being the number of monolayers which are considered equal to 1, 2, ..., 10 are studied. A half-metallic behavior is observed for the (CrO2)n(SnO2)n SLs for all values of n. The ground state is found to be FM with a magnetic moment of 2 μB per chromium atom, and this result does not depend on the number of monolayers n. As the FM rutile-CrO2 is unstable at ambient temperature, and known to be stabilized when on top of SnO2, the authors suggest that (CrO2)n(SnO2)n SLs may be applied to spintronic technologies since they provide efficient spin-polarized carriers. Introduction A variety of heterostructures have been studied for spintronics applications, and they have proved to have a great potential for high-performance spin-...

Journal of Crystal Growth

The European Physical Journal B

Recent theoretical works on NiO have not agreed upon the nature of the first optical transition. ... more Recent theoretical works on NiO have not agreed upon the nature of the first optical transition. By altering band filling – with highly concentrated O vacancies and Fe impurities – here, the orbital density of states is changed near the Fermi energy. The variation in optical properties, relative to the changes in orbital character, along with group theory analysis of hybridized orbitals, provides new insight when evaluating the first optical transition of NiO. Here, based on density functional theory, the first optical transition is found to have two possibilities – either superexchange site-hopping or a transition from the hybridized eg state to the hybridized a1u state, rather than the intra-atomic transitions which are causing disagreement in the recent literature.Graphical abstract

ACS Omega

First-principles calculations done via density functional theory were used to study the structura... more First-principles calculations done via density functional theory were used to study the structural and electronic properties of sodium montmorillonite clay (Mt-Na +) of general formula M x Al 3 Si 8 O 24 H 4 Na•nH 2 O (M x : Mg or Fe). The final position of the interlamellar sodium atom is found to be close to the oxygen atoms located on the upper surface of silica. Following Fe-Mt-Na + system relaxation, with subsequent analysis of magnetic moment and magnetic states, the electroneutrality of the system established that both Fe 2+ and Fe 3+ oxidation states are possible to occur. The Mg 2+-Mt-Na + material shows a band gap energy greater than that of Fe 2+-Mt-Na + when iron is in the octahedral site. It is found that the valence-band maximum and the conduction-band minimum of iron-doped montmorillonite are both at the Γ-point, while it is at V → Γ for magnesium-doped montmorillonite. The calculated band gap from hybrid functional (HSE06) of Fe 2+-Mt-Na + is equal to 4.3 eV, exhibiting good agreement with experimental results obtained from ultraviolet−visible spectroscopy of the natural Mt-Na + (Cloisite-Na +).

Journal of Applied Physics

First principles band structure calculations are employed to perform a comprehensive study of the... more First principles band structure calculations are employed to perform a comprehensive study of the electronic and optical properties of the 2H-phase XTO2 (X=Cu, Ag; T=Al, Cr) delafossites, and the effects on those properties due to Cu, Ag, and O vacancies, as well as p-type doping with magnesium. From the band structures, the frequency-dependent real and imaginary parts of the complex dielectric function, (), are obtained, together with refractive index, extinction coefficient, optical conductivity, reflectivity, absorption coefficient, and energy loss function. Moreover, hole effective masses are calculated for all systems and are shown to be highly anisotropic. Comparisons between the results obtained in this work and previous experimental and other theoretical results are made, when available. X vacancies and Mg doping are predicted to only marginally affect transparent character, but are also predicted to increase the hole effective mass. O vacancies are predicted to lower the hole effective mass, but at the cost of increased absorption in the optical portion of the spectrum, harming transparent character.

Applied Clay Science

The adsorption of textile dyes onto Layered Double Hydroxides (LDH) and their thermally decompose... more The adsorption of textile dyes onto Layered Double Hydroxides (LDH) and their thermally decomposed products is a promising strategy for the treatment of contaminated effluents-combining high removal efficiency with reasonable cost. The main purpose of this paper was to investigate the adsorption of textile azo dye Acid Yellow 42 (AY) onto calcined and uncalcined Mg-Al-CO 3-LDH. A set of analytical techniques was used to characterize the materials, namely X-ray diffraction (XRD), Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), thermogravimetric analyses (TGA), N 2 adsorption-desorption isotherms and Scanning Electron Microscopy (SEM). In the study of azo dye adsorption, the following factors were assessed: kinetics, adsorption capacity, effect of temperature, initial pH value, and recyclability of the adsorbent material. The adsorption capacity of calcined LDH (CLDH) was almost four times greater than that of the LDH precursor: 1266 mg•g −1 (1.669 mmol•g −1) and 330.0 mg•g −1 (0.4350 mmol•g −1), respectively, at pH equal to 7.0 and 25°C. The greater adsorption capacity for CLDH is related to the recovery property of these materials in light of the so called "memory effect", which allows an intercalation process of the anionic dye, as demonstrated by XRD data.

Journal of Solid State Chemistry

ABSTRACT Doping in a manner that introduces anisotropy in order to reduce thermal conductivity is... more ABSTRACT Doping in a manner that introduces anisotropy in order to reduce thermal conductivity is a significant focus in thermoelectric research today. By solving the semiclassical Boltzmann transport equations in the constant scattering time (τ) approximation, in conjunction with ab initio electronic structure calculations, within Density Functional Theory, we compare the Seebeck coefficient (S) and figure of merit (ZT) of bulk PbTe to PbTe/SnTe/PbTe heterostructures and PbTe doping superlattices (SLs) with periodically doped planes. Bismuth and Thallium were used as the n- and p-type impurities, respectively. The effects of carrier concentration are considered via chemical potential variation in a rigid band approximation. The impurity bands near the Fermi level in the electronic structure of PbTe SLs are of Tl s- and Bi p-character, and this feature is independent of the doping concentration or the distance between impurity planes. We observe the impurity bands to have a metallic nature in the directions perpendicular to the doping planes, yet no improvement on the values of ZT is found when compared to bulk PbTe. For the PbTe/SnTe/PbTe heterostructures, the calculated S presents good agreement with recent experimental data, and an anisotropic behavior is observed for low carrier concentrations (n<1018 cm−3). A large value of ZT|| (parallel to the growth direction) of 3.0 is predicted for n=4.7×1018 cm−3 and T=700 K, whereas ZTp (perpendicular to the growth direction) is found to peak at 1.5 for n=1.7×1017 cm−3. Both electrical conductivity enhancement and thermal conductivity reduction are analyzed.

AIP Advances

Antiferromagnetic NiO is a candidate for next generation high-speed and scaled RRAM devices. Here... more Antiferromagnetic NiO is a candidate for next generation high-speed and scaled RRAM devices. Here, electronic and optical properties of antiferromagnetic NiO: Fe 25% in the rock salt structure are studied and compared to intrinsic NiO. From density of states and complex dielectric function analysis, the first optical transition is found to be at lower frequency than intrinsic NiO due to an Fe impurity level being the valence band maximum. The resulting effects on refractive index, reflectivity, absorption, optical conductivity and loss function for Fe-doped NiO are compared to those of intrinsic NiO, and notable differences are analyzed. The electronic component of the static dielectric constant of NiO: Fe 25% is calculated to be about 2% less than that of intrinsic NiO.

AIP Conference Proceedings, 2007

The electronic and optical properties of rutile SnO2 are studied by means of the ab initio full-p... more The electronic and optical properties of rutile SnO2 are studied by means of the ab initio full-potential linear augmented plane-wave method and within a full relativistic approach. Relevant relativistic and spin-orbit coupling effects are observed. We provide values for the dielectric constant and refraction index, as well as for conduction- and valence-band effective masses. These later are shown to be highly anisotropic, and with the holes heavier than the electrons. The values obtained for the electronic part of the dielectric constant are &Vegr;xx(0) = 4.5 and &Vegr;zz(0) = 4.8. For the refraction index nxx and nzz we obtained the values of 2.15 and 2.20, respectively.

Lead chalcogenides, most notably PbTe and PbSe, have become an active area of research due to the... more Lead chalcogenides, most notably PbTe and PbSe, have become an active area of research due to their thermoelectric (TE) properties. The high figure of merit (ZT) of these materials has brought much attention to them, due to their ability to convert waste heat into electricity, with a possible application being in engine exhaust. Here, we examine the effects of altering the lattice parameter on total ground state energy and the band gap using first principles calculations performed within Density Functional Theory and the Projector Augmented Wave approach and the Vienna Ab-initio Simulation Package (VASP-PAW) code. Both PbTe and PbSe, in NaCl, orthorhombic, and CsCl structures are considered. It is found that altering the lattice parameter, which is analogous to applying external pressure on the material experimentally, has notable effects on both ground state energy and the band gap. The implications of this behavior in the TE properties of these materials are analyzed.

Journal of Solid State Chemistry

Doping in a manner that introduces anisotropy in order to reduce thermal conductivity is a signif... more Doping in a manner that introduces anisotropy in order to reduce thermal conductivity is a significant focus in thermoelectric research today. By solving the semiclassical Boltzmann transport equations in the constant scattering time (τ) approximation, in conjunction with ab initio electronic structure calculations, within Density Functional Theory, we compare the Seebeck coefficient (S) and figure of merit (ZT) of bulk PbTe to PbTe/SnTe/PbTe heterostructures and PbTe doping superlattices (SLs) with periodically doped planes. Bismuth and Thallium were used as the n- and p-type impurities, respectively. The effects of carrier concentration are considered via chemical potential variation in a rigid band approximation. The impurity bands near the Fermi level in the electronic structure of PbTe SLs are of Tl s- and Bi p-character, and this feature is independent of the doping concentration or the distance between impurity planes. We observe the impurity bands to have a metallic nature i...

The primary relaxation in glass forming supercooled liquids (SCLs) above the glass transformation... more The primary relaxation in glass forming supercooled liquids (SCLs) above the glass transformation temperature T_g is discussed in terms of the first-order (steepness) and the second-order (curvature) temperature derivatives of the observed primary relaxation timescale. We report new insights into the problem of the domain of the Vogel-Fulcher-Tamman (VFT) equation, raised by Stickel et al. (J. Chem. Phys.,1995, 1996) and

Theoretical Chemistry Accounts, 2010

We report results on the electronic, vibrational, and optical properties of SnO 2 obtained using ... more We report results on the electronic, vibrational, and optical properties of SnO 2 obtained using first-principles calculations performed within the density functional theory. All the calculated phonon frequencies, real and imaginary parts of complex dielectric function, the energyloss spectrum, the refractive index, the extinction, and the absorption coefficients show good agreement with experimental results. Based on our calculations, the SnO 2 electron and hole effective masses were found to be strongly anisotropic. The lattice contribution to the low-frequency region of the SnO 2 dielectric function arising from optical phonons was also determined resulting the values of e 1\ latt (0) = 14.6 and e 1k latt (0) = 10.7 for directions perpendicular and parallel to the tetragonal c-axis, respectively. This is in excellent agreement with the available experimental data. After adding the electronic contribution to the lattice contribution, a total average value of e 1 (0) = 18.2 is predicted for the static permittivity constant of SnO 2 .

Physical Review B, 2013

ABSTRACT The phonon dispersion and the lattice contribution to the dielectric function of PbTe in... more ABSTRACT The phonon dispersion and the lattice contribution to the dielectric function of PbTe in the NaCl structure are calculated ab initio. The results obtained are in agreement with the available experimental data and reproduce the main features observed in this material, such as an anharmonic LA-TO phonon coupling in the �-X direction, as well as high values for the dielectric constants �e1(0) and e�1(∞). Calculations include the pressure dependence and indicate that the anharmonic phonon coupling is very sensitive to applied pressure. The calculations indicate that the higher values for the dielectric constant together with the anharmonic LA-TO coupling reduce the lattice thermal conductivity and increase the internal electric field, thereby enhancing the electronic conductivity in PbTe, key conditions to increase the thermoelectric figure of merit.

Nanoscale Research Letters, 2012

Transition-metal (TM)-doped diluted magnetic oxides (DMOs) have attracted attention from both exp... more Transition-metal (TM)-doped diluted magnetic oxides (DMOs) have attracted attention from both experimental and theoretical points of view due to their potential use in spintronics towards new nanostructured devices and new technologies. In the present work, we study the magnetic properties of Sn0.96TM0.04O2 and Sn0.96TM0.04O1.98(V O)0.02, where TM = Fe and Co, focusing in particular in the role played by the presence of O vacancies nearby the TM. The calculated total energy as a function of the total magnetic moment per cell shows a magnetic metastability, corresponding to a ground state, respectively, with 2 and 1 μB/cell, for Fe and Co. Two metastable states, with 0 and 4 μB/cell were found for Fe, and a single value, 3 μB/cell, for Co. The spin-crossover energies (E S) were calculated. The values are E S 0/2 = 107 meV and E S 4/2 = 25 meV for Fe. For Co, E S 3/1 = 36 meV. By creating O vacancies close to the TM site, we show that the metastablity and E S change. For iron, a new s...