First-principles modeling of the thermoelectric properties of SrTiO3/SrRuO3 superlattices

in Physical Review. B, Condensed Matter and Materials Physics (2012), 86

Using a combination of first-principles simulations, based on density functional theory and Boltzmann’s semiclassical theory, we have calculated the transport and thermoelectric properties of the half ... [more ▼]

Using a combination of first-principles simulations, based on density functional theory and Boltzmann’s semiclassical theory, we have calculated the transport and thermoelectric properties of the half-metallic twodimensional electron gas confined in single SrRuO3 layers of SrTiO3/SrRuO3 periodic superlattices. Close to the Fermi energy, we find that the semiconducting majority-spin channel displays a very large in-plane component of the Seebeck tensor at room temperature, S ∼ 1500 μV/K, and the minority-spin channel shows good in-plane conductivity, σ = 2.5 (m cm)−1. However, we find that the total power factor and thermoelectric figure of merit for reduced doping is too small for practical applications. Our results support that the confinement of the electronic motion is not the only thing that matters to describe the main features of the transport and thermoelectric properties with respect the chemical doping, but the shape of the electronic density of states, which in our case departs from the free-electron behavior, is also important. The evolution of the electronic structure, electrical conductivity, Seebeck coefficient, and power factor as a function of the chemical potential is explained by a simplified tight-binding model. We find that the electron gas in our system is composed by a pair of one-dimensional electron gases orthogonal to each other. This reflects the fact the physical dimensionality of the electronic system (1D) can be even smaller than that of the spacial confinement of the carriers (2D). [less ▲]

First-principles study of a pressure-induced spin transition in multiferroic Bi2FeCrO6

in Physical Review. B, Condensed Matter and Materials Physics (2012)

We report on a first-principles study of the structural, electronic, andmagnetic properties of multiferroic double perovskite Bi2FeCrO6, using density functional theory within the local spin-density ... [more ▼]

We report on a first-principles study of the structural, electronic, andmagnetic properties of multiferroic double perovskite Bi2FeCrO6, using density functional theory within the local spin-density approximation (LSDA), the LSDA+Uapproximation as well as a hybrid functional scheme.We showthat Bi2FeCrO6 presents two competing ferrimagnetic phases, sharing the same total magnetic moment of 2μB per unit cell but with a different electronic configuration for the Fe3+ species. The phase with high-spin iron is the ground state at ambient conditions, but we predict that low-spin iron gets stabilized under compression.We also investigate the corresponding ferromagnetic phases, and show that the magnetic couplings sharply decrease when moving form high- to low-spin Fe3+. [less ▲]

Thermopower in oxide heterostructures: The importance of being multiple-band conductors

in Physical Review. B, Condensed Matter and Materials Physics (2012)

Noncollinear magnetism and single-ion anisotropy in multiferroic perovskites

in Physical Review. B, Condensed Matter and Materials Physics (2012), 86(9),

The link between the crystal distortions of the perovskite structure and the magnetic exchange interaction, the single-ion anisotropy (SIA), and the Dzyaloshinsky-Moriya (DM) interaction are investigated ... [more ▼]

The link between the crystal distortions of the perovskite structure and the magnetic exchange interaction, the single-ion anisotropy (SIA), and the Dzyaloshinsky-Moriya (DM) interaction are investigated by means of density functional calculations. Using BiFeO 3 and LaFeO 3 as model systems, we quantify the relationship between the oxygen octahedra rotations, the ferroelectricity, and the weak ferromagnetism (wFM). We recover the fact that the wFM is due to the DM interaction induced by the oxygen octahedra rotations. We find a simple relationship between the wFM, the oxygen rotation amplitude, and the ratio between the DM vector and the exchange parameter such that the wFM increases with the oxygen octahedra rotation when the SIA does not compete with the DM forces induced on the spins. Unexpectedly, we also find that in spite of the d5 electronic configuration of Fe3 +, the SIA is very large in some structures and is surprisingly strongly sensitive to the chemistry of the A-site cation of the ABO 3 perovskite. In the ground R3c state phase we show that the SIA shape induced by the ferroelectricity and the oxygen octahedra rotations are in competition such that it is possible to turn the wFM "on" and "off" through the relative size of the two types of distortion. © 2012 American Physical Society. [less ▲]

Flux avalanches triggered by microwave depinning of magnetic vortices in Pb superconducting films

in Physical Review. B, Condensed Matter and Materials Physics (2011), 84(22), 224511

We observe abrupt changes in broadband microwave permeability of thin Pb superconducting films as functions of the microwave frequency and intensity, as well as of external magnetic field. These changes ... [more ▼]

We observe abrupt changes in broadband microwave permeability of thin Pb superconducting films as functions of the microwave frequency and intensity, as well as of external magnetic field. These changes are attributed to vortex avalanches generated by microwave induced depinning of vortices close to the sample edges. We map the experimental results on the widely used theoretical model assuming reversible response of the vortex motion to ac drive. It is shown that our measurements provide an efficient method of extracting the main parameter of the model—depinning frequencies—for different pinning centers. The observed dependences of the extracted depinning frequencies on the microwave power, magnetic field, and temperature support the idea that the flux avalanches are generated by microwave induced thermomagnetic instabilities. [less ▲]

Band structure modulation of ZnSe/ZnTe nanowires under strain

in Physical Review. B, Condensed Matter and Materials Physics (2011), 84(16), 1654448-16544481654448

First-principles calculations have been used to investigate the structural and electronic properties of unpassivated ZnSe, ZnTe and ZnX/ZnY [X = Se(Te),Y = Te(Se)] core/shell nanowires oriented along the ... [more ▼]

First-principles calculations have been used to investigate the structural and electronic properties of unpassivated ZnSe, ZnTe and ZnX/ZnY [X = Se(Te),Y = Te(Se)] core/shell nanowires oriented along the [111] direction with hexagonal cross sections. The effects of quantum confinement and strain on the electronic properties of the nanowires have been explored for different diameters and core/shell thicknesses. We observe that strong band structure modulation is achievable through uniaxial strain. While for ZnTe nanowires, compression induces a direct-to-indirect band transition for diameters larger than 1.4 nm, there is no sign for a similar transition either for single component ZnSe or core/shell nanowires. The wave function analysis reveals a strong preference for localizing the electron states inside ZnSe rich regions. [less ▲]

First-principles design of efficient solar cells using two-dimensional arrays of core-shell and layered SiGe nanowires

in Physical Review. B, Condensed Matter and Materials Physics (2011), 83(3), 0353176-03531760353176

Research for third generation solar cell technology has been driven by the need to overcome the efficiency and cost problems encountered by current crystalline Si- and thin-film-based solar cells. Using ... [more ▼]

Research for third generation solar cell technology has been driven by the need to overcome the efficiency and cost problems encountered by current crystalline Si- and thin-film-based solar cells. Using first-principles methods, Ge/Si and Si/Ge core/shell and Si-Ge layered nanowires are shown to possess the required qualities for an efficient use in photovoltaic applications. We investigate the details of their band structure, effective mass, absorption property, and charge-carrier localization. The strong charge separation and improved absorption in the visible spectrum indicate a remarkable quantum efficiency that, combined with new designs, compares positively with bulk Si. [less ▲]

Equilibrium and metastable phase transitions in silicon nitride at high pressure: A first-principles and experimental study

in Physical Review. B, Condensed Matter and Materials Physics (2011), 84(1),

We have combined first-principles calculations and high-pressure experiments to study pressure-induced phase transitions in silicon nitride (Si 3N 4). Within the quasi-harmonic approximation, we predict ... [more ▼]

We have combined first-principles calculations and high-pressure experiments to study pressure-induced phase transitions in silicon nitride (Si 3N 4). Within the quasi-harmonic approximation, we predict that the α phase is always metastable relative to the β phase over a wide pressure-temperature range. Our lattice vibration calculations indicate that there are two significant and competing phonon-softening mechanisms in the β-Si 3N 4, while phonon softening in the α-Si 3N 4 is rather moderate. When the previously observed equilibrium high-pressure and high-temperature β → γ transition is bypassed at room temperature (RT) due to kinetic reasons, the β phase is predicted to undergo a first-order structural transformation to a denser P6̄ phase above 39 GPa. The estimated enthalpy barrier height is less than 70 meV/atom, which suggests that the transition is kinetically possible around RT. This predicted new high-pressure metastable phase should be classified as a "postphenacite" phase. Our high-pressure x-ray diffraction experiment confirms this predicted RT phase transition around 34 GPa. No similar RT phase transition is predicted for α-Si 3N 4. Furthermore, we discuss the differences in the pressure dependencies of phonon modes among the α, β, and γ phases and the consequences on their thermal properties. We attribute the phonon modes with negative Grüneisen ratios in the α and β phases as the cause of the predicted negative thermal expansion coefficients (TECs) at low temperatures in these two phases, and predict no negative TECs in the γ phase. © 2011 American Physical Society. [less ▲]

Spin transport in carbon nanotubes with magnetic vacancy-defects

in Physical Review. B, Condensed Matter and Materials Physics (2010), 81(16),

The spin-polarized electron transport properties of metallic carbon nanotubes containing vacancies are investigated using first-principles and nonequilibrium Green’s function techniques. Reconstructed ... [more ▼]

The spin-polarized electron transport properties of metallic carbon nanotubes containing vacancies are investigated using first-principles and nonequilibrium Green’s function techniques. Reconstructed mono- and trivacancies, containing carbon atoms with unsaturated bonds, behave like quasilocalized magnetic impurities. However, in conventional ab initio simulations, these magnetic defects are artificially repeated periodically (supercell method) and are thus incorrectly coupled by long range interactions. Consequently, a technique based on an open system with an isolated magnetic impurity is used here to accurately describe the local magnetic properties of these defects, revealing spin-dependent conductances in tubes, which could be exploited in spintronic nanodevices. [less ▲]

J dependence in the LSDA plus U treatment of noncollinear magnets

in Physical Review. B, Condensed Matter and Materials Physics (2010), 82(22),

We re-examine the commonly used density-functional theory plus Hubbard U (DFT+U) method for the case of noncollinear magnets. While many studies neglect to explicitly include the exchange-correction ... [more ▼]

We re-examine the commonly used density-functional theory plus Hubbard U (DFT+U) method for the case of noncollinear magnets. While many studies neglect to explicitly include the exchange-correction parameter J, or consider its exact value to be unimportant, here we show that in the case of noncollinear magnetism calculations the J parameter can strongly affect the magnetic ground state. We illustrate the strong J dependence of magnetic canting and magnetocrystalline anisotropy by calculating trends in the magnetic lithium orthophosphate family LiMPO(4) (M=Fe and Ni) and difluorite family MF(2) (M=Mn, Fe, Co, and Ni). Our results can be readily understood by expanding the usual DFT+U equations within the spinor scheme, in which the J parameter acts directly on the off-diagonal components which determine the spin canting. [less ▲]