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See detailFirst-principles modeling of the thermoelectric properties of SrTiO3/SrRuO3 superlattices
Garcia-Fernandez, Pablo; Verissimo-Alves, Marcos; Bilc, Daniel ULg et al

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 ▲]

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See detailElectroresistance Effect in Ferroelectric Tunnel Junctions with Symmetric Electrodes
Bilc, Daniel ULg; Novaes, F. D.; Iniguez, J. et al

in ACS Nano (2012), 6(2), 1473-1478

Understanding the effects that govern electronic transport in ferroelectric tunnel junctions (FTJs) is of vital importance to improve the efficiency of devices such as ferroelectric memories with ... [more ▼]

Understanding the effects that govern electronic transport in ferroelectric tunnel junctions (FTJs) is of vital importance to improve the efficiency of devices such as ferroelectric memories with nondestructive readout. However, our current knowledge (typically based on simple semiempirical models or first-principles calculations restricted to the limit of zero bias) remains partial, which may hinder the development of more efficient systems. For example, nowadays it is commonly believed that the tunnel electroresistance (TER) effect exploited in such devices mandatorily requires, to be sizable, the use of two different electrodes, with related potential drawbacks concerning retention time, switching, and polarization imprint. In contrast, here we demonstrate at the first-principles level that large TER values of about 200% can be achieved under finite bias in a prototypical FTJ with symmetric electrodes. Our atomistic approach allows us to quantify the contribution of different microscopic mechanisms to the electroresistance, revealing the dominant role of the inverse piezoelectric response of the ferroelectric. On the basis of our analysis, we provide a critical discussion of the semiempirical models traditionally used to describe FTJs. [less ▲]

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See detailHighly Confined Spin-Polarized Two-Dimensional Electron Gas in SrTiO3/SrRuO3 Superlattices
Verissimo-Alves, M.; Garcia-Fernandez, P.; Bilc, Daniel ULg et al

in Physical Review Letters (2012), 108

We report first-principles characterization of the structural and electronic properties of (SrTiO3)5/(SrRuO3)1 superlattices. We show that the system exhibits a spin-polarized two-dimensional electron gas ... [more ▼]

We report first-principles characterization of the structural and electronic properties of (SrTiO3)5/(SrRuO3)1 superlattices. We show that the system exhibits a spin-polarized two-dimensional electron gas, extremely confined to the 4d orbitals of Ru in the SrRuO3 layer. Every interface in the superlattice behaves as a minority-spin half-metal ferromagnet, with a magnetic moment of µ = 2.0µB/SrRuO3 unit. The shape of the electronic density of states, half-metallicity, and magnetism are explained in terms of a simplified tight-binding model, considering only the t2g orbitals plus (i) the bidimensionality of the system and (ii) strong electron correlations. [less ▲]

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See detailHydrogen-Saturated Silicon Nanowires Heavily Doped with Interstitial and Substitutional Transition Metals
Durgun, E.; Bilc, Daniel ULg; Ciraci, S. et al

in Journal of Physical Chemistry C: Nanomaterials, Interfaces, and Hard Matter (2012), 116

We report a first-principles systematic study of atomic, electronic, and magnetic properties of hydrogen-saturated silicon nanowires (H-SiNW) that are heavily doped by transition metal (TM) atoms placed ... [more ▼]

We report a first-principles systematic study of atomic, electronic, and magnetic properties of hydrogen-saturated silicon nanowires (H-SiNW) that are heavily doped by transition metal (TM) atoms placed at various interstitial and substitutional sites. Our results obtained within the conventional GGA+U approach have been confirmed using a hybrid functional. To reveal the surface effects, we examined three different possible facets of HSiNW along the [001] direction with a diameter of ∼2 nm. The energetics of doping and resulting electronic and magnetic properties are examined for all alternative configurations. We found that except Ti, the resulting systems have a magnetic ground state with a varying magnetic moment. Whereas H-SiNWs are initially nonmagnetic semiconductor, they generally become ferromagnetic metal upon TM doping. They can even exhibit half-metallic behavior for specific cases. Our results suggest that H-SiNWs functionalized by TM impurities form a new type of dilute magnetic semiconductor potentially attractive for new electronic and spintronic devices on the nanoscale. [less ▲]

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See detailSpontaneous 2-Dimensional Carrier Confinement at the n-Type SrTiO3=LaAlO3 Interface
Delugas, P.; Filippetti, A.; Fiorentini, V. et al

in Physical Review Letters (2011), 106

We describe the intrinsic mechanism of 2-dimensional electron confinement at the n-type SrTiO3=LaAlO3 interface as a function of the sheet carrier density ns via advanced first-principles calculations ... [more ▼]

We describe the intrinsic mechanism of 2-dimensional electron confinement at the n-type SrTiO3=LaAlO3 interface as a function of the sheet carrier density ns via advanced first-principles calculations. Electrons localize spontaneously in Ti 3dxy levels within a thin (& 2 nm) interface-adjacent SrTiO3 region for ns lower than a threshold value nc 1014 cm 2. For ns >nc a portion of charge flows into Ti 3dxz-dyz levels extending farther from the interface. This intrinsic confinement can be attributed to the interface-induced symmetry breaking and localized nature of Ti 3d t2g states. The sheet carrier density directly controls the binding energy and the spatial extension of the conductive region. A direct, quantitative relation of these quantities with ns is provided. [less ▲]

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See detailElectronic and thermoelectric properties of Fe2VAl: The role of defects and disorder
Bilc, Daniel ULg; Ghosez, Philippe ULg

in Physical Review. B, Condensed Matter and Materials Physics (2011), 83

Using first-principles calculations, we show that Fe2VAl is an indirect band-gap semiconductor. Our calculations reveal that its semimetallic character (which is sometimes assigned) is not an ntrinsic ... [more ▼]

Using first-principles calculations, we show that Fe2VAl is an indirect band-gap semiconductor. Our calculations reveal that its semimetallic character (which is sometimes assigned) is not an ntrinsic property but originates from magnetic antisite defects and site disorder, which introduce localized in-gap and resonant states changing the electronic properties close to the band gap. These states negatively affect the thermopower S and the power factor equal to S2σ, decreasing the good thermoelectric performance of intrinsic Fe2VAl. [less ▲]

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See detailFirst-principles calculations of the structural and dynamical properties of ferroelectric YMnO3
Prikockyte, Alina ULg; Bilc, Daniel ULg; Hermet, Patrick ULg et al

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

We report the structural and dynamical properties of the ground state ferroelectricP63cm structure of hexagonal YMnO3. The lattice parameters, atomic positions, local magnetic moment of Mn atoms, band gap ... [more ▼]

We report the structural and dynamical properties of the ground state ferroelectricP63cm structure of hexagonal YMnO3. The lattice parameters, atomic positions, local magnetic moment of Mn atoms, band gap, and -point phonons are calculated within the local spin-density approximation plus Hubbard U term and the B1-WC hybrid functional. Our results are discussed in comparison to theoretical and experimental values available in the literature. Based on our first-principles calculations, we propose a reassignment of certain modes. We also discuss the relation between the phonon modes of the ferroelectric P63cm phase and those of the paraelectric P63/mmc phase that condense at the phase transition. [less ▲]

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See detailHybrid functional study of prototypical multiferroic bismuth ferrite
Goffinet, Marco ULg; Hermet, Patrick ULg; Bilc, Daniel ULg et al

in Physical Review. B, Condensed Matter and Materials Physics (2009), 79(1),

We report a systematic comparison of various exchange-correlation functionals for the prediction of the structural, magnetic, electronic, and dynamical (phonons and Born effective charge tensors ... [more ▼]

We report a systematic comparison of various exchange-correlation functionals for the prediction of the structural, magnetic, electronic, and dynamical (phonons and Born effective charge tensors) properties of bismuth ferrite, a prototypical multiferroic compound. We have not only considered the usual approximations to density-functional theory such as the local-density approximation (LDA), generalized (GGA), and LDA+U, but also hybrid approaches such as B3LYP and B1. The recent B1-WC hybrid functional of Bilc [Phys. Rev. B 77, 165107 (2008)], with the GGA functional of Wu and Cohen and an exact exchange mixing parameter of 0.16, provides very good overall agreement with experiments and can be considered as a valuable alternative to LDA, GGA, and DFT+U for the study of bismuth ferrite. This does not only allow a reliable interpretation of the physical properties of this specific compound but also opens perspectives for further and more predictive first-principles investigations of multiferroic materials. [less ▲]

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See detailA new hybrid exchange-correlation functional for accurate prediction of the electronic and structural properties of ferroelectric oxides
Bilc, Daniel ULg; Orlando, R.; Shaltaf, R. et al

in Physical Review. B, Condensed Matter and Materials Physics (2008), 77

Using a linear combination of atomic orbitals approach, we report a systematic comparison of various density functional theory (DFT) and hybrid exchange-correlation functionals for the prediction of the ... [more ▼]

Using a linear combination of atomic orbitals approach, we report a systematic comparison of various density functional theory (DFT) and hybrid exchange-correlation functionals for the prediction of the electronic and structural properties of prototypical ferroelectric oxides. It is found that none of the available functionals is able to provide, at the same time, accurate electronic and structural properties of the cubic and tetragonal phases of BaTiO3 and PbTiO3. Some, although not all, usual DFT functionals predict the structure with acceptable accuracy, but always underestimate the electronic band gaps. Conversely, common hybrid functionals yield an improved description of the band gaps, but overestimate the volume and atomic distortions associated with ferroelectricity, giving rise to an unacceptably large c/a ratio for the tetragonal phases of both compounds. This supertetragonality is found to be induced mainly by the exchange energy corresponding to the generalized gradient approximation (GGA) and, to a lesser extent, by the exact exchange term of the hybrid functional. We thus propose an alternative functional that mixes exact exchange with the recently proposed GGA of Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] which, for solids, improves over the treatment of exchange of the most usual GGA's. The new functional renders an accurate description of both the structural and electronic properties of typical ferroelectric oxides. [less ▲]

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