First-principles study of the ferroelectric Aurivillius phase Bi2WO6

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

In order to better understand the reconstructive ferroelectric-paraelectric transition of Bi2WO6, which is unusual within the Aurivillius family of compounds, we performed first-principles calculations of ... [more ▼]

In order to better understand the reconstructive ferroelectric-paraelectric transition of Bi2WO6, which is unusual within the Aurivillius family of compounds, we performed first-principles calculations of the dielectric and dynamical properties of two possible high-temperature paraelectric structures—the monoclinic phase of A2/m symmetry observed experimentally and the tetragonal phase of I4/mmm symmetry—common to most Aurivillius-phase components. Both paraelectric structures exhibit various unstable modes, which, after their condensation, bring the system toward more stable structures of lower symmetry. The calculations confirm that, starting from the paraelectric A2/m phase at high temperatures, the system must undergo a reconstructive transition to reach the P21ab ferroelectric ground state. [less ▲]

Tunable conductivity threshold at polar oxide interfaces

in Nature Communications (2012)

The physical mechanisms responsible for the formation of a two-dimensional electron gas at the interface between insulating SrTiO3 and LaAlO3 have remained a contentious subject since its discovery in ... [more ▼]

The physical mechanisms responsible for the formation of a two-dimensional electron gas at the interface between insulating SrTiO3 and LaAlO3 have remained a contentious subject since its discovery in 2004. Opinion is divided between an intrinsic mechanism involving the build-up of an internal electric potential due to the polar discontinuity at the interface between SrTiO3 and LaAlO3, and extrinsic mechanisms attributed to structural imperfections. Here we show that interface conductivity is also exhibited when the LaAlO3 layer is diluted with SrTiO3, and that the threshold thickness required to show conductivity scales inversely with the fraction of LaAlO3 in this solid solution, and thereby also with the layer’s formal polarization. These results can be best described in terms of the intrinsic polar-catastrophe model, hence providing the most compelling evidence, to date, in favour of this mechanism. [less ▲]

First-Principles Seebeck Calculation: Implementing a general formula for the calculation of the Seebeck coefficient from first-principles

Conference (2012, May)

Electrical properties of improper ferroelectrics from first principles

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

We study the interplay of structural and polar distortions in hexagonalYMnO3 and short-period PbTiO3/SrTiO3 (PTO/STO) superlattices by means of first-principles calculations at constrained electric ... [more ▼]

We study the interplay of structural and polar distortions in hexagonalYMnO3 and short-period PbTiO3/SrTiO3 (PTO/STO) superlattices by means of first-principles calculations at constrained electric displacement field D. We find that in YMnO3 the tilts of the oxygen polyhedra produce a robustly polar ground state, which persists at any choice of the electrical boundary conditions. Conversely, in PTO/STO the antiferrodistortive instabilities alone do not break inversion symmetry, and open-circuit boundary conditions restore a nonpolar state.We suggest that this qualitative difference naturally provides a route to rationalizing the concept of “improper ferroelectricity” from the point of view of first-principles theory. We discuss the implications of our arguments for the design of novel multiferroic materials with enhanced functionalities and for the symmetry analysis of the phase transitions. [less ▲]

Hydrogen-Saturated Silicon Nanowires Heavily Doped with Interstitial and Substitutional Transition Metals

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 ▲]

Electroresistance Effect in Ferroelectric Tunnel Junctions with Symmetric Electrodes

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 ▲]

First-principles study of thermoelectric properties in SrTiO3 and FeSb2

Poster (2011, October)

First-Principles Study of Electron-Phonon Coupling in STO and 2DEG

Conference (2011, May)

Spontaneous 2-Dimensional Carrier Confinement at the n-Type SrTiO3=LaAlO3 Interface

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 ▲]

Interface Physics in Complex Oxide Heterostructures

in Annual Review of Condensed Matter Physics (2011), 2

Complex transition metal oxides span a wide range of crystalline structures and play host to an incredible variety of physical phenomena. High dielectric permittivities, piezo-, pyro-, and ... [more ▼]

Complex transition metal oxides span a wide range of crystalline structures and play host to an incredible variety of physical phenomena. High dielectric permittivities, piezo-, pyro-, and ferroelectricity are just a few of the functionalities offered by this class of materials, while the potential for applications of the more exotic properties like high temperature superconductivity and colossal magnetoresistance is still waiting to be fully exploited. With recent advances in deposition techniques, the structural quality of oxide heterostructures now rivals that of the best conventional semiconductors, taking oxide electronics to a new level. Such heterostructures have enabled the fabrication of artificial multifunctional materials. At the same time they have exposed a wealth of phenomena at the boundaries where compounds with different structural instabilities and electronic properties meet, giving unprecedented access to new physics emerging at oxide interfaces. Here we highlight some of these exciting new interface phenomena. [less ▲]