arXiv subjects

Emilio Artacho

Publications and source records attributed to Emilio Artacho.

At least 73 records · Page 4Linked to original sources

arXiv2012

Van der Waals interaction in magnetic bilayer graphene nanoribbons

We study the interaction energy between two graphene nanoribbons by first principles calculations, including van der Waals interactions and spin polarization. For ultranarrow zigzag nanoribbons, the direct stacking is even more stable than Bernal, competing in energy for wider ribbons. This behavior is due to the magnetic interaction between edge states. We relate the reduction of the magnetization in zigzag nanoribbons with increasing ribbon width to the structural changes produced by the magnetic interaction, and show that when deposited on a substrate, zigzag bilayer ribbons remain magnetic for larger widths.

cond-mat.mes-hall↗

arXiv2012

Electronic stopping power in gold: The role of d electrons and the H/He anomaly

The electronic stopping power of H and He moving through gold is obtained to high accuracy using time-evolving density-functional theory, thereby bringing usual first-principles accuracies into this kind of strongly coupled, continuum non-adiabatic processes in condensed matter. The two key unexplained features of what observed experimentally have been reproduced and understood: (i) The non-linear behaviour of stopping power versus velocity is a gradual crossover as excitations tail into the d-electron spectrum; and (ii) the low-velocity H/He anomaly (the relative stopping powers are contrary to established theory) is explained by the substantial involvement of the d electrons in the screening of the projectile even at the lowest velocities where the energy loss is generated by s-like electron-hole pair formation only.

cond-mat.mtrl-sci↗

arXiv2012

Electrochemical ferroelectric switching: The origin of polarization reversal in ultrathin films

Against expectations, robust switchable ferroelectricity has been recently observed in ultrathin (1 nm) ferroelectric films exposed to air [V. Garcia $et$ $al.$, Nature {\bf 460}, 81 (2009)]. Based on first-principles calculations, we show that the system does not polarize unless charged defects or adsorbates form at the surface. We propose electrochemical processes as the most likely origin of this charge. The ferroelectric polarization of the film adapts to the bound charge generated on its surface by redox processes when poling the film. This, in turn, alters the band alignment at the bottom electrode interface, explaining the observed tunneling electroresistance. Our conclusions are supported by energetics calculated for varied electrochemical scenarios.

cond-mat.mes-hall↗

arXiv2011

Proposal of a one-dimensional electron gas in the steps at the LaAlO$_3$-SrTiO$_3$ interface

The two-dimensional electron gas (2DEG) at the interface between LaAlO$_3$ (LAO) and SrTiO$_3$ (STO) has become one of the most fascinating and highly-debated oxide systems of recent times. Here we propose that a one-dimensional electron gas (1DEG) can be engineered at the step edges of the LAO/STO interface. These predictions are supported by first principles calculations and electrostatic modeling which elucidate the origin of the 1DEG as an electronic reconstruction to compensate a net surface charge in the step edge. The results suggest a novel route to increasing the functional density in these electronic interfaces.

cond-mat.mes-hall↗

arXiv2011

Surface defects and conduction in polar oxide heterostructures

The polar interface between LaAlO$_{3}$ and SrTiO$_{3}$ has shown promise as a field effect transistor, with reduced (nanoscale) feature sizes and potentially added functionality over conventional semiconductor systems. However, the mobility of the interfacial two-dimensional electron gas (2DEG) is lower than desirable. Therefore to progress, the highly debated origin of the 2DEG must be understood. Here we present a case for surface redox reactions as the origin of the 2DEG, in particular surface O vacancies, using a model supported by first principles calculations that describes the redox formation. In agreement with recent spectroscopic and transport measurements, we predict a stabilization of such redox processes (and hence Ti 3$d$ occupation) with film thickness beyond a critical value, which can be smaller than the critical thickness for 2D electronic conduction, since the surface defects generate trapping potentials that will affect the interface electron mobility. Several other recent experimental results, such as lack of core level broadening and shifts, find natural explanation. Pristine systems will likely require changed growth conditions or modified materials with a higher vacancy free energy.

cond-mat.mes-hall↗

arXiv2011

The net charge at interfaces between insulators

The issue of the net charge at insulating oxide interfaces is shortly reviewed with the ambition of dispelling myths of such charges being affected by covalency and related charge density effects. For electrostatic analysis purposes, the net charge at such interfaces is defined by the counting of discrete electrons and core ion charges, and by the definition of the reference polarisation of the separate, unperturbed bulk materials. The arguments are illustrated for the case of a thin film of LaAlO$_3$ over SrTiO$_3$ in the absence of free carriers, for which the net charge is exactly 0.5$e$ per interface formula unit, if the polarisation response in both materials is referred to zero bulk values. Further consequences of the argument are extracted for structural and chemical alterations of such interfaces, in which internal rearrangements are distinguished from extrinsic alterations (changes of stoichiometry, redox processes), only the latter affecting the interfacial net charge. The arguments are reviewed alongside the proposal of Stengel and Vanderbilt [Phys. Rev. B {\bf 80}, 241103 (2009)] of using formal polarisation values instead of net interfacial charges, based on the interface theorem of Vanderbilt and King-Smith [Phys. Rev. B {\bf 48}, 4442 (1993)]. Implications for non-centrosymmetric materials are discussed, as well as for interfaces for which the charge mismatch is an integer number of polarisation quanta.

cond-mat.mtrl-sci↗

arXiv2010

Density, structure and dynamics of water: the effect of Van der Waals interactions

It is known that ab initio molecular dynamics (AIMD) simulations of liquid water, based on the generalized gradient approximation (GGA) to density functional theory (DFT), yield structural and diffusive properties in reasonable agreement with experiment only if artificially high temperatures are used in the simulations. The equilibrium density, at normal conditions, of DFT water has been recently shown by Schmidt et al. [J. Phys. chem. B, 113, 11959 (2009)] to be underestimated by different GGA functionals for exchange and correlation, and corrected by the addition of interatomic pair potentials to describe van derWaals (vdW) interactions. In this contribution we present a DFTAIMD study of liquid water using several GGA functionals as well as the van der Waals density functional (vdW-DF) of Dion et al. [Phys. Rev. Lett. 92, 246401(2004)]. As expected, we find that the density of water is grossly underestimated by GGA functionals. When a vdW-DF is used, the density improves drastically and the experimental diffusivity is reproduced without the need of thermal corrections. We analyze the origin of the density differences between all the functionals. We show that the vdW-DF increases the population of non-H-bonded interstitial sites, at distances between the first and second coordination shells. However, it excessively weakens the H-bond network, collapsing the second coordination shell. This structural problem is partially associated to the choice of GGA exchange in the vdW-DF. We show that a different choice for the exchange functional is enough to achieve an overall improvement both in structure and diffusivity.

cond-mat.mtrl-sci↗

arXiv2009

Topology of the polarization field in ferroelectric nanowires from first principles

The behaviour of the cross-sectional polarization field is explored for thin nanowires of barium titanate from first-principles calculations. Topological defects of different winding numbers have been obtained, beyond the known textures in ferroelectric nanostructures. They result from the inward accommodation of the polarization patterns imposed at the surface of the wire by surface and edge effects. Close to a topological defect the polarization field orients out of the basal plane in some cases, maintaining a close to constant magnitude, whereas it virtually vanishes in other cases.

cond-mat.mtrl-sci↗

arXiv2009

Oxide superlattices with alternating p and n interfaces

The physics of oxide superlattices is considered for pristine (001) multilayers of the band insulators LaAlO3 and SrTiO3 with alternating p and n interfaces. First principles results and a model of capacitor plates offer a simple paradigm to understand their dielectric properties and the insulator to metal transition (IMT) at interfaces with increasing layer thickness. The charge at insulating interfaces is found to be as predicted from the formal ionic charges, not populations. Different relative layer thicknesses produce a spontaneous polarization of the system, and allow manipulation of the interfacial electron gas. Large piezoresistance effects can be obtained from the sensitivity of the IMT to lateral strain. Carrier densities are found to be ideal for exciton condensation.

cond-mat.mes-hall↗

arXiv2008

Novel structural features of CDK inhibition revealed by an ab initio computational method combined with dynamic simulations

The rational development of specific inhibitors for the ~500 protein kinases encoded in the human genome is impeded by a poor understanding of the structural basis for the activity and selectivity of small molecules that compete for ATP binding. Combining classical dynamic simulations with a novel ab initio computational approach linear-scalable to molecular interactions involving thousands of atoms, we have investigated the binding of five distinct inhibitors to the cyclin-dependent kinase CDK2. We report here that polarization and dynamic hydrogen bonding effects, so far undetected by crystallography, affect both their activity and selectivity. The effects arise from the specific solvation patterns of water molecules in the ATP binding pocket or the intermittent formation of hydrogen bonds during the dynamics of CDK/inhibitor interactions and explain the unexpectedly high potency of certain inhibitors such as 3-(3H-imidazol-4-ylmethylene)-5-methoxy-1,3-dihydro-indol-2-one (SU9516). The Lys89 residue in the ATP-binding pocket of CDK2 is observed to form temporary hydrogen bonds with the three most potent inhibitors. This residue is replaced in CDK4 by Thr89, whose shorter side-chain cannot form similar bonds, explaining the relative selectivity of the inhibitors for CDK2. Our results provide a generally applicable computational method for the analysis of biomolecular structures and reveal hitherto unrecognized features of the interaction between protein kinases and their inhibitors

physics.bio-ph↗

arXiv2008

Nature of Structural Transformations in the B2O3 Glass under High Pressure

We report the results of the X-ray diffraction study of B2O3 glass in the pressure interval up to 10 GPa in the 300-700 K temperature range, the results of in-situ volumetric measurements of the glass at pressures up to 9 GPa at room temperature, and first-principles simulations data at pressures up to 250GPa. The behavior of B2O3 glass under pressure can be described as two broad pressure-overlapping transitions. The first transition starts at P > 1 GPa and proceeds without any changes in thecoordination number of the boron atoms; the other one starts at P > 5 GPa and is accompanied by a gradual increase of coordination number. The second transition is completely reversible; the residual densification of the B2O3 glass after decompression is associated with the incomplete reversibility of the first transformation. The fraction of boron atoms transferred to the 4-coordination state in the glass at P < 10 GPa is much smaller than was assumed from indirect experimental data [4], but is considerably larger (by tens of times) than was previously suggested by the classical molecular dynamics simulations [5,6]. The observed transformations under both compression and decompression are quite broad, contrary to the assumption in Ref. [7]. On the basis of ab-initio results, we also predict the one more transformation under higher pressures to a super-dense phase, in which B atoms become 5- and 6-fold coordinated.

cond-mat.dis-nn↗

arXiv2007

Electronic stopping power in insulators from first principles

Using time-dependent density-functional theory we calculate from first principles the rate of energy transfer from a moving proton or antiproton to the electrons of an insulating material, LiF. The behavior of the electronic stopping power versus projectile velocity displays an effective threshold velocity of ~0.2 a.u. for the proton, consistent with recent experimental observations, and also for the antiproton. The calculated proton/antiproton stopping-power ratio is ~2.4 at velocities slightly above the threshold (v~0.4 a.u.), as compared to the experimental value of 2.1. The projectile energy loss mechanism is observed to be stationary and extremely local.

cond-mat.mtrl-sci↗

arXiv2006

Resistive and rectifying effects of pulling gold atoms at thiol-gold nano-contacts

We investigate, by means of first-principles calculations, structural and transport properties of junctions made of symmetric dithiolated molecules placed between Au electrodes. As the electrodes are pulled apart, we find that it becomes energetically favorable that Au atoms migrate to positions between the electrode surface and thiol terminations, with junction structures alternating between symmetric and asymmetric. As a result, the calculated $\emph{IV}$ curves alternate between rectifying and non-rectifying behaviors as the electrodes are pulled apart, which is consistent with recent experimental results.

cond-mat.mtrl-sci↗

arXiv2006

Transformation of spin information into large electrical signals via carbon nanotubes

Spin electronics (spintronics) exploits the magnetic nature of the electron, and is commercially exploited in the spin valves of disc-drive read heads. There is currently widespread interest in using industrially relevant semiconductors in new types of spintronic devices based on the manipulation of spins injected into a semiconducting channel between a spin-polarized source and drain. However, the transformation of spin information into large electrical signals is limited by spin relaxation such that the magnetoresistive signals are below 1%. We overcome this long standing problem in spintronics by demonstrating large magnetoresistance effects of 61% at 5 K in devices where the non-magnetic channel is a multiwall carbon nanotube that spans a 1.5 micron gap between epitaxial electrodes of the highly spin polarized manganite La0.7Sr0.3MnO3. This improvement arises because the spin lifetime in nanotubes is long due the small spin-orbit coupling of carbon, because the high nanotube Fermi velocity permits the carrier dwell time to not significantly exceed this spin lifetime, because the manganite remains highly spin polarized up to the manganite-nanotube interface, and because the interfacial barrier is of an appropriate height. We support these latter statements regarding the interface using density functional theory calculations. The success of our experiments with such chemically and geometrically different materials should inspire adventure in materials selection for some future spintronics

cond-mat.mes-hall↗

arXiv2005

Density functionals and half-metalicity in La2/3Sr1/3MnO3

The electronic structure and equilibrium geometry of La2/3Sr1/3MnO3 are studied theoretically by means of density functional calculations. The doping is treated by introducing holes and a compensating jellium background. The results for the local density approximation (LDA) agree with previous LDA calculations, with an equilibrium volume 5.3% too small and with both majority and minority spin states present at the Fermi level for the relaxed system. The generalised gradient approximation (GGA) offers a qualitatively improved description of the system, with a more realistic volume, and a half-metallic behaviour for the relaxed structure, which enables studies needing theoretical relaxation.The ideal MnO2-terminated (001) surface is then described with explicit doping.

cond-mat.mtrl-sci↗

arXiv2005

On electrons and hydrogen-bond connectivity in liquid water

The network connectivity in liquid water is revised in terms of electronic signatures of hydrogen bonds (HBs) instead of geometric criteria, in view of recent X-ray absorption studies. The analysis is based on ab initio molecular-dynamics simulations at ambient conditions. Even if instantaneous thread-like structures are observed in the electronic network, they continuously reshape in oscillations reminiscent of the r and t modes in ice (tau~170 fs). However, two water molecules initially joint by a HB remain effectively bound over many periods regardless of its electronic signature.

cond-mat.other↗

arXiv2005

Two exchange-correlation functionals compared for first-principles liquid water

The first-principles description of liquid water using ab initio molecular dynamics (AIMD) based on Density Functional theory (DFT) has recently been found to require long equilibration times, giving too low diffusivities and a clear over-structuring of the liquid. In the light of these findings we compare here the room-temperature description offered by two different exchange correlation functionals: BLYP, the most popular for liquid water so far, and RPBE, a revision of the widely used PBE. We find for RPBE a less structured liquid with radial distribution functions closer to the experimental ones than the ones of BLYP. The diffusivity obtained with RPBE for heavy water is still 20% lower than the corresponding experimental value, but it represents a substantial improvement on the BLYP value, one order of magnitude lower than experiment. These characteristics and the hydrogen-bond (HB) network imperfection point to an effective temperature ~3% lower than the actual simulation temperature for the RPBE liquid, as compared with BLYP's ~17% deviation. The too long O--O average nearest-neighbor distance observed points to an excessively weak HB, possibly compensating more fundamental errors in the DFT description.

cond-mat.soft↗

arXiv2005

Network equilibration and first-principles liquid water

Motivated by the very low diffusivity recently found in ab initio simulations of liquid water, we have studied its dependence with temperature, system size, and duration of the simulations. We use ab initio molecular dynamics (AIMD), following the Born-Oppenheimer forces obtained from density-functional theory (DFT). The linear-scaling capability of our method allows the consideration of larger system sizes (up to 128 molecules in this study), even if the main emphasis of this work is in the time scale. We obtain diffusivities that are substantially lower than the experimental values, in agreement with recent findings using similar methods. A fairly good agreement with D(T) experiments is obtained if the simulation temperature is scaled down by ~20%. It is still an open question whether the deviation is due to the limited accuracy of present density functionals or to quantum fluctuations, but neither technical approximations (basis set, localization for linear scaling) nor the system size (down to 32 molecules) deteriorate the DFT description in an appreciable way. We find that the need for long equilibration times is consequence of the slow process of rearranging the H-bond network (at least 20 ps at AIMD's room temperature). The diffusivity is observed to be very directly linked to network imperfection. This link does not appear an artefact of the simulations, but a genuine property of liquid water.

cond-mat.soft↗