Inelastic electron injection in a water chain

Irradiation of biological matter triggers a cascade of secondary particles that interact with their surroundings, resulting in damage. Low-energy electrons are one of the main secondary species and electron-phonon interaction plays a fundamental role in their dynamics. We have developed a method to capture the electron-phonon inelastic energy exchange in real time and have used it to inject electrons into a simple system that models a biological environment, a water chain. We simulated both an incoming electron pulse and a steady stream of electrons and found that electrons with energies just outside bands of excited molecular states can enter the chain through phonon emission or absorption. Furthermore, this phonon-assisted dynamical behaviour shows great sensitivity to the vibrational temperature, highlighting a crucial controlling factor for the injection and propagation of electrons in water

Dipole-quadrupole interactions and the nature of phase III of compressed hydrogen

A new class of strongly infrared active structures is identified for phase III of compressed molecular H2 by constant-pressure ab initio molecular dynamics and density-functional perturbation calculations. These are planar quadrupolar structures obtained as a distortion of low-pressure quadrupolar phases, after they become unstable at about 150 GPa due to a zone-boundary soft phonon. The nature of the II-III transition and the origin of the IR activity are rationalized by means of simple electrostatics, as the onset of a stabilizing dipole-quadrupole interaction.Comment: 4 pages, 3 figures. To appear in Phys. Rev. Let

Molecular dynamics simulation of surface phenomena due to high electronic excitation ion irradiation in amorphous silica

We studied by means of an atomistic model based on molecular dynamics the thermal evolution of surface atoms in amorphous silica under high electronic excitation produced by irradiation with swift heavy ions. The model was validated with the total and differential yields measured in sputtering experiments with different ions and ion energies showing a very good quantitative prediction capability. Three mechanisms are behind the evolution of the surface region: (1) an ejection mechanism of atoms and clusters with kinetic energy exceeding their binding energy to the sample surface, which explains the experimentally observed angular distributions of emitted atoms, and the correlation of the total sputtering yield with the electronic stopping power and the incidence angle. (2) A collective mechanism of the atoms in the ion track originated by the initial atom motion outwards the track region subsequently followed by the return to the resulting low-density region in the track center. The collective mechanism describes the energy dissipation of bulk atoms and the changes in density, residual stress, defect formation and optical properties. (3) A flow mechanism resulting from the accumulation and subsequent evolution of surface atoms unable to escape. This mechanism is responsible for the crater rim formation.Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. This work was funded by the projects Radiafus-5 (PID2019-105325RB-C32) of Spanish Ministry of Science, Technofusion (S2018/EMT-4437) of Madrid Regional Government and Eurofusion (EH150531176). The authors acknowledge the computer resources and technical assistance provided by the Centro de Supercomputación y Visualización de Madrid (CeSViMa) CESVIMA-MAGERIT. AP acknowledges the support of FONDECYT under grants 3190123. EMB thanks support from grant ANPCyT PICTO-UUMM-2019-00048. JK was supported by the Beatriz Galindo Program (BEAGAL18/00130) from the Ministerio de Educación y Formación Profesional of Spain

Solid molecular hydrogen: The Broken Symmetry Phase

By performing constant-pressure variable-cell ab initio molecular dynamics simulations we find a quadrupolar orthorhombic structure, of $Pca2_1$ symmetry, for the broken symmetry phase (phase II) of solid H2 at T=0 and P =110 - 150 GPa. We present results for the equation of state, lattice parameters and vibronic frequencies, in very good agreement with experimental observations. Anharmonic quantum corrections to the vibrational frequencies are estimated using available data on H2 and D2. We assign the observed modes to specific symmetry representations.Comment: 5 pages (twocolumn), 4 Postscript figures. To appear in Phys. Rev. Let

Production of H2 by water radiolysis in cement paste under electron irradiation: A joint experimental and theoretical study

International audienceLong-term confinement of nuclear waste is one of the main challenges faced by the nuclear industry. Fission products such as 90 Sr and 137 Cs, both β − emitters known to induce serious health hazards, represent the largest fraction of nuclear waste. Cement is a good candidate to store them, provided it can resist the effects of irradiation over time. Here, we have investigated the effects of β − decay on cement by performing electron irradiation experiments on different samples. We show that H 2 production in cement, the main effect of water radiolysis, depends strongly on composition and relative humidity. First-principles calculations indicate that the water-rich interlayer regions with Ca 2+ ions act as electron traps that promote the formation of H 2. They also show that holes localize in water-rich regions in low Ca content samples and are then able to participate in H 2 production. This work provides new understanding of radiolysis effects in cements

Second-principles method for materials simulations including electron and lattice degrees of freedom

We present a first-principles-based (second-principles) scheme that permits large-scale materials simulations including both atomic and electronic degrees of freedom on the same footing. The method is based on a predictive quantum-mechanical theory - e.g., density functional theory - and its accuracy can be systematically improved at a very modest computational cost. Our approach is based on dividing the electron density of the system into a reference part - typically corresponding to the system's neutral, geometry-dependent ground state - and a deformation part - defined as the difference between the actual and reference densities. We then take advantage of the fact that the bulk part of the system's energy depends on the reference density alone; this part can be efficiently and accurately described by a force field, thus avoiding explicit consideration of the electrons. Then, the effects associated to the difference density can be treated perturbatively with good precision by working in a suitably chosen Wannier function basis. Further, the electronic model can be restricted to the bands of interest. All these features combined yield a very flexible and computationally very efficient scheme. Here we present the basic formulation of this approach, as well as a practical strategy to compute model parameters for realistic materials. We illustrate the accuracy and scope of the proposed method with two case studies, namely, the relative stability of various spin arrangements in NiO (featuring complex magnetic interactions in a strongly-correlated oxide) and the formation of a two-dimensional electron gas at the interface between band insulators LaAlO3 and SrTiO3 (featuring subtle electron-lattice couplings and screening effects). We conclude by discussing ways to overcome the limitations of the present approach (most notably, the assumption of a fixed bonding topology), as well as its many envisioned possibilities and future extensions.We thank M. Moreno and J. A. Aramburu for use-ful discussions. P.G.F. and J.J. acknowledge financial sup-port from the Spanish Ministry of Economy and Competitiveness through the MINECO Grant No. FIS2012-37549-C05-04. P.G.F. also acknowledges funding from the Ram ́on y Cajal FellowshipRYC-2013-12515. J.I. is funded by MINECO-Spain Grant MAT2013-40581-P and Fonds National de la Recherche (FNR) Luxembourg Grant FNR/P12/4853155/Kreise