arXiv subjects

Jon Zubeltzu

Publications and source records attributed to Jon Zubeltzu.

6 recordsLinked to original sources

arXiv2026

Unambiguous characterization of in-plane dielectric response in nanoconfined liquids: water as a case study

The in-plane dielectric constant of nanoconfined water has attracted growing interest over the last years. Nevertheless, this magnitude is not well-defined at the nanoscale due to its dependence on the arbitrary choice of water width. We propose the in-plane 2D polarizability, $α_{\parallel}$, as an unambiguous characterization of the in-plane dielectric response under 2D confinement, in analogy to what has been recently done for the perpendicular response. Using classical molecular dynamics simulations, we compute $α_{\parallel}$ via two independent and consistent methods: based on fluctuation--dissipation theory, and from the induced dipole moment when water is placed in a capacitor. Our results provide the framework to quantify the in-plane dielectric response of polar liquids across simulations and experiments.

cond-mat.soft↗

arXiv2025

Redefining the dielectric response of nanoconfined liquids: insights from water

Recent experiments show that the relative dielectric constant $ε$ of water confined to a film of nanometric thickness reaches a strikingly low value of 2.1, barely above the bulk's 1.8 value for the purely electronic response. We argue that $ε$ is not a well-defined measure for dielectric properties at sub-nanometer scales due to the ambiguous definition of confinement width. Instead we propose the 2D polarizability $α_{\perp}$ as the appropriate, well-defined response function whose magnitude can be directly obtained from both measurements and computations. Once the appropriate description is used, understanding the interplay between electronic and ionic contributions becomes critical, contrary to what is widely assumed. This highlights the importance of electronic degrees of freedom in interpreting the dielectric response of polar fluids under nanoconfinement conditions, as revealed by molecular dynamics simulations.

cond-mat.mes-hall↗

arXiv2017

Simulations of water nano-confined between corrugated planes

Two-dimensionally nanoconfined water between ideal planar walls has been the subject of ample study, aiming at understanding the intrinsic response of water to confinement, avoiding the consideration of the chemistry of actual confining materials. In this work, we study the response of such nanoconfined water under a periodic confining potential by means of computer simulations, both using empirical potentials and from first-principles. We propose a periodic confining potential emulating the atomistic oscillation of the confining walls, which allows varying the lattice parameter and amplitude of the oscillation. We do it for a triangular lattice, with several values of the lattice parameter: one which is ideal for commensuration with layers of Ih ice, and other values that would correspond to more realistic substrates. For the former, an overall rise of the melting temperature is observed. The liquid maintains a bi-layer triangular structure, however, despite the fact that it is not favoured by the external periodicity. The first-principles liquid is significantly affected by the modulation in its layering and stacking even at relatively small amplitudes of the confinement modulation. Beyond some critical modulation amplitude the hexatic phase present in flat confinement is replaced by a trilayer crystalline phase unlike any of the phases encountered for flat confinement. For more realistic lattice parameters, the liquid does not display higher tendency to freeze, but it clearly shows inhomogeneous behaviour as the strength of the rugosity increases. In spite of this expected inhomogeneity, the structural and dynamical response of the liquid is surprisingly insensitive to the external modulation. Although the first-principles calculations give a more triangular liquid than the one observed with empirical potentials, both agree remarkably well for the main conclusions of the study.

cond-mat.soft↗

arXiv2016

Continuous melting through a hexatic phase in confined bilayer water

Liquid water is not only of obvious importance but also extremely intriguing, displaying many anomalies that still challenge our understanding of such an a priori simple system. The same is true when looking at nanoconfined water: The liquid between constituents in a cell is confined to such dimensions, and there is already evidence that such water can behave very differently from its bulk counterpart. A striking finding has been reported from computer simulations for two-dimensionally confined water: The liquid displays continuous or discontinuous melting depending on its density. In order to understand this behavior, we have analyzed the melting exhibited by a bilayer of nanoconfined water by means of molecular dynamics simulations. At high density we observe the continuous melting to be related to the phase change of the oxygens only, with the hydrogens remaining liquid-like throughout. Moreover, we find an intermediate hexatic phase for the oxygens between the liquid and a triangular solid ice phase, following the Kosterlitz-Thouless-Halperin-Nelson-Young theory for two-dimensional melting. The liquid itself tends to maintain the local structure of the triangular ice, with its two layers being strongly correlated, yet with very slow exchange of matter. The decoupling in the behavior of the oxygens and hydrogens gives rise to a regime in which the complexity of water seems to disappear, resulting in what resembles a simple monoatomic liquid. This intrinsic tendency of our simulated water may be useful for understanding novel behaviors in other confined and interfacial water systems.

cond-mat.soft↗

arXiv2016

Enhanced configurational entropy in high-density nanoconfined bilayer ice

A novel kind of crystal order in high-density nanoconfined bilayer ice is proposed from molecular dynamics and density-functional theory simulations. A first-order transition is observed between a low-temperature proton-ordered solid and a high-temperature proton-disordered solid. The latter is shown to possess crystalline order for the oxygen positions, arranged on a close-packed triangular lattice with AA stacking. Uniquely amongst the ice phases, the triangular bilayer is characterized by two levels of disorder (for the bonding network and for the protons) which results in a configurational entropy twice that of bulk ice.

cond-mat.soft↗

arXiv2013

Knock-on damage in bilayer graphene: indications for a catalytic pathway

We study by high-resolution transmission electron microscopy the structural response of bilayer graphene to electron irradiation with energies below the knock-on damage threshold of graphene. We observe that one type of divacancy, which we refer to as the butterfly defect, is formed for radiation energies and doses for which no vacancies are formed in clean monolayer graphene. By using first principles calculations based on density-functional theory, we analyze two possible causes related with the presence of a second layer that could explain the observed phenomenon: an increase of the defect stability or a catalytic effect during its creation. For the former, the obtained formation energies of the defect in monolayer and bilayer systems show that the change in stability is negligible. For the latter, ab initio molecular dynamics simulations indicate that the threshold energy for direct expulsion does not decrease in bilayer graphene as compared with monolayer graphene, and we demonstrate the possibility of creating divacancies through catalyzed intermediate states below this threshold energy. The estimated cross section agrees with what is observed experimentally. Therefore, we show the possibility of a catalytic pathway for creating vacancies under electron radiation below the expulsion threshold energy.

cond-mat.mes-hall↗