arXiv subjects

Mauricio Sevilla

Publications and source records attributed to Mauricio Sevilla.

5 recordsLinked to original sources

arXiv2026

A hidden bulk polymorph governs charge transport dimensionality in an organic semiconductor

Organic semiconductors (OSCs) are widely explored for flexible optoelectronic technologies, with performance governed not only by molecular design, but also by solid-state packing, which can give rise to polymorphism. Dinaphthothienothiophene (DNTT) is a benchmark OSC that has long been considered monomorphic. Here, we discover, isolate, and resolve the crystal structure of a previously unrecognised bulk polymorph of DNTT, termed blue DNTT owing to its characteristic blue emission. Coexisting with the well-known (green) DNTT in commercial powders, yet previously overlooked, blue DNTT represents the thermodynamically stable form. By combining X-ray diffraction, Raman, and THz spectroscopy with simulations, we demonstrate that polymorphism in DNTT reshapes the low-frequency phonon landscape and transfer-integral network, impacting charge transport. While green DNTT exhibits two-dimensional charge transport with holes more mobile than electrons, blue DNTT shows charge transport along all crystallographic directions enabled by a distinct herringbone packing. Electron mobility along the crystallographic a and b-axes in blue DNTT exceeds twice the hole mobility in the green phase. To our knowledge, this is the first reported acene-based semiconductor exhibiting three-dimensional charge transport. Polymorphism emerges as a key lever to tune charge transport dimensionality and carrier efficiency in organic semiconductors.

cond-mat.mtrl-sci↗

arXiv2024

Density Fluctuations, Solvation Thermodynamics and Coexistence Curves in Grand Canonical Molecular Dynamics Simulations

Fluid transport across nanometric channels induced by electric, pressure and concentration gradients is ubiquitous in biological systems and fosters various applications. In this context, computer simulation setups with well-defined open-boundary equilibrium starting states are essential in understanding and assisting experimental studies. However, open-boundary computational methods are scarce and typically do not satisfy all the equilibrium conditions imposed by reality. Namely, in the absence of external gradients, 1) the system of interest (SoI) must be at thermodynamic and chemical equilibrium with an infinite reservoir of particles, 2) the fluctuations of the SoI in equilibrium should sample the grand canonical ensemble, 3) the local solvation thermodynamics, which is extremely sensitive to finite-size effects due to solvent depletion, should be correctly described. This point is particularly relevant for out-of-equilibrium systems. Finally, 4) the method should be robust enough to deal with phase transitions and coexistence conditions in the SoI. In this study, we demonstrate with prototypical liquid systems embedded into a reservoir of ideal gas particles that the adaptive resolution simulation (AdResS) method, coupled with particle insertion/deletion steps, satisfies all these requirements. Therefore, this AdResS setup is suitable for performing equilibrium and non-equilibrium simulations of open systems.

cond-mat.soft↗

arXiv2023

Finite-size excess-entropy scaling for simple liquids

We introduce and validate a finite-size two-body excess entropy integral equation. By using analytical arguments and computer simulations of prototypical simple liquids, we show that the excess entropy $s_2$ exhibits a finite-size scaling with the inverse of the linear size of the simulation box. Since the self-diffusivity coefficient $D^*$ displays a similar finite-size effect, we show that the scaling entropy relation $D^*=A\exp(αs_2)$ also depends on the simulation box size. By extrapolating to the thermodynamic limit, we report values for the coefficients $A$ and $α$ that agree well with values available in the literature. Finally, we find a power law relation between the scaling coefficients for $D^*$ and $s_2$, suggesting a constant viscosity to entropy ratio.

cond-mat.soft↗

arXiv2022

Finite-size scaling and thermodynamics of model supercooled liquids: Long-range concentration fluctuations and the role of attractive interactions

We compute partial structure factors, Kirkwood-Buff integrals (KBIs) and chemical potentials of model supercooled liquids with and without attractive interactions. We aim at investigating whether relatively small differences in the tail of the radial distribution functions result in contrasting thermodynamic properties. Our results suggest that the attractive potential favours the nucleation of long-range structures. Indeed, upon decreasing temperature, Bathia-Thornton structure factors display anomalous behaviour in the $k\to 0$ limit. KBIs extrapolated to the thermodynamic limit confirm this picture, and excess coordination numbers identify the anomaly with long-range concentration fluctuations. By contrast, the purely repulsive system remains perfectly miscible for the same temperature interval and only reveals qualitatively similar concentration fluctuations in the crystalline state. Furthermore, differences in both isothermal compressibilities and chemical potentials show that thermodynamics is not entirely governed by the short-range repulsive part of the interaction potential, emphasising the nonperturbative role of attractive interactions. Finally, at higher density, where both systems display nearly identical dynamical properties and repulsive interactions become dominant, the anomaly disappears, and both systems also exhibit similar thermodynamic properties.

cond-mat.soft↗

arXiv2021

Connecting density fluctuations and Kirkwood-Buff integrals for finite-size systems

Kirkwood-Buff integrals (KBI) connect the microscopic structure and thermodynamic properties of liquid solutions. KBI are defined in the grand canonical ensemble and evaluated assuming the thermodynamic limit (TL). In order to reconcile analytical and numerical approaches, finite-size KBI have been proposed in the literature, resulting in two strategies to obtain their TL values from computer simulations. (i) The spatial block-analysis method in which the simulation box is divided into subdomains of volume $V$ to compute fluctuations of the number of particles. (ii) A direct integration method where a corrected radial distribution function and a kernel that accounts for the geometry of the integration subvolumes are combined to obtain KBI as a function of $V$. In this work, we propose a method that connects both strategies into a single framework. We start from the definition of finite-size KBI, including the integration subdomain and an asymptotic correction to the radial distribution function, and solve them in Fourier space where periodic boundary conditions are trivially introduced. The limit $q\to 0$, equivalent to the value of the KBI in the TL, is obtained via the spatial block-analysis method. When compared to the latter, our approach gives nearly identical results for all values of $V$. Moreover, all finite-size effect contributions (ensemble, finite-integration domains and periodic boundary conditions) are easily identifiable in the calculation. This feature allows us to analyse finite-size effects independently and extrapolate the results of a single simulation to different box sizes. To validate our approach, we investigate prototypical systems, including SPC/E water and aqueous urea mixtures.

cond-mat.stat-mech↗