arXiv subjects

Daniele Colognesi

Publications and source records attributed to Daniele Colognesi.

7 recordsLinked to original sources

arXiv2026

Negative thermal expansion in ice I polytypes

The fundamental properties of ice have always attracted a lot of interest due to omnipresence of ice in many different natural contexts. Since cubic ice recently become experimentally accessible from a low-density gas hydrate precursor [1, 2], it has been possible to measure its density as a function of temperature in the whole thermodynamic range of metastability. We found strong analogies with respect to the other ice I polytype, i.e., hexagonal ice Ih [3], including the presence of a negative thermal expansion behavior at low temperature. Based on these results, a new enthalpy calculation quantifies the metastable nature of the cubic form and, consequently its inaccessibility from a "normal" ice Ih precursor.

cond-mat.mtrl-sci↗

arXiv2023

Role of the single-particle dynamics in the transverse current autocorrelation function of a liquid metal

A very recent simulation study of the transverse current autocorrelation of the Lennard-Jones fluid revealed, as expected, that this function can be perfectly described within the exponential expansion theory. However, above a certain wavevector $Q$, not only transverse collective excitations are found to propagate in the fluid, but a second oscillatory component of unclear origin (thereby called X) must be considered to properly account for the time behavior of the correlation. Here we present an extended investigation of the transverse current autocorrelation of liquid Au as obtained by ab initio molecular dynamics in the very wide range 5.7 nm$^{-1}$ $\le Q \le$ 32.8 nm$^{-1}$ in order to follow the behavior of the X component, if present, also at large $Q$ values. By combining the study of the transverse current autocorrelation with the analogous analysis of its self part, we show that the second oscillatory component originates from the longitudinal dynamics and appears in the same form as a collective excitation is represented in the single-particle behavior. Therefore, the signature of the longitudinal processes (sound waves) in the transverse current autocorrelation is not due to often conjectured couplings of longitudinal and trasverse modes, but descends from the self part of the function, which contains the traces of all processes acting in the fluid as the density of states, that is the spectrum of the velocity autocorrelation function, does.

cond-mat.stat-mech↗

arXiv2022

Onset of two collective excitations in the transverse dynamics of a simple fluid

A thorough analysis of the transverse current autocorrelation function obtained by molecular dynamics simulations of a dense Lennard-Jones fluid reveals that even such a simple system is characterized by a varied dynamical behavior with changing length scale. By using the exponential expansion theory, we provide a full account of the time correlation at wavevectors $Q$ between the upper boundary of the hydrodynamic region and $Q_p/2$, with $Q_p$ the position of the main peak of the static structure factor. In the $Q$ range studied we identify and accurately locate the wavevector at which shear wave propagation starts to take place, and show clearly how this phenomenon may be represented by a damped harmonic oscillator changing, in a continuous way, from an overdamped to an underdamped condition. The decomposition into exponential modes allows one to convincingly establish not only the crossover related to the onset of transverse waves but, surprisingly, also the existence of a second pair of modes equivalent to another oscillator that undergoes, at higher $Q$ values, a similarly smooth over- to underdamped transition.

cond-mat.stat-mech↗

arXiv2019

Wavelet imaging of transient energy localization in nonlinear systems at thermal equilibrium: the case study of NaI crystals at high temperature

In this paper we introduce a method to resolve transient excitations in time-frequency space from molecular dynamics simulations. Our technique is based on continuous wavelet transform of velocity time series coupled to a threshold-dependent filtering procedure to isolate excitation events from background noise in a given spectral region. By following in time the center of mass of the reference frequency interval, the data can be easily exploited to investigate the statistics of the burst excitation dynamics, by computing, for instance, the distribution of the burst lifetimes, excitation times, amplitudes and energies. As an illustration of our method, we investigate transient excitations in the gap of NaI crystals at thermal equilibrium at different temperatures. Our results reveal complex ensembles of transient nonlinear bursts in the gap, whose lifetime and excitation rate increase with temperature. The method described in this paper is a powerful tool to investigate transient excitations in many-body systems at thermal equilibrium. Our procedure gives access to both the equilibrium and the kinetics of transient excitation processes, allowing one in principle to reconstruct the full picture of the dynamical process under examination.

cond-mat.other↗

arXiv2017

Dynamics of hydrogen guests in ice XVII nanopores

The present high-resolution inelastic neutron scattering experiment on ice XVII, containing molecular hydrogen with different ortho/para ratio, allows to assign the H$_2$ motion spectral bands to rotational and center-of-mass translational transitions of either {\it para}- or {\it ortho}-H$_2$. Due to its structure, ice XVII confines H$_2$ molecules to move in spiral channels of molecular size. Reported data demonstrate that H$_2$ molecules rotate almost freely in these nanometric channels, though showing larger perturbation than in clathrate hydrates, and perform a translational motion exhibiting two low frequency excitations. The good agreement between experimental spectra and corresponding molecular dynamics results clearly enables to portray a picture of the confined motions of a hydrophobic guest within a metastable ice framework, i.e. ice XVII.

cond-mat.other↗

arXiv2016

Refined Structure of Metastable Ice XVII from Neutron Diffraction Measurements

The structure of the recently identified metastable ice XVII, obtained by release of hydrogen from the C$_0$ D$_2$O-H$_2$ compound (filled ice), has been accurately measured by neutron powder diffraction. The diffraction pattern is indexed with a hexagonal cell and can be refined with space group $P6_122$ so to obtain accurate values of the oxygen and deuterium positions. The values of the lattice constants at three temperatures between 25 to 100 K are reported, and their behavior is compared with that of ice Ih. Ice XVII is a porous solid that, if exposed to H$_2$ gas, may adsorb a substantial amount of it. Monitoring this effect at a constant temperature of 50 K, we have observed that the two lattice constants show opposite behavior, $a$ increases and $c$ decreases, with the volume showing a linear increase. At temperatures higher than 130 K the metastability of this form of porous ice is lost and the sample transforms into ice Ih.

cond-mat.mtrl-sci↗

arXiv2010

Nuclear quantum effects in ab initio dynamics: theory and experiments for lithium imide

Owing to their small mass, hydrogen atoms exhibit strong quantum behavior even at room temperature. Including these effects in first principles calculations is challenging, because of the huge computational effort required by conventional techniques. Here we present the first ab-initio application of a recently-developed stochastic scheme, which allows to approximate nuclear quantum effects inexpensively. The proton momentum distribution of lithium imide, a material of interest for hydrogen storage, was experimentally measured by inelastic neutron scattering experiments and compared with the outcome of quantum thermostatted ab initio dynamics. We obtain favorable agreement between theory and experiments for this purely quantum mechanical property, thereby demonstrating that it is possible to improve the modelling of complex hydrogen-containing materials without additional computational effort.

cond-mat.mtrl-sci↗