We implement and optimize a particle-swap Monte Carlo algorithm that allows us to thermalize a polydisperse system of hard spheres up to unprecedentedly large volume fractions, where previous algorithms and experiments fail to equilibrate. We show that no glass singularity intervenes before the jamming density, which we independently determine through two distinct nonequilibrium protocols. We demonstrate that equilibrium fluid and nonequilibrium jammed states can have the same density, showing that the jamming transition cannot be the end point of the fluid branch.

The dynamics of glass forming liquids shows a tremendous slowing down if temperature is decreased: Relaxation times, transport coefficients such as the diffusion constant or the shear viscosity grow by more than 15 orders of magnitude within narrow temperature range. One of the central goals in the field of glass physics is to understand the origin of this dramatic slowing down of the dynamics.There are several scenarios that seem to be able to explain the slowing down of the dynamics and the low temperature state of the glass former. A very popular one is to rationalize the slow dynamics by invoking the existence of a thermodynamic transition, the so-called ideal glass transition from the liquid to an ideal glass states. This ideal glass transition occurs when the configurational entropy, defined as the logarithm of the number of available states, becomes zero. Due to thissingular behavior of the configurational entropy, the relaxation time andtransport coefficients diverge if the transition point is approached [1].However, confirming the existence of the ideal glass transition is a verydifficult task since in practice the system falls out of equilibriumbefore reaching this putative ideal glass transition temperature,thus making the test of this theory most difficult. Furthermore, otherscenarios are also able to explain the slow dynamics without resortingany thermodynamic transition [2].In the present study, we investigate the existence of the idealglass transition by using computer simulation of a canonical binaryLennard-Jones glass former in three dimensions. Massive computationaleffort and efficient sampling algorithm (parallel tempering) allow us toreach equilibrium states that are very deeply supercooled. By measuringthe configurational entropy directly (obtained via thermodynamicintegration), we find that the configuration entropy does not go to zeroat a finite temperature. This implies that the thermodynamic singularityis indeed avoided before reaching a putative ideal glass transitiontemperature predicted by previous studies. Our results indicates thatcontrary to the previous results the system remains in a liquid statedown to zero temperature without showing an ideal glass transition.Furthermore, we analyze the microscopic structure and the potentialenergy landscape of the system to clarify the mechanism that leads tothe avoidance of the ideal glass transition. We find that locallyfavored structures present in the liquid state hardly change in thetemperature range we consider, which rules out another possibility forthe slow dynamics, e.g. the transformation between two distinct liquids,so-called liquid-liquid transition [3]. Instead, we find the potentialenergy landscape contains extensive number of minima even very lowtemperature, which rationalize the avoided singularity.Our results support a theoretical argument that the ideal glasstransition can not exist at bulk system in finite dimensions [5].[1] Adam G and Gibbs JH, J. Chem. Phys., 43, 139 (1965)[2] Chandler D and Garrahan JP, Annu. Rev. Phys. Chem., 61, 191 (2010)[3] Speck T, Royall CP, and Williams SR, arXiv:1409.0751, (2014)[4] Saksaengwijit A and Heuer A, Phys. Rev. Lett., 93, 235701 (2004)[5] Stillinger FH, J. Chem. Phys., 88, 7818 (1988)

During the last two decades a lot of effort has been devoted to develop a material that could store an applicable amount of hydrogen by physisorption. All these attempts have failed. Therefore, computer simulations have been used to guide the experiment and to determine in advance the potential storage capacity of a particular structure.Usually, to simplify the interaction model and to spare the computation time, the simulations of hydrogen adsorption in nanoporous materials use the superatom representation of H2 molecule with semi-empirical values of interaction model. This approach totally neglects the non-spherical shape of the molecule. However, this information may be crucial for the precise evaluation of the amount stored and the structure of the adsorbed layers, as packing of the spherical and elongated molecules is not the same. Therefore in the present work we compare the structure and storage of H2 in slit-shaped, infinite carbon pores of nanometric width (from 0.6 nm to 2.5 nm), modeled using united atom (UA) and all atom (AA) representation of H2 molecule.We used Grand Canonical Monte Carlo technique to simulate H2 adsorption isotherms at T = 77 K, either within Material Studio software (for AA model) or home-made code (for UA model). We shows that in both models the calculated amount of stored hydrogen is similar. This results confirm the validity of previous UA model-based estimations of storage capacity reported in the literature. Moreover, our simulation shows that UA model slightly overestimates the stored amount in narrowest pores (0.6 – 0.8 nm) and underestimates it in pores of width of 1.0 -1.2 nm. For pores larger than 1.5nm both models give the same results, at least at the adsorption pressure range studied here (1 – 700 bar). In particular, these observations do not depend on pressure.Both models shows that the H2 layer directly in contact with the pore wall is dense, with density largely exceeding the bulk density of liquid hydrogen at 33 K. This results confirm the recent experimental observations of hydrogen densification under confinement in carbon-based nanospaces [1, 2].

In recent years, Raman spectroscopy has become a toolof choice for the structural analysis of glasses, due to its simplicity and thelow cost of acquisition. However, the broad and overlapping peaks observedin the spectra result in a phenomenological and often qualitativeinterpretation. In order to improve the analysis of Raman spectra, and to startto obtain a more quantitative interpretation, one needs to identify the exactcontributions arising from individual structural units.We have used first-principles and combined classical/first-principlesapproaches to create representative atomistic models of some simple binarysoda- and lime-silicates, as well as for some ternary aluminosilicates. Withinthe density functional theory framework, we have calculated the vibrationaldensity of states as well as the IR and Raman spectra of these glass models,and we have found a good agreement with the experimental spectra.The knowledge of the theoretical spectra and the atomic structure has madepossible to identify the signatures of the various constituents of the glasses inthe spectra, as for example the Qi species. The obtained correlations can bethen used to better assign the main bands present in the spectra of morecomplex silicate glasses.

No abstract available

We have recently described how metastable aqueous dispersions of single layer graphene (SLG) can beprepared simply by transferring SLG, prepared by oxidation of fully exfoliated graphenide, into water, with nosurfactant.1 The aqueous graphene dispersions are named eau de Graphene (EdG) to convey the idea of waterwith only graphene inside.Here, we report the intrinsic Raman signatures of graphene dispersed in EdG and we show that theycorrespond to all the expected characteristics of SLG. We stress the difference in these signatures with respect tothose of the natural graphite precursor and to those of some aqueous dispersions of few-layers graphene (FLG)stabilized by surfactants. We discuss the Raman shifts of the G and 2D band in terms of doping and strain of thegraphene flakes2,3. Finally, by comparing the second order Raman signatures (D and D’ bands)3 of EdG and thoseof corresponding thin films, we discuss the nature and amount of defects on the graphene sheets.4,5 All togetherthis provides a full description of the structure and properties of graphene flakes dispersed in water without anyadditive.