Nowadays powerful X-ray sources like synchrotrons and free-electron lasers are considered as ultimate tools for probing microscopic properties in materials. However, the correct interpretation of such experiments requires a good understanding on how the beam affects the properties of the sample, knowledge that is currently lacking for intense X-rays. Here we use X-ray photon correlation spectroscopy to probe static and dynamic properties of oxide and metallic glasses. We find that although the structure does not depend on the flux, strong fluxes do induce a non-trivial microscopic motion in oxide glasses, whereas no such dependence is found for metallic glasses. These results show that high fluxes can alter dynamical properties in hard materials, an effect that needs to be considered in the analysis of X-ray data but which also gives novel possibilities to study materials properties since the beam can not only be used to probe the dynamics but also to pump it.

Recent diffraction experiments on metallic glasses have unveiled an unexpected non-cubic scaling 14 law between density and average interatomic distance, which lead to the speculations on the presence of 15 fractal glass order. Using X-ray tomography we identify here a similar non-cubic scaling law in 16 disordered granular packing of spherical particles. We find that the scaling law is directly related to the 17 contact neighbors within first nearest neighbor shell, and therefore is closely connected to the 18 phenomenon of jamming. The seemingly universal scaling exponent around 2.5 arises due to the isostatic 19 condition with contact number around 6, and we argue that the exponent should not be universal. .

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)