We use X-Ray Photon Correlation Spectroscopy to investigate the structural relaxation process in a metallic glass on the atomic length scale. We report evidence for a dynamical crossover between the supercooled liquid phase and the metastable glassy state, suggesting different origins of the relaxation process across the transition. Furthermore, using different cooling rates we observe a complex hierarchy of dynamic processes characterized by distinct aging regimes. Strong analogies with the aging dynamics of soft glassy materials, such as gels and concentrated colloidal suspensions, point at stress relaxation as a universal mechanism driving the relaxation dynamics of out-of-equilibrium systems.PACS numbers: 64.70.pe,65.60.+a,64.70.pv Glasses are usually defined as liquids that are trapped in a metastable state from which they slowly evolve toward the corresponding equilibrium phase [1,2]. Although aging is known since centuries, a clear picture of the dynamics in the glassy state is still missing [3,4]. Most of the experimental information available on aging concerns macroscopic quantities, such as viscosity or elastic moduli [5][6][7][8][9][10][11], or focuses on dielectric relaxation [12][13][14][15][16][17], a quantity that is often difficult to relate directly to the particle-level dynamics. From these measurements, a characteristic time for the evolution towards equilibrium can be extracted, but no direct information on the connection between aging and the underlying microscopic dynamics is available. By contrast, a full understanding of aging requires a detailed description of the particle-level dynamics, and in particular of the structural relaxation time τ , the characteristic time for a system to rearrange its structure on the length scale of its constituents. While the structural relaxation has been widely investigated in the liquid phase, only a few studies report on the behaviour of τ below the glass transition temperature, T g [14-16, 18]. Molecular dynamics simulations show that τ increases linearly or sub-linearly with the waiting time, t w , and suggest the possibility of rescaling the measured quantities on a single master curve (time-waiting time superposition principle) [18]. Studies in this direction, however, have led to debated results [5,12,13,17,19]. Consequently, several key questions remain unanswered: what is the fate of the structural relaxation process when the system falls out of equilibrium in the glassy state? How does it depend on the thermal history and the waiting time? What physical mechanism is responsible for structural relaxation? Here, we address these questions by presenting an experimental investigation of the structural relaxation on the atomic length scale in a metallic glass former, in both the supercooled liquid and the glassy state, as a function of temperature and waiting time, and for different thermal histories. We use X-Ray Photon Correlation Spectroscopy (XPCS) to study Mg 65 Cu 25 Y 10 , a well-known glass former with a relatively low T g ∼ 405 K and a st...

We tune the thermodynamics of hydrogen absorption in Mg by means of elastic clamping. The loading isotherms measured by hydrogenography show that Mg films covered with Mg-alloy-forming elements, such as Pd and Ni, have hydrogen plateau pressures more than 2 orders of magnitude higher than bulk Mg at the same temperature. An elastic model allows us to interpret the Mg thickness dependence of the hydrogen plateau pressure. Our results suggest an alternative route for the development of new hydrogen storage materials with optimized thermodynamic properties.

Organic light-emitting diodes with ultrastable glass emission layers show increased efficiency and device stability.

Ultrastable thin film glasses transform into supercooled liquid via propagating fronts starting from the surface and/or interfaces. In this paper, we analyze the consequences of this mechanism in the interpretation of specific heat curves of ultrastable glasses of indomethacin for samples with varying thickness from 20 nm up to several microns. We demonstrate that ultrastable films above 20 nm have identical fictive temperatures and that the apparent change of onset temperature in the specific heat curves originates from the mechanism of transformation and the normalization procedure. An ad hoc surface normalization of the heat capacity yields curves which collapse into a single one irrespective of their thickness. Furthermore, we fit the surface-normalized specific heat curves with a heterogeneous transformation model to evaluate the velocity of the growth front over a much wider temperature interval than previously reported. Our data expands previous values up to Tg + 75 K, covering 12 orders of magnitude in relaxation times. The results are consistent with preceding experimental and theoretical studies. Interestingly, the mobility of the supercooled liquid in the region behind the transformation front remains constant throughout the thickness of the layers.

The thermodynamics of hydrogen absorption in Pd-capped Mg films are strongly dependent on the magnesium thickness. In the present work, we suppress such dependency by inserting a thin Ti layer between Mg and Pd. By means of optical measurements, we show that the surface energy contribution to the destabilization of MgH 2 is negligible. The inserted Ti layer prevents Mg-Pd alloy formation at the Mg/Pd interface, leading to quasifree Mg films and enhancing the kinetics of hydrogen desorption. Our observations are important for the development of thin film devices.

Mg-Ti alloys have uncommon optical and hydrogen absorbing properties, originating from a "spinodal-like" microstructure with a small degree of chemical short-range order in the atomic distribution. In the present study we artificially engineer short-range order by depositing Pd-capped Mg/Ti multilayers with different periodicities. Notwithstanding the large lattice mismatch between Mg and Ti, the as-deposited metallic multilayers show good structural coherence. On exposure to H 2 gas a two-step hydrogenation process occurs with the Ti layers forming the hydride before Mg. From in situ measurements of the bilayer thickness ⌳ at different hydrogen pressures, we observe large out-of-plane expansions of Mg and Ti layers on hydrogenation, indicating strong plastic deformations in the films and a consequent shortening of the coherence length. On unloading at room temperature in air, hydrogen atoms remain trapped in the Ti layers due to kinetic constraints. Such loading/ unloading sequence can be explained in terms of the different thermodynamic properties of hydrogen in Mg and Ti, as shown by diffusion calculations on a model multilayered systems. Absorption isotherms measured by hydrogenography can be interpreted as a result of the elastic clamping arising from strongly bonded Mg/Pd and broken Mg/Ti interfaces.

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We report the thermodynamic measurement of the enthalpy released during the aging of supported films of a molecular glass former, toluene, at temperatures well below the glass transition temperature. By using microfabricated devices with very short equilibration times (below 1 s), we evidence a remarkable variation of the relaxation rate on decreasing film thickness from 100 nm down to a 7 nm thick film. Our results demonstrate that surface atoms are more efficient than bulk atoms in attaining low energy configurations within the potential energy landscape.