We use coherent X-rays to probe the aging dynamics of a metallic glass directly on the atomic level. Contrary to the common assumption of a steady slowing down of the dynamics usually observed in macroscopic studies, we show that the structural relaxation processes underlying aging in this metallic glass are intermittent and highly heterogeneous at the atomic scale. Moreover, physical aging is triggered by cooperative atomic rearrangements, driven by the relaxation of internal stresses. The rich diversity of this behavior reflects a complex energy landscape, giving rise to a unique type of glassy-state dynamics.Physical aging is not only of scientific interest [1,2] but also of great practical importance, as the performance stability of many technologically relevant amorphous materials such as oxide glasses, polymers and metallic glasses (MGs) depends on how aging affects their properties during the expected service life. For MGs in particular, aging can result in severe embrittlement [3] and bring about profound changes to fracture and fatigue properties [4]. Therefore, the ability to understand and characterize the micromechanisms of aging in MGs will play a central role in ensuring their success in a broad range of industrial and commercial applications.A key to understanding aging on the microscopic level is measuring the time scale on which the system rearranges its internal structure. These structural rearrangements are directly related to how fast microscopic density fluctuations in the system decay [5]. The physical quantity describing this is the density correlation function, which can be measured over a wide range of time scales using a variety of complimentary experimental techniques [6]. Additionally, it can be formally defined within the framework of statistical mechanics, allowing for detailed comparisons between experiment, theory [7] and simulation [8]. Access to the density correlation function is therefore fundamental for describing the dynamical behavior of glasses and achieving a consistent picture of structural relaxation on microscopic length scales.X-ray photon correlation spectroscopy (XPCS) has recently emerged as a novel technique for studying the microscopic dynamics of condensed matter [6,[9][10][11]. In XPCS, coherent beams of X-rays are scattered from a sample and the resulting fluctuations in intensity correlated over time. This enables measurement of the density correlation function on the atomic scale. of a unique stress-dominated dynamics characterized by subsequent aging regimes: a fast exponential growth for short waiting times, followed by a long, almost stationary regime. The latter has been observed also in network glasses [13] and is contrary to the steady slowing down of the dynamics during aging observed in macroscopic studies.Here, we investigate the atomic motion in a Pd 43 Cu 27 Ni 10 P 20 MG ribbon using XPCS and show that this system ages in a highly heterogeneous manner, consisting of quiescent periods of stationary dynamics interspersed with cooperative, avalanch...

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