Phonons are lattice vibration arising from collective atomic motion and one of the questions popping up is whether the associated physical properties can be explained by the particle picture or by the wave nature of these excitations. Why does this matter? It matters since it is intimately related to energy dissipation, rendering inadequate attempts to transmit useful signals, from few GHz to 100s of THz impacting, e.g., information processing.

We investigated propagating phonons in two-dimensional silicon crystals with various positional disorder of otherwise periodically positioned holes, to ascertain to what degree disorder matters in their propagation, since they carry most of the heat in semiconductors materials, underpinning today’s (opto)electronics and nanoelectronics. We used a state-of-the-art pump-and-probe spectroscopy (see fig. 1) and a home-built two-laser Raman thermometry (2LRT) set up to extract the thermal conductivity. We measure at room temperature and simulate by finite element methods. We found that disorder lowers phonon energies and facilitates their in-plane propagation becoming impervious to disorder.  Surprisingly, disorder was found to be unaffected by disorder. We thus propose a practical criterion for predicting phonon coherence as a function of roughness and disorder: (i) phonon coherence is unaffected if the roughness R is smaller than 1/25 of the phonon wavelength and (ii) phonon coherence is destroyed if R is greater than 1/10 of the phonon wavelength.

Our results have repercussions well beyond our own research in nano-scale thermal transport and thermoelectricity. They provide a hint to the factors affecting vibrational energy transport in condensed matter with variations in the nm-scale, the length scale of acoustic phonons relevant to electronics but also to biology.

As time goes by, continuing revolutions of the largest gamma-ray astronomy satellite ever flown, Fermi, allows for deeper and deeper studies of the sky. Joined by mature ground-based observational facilities at even higher gamma-ray frequencies, like MAGIC at TeV energies (TeV = 1e12 eV), as well as by a plethora of concurrent observations at lower energies (from radio to optical to X-rays), significant discoveries are driving theoretical research.

This year has seen the announcement of the most energetic light ever observed from a pulsar, for the first time with energies of more than trillion electron volts, about a thousand times larger than previously observed, arriving at the detector concurrently with the pulsar period. These photons are thus coming from the close proximity of the rotating neutron star, but exactly how and where they are generated is unknown. No theory to date can cope with such measurements.

Another first-timer this year has been the detection, at similarly such high energies, of a years-long recurrent variability. Every 4.2 years, the TeV emission from a gamma-ray binary has been seen to oscillate, in an effect theoretically predicted a few years before (Torres, D. F., et al. 2012, ApJ, 744, 106). Much is yet to learn from the recurrence of such oscillation and how can, perhaps, be driven by oscillations in the surrounding circumstellar disk of the companion.

Finally, the spatial connection of the archetypical, accreting millisecond pulsar with a gamma-ray source has been also noted for the first time. If this connections proves real, for instance via a detection of gamma-ray pulsations, it would imply rotationally-powered activity in quiescence mode, showing evidence of a transition to a rotation-powered radio pulsar state in X-ray quiescence, whilst it is observed as an accreting pulsar when it has a disk.