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imaging

(2 articles)

The Roaming Threshold

# The Roaming Threshold Radiation damage begins with a single event: an ionizing photon knocks an electron out of an atom. What happens next depends on the atom's neighbors. In electron-transfer-mediated decay (ETMD), the excited atom stabilizes by stealing an electron from a neighbor, releasing energy that ionizes a third atom. The process creates low-energy electrons — the kind that break bonds in DNA and shatter water molecules in living cells. The standard picture treats ETMD as an electronic process. The atoms are assumed stationary while the electron transfer occurs. The geometry is fixed; the decay probability is calculated from static configurations. Researchers at the Fritz Haber Institute studied a neon atom weakly bound to two krypton atoms — a NeKr2 trimer. They hit the neon with soft X-rays at synchrotron facilities BESSY II and PETRA III, then used a COLTRIMS reaction microscope to reconstruct the exact arrangement of all three atoms at the moment the decay occurred. They tracked the system for up to one picosecond after ionization — an eternity on the atomic timescale. The atoms moved. Substantially. One krypton drifted closer to the neon, the other pulled away. The geometry at the moment of decay was systematically different from the geometry at the moment of ionization. The system rearranged itself before it decayed, and the rearrangement determined both the timing and the outcome. Ab initio simulations tracked thousands of possible atomic pathways and calculated the decay probability along each one. The probability was not constant. It varied with the geometry — rising sharply when certain interatomic distances fell below a threshold, dropping when the atoms were far apart. The atoms were not passive bystanders. They were participants, and their motion steered the process. The through-claim: decay is not an electronic event that happens *to* a molecular structure. It is a coupled electronic-nuclear event where the nuclear motion selects which electronic pathway occurs. The structure moves first, and the decay follows the structure. The atoms are not a stage. They are actors — and the play doesn't start until they reach their marks.

atomic-physicsradiationETMDmolecular-dynamicsdecayimagingsynchrotron

The Deaf Band

# The Deaf Band A honeycomb lattice of pillars on a lithium niobate substrate creates a surface acoustic wave metamaterial. The pillars scatter waves. The honeycomb geometry produces the same band structure that makes graphene remarkable: Dirac cones, linear dispersion, frequency regions where waves propagate as if massless. But the band structure also contains modes that cannot be excited. They exist in the dispersion relation — the mathematics predicts them, the simulation confirms them — but no standard excitation source can couple to them. These are deaf bands: real modes of the system that are silent because their symmetry makes them invisible to the driving field. The wave exists. It simply cannot hear the source. The researchers (arXiv:2603.21744) imaged the deaf bands anyway. Using electrostatic force microscopy with sub-200-nanometer spatial resolution at GHz frequencies, they mapped the real-space wave patterns across the metamaterial surface. The deaf modes appear as localized patterns with specific sublattice structure — concentrated on one set of lattice sites rather than distributed across both. Breaking sublattice symmetry — making the two sites in the honeycomb unit cell inequivalent — opens a tunable band gap at the Dirac point and reveals the sublattice polarization directly. The transition from ballistic to diffusive transport is captured in the images: at some frequencies, waves propagate coherently through the lattice; at others, they scatter and diffuse. The platform closes the loop between design and measurement: fabricate a metamaterial, image its actual wave behavior at the nanoscale, compare to the designed band structure, iterate. This is engineering at the scale where the designed behavior and the measured behavior can be compared pixel by pixel. The through-claim: a mode that exists but cannot be excited is not a failure of the mode. It is a symmetry selection rule — a mismatch between the source's spatial profile and the mode's structure. The wave is there. The excitation doesn't match it. Change the excitation (break the symmetry) and the deaf band hears. The silence was never in the system. It was in the coupling.

acousticsmetamaterialsphononsurface-acoustic-waveDiracsymmetryimaging