Lindhard continuum observed directly for the first time with X-rays

A resonant inelastic X-ray scattering experiment has made it possible to observe the Lindhard continuum directly, the theoretical signature of collective electron excitations in a metal. It is the first direct detection of this structure, predicted decades ago by Jens Lindhard as part of his description of the interacting electron gas, but until now out of reach of conventional spectroscopic techniques. The Lindhard continuum describes how an electron travelling through a conducting medium excites electron-hole pairs across a continuous range of energies and momenta. Although the theory is a pillar of condensed-matter physics and appears in any solid-state textbook, testing it experimentally proved difficult: the individual excitations overlap into a broad, weak background, easily masked by stronger signals such as plasmons or elastic scattering peaks. The key was to combine hard X-rays with resonance: by tuning the incident photon energy to a specific electronic threshold of the material, the scattering cross-section increases enough for the continuum of individual electrons to stand out above the noise. The technique, resonant inelastic X-ray scattering (RIXS), also resolves the energy and momentum transferred to the system simultaneously, so that the two-dimensional shape of the continuum can be compared directly with the calculated Lindhard response function. The results reproduce the shape and extent of the continuum predicted by theory, with quantitative agreement within experimental uncertainty. The measurement opens the door to studying how this continuum is modified in materials where electronic correlations or spin-orbit coupling alter the electron gas response, and to using it as a reference to separate contributions in more complex RIXS spectra.

Vicsek model links coupled structural dynamics and collective order

A quantitative analysis of the original Vicsek model establishes an explicit relationship between the coupled structural dynamics of the particles and the degree of collective order that emerges in the system. The work addresses an open question in the physics of active systems: how correlations in the position and orientation of the agents translate into the onset of ordered collective motion. The Vicsek model, proposed in 1995, describes particles moving at constant speed that adjust their direction towards the average of their neighbours within a given radius, with added angular noise. Despite its simplicity, it reproduces the transition between a disordered state and a state of collectively aligned motion, and has become the reference paradigm for studying active matter. Quantifying precisely how structural correlations feed the global order has, however, proved difficult. The study examines the Vicsek model in its original formulation and quantifies the coupling between the structural dynamics of the system and the order parameter that measures the alignment of the velocities. The resulting relationship allows the collective behaviour to be predicted from the coupled structural fluctuations of the particles. The significance of the result lies in providing a quantitative framework to connect microscopic and macroscopic descriptions in active systems, a bridge that could be extended to bird flocks, fish schools, bacterial suspensions and autonomous robots. Understanding how collective order emerges from local interactions remains one of the central questions of active matter.

NASA improves shock and vibration analysis for launches

Rockets vibrate, shake and flex intensely as they climb into space, and spacecraft packed with delicate scientific instruments must survive those forces. Before any rocket leaves the ground, NASA engineers work to predict how those stresses will affect every part of a spacecraft. To do that, the Dynamic Environments Group within the agency's Launch team has refined its shock and vibration analysis methods, aiming to anticipate with greater fidelity the mechanical environment the payload will face. Predicting these dynamic environments is a critical step in the design of any mission: if the loads are underestimated, an instrument can be damaged during launch; if they are overestimated, the hardware is overbuilt and valuable mass that could be devoted to science is lost. The group's work focuses on characterising how vibrations and shocks propagate through the launch vehicle structure to the spacecraft, a problem that combines ground-test data, structural models and measurements from previous flights. The improvement introduced by NASA engineers allows the response of systems to these loads to be analysed more precisely, refining the tools used to estimate the acoustic and mechanical vibration levels the payload will experience. A more reliable analysis translates into prototypes better prepared for the real conditions of liftoff and into a reduced risk of failure during the most demanding phases of flight. These tools are particularly relevant for upcoming scientific missions, whose instruments — telescopes, detectors and highly sensitive sensors — must arrive intact at their destination orbit. With a better characterised launch environment, design teams can adjust the isolation and mounting of each component with greater confidence, and the agency will have a firmer basis for planning future launches.

Layered Cr₂Ge₂Te₆ shows spin–phonon coupling and isotope-related pseudo-molecular vibrations

The layered compound Cr₂Ge₂Te₆ exhibits coupling between its spin excitations and lattice vibrations, according to a study examining its structural and magnetic dynamics. This kind of spin–phonon interaction is relevant to understanding how excitations propagate in two-dimensional magnetic materials and how they might be controlled through the crystal structure. The work also reveals the presence of pseudo-molecular vibrations linked to the isotope, a phenomenon suggesting that the mass of individual atoms appreciably modulates certain vibrational modes of the crystal. The isotopic dependence of these vibrations provides a way to disentangle which modes are localized on specific sublattices and which correspond to collective motions of the material. Cr₂Ge₂Te₆ is a ferromagnetic semiconductor of interest as a platform for layered magnetic materials, closely related to the family of transition metal dichalcogenides. In these systems, spin–phonon coupling plays a role in phenomena such as magnetic anisotropy, spin transport and domain dynamics, so characterising it precisely is a necessary step for any future application. The identification of isotope-dependent pseudo-molecular vibrations opens up the possibility of tuning lattice dynamics through isotopic substitution, a strategy that does not alter the chemistry of the material but does change its vibrational properties. Combined with the observed spin–phonon coupling, this tool could be used to modulate the magnetic response of the compound.