That neutrinos carry a nonvanishing rest mass is evidence of physics beyond the Standard Model of elementary particles. Their absolute mass holds relevance in fields from particle physics to cosmology. We report on the search for the effective electron antineutrino mass with the KATRIN experiment. KATRIN performs precision spectroscopy of the tritium β-decay close to the kinematic endpoint. On the basis of the first five measurement campaigns, we derived a best-fit value of m ν 2 = − 0.14 − 0.15 + 0.13 eV ² , resulting in an upper limit of m _ν < 0.45 eV at 90% confidence level. Stemming from 36 million electrons collected in 259 measurement days, a substantial reduction of the background level, and improved systematic uncertainties, this result tightens KATRIN’s previous bound by a factor of almost two.

A twinned microstructure is frequently observed after a martensitic phase transition. In this paper we investigate the atomic-level processes associated with the twin formation in a model system, using a many-body potential parametrized to represent zirconium. Molecular-dynamics simulations of the martensitic phase transition from bcc to hcp in zirconium show the evolution of a laminated twinned microstructure. Plastic deformation also occurs, creating basal stacking faults. The plastic deformation is such as to cause a rotation of the twins. This alters the twinning angle to the $61.5°$ angle of the low-energy $(10\mathrm{}\overline{1}{1)}_{\mathrm{hcp}}$ twins. These are thus identified as a cause of microscopic irreversibility in the transition.

Using molecular dynamics with a many-body potential fitted to properties of zirconium we study the behavior of phonons at high temperature in the bcc lattice; in particular, the $T_{1N}$ mode which according to the Nishiyama-Wassermann mechanism is the cause of the martensitic transition to hcp in group-IV transition metals. This phonon frequency softens towards the transition but does not tend to zero frequency. In contrast to the fast kinetics of the $\mathrm{bcc}\to \mathrm{hcp}$ transition, the reverse process, i.e., $\mathrm{hcp}\to \mathrm{bcc}$, is not observed in standard molecular dynamics: it can be induced if the $T_{1N}$ phonon is perturbatively excited in the martensitic phase. The sluggishness of the reverse process is attributed to the fact that in the low-temperature hcp phase, the equivalent oscillation to $T_{1N}$ involves two modes with different frequency. The bcc to hcp transition is a first-order transition and occurs in such a way that most of the reversed bcc atoms obey Nishiyama-Wassermann rules, i.e., the path of transformation is reversible. However, there is some plastic damage which is not recovered.

A large class of metal superhydrides was found to be a conventional BCS superconductor under high pressures. In this work, we focused on cerium decahydride, CeH₁₀. Ce is a member of the so–called lability belt in the periodic table, where the physical properties can be largely affected by pressure. It was reported and we confirmed that CeH₁₀ can be formed with the Fm-3m structure, where a cerium atom is embedded in a H₃₂ clathrate cage. Our phonon calculations show that it is dynamically stable at around 300 GPa onwards. We examined the evolution under pressures of the phonons, the electronic states, the Fermi surface, and the electron localization function (ELF). There exists a small van Hove singularity (vHs), and it gradually moves to below the Fermi surface as pressure increases. This behaviour associating with the reducing value of the electron-phonon coupling strength (λ), causes the superconductivity transition temperature (T_c) to gradually reduce under pressures. The maximum T _c is 45 K at 300 GPa.