Lattice distortion and electronic structure of BaAg2As2 across its nonmagnetic phase transition

${\mathrm{BaAg}}_{2}{\mathrm{As}}_{2}$, a sibling compound of ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ with a nonmagnetic phase transition around 150 K, is studied by the comprehensive measurements of angle-resolved photoemission spectroscopy, synchrotron x-ray diffraction, and resonant soft x-ray scattering. The Fermi surfaces and electronic structure of ${\mathrm{BaAg}}_{2}{\mathrm{As}}_{2}$ are revealed, with strong ${k}_{z}$ dispersion, consistent with the strongly contracted $c/a$ ratio in ${\mathrm{BaAg}}_{2}{\mathrm{As}}_{2}$. Across the phase transition, splitting of [101] Bragg peak is observed, showing a structural distortion with the in-plane distortion magnitude $\ensuremath{\delta}=|a\ensuremath{-}b|/(a+b)=0.0052$. Although the nesting condition is satisfied in some parallel Fermi-surface sectors, there is no signature of charge density wave order at the nesting wave vector. Moreover, neither a charge density wave gap opening nor band reconstruction are observed across the phase transition. Instead, an enhancement on the spectral weight of dispersive bands is observed across the structural phase transition, which can explain the sharp drop of resistivity below the phase transition temperature. These studies could enrich the understanding of the variable and common features of the structural transition in transition metal pnictide layered materials.