Quantum high energy density physics

Abstract:

We present in situ x-ray diffraction and velocity measurements of Fe2⁢O3 under laser shock compression at pressures between 38–122 GPa. None of the high-pressure phases reported by static compression studies were observed. Instead, we observed an isostructural phase transition from 𝛼−Fe2⁢O3 to a new 𝛼′−Fe2⁢O3 phase at a pressure of 50–62 GPa. The 𝛼′−Fe2⁢O3 phase differs from 𝛼−Fe2⁢O3 by an 11% volume drop and a different unit cell compressibility. We further observed a two-wave structure in the velocity profile, which can be related to an intermediate regime where both 𝛼 and 𝛼′ phases coexist. Density functional theory calculations with a Hubbard parameter indicate that the observed unit cell volume drop can be associated with a spin transition following a magnetic collapse.

Abstract:

Spectroscopic techniques are essential for studying material properties, but the small cross-sections of some methods may result in low signal-to-noise ratios (SNRs) in the collected spectra. In this article we present methods, based on combining Bragg spectroscopy with photon counting and deconvolution algorithms, which increase the SNRs, making the spectra better suited to further analysis. We aim to provide a comprehensive guide for constructing spectra from camera images. The efficacy of these methods is validated on synthetic and experimental data, the latter coming from the field of high-energy density (HED) science, where x-ray spectroscopy is essential for the understanding of materials under extreme thermodynamic conditions.

Abstract:

We present new exclusion bounds obtained at the European X-Ray Free Electron Laser facility (EuXFEL) on axionlike particles in the mass range $10^{-3}\,\mathrm{eV} \lesssim m_a \lesssim 10^{4}\,\mathrm{eV}$. Our experiment exploits the Primakoff effect via which photons can, in the presence of a strong external electric field, decay into axions, which then convert back into photons after passing through an opaque wall. While similar searches have been performed previously at a third-generation synchrotron [Yamaji et al., Phys.\ Lett.\ B 782, 523 (2018)], our work demonstrates improved sensitivity, exploiting the higher brightness of x-rays at EuXFEL.