Quasiparticle random-phase approximation in valence spaces: The case of magnetic dipole transitions

Background: The low-energy enhancement observed recently in the deexcitation -ray strength functions, suggested to arise due to the magnetic dipole (M1) radiation, motivates theoretical efforts to improve the description of M1 strength in available nuclear structure models. Reliable theoretical predictions of nuclear dipole excitations are of interest for different nuclear applications and in particular for nuclear astrophysics, where the calculations of radiative capture cross sections often resort to theoretical strength functions. The quasiparticle random-phase approximation (QRPA) approach is arguably the most widely spread microscopic tool in this context since it can be applied to heavy nuclei and at the scale of the entire chart. Purpose: We aim to benchmark the performance of QRPA calculations with respect to M14pt{0ex} strength functions, with a special emphasis on the description of the low-energy effects observed in the deexcitation strength. Methods: We investigate the zero-temperature and finite-temperature (FT) magnetic dipole strength functions computed within the QRPA and compare them to those obtained from exact diagonalizations of the same Hamiltonian in restricted orbital spaces. Our sample consists of 25 spherical and deformed nuclei, with masses ranging from A=26 to A=136, for which the exact diagonalization of the respective effective Hamiltonian in three different valence spaces remains feasible. Results: We find a reasonable agreement for the total photoabsorption strengths between both many-body methods but show that the QRPA distributions are shown to be systematically shifted down in energy with respect to exact results. Photoemission strengths obtained within the FT-QRPA formalism appear insufficient to explain the low-energy enhancement of the M1 strength functions evidenced by the exact diagonalization approach. Conclusions: We ascribe the problems encountered in the zero- and finite-temperature QRPA calculations to the lack of correlations in the nuclear ground state and to the truncation of the many-body space. In particular, the latter prevents obtaining the sufficiently high level density to produce the low-energy enhancement of the M1 strength function, making the (FT-) QRPA approach unsuitable for predictions of such effects across the nuclear chart.