Search

Many of the most sensitive physics experiments searching for rare events, like neutrinoless double beta ($0\nu\beta\beta$) decay and dark matter interactions, rely on cryogenic macro-calorimeters operating at the mK-scale. Located underground at the Gran Sasso National Laboratory (LNGS), in central Italy, CUORE (Cryogenic Underground Observatory for Rare Events) is one of the leading experiments for the search of $0\nu\beta\beta$ decay, implementing the low-temperature calorimetric technology. We present a novel multi-detector analysis to correlate environmental phenomena with the low-frequency noise of low-temperature calorimeters. Indeed, the correlation of marine and seismic data with data from a pair of CUORE detectors indicates that cryogenic detectors are sensitive not only to intense vibrations generated by earthquakes, but also to the much fainter vibrations induced by marine microseisms in the Mediterranean Sea due to the motion of sea waves. Proving that cryogenic macro-calorimeters are sensitive to such environmental sources of noise opens the possibility of studying their impact on the detectors physics-case sensitivity. Moreover, this study could pave the road for technology developments dedicated to the mitigation of the noise induced by marine microseisms, from which the entire community of cryogenic calorimeters can benefit.

By introducing electron screening in the extended Koopmans' theorem we correctly describe the band gap opening in weakly as well as strongly correlated systems. We show this by applying our method to bulk LiH, Si, and paramagnetic as well as antiferromagnetic NiO. Although incorrect features remain in the full photoemission spectra, this is a remarkable result for an ab-initio electronic structure method and it opens the way to a unified description of photoemission spectra at weak and strong correlation.

In this work we get insight into the impact of reduced density matrix functionals on the quality of removal/addition energies obtained using the Extended Koopmans' Theorem (EKT). Within reduced density matrix functional theory (RDMFT) the EKT approach reduces to a matrix diagonalization, whose ingredients are the one- and two-body reduced density matrices. A striking feature of the EKT within RDMFT is that it opens a band gap, although too large, in strongly correlated materials, which are a challenge for state-of-the-art methods such as GW . Using the one-dimensional Hubbard model and the homogeneous electron gas as test cases, we find that: i) with exact or very accurate density matrices the EKT systematically overestimates the band gap in the Hubbard model and the bandwidth in the homogeneous electron gas; ii) with approximate density matrices, instead, the EKT can benefit from error cancellation. In particular we test a new approximation which combines RPA screening with the Power functional (PF) approximation to the two-body reduced density matrix introduced by Sharma et al. [Phys. Rev. B 78, 201103(R) (2008)]. An important feature of this approximation is that it reduces the EKT band gap in the studied models; it can hence be a promising approximation for correcting the EKT band-gap overestimation in strongly correlated materials.

The Extended Koopman's Theorem (EKT) provides a straightforward way to compute charged excitations from any level of theory. In this work we make the link with the many-body effective energy theory (MEET) that we derived to calculate the spectral function, which is directly related to photoemission spectra. In particular, we show that at its lowest level of approximation the MEET removal and addition energies correspond to the so-called diagonal approximation of the EKT. Thanks to this link, the EKT and the MEET can benefit from mutual insight. In particular, one can readily extend the EKT to calculate the full spectral function, and choose a more optimal basis set for the MEET by solving the EKT secular equation. We illustrate these findings with the examples of the Hubbard dimer and bulk silicon., Comment: 7 pages, 3 figures

In this work we provide an exhaustive study of the photemission spectrum of paramagnetic FeO under pressure using a refined version of our recently derived many-body effective energy theory (MEET). We show that, within a nonmagnetic description of the paramagnetic phase, the MEET gives an overall good description of the photoemission spectrum at ambient pressure as well as the changes it undergoes by increasing pressure. In particular at ambient pressure the band gap opens between the mixed Fe $t_{2g}$ and O $2p$ states and the Fe 4s states and, moreover, a $d$-$d$ gap opens, which is compatible with a high-spin configuration (hence nonzero local magnetic moments as observed in experiment), whereas decreasing pressure the band gap tends to close, $t_{2g}$ states tend to become fully occupied and $e_{g}$ fully unoccupied, which is compatible with a low-spin configuration (hence a collapse of the magnetic moments as observed in experiment). This is a remarkable result, since, within a nonmagnetic description of the paramagnetic phase, the MEET is capable to correctly describe the photoemission spectrum and the spin configuration at ambient as well as high pressure. For comparison we report the band gap values obtained using density-functional theory with a hybrid functional containing screened exchange (HSE06) and a variant of the $GW$ method (self-consistent COHSEX), which are reliable for the description of the antiferromagnetic phase. Both methods open a gap at ambient pressure, although, by construction, they give a low-spin configuration; increasing pressure they correctly describes the band gap closing. We also report the photoemission spectrum of the metallic phase obtained with one-shot fully-dynamical $GW$ on top of LDA, which gives a spectrum very similar to DMFT results from literature., Comment: 8 pages, 6 figures

In this work we explore the performance of a recently derived many-body effective energy theory for the calculation of photoemission spectra in the regime of strong electron correlation. We apply the theory to paramagnetic MnO, FeO, CoO, and NiO, which are typical examples of strongly correlated materials and, therefore, a challenge for standard theories. We show that our methods open a correlation gap in all the oxides studied without breaking the symmetry. Although the materials seem similar, we show that an analysis of the occupation numbers reveals that the nature of the gap is not the same for these materials. Overall the results are very promising, although improvements are clearly required, since the band gap is overestimated for all the systems studied. We indicate some possible strategies to further develop the theory.