Multiple electronic-valence elements in A-site perovskite manganites: a route to high metallicity and an orbital order coexistence

With respect to the double-exchange hopping mechanism, the role of chemical element sitting at perovskite A-site (generally hosting an alkali-earth or a rare-earth atom) has always been considered as “silent,” being the corresponding conduction band of the A-site element too far from Fermi’s energy level. In order to make such an atomic site active within the transport mechanism, a possible strategy calls for a partial insertion of multiple-valence ions which also show the requested electronic properties (i.e. conduction band crossing EF). In manganites, the ideal candidate isindeed Mn-ions themselves. Here we show that a partial substitution of Mn ions at perovskite A-site (therefore named as A-site perovskite manganites) is indeed possible in both La-deficient LaxMnO3 and off-stoichiometric LaxSryMnO3 manganite thin films. By combining polarization-dependent x-ray absorption spectroscopy and resonant inelastic x-ray spectroscopy, the relevant Mn2+ content is demonstrated, and it is unambiguously assigned its crystallographic site (namely, the perovskite A-site). Similarly to traditional manganites, Mn2+ substitution induces the required Mn3+/Mn4+ mixed population. However, differently from the latter, the Mn2+ ions at perovskite A-site are electronically involved in the transport mechanisms, having their electronic bands crossing the Fermi energy. Such an energetic configuration favours the hopping of electrical charge through that site (usually silent), in addition to the traditional Mn3+/Mn4+ hopping path (named Multiple double-exchange mechanism), thus contributing to the ferromagnetic and metallic state. Furthermore, to an highly metallic and ferromagnetic state, it surprisingly corresponds also a strong Mn orbital order. Indeed, the tendency of the orbitals to order locally usually strongly compete with the kinetic energy of the free charge carriers, which however tends to destroy long range orbital order. Multiple double-exchange mechanism is here demonstrated that destroys such a dichotomy by sustaining the co-existence of highly metallic states with orbital ordered phase. This will open unexplored perspectives in both theoretical and experimental possibilities based on such a coexistence of spin/orbital order (generally in competition with each other), and more in general in fundamental studies on transport mechanism in strongly correlated electrons systems

Directionality for nuclear recoils in a Liquid Argon TPC

International audienceDirectional sensitivity to nuclear recoils would provide a smoking gun for a possible discovery of dark matter in the form of WIMPs (Weakly Interacting Massive Particles). A hint of directional dependence of the response of a dual-phase argon Time Projection Chamber (TPC) was found in the SCENE experiment. Given the potential importance of such a capability in the framework of dark matter searches, a new dedicated experiment, ReD (Recoil Directionality), was designed by the Global Argon Dark Matter Collaboration, in order to scrutinise this hint. A small dual-phase argon TPC was irradiated with neutrons produced by the p($^{7}$Li,$^{7}$Be)n reaction using the 15 MV TANDEM accelerator of the INFN - Laboratori Nazionali del Sud, Catania, Italy, so as to produce argon nuclear recoils in the range (20 - 100) keV of interest for dark matter searches. Energy and direction of nuclear recoils are inferred by the detection of the elastically-scattered neutron by a set of scintillation detectors. Events were selected by gating of the associated $^{7}$Be, which is detected by a telescope of Si detectors

Directionality for nuclear recoils in a LAr TPC

International audienceIn the direct searches for Weakly Interacting Massive Particles (WIMPs) as Dark Matter candidates, the sensitivity of the detector to the incom- ing particle direction could provide a smoking gun signature for an interesting event. The SCENE collaboration firstly suggested the possible directional de- pendence of a dual-phase argon Time Projection Chamber through the columnar recombination effect. The Recoil Directionality project (ReD) within the Global Argon Dark Matter Collaboration aims to characterize the light and charge re- sponse of a liquid Argon dual-phase TPC to neutron-induced nuclear recoils to probe for the hint by SCENE. In this work, the directional sensitivity of the de- tector in the energy range of interest for WIMPs (20-100 keV) is investigated with a data-driven analysis involving a Machine Learning algorithm

Recoil Directionality Experiment

International audienceDirectional sensitivity to nuclear recoils could provide a smoking gun for a possible discovery of dark matter in the form of WIMPs. A hint of directional dependence of the response of a dual-phase liquid argon Time Projection Chamber was found in the SCENE experiment. Given the potential importance of such a capability in the frame work of dark matter searches, a new dedicated experiment, ReD (Recoil Directionality), was designed in the framework of the DarkSide Collaboration, in order to scrutinize this hint. This contribution will describe the performance of the detectors achieved during the first test-beam, the current status of ReD and the perspectives for physics measurements during the forthcoming beam-time