Direct detection experiments for light dark matter are making enormous leaps in reaching previously unexplored model space. Several recent proposals rely on collective excitations, where the experimental sensitivity is highly dependent on detailed properties of the target material, well beyond just nucleus mass numbers as in conventional searches. It is thus important to optimize the target choice when considering which experiment to build. We carry out a comparative study of target materials across several detection channels, focusing on electron transitions and single (acoustic or optical) phonon excitations in crystals, as well as the traditional nuclear recoils. We compare materials currently in use in nuclear recoil experiments (Si, Ge, NaI, CsI, CaWO4), a few which have been proposed for light dark matter experiments (GaAs, Al2O3, diamond), as well as 16 other promising polar crystals across all detection channels. We find that target-and dark matter modeldependent reach is largely determined by a small number of material parameters: speed of sound, electronic band gap, mass number, Born effective charge, high frequency dielectric constant, and optical phonon energies. We showcase, for each of the two benchmark models, an exemplary material which has a better reach than in any currently proposed experiment.

We present a unified theoretical framework for computing spin-independent direct detection rates via various channels relevant for sub-GeV dark matter -nuclear recoils, electron transitions and single phonon excitations. Despite the very different physics involved, in each case the rate factorizes into the particle-level matrix element squared, and an integral over a target material-and channel-specific dynamic structure factor. We show how the dynamic structure factor can be derived in all three cases following the same procedure, and extend previous results in the literature in several aspects. For electron transitions, we incorporate directional dependence and point out potential daily modulation signals in anisotropic target materials. For single phonon excitations, we present a new derivation of the rate formula from first principles for generic spin-independent couplings, and include the first calculation of phonon excitation through electron couplings. We also discuss the interplay between single phonon excitations and nuclear recoils, and clarify the role of Umklapp processes, which can dominate the single phonon production rate for dark matter heavier than an MeV. Our results highlight the complementarity between various search channels in probing different kinematic regimes of dark matter scattering, and provide a common reference to connect dark matter theories with ongoing and future direct detection experiments.

The electronic properties of MoO and reduced molybdenum oxide phases are studied by density functional theory (DFT) alongside characterization of mixed phase MoO films. Molybdenum oxide is utilized in compositions ranging from MoO to MoO with several intermediary phases. With increasing degree of reduction, the lattice collapses and the layered MoO structure is lost. This affects the electronic and optical properties, which range from the wide band gap semiconductor MoO to metallic MoO. DFT is used to determine the stability of the most relevant molybdenum oxide phases, in comparison to oxygen vacancies in the layered MoO lattice. The non-layered phases are more stable than the layered MoO structure for all oxygen stoichiometries of MoO studied where 2 ≤ x < 3. Reduction and lattice collapse leads to strong changes in the electronic density of states, especially the filling of the Mo 4d states. The DFT predictions are compared to experimental studies of molybdenum oxide films within the same range of oxygen stoichiometries. We find that whilst MoO is easily distinguished from MoO, intermediate phases and phase mixtures have similar electronic structures. The effect of the different band structures is seen in the electrical conductivity and optical transmittance of the films. Insight into the oxide phase stability ranges and mixtures is not only important for understanding molybdenum oxide films for optoelectronic applications, but is also relevant to other transition metal oxides, such as WO, which exist in analogous forms.