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.

The electronic structure of layered molybdenum trioxide MoO 3 is highly sensitive to changes in oxygen stoichiometry as Mo 6+ has an empty 4d shell. Applications of MoO 3 are responsive to small changes in vacancy concentration, with some functions relying on a narrow window of oxygen non-stoichiometry. Difficulties in analyzing the energetics of oxygen vacancies by computational methods stem from the inability to accurately model the layered structure of MoO 3 . One unit cell parameter is governed by long range forces across the structural gaps and these dispersed interactions are not well described by conventional density functional theory (DFT) methods. With the exchange functional vdW-DF2 we accurately model the structure, in good agreement with experimental data. This basis allows exploration of the effect of oxygen non-stoichiometry on the electronic structure and properties of the oxygen deficient material.The layered structure efficiently screens the structural perturbations caused by oxygen vacancies.The enthalpies of formation are calculated for oxygen vacancies at the three symmetry inequivalent oxygen sites. The oxygen deficiency in MoO 3 gives rise to Mo 4d gap states with energy levels dependent on the type of oxygen vacancy.

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Ferroelectric domain walls are attracting broad attention as atomic-scale switches, diodes and mobile wires for next-generation nanoelectronics. Charged domain walls in improper ferroelectrics are particularly interesting as they offer multifunctional properties and an inherent stability not found in proper ferroelectrics. Here we study the energetics and structure of charged walls in improper ferroelectric YMnO3, InMnO3 and YGaO3 by first principles calculations and phenomenological modeling. Positively and negatively charged walls are asymmetric in terms of local structure and width, reflecting that polarization is not the driving force for domain formation. The wall width scales with the amplitude of the primary structural order parameter and the coupling strength to the polarization. We introduce general rules for how to engineer n-and p-type domain wall conductivity based on the domain size, polarization and electronic band gap. This opens the possibility of fine-tuning the local transport properties and design p-n-junctions for domain wall-based nano-circuitry.

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Understanding growth mechanisms and the role of surface functionalization is of key importance to control shape and morphology of nanoparticles and their properties. Here, we describe the growth mechanism and the effect of hydrothermal synthesis parameters (pH, time and precursor/functionalization agent ratio) during in situ functionalization of anatase TiO2 nanoparticles with 3-aminopropyltriethoxysilane. Elongated crystallographically oriented TiO2 nanoparticles were formed by oriented attachment mechanism in addition to spherical nanoparticles. The growth mechanism is determined by a combination of ex situ techniques such as high-resolution transmission electron microscopy combined with in situ synchrotron X-ray diffraction and density functional theory calculations. Oriented attachment induced by the functionalization agent is shown to be the origin of the elongation of the nanoparticles, as only spherical nanoparticles were formed in the absence of surface functionalization. Finally, it was shown that the amount and the size of the elongated nanoparticles can be tuned by adjusting pH.