Hello, A yearly allocation for the LCRC cluster has been requested with the following updated information: Submitter/PI: Peter Zapol Project Name: SRF-materials Division: MSD Project title: Modeling of Impurity Structures in Superconducting Radio-Frequency Cavities Associated funding: DOE-HEP Other Systems: ANL CNM - Carbon Science: This objective of this project is to model the effects of chemical impurities and interface properties on the materials used to construct superconducting radio-frequency (SRF) cavities. Niobium SRF cavities are a key technology for high-performance linear particle accelerators, and will be used in high energy physics applications such as a proposed 8 GeV proton source at Fermilab, and potentially in accelerators for waste management from nuclear power sources, for synchrotron light sources, and for neutron sources. SRF cavity performance is characterized by the accelerating gradient and the quality factor (a measure of the stored energy versus the dissipated power), both of which are highly sensitive to the structure and composition of the top ~100 nm of a cavity’s surface. Absorption of chemical impurities during processing can affect both the maximum accelerating gradient and the quality factor. In addition, materials synthesis and processing affects these properties. Two examples, which will be the focus of this project, are (i) the formation of a surface oxide film, approximately 20 nm thick, which is composed of layers of Nb2O5, NbO2, NbO, and a gra dient of dissolved oxygen in the niobium bulk; and (ii) the formation of hydride precipitates. The niobium oxides can accommodate substantial off-stoichiometry, which results in some cases in local magnetic moments. This is detrimental for superconductivity because magnetic impurities create normal conducting states within the superconducting energy gap. Hydride precipitation is detrimental because the hydrides with an ordered structure are not superconducting above 2 K. On the other hand, the presence of interstitial oxygen, nitrogen, and carbon may be beneficial to SRF cavity performance at low concentrations. A steep drop-off of the quality factor at the onset of the RF field can be observed in cavities made from high purity niobium, but not typically in cavities from lower purity reactor grade niobium. This is because oxygen, nitrogen, and possibly carbon can trap hydrogen atoms, which prevents their diffusion and precipitate formation. The tetragonal and cubic niobium nitrides also have superconducting transition temperatures that are higher than that of pure niobium, so formation of these phases may potentially benefit niobium SRF cavities. In addition, different nitride phases form interfaces that can have either beneficial or detrimental properties. Understanding of these interfaces is important for both Nb and other superconducting nitrides. This project will utilize VASP to model the interactions of chemical impurities in niobium and other superconducting metals the formation of the phases described above to shed light on processes that may positively or negatively affect the performance of SRF cavities. Project description: Our previous studies focused on chemical impurities in Nb. This year the project will utilize VASP to model the interfaces of different nitride phases and the formation of the phases described above to shed light on processes that may positively or negatively affect the performance of SRF cavities. This phase of the project will focus on surface and interface properties. We are particularly interested in the change in superconducting properties at surfaces and across the interfaces between the nitrides and other materials. Since the crystal lattices may deform considerably near the interfaces, we will consider bulk models of under strain, as well as full interface models. The models will consist of up ~100 atoms. Density functional theory in VASP will be used to optimize the geometries of the models and calculate their electronic properties. Phonon DOS and dispersions will be evaluated using Phonopy, which uses the force constants calculated for atomic perturbations via density functional theory in VASP. Since defects, including interfaces, reduce the symmetry of the systems, many perturbations will need to be performed for some of these models to accurately assess the phonon properties. Abinit will also be used to calculate the superconducting properties (elec tron-phonon coupling and transition temperature) of select structures. Accurate determination of the electron-phonon coupling is very computationally demanding, requiring dense k-point and q-point grids. Abinit parallelizes over k-points, bands, and perturbations; so it is particularly suitable for use on large machines with many cores. Industry partnership: Project URL: Current FY Hours Used: undetermined amount New FY Requested allocation: 400000 Q1: 100000 Q2: 100000 Q3: 100000 Q4: 100000 Justification: Storage requirements: Thank You, The LCRC Accounts System