Fundamental R&D - Nb
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TUP043 Ab-initio Study of Atomic Scale Interaction Among Nb, Sn, Cl, and O 518
  • A.B. Tesfamichael, T. Arias
    Cornell University, Ithaca, New York, USA
  Funding: Center for Bright Beam (CBB)
We employed a combination of ab-initio calculations and statistical mechanical models to understand the nature of atomic scale interaction among Nb, Sn, Cl, and O. Because of the profound nature of the interaction, we began our study by focusing only on the interaction of Nb with Sn in the absence of Cl and O. Using Density Functional Theory (DFT) we calculated: (1), binding energy of both vacant and interstitial of the super cell for both Nb and Sn atoms (2), rate of diffusion and re-evaporation upon transportation of Sn atom across z-axis from bulk Nb layer (3), electron transfer and electric field upon transportation of Sn atom both across z-axis and xy-plane from bulk Nb layer. Our calculation indicated 30-40% difference from experimental results. Therefore, we conclude that the presence of oxides is important and also Cl impurity can not not be avoided.
DOI • reference for this paper ※  
About • paper received ※ 24 June 2019       paper accepted ※ 04 July 2019       issue date ※ 14 August 2019  
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TUP045 Ab Initio Calculations on Impurity Doped Niobium and Niobium Surfaces 523
  • N. Sitaraman, T. Arias
    Cornell University, Ithaca, New York, USA
  • R.G. Farber, S.J. Sibener, R.D. Veit
    The University of Chicago, Chicago, Illinois, USA
  • M. Liepe, J.T. Maniscalco
    Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
  Funding: This work was funded by the Center for Bright Beams
We develop and apply new tools to understand Nb surface chemistry and fundamental electronic processes using theoretical ab initio methods. We study the thermodynamics of impurities and hydrides in the near-surface region as well as their effect on the surface band gap. This makes it possible for experimentalists to relate changes in STM dI/dV measurements resulting from different preparations to changes in subsurface structure. We also calculate matrix elements for electron-impurity scattering in Nb for common impurities O, N, C, and H. By transforming these matrix elements into a Wannier function basis, we calculate lifetimes for a dense set of states on the Fermi surface and determine the mean free path as a function of impurity density. This technique can be generalized to calculate other scattering amplitudes and timescales relevant to SRF theory.
DOI • reference for this paper ※  
About • paper received ※ 02 July 2019       paper accepted ※ 03 July 2019       issue date ※ 14 August 2019  
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Field-Dependent Nonlinear Surface Resistance and Its Optimization by Surface Nano-Structuring of the SRF Cavities  
  • T. Kubo
    KEK, Ibaraki, Japan
  • A.V. Gurevich
    ODU, Norfolk, Virginia, USA
  Funding: The work of T. K. was supported by JSPS KAKENHI Grant Number JP17H04839 and JP17KK0100. The work of A. G. was supported by NSF under Grant No. PHY-1416051 and DOE under Grant No DE-SC100387-020.
We propose a theory of nonlinear surface resistance of a dirty superconductor in a strong RF field (H0), taking into account magnetic and nonmagnetic impurities, finite quasiparticle lifetimes, and a thin proximity-coupled normal layer characteristic of the oxide surface of many materials. It is shown that the interplay of the broadening of the quasiparticle density of states (DOS) peaks and a decrease of a quasiparticle gap caused by the RF currents produces a minimum in Rs(H0) and an extended rise of the quality factor Q(H0) with the RF field. Paramagnetic impurities shift the minimum in Rs(H0) to lower fields and can reduce Rs(H0) in a wide range of H0. Subgap states in the DOS can give rise to a residual surface resistance while reducing Rs at higher temperatures. A proximity-coupled normal layer can shift the minimum to either low and high fields and can reduce Rs below that of an ideal surface. The field dependence of Q(H0) can be very sensitive to the materials processing. The nonlinear RF losses can be minimized by tuning pairbreaking effects using impurity management or surface nanostructuring.
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Effect of Inhomogeneous Disorder on the Superheating Field of SRF Cavities  
  • J.A. Sauls
    NU, Evanston, Illinois, USA
  Funding: The research of the authors is supported by National Science Foundation Grant PHY-1734332 and the Northwestern-Fermilab Center for Applied Physics and Superconducting Technologies.
Recent advances in surface treatments of Niobium SRF cavities have led to increased Q-factors and maximum surface field. This poses theoretical challenges to identify the mechanisms responsible for performance enhancements. I report theoretical results for the effects of inhomogeneous surface disorder on the superheating field.* We find that inhomogeneous disorder, such as that introduced by infusion of Nitrogen into the surface layers of Niobium SRF cavities, can increase the superheating field above the maximum for superconductors in the clean limit or with homogeneously distributed disorder. Disorder increases the penetration of screening current, but also suppresses the maximum supercurrent. Inhomogeneous disorder in the form of an impurity diffusion layer biases this trade-off by increasing the penetration of the screening currents into cleaner regions with larger critical currents, thus limiting the suppression of the screening current to a thin dirty region close to the surface. Our results suggest that the impurity diffusion layers play a role in enhancing the maximum accelerating gradient of Nitrogen treated Niobium SRF cavities.
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