SRF2021 - List of Authors (Kelley, M.M.)

Paper	Title	Page
SUPFDV002	Ab Initio Theory of the Impact of Grain Boundaries on the Superconducting Properties of Nb₃Sn	62
	M.M. Kelley, T. Arias, N. Sitaraman Cornell University, Ithaca, New York, USA
	Funding: This work was supported by the US National Science Foundation under award PHY-1549132, the Center for Bright Beams. For over 50 years experiments have repeatedly demonstrated that the superconducting performance of Nb₃Sn is profoundly sensitive to grain boundaries (GBs), but only recently has a microscopic theory emerged. Here we present the first comprehensive, ab initio study of GBs in Nb₃Sn. While most conventional superconductors, such as Nb, are not significantly impacted by GBs, Nb₃Sn is much more sensitive to defects and disorder owing to its short coherence length of ~3 nm. Indeed, experiments suggest a link between GB stoichiometry and the performance of Nb₃Sn superconducting radio frequency (SRF) cavities, and mesoscopic simulations point to GBs as a candidate mechanism that lowers the vortex-entry field in Nb₃Sn SRF cavities. Our density-functional theory (DFT) calculations on tilt and twist GBs provide direct insight into antisite defect formation near GBs and how local electronic properties are impacted by clean GBs and by GBs with added point defects. Ultimately, we will show how GB composition affects the local Tc around GBs in Nb₃Sn to elucidate recent SRF experiments and provide insight on promising modifications to the growth procedure of Nb₃Sn to optimize its SRF performance. [] Michelle M Kelley et al 2021 Supercond. Sci. Technol. 34 015015
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-SUPFDV002
About •	Received ※ 01 July 2021 — Accepted ※ 02 December 2021 — Issue date ※ 09 April 2022
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUPFDV010	New Recipes to Optimize the Niobium Oxide Surface From First-Principles Calculations	426
	N. Sitaraman, T. Arias, Z. Baraissov, M.M. Kelley, D.A. Muller Cornell University, Ithaca, New York, USA M. Liepe, R.D. Porter, Z. Sun Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: This work was supported by the U.S. National Science Foundation under Award No. PHY-1549132, the Center for Bright Beams The properties of niobium oxide are of critical importance for a wide range of topics, from the behavior of nitrogen during infusion treatments, to the nucleation of Nb₃Sn, to the superconducting properties of the surface. However, the modeling of the oxide is often much simplified, ignoring the variety of niobium oxide phases and the extremely different properties of these phases in the presence of impurities and defects. We use density functional theory (DFT) to investigate how electrochemical treatments and gas infusion procedures change the properties of niobium oxide, and to investigate how these properties could be optimized for Nb₃Sn nucleation and for niobium SRF performance.
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-TUPFDV010
About •	Received ※ 01 July 2021 — Accepted ※ 18 November 2021 — Issue date ※ 22 February 2022
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

WEOTEV03	Toward Stoichiometric and Low-Surface-Roughness Nb₃Sn Thin Films via Direct Electrochemical Deposition	710
	Z. Sun, G. Gaitan, M. Ge, K. Howard, M. Liepe, T.E. Oseroff, R.D. Porter Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA T. Arias, Z. Baraissov, M.M. Kelley, D.A. Muller, J.P. Sethna, N. Sitaraman Cornell University, Ithaca, New York, USA K.D. Dobson University of Delaware, Newark, Delaware, USA
	Reducing surface roughness and attaining stoichiometry of Nb₃Sn superconducting films are required to push their superheating field to the theoretical limit in SRF cavities. As such, we explore direct electrochemical processes that minimize involving foreign elements to deposit high-quality Sn, Nb, and NbxSn films on Nb and Cu surfaces. These films are then thermally annealed to Nb₃Sn. We find that smooth Sn pre-depositions via electroplating on Nb surfaces significantly reduce the average roughness of resultant Nb₃Sn to 65 nm, with a dramatic reduction in power intensity at medium special frequencies. Structural and superconducting properties demonstrate a Nb₃Sn A15 phase with a stoichiometry of 25 at% Sn. This process is being scaled-up to a 3.9 GHz cavity. Moreover, preliminary results on electroplating on Cu surface show that Nb plating undergoes a slow growth rate while subsequent Sn plating on the plated Nb surface can be controlled with varied thickness. The Nb plating process is currently being optimized.
DOI •	reference for this paper ※ doi:10.18429/JACoW-SRF2021-WEOTEV03
About •	Received ※ 09 July 2021 — Revised ※ 09 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 16 January 2022
Cite •	reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

Paper

Title

Page

Ab Initio Theory of the Impact of Grain Boundaries on the Superconducting Properties of Nb₃Sn

62

M.M. Kelley, T. Arias, N. Sitaraman
Cornell University, Ithaca, New York, USA

Funding: This work was supported by the US National Science Foundation under award PHY-1549132, the Center for Bright Beams.
For over 50 years experiments have repeatedly demonstrated that the superconducting performance of Nb₃Sn is profoundly sensitive to grain boundaries (GBs), but only recently has a microscopic theory emerged. Here we present the first comprehensive, ab initio study of GBs in Nb₃Sn*. While most conventional superconductors, such as Nb, are not significantly impacted by GBs, Nb₃Sn is much more sensitive to defects and disorder owing to its short coherence length of ~3 nm. Indeed, experiments suggest a link between GB stoichiometry and the performance of Nb₃Sn superconducting radio frequency (SRF) cavities, and mesoscopic simulations point to GBs as a candidate mechanism that lowers the vortex-entry field in Nb₃Sn SRF cavities. Our density-functional theory (DFT) calculations on tilt and twist GBs provide direct insight into antisite defect formation near GBs and how local electronic properties are impacted by clean GBs and by GBs with added point defects. Ultimately, we will show how GB composition affects the local Tc around GBs in Nb₃Sn to elucidate recent SRF experiments and provide insight on promising modifications to the growth procedure of Nb₃Sn to optimize its SRF performance.
[*] Michelle M Kelley et al 2021 Supercond. Sci. Technol. 34 015015

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-SUPFDV002

About •

Received ※ 01 July 2021 — Accepted ※ 02 December 2021 — Issue date ※ 09 April 2022

Cite •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

TUPFDV010

New Recipes to Optimize the Niobium Oxide Surface From First-Principles Calculations

426

N. Sitaraman, T. Arias, Z. Baraissov, M.M. Kelley, D.A. Muller
Cornell University, Ithaca, New York, USA
M. Liepe, R.D. Porter, Z. Sun
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: This work was supported by the U.S. National Science Foundation under Award No. PHY-1549132, the Center for Bright Beams
The properties of niobium oxide are of critical importance for a wide range of topics, from the behavior of nitrogen during infusion treatments, to the nucleation of Nb₃Sn, to the superconducting properties of the surface. However, the modeling of the oxide is often much simplified, ignoring the variety of niobium oxide phases and the extremely different properties of these phases in the presence of impurities and defects. We use density functional theory (DFT) to investigate how electrochemical treatments and gas infusion procedures change the properties of niobium oxide, and to investigate how these properties could be optimized for Nb₃Sn nucleation and for niobium SRF performance.

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-TUPFDV010

About •

Received ※ 01 July 2021 — Accepted ※ 18 November 2021 — Issue date ※ 22 February 2022

Cite •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)

WEOTEV03

Toward Stoichiometric and Low-Surface-Roughness Nb₃Sn Thin Films via Direct Electrochemical Deposition

710

Z. Sun, G. Gaitan, M. Ge, K. Howard, M. Liepe, T.E. Oseroff, R.D. Porter
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
T. Arias, Z. Baraissov, M.M. Kelley, D.A. Muller, J.P. Sethna, N. Sitaraman
Cornell University, Ithaca, New York, USA
K.D. Dobson
University of Delaware, Newark, Delaware, USA

Reducing surface roughness and attaining stoichiometry of Nb₃Sn superconducting films are required to push their superheating field to the theoretical limit in SRF cavities. As such, we explore direct electrochemical processes that minimize involving foreign elements to deposit high-quality Sn, Nb, and NbxSn films on Nb and Cu surfaces. These films are then thermally annealed to Nb₃Sn. We find that smooth Sn pre-depositions via electroplating on Nb surfaces significantly reduce the average roughness of resultant Nb₃Sn to 65 nm, with a dramatic reduction in power intensity at medium special frequencies. Structural and superconducting properties demonstrate a Nb₃Sn A15 phase with a stoichiometry of 25 at% Sn. This process is being scaled-up to a 3.9 GHz cavity. Moreover, preliminary results on electroplating on Cu surface show that Nb plating undergoes a slow growth rate while subsequent Sn plating on the plated Nb surface can be controlled with varied thickness. The Nb plating process is currently being optimized.

DOI •

reference for this paper ※ doi:10.18429/JACoW-SRF2021-WEOTEV03

About •

Received ※ 09 July 2021 — Revised ※ 09 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 16 January 2022

Cite •

reference for this paper using ※ BibTeX, ※ LaTeX, ※ Text/Word, ※ RIS, ※ EndNote (xml)