An experimental and computational study of srain sensitivity in superconducting Nb3Sn

Abstract

This work is the result of efforts by many people, both in a professional and a personal sense. During my time in Berkeley I had the opportunity to work with friendly and supportive people who were pushing back scientific boundaries, regarding the limits of superconducting high field technology. It is an odd but good experience to read papers from various authors detailing one scientific breakthrough or another, and then to meet them in person. Being a multi-cultural hub, Berkeley also allowed me to meet and become friends with people from all corners of this planet.Along the way, people have been instrumental in making this research happen. At LBNL, Jonathan Slack, Reuben Mendelsberg, and Andre Anders allowed me to use their lab for a rather extended period of time thus allowing for the fabrication of thin films, and have helped in various other ways along the years. The people at Ohio State University have helped by repeatedly performing heat capacity measurements on Nb 3 Sn bulk samples which were kindly supplied by Wilfried Goldacker of the Karlsruhe Institute of Technology. In particular I would like to thank Mike Susner of the Ohio State University, who came to the lab in the weekends just to make sure my samples would get measured. John Bonevich of the National Institute of Science and Technology was kind enough to perform characterization work on some of the Nb-Sn thin films we made, which included the STEM image on the cover. A lot of the experimental work was made possible with software and hardware designed at the University of Twente (such as VI by Bennie ten Haken), and I have gotten useful recommendations and assistance along the way, such as the advice on thin film fabrication from Frank Roesthuis. Professors Marcel ter Brake, Herman ten Kate, David Larbalestier, and Frances Hellman were kind enough to write letters of recommendation which contributed to getting the CSC student fellowship award. Vladimir Kresin of LBNL was kind enough to answer various questions related to the microscopic properties of Nb 3 Sn and comment on the description of microscopic theory in this thesis.Shane Cybart and Stephen Wu have assisted Edwin Dollekamp and myself in attempting to do some very fancy experiments. Edwin, from the University of Twente, took on a complicated research topic and I was impressed with how far he managed to get in that work, and Shane and Stephen invested time in the evenings and weekends to make it happen. Professor Orlandos previous investigations of Nb 3 Sn were an inspiration for some of the work described in this thesis, and he was kind enough to answer questions by email and over the phone. Derek Stewart of the Cornell NanoScale Science and Technology Facility helped me to get familiar with density functional theory calculations and computing clusters and made the suggestion of using Quantum Espresso ab-initio software. Robert Ryne and Steve Gourlay of LBNL helped me getting access to sizeable supercomputing resources at NERSC which were needed for this research.My friends and family st...

Fundamental origin of the large impact of strain on superconducting Nb3Sn

“…Although the experimental strain range is limited, two trends can be distinguished: the off-stoichiometric thin films have a similar strain sensitivity to the wires, which is in-line with their random [110]-[100] in-plane crystal orientation, and the off-stoichiometric A15 compositions that also determine the critical properties in wires. For the stoichiometric thin film and bulk samples a reduced strain sensitivity is observed, which is in-line with our observations that when approaching stoichiometry, the sensitivity to strain rapidly reduces (see figure 2.24 in [36]). The reason for this reduced sensitivity, however, should not be sought in crystal alignment, but in the presence of the spontaneous tetragonal distortion, which affects the way that externally applied strain enters the lattice structure.…”

“…These findings are in contrast to other analyses in which mostly an a priori assumption was made that changes in the phonon spectrum [28,[45][46][47][48][49], or changes in the electron DOS [50][51][52], are the dominant cause for the large strain sensitivity of Nb 3 Sn. We also note that an assumption that changes in the phonon spectrum dominate the effects of strain, is in conflict with the experimental evidence that ρ n changes substantially with strain [33,36]: ρ n is at temperatures below 18K mainly determined by defect scattering and not by the phonons.…”

“…This means that the Nb sub-lattice distortion is the leading cause (for about 90%) for the reduced N(E F ). Similar effects are seen for [110] strain and for other directions, although the sub-lattice distortions are more complex [36].…”

“…To validate our calculations, we compared to experimental data from literature, and to own measurements on bulk material, thin films, and wires. Details of sample preparation and initial characterization of the bulk and thin film samples, including our finding that the normal-state resistivity ρ n is significantly affected by strain [33], were published earlier [33][34][35], with further experimental characterization to be published elsewhere [36]. Here we summarize the sample details that are relevant for our present analyses.…”

Transverse pressure effect on superconducting Nb3Sn Rutherford and ReBCO Roebel cables for accelerator magnets

“…Ten Haken et al [121] picked up on this idea and showed that the key factor in this description is the deviatoric strain component dev . Meanwhile, this type of scaling description has been refined by further experimental observations [216,217] and the role of deviatoric strain has been understood a-priori in terms of the disruption that it causes in the crystal structure and hence in the electronic density of states [181,[218][219][220].…”