PhD defence by Kristine Marie Løfgren Krighaar
Probing Low Energy Magnetic Dynamics in Cuprates – Neutron Spectroscopy and Instrumentation
Abstract:
Understanding unconventional superconductivity in cuprate materials remains a central challenge in condensed matter physics, where magnetic excitations are believed to play a key role in the pairing mechanism. Progress in this field, however, is fundamentally constrained by both the complexity of the materials and the capabilities of available experimental techniques. This thesis addresses this dual challenge through a combined investigation of magnetic excitations in cuprate superconductors and the development of advanced neutron spectroscopy instrumentation required to probe them.
On the materials side, the magnetic excitation spectra of three representative cuprates, Nd2 –xCexCuO4 –δ(NCCO), La2 –xBaxCuO4(LBCO), and La2 –xSrxCuO4(LSCO), are investigated using neutron spectroscopy. These compounds span both electron- and hole-doped regimes, enabling a broad overview of spin dynamics behaviour across the family. The results reveal previously unresolved features in the excitation spectra, including modifications to dispersion and spectral weight distributions. Together, these results provide additional information to understanding the interplay between magnetism and superconductivity and offer new constraints for theoretical models of cuprate superconductivity.
Complementing these studies, this thesis develops a computational framework for virtual neutron scattering experiments using the McStas package, with specific application to the BIFROST spectrometer at the European Spallation Source. This framework enables detailed characterization of instrument performance, including resolution and flux optimization, and allows realistic simulation of experiments on quantum materials. By linking instrument design directly to scientific requirements, the approach provides a pathway for optimizing future measurements and instruments. By integrating neutron studies of cuprate magnetism with the development of novel neutron instrumentation, this thesis emphasizes that advances in understanding unconventional superconductivity are inseparable from advances in experimental capability.
The combined contributions presented here establish both new physical insights and practical tools that will be essential for future high-precision studies of cuprates and related quantum materials.
Assessment Committee:
(Chairperson) Professor Jens Paaske, NBI, UCPH
Professor Elizabeth Blackburn, Lund University
Instrument Scientist Manh Duc L, ISIS, Science and Technology Facilities Council
Principal supervisor:
Professor Kim Lefmann, NBI, UCPH
Co-supervisor:
Assistant Professor Machteld E. Kamminga, Utrecht University