TY - BOOK

T1 - Magnetic structure and dynamics of cuprate superconductors studied by neutron scattering

AU - Tutueanu, Ana-Elena

PY - 2021

Y1 - 2021

N2 - The research presented in this thesis is centred around two compounds of the cuprates family. In both cases the aim has been to understand the relationship between magnetism and superconductivity by means of neutron scattering experiments.The main focus of my experimental investigations has been the underdoped region of the La2-xSrxCuO4 (LSCO) phase diagram around the doping dependent transition from an insulator to a superconductor. The consistent study of crystals with different levels of strontium doping has revealed a number of similarities and differences between superconducting and non-superconducting samples. Our experiments revealed a spectralweight shift from low-energy spin fluctuations towards the elastic or quasi-elastic magnetic signals. This, corroborated with a lack of spin gap in all our samples, regardless of conduction properties, and a drastic decrease of both elastic and inelastic signals at the onset of superconductivity as a function of doping led us to propose a model composed of several coexisting electronic orders. We argue that the phase diagram of LSCO displays coexisting pair density wave and uniform d-wave superconducting regions the ratio of which can be tuned by adjusting the doping concentration.In addition to the studies of the magnetic structure we have also investigated the doping distribution in one of our superconducting LSCO single crystals by means of polarised neutron imaging. The temperature dependence of the trapped magnetic flux inside our sample revealed a doping variation in the growth direction of our crystals also confirmed by means of magnetic susceptibility measurements. Finally, we have identified the origin of an incommensurate signal appearing at the same position in reciprocal space as the spin stripes signal. Calculations have revealed the signal to be a spurious double scattering event.The second compound that we focused on was the oxygen doped La2CuO4+y (LCO+O). A significant amount of data has been acquired on these samples by our group over the years. I have gathered and combined the data and performed additional measurements which led to an improved general understating of the magnetism in this type of compounds. Through polarisation analysis we showed that charged stripes run along both tetragonal a and b axis and that magnetic fluctuations are most probably isotropic in nature. Furthermore, the lack of aspin gap or a significant applied magnetic field effect on low-energy magnetic excitations have been interpreted as manifestations of a microscopical co-existence between superconductivity and spin fluctuations. In this picture we argue that magnetic order originates from a separate electronic phase competing with with superconductivity.Besides the details of each conclusion we draw, our study also reveals that seemingly similar samples, which have the emergence of superconductivity in common, also exhibit significantly different magnetic structures. This points to a fairly complex interplay between superconductivity and magnetism that needs to be further investigated and understood in order to explain the pairing mechanism in these compounds.

AB - The research presented in this thesis is centred around two compounds of the cuprates family. In both cases the aim has been to understand the relationship between magnetism and superconductivity by means of neutron scattering experiments.The main focus of my experimental investigations has been the underdoped region of the La2-xSrxCuO4 (LSCO) phase diagram around the doping dependent transition from an insulator to a superconductor. The consistent study of crystals with different levels of strontium doping has revealed a number of similarities and differences between superconducting and non-superconducting samples. Our experiments revealed a spectralweight shift from low-energy spin fluctuations towards the elastic or quasi-elastic magnetic signals. This, corroborated with a lack of spin gap in all our samples, regardless of conduction properties, and a drastic decrease of both elastic and inelastic signals at the onset of superconductivity as a function of doping led us to propose a model composed of several coexisting electronic orders. We argue that the phase diagram of LSCO displays coexisting pair density wave and uniform d-wave superconducting regions the ratio of which can be tuned by adjusting the doping concentration.In addition to the studies of the magnetic structure we have also investigated the doping distribution in one of our superconducting LSCO single crystals by means of polarised neutron imaging. The temperature dependence of the trapped magnetic flux inside our sample revealed a doping variation in the growth direction of our crystals also confirmed by means of magnetic susceptibility measurements. Finally, we have identified the origin of an incommensurate signal appearing at the same position in reciprocal space as the spin stripes signal. Calculations have revealed the signal to be a spurious double scattering event.The second compound that we focused on was the oxygen doped La2CuO4+y (LCO+O). A significant amount of data has been acquired on these samples by our group over the years. I have gathered and combined the data and performed additional measurements which led to an improved general understating of the magnetism in this type of compounds. Through polarisation analysis we showed that charged stripes run along both tetragonal a and b axis and that magnetic fluctuations are most probably isotropic in nature. Furthermore, the lack of aspin gap or a significant applied magnetic field effect on low-energy magnetic excitations have been interpreted as manifestations of a microscopical co-existence between superconductivity and spin fluctuations. In this picture we argue that magnetic order originates from a separate electronic phase competing with with superconductivity.Besides the details of each conclusion we draw, our study also reveals that seemingly similar samples, which have the emergence of superconductivity in common, also exhibit significantly different magnetic structures. This points to a fairly complex interplay between superconductivity and magnetism that needs to be further investigated and understood in order to explain the pairing mechanism in these compounds.

M3 - Ph.D. thesis

BT - Magnetic structure and dynamics of cuprate superconductors studied by neutron scattering

PB - Niels Bohr Institute, Faculty of Science, University of Copenhagen

ER -