Astrid Tranum Rømer
A thesis submitted July 1st, 2015 for the degree of Doctor of Philosophy and defended September 16, 2015.
The PhD School of Science
Faculty of Science
Niels Bohr Institute, Condensed Matter Theory, University of Copenhagen
Brian Møller Andersen
Henrik M. Rønnow
Spin-fluctuation mediated superconductivity and magnetic order in the cuprate La1.88Sr0.12CuO4
High-temperature superconductivity in cuprates emerges as one out of many electronic phases when doping the antiferromagnetic Mott insulator La2CuO4 away from half _lling. The description of the superconducting phase is therefore complicated by intertwined electronic orders that compete with superconductivity. However, it is possible that the tendency towards additional ordering phenomena is a necessary condition for unconventional superconductivity to develop. Indeed most superconductors discovered throughout the last 29 years, including also the recently discovered class of iron-based superconductors, show a very rich electronic phase diagram. A common feature that characterizes both cuprates, heavy fermions, and iron pnictides is the proximity to magnetic order. Therefore, the idea of spin-uctuation mediated pairing is a popular paradigm proposed for unconventional superconductivity. A _ngerprint of the pairing mechanism is found in the superconducting gap symmetry. Therefore the study of gap symmetries constitutes one of the most important parts of resolving the superconducting puzzle.
This thesis consists of a theoretical and an experimental part. In the theoretical part, we address spin-uctuation mediated pairing in the weak-coupling limit by a systematic study of the one-band Hubbard model. We consider the problem of spin-uctuation mediated pairing in a paramagnetic system, as well as a system in which spin-density wave order is wellestablished. In addition, we describe how disorder locally a_ects the spin susceptibility and its implications on the local superconducting gap.
In the experimental part, we use neutron scattering experiments to investigate magnetic order and low-energy spin uctuations in the cuprate superconductor La1:88Sr0:12CuO4. This crystal is superconducting with Tc = 27 K and shows incommensurate magnetic order as well as spin uctuations which are "stripe-like" and distinct from the antiferromagnetic order of the parent compound La2CuO4. Yet some remnant features of the original La2CuO4 spin order remain. For instance, we _nd that despite the glassy nature of the low-energy spin uctuations in La1:88Sr0:12CuO4, the spectrum supports small anisotropy gaps similar to the observations of anisotropy gaps in the parent compound as well as stripe-ordered La1:875Ba0:125CuO4.