Master thesis defense by William H. P. Nielsen – Niels Bohr Institute - University of Copenhagen

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Master thesis defense by William H. P. Nielsen

STM predictions for the loop-current theory of the cuprates 

In the field of high-temperature superconductivity a certain class of superconductors stands out: the cuprates. Discovered in 1986, they have been studied intensively for more than two decades, but there is still no consensus on the microscopic origin of the superconductivity, or, for that matter, the non-superconducting phases of these materials. In particular, the so-called pseudogap phase in which the material is not yet superconducting has received a lot of interest.

In this thesis, one particular theory for the pseudogap, the so-called loop-current theory, is studied. The goal of the thesis has been to produce an experimental prediction of the loop-current theory; a potential ``smoking gun''. The experiment best believed to reveal such evidence is the STM-experiment, which takes place at low temperatures where superconductivity is present. The thesis builds a simple model for the copper-oxygen planes of cuprates, taking both superconductivity and loop-currents into account. By calculating the local density of states around a single point-like impurity, a clear loop-current fingerprint is
seen: the 90-degrees rotational symmetry is broken.

As STM-experiments are in principle able to probe superconducting samples with and without loop-currents, the conclusion is that a falsifiable prediction of the loop-current theory has been provided.

Master thesis defense by William H. P. Nielsen