Master's thesis: Structural investigation of La(2-x)Sr(x)CuO(4+y) - Following staging as a function of temperature

2016-02-09T12:34:40Z (GMT) by Pia Jensen Ray
A thesis submitted to the Niels Bohr Institute at the Faculty of Science at the University of Copenhagen, Denmark, in partial fulfilment of the requirements for the degree of Master of Science in physics. Submission date was November 19, 2015, and the defence was held on November 30, 2015, where the degree was also awarded.<br><br>The cuprate La<sub>2-<i>x</i></sub>Sr<sub><i>x</i></sub>CuO<sub>4+<i>y</i></sub> – a high-temperature superconductor – was discovered almost three decades ago. However the mechanisms behind the superconductivity in the material for different doping values <i>x</i> and y are still not fully understood. A small part of this large puzzle is added to the pile with this thesis, where results on the structure for several different samples are presented.<br><br>The emphasis in this thesis is on a specific superstructure thought to be connected to the ordering of interstitial oxygen, known from the isostructural compound La<sub>2</sub>NiO<sub>4+<i>y</i></sub> as staging. Four single crystal samples with different co-doping values are investigated by the use of both X-rays and neutrons.<br><br>Staging is observed for all four samples at low temperatures with X-ray measurements. The sample with strontium doping <sub>x</sub> = 0.00 shows several coexisting staging levels with staging numbers between 2 and 8, with the highest contribution from a staging level between 4 and 5. The co-doped samples show increasing staging number with increasing <i>x</i>. It is found that the staging belongs to a structural phase assumed in space group <i>Fmmm</i>, while the unstaged fraction of the samples are in the <i>Bmab </i>space group. These two structural phases are found to have significantly different lengths of the long crystal axis for the two low <i>x</i> samples, in the order of a fraction of a percent, while the two higher <i>x</i> samples had a difference of only a small fraction of a permille.<br><br>The temperature dependent phase transitions for both the Bmab structure and the staging reflections are investigated between 5 and 300 K. The critical exponents for the <i>Bmab</i> reflections are found to be significantly lower than results from similar materials in literature, although with transition temperatures consistent with literature for comparable sample compositions. It is found that the critical exponents for the staging reflections increase for increasing doping while the transition temperatures decrease, both consistent with results on the isostructural La<sub>2</sub>NiO<sub>4+<i>y</i></sub>.<br><br>Results from previous neutron measurements are found to be consistent with the X-ray measurements in this work, and measured reciprocal space maps from this work show a large variety of other superstructure reflections which will be interesting to investigate in the future.