In order to achieve the objectives listed above I used a range of different resources. Of high important for several of the objective was access to high magnetic fields. Very high fields are needed both to suppress the superconductivity and also to overcome impurity damping in quantum oscillation measurements. Primary access to high magnetic fields in Toulouse (pulsed field), and Nijmegen (dc field) was obtain through the European magnetic fields laboratory (EMFL). A total of six experiments were performed in Nijmegen or Toulouse, roughly one visit (typically two weeks in duration) every six months.
A central issue was the development of pressure cells to work in pulse magnetic fields. The challenge was to have a cell which has been miniaturized to an outside diameter of 13mm and produced minimal heating at the sample position during the pulse. The cells consist of a body machined from a high strength, non-magnetic material Ti-6Al-4V alloy (figure 1).
The gasket consists of a phosphor-bronze 8mm diameter disc. The disc is punch out from a 400 or 500 µm thick sheet of phosphor-bronze. The gasket is then indented to the maximum pressure we will use. Slots are made in the gasket in order to reduce eddy currents (figure 2). The sample space is small, so the sample size needs to be less than 200x200 µm. The tracks are made by evaporating gold directly on the moissanite anvil (figure 2).
The anvil cells have been tested successfully up to 6GPa and fit into the bore of the 60T coil in Toulouse. To test the heating issue, we used a thin film of AuGe made in Bristol to act as a accurate thermometer. Our measurements showed that the pressure cell was not heating significantly during the measurement (figure 3) at low temperature (4.2K).
After we successfully tested the pressure cells up to 60T we perform measurements on different samples as discuss in the results chapter. The design of these pressure cells will be made available to the laboratories in the EMFL so that these cells can be made available for other users. This way the results will benefit a wide range of disciplines (just a semiconductors and low dimensional materials like graphene) which use pulsed fields and could benefit from using high pressure.
Another important aspect of this project was to have good quality samples of YBa2Cu3O7-x (Y123) and Y123 doped with Zn. The samples were grown by myself in Bristol. The high quality of our samples was directly evidence by our measurement of quantum oscillation (figure 4) and by the observation of the x-ray peak related to the ordering of the chains (figure 4). Furthermore, as we describe later, we manage to track the CDW in our samples which is also an indication of the high quality of our samples.
A second type of cell was developed to study the uniaxial application of pressure. This is important because the direction of the stress can be used to break the symmetry of crystal and thus gain new important information about the origin of the CDW and pseudogap phases. For this I developed a mechanical screw actuated cell for the high field measurement that could fit in the 37T coil in Nijmegen (figure 5). My mechanical device allows tuning of the stress at low temperature via a screwdriver mechanism built into a specially constructed probe for the Nijmegen cryostats. The strain device was successfully tested in Bristol. An example of the suppression of Tc achieved, using this device, with a YBCO sample can be seen the Figure 5. The increase in structure of the resistance- temperature R(T) curve under strain shows the non-uniformity of the strain, however, evidently there is a significant portion (the middle of the sample) where it is uniform. We also show that the strain effect on the sample was relaxing with time at room temperature (figure 5).
Finally, the effect of disorder on the CDW was studied directly using x-ray diffraction. We were awarded two sessions (each week long) of beam time at Diamond light source to investigate this. The disorder was introduced using either electron irradiation (done at the Sirius beam of the Laboratoire des Solides Irradiés (Ecole Polytechnique, France) in collaboration with M. Konczykowski) or Zn doping. Five days of beam time on I16 were completed in June 2019, which gave some encouraging results. A second week of beam time was awarded for April 2020 where we planned to complete the experiments but unfortunately this was not possible because of the Covid-19 shutdown.
