Periodic Reporting for period 1 - CUPRES (High pressure study of pairing or competing orders in high Tc cuprates)
Reporting period: 2018-04-01 to 2020-03-31
This project aims to get new insights into to origin of high temperature superconductivity in copper oxide based (cuprate) materials. Superconductivity is a phenomena that has the potential to radically transform almost all applications of electrical power, from high field magnets, to power transmission, motors and generators. Using superconducting materials can significantly reduce our energy usage and enable technologies (such as medical MRI scanners or nuclear fusion reactors) which would be impossible without them. The key to fully realizing this potential is to develop materials which can be operated at high temperature and carry high currents.
The materials class with the best current prospects for developing superconducting applications at high temperature are the cuprates, as these have the highest critical temperatures at ambient pressure. It is therefore important to fully understand the phase diagram of these materials in order to be able to use them for applications. As the materials are chemically tuned across the phase diagram two different electronic phases are seen to appear in addition to the superconductivity. The pseudogap phase and the charge density wave (CDW) phases both have end points close to optimal doping (where the superconducting transition temperature Tc is maximum). Hence, both of these phases are of very great interest because critical fluctuations associated with either or both of these phases could be the driving force behind high Tc.
Although the CDW as recently be widely studied his relationship with superconductivity is still under debate. Here we used new approaches (high magnetic field, high pressure, disorder and x-ray) to address this problem.
The materials class with the best current prospects for developing superconducting applications at high temperature are the cuprates, as these have the highest critical temperatures at ambient pressure. It is therefore important to fully understand the phase diagram of these materials in order to be able to use them for applications. As the materials are chemically tuned across the phase diagram two different electronic phases are seen to appear in addition to the superconductivity. The pseudogap phase and the charge density wave (CDW) phases both have end points close to optimal doping (where the superconducting transition temperature Tc is maximum). Hence, both of these phases are of very great interest because critical fluctuations associated with either or both of these phases could be the driving force behind high Tc.
Although the CDW as recently be widely studied his relationship with superconductivity is still under debate. Here we used new approaches (high magnetic field, high pressure, disorder and x-ray) to address this problem.
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.
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.
Our measurements as a function of pressure and disorder show new and important link between the CDW and the superconductivity. These results will be published in two papers.
Concerning the uniaxial experiment, the results are very interesting but not yet enough to lead to a publication. More time have been allocated to complete these measurements in Nijmegen where we have two weeks of measurement planned for April 2020 to complete that measurement.
Concerning the uniaxial experiment, the results are very interesting but not yet enough to lead to a publication. More time have been allocated to complete these measurements in Nijmegen where we have two weeks of measurement planned for April 2020 to complete that measurement.