Fermi Surface Reconstruction in Cuprate High Temperature Superconductors

Lead Research Organisation: University of Bristol
Department Name: Physics


Superconductors have the potential to revolutionise the way the world uses electricity. There are already many practical applications of these materials, ranging from energy transport to uses in medical diagnosis, communications and mass people transport. However for more wide-ranging impact we need to discover materials which have even better properties than are already known today. In order to tune these properties and to guide the search for new materials, knowledge of the fundamental physical reasons why these materials are superconducting is highly desirable. Although this is known for so-called conventional materials, mostly discovered before 1980, an understanding of the superconducting mechanism responsible for copper oxide based high temperature superconductivity, discovered in 1986, is still lacking. The research in this proposal aims to advance our understanding of the electronic structure of copper oxide high-temperature superconductors. We believe this knowledge will provide a major step forward in the world-wide quest to understand and hence improve these materials.
We are proposing a wide ranging programme which will study the thermodynamic and quantum coherent properties of extremely well ordered samples of these materials. In less well ordered samples, the signatures of the fundamental symmetry-breaking phase transitions may be smeared out, making them invisible to experiment. Also quantum coherence effects which give unique information about the electronic structure are made unobservable by disorder. We will use techniques developed over the last twenty years to grow highly ordered single crystal samples and study their behaviour under the world's highest available magnetic fields of up to 100 T (which is roughly 2 million times larger than the earth's field) at temperatures less than one degree above absolute zero. From these measurements we will discover how the Fermi surface, which characterises the momentum distribution of the current carrying electrons in the material, evolves with electron concentration. This will give unique and important information to guide the development of a theory of superconductivity in these materials.

Planned Impact

The work outlined in this proposal is fundamental research into the mechanism of high temperature superconductivity. Superconductors are used already in numerous applications, the most important (by economic value) is currently in magnetic resonance imaging (MRI) scanners. However, the future use could be much wider-ranging if the properties of the available materials could be improved over what is available today. The key parameters are the critical temperature and the ability of the material to carrier a high electrical current. If these parameters could be suitably enhanced the societal and economic benefits would be immense as these materials could then be used to improve the vast majority of devices which currently use electricity as well as facilitate the development of completely new devices. Possible applications range from the generation, transport and distribution of electricity to improving the speed of computers by reducing the power dissipation in integrated circuits, improving the diagnostic abilities of MRI scanners (by increasing the available magnetic field and possibly reducing the need for cryogenic refrigeration) and also mass people transport through the development of superconducting levitating trains, such as the Linear Chuo Shinkansen Project presently under development in Japan.

A crucial step on the pathway to achieving this goal is to gain a more comprehensive picture of the fundamental physics that causes materials to superconduct at high temperature. If this were known, it would clarify the route towards optimising current materials as well as inform the search for new materials. Our programme seeks to identify the source of electronic order which may compete with, or be the cause of, superconductivity in the cuprate high temperature superconductors. Although this order is notoriously difficult to pin down we believe our programme of using high magnetic field to investigate the thermodynamic and quantum coherent properties of very finely tuned and very well ordered single crystal samples will shed considerable new light on the issues.

A recent comprehensive report(*), from a panel commissioned by the US Department of Energy on the basic research needs for superconductivity, identifies the following as priority research directions: i) Understand and exploit competing electronic phases, ii) Identify the essential interactions that give rise to high-Tc superconductivity, iii) Develop a comprehensive and predictive theory of superconductivity. Our programme impacts strongly on all three of these themes.

* Available from http://science.energy.gov/~/media/bes/pdf/reports/files/sc_rpt.pdf
Description A major part of this project was devoted to measuring the properties of cuprate high temperature superconductors at very high magnetic field under hydrostatic pressure. High magnetic field (up to 80 Tesla) can be used to investigate the electronic structure of the materials by suppressing the superconductivity and enabling the observation of quantum oscillations. Pressure is used to tune the electronic structure, avoiding the complications of charge doping which introduces disorder. A main result was that we succeeded to measure Shubnikov-de Haas quantum oscillations in the cuprate YBa2Cu4O8 at pressures above 8kBar, and fields up to 70 Tesla, and so we could follow how the detailed changes in the electronic structure correlated with the changes in the superconducting transition temperature. A surprising result was that the effective mass decreases as Tc increased under pressure. This is exactly the opposite correlation to that observed when Tc in YBa2Cu3O7+x is increased by doped (changing x). The result suggests that the correlation between the mass enhancement and maximum Tc observed in YBa2Cu3O7+x is probably accidental, perhaps resulting from strong fluctuations in the charge density wave order which are therefore do not boost superconductivity.

A second strand of research searched for microscopic evidence for change order in the cuprates. Our x-ray experiments showed that such charge order exists in the form of a charge density wave (CDW) in the canonical HTC systems YBa2Cu3O7+x and La2-xSrxCuO4 near doping, p= 0.12. This work yielded one of the most highly cited papers in the field in the last few years and has led to numerous follow on experiments. Our experiments also showed that there is a strong coupling and competition between he CDW and the superconducting order parameters visible in the field and temperature dependence of the CDW order parameter. More recently, we have shown how the application of a 17T magnetic field to YBa2Cu3O7+x causes 2D CDW fluctuations to become 3D order. This transition coincides with anomalies in other physical properties such as the Hall effect.
The award also supported some work on a related class of materials know as Dirac semi-metals. The material investigated, ZrSiS is a semimetal with linearly dispersing bands around a 'Dirac-nodal loop'. Such materials have many potential applications the simulation of long-sought phenomena of high-energy physics, as well as for the realization of novel (quantum) information scheme. The work looked for evidence for enhanced many-body effects which may be a precursor to a new broken symmetry phase. Through measurements of quantum oscillations at fields beyond 30T, we found that above a threshold field, magnetic breakdown occurs across gaps in the loop structure with orbits that enclose different windings around its vertices, each winding accompanied by an additional Berry phase. The amplitudes of these breakdown orbits exhibit an anomalous temperature dependence, demonstrating the emergence of novel, correlation-driven physics in ZrSiS associated with the Dirac-like quasiparticles.
Exploitation Route The research forms part of a body of worldwide work to understand the electronic structure of cuprate high temperature superconductors, which will eventually lead to the development of materials with better superconducting properties - such as higher transition temperauture.
Sectors Other