Fermi surface instabilities and quantum order at high pressure

Lead Research Organisation: University of Cambridge
Department Name: Physics

Abstract

One of the biggest scientific surprises of the twentieth century was the discovery of superconductivity, whereby some metals can carry electrical currents with absolutely no energy loss. Frictionless flow of electrons appears impossible in classical physics, but does in fact occur in quantum mechanical systems such as atoms or molecules. By taking phenomena we normally associate with the unusual micro-world of quantum physics into the practical macro-world of cables and switches, the discovery of superconductivity has paved the way for new devices and applications, in magnetic resonance imaging (MRI) scanners, high current fault limiting switches, high frequency filters and ultrasensitive measurement devices based on the Josephson effect. High-technology industries and the associated need for skilled labour are germinated by fundamental discoveries such as superconductivity. Future solutions for pressing problems, particularly in the fields of energy and sustainability, demand new materials with unusual electronic properties.

Real materials contain electronic quantum liquids. Because electrons have a low mass and are present at high density, the effects of quantum physics persist up to high temperatures, in many cases far exceeding room temperature. Interactions between the electrons cause them to correlate their motion and can induce new ordered states, of which an increasing variety - including various forms of superconductivity - have been discovered in recent years. The effective interactions depend on details of the specific material and thereby become highly tunable: they can be varied by changing material composition, by applying magnetic or electric fields, or by changing the lattice spacing through applied pressure. The transition into a new ordered phase as a function of this form of quantum tuning is called a quantum phase transition. The vicinity of quantum phase transitions is a fertile ground for unexpected and often spectacular discoveries. Examples include high temperature superconductivity in the iron-pnictide materials, the quantum nematic state in Sr3Ru2O7, and unconventional superconductivity in ferromagnets.

To pave the way for future discoveries, we need to know more about the mechanisms operating near such electronic instabilities. In this project, we will examine the electronic structure of selected materials close to quantum phase transitions, which can best be accessed under pressure. In some ways this is similar to deducing a crystal structure, but because the electrons are always in motion, we do not determine their position but rather their velocity, energy and effective mass. This is achieved by observing oscillations in the magnetic field dependence of the electrical resistivity, the magnetic susceptibility or other properties. These quantum oscillation measurements are a powerful tool for examining the electronic structure of a wide range of materials of current interest. To achieve the required ultra-sensitive measurements in a high pressure environment of more than 100,000 atmospheres is challenging, but recent technical developments in our group and elsewhere suggest that such experiments are now possible and will be justified by the resulting benefits.

We will investigate the correlated metallic state on approaching metal-insulator transitions, the transition from density wave order to the normal metallic state, the local moment to itinerant electron cross-over in heavy fermion systems, and other topics which are timely and of particular theoretical and practical interest. We will also use high precision heat capacity measurements under pressure to examine the electronic density of states near quantum phase transitions and to identify thermodynamic signatures of Fermi liquid breakdown in certain high-profile cases. Our electronic structure measurements will be complemented by high pressure lattice structure determination in the new Diamond Light Source synchrotron facility.

Planned Impact

There are already important applications for some of the correlated electron states related to this project. Magnetism, the best-known manifestation of electronic order in solids, is applied throughout the engineering disciplines. For example, modern permanent magnets are essential for efficient electric motors or generators, and magnetoresistive sensors permit high storage density in computer hard drives, which themselves store information in magnetically polarised domains. Superconductivity, on the other hand, enables loss-free high field electromagnets. Such magnets operate in the many medical MRI scanners, as well as in numerous research facilities, including particle accelerators such as the LHC. It also has found important applications in power distribution, especially where space is at a premium, in microwave filters for mobile-phone base stations, and provides the basis for schemes to achieve quantum computation.

The diversity of electronic states in correlated systems, their intrinsic quantum nature, their reach into practicable temperature regions, and their tunability by varying the crystal structure, or by applying external fields or pressure, can be exploited for next generation quantum innovations. These may be novel devices, for instance for sensing, computation or storage, or new materials with potentially major impact on current engineering challenges. This programme of fundamental research helps to address key concerns of our industrialised economy, in particular with regard to energy and sustainability. Progress in this area benefits the general population as well as specialised companies poised to exploit technological advances:

* More efficient batteries and supercapacitors: key battery materials such as LiCoO2 exist on the boundary of a Mott insulator transition, one of the central topics of this project. The high electron density and strong interactions present in Mott insulators benefit energy storage applications, and the wide variety of structures available in complex materials can be exploited for efficient ion transport. Better battery and capacitor materials strongly affect the use of increasingly ubiquitous mobile devices, as well as the performance of electrical motorcars. On a larger scale, efficient energy storage is required to secure a reliable energy supply in the presence of fluctuations of supply and demand.

* Cooling and thermal energy harvesting: materials with high thermoelectric figure of merit, including the skutterudite compounds to be investigated in this project, can be used for refrigeration and, conversely, for thermal energy harvesting. Both technologies are relevant in computing and other high-technology applications, which are increasingly limited by waste heat production. Moreover, materials with strongly enhanced magnetocaloric effects, such as some of the rare earth compounds in this work, can be used for magnetic refrigeration. Present cooling and air conditioning methods account for a substantial fraction of our energy consumption, and even modest gains in efficiency would produce major savings. Electric and magnetic refrigeration methods also have a role to play in cooling sensors and other appliances which benefit from a quiet, low temperature environment, efficiently and conveniently to their ideal operating temperature.

* Power distribution: this research contributes to the search for new superconductors with high transition temperatures, which are expected near the Mott insulating state or near other quantum phase transitions. Materials which superconduct at high temperatures, potentially even at room temperature, could trigger a technological revolution, but even modest advances benefit power transmission, distribution and switching.

The development of experimental techniques associated with this work will feed innovative solutions as well as skilled problem solvers and entrepreneurs into the UK network of high technology companies.

Publications

10 25 50
 
Description 1) First observation of the electronic Fermi surface and effective carrier mass in the correlated metallic state on the threshold of Mott localisation. This required high frequency tank circuit measurements on the Mott insulator NiS2, which was metallised in a pressure device at about 40,000 atmospheres.

2) Discovery of unconventional superconductivity in the layered iron-germanide YFe2Ge2. This material shares key structural and electronic characteristics of the high temperature superconducting iron pnictides and chalcogenides, but is the first example of an iron-based unconventional superconductor outside those families of materials.

3) Discovery and investigation of the structural quantum critical point in the cage compounds (Ca/Sr)3(Ir/Rh)4Sn13. These complex materials undergo superlattice formation on cooling. It is possible to reduce the superlattice transition temperature towards zero, either by changing the chemical composition or by applying hydrostatic pressure. In the vicinity of the critical point, where the superlattice transition temperature extrapolates to zero Kelvin, superconductivity is enhanced and soft lattice vibrations cause an unconventional temperature dependence of the electrical resistivity as well as enhancing the specific heat capacity.
Exploitation Route They have already motivated further fundamental research. The techniques developed for the high pressure work on NiS2 can now be applied to similar, challenging experiments in other materials. The improved understanding of a structural quantum critical point will have implications for the investigation and exploitation of structure-property relations, in particular in thermoelectrics. The discovery of a new unconventional superconductor adds a fresh perspective in the endeavour to understand and master pairing mechanisms that can lead to robust superconductivity at elevated temperatures.
Sectors Electronics

 
Description We have identified materials which have potential for solid state refrigeration at low temperature. This line is now pursued in further work towards a demonstrator system and, ultimately, commercialisation.
First Year Of Impact 2015
Sector Aerospace, Defence and Marine,Electronics,Energy
Impact Types Economic

 
Description IAA Follow on Fund
Amount £51,394 (GBP)
Funding ID KIZA/054 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 07/2014 
End 09/2015
 
Title Tank circuit oscillator techniques for Fermi surface determination in high pressure anvil cells 
Description We have developed methods and hardware for measuring the electronic Fermi surface under very high applied pressures of up to about 100 kbar. The methods involve inserting the sample into a miniature coil, which forms the inductor in a tunnel-diode driven LC tank circuit oscillator. The resonance frequency of the oscillator shifts with changes in the skin depth of the sample, reflecting changes in the electrical resistivity. This enables high resolution measurements of Shubnikov-de Haas oscillations in applied fields, and thereby gives valuable information on the Fermi surface and carrier mass under high applied pressure. 
Type Of Material Improvements to research infrastructure 
Year Produced 2016 
Provided To Others? Yes  
Impact We have been able to resolve key elements of the electronic Fermi surface of the Mott insulator NiS2, when it was metallised under pressures exceeding 30,000 atmospheres. The initial data has been published in Nature Scientific Reports (2016), and further publications are in preparation. 
 
Title Research data supporting "Strong coupling superconductivity in a quasiperiodic host-guest structure" 
Description Data underlying the figures shown in the publication 'Strong coupling superconductivity in a quasiperiodic host-guest structure', including resistivity versus temperature at different pressures and magnetic fields, the critical-field curve of high pressure bismuth, the high pressure magnetisation of bismuth, and the results of phonon dispersion calculations. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Impact The published datasets allow colleagues to conduct their own analysis and compare new theories for the electronic and vibrational excitations of a quasiperiodic material against experimental data. 
URL https://www.repository.cam.ac.uk/handle/1810/278535
 
Title Research data supporting 'Quantum Tricritical Points in NbFe2' 
Description Magnetisation, magnetic susceptibility and electrical resistivity data obtained on single crystals of NbFe2 with varying levels of Fe or Nb excess. Analysis in terms of Arrott plots. Resulting phase diagram showing buried ferromagnetic quantum critical point and quantum tricritical points. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
 
Description Growth and characterisation of quantum materials 
Organisation University of Central Lancashire
Department Jeremiah Horrocks Institute for Mathematics, Physics and Astronomy
Country United Kingdom 
Sector Academic/University 
PI Contribution We have grown high quality crystals and polycrystals of quantum materials, such as YFe2Ge2, NiS2 and NbSiSb and investigated their properties by a range of transport, thermodynamic, magnetic and spectroscopic probes.
Collaborator Contribution Colleagues at the University of Central Lancashire have carried out high precision structural studies by single crystal and powder x-ray diffraction, in order to resolve details of the crystal structure and its defect concentration.
Impact The collaboration has led to crystals of world-leading quality of both NiS2 and YFe2Ge2. This in turn has enabled us to resolve the electronic structure of pressure-metallised NiS2, the first such measurement near the threshold of Mott localisation, and it has produced clear thermodynamic evidence for the unconventional superconducting state in YFe2Ge2. These outcomes are recorded in a number of joint publications.
Start Year 2015
 
Description Joint low temperature measurements on high quality crystals 
Organisation Max Planck Society
Country Germany 
Sector Charity/Non Profit 
PI Contribution Provided crystals and suggested aspects of the experiment
Collaborator Contribution Carried out low temperature magnetic torque measurements at the MPI-CPfS in Dresden
Impact Detailed low temperature magnetic measurements in NbFe2 samples of varying stoichiometry have improved our understanding of the origins of magnetic anisotropy in this complex material and will contribute to a future publication.
Start Year 2013
 
Description Single crystals for low temperature, high field, high pressure measurements 
Organisation University of Warwick
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Sharing of low temperature, high field, high pressure measurement data. Future joint publications.
Collaborator Contribution Provision of high quality single crystals of two materials of intense current interest.
Impact A series of successful high pressure, low temperature, high magnetic field measurements to elucidate the change in the electronic structure as a material is tuned across a band inversion transition. The data will form part of a PhD thesis, has in parts been presented at conferences, and is due to be published as soon as the measurements are complete.
Start Year 2013