Electrons in superconductors and other quantum states of matter
Project supervisor: Dr Paul Goddard∗
Resistance is futile: lightbulbs and heaters aside, the majority of electronic components are at their
most efficient when their electrical resistance is minimized. In the present climate, with energy
sustainability regularly topping the international agenda, reducing the power lost in conducting
devices or transmission lines is of worldwide importance. Research into the nature of superconducting
materials is hence more vital than ever before.
Superconductivity, the phenomenon of zero
electrical resistance which occurs below a critical temperature in certain materials, remains
inadequately explained. At present, many superconductors have unfavourable physical attributes that stymies their use in technology.
A more complete understanding of what causes
the superconducting state (an example of a cooperative macroscopic quantum state) to form
could result in the design of superconducting
materials with the mechanical properties required for mass technological exploitation. Unfortunately, it is the very materials which are
most likely to lead us to this end, the so-called
high-temperature superconductors, that are the
least understood.
Furthermore, there are several other classes (a) Quantum oscillation data in a high-temperature
of material, which display useful and interesting superconductor taken in fields up to 100 T and from
properties that cannot be adequately explained which the Fermi surface can be deduced [1]. (b) Fermi
semiclassically and that remain to be charac- surface of an organic superconductor as determined
terised experimentally. In particular, systems from angle-dependent magnetoresistance [2]. (c) Fermi
surface of the shape memory alloy AuZn [3]. (d) Strucof interacting electronic spins can be arranged
ture of [Cu(pyrazine)(glycine)]ClO4 , in which pairs of
in such a way that a cooperative quantum state spin = 1/2 moments form a cooperative ground state
develops at low temperatures. In many mate- akin to a Bose-Einstein condensate [4].
rials, the phase which exhibits the interesting
properties is found to be in competition with other phases of matter and it is a delicate balance of
electronic interactions that governs which phase ultimately wins out. This project aims to figure
out what the electrons are doing in these materials at the point at which the transition into the
interesting phase takes place. What it is that occurs at the vital tipping point where the ground
state is established? Key to this aim in the conducting systems is a determination of the Fermi
surface, which in many of these materials is yet to be established.
The project will make use of new developments in materials growth and in the provision of ultrahigh magnetic fields to explore the electronic properties of a range of novel systems, including cuprate,
iron-based, topological and non-centrosymmetric superconductors, as well as magnetic materials
based on tailored networks of interacting spins. Where appropriate, Fermi surfaces will be determined
using measurements of magnetic quantum oscillations and angle-dependent magnetoresistance. The
electronic interaction strengths will be adjusted by doping and/or the application of high applied
pressures and the resulting change in properties investigated. Experimental results will be compared
with existing theoretical models as well as those developed during the course of the project. The
experiments will performed in-house and at high-field user facilities located in Europe and the USA.
[1]
[2]
[3]
[4]
S. Sebastian et al., Nature 511, 61 (2014).
P. Goddard et al., Phys. Rev. B 69, 174509 (2004).
P. Goddard et al., Phys. Rev. Lett. 94, 116401 (2005).
T. Lancaster et al., Phys. Rev. Lett. 112, 207201 (2014).
∗
For further information do not hesitate to contact p.goddard@warwick.ac.uk.