National Physical Laboratory

Anomalous critical fields in quantum critical superconductors

Strategic Research contribution to article in Nature Communications - Anomalous critical fields in quantum critical superconductors.

Contour plot

Contour plot of measured Hc1 in
BaFe2(As1-xPx)2 as a function of
temperature and isovalent substitution
(P for As). This is enhanced near the
quantum critical composition x = 0.3
(image courtesy of Professor A Carrington,
University of Bristol).

Superconductivity is a distinctive electronic 'state' where a cryogenically cooled material has zero electrical resistance. The current-carrying properties of superconductors enable large magnetic fields to be created, for example in medical imaging, while other properties make possible super-sensitive detectors and voltage reference standards and many other devices.

Some applications are limited by the 'envelope' of superconducting operating parameters outside of which superconductivity is destroyed (e.g. operating temperature, magnetic field and current). Understanding why superconductors have different limits, or even why they are superconductors at all, is challenging. The problem is particularly tricky where superconductivity emerges from a material that is 'badly-behaved' in its normal state, which are often the materials with the most promising properties. To solve this puzzle, detailed studies of 'weird superconductors' are required. Dr Jonathan Fletcher and Dr Patrick See are co-authors of a work published in Nature Communications which draws on work from an NPL Exploratory Strategic Research project.

Teams from several institutions (University of Bristol, LMNCI Toulouse, Kyoto University, University of Tokyo and Cambridge University) used different measurement techniques to measure the critical magnetic parameters of an iron-based superconductor as it is tuned towards a quantum critical point - a point of instability between phases where electronic properties can be perturbed by quantum fluctuations.

Simplified picture of superconducting vortices
Simplified picture of superconducting
vortices: Vortex energy can be separated
into an electromagnetic part (screening
currents and magnetic field) and the
non-superconducting vortex core
(image courtesy of Adwaele / Wikipedia).

The iron-pnictide superconductor BaFe2(As1-xPx)2 is a high-temperature quantum critical superconductor that is particularly suitable to study how quantum critical fluctuations affect the superconducting state. Measurements of the (superconductivity-destroying) upper critical magnetic field Hc2 were made using heat capacity and torque measurements in fields up to 60 T at labs in Bristol and Toulouse for a range of compositions. The lower critical magnetic field Hc1, which marks the point where magnetic vortices first penetrate the sample, is 1,000 times smaller than Hc2. This was also measured in Bristol using micron-scale magnetic field sensor arrays designed and fabricated by Jonathan Fletcher and Patrick See using MBE-grown semiconductor wafers from the University of Cambridge.

The authors found that the critical fields do not at all behave in the way expected according to the standard theory. Near the quantum critical point an enhancement in Hc1 is seen rather than the suppression expected from the reduced magnetic energy of a vortex where the electron mass is enhanced. The discrepancy points to a substantial increase of the vortex core energy. In textbook conventional superconductors this has only a small effect on c1, but is clearly more important in this 'weird' quantum-critical case. This result suggests that it is the increased energy of the normal state caused by quantum critical fluctuations that provides the boost which stabilises superconductivity at high temperature.

For additional information, please contact Dr Jonathan Fletcher

Last Updated: 15 Jan 2015
Created: 14 Jan 2015

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