Laboratoire National des Champs Magnétiques Pulsés CNRS – INSA

Pnictides
Superconductivity with unexpectedly high critical temperatures was recently discovered in a whole
new class of materials, iron-based pnictides. In some of them, superconductivity emerges already at
ambient pressure, while an application of high pressure is required to induce a superconducting
transition in some others. In spite of considerable experimental and theoretical efforts, the
understanding of underlying mechanism of superconductivity in such compounds is still far from
being achieved. One reason of slow progress is that high quality single crystals of these materials
became available only very recently. Another one is that some of the key experiments require extreme
experimental conditions such as very low temperatures, high magnetic fields and high pressures, as
well as combinations of those. Such experimental conditions are not readily available in most of the
laboratories worldwide. Built on our expertise of measurements under extreme conditions, we plan to
contribute to answering some of the fundamental questions related to iron-based superconductors.
Transport and specific heat measurements both under magnetic field and pressure will allow us to
explore in some details the phase diagrams of these materials and investigate their superconducting
and normal state electronic properties. de Haas-van Alphen effect measurements will be employed for
the direct exploration of the Fermi-surface and determination of the quasiparticle effective masses,
which are directly related to the strength of the electronic correlations. What's more, in
superconductors, the comparison of the measured effective masses with the calculated band masses
provides a direct access to the strength of the electron-phonon coupling. Such coupling is suggested as
the origin of superconductivity in iron-based pnictides by some theories. Other theoretical models
suggest a multi-band superconductivity in Fe-based superconductors. Moreover, they suggest that the
pairing mechanism may be mediated by magnetic fluctuations due to the proximity to a spin density
wave. Knowing the fine details of the Fermi surface topology, its tendency towards instabilities as
well as the strength of the coupling of the quasiparticles to excitations is, therefore, essential for
understanding the superconductivity in these compounds.
To realise the scientific objectives described above, we will both use the existing experimental
set-ups and techniques and develop new ones. We plan to improve the performance of some of the
existing set-ups (better sensitivity, wider range of temperatures/fields, etc). For instance, our set-up for
specific heat measurements by relaxation is currently limited to about 1.5 K. We are going to adopt it
to lower temperatures, 3 He system (about 0.4 K) in the beginning with an ultimate goal of 0.1 K in the
dilution refrigerator. Built on our experience in high pressures, we are going to develop a whole set of
different type pressure cells for transport and specific heat measurements covering a wide range of
pressures, and suitable for high fields and low temperatures. One of the techniques we are going to
develop shortly is absolute measurements of magnetisation with high resolution and sensitivity in
magnetic field gradients, both pulsed and DC. The objective is to develop such measurements up 35
Tesla DC and 70 Tesla pulsed and down to 40 mK.
Cuprates
The cuprates, although known as high Tc superconductors for over 20 years, continue to provide
surprising experimental data, and a complete theory for this phenomenon is still lacking. Recent
advances in crystal growth and improvements in measurement techniques now allow the observation
of quantum oscillations in these systems. We will continue our pioneering studies of the Fermi surface
of these materials through quantum oscillation measurements in order to obtain a more complete
picture of the fermiology of these compounds.
Other techniques, like Nernst effect measurements, ultrasound spectroscopy and pulsed field NMR
will be developed and applied to these systems, in the hope to finally resolve the mystery of high Tc
superconductivity in the cuprates.
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