In March 2008, the discovery of superconductivity with a critical temperature at 26 K in the fluorine doped layered iron arsenides [1] has encouraged broader exploration of phases with iron in a similar tetrahedron environment in the hope of finding new superconducting compounds that would possess an even higher critical temperature. This discovery had great impact on the condensed matter community, not only due to the presence of iron in these compounds but also because of the enormous amount of compounds that could be synthesised by doping in the many possible atomic locations (see Figure 2a). Consequently, several hundreds of articles have since been published in the most important journals. Over the same period, high pressure structural, magnetic and transport property measurements have seen a very important technical development. X-ray diffraction, ac susceptibility and resistivity studies can be routinely carried out at Mbar conditions (1 Mbar = 100 GPa ~ 106 atm). Furthermore, studying samples using a combination of different techniques allows greater comprehension of the physical properties.

Fig. 2: Crystallographic structures of Fe based compounds at ambient conditions: a) LaFeAsO; b) FeSe.

 In earlier studies, we performed the first diffraction experiment at high pressure on LaFeAsO1-xFx (x~0.1) [2], where we correlated the structure and transport properties under pressure. One of the simplest examples of iron based compounds with tetragonal PbO structure is FeSe. In this system, the report of superconductivity in the tetragonal phase [3] (see Figure 2b) is tantalising, as pronounced pressure effects at relatively low pressures seem to strongly increase TC up to around 25 K for the onset of the transition. Furthermore, partial replacement with tellurium also noticeably increases TC. However, superconductivity seems to coexist with magnetically-ordered states accompanied by lattice distortions that are much more stable than superconductivity. Here, we report one of the first simultaneous studies of the evolution of both structure and superconductivity under pressure on the compound FeSe. These studies have been developed at beamline ID27 and at the Matière Condensée et Basses Témperatures Department of the Institut Néel. We find an orthorhombic high pressure phase, which develops above 12 GPa in our samples (see Figure 3b). The high pressure resistivity measurements show that TC has a linear increase below 12 GPa (in the tetragonal phase) with a kink at higher pressure reaching its maximum of 34 K at 22 GPa (see Figure 3a). Analysis of the pressure evolution of the crystallographic structure shows that the separation between FeSe layers is reduced by 30% at 12 GPa, while the thickness of the FeSe layer remains constant. The anisotropic compression, that is typical of layered compounds, may reach a threshold at this pressure, forcing the phase transformation observed at high pressures. A detailed study as a function of subtle changes in Fe stoichiometry is necessary to understand the differences found in several reports on this material.

Fig. 3: Pressure evolution of a) the superconducting transition temperature, and b) the unit cell volume. The change of pressure dependence of TC at the crystallographic phase transition can be seen clearly.

In summary, we have revealed a linear relationship between structural and superconducting properties under pressure for the FeSe system. We obtained one of the highest critical temperatures in this Fe based compounds, which opens the possibility to design new materials with even higher TC with perspective of greater comprehension of the superconducting state.

 

References

[1] H. Hosono, J. Phys. Soc. Jpn. 77, Suppl. C, 1 - 8 (2008).
[2] G. Garbarino, P. Toulemonde, M. Álvarez-Murga, A. Sow, M. Mezouar and M. Núñez-Regueiro, Phys. Rev. B (RC) 78, 100507 (2009).
[3] F.C. Hsu, J.Y. Luo, K.W. Yeh, T.K. Chen, T.W. Huang, P. M. Wu, Y.C. Lee, Y.L. Huang, Y.Y. Chu, D.C. Yan and M.K. Wu, PNAS 105, 14262 - 14264 (2008).

 

Principal publication and authors

G. Garbarino (a), A. Sow (b), P. Lejay (b), A. Sulpice (b), P. Toulemonde (b), M. Mezouar (a) and M. Núñez-Regueiro (b), European Physics Letters, 86, 27001 (2009).
(a) ESRF
(b) Institut NEEL, CNRS & Université Joseph Fourier, Grenoble (France)