Neven Barisic, SPEC, CEA Saclay

Jeudi 04 Octobre, 16h
Amphi Howleck, Esc C, 1ème etage

Conductivity in the Normal State of High-Tc Superconductors

Neven Barisic
Service de Physique de l’Etat Condensé, CEA Sacaly

Over the past quarter of a century, the understanding the enigmatic metallic state of the cuprates has been confirmed as one of the pivotal issues in physics. This strange metallic state, from which superconductivity evolves upon cooling, is characterized by remarkable
physical properties, such as the pseudogap phenomenon and the linear temperature dependence of the resistivity. The enormous amount of associated experimental work has been plagued by non-universal, material-specific results, which has obfuscated the true nature
of these materials. Our new findings for the metallic state of the cuprates are both remarkably simple and
profound. Contrary to conventional wisdom, we find that the in-plane resistivity in the pseudogap phase of HgBa2CuO4+δ, arguably the model cuprate superconductor (1), exhibits a quadratic T dependence (  T2), as in conventional Fermi liquids (2, 3). By combining our
results with published data on three structurally more complex cuprates we have been able to obtain a universal, quantitative resistivity value per copper-oxygen plane (or plaquette)
throughout most of the temperature-doping phase diagram. Furthermore, we demonstrate that only the near-nodal states contribute to the planar transport, even outside the pseudogap regime. We derive a novel phase diagram containing four characteristic temperatures (Tc < T ’
< T** < T*). T* is coincident with the onset of a novel q = 0 magnetic order (4) ; T** is tentatively associated to a separate magnetic transition (2) ; T ’ lies only 10-20 K above Tc and marks the onset of critical superconducting fluctuations. Conversely, it appears that deep
inside the pseudogap regime, close to the Mott-insulating state at zero doping, the conductivity of cuprates may in fact behave like that of Fermi liquids. These findings call for
a paradigm shift in the interpretation of the phase diagram of cuprates. A combined analysis of the dc and optical conductivities in the optimally doped 122-iron pnictides also reveals a T2 dependence of the normal state scattering rate (5). Although, ironpnictides
are in many respects different from cuprates, such a similarity raises the question whether superconductivity in “unconventional” superconductors exhibiting the highest-Tc’s evolves from a conventional Fermi-liquid-like state.

References
1. N. Barišić, Y. Li, X. Zhao, Y. Cho, G. Chabot-Couture, G. Yu, M. Greven. Phys. Rev. B 78, 054518 (2008).
2. N. Barišić, Y. Li, G. Yu, X. Zhao, M. Dressel, A. Smontara, M. Greven. arXiv 1207.1504.
3. S. I. Mirzaei, D. Stricker, J. N. Hancock, C. Berthod, A. Georges, E. van Heumen, M. K. Chan, X. Zhao, Y. Li, M. Greven, N. Barišić, D. van der Marel. arXiv 1207.6704.
4. Y. Li, V. Baledént, N. Barišić, Y. Cho, B. Fauque, Y. Sidis, G. Yu, X. Zhao, P. Bourges, M. Greven. Nature 455, 372 (2008).
5. N. Barišić, D. Wu, M. Dressel, L. J. Li, G. H. Cao, and Z. A. Xu. Phys. Rev. B 82, 054518 (2010).

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