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A new universal law for high temperature superconductivity


Cuprates, a new class of superconducting compounds discovered in 1986, some of which able to superconduct above liquid nitrogen temperature(-195,79°C), still keep their mystery. Besides the fact they conduct electricity without resistance below a high critical temperature Tc, another intriguing property, which gave them the name of strange metal, is the linear decrease of their resistance at low temperature, instead of a more rapid decrease as observed in conventional metals. This behaviour was first observed in the cuprates with only one copper-oxide plane per unit cell with the lower Tc values. In order to show if this is a generic property of cuprates, a team [1] of the Laboratoire International Associé “Circuits and Quantum Materials” have studied the low temperature resistivity of the two copper–oxide plane cuprate Bi2Sr2CaCu2O8+d (see figure).

The experiment reported in Nature Physics was done on an epitaxial c-axis oriented Bi2Sr2CaCu2O8+d thin film, deposited by sputtering in LPS, patterned into a small Hall bar equipped with contacts for current injection and voltage measurements (see figure). The film overdoped with oxygen was superconducting below 50K. It was measured at LNCMI, Laboratoire des Champs Magnétiques Intenses, in a 60 teslas magnet, which creates a magnetic field more than one million times as strong as the earth field and necessary to kill the superconductivity in the cuprate sample. With the superconductivity lifted, a perfect linear variation of the resistance was unveiled continuing the linear variation present in zero magnetic field from 120K. Importantly, it was found that the T-linear coefficient is given by a universal relation obeyed in all cuprates where a pure T-linear resistivity has so far been observed. This relation was obtained by assuming that the scattering rate of carriers (holes or electrons), 1/τ reaches the Planckian limit [2] (given by quantum uncertainty principle : ħ=kBT* τ, where ħ is the reduced Planck constant).

We may hope that understanding the strange metal behaviour from which superconductivity emerges will help to understand why superconductivity is so high in cuprates.

Left : Bi2Sr2CaCu2O8+d thin film epitaxially grown on SrTiO3 single crystal substrate (5mmx3mm), patterned by mechanical scribing for transport measurements. Centre : Crystal structure of Bi2Sr2CaCu2O8 . Right : Resistivity as a function of T at H=0 (blue) and (red diamond) high field values extracted from resistivity versus field curves.

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Hélène Raffy

Referencs

Universal T-linear resistivity and Planckian dissipation in overdoped cuprate
A. Legros, S. Benhabib, W. Tabis, F. Laliberté, M. Dion, M. Lizaire, B.Vignolle, D. Vignolles, H. Raffy, Z.Z. Li, P. Auban-Senzier, N. Doiron-Leyraud, P. Fournier, D. Colson, L. Taillefer et C. Proust
Nature Physics 15, 142-145 (2019). DOI:10.1038/s41567-018-0334-2
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[1The experiment involved a team of the international associated laboratory “Circuits and Quantum materials” composed of researchers from ” Institut Quantique”, University of Sherbrooke (Canada), LNCMI (Toulouse), SPEC-UMR (Ormes-des-Merisiers), and the LPS.

[2Why the temperature is high, J. Zaanen, Nature News & Views 430, 512-513 (2004)