Selective mass enhancement close to the quantum critical point in BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$

Abstract

A quantum critical point (QCP) is currently being conjectured for the BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ system at the critical value $x_{\mathrm{c}}\approx$ 0.3. In the proximity of a QCP, all thermodynamic and transport properties are expected to scale with a single characteristic energy, given by the quantum fluctuations. Such an universal behavior has not, however, been found in the superconducting upper critical field $H_{c2}$. Here we report $H_{c2}$-data for epitaxial thin films extracted from the electrical resistance measured in very high magnetic fields up to 67 Tesla. Using a multi-band analysis we find that $H_{c2}$ is sensitive to the QCP, implying a significant charge carrier effective mass enhancement at the doping-induced QCP that is essentially band-dependent. Our results point to two qualitatively different groups of electrons in BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$. The first one (possibly associated to hot spots or whole Fermi sheets) has a strong mass enhancement at the QCP, and the second one is insensitive to the QCP. The observed duality could also be present in many other quantum critical systems.

arXiv comment9 pages, 3 figures, and Supplementary materials

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