Events

Department of Magnetic Reseach Seminar

Two-band superconductivity in electron-irradiated PrOs4Sb12 and CeCu2Si2 studied by local magnetization measurements

prof. dr hab. Tomasz Cichorek

Oddział Badań Magnetyków INTiBS PAN we Wrocławiu

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Streszczenie:

Phases of matter are usually identified through spontaneous symmetry breaking, especially regarding unconventional superconductivity and the interactions from which it originates. As the introductory part, we will briefly discuss the superconducting state of Sr2RuO4 that for more than 25 years is discussed as a solid-state analogue of the superfluid 3He-A phase - the only well documented chiral spin-triplet state.
The main focus will be directed to the heavy-fermion and multiband superconductor PrOs4Sb12 with broken time-reversal symmetry, spontaneously developing below the critical temperature Tc≃1.85K, which is a leading candidate to display chiral superconductivity. Based on measurements of the temperature dependence of the lower critical field Hc1(T), we have recently found evidence for the order parameters composed of a sign-changing smaller gap and a large isotropic s-wave gap [1].
To develop a detailed understanding of multicomponent superconductivity in PrOs4Sb12, we have extended measurements of Hc1(T) down to temperatures as low as 7 mK utilizing a 2DEG Hall magnetometry. We observe a sudden increase in Hc1(T) deep in the superconducting state, indicative of a rare case of two nearly decoupled bands. Furthermore, a non-saturating and concave behaviour of Hc1(T) below about 0.45 K clearly points at a sign-changing symmetry of the smaller gap. Equally remarkable is a high sensitivity of this characteristic to electron irradiation. Indeed, a concentration of artificial atomic defects as small as a few 0.1% results in both a saturation of Hc1(T) at T < 0.15 K and a strong suppression of the anomalous enhancement below ≃ 0.25Tc, consistent with a destruction of an unconventional order parameter due to the smaller gap. Theoretical description of possible symmetries of the smaller gap as well as results of a comparative study on the two-band isotropic s-wave homologue LaRu4As12 (Tc =10.4 K) will be discussed in the context of a putative chiral spin-triplet pairing state in PrOs4Sb12.
In addition, we report in- and out-of-plane Hc1(T) dependencies of the prototypical heavy-fermion material CeCu2Si2 (Tc≃0.58 K, S-type), for which two-band and fully gapped superconductivity has been recently observed. For both [100] and [001] directions, we found sharp anomalies (at ≃ 0.34Tc and ≃ 0.41Tc, respectively) followed by moderate enhancements. Unlike to PrOs4Sb12, however, both Hc1a(T) and Hc1c(T) curves saturate in the limit T=0, being more consistent with an s±-wave scenario [2] than a dxy + dx2-y2 band-mixing paring model [3]. Besides, the effect of electron-irradiation on the anomalous enhancement of Hc1(T) at T << Tc will be shown.
References
[1] J. Juraszek et al., Phys. Rev. Lett. 124, 027001 (2020).
[2] T. Yamashita et al., Sci. Adv. 3 : e1601667 (2017).
[3] G. Pang et al., PNAS 115, 5345 (2018).