Northwestern Events Calendar

Oct
25
2018

CMP Seminar: Professor John Saunders: Superconductivity in YbRh2Si2; electrical transport and noise experiments

When: Thursday, October 25, 2018
4:00 PM - 5:30 PM CT

Where: Technological Institute, F160, 2145 Sheridan Road, Evanston, IL 60208 map it

Audience: Faculty/Staff - Student - Public - Post Docs/Docs - Graduate Students

Contact: Cristian Pennington  

Group: Physics and Astronomy Condensed Matter Physics Seminars

Category: Academic

Description:

We report electrical transport measurements on the putative heavy fermion superconductor YbRh2Si2 [1], over the temperature range 1 K to 0.6 mK, which clearly identify a superconducting state in high quality single crystal samples. There is evidence for strong anisotropy of the superconductivity in this material. With modest in-plane magnetic fields (above 4 mT), applied perpendicular to the c-axis, we observe “re-entrance” of the normal state resistance at the lowest temperature. This is attributed to the interplay between superconductivity and electro-nuclear magnetism. The influence of in-plane magnetic fields up to 70 mT, through the quantum-critical field, has also been investigated. We discuss the prospect of tuning the magnetism by choice of Yb isotope in enriched samples, coupled to studies at ultralow temperatures of electric transport at the field-tuned quantum critical point. Furthermore a superconducting transition has been detected in a focussed-ion-beam machined sample. This opens the way to: improved studies of anisotropy in superconducting transport, and its likely intrinsic origin; the control of strain; the use of strain as a tuning parameter; devices exploiting the relatively long superconducting coherence length.


The rather low superconducting transition temperatures found in YbRh2Si2 requires the use of sensitive ultra-low dissipation measurement techniques. An experimental set-up with SQUID-based detection was employed in which samples were contacted via ultrasonically bonded aluminium wire. This has enabled measurements of Nyquist noise, which showed that on cooling below around 12 mK (sample I), there is a clear transition from normal metal into a state in which the resistance gradually decreases with decreasing temperature. With the same set-up, the detection of persistent currents demonstrated the transition to a zero-resistance phase-coherent superconducting state of the current loop including the aluminium-YbRh2Si2 contact region. This transition occurred at 3.6 mK (sample I) and 6.0 mK (sample II). Subsequently a SQUID-based impedance measurement technique was applied to sample II. This unambiguously identified a superconducting transition in the ab plane, occurring at 6 mK in zero magnetic field. These measurements also provide evidence for anisotropy in electrical transport, between 6 mK and 11 mK, and are consistent with the noise measurements. The field dependence of these features demonstrate heavy fermion superconductivity.

We propose that superconductivity coexists with magnetic order in this system, and that quantum criticality plays no role in the superconductivity observed so-far. However we observe a very weakly field dependent feature in the electrical response of the sample, at around 2 mK, which we identify with the magnetic transition proposed in [1]. The nature of the superconducting order; the pairing symmetry; the possibilities of multiple superconducting phases, quantum critical superconductivity and distinct phases at the interface between an s-wave superconductor and YbRh2Si2 will be discussed.
Of particular interest is to establish whether or not YbRh2Si2 is a topological superconductor, which would be significant in view of its availability as high quality single crystals, and in view of its potential tuneability.

[1] E. Schuberth et al., Science, 351, 495 (2016).

Professor John Saunders, London Low Temperature Laboratory, Department of Physics, Royal Holloway University

Host: James Sauls

Keywords: Physics, Astronomy, Condensed Matter

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