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1. Thermal transport of helium-3 in a strongly confining channel

   https://doi.org/10.1038/s41467-020-18662-8  

   Lotnyk, D. ; Eyal, A. ; Zhelev, N. ; Abhilash, T. S. ; Smith, E. N. ; Terilli, M. ; Wilson, J. ; Mueller, E. ; Einzel, D. ; Saunders, J. ; et al ( December 2020 , Nature Communications)

   null (Ed.)

   Abstract The investigation of transport properties in normal liquid helium-3 and its topological superfluid phases provides insights into related phenomena in electron fluids, topological materials, and putative topological superconductors. It relies on the measurement of mass, heat, and spin currents, due to system neutrality. Of particular interest is transport in strongly confining channels of height approaching the superfluid coherence length, to enhance the relative contribution of surface excitations, and suppress hydrodynamic counterflow. Here we report on the thermal conduction of helium-3 in a 1.1  μ m high channel. In the normal state we observe a diffusive thermal conductivity that is approximately temperature independent, consistent with interference of bulk and boundary scattering. In the superfluid, the thermal conductivity is only weakly temperature dependent, requiring detailed theoretical analysis. An anomalous thermal response is detected in the superfluid which we propose arises from the emission of a flux of surface excitations from the channel. 

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2. Superfluid response of an atomically thin gate-tuned van der Waals superconductor

   https://doi.org/10.1038/s41467-023-37210-8  

   Jarjour, Alexander ; Ferguson, G. M. ; Schaefer, Brian T. ; Lee, Menyoung ; Loh, Yen Lee ; Trivedi, Nandini ; Nowack, Katja C. ( April 2023 , Nature Communications)

   A growing number of two-dimensional superconductors are being discovered in the family of exfoliated van der Waals materials. Due to small sample volume, the superfluid response of these materials has not been characterized. Here, we use a local magnetic probe to directly measure this key property of the tunable, gate-induced superconducting state in MoS₂. We find that the backgate changes the transition temperature non-monotonically whereas the superfluid stiffness at low temperature and the normal state conductivity monotonically increase. In some devices, we find direct signatures in agreement with a Berezinskii-Kosterlitz-Thouless transition, whereas in others we find a broadened onset of the superfluid response. We show that the observed behavior is consistent with disorder playing an important role in determining the properties of superconducting MoS₂. Our work demonstrates that magnetic property measurements are within reach for superconducting devices based on exfoliated sheets and reveals that the superfluid response significantly deviates from simple BCS-like behavior.

    

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3. Possible transport evidence for three-dimensional topological superconductivity in doped β-PdBi2

   https://doi.org/10.1038/s41598-019-48906-7  

   Kolapo, Ayo ; Li, Tingxin ; Hosur, Pavan ; Miller, Jr., John H. ( August 2019 , Scientific Reports)

   Interest in topological states of matter burgeoned over a decade ago with the theoretical prediction and experimental detection of topological insulators, especially in bulk three-dimensional insulators that can be tuned out of it by doping. Their superconducting counterpart, the fully-gapped three-dimensional time-reversal-invariant topological superconductors, have evaded discovery in bulkintrinsic superconductorsso far. The recently discovered topological metalβ-PdBi₂is a unique candidate for tunable bulk topological superconductivity because of its intrinsic superconductivity and spin-orbit-coupling. In this work, we provide experimental transport signatures consistent with fully-gapped 3D time-reversal-invariant topological superconductivity in K-dopedβ-PdBi₂. In particular, we find signatures of odd-parity bulk superconductivity via upper-critical field and magnetization measurements— odd-parity pairing can be argued, given the band structure ofβ-PdBi₂, to result in 3D topological superconductivity. In addition, Andreev spectroscopy reveals surface states protected by time-reversal symmetry which might be possible evidence of Majorana surface states (Majorana cone). Moreover, we find that the undoped bulk system is a trivial superconductor. Thus, we discoverβ-PdBi₂as a unique bulk material that, on doping, can potentially undergo an unprecedented topological quantum phase transition in the superconducting state.

    

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4. Evidence for even parity unconventional superconductivity in Sr 2 RuO 4

   https://doi.org/10.1073/pnas.2025313118  

   Chronister, Aaron ; Pustogow, Andrej ; Kikugawa, Naoki ; Sokolov, Dmitry A. ; Jerzembeck, Fabian ; Hicks, Clifford W. ; Mackenzie, Andrew P. ; Bauer, Eric D. ; Brown, Stuart E. ( June 2021 , Proceedings of the National Academy of Sciences)

   Unambiguous identification of the superconducting order parameter symmetry in${\text{Sr}}_2{\text{RuO}}_4$has remained elusive for more than a quarter century. While a chiral p-wave ground state analogue to superfluid³He-A was ruled out only very recently, other proposed triplet-pairing scenarios are still viable. Establishing the condensate magnetic susceptibility reveals a sharp distinction between even-parity (singlet) and odd-parity (triplet) pairing since the superconducting condensate is magnetically polarizable only in the latter case. Here field-dependent¹⁷O Knight shift measurements, being sensitive to the spin polarization, are compared to previously reported specific heat measurements for the purpose of distinguishing the condensate contribution from that due to quasiparticles. We conclude that the shift results can be accounted for entirely by the expected field-induced quasiparticle response. An upper bound for the condensate magnetic response of <10% of the normal state susceptibility is sufficient to exclude all purely odd-parity candidates.

    

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5. Multicomponent superconducting order parameter in UTe 2

   https://doi.org/10.1126/science.abb0272  

   Hayes, I. M. ; Wei, D. S. ; Metz, T. ; Zhang, J. ; Eo, Y. S. ; Ran, S. ; Saha, S. R. ; Collini, J. ; Butch, N. P. ; Agterberg, D. F. ; et al ( July 2021 , Science)

   An unconventional superconducting state was recently discovered in uranium ditelluride (UTe₂), in which spin-triplet superconductivity emerges from the paramagnetic normal state of a heavy-fermion material. The coexistence of magnetic fluctuations and superconductivity, together with the crystal structure of this material, suggests that a distinctive set of symmetries, magnetic properties, and topology underlie the superconducting state. Here, we report observations of a nonzero polar Kerr effect and of two transitions in the specific heat upon entering the superconducting state, which together suggest that the superconductivity in UTe₂is characterized by a two-component order parameter that breaks time-reversal symmetry. These data place constraints on the symmetries of the order parameter and inform the discussion on the presence of topological superconductivity in UTe₂.

    

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