Heavy Fermion Superconductivity

In the quest to unravel the mysteries of superconductivity, our research is centred on two pivotal challenges: uncovering a crystalline superconductor that exhibits topological properties, and understanding the complex relationship between magnetism and superconductivity. We focus on YbRh2Si2, a heavy fermion metal known for its quantum critical behaviour, which has shown early signs of superconductivity at temperatures below 10 mK—a realm inaccessible to many.

 

Our specialized measurement techniques, such as SQUID impedance and magnetic susceptibility, along with noise thermometry, are tailored to operate in these extreme conditions. Our ambition is to determine the nature of the superconducting order in YbRh2Si2 with preliminary evidence pointing towards odd-parity superconductivity. Collaborating with leading institutes, our research extends to studying other strongly correlated electron systems, offering fresh insights into the interplay of nuclear interactions and quantum states.

Heavy fermion superconductivity

Two of the most significant questions in the study of superconductivity are:

  • To identify a crystalline superconductor that is a topological superconductor
  • To understand the interplay of magnetism and superconductivity

Our research focusses on YbRh2Si2, a canonical heavy fermion metal which also exhibits quantum criticality. The first indications of superconductivity in this material appear below 10 mK, beyond the reach of most researchers.

We have developed a portfolio of measurement techniques with high sensitivity and low dissipation which match the requirements of this challenging temperature regime:

  • SQUID based measurements of the sample impedance have mapped the phase diagram in a magnetic field, revealing multiple superconducting phases.
  • Precise heat capacity measurements using noise thermometry have revealed a phase transition in the electronic magnetism driven by hyperfine coupling to Yb nuclei [1]
  • SQUID based measurements of magnetic susceptibility
  • Transport measurements in samples micro-structured by focussed ion beam (FIB) [2]

The target of this project is to firmly identify the nature of the superconducting order in this system, and to exploit it in devices. Results so far point to odd-parity superconductivity.

This work is in collaboration with Max Planck Institute for Chemical Physics of Solids, Dresden and Physikalisches Institut, Frankfurt.
We also exploit the high B/T facility of ND3 to study a variety of strongly correlated electron systems. Research on PrOs4Sb12 has revealed the importance of the nuclear hyperfine interaction on antiferroquadrupolar order and superconductivity, leading to modified quantum criticality [3].

HeatCapacity
Figure caption: Molar heat capacity in zero field exhibits two sharp anomalies at TA and TN identified with magnetic transitions.

References

  1. Electronuclear transition into a Spatially Modulated State in YbRh2Si2. J. Knapp, L. Levitin, J. Nyeki, A. F. Ho, B. Cowan, Saunders, M. Brando, C. Geibel ,K. Kliemt and C. Krellner, Phys. Rev. Lett. 130, 126802 (2023).  https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.126802
  2. Microstructuring YbRh2Si2for resistance and noise measurements down to ultralow tempeartures. A. Steppke, S. Hamann, M Konig. A. P. Mackenzie, K. Kliemt, C. Krellner, M. Kopp, M. Lonsky, J. Muller, L. Levitin, J. Saunders and M. Brando. New J. of Physics 24 123033 (2022)
  3. Diverse influences of hyperfine interactions on strongly correlated electron states. F. Bangma, L. Levitin, A. Casey, J. Nyeki, I. Broeders, A. Sutton, B. Andraka, S. Julian, J. Saunders and A. McCollam https://arxiv.org/pdf/2305.17088.pdf