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Like Moses parting the sea: spontaneous segregation of a superconducting quantum fluid revealed
(Nanowerk Spotlight) More than 25 years since the discovery of superconductivity in strontium ruthanate (Sr2RuO4, abbr. SRO), understanding the properties of this material remains as challenging as ever.
Although the material bears some structural resemblance to high-temperature superconducting cuprates, its superconducting behavior stands entirely on its own, without any analogue among the transition metal oxides or elsewhere. On the other hand, it seems much closer to superfluid helium-3, presenting a unique such case among unconventional superconductors.
Arguably the defining property of any unconventional superconducting state is that the superconducting order parameter has a non-trivial symmetry, stemming from the formation of Cooper-pairs (carriers of superconductivity) beyond the conventional attraction of electrons via interaction with phonons.
To date, the exact symmetry of the order parameter in SRO is not settled. The analogy to superfluid He-3 suggests the p-wave symmetry.
Furthermore, evidence exists that the order parameter in SRO breaks the time-reversal symmetry, making SRO one of the rare materials exhibiting traces of chiral superconductivity.
Let us try to explain. In a magnetic material, the spins must choose between two equivalent states: spin-up and spin-down. When ferromagnetic ordering sets in, the magnetic crystal takes a definite spin direction and not the reverse one. This breaks the time-reversal symmetry.
An analogous – admittedly oversimplified – picture can be drawn in SRO: having two degenerate states in absence of a magnetic field, with Cooper pairs having angular momentum either +1 or -1 (i.e. having opposite internal motion of there paired electrons).
In an international effort spanning from experiment to theory, researchers from Kyoto, Leiden and Antwerp postulated that instead of choosing one over the other, a SRO sample can allow the two energetically degenerate condensates of Cooper-pairs with opposite momentum to segregate in domains, with a domain wall in between where chirality changes.
To use the analogy to Moses parting the sea, here the Red Sea would spontaneously part into 'magenta' and 'yellow' seas on either side.
Spontaneous emergence of Josephson junctions in homogeneous rings of single-crystal Sr2RuO4
(Image courtesy of the researchers) (click on image to enlarge)
To prove the point, the scientists fabricated submicrometer SRO rings so that the chiral domain walls form in the middle of the arms of the loop, and thereby take the role of Josephson junctions in a standard geometry of a Superconducting Quantum Interference Device (SQUID).
The hypothesis was then validated in subsequent transport measurements in the presence of magnetic field, where clear SQUID-like behavior of the sample was found without any physical junctions being made!
"This observation goes beyond just supporting the claims of chirality in SRO," says Milorad Milosevic, one of the team leaders of the study (npj Quantum Materials, "Spontaneous emergence of Josephson junctions in homogeneous rings of single-crystal Sr2RuO4"). "This is a prime example of spontaneous segregation of a superconducting quantum fluid into two spatially separated and different components, with a new topological item, a chiral junction, between them. Knowing how much the traditional Josephson junctions mean to the superconducting electronics, our findings open an entirely new prospect of chiral Josephson devices – worthy of further characterization and functionalization towards advanced sensing abilities."
Provided by the University of Antwerp as a Nanowerk exclusive
 

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