Physicists have found the first type I superconductor that breaks time-reversal symmetry, a property that every previous example of the effect had shown only in the other major class of superconductors. The material is ytterbium diantimonide, written YbSb2, and the work appeared Oct. 2 in Physical Review Letters, according to a report from Xenospectrum.
What the Type I Superconductor Did
A team led by Anshu Kataria at the Indian Institute of Science Education and Research Bhopal cooled single crystals of YbSb2 until they stopped resisting electric current. That happens at 0.95 kelvin, or about minus 272.2 degrees Celsius, according to the preprint posted on arXiv. With no outside magnetic field applied, a small magnetic field appeared inside the crystal at the moment it became superconducting.
The researchers measured the field at about 0.44 gauss. A material with no outside field should have none of its own. A field that shows up spontaneously means the system no longer behaves the same when the direction of time is flipped, which is what physicists call broken timereversal symmetry.
The authors wrote that all known systems with this feature “exhibit type-II superconducting character” before their result. Type I and type II describe how a superconductor responds to a magnetic field. A type I material pushes the field out almost completely until the field gets too strong, then loses superconductivity. A type II material lets the field in through tiny tubes.
How the Muon Spin Spectroscopy Worked
The team used muon spin spectroscopy, which fires subatomic particles called muons into a sample and tracks how their spin changes as they feel the magnetic field around them. The measurements ran at the ISIS Neutron and Muon Source at the Rutherford Appleton Laboratory in England, per the arXiv paper.
The same technique, run with an applied field, confirmed that the crystal behaves as a type I superconductor, with superconducting and normal regions side by side. The paper reports a critical field of 51 gauss at 0.1 kelvin. That is the field strength above which superconductivity disappears. The spontaneous field of 0.44 gauss equals less than 1% of that figure.
The 0.44 gauss reading also sits inside the range of the planet’s own field. The National Centers for Environmental Information at NOAA puts Earth’s magnetic field at roughly 0.25 to 0.65 gauss at the surface. The signal the team reports is therefore no stronger than the ambient field everyone lives in. The authors also used a statistical method called principal component analysis to test whether the signal was real, according to the arXiv paper.
Why Majorana Modes Matter for YbSb2
Calculations in the paper identify YbSb2 as a topological metal with a feature called a Dirac nodal line. The authors propose that its electrons pair in an unusual state that they call an internally antisymmetric nonunitary triplet. In that state, the paper says, electrons on the same atomic site pair across different orbitals.
The crystal structure is layered. The arXiv paper describes quintuple layers stacked along one axis and held together by weak van der Waals forces, in the space group Cmcm. The same paper says the spontaneous field appears even though the crystal has a center of symmetry. Bandstructure calculations in the paper show strong spin-orbit coupling, with a band splitting of about 80 millielectronvolts, and classify the material as a Z2 topological metal.
If that proposal holds, the material’s surface should host Majorana modes. Physicists study these quantum states because they could store information in a way that resists heat and electromagnetic noise, ScienceAlert reported. The arXiv paper says Majorana modes would show up as zero-bias conductance peaks in tunneling or point-contact experiments. Nobody has run those experiments on YbSb2 yet.
Limits of the YbSb2 Result
Xenospectrum reports that the proposed pairing state still needs direct confirmation from experiments such as tunneling spectroscopy. The authors use cautious language in the preprint, where the abstract says they report evidence of broken time-reversal symmetry rather than proof. ScienceAlert’s headline went further and said physicists “created” such a material. The two descriptions differ on how settled the finding is, and the paper’s own wording is the narrower one.
The material also has no practical use in its current form. Xenospectrum notes that its transition temperature is far too low and its critical field too weak for conventional superconductor applications. Xenospectrum adds that the finding suggests complex quantum behavior can hide in materials physicists thought they understood.
The preprint calls for follow-up work with muon spin spectroscopy, angle-resolved photoemission and scanning tunneling microscopy. It also proposes a systematic study of related compounds in the RSb2 family, in which the R stands for calcium or ytterbium. The paper also lists the related nonsymmorphic compounds as targets. Each of those would test whether YbSb2 is a one-off or the first of a group.
Timeline and authors of the YbSb2 paper
The preprint went onto arXiv on Jan. 12, so peer review at the journal took nearly nine months before publication on Oct. 2. The author list has 12 names, including Kataria, Adrian D. Hillier, Amit Agarwal, Sudeep Kumar Ghosh and Ravi Prakash Singh. Xenospectrum lists IISER Bhopal, the Indian Institute of Technology Kanpur, the University of Warwick and the ISIS facility as the participating institutions.
The crystals came from a modified Bridgman method, in which a molten mixture cools slowly along a temperature gradient until a single crystal forms. The arXiv paper reports that X-ray diffraction and Laue patterns confirmed their quality. Image: An original illustration of a muon spin experiment drawn by Onsite’s own design staff, which carries no outside rights. Figures from the Physical Review Letters paper and ISIS facility photos belong to the publisher and the facility and need written permission.
