Superconductivity is full of surprises. One of its most intriguing hallmarks is the spontaneous breaking of time-reversal symmetry (TRS), a phenomenon usually associated with unconventional, type-II superconductors. What happens when a property usually tied to exotic superconductors suddenly shows up in a material long thought to be ordinary?
But now, a study led by Prof. Ravi Prakash Singh and his group at IISER Bhopal, in collaboration with researchers from the Indian Institute of Technology Kanpur (IITK) and the ISIS Neutron and Muon Source, has found evidence of TRS breaking in a far more unusual setting: the type-I superconductor YbSb₂.
Superconducting materials allow electrical current to flow through them with zero resistance once they have been brought down to a specific temperature. The two major types of superconducting materials include Type I and Type II. While most people find Type I to be easier to understand than Type II, many of the strange phenomena associated with superconductivity occur in Type II materials.
After the superconducting transition in YbSb2 took place, scientists were able to observe the creation of internal spontaneous magnetic fields through the use of zero-field muon spin relaxation/rotation (μSR).
These results provide evidence of time-reversal symmetry breaking. Additionally, results from the use of transverse-field μSR measurements show that YbSb2 is a full-gap type-I superconductor.
Unconventional superconductor can be used to make future quantum computers
The combination of TRS being broken in a type-I superconductor is extremely rare; therefore, it indicates that there is some form of “unconventional” physics that is occurring within the system.
First-principles calculations revealed that YbSb₂ is a ℤ₂ topological metal, with a Dirac nodal line near the Fermi level. This electronic structure sets the stage for exotic superconducting states.
Credit: IISER BHOPAL
The symmetry of the Ginzburg-Landau equations indicates that the ground state of the system will be a non-unitary triplet (INT) state that has internal asymmetry. This internal asymmetry allows the TRS to be broken in a natural way as opposed to being broken by external factors.
Low-energy effective models predict that the INT state of YbSb₂ can harbor gapless Majorana surface states. These states are very important in quantum computation, as they have been suggested to be used as non-Abelian statistics and potentially fault-tolerant qubits, based upon the prediction of Majorana states in topological superconductors.
New superconducting material offers unconventional superconductivity
The discovery of YbSb2 provides a new platform to study superconductors with both type-I properties and non-trivial topology; this may be further improved with the use of triplet pairing, and evidence of time-reversal symmetry breaking suggests that there could be other physical mechanisms at play that are not part of our current understanding of superconductivity classification.
The breakdown of the historical distinction between type-I and type-II superconductivity has been made possible by this finding. Additionally, researchers may now explore the possibility of topological superconductivity in systems which had historically been defined as “conventional.”
YbSb₂ could be the bridge from traditional superconductivity to the front line of research on quantum materials that will ultimately lay down the groundwork for future advancements in technology through the use of Majorana modes and topological orders.
Journal Reference:
- Anshu Kataria, Shashank Srivastava, Dibyendu Samanta, Pushpendra Yadav et al. Observation of Time-Reversal Symmetry Breaking in the Type-I Superconductor YbSb2.
Physical Review Letters. DOI: https://doi.org/10.1103/drzq-lfn5
