Researchers at the Indian Institute of Science Education and Research Bhopal have discovered that the material YbSb₂ spontaneously breaks time-reversal symmetry upon entering a superconducting state, marking the first known instance of this phenomenon in a type-I superconductor. The findings, published in the journal Physical Review Letters, reveal that the material creates a tiny internal magnetic field of about 0.44(3) gauss as electrical resistance vanishes.
Growing Single Crystals of YbSb₂
The research team grew single crystals of YbSb₂ to observe how the material behaves under extreme cooling. Laboratory tests showed that the compound becomes superconducting at approximately 0.95 kelvin, which sits just above absolute zero. To establish its classification, the team measured its response to external magnetic fields at low temperatures.
At 20 gauss, YbSb₂ remained in the Meissner state, expelling applied magnetic fields from its interior. When the external field reached 40 gauss, the material entered an intermediate state where normal and superconducting regions coexisted. Measurements recorded a critical field of about 51 gauss at 0.1 kelvin, confirming its type-I classification. Heat-capacity data further indicated a fully opened superconducting gap, meaning the material maintains a finite energy barrier before paired electrons can be excited.
Detecting Spontaneous Internal Magnetic Fields
To investigate the microscopic behavior of the material, the researchers used muons to track changes in spin as YbSb₂ crossed the superconducting transition. Zero-field muon spin relaxation measurements detected a new magnetic signal emerging directly below the critical temperature. This signal indicated an internal field of roughly 0.44(3) gauss produced entirely by the material itself.
Applying a small external field of about 10 millitesla successfully suppressed the muon relaxation, supporting the conclusion that the internal field was static or slowly changing. Because the signal appeared precisely as the material became superconducting, the data confirmed that YbSb₂ was breaking time-reversal symmetry rather than simply reacting to an outside magnetic source.
Theoretical Paths to Majorana Surface Modes
The research team attributes this unusual behavior to an unconventional form of superconductivity known as an internally antisymmetric nonunitary triplet state. According to the study authors, YbSb₂ acts as a unique platform where type-I superconductivity coexists with triplet-pairing and nontrivial topology. This indicates that electrons pair in a manner that imparts an internal magnetic character to the superconducting state.
Calculations performed by the group identify YbSb₂ as a Z₂ topological metal. The theoretical models suggest the superconducting state could potentially host Majorana surface modes, which are exotic quantum states of interest for future quantum technologies. However, the researchers noted that detecting these Majorana modes remains a theoretical prediction, as they have not yet been observed experimentally in YbSb₂.
Frequently Asked Questions
What temperature does YbSb₂ become superconducting?
YbSb₂ becomes superconducting at approximately 0.95 kelvin, which is just above absolute zero.
How large is the internal magnetic field created by the material?
The researchers measured an internal magnetic field of about 0.44(3) gauss that emerges spontaneously as the material crosses into its superconducting state.
What type of superconductor is YbSb₂?
YbSb₂ behaves as a type-I superconductor, expelling applied magnetic fields in the Meissner state below its critical threshold of about 51 gauss at 0.1 kelvin.