A recently synthesized material, calcium-platinum-arsenide (CaPtAs), is challenging conventional descriptions of superconductivity. Researchers have found that this material exhibits a rare combination of properties that places it at the intersection of several classes of unconventional superconductors. The discovery, detailed in a study published in *Physical Review Letters*, provides a new platform for exploring the fundamental physics of superconductivity and its potential applications in next-generation computing.
Unlike conventional superconductors, which are well-described by the Bardeen-Cooper-Schrieffer (BCS) theory, CaPtAs belongs to a class of materials known as noncentrosymmetric superconductors. The crystal structure of these materials lacks a center of inversion symmetry, which allows for the mixing of different types of electron pairing. This leads to exotic behaviors not seen in their centrosymmetric counterparts. The study on CaPtAs has revealed that it not only has a complex superconducting gap structure but also breaks time-reversal symmetry, a combination of features that has been sought after by physicists.
A Material with a Unique Crystal Structure
CaPtAs was first reported as a new noncentrosymmetric superconductor in a paper published in late 2019. It has a tetragonal crystal structure characterized by three-dimensional honeycomb networks of platinum and arsenic atoms, with a notably elongated c-axis. The lack of inversion symmetry in this structure is a key factor in its unconventional properties. The superconductivity in CaPtAs emerges at a critical temperature (Tc) of 1.47 K, a relatively low temperature that requires specialized equipment for study. The initial characterization of the material was performed using electrical resistivity, specific heat, and magnetic susceptibility measurements, all of which confirmed its superconducting nature.
Evidence of Unconventional Pairing
The research team behind the *Physical Review Letters* study employed a range of advanced experimental techniques to probe the superconducting state of CaPtAs. Their findings point towards a departure from the simple s-wave pairing found in conventional superconductors. Instead, the evidence suggests a more complex scenario involving a mix of pairing states, which is a direct consequence of the material’s noncentrosymmetric nature.
A Nodal Superconducting Gap
One of the key findings is the presence of nodes in the superconducting energy gap. In a conventional superconductor, there is a uniform energy gap across the entire Fermi surface. However, in CaPtAs, the gap appears to vanish at certain points or lines on the Fermi surface. This was revealed by measurements of the magnetic penetration depth at very low temperatures. The penetration depth, a measure of how far a magnetic field can penetrate into the superconductor, showed a temperature dependence characteristic of a nodal gap structure.
A Two-Gap Superconductor
Further analysis of the superfluid density and electronic specific heat provided more detailed insights into the gap structure. The data could not be explained by a single-gap model, whether it was a simple s-wave or a more complex p- or d-wave model. Instead, the best fit to the experimental data was achieved with a two-gap model. Specifically, a model comprising a nodeless s-wave gap and a nodal p-wave gap (an s+p-wave model) provided the most accurate description of the material’s behavior. This suggests that CaPtAs has a complex superconducting order parameter with contributions from different pairing channels.
Breaking Time-Reversal Symmetry
Perhaps the most striking discovery is that CaPtAs breaks time-reversal symmetry (TRS) in its superconducting state. This was detected using a technique called zero-field muon spin relaxation (μSR). In these experiments, muons are implanted into the material, and their spins precess in the presence of a magnetic field. If there are no internal magnetic fields, the muon spins will relax at a certain rate due to the random magnetic fields from the nuclei. The researchers observed an increase in the muon spin relaxation rate below the superconducting transition temperature, which indicates the emergence of spontaneous internal magnetic fields. Since these fields appear with the onset of superconductivity, it is a clear sign that TRS is broken.
A Bridge Between Disparate Classes
The simultaneous observation of a nodal gap and broken TRS is highly unusual. Most noncentrosymmetric superconductors that break TRS have fully gapped superconductivity, while those with nodal gaps often preserve TRS. CaPtAs, therefore, serves as a bridge between these two distinct classes of materials. This discovery provides a new avenue for understanding the interplay between the lack of inversion symmetry, mixed-parity pairing, and the breaking of fundamental symmetries in superconductors. The properties of CaPtAs are thought to be influenced by a significant mixing of spin-singlet and spin-triplet pairing states, which is made possible by the material’s crystal structure and the strong spin-orbit coupling of the platinum atoms.
Implications for Topological Superconductivity
The unique combination of properties in CaPtAs makes it a strong candidate for a topological superconductor. Topological superconductors are a class of materials that have a conventional superconducting gap in their bulk but host exotic, protected states on their surfaces or edges. These surface states are predicted to be Majorana fermions, which are their own antiparticles. The search for Majorana fermions is a major area of research in condensed matter physics, as they could be used to build fault-tolerant quantum computers. The broken TRS and nodal gap structure in CaPtAs are consistent with theoretical predictions for some types of topological superconductors, making it a promising material for future studies in this area. Researchers are hopeful that further investigations, such as scanning tunneling microscopy and angle-resolved photoemission spectroscopy, will provide more direct evidence for topological surface states in this material.