Chromium material hosts three spin-triplet

A chromium-based superconductor has been found to host three distinct quantum phases, a hallmark of rare spin-triplet superconductivity. The discovery was made in the material potassium chromium arsenide (K2Cr3As3), which becomes superconducting below 6.2 Kelvin.

Spin-triplet superconductors, where paired electrons have a combined spin of 1, are highly sought after. They can support multiple superconducting phases and host strong Majorana bound states, which are quasiparticles considered promising for building fault-tolerant quantum computers. Professor Guo-qing Zheng, the study's senior author, told Phys.org that while spin-triplet states have been seen before, finding multiple phases due to the internal degree of freedom was a key missing piece. "The objective of our study was to find multiple phases in K2Cr3As3," Zheng said.

Probing the material's phases

The researchers grew single crystals of K2Cr3As3 and probed them using nuclear magnetic resonance (NMR). This technique uses magnetic fields and radio waves to examine atomic nuclei, revealing details about the local electronic environment. By systematically varying temperature and the strength of an applied magnetic field aligned with the crystal's c-axis, the team mapped out three different superconducting states.

Zheng explained the critical role of their method. "In this study, nuclear magnetic resonance played a important role. This technique is realistically the only method to probe spin susceptibility in the superconducting state."

The three superconducting states

The experiments revealed three phases, labeled A, B, and C. Each emerges under specific conditions of temperature and magnetic field strength.

Under relatively low magnetic fields, the material enters phase A. In this state, the arrangement of electron spins changes direction with the electrons' momentum, but the state preserves time-reversal symmetry.

When cooled further under these low fields, the system transitions to phase B. This state breaks time-reversal symmetry and is characterized by a handed, momentum-dependent superconducting structure.

At higher magnetic fields, the material enters phase C. This phase is distinguished by a superconducting gap that vanishes along a continuous line in momentum space, rather than at isolated points.

Implications for quantum research

The observation of three phases provides compelling evidence that K2Cr3As3 is a spin-triplet superconductor. Zheng emphasized the significance of their findings. "By observing and elucidating the multiple superconducting phases, we definitely proved spin-triplet pairing in this compound with the highest transition temperature ever (above liquid helium temperature)," he stated. The team also clarified the topological nature of these phases, noting that one can host Majorana excitations.

This work establishes K2Cr3As3 as a new platform for manipulating topological superconductivity. The researchers plan to apply similar NMR methods to other candidate materials, with the goal of discovering spin-triplet superconductors with even higher transition temperatures. The study's detailed phase diagram, published alongside the findings, serves as a guide for future experiments aiming to use these exotic quantum states for advanced technologies.