Dr Sheng Ran is recognised for his research on unconventional superconductivity and electronic phases, particularly his seminal contributions to the discovery of exotic and extremely high-field re-entrant superconductivity in uranium ditelluride.
"Re-entrant superconductivity in such high magnetic field is an extremely rare quantum phenomenon and I feel very lucky to be the one to observe it, and to be the recipient of the Lee Osheroff Richardson Prize", commented Dr Ran.
Sheng has a long history of working on quantum materials. In the field of iron superconductors, he made a breakthrough in the synthesis of CaFe2As2, known for its spectacular volume collapse as a function of applied pressure. Sheng discovered that single crystals of FeAs-flux grown CaFe2As2 could be stabilized in the collapsed phase at ambient pressure, which opened the door to the first ARPES and inelastic neutron scattering studies. Sheng has also added to our understanding of the enigmatic Hidden Order phase in heavy fermion URu2Si2. By chemically tuning single crystals via Fe substitution and performing thermal expansion measurements and transport measurements in high magnetic field, he uncovered important differences between the hidden order phase and a competing antiferromagnetic phase, offering insight into the underlying physical mechanisms.
The discovery of exotic spin-triplet superconductivity in UTe2 is a thrilling experimental development. This remarkable superconducting phase has the highest transition temperature among spin-triplet superconductors of 1.6 K and has incredibly large, anisotropic upper critical field values, up to 35 T. Of particular interest is that this superconductivity also appears to have nontrivial topology. Topology-driven quantum information science is currently at the heart of the National Quantum Initiative Act in the US. While there are many theoretical proposals for using spin-triplet superconductors in devices for quantum information, there are a limited number of unambiguous spin-triplet superconductors supported by concrete experimental evidence. UTe2 adds another member to this very short list.
This discovery was published in Science in August 2019 and is already having a huge impact on quantum materials and quantum information research. An international experimental race was actually begun earlier by the preprint on arXiv well before the date of publication. UTe2 is a worldwide phenomenon, and it has caught the condensed matter physics community’s attention because of its potential as an intrinsic topological superconductor. It is possible that UTe2 will replace Sr2RuO4 as the spin-triplet superconductor archetype. Sheng has been invited to international workshops and conferences to present these exciting results.
Perhaps even more extraordinary is the discovery of an unprecedented, ultra-high magnetic field reentrant superconducting phase, published in Nature Physics in October 2019 (online). Whereas magnetic fields typically limit superconductivity, in this case, magnetic fields induce superconductivity between 40 T and 65 T. These are such high magnetic fields that the physical mechanism behind the stability of superconductivity is currently a very open question. “The discovery of this ‘Lazarus superconductivity’ at record-high fields is likely to be among the most important discoveries to emerge from this lab in its 25-year history,” according to National High Magnetic Field Laboratory director Greg Boebinger.
Sheng has been involved in every aspect of this research. He chemically synthesized the single crystals, oriented and prepared samples, and performed most of the initial structural and electronic property characterization. He travelled to the National High Magnetic Field Laboratory and NIST to perform numerous high-field and neutron scattering measurements. He then performed the data analysis and wrote the manuscripts.