Our postdoctoral researcher Ertugrul Karaca, in collaboration with scientist from China and Spain, has published a paper in Physical Review Letters, in which they report the multiple superconducting state transitions in 1T-HfS₂ under pressure and reveal its structural origin.

Pressure-Sensitive Multiple Superconducting Phases and Their Structural Origin in Van der Waals HfS₂ Up to 160 GPa

Wei Zhong, He Zhang, Ertugrul Karaca, Jie Zhou, Saori Kawaguchi, Hirokazu Kadobayashi, Xiaohui Yu, Daniel Errandonea, Binbin Yue, and Fang Hong,  Phys. Rev. Lett. 133, 066001 (2024).

DOI: 10.1103/PhysRevLett.133.066001

Transition metal dichalcogenides (TMDCs) show complex superconductivity under ultra-high pressure, which could mean that the sensitive structural phase transitions possibly exist under ultra-high pressure and low temperature. However, the structures of these superconducting (SC) phases are still unknown, due to the absence of direct structural study at low temperature and ultra-high pressure. This lack of structural analyses for SC states also raises a longstanding question: whether the superconductivity is from the chalcogens decomposed from TMDCs under ultrahigh pressure and low temperature? Especially for the SC TMDCs with a T_c close to the corresponding chalcogens.

To address these questions, in addition to the electrical transport measurements and analysis, we have conducted a challenging in situ low-temperature synchrotron X-ray diffraction in 1T-HfS₂ up to 160 GPa. In this work, we demonstrated multiple SC states in HfS₂ under ultra-high pressure (up to ~158 GPa), including an exciting SC-I state with T_c around 16.4 K and an upper critical field surpassing the weak-coupling Pauli limit (μ₀H_c2(0) ≈ 29.7 T for a T_c = 15.2 K). Both T_c and μ₀H_c2(0) are the highest records in bulk TMDCs. It is distinct from previous reports.

Fig. 1 Proposed phase diagram of HfS₂ (a), and the summarized T_c (b), and μ₀H_c2 (c) for different bulk TMDCs and sulfur. The T_c of S_mag and S_RT are obtained from magnetic susceptibility and R-T curves, respectively.

In situ high-pressure XRD results reveal that there are multiple structural phase transitions at room temperature, and an extra structural transition from I4/mmm to R-3m at low temperature. The coexistence of I4/mmm and R-3m phases near the T_c is the reason for the emergence of multiple SC states. More importantly, our XRD data directly excludes the existence of sulfur. The most special SC-I with a large μ₀H_c2(0) may originate from the R-3m structure, which can be regarded as β-Po type S doped with Hf atoms. This suggests that the 4f electrons of Hf potentially contribute to the unconventional superconductivity, resulting in a large μ₀H_c2(0). These results demonstrate the sensitive structure behavior in such kinds of TMDCs under pressure, which is responsible for their complex superconductivity. To our knowledge, this work is the first in situ temperature-dependent structural study under ultrahigh pressure, and it establishes the close correlation between the low-temperature structures and SC states in TMDCs. In addition, the extremely large upper critical field and the unconventional superconductivity features in HfS₂ will extend our understanding on the superconductivity in TMDCs.