Structural origin of the pressure-sensitive multiple superconducting phases in HfS2

New research from a research team of scientists led by Dr. Binbin Yue from HPSTAR and Dr. Fang Hong from Institute of Physics, CAS, have revealed the pressure-sensitive multiple superconducting phases and their structural origin in van der Waals HfS2 up to 160 GPa. An unsaturated Tc of 16.4 K at 158 GPa is reported with the largest upper critical field among the bulk TMDCs, exceeding the weak-coupling Pauli limit. Published in Physical Review Letters, this work excludes the existence of decomposed sulfur under high pressure and low temperature, and demonstrates the sensitive structure behavior in such kinds of TMDCs under pressure, which is responsible for their complex superconductivity. The extremely large upper critical field and the unconventional superconductivity features in HfS2 will extend our understanding on the superconductivity in TMDCs.  

Due to their layered structure and exceptional electronic properties, transition metal dichalcogenides (TMDCs) have garnered broad attention. Pressure can enhance interlayer interactions in TMDCs, which finally induces significant structural/electronic transitions and even superconductivity. Recently, complex superconducting behaviors in TMDCs have been reported under ultra-high pressure, which suggests the existence of possible phase transitions 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 of SC states also raises a longstanding question: whether the superconductivity of this group of materials is from the chalcogens decomposed from TMDCs under ultrahigh pressure and low temperature? Especially for the SC TMDCs with a Tc close to that of the corresponding chalcogens.

To address these questions, the team studied the electrical and structural evolution of 1T-HfS2 up to a pressure of 160 GPa and a low temperature down to 11 K. The electrical transport measurements demonstrated a pressure-induced multiple SC state in HfS2, including an exciting SC-I state with Tc above 16.4 K and an upper critical field surpassing the weak-coupling Pauli limit (μ0Hc2(0) ≈ 29.7 T for a Tc = 15.2 K). Both Tcand μ0Hc2(0) are the highest records in bulk TMDCs. And the Tc increases with pressure, remained unsaturated up to 158 GPa, suggesting a potential for achieving a higher Tc under higher pressure. The high μ0Hc2(0) also suggests that the SC-I state in HfS2 may be unconventional superconductivity.

Caption: The multiple superconducting transition in HfS2 under pressure and the high upper critical field of SC-I.

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 Tc leads to the emergence of multiple SC states in the sample. More importantly, the XRD results directly exclude the  the presence of decomposed sulfur. Based on the theoretical calculation, the most special SC-I with a large μ0Hc2(0) may originate from the R-3m  structure, which can be regarded as β-Po type S doped with Hf atoms.

Caption: The original diffraction patterns at 11 K (154 GPa) and 295 K (165 GPa), showing the phase transition features and the atomic structures of trigonal R-3m and tetragonal  I4/mmm  phases. To enhance the visual clarity, the diffraction peaks of I4/mmm phase at 11 K and 295 K are aligned.

Their results show that the abundant transport behavior and sensitive structures of HfS2 under high pressure, and opens a new window for probing novel superconductivity in other TMDC compounds.

北京高压科学研究中心的岳彬彬研究员等，与中国科学院物理所的洪芳副研究员等团队合作，通过高压原位电输运，同步辐射高压低温X射线衍射，以及理论计算等相结合，揭示了范德华材料HfS2中多重超导转变的结构起源。发现HfS2在高压下存在与新型结构相关的非常规超导电性，这对探索类似4f电子系统中的非常规超导性具有重要意义，将有助于研究非常规超导性、结构和电子构型之间的相互作用。此外，HfS2创造了目前过渡金属二硫化物体材料中最高的Tc和上临界场记录，其结构演变导致的超导电性变化可以为此类压力敏感材料的物性调控提供重要参照。相关工作以“Pressure-sensitive multiple superconducting phases and their structural origin in van der Waals HfS2 up to 160 GPa”为题发表于《Physical Review Letters》。