Researchers discovered the second type of Ising superconductor in a two-dimensional crystalline superconducting system

[ Instrument R&D of Instrument Network ] Two-dimensional crystalline superconductors are emerging frontier research directions in condensed matter physics and materials science. In 2016, Professor Iwasa of the University of Tokyo, Japan, inventor of liquid gate technology, pointed out in the Nature Review Materials article that quantum Griffith singularities, abnormal metal states and superconductivity (such as Ising superconductivity) exist in strong parallel magnetic fields ) Is the most important research topic in the field of two-dimensional crystalline superconductors [Nat. Rev. Mater. 2, 16094 (2016)].

Ising superconductor is a strong spin-orbit coupled superconducting system. It has been reported that in the transition group metal sulfide systems such as MoS2 and single-layer NbSe2 flakes modulated by liquid gates, the inversion of the in-plane center inversion symmetry produces a Zeeman-type spin-orbit coupling, making the electrons vertical Due to the spin polarization in a two-dimensional plane, this special superconductivity is called Ising superconductivity.

Due to the existence of spin-orbit coupling and spin polarization, Ising superconductivity has a very large parallel critical field, which can often reach several times the Pauli limit, often corresponding to a strong magnetic field of tens of Tesla or even higher . The so-called Pauli limit refers to the magnetic field required to destroy the superconducting Cooper pair through the spin effect in conventional superconductors.

Professor Wang Jian's research team and collaborators at the Quantum Materials Science Center of the School of Physics at Peking University reported in their previous work that for the first time, Ising superconductivity in a macroscopic single-layer NbSe2 thin film grown by ultrahigh vacuum molecular beam epitaxy [Nano Lett . 17, 6802 (2017)] and the interface-induced Ising superconductivity in ultra-thin crystalline lead films [Phys. Rev. X 8, 021002 (2018)]. A second type of Ising superconductor and anomalous metal states found in the two-dimensional crystalline superconducting system

In addition, the existence of two-dimensional anomalous metal states has always been a core issue in the field of condensed matter physics. In the past three decades, experimentally found possible signs of abnormal metal states in various two-dimensional superconducting systems [Rev. Mod. Phys. 91, 11002 (2019)]. However, due to the interference of external high-frequency noise on the experiment, the experimental results of the existence of two-dimensional abnormal metal states have been widely questioned and have never been truly confirmed [Sci. Adv. 5, 3826 (2019)].

Wang Jian's research team and collaborators confirmed the existence of two-dimensional anomalous metal states in high-temperature superconducting YBCO thin films modulated by periodic hole arrays through high-frequency filter extremely low-temperature electrical transport experiments [Science 366, 1505( 2019)]. However, in high-quality crystalline thin films grown by molecular beam epitaxy, there is still a lack of reliable experimental evidence for the existence of abnormal metal states.

Recently, Wang Jian cooperated with Researcher Lin Xi, Academician Xue Qikun of Tsinghua University, Associate Researcher Wang Lili, Associate Professor Xu Yong, Professor Yao Hong, and Researcher Liu Haiwen of Beijing Normal University, etc., to prepare two-dimensional crystalline superconductors in ultra-high vacuum molecular beam epitaxy A new class of Ising superconductivity was found in the PdTe2 film, and the existence of abnormal metal states was confirmed. The system's extremely low temperature experiments show that 6 PdTe2 thin films with a thickness of about 3nm (about 3nm) have a very large parallel critical field, exceeding the Pauli limit of 7 times, which is a typical feature of Ising superconductivity.

Unlike the previously reported Ising superconductivity, the PdTe2 film has in-plane center inversion symmetry, indicating that there is a new Ising superconducting pairing mechanism in the system. After discussion by Wang Jian and Xu Yong, the name is Type II Ising superconductivity. Energy band calculation and theoretical analysis show that the triple rotation symmetry of the PdTe2 thin film keeps the equivalent magnetic field of the spin-orbit coupling of the system always in the direction perpendicular to the plane, causing the electrons to spin-polarize perpendicular to the plane. The superconducting Cooper pair is difficult to be destroyed by the magnetic field parallel to the surface, so it generates a second type of Ising superconductivity with extremely high in-plane superconducting critical field.

The calculation shows that for the in-plane center-reversed symmetric 2D superconductor, not only the triple rotation symmetry, but also the six-fold rotation symmetry and the quadruple rotation symmetry can keep the spin-orbit coupling equivalent magnetic field of the two-dimensional system in the vertical In the direction of the surface, it shows the second type of Ising superconductivity. Therefore, the discovery of the second type of Ising superconductor is expected to develop a new research direction.

More interestingly, under the vertical magnetic field, the research team used a filter to filter the high-frequency noise in the measurement circuit, and found that the resistance of the PdTe2 film first decreased as the temperature decreased, and then tended to a temperature-independent constant. This is the first experimental evidence that an abnormal metal state has been observed in a high-quality two-dimensional crystal sample grown by molecular beam epitaxy. It is further shown that the anomalous metal state is another quantum ground state of the two-dimensional superconducting or Bose system in addition to the superconducting state and the insulating state.

In addition, most two-dimensional superconducting systems are very sensitive to air and easily lose their superconductivity due to oxidation. The research team found that the superconducting properties of the PdTe2 film can be maintained for more than 20 months without any protective layer. This kind of strong spin orbit coupled superconducting system with macroscopic area and stable properties has potential application value in superconducting sub-devices and spin electronic devices.

Wang Jian, Wang Lili and Lin Xi are the co-corresponding authors of this article. Boyi postdoctoral fellow Liu Yi, Xu Yong, Peking University doctoral student Sun Jian, and Tsinghua University doctoral student Liu Chong (graduated) are co-first authors of this article. The main collaborators of this work also included Xue Qikun, Yao Hong, Liu Haiwen, etc.

The work was supported by the National Key R&D Program, the National Natural Science Foundation of China, the Center for Excellence in Innovation of the Chinese Academy of Sciences, the Beijing Natural Science Foundation, the Beijing Future Chip Technology Advanced Innovation Center, and the Postdoctoral Science Fund.