Recently, Professor Wang Jian's research group from the Center for Quantum Materials Science of the School of Physics of Peking University cooperated with Professor Qiao Liang from the School of Physics of the University of Electronic Science and Technology of China, Associate Professor Liu Yi from the Department of Physics of the Chinese People's University, Professor Zhang Guangming from the Department of Physics of Tsinghua University, Researcher Yang Yifeng from the University of Chinese Academy of Sciences, and Professor Gao Peng from the School of Physics of Peking University to observe the unconventional superconducting state with broken rotational symmetry in the nickel oxide superconductor Nd0.8Sr0.2NiO2 film with infinite layer structure. Experimental results show that with the increase of the applied parallel magnetic field, the in-plane magnetic resistance of Nd0.8Sr0.2NiO2 films undergoes strange behavior from isotropic to quadruple rotational symmetry (C4) and then to quadruple superimposed double rotational symmetry (C4+C2). In the magnetic field interval corresponding to the double symmetry (C2) anisotropy, the in-plane superconducting critical magnetic field curve of Nd0.8Sr0.2NiO2 film shows abnormal upturning behavior with temperature. This work reveals the evolution of novel quantum states of matter in nickel-based superconducting thin films with multiple symmetry-breaking defects, and provides a new perspective for in-depth understanding of superconducting pairing symmetry, unconventional superconductivity, and the competition between various ordered states in nickel-based superconductors. The work, titled "Rotational symmetry breaking in superconducting nickelate Nd0.8Sr0.2NiO2 films", was published online in the academic journal Nature Communications on November 7, 2023.
Since the discovery of copper oxide high-temperature superconductors in 1986, the exploration of new unconventional high-temperature superconducting systems and the understanding of their intrinsic superconductivity mechanism have been important research frontiers in the field of condensed matter physics. In 2019, superconductivity was observed in nickel oxide films with infinite layer structures, with a superconducting transition temperature of about 15 K. The infinite layer nickel oxide has a similar crystal structure to the copper oxide high-temperature superconductor, and the Ni+ in the system and Cu2+ in the copper oxide are in the same 3d9 outermost electron orbital arrangement. Therefore, the emergence of nickel-based superconducting systems is of great significance for studying the physical mechanism of unconventional high-temperature superconductivity.
The Bardeen-Cooper-Schrieffer (BCS) theory states that two electrons with opposite spins and momentum are paired to form Cooper pairs by electroacoustic coupling, and the coherent condensation of Cooper pairs leads to the emergence of superconductivity, and the superconducting sequence parameters describing this superconductivity have isotropic s-wave symmetry. However, copper oxide high-temperature superconductors (Tc>40 K) exhibit many novel properties that go beyond the traditional BCS theoretical framework, such as the superconducting electron pairing may be driven by antiferromagnetic spin fluctuations, and the superconducting sequence parameters satisfy the d-wave symmetry. Several theoretical works have pointed out that nickel-based superconductors are also likely to exhibit D-wave pairing symmetry similar to copper-based superconductors. However, the pairing symmetry of nickel-based superconductors is still experimentally controversial. In addition, previous experiments have reported the existence of charge order and antiferromagnetic interaction in nickel-based superconducting parent materials. Therefore, it has become an important scientific issue in this field to reveal the physical mechanism of nickel oxide superconductivity and clarify the correlation and competition between superconductivity and other ordered states such as charge order and antiferromagnetic order in the system.
In this work, Jian Wang's group prepared a ring electrode structure (Corbino-disk) on an infinite layer of nickel oxide superconductor Nd0.8Sr0.2NiO2 thin films by microfabrication, and systematically studied the in-plane anisotropic magnetoresistive properties of Nd0.8Sr0.2NiO2 superconducting films (Fig. 1a). Among them, the annular electrode structure can make the current flow uniformly from the central electrode to the outermost electrode, which is conducive to revealing the anisotropy characteristics of the sample. The research team found that the in-plane reluctance R(φ) of Nd0.8Sr0.2NiO2 superconducting films is basically isotropic under a small magnetic field. With the increase of the external magnetic field, the in-plane reluctance R(φ) changes from isotropic to quadruple rotational symmetry (C4), which may be related to the superconducting pairing symmetry of the system from isotropic s-wave pairing to quadruple symmetric d-wave pairing. Further experiments confirmed that the quadruple rotational symmetry (C4) in the in-plane magnetoresistance disappears simultaneously with the suppression of superconductivity at higher temperatures, indicating that the quadruple symmetry (C4) is derived from the superconductivity in the Nd0.8Sr0.2NiO2 film (Figs. 1b, 1c). An in-depth analysis of the intra-plane magnetoresistance also well excludes the possibility that the quadruple symmetry (C4) is derived from the Nd3+ magnetic moment in the material.
At lower temperatures and higher magnetic fields, the research team found that a new double rotational symmetry (C2) component of the in-plane magnetoresistive R(φ) appeared on the basis of quadruple rotational symmetry (C4), indicating that the superconducting state of the Nd0.8Sr0.2NiO2 film has further broken the rotational symmetry (Figs. 1d, 1e). The research team quantitatively analyzed the anisotropy of in-plane magnetoresistiveness and found that the temperature-dependent behavior of quadruple symmetry (C4) and quadruple symmetry (C2) was inconsistent, and the magnetic field-dependent behavior of the two was opposite, indicating that the two anisotropies have different physical origins and there is a competitive relationship between magnetic field modulation (Fig. 1f, 1g). These results suggest that the anisotropy of double symmetry (C2) may be due to the fluctuation of charge fringe order in the Nd0.8Sr0.2NiO2 system after magnetic field suppression superconductivity.
Fig.1 (a) Schematic diagram of the measurement structure of Corbino-disk electric transport. (b) (c) The in-plane reluctance R(φ) measurements at different temperatures under the 8T magnetic field show the characteristics of quadruple symmetry (C4). (d) (e) The in-plane magnetoresistance R(φ) measurements at different temperatures under a 16T magnetic field, and the double symmetry (C2) component can be observed in the in-plane magnetoresistance with quadruple symmetry (C4) (as shown by the light blue ellipse in the d figure). (f), (g) The anisotropic amplitudes of C4 and C2 (ΔRC4, ΔRC2) and Ravg/RN in the internal magnetoresistive R(φ) under different magnetic fields were extracted by trigonometric fitting. Ravg is the average value of the magnetoresistance in different azimuth angles (φ), and RN is the normal resistance of the Nd0.8Sr0.2NiO2 film.
Based on the experimental results of in-plane magnetoresistive R(φ) and in-plane superconducting critical magnetic field-temperature (Bc-T), the research team drew a phase diagram of the Nd0.8Sr0.2NiO2 superconducting film (Fig. 2a), revealing two phase transitions in the superconducting state of matter under the modulation of an applied in-plane magnetic field, characterized by the breaking of rotational symmetry, and depicting the superconductivity, charge order, antiferromagnetic interaction and Kondo in the system The delicate balance and interweaving between the interactions: the transition from the first phase to isotropic superconductivity to quadruple symmetry (C4) anisotropic superconductivity, which may correspond to the S-wave superconductivity under the local spin fluctuations of the Kondo interaction, to the transition from d-wave superconductivity after the antiferromagnetic interaction is enhanced by the magnetic field. The second phase change is further broken by the quadruple symmetry (C4), resulting in the double symmetry (C2) characteristic, which corresponds to the occurrence of charge fringe order fluctuations after the superconductivity is suppressed by a strong magnetic field. At the same time, this phase transition is also accompanied by the anomalous upturning behavior of the superconducting critical magnetic field in the plane, which may be the result of electrons pairing into Cooper pairs of finite momentum in the periodic potential of the charge order and eventually forming a secondary pair density wave (PDW) state (or a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state with broken time-reversal symmetry). The physical image is of great guiding significance for the unified understanding of the unconventional superconductivity in the infinite layer nickel-based superconducting Nd0.8Sr0.2NiO2 film, and the novel quantum state of matter under multiple competitive sequences and interactions, and also provides a new idea for the study of the physical mechanism of unconventional high-temperature superconductivity.
Fig. 2 (a) The phase diagram of magnetic field B and temperature T comprehensively summarizes the temperature-dependent behavior of the in-plane superconducting critical magnetic field and the anisotropy characteristics of the in-plane magnetoresistance. (b)-(e) In-plane reluctance measurements at partial magnetic fields versus temperature. As the applied magnetic field increases, the R(φ) curve exhibits a rotational symmetry break from isotropy (e) to C4 anisotropy (d) to C4+C2 anisotropy (b, c), and the black arrows indicate their corresponding regions in the phase diagram.