The microscopic mechanism of high-temperature superconductivity in copper oxides is one of the most important and challenging problems in modern condensed matter physics. Unlike the isotropic s -wave pairing symmetry in conventional superconductors, copper oxide superconductors are generally believed to exhibit d -wave pairing symmetry, with a superconducting order parameter that possesses cloverleaf-like C 4 symmetry. This symmetry is the key to the pairing mechanism of high-temperature superconductivity. Meanwhile, electron correlation effects play a crucial role in high-temperature superconductors, giving rise to other phases such as the strange metal state, the pseudogap, and various ordered electronic states. As "neighbors" adjacent to the superconducting phase, some microscopic characteristics of these states are likely related to the formation of superconductivity. Unraveling the mechanisms of these "neighbors" will help solve the mystery of high-temperature superconductivity.
Electronic nematicity, as a type of ordered electronic state, spontaneously breaks the rotational symmetry of electron motion, reducing it to C 2 symmetry, analogous to the molecular alignment in nematic liquid crystals. It has been found to be widespread in the normal state of unconventional superconductors and is very likely the parent state of unconventional superconductivity. Recently, electronic nematicity has been experimentally observed in the copper oxide superconductor La₁.₈₄Sr₀.₁₆CuO₄ (LSCO) through electrical transport measurements. Clarifying how nematicity evolves upon entering the superconducting state, whether other quantum orders exist, and how these quantum orders interact with each other are likely to provide important clues for understanding the origin of electronic nematicity and its relationship with the superconducting state.
To address these questions, Jie Wu’s group developed a high-precision angle-resolved resistivity method to measure in-plane resistivity anisotropy and performed measurements on optimally doped LSCO thin films in the superconducting state. The experimental results show that the superconducting state of LSCO is also nematic. At zero magnetic field, as the temperature decreases, superconducting fluctuations enhance the magnitude of nematicity and change the nematic director. Consequently, the system first enters the superconducting state along a particular direction, which is corroborated by the anisotropy of the critical current in the superconducting state, evidence the nematicity in the superconducting state. By further combining in-plane rotated magnetic fields, they successfully separated the magnetoresistance contribution arising from superconducting vortex motion and uncovered the d -wave superconducting order with C 4 symmetry originally hidden within the superconducting state. From the magnetoresistance measurements and critical current results, they revealed the intertwined nematic and d-wave superconductive order, and established a B-T phase diagram for different types of superconducting fluctuations.
This discovery reveals the evolution of nematic order and superconducting order within the superconducting state of copper oxide high-temperature superconductors, indicating that nematicity not only appears in the normal state of unconventional superconductors but also plays an important role in the superconducting condensation process. It provides guidance for a deeper understanding of the mechanism of electronic nematicity, the pairing mechanism in high-temperature superconductivity, and the formation of phase coherence.
National Science Review
Experimental study