Recently, Prof. Jian Wang’s group at International Center of Quantum Materials, School of Physics, Peking University, in collaboration with Prof. Ziqiang Wang at Boston College and Prof. Yi Liu at Renmin University of China, observed high-temperature field-free superconducting diode effect in Bi2Sr2CaCu2O8+δ (BSCCO) flake devices with the working temperature up to 72 K (around liquid nitrogen temperature regime). This discovery offers promising developments for potential applications in non-dissipative electronics and provides new insights into the mechanism of field-free SDE and symmetry breakings in high-Tc superconductors. This article entitled “High-temperature field-free superconducting diode effect in high-Tc cuprates” was published in Nature Communications on January 9, 2025 (Nat. Commun. 16, 531 (2025)).
Nonreciprocal charge transport, arising from the symmetry breaking properties of materials, describes the asymmetric behavior of voltage with currents flowing in opposite directions. One typical example of nonreciprocity is the p-n junction in semiconductors, where the inversion symmetry is naturally broken due to the imbalance of chemical potential distribution. When the electric current flows through such a semiconductor diode, the resistance is small in one direction but becomes dramatically large in the opposite direction, giving rise to the half-wave rectification effect. The stable half-wave rectification effect enables the semiconductor diode to serve as a fundamental component in electronic circuits. However, the unavoidable Joule heating in the semiconductor diode increases the energy consumption and impedes further integration of the circuit. The superconducting diode effect (SDE), whose critical current in one direction (positive) is different in magnitude from that in the opposite (negative) direction, offers a promising approach to achieve low power consumption in electronic circuits. As a unidirectional current with amplitude in between the two disparate critical currents is applied, the superconducting diode remains in the zero-resistance state in one direction and becomes resistive when the current direction is reversed. Therefore, superconducting diodes have the potential to become the basic computing element of low-consumption logic circuits.
In general, the emergence of the SDE requires simultaneous breaking of inversion symmetry and time-reversal symmetry (TRS). In superconducting systems, non-centrosymmetric superconductors and artificially constructed heterostructures with asymmetric structures are commonly used to break inversion symmetry, and applying magnetic field is an effective way to break TRS. Up to now, the SDE has been observed in various non-centrosymmetric superlattices, Josephson junctions, nano-fabricated devices, and films under magnetic field, while the necessity of applied magnetic field is still an obstacle to its application in electronic circuits. Therefore, developing the zero-field superconducting diode will enhance its application potential in low-dissipation electronic circuits. The zero-field SDE has been reported in some superconducting systems where the asymmetric device configuration leads to the inversion symmetry breaking, and the TRS breaking arises from the ferromagnetism of the magnetic layers, valley polarization, and current induced reduction of degeneracy. However, the operating temperature of these superconducting diodes is relatively low, and/or the structure of the diode devices is rather complex, which impede its implementation in the circuit. Field-free high-temperature superconducting diode with significant efficiency in a simple structure is highly desired for exploring potential applications in low-power consumption electronics.
Prof. Jian Wang’s group successfully fabricated the BSCCO flake deices by the electron-beam lithography technique and the cryogenic exfoliation technique, and performed the systematic transport measurements on these devices. In zero magnetic field, the positive critical current does not equal to the negative one (Fig. 1), which is the characteristic of the SDE. When square wave current with amplitude between positive and negative critical currents is applied, half-wave rectification effect appears in BSCCO devices, which further confirms the existence of SDE. The robustness of the SDE is further demonstrated by the highly stable performance of the rectification effect over 200 cycles. In addition, the SDE can persist up to 72 K, and finally disappear at 75 K, reaching a maximum efficiency of 22% at 53 K. The high operating temperature up to 72 K, large efficiency, zero applied magnetic field, and the simple flake configuration dramatically increase the practicability of superconducting electronic device fabricated using high-Tc cuprate superconductors. Eight BSCCO devices fabricated by two techniques show the SDE in different measurement systems, indicating that the appearance of SDE is independent of the fabrication method and measurement system.
Fig. 1 a, V-I curves measured by ramping the current from negative to positive and from positive to negative at 53 K in device s1. b, Evidence of SDE in device s1 at 53 K: The nonreciprocal critical currents. c, Half-wave rectification measured at 53 K under zero magnetic field. d, Rectification response observed in 200 cycles (over 3 hours), illustrating high stability of the SDE. e, Critical currents obtained from the 0–P and 0–N branches as a function of temperature at zero magnetic field. f, Temperature dependence of the diode efficiency.
To further confirm the zero magnetic field nature of the SDE, Jian Wang’s group conducted a detailed characterization of the magnetic field response of the SDE in BSCCO devices. It is found that both the critical currents of BSCCO devices and the efficiency of the SDE show a symmetrical structure with respect to the zero magnetic field, and the diode efficiency reaches its maximum at the zero magnetic field (Fig. 2e), which is quite different from the magnetic field response of field-induced SDE. The efficiency of a field-induced SDE is antisymmetric with respect to the magnetic field and should be zero at zero magnetic field. Therefore, the observed field-free SDE in BSCCO should be induced by the nonreciprocity of BSCCO flakes, rather than external factors. When turning the BSCCO device upside down inside the measurement system, the polarity of the SDE remains the same as that before flipping, ruling out the possibility of external remanence induced SDE (Figs. 2b and 2d). In addition, the research team used the same measuring equipment and measurement procedure to carry out a control experiment in the Nb film devices. The SDE is absent in the Nb devices in zero magnetic field. Under applied magnetic field, the SDE can be induced and the diode efficiency is antisymmetric around 0 mT, further demonstrating the field-free nature of the SDE in BSCCO flake devices.
Fig. 2 a, Temperature dependence of the resistance in zero magnetic field of BSCCO flake device s6. b, V-I curves at 30 K and 0°. The schematic position of the device is shown in the inset. c, Half-wave rectification measured at 30 K under zero magnetic field. d, V-I curves at 30 K and 180°. The inset is the schematic position of the “flipped” device. e, Critical currents obtained from the 0–P and 0–N branches and the diode efficiency as a function of perpendicular magnetic fields at 30 K.
The observation of the zero-field SDE in BSCCO devices suggests that the inversion and TRS may be broken in cuprates. It is theoretically proposed that the loop current along the copper oxygen atoms forms below the critical temperature of the pseudogap phase, which lowers the symmetry of the crystal and has the possibility to persist into the superconducting phase. This model provides a theoretical explanation for the possible inversion and TRS breakings in cuprates. In the last few decades, possible evidence of inversion and TRS breakings in the pseudogap phase has been observed in Bi2Sr2CaCu2O8+δ, YBa2Cu3O6+x, La2-xBaxCuO4 and other cuprates by angular resolved photoelectron spectroscopy, polarized neutron scattering, magneto-optical Kerr effect measurement and other experimental techniques. However, the relevant experimental results and analysis are still controversial, and there is no consensus on whether the symmetry is spontaneously broken. Therefore, the discovery of the zero-field SDE in BSCCO devices not only raises the possibility of the application of high-Tc superconductors and the SDE in low-dissipation devices, but also will stimulate more theoretical and experimental studies on the intrinsic inversion symmetry and TRS breakings in cuprates.
Shichao Qi, Dr. Jun Ge, and Chengcheng Ji at Peking University contribute equally to this work, and Prof. Jian Wang is the corresponding author of this paper. This work is financially supported by the National Natural Science Foundation of China and Beijing Natural Science Foundation.
Paper link: https://www.nature.com/articles/s41467-025-55880-4
