Abstract Body

Many superconducting systems with broken time-reversal and inversion symmetry show a superconducting diode effect, a non-reciprocal phenomenon analogous to semiconducting p–n-junction diodes. While the superconducting diode effect lays the foundation for realizing ultralow dissipative circuits, Josephson-phenomena-based diode effect (JDE) can enable the realization of protected qubits. The superconducting diode effect and JDE reported thus far are at low temperatures (~4 K), limiting their applications. Here we demonstrate JDE persisting up to 77 K using an artificial Josephson junction of twisted layers of Bi₂Sr₂CaCu₂O_8+δ. JDE manifests as an asymmetry in the magnitude and distributions of switching currents, attaining the maximum at 45° twist. The asymmetry is induced by and tunable with a very small magnetic field applied perpendicular to the junction and arises due to interaction between Josephson and Abrikosov vortices. We report a large asymmetry of 60% at 20 K. Our results provide a path towards realizing superconducting Josephson circuits at liquid-nitrogen temperature.

References

[1] "High-temperature Josephson diode," Sanat Ghosh et al. Nature Materials 23, 612 (2024).

Acknowledgment

We acknowledge the Department of Science and Technology (DST), Nanomission grant SR/NM/NS-45/2016, CORE grant CRG/2020/003836 and Department of Atomic Energy (DAE) of the Government of India (12-R&D-TFR-5.10-0100) for support.

Keywords: Superconducting diode, Josephson diode, high Tc superconductor, BSCCO

Abstract Body

Communication using terahertz (~10¹² Hz) electromagnetic waves is critical for developing 6^th-generation wireless network infrastructures. Conflictions between stable radiation and the modulation frequency of terahertz sources impede the superposing of transmitting signals on carrier waves. The Josephson junctions included in a cuprate superconductor radiate terahertz waves [1] with frequencies proportional to the bias voltages [2][3]. Thus, the modulation of the bias voltage leads to the modulation of the Josephson plasma emission (JPE) frequency. This study aims to demonstrate the generation of frequency-modulated (FM) terahertz continuous waves from Josephson junctions. We achieved to demonstrate sinusoidal signal up to 4.5 GHz superimposed on sub-terahertz carrier waves in two JPE devices. One device radiates at 840–890 GHz with the maximum FM bandwidth of 40 GHz [4] and the other radiates at 400-500 GHz with high- and low bias regions. The results verify that the instantaneous JPE frequency follows the gigahertz-modulated bias voltage. The on-chip FM terahertz generation shows a sharp contrast to the mode-lock frequency comb constructed by highly sophisticated optics on a bench. A further increase of the modulation amplitude facilitates up- or down-frequency conversion over more than one octave. The obtained FM bandwidth exhibited an improvement of two orders of magnitude in the demodulation signal-to-noise ratio compared to the amplitude-modulated signal. The demonstrated FM-JPE stimulates further research on terahertz communication technology and metrology using superconducting devices.

References

Ozyuzer, L. et al. Emission of coherent THz radiation from superconductors. Science 318, 1291 (2007).

Tsujimoto, M. et al. Broadly Tunable Subterahertz Emission from Internal Branches of the Current-Voltage Characteristics of Superconducting Bi2Sr2CaCu2O8+δSingle Crystals. Phys Rev Lett 108, 107006 (2012).

Kakeya, I. & Wang, H. Terahertz-wave emission from Bi2212 intrinsic Josephson junctions: a review on recent progress. Supercond Sci Technol 29, 073001 (2016).

Miyamoto, M., Kobayashi, R., Kuwano, G., Tsujimoto, M. & Kakeya, I. Wide-band frequency modulation of a terahertz intrinsic Josephson junction emitter of a cuprate superconductor. Nat Photonics 18, 267 (2024).

Acknowledgment

The results were obtained by the collaboration with M. Miyamoto, R. Kobayashi, N. Yagyu, G. Kuwano, and M. Tsujimoto. This work has been partly supported by JSPS KAKENHI (grant numbers JP20H02606, 23K17747, and 24K00946) and ISHIZUE 2023 of Kyoto University.

Radiation spectrum (red lines and symbols) at temperature of 25 K, mesa voltage of 2.05 V, superimposed modulation frequency of 3 GHz, and modulation power of 25 dBm. The ideal FM spectrum for a carrier frequency of 856 GHz and modulation index of 5.7 is shown with blue bars.

Keywords: Cuprate, Intrinsic Josephson junction, terahertz radiation

Abstract Body

The RE-123 system of REBa₂Cu₃O_y(RE = rare-earth elements) with the superconducting transition temperature (T_c) of around 90 K is a promising candidate as a practical superconducting magnet. The enhancement of T_c is desired for the operation at liquid nitrogen temperatures. The oxygen content y in REBa₂Cu₃O_yis around 6.9, indicating that ~10% of the oxygen ions (O^2-) in the Cu-O chain is deficient. Since the disorder in the oxygen-deficient Cu-O chain will suppress the superconductivity in the adjacent CuO₂ plane, the reduction of the oxygen deficiencies is crucial for the enhancement of T_c. Additional O^2- can be incorporated into the oxygen deficient Cu-O chain through the RE³⁺-substitution for Ba²⁺ to maintain the charge neutrality. However, it has been reported that T_c decreases with increasing xfor x ≥ 0.1 in Nd_1+xBa_2-xCu₃O_y, which has the highest T_c in RE-123 system [1]. In this work, we report the enhancement of T_c in the narrow range of x ≤ 0.05 for Nd_1+xBa_2-xCu₃O_y.

Polycrystalline samples of Nd_1+xBa_2-xCu₃O_y with x ≤ 0.05 were prepared by sintering in flowing Ar gas followed by annealing in flowing O₂ gas. It has been found that the oxygen content y increases with increasing x without any change in the hole concentration in the CuO₂ plane p as shown in Fig. 1. Furthermore, T_c also increased to 96.4 K for x = 0.05 from 94.8 K for x = 0. The slight enhancement of T_c through the Nd-substitution for Ba may be attributed to the reduction of the disorder in the oxygen-deficient in the Cu-O chain.

References

[1] N. Chikumoto et al., Appl. Supercond. 2, 163 (1998).

Acknowledgment

This work was supported by JST SPRING, Grant Number JPMJSP2114.

Figure 1. x dependence of T_c, oxygen content y and hole concentration in the CuO₂ plane p in Nd_1+xBa_2-xCu₃O_y with x ≤ 0.05.

Keywords: RE-123, substitution, oxygen deficiency, T_c