Field-free Superconducting Diode Effect in FeTe$_{0.55}$Se$_{0.45}$

TL;DR

The paper tackles the challenge of achieving field-free nonreciprocal superconducting transport in iron-based superconductors with simple device geometries. Using thin FeTe$_{0.55}$Se$_{0.45}$ flakes, the authors measure zero-field nonreciprocity in critical currents (I_c) and observe a finite second-harmonic resistance at the superconducting transition, indicative of SDE. They systematically rule out magnetic chirality, dynamic superconducting domains, thermal gradients, and geometric effects, demonstrating that localized stress amplifies the rectification and that the effect persists without external magnetic fields. The findings establish FeTe$_{0.55}$Se$_{0.45}$ as a robust, structurally simple platform for field-free SDE, with implications for superconducting electronics and the study of symmetry-breaking and correlated transport in iron-based superconductors.

Abstract

The superconducting diode effect (SDE) - the asymmetry of critical currents with respect to current direction - is a pivotal advancement in non-reciprocal superconductivity. While SDE has been realized in diverse systems, a fundamental challenge remains achieving field-free operation in iron-based superconductors with simple device geometries. Here, we report a non-volatile, field-free SDE in thin crystalline FeTe$_{0.55}$Se$_{0.45}$(FTS), showing asymmetric critical currents with a rectification coefficient of 1.9% and operating temperatures up to 9 K. Intriguingly, a pronounced non-zero second harmonic resistance emerges at the superconducting transition, exhibiting a sign reversal under varying current and temperature. The SDE persists at zero magnetic field and the rectification coefficient($η$) exhibits an even symmetric dependence on the magnetic field, distinguishing it from magnetic chirality anisotropy mechanisms. In addition to this, we systematically ruled out influences from dynamic superconducting domains, thermal gradients, and sample geometry, while establishing that localized stress amplifies the rectification coefficient, likely constituting one of the principal contributing factors. These results establish FTS as a robust platform for realizing field-free superconducting diodes in a structurally simple platform.

Figures (5)

• Figure 1: Nonreciprocal transport properties in FeTe0.55Se0.45(FTS) under zero field (B = 0 T) . (a) Crystal structure of FTS inversion symmetry is broken along the c axis. Orange denotes Fe atoms, green signifies Te atoms, and blue represents Se atoms. (b) Optical image of FTS sample. AC or DC current is administered through the electrode positioned at the farthest point to ensure uniform distribution, while the central planar section is designated as the voltage measurement area. In this configuration, the precisely defined rectangular structure comprises the FTS encapsulated with boron nitride(BN), where the violet hue arises from the combined effect of Au/Ti electrodes and SiO2 etched to a specific thickness. (c) The temperature dependence of first harmonic resistance($R_{\text{xx}}^{\omega}$) under zero field cooling by applying different current range from 50 $\mu$A to 1 mA. Zero-resistance state is attained, at $T_{c}^{0}$ = 13 K under low current excitation in device A. (d) The coresponding second harmonic resistance($R_{\text{xx}}^{2\omega}$) as a function of temperature using AC measurement. Obvious non-zero signal is developed at the superconducting transition and the sign of the resistance even reverses under low current excitation in device A.
• Figure 2: Zero field SDE in Device A. (a) The negative side (from 0 mA to -0.8 mA) and positive side (from 0 mA to 0.8 mA) of current-votage curves at different temperature. A clear difference of critical current is observed. (b) The disparity in critical current (purple line) diminishes progressively with increasing temperature, whereas the rectification coefficient (orange line) remains invariant at low temperature, undergoes initial amplification to 4% with temperature increasing, and ultimately attenuates to zero. (c) Resistance switch is realized by applying a current of $\pm$0.6575 mA at 4 K and zero magnetic field. The half-wave rectification is achieved with high stability and robustness.
• Figure 3: Comparative analysis of rectification coefficients with and without localized stress. (a)Schematic illustration of the sample structure in the stress-application experiment. The right electrode underwent additional photolithography and reactive ion beam etching to create a trench approximately 1.3 $\mu$m deep. Mechanical stress was induced in the sample through compressive deformation during the transfer process. (b)The region devoid of localized stress exhibits an even-symmetry dependence for both rectification coefficient and voltage differential with respect to magnetic field variations. (c)In contrast, the stressed region demonstrates an even-symmetry characteristic in the rectification coefficient, accompanied by a 1% enhancement, while the voltage differential manifests no discernible dependence on the magnetic field.
• Figure 4: The dependence of rectification coefficient ($\eta$) and thermoelectric potential ($\Delta V_{heat}$) on magnetic fields in symmetric and asymmetric configurations. (a) Optical image of symmetric and asymmetric sample with a scale bar denoting 10 $\mu$m.The device was meticulously transferred via a dry-transfer process onto pre-fabricated electrodes and subsequently encapsulated with boron nitride for protection. This was followed by sophisticated micro-nano fabrication techniques to selectively etch the same sample into both symmetrical and asymmetrical configurations. (b) Illustration of the intrinsic potential voltage difference ($\Delta V_{heat}$) arising from thermal gradient in the specimen.The voltage disparity for equivalent currents of opposite polarity in the normal state manifests as twice the $\Delta V_{heat}$. (c) The rectification coefficient $\eta$ dependence of applied magnetic fields manifests unequivocal even-symmetry. (d) The voltage differential($\Delta V_{heat}$) calculated by $| V_{+0.8 mA} - V_{-0.8 mA}|$ exhibits no discernible dependence on the magnetic field. (e) The rectification coefficient $\eta$ dependence of applied magnetic fields manifests unequivocal even-symmetry. (f) The voltage differential manifests a pronounced even-symmetry characteristic.
• Figure 5: Repeated temperature cycles from superconducting state to normal state were implemented to eliminate potential influences from internal dynamic SC domains.