Observation of a Zero-Field Josephson Diode Effect in a Helimagnet Josephson Junction

TL;DR

This study reports a zero-field Josephson diode effect in Cr1/3NbS2-based junctions, revealing non-reciprocal superconductivity in a non-centrosymmetric helimagnet. The authors observe a diode efficiency up to $η = 20\%$ and magnetic-diffraction patterns with offsets and asymmetries, persisting without an external field. They attribute these features to a combination of pinned Abrikosov vortices in the leads and the intrinsic spin chirality of the helimagnet, supported by CPR and vortex- Fraunhofer simulations that reproduce key pattern offsets and asymmetries. The work advances understanding of non-reciprocal superconductivity in chiral magnets and highlights vortex physics and chiral spin textures as joint drivers of the diode behavior.

Abstract

Cr$_{1/3}$NbS$_{2}$ is a transition metal dichalcogenide that is also a chiral helimagnet, and so lacks inversion symmetry and has non-zero Berry curvature in position and momentum space. It is well known that the combination of broken time-reversal symmetry and broken inversion symmetry can generate non-reciprocal phenomena, but the interplay between these kinds of systems and superconductivity is not well known. We present Josephson junctions fabricated from Cr$_{1/3}$NbS$_{2}$ that give magnetic diffraction patterns with asymmetry in both the magnetic field and the critical current. The non-reciprocity in positive critical current and negative critical current, generally called the Josephson diode effect, has an efficiency of up to $η=20\%$ in some parts of the magnetic diffraction pattern and persists even at zero applied field. We propose that pinned Abrikosov vortices are a main mechanism for the asymmetric magnetic field response in this system, and that the non-zero spin chirality of the Cr$_{1/3}$NbS$_{2}$ causes the diode effect. Simulations of magnetic diffraction patterns from Josephson junctions with vortices present show offsets from zero-field consistent with observations, while simulations of chiral spin structures with an out-of-plane canting show a diode effect.

Figures (8)

• Figure 1: (a) The crystal structure of Cr1/3NbS2, Cr atoms (purple) are intercalated between the planes of NbS2 (blue, yellow). (b) Applied magnetic field vs magnetization data showing magnetic hysteresis in bulk Cr1/3NbS2. Above the magnetic transition temperature there is a purely paramagnetic response. The hysteresis in the measurements below the transition temperature indicates the presence of solitons. (c) Magnetic susceptibility data for field-cooled and zero-field-cooled bulk Cr1/3NbS2, with a clear transition at ∼ 132K.
• Figure 2: (a) An optical microscope image of a quasi-four-point Josephson junction, with a Cr1/3NbS2 weak link and NbTiN superconducting leads. (b) The resistance across the same junction as a function of temperature. The blue points are the raw data, and the red line is a fit. (c) The resistance of the junction as a function of applied current. This measurement was taken immediately after the initial cooldown to superconducting temperatures, with no applied magnetic field. There is a slight asymmetry in the positive and negative critical currents, as well as a difference in resistance peak amplitude at the critical currents.
• Figure 3: (a) The magnetoresistance of the junction as a magnetic field is applied perpendicular to the plane of the junction. The left plots show magnetic field sweeps going from positive magnetic field to negative magnetic field, and the right plots from negative to positive. The lighter yellow curves begin at larger field magnitudes and are further offset from zero, while the darker blue curves begin at smaller magnetic field values and show less hysteresis. (b) Magnetoresistance as a function of out-of-plane magnetic field strength for a second junction. The hysteresis is more apparent when plotted on a logarithmic scale, and multiple dips in resistance are seen at larger field values. The reproducibility of the curves suggests that the hysteresis and local extrema are part of the larger magnetic diffraction pattern.
• Figure 4: (a) Magnetic diffraction pattern in a Cr1/3NbS2 junction for a field applied perpendicular to the plane of the junction. The field has been normalized to units of flux divided by flux quanta using the cross-sectional area of the junction. There are several notable features: there is a shift of the patterns largest peak from zero magnetic field, a large asymmetry with respect to field, a decay in peak amplitude that does not follow the standard $\sin(\pi \Phi/\Phi_0)/(\pi \Phi/\Phi_0)$ Fraunhofer behavior, and a small diode effect. (b) A zoomed-in section of a magnetic diffraction measurement similar to a. In this section the superconducting diode effect is easily visible as a tilting of the lobes in the pattern. (c) Line cuts at specific flux values from b. The left image shows the diode effect with a larger magnitude positive critical current, while the right image shows a larger magnitude negative critical current. Note that both of these are at negative field values within a single flux quanta of each other.
• Figure 5: Magnetic diffraction patterns with a magnetic field sweeping from negative to positive, and positive to negative. The patterns are not perfect mirror images of each other, but do have the same overall shape and features. The central node shift away from zero field is not consistent with ferromagnetic hysteresis, where a positive (negative) coercive field is necessary to return from negative (positive) remnant magnetization to zero magnetization.10.1063/5.0195229