Intrinsic magnetization of the superconducting condensate in Fe(Te,Se)

Abstract

A spin-polarized superconducting condensate generates a net magnetization with measurable signatures. We present evidence for an intrinsic magnetic field in mesoscopic Fe(Te,Se) rings. The intrinsic field, encoded in the phase of superconducting quantum oscillations, scales linearly with the DC bias current, and its orientation exhibits an anomalous dependence on polarity and magnitude of the applied current. The magnetoresistance displays a dual flux-quantization effect with respect to the external magnetic field and the DC current. A minimal model incorporating Rashba coupling with an effective anisotropic out-of-plane interaction accounts for the experimental observations. These results provide evidence for spin-polarized superconductivity at the device scale and open new opportunities for superconducting spintronic and quantum information platforms.

Figures (6)

• Figure 1: (a) Schematic illustration of the external field $B_z$ and the intrinsic effective field $B_\text{eff}$ contributing to the magnetic flux through a superconducting ring. (b) SEM image of a representative device. The layer stack from top to bottom is hBN/Fe(Te,Se)/SiO$_2$(300 nm)/Si.
• Figure 2: (a) Normalized MR as a function of external field and DC bias current at 8 K. The winding number is defined as $n = B_z / \Delta B_z$, with $\Delta B_z\approx66$ G. The white and yellow lines are constant-phase trajectories that illustrate the gradual shift of the LP oscillations with bias current. Dashed horizontal lines mark $I_F$, the bias current at which the polarity of $B_\text{eff}$ reverses. (b, c) Enlarged MR maps from the third and second quadrants of panel a, respectively. The effective field is determined from the slope of constant-phase lines. (d) Schematic illustrating the relative orientation of the external ($B_z$) and intrinsic ($B_\text{eff}$) fields in the four quadrants of panel a.
• Figure 3: (a) MR spectra over a range of temperatures near $T_c$. Dashed lines mark the characteristic currents $I_F$. (b) FFT of the MR spectra over a wide temperature range. The FFT peaks at $\Phi_0/\Phi=1~(2)$ correspond to the IW (HIW) of the order parameter. The inner (outer) dashed lines trace the quantum oscillations associated with $\alpha_-$ ($\alpha_+$) coupling regimes.
• Figure 4: (a) Phase diagram of coupling factor. Dashed lines are linear fits. Magnetoelectric coupling strength $|\alpha_\pm|$ as a function of (b) temperature and (c) ring size. The latter is collected over multiple devices. Inset in c shows the ring geometry. (d) Magnetoelectric coefficient per current-density as function of ring size.
• Figure 5: Schematics of two concentric Fermi surfaces with opposite spin–momentum locking. (a) Rashba SOC in equilibrium, where the opposite helicity bands carry in-plane spin textures. (b) Rashba SOC with an effective out-of-plane interaction $g_z(\mathbf{k})$ in the presence of a DC current. The current drives a non-equilibrium imbalance and $g_z(\mathbf{k})$ cants the spin textures to generate opposite $s_z(\mathbf{k})$ components.