Switching Magnetism and Superconductivity with Spin-Polarized Current in Iron-Based Superconductor

We explore a new mechanism for switching magnetism and superconductivity in a magnetically frustrated iron-based superconductor using spin-polarized scanning tunneling microscopy (SPSTM). Our SPSTM study on single-crystal ${\mathrm{Sr}}_2{\mathrm{VO}}_3\mathrm{FeAs}$ shows that a spin-polarized tunneling current can switch the Fe-layer magnetism into a nontrivial $C_4$ ($2\times 2$) order, which cannot be achieved by thermal excitation with an unpolarized current. Our tunneling spectroscopy study shows that the induced $C_4$ ($2\times 2$) order has characteristics of plaquette antiferromagnetic order in the Fe layer and strongly suppresses superconductivity. Also, thermal agitation beyond the bulk Fe spin ordering temperature erases the $C_4$ state. These results suggest a new possibility of switching local superconductivity by changing the symmetry of magnetic order with spin-polarized and unpolarized tunneling currents in iron-based superconductors.

Figure 1

(a)–(d) Schematic illustrations of FeAs-layer configuration potential landscapes for ${\mathrm{Sr}}_2{\mathrm{VO}}_3\mathrm{FeAs}$ in various situations. (a) The imaginary case of FeAs and ${\mathrm{Sr}}_2{\mathrm{VO}}_3$ layers being separated sufficiently apart while the electron doping from the Sr layer retained near optimal. The $C_2$ magnetism in the Fe layer with strong superconductivity is preferred. (b) The natural separation found in ${\mathrm{Sr}}_2{\mathrm{VO}}_3\mathrm{FeAs}$ results in interlayer coupling and near degeneracy among the magnetic states with different symmetries, with the $C_2$ magnetism with strong superconductivity still being the ground state. (c) If a sufficiently strong spin-polarized current is injected, the balances among these states may change, possibly resulting in $C_4$ magnetic states with weak superconductivity in the FeAs layer. (d) When the sample is thermally annealed globally or locally with a high bias tunneling current injection, it may return to the ground states with $C_2$ magnetism and strong superconductivity.

Figure 2

(a) The structure of the Fe magnetic moments in the $C_4$ (plaquette) order. Each red dot represents the oxygen at the top of each vertical O-V-As atomic chain acting as the tunneling path. (b) The theoretical electron density plot near the Fermi level (integrated over $[-50,0]\mathrm{meV}$). (c) A typical 4.6 K topograph taken with W tip showing quasi-$C_2$ SRs with random orientations. (d) A spin-polarized STM image taken at a low junction resistance with a nearly in-plane polarized Cr tip showing the induced $C_4$-symmetric ($2\times 2$) order. The orange, green, and blue circles in the Fourier-transformed images (the insets) in (c) and (d) indicate $|\mathit{q}|=2\pi /a$ (Bragg peaks), $3\pi /4a$, and $\pi /a$, respectively. (e) The magnified view of the area in a white square in (d), with the ($2\times 2$) magnetic unit cells with a $C_4$ plaquette spin model overlaid. Its inset shows the cross sections along the black and blue arrows. (f) The spin-wave dispersion of the $C_4$ plaquette order and its two momentum transfer vectors ($\mathit{Q}$ and ${\mathit{Q}}^{*}$) from the localized moment picture [8], shown together with the ARPES-based Fermi surfaces (dark curves) [34].

Figure 3

(a)–(d) Bias dependence of W-tip topograph images showing a threshold voltage for SR fluctuation near $V_{\text{th}}^N\approx 300\mathrm{meV}$. (e)–(h) Bias dependence of Cr-tip topograph images. $C_4$ symmetric ($2\times 2$) domains and phase domain walls (g) are induced at a significantly lower threshold ($V_{\text{th}}^{\mathrm{SP}}\approx 30\mathrm{meV}$), indicating a qualitatively different final state from that obtained with the W tip (d). The inset in (g) is taken at a slightly higher junction conductance [$-50\mathrm{mV}$, 100 pA]. In all the FFT insets, the blue (red, green) arrows correspond to $|\mathit{q}|=\pi /a_0$ ($5\pi /4a_0$, $3\pi /4a_0$). (i)–(l) Temperature-dependent Cr-tip topographs taken at bias [$-50\mathrm{mV}$, 100 pA] showing the erasure of the $C_4$ order beyond the Fe magnetic ordering temperature.

Figure 4

(a) [(b)] W-tip (Cr-tip) topograph with bias conditions below threshold $V_{\text{th}}^N$ ($V_{\text{th}}^{\mathrm{SP}}$) taken after the higher bias scans shown in Figs. 3 [Figs. 3] performed only in the area inside the dotted square. The inset image in the solid square in (b) is a Cr-tip topograph with a higher bias above $V_{\text{th}}^{\mathrm{SP}}$ showing the domains and the domain walls more clearly (see Fig. S8). (c) [(d)] shows the tunneling spectra measured at marked positions with corresponding marker colors in (a) with a W tip [(b) with a Cr tip]. The W-tip spectra were measured at bias [40 meV, 300 pA]. The Cr-tip spectra were measured at the set point of [50 meV, 120 pA] inside the $C_4$ region and at the set point of [30 meV, 5 pA] in the pristine region with a larger averaging time to avoid inducing a $C_4$ state during the $dI/dV$ measurement. All the data are taken at 4.6 K.