Spin-Polarized Yu-Shiba-Rusinov States in an Iron-Based Superconductor

Yu-Shiba-Rusinov (YSR) bound states appear when a magnetic atom interacts with a superconductor. Here, we report on spin-resolved spectroscopic studies of YSR states related with Fe atoms deposited on the surface of the topological superconductor ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ using a spin-polarized scanning tunneling microscope. We clearly identify the spin signature of pairs of YSR bound states at finite energies within the superconducting gap having opposite spin polarization as theoretically predicted. In addition, we also observe zero-energy bound states for some of the adsorbed Fe atoms. In this case, a spin signature is found to be absent indicating the absence of Majorana bound states associated with Fe adatoms on ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$.

Figure 1

(a) STM topography image of the cleaved ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ surface with deposited Fe atoms (yellow spots). Scale bar, 5 nm. (b) Two STS curves taken on clean ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ surface regions. (c),(d) STS data taken above two different Fe adatoms exhibiting either (c) YSR states at finite energies or (d) a zero-bias peak. Tunneling parameters: (a) $V=-10\mathrm{mV}$, $I=200\mathrm{pA}$; (b) $V_{\mathrm{stab}}=5\mathrm{mV}$, $I_{\mathrm{stab}}=100\mathrm{pA}$ (black spectrum), $V_{\mathrm{stab}}=-10\mathrm{mV}$, $I_{\mathrm{stab}}=200\mathrm{pA}$ (pink spectrum); (c) $V_{\mathrm{stab}}=-10\mathrm{mV}$, $I_{\mathrm{stab}}=400\mathrm{pA}$; (d) $V_{\mathrm{stab}}=-10\mathrm{mV}$, $I_{\mathrm{stab}}=600\mathrm{pA}$.

Figure 2

(a) Atomic-resolution STM topography image of the ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ surface including an Fe adatom (bright spot) exhibiting finite-energy YSR states. The white dashed arrow indicates the line with a length of 8.45 nm along which $dI/dV$ spectra in (b) have been taken and the green lines are on top of the Se/Te atoms in two different directions. Scale bar, 1 nm. (b) From the bottom to top, a total of $66dI/dV$ spectra are presented taken along the line shown in (a). The black spectrum has been recorded above the center of the Fe atom. The black dashed vertical lines at $\pm 0.5\mathrm{meV}$ indicate the energy position of the $\mathrm{YSR}+$ and $\mathrm{YSR}-$ peaks. (c) The peak amplitudes of $\mathrm{YSR}+$ (circle) and $\mathrm{YSR}-$ (triangle) states extracted from the $dI/dV$ spectra shown in (b). The black dashed vertical line indicates the center position of the Fe atom. Tunneling parameters in (a),(b) $V_{\mathrm{stab}}=-10\mathrm{mV}$, $I_{\mathrm{stab}}=400\mathrm{pA}$.

Figure 3

(a),(b) Illustration of the spin-polarized tunneling experiment on individual Fe adatoms on an ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ surface. The SP-STM tip is magnetized (a) up and (b) down, as indicated by the blue and red arrows, respectively. The magnetic moment orientation of the Fe atom is controlled by a very small external magnetic field of $\pm 20\mathrm{mT}$. The tip magnetization orientation remains unchanged in such a small external field. (c),(d) The spin-resolved $dI/dV$ spectra taken at an external field of $\pm 20\mathrm{mT}$, when the tip magnetization has been prepared (c) up or (d) down. All spectra were recorded above the center of the Fe atom. Tunneling parameters in (a),(b): $V_{\mathrm{stab}}=-10\mathrm{mV}$, $I_{\mathrm{stab}}=400\mathrm{pA}$.

Figure 4

(a) STM topography image of the ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ surface including an Fe atom exhibiting a ZBP. The white dashed arrow indicates the line with a length of 8.34 nm along which the $dI/dV$ spectra in (b) have been taken. Scale bar, 1 nm. (b) From the bottom to top, a total of $66dI/dV$ spectra are displayed taken along the line shown in (a). The black spectrum was measured above the center of the Fe atom. (c) Magnetic field-dependent measurements of the $dI/dV$ spectrum exhibiting a ZBP. The SP-STM tip has been polarized up or down in an external magnetic field of $\pm 1.5\mathrm{T}$. The orientation of the Fe atom’s magnetic moment is controlled by a small external magnetic field of $\pm 20\mathrm{mT}$, which does not affect the magnetization direction of the SP-STM probe tip. All tunneling spectra were measured above the center of the Fe atom. Tunneling parameters in (a),(b): $V_{\mathrm{stab}}=-10\mathrm{mV}$, $I_{\mathrm{stab}}=600\mathrm{pA}$.