High-Resolution Scanning Tunneling Spectroscopy of Magnetic Impurity Induced Bound States in the Superconducting Gap of Pb Thin Films

Tunneling spectra for individual atoms and dimers of Mn and Cr adsorbed on superconducting Pb thin films were measured by a low temperature scanning tunneling microscope. Multiple-resonance structures within the superconducting gap on the adsorbates were resolved and interpreted as the magnetic impurity-induced bound states associated with different scattering channels. The experiment demonstrates a spectroscopic approach to characterizing the spin states of magnetic structures and exploring the competition between superconductivity and magnetism at the nanometer scale.

Figure 1

Single magnetic atom-induced bound states in the superconducting gap of Pb thin films. (a) Schematic and (b) differential conductance of the $S\mathrm{\text{−}}S$ tunneling junction. The tunneling junction was set at $V_{\mathrm{bias}}=10\mathrm{mV}$, $I=0.4\mathrm{nA}$. The bias modulation was 0.05 mV (rms) at 1.991 kHz. (c)–(f) Topographic images and normalized $dI/dV$ spectra of single Mn and Cr atoms at 0.4 K. For a Mn atom (c), the four peaks at energies of $-2.35\mathrm{meV}$, $-1.84\mathrm{meV}$, $+1.86\mathrm{meV}$, and $+2.47\mathrm{meV}$ correspond to the first two angular momentum channels, respectively. For a Cr atom (e), six peaks (f) were detected at energies of $-2.44\mathrm{meV}$, $-1.92\mathrm{meV}$, $-1.49\mathrm{meV}$, $+1.49\mathrm{meV}$, $+1.89\mathrm{meV}$, and $+2.38\mathrm{meV}$, respectively. The plus and minus signs correspond to the electronlike and holelike excitations. Suppressing of the coherence peaks of superconductor by the magnetic atoms is noted.

Figure 2

(a) The topographic image of Mn dimer I with 6.1 Å interatomic distance. (b) The topographic image of Mn dimer II with 4.2 Å interatomic distance. (c) Typical $dI/dV$ spectra taken on Pb (away from adsorbates), an isolated Mn atom, Mn dimer I, and Mn dimer II. The curves in (c) are offset vertically for clarity. All STS were acquired at a set point of $V=10\mathrm{mV}$ and $I=0.2\mathrm{nA}$ at 0.4 K.

Figure 3

(a)–(c) The $dI/dV$ mappings for an isolated Mn atom at various energies. The holelike $l=1$ state (not shown) is not strong enough for clear $dI/dV$ imaging. (d)–(g) $dI/dV$ mappings of Mn dimer I. (h)–(o) $dI/dV$ mappings of Mn dimer II. The black dots indicate the centers of the atoms. Imaging conditions: $V=10\mathrm{mV}$ and $I=0.15\mathrm{nA}$.

Figure 4

(a) The topographic image of Cr dimer I with 6.6 Å interatomic distance. (b) The topographic image of Cr dimer II with 3.0 Å interatomic distance. (c) Typical $dI/dV$ spectra taken on Pb (away from adsorbates), an isolated Cr atom, Cr dimer I and Cr dimer II. The curves in (c) are offset vertically for clarity. All STS were acquired at a set point of $V=10\mathrm{mV}$ and $I=0.2\mathrm{nA}$ at 0.4 K.