Correlating Josephson Supercurrents and Shiba States in Quantum Spins Unconventionally Coupled to Superconductors
Result of the Month
a–e Scanning tunneling spectroscopy for V, Cr, Mn, Fe, and Co. While no YSR state is detected for V and Co, a rich set of in-gap states emerge for Cr, Mn, and Fe. The insets show the spatial distribution of the YSR states, which reflect their orbital character and allows one to quantify their spatial extension. Stabilization parameters: V = 5 meV, I = 500 pA.
Supplementary Figure Adsorption sites. All adatoms are characterized by a single and the same adsorption site, which is found to be the hollow site of the Nb(110) surface. The existence of a single adsorption site can be verified in two distinct way: (i) topographic imaging: as shown in the main text, Fig. 1 e, for each element, all adatoms have always the very same appearance on the surface; (ii) spectroscopic measurements: it is well-known that different adsorption sites result in very different hybridization strength, ultimately leading to distinct spectroscopic features. We analyzed several different adatoms for each element, and they all show the very same spectroscopic signatures, which is also found constant across different sample preparation (see Supplementary Note 4). The exact determination of the adsorption sites rely on atomically resolved images. Panel (a) show how adatoms are adsorbed, as expected, in the energetically favourable hollow site, which is the same adsorption site consistently used in our theoretical calculations (see Supplementary Note 9). However, depending on the tip used in the measurements and the set-point parameter, a corrugation reversal effect can be observed, as visualized in panel (b). This is in line with theoretical predictions suggesting strong corrugation reversal at (110) surfaces of bcc metals, with the effect expected to be particularly prominent especially for Nb [2].
Local spins coupled to superconductors give rise to several emerging phenomena directly linked to the competition between Cooper pair formation and magnetic exchange. These effects are generally scrutinized using a spectroscopic approach which relies on detecting the in-gap bound modes arising from Cooper pair breaking, the so-called Yu-Shiba-Rusinov (YSR) states. However, the impact of local magnetic impurities on the superconducting order parameter remains largely unexplored. Here, we use scanning Josephson spectroscopy to directly visualize the effect of magnetic perturbations on Cooper pair tunneling between superconducting electrodes at the atomic scale. By increasing the magnetic impurity orbital occupation by adding one electron at a time, we reveal the existence of a direct correlation between Josephson supercurrent suppression and YSR states. Moreover, in the metallic regime, we detect zero bias anomalies which break the existing framework based on competing Kondo and Cooper pair singlet formation mechanisms. Based on first-principle calculations, these results are rationalized in terms of unconventional spin-excitations induced by the finite magnetic anisotropy energy. Our findings have far reaching implications for phenomena that rely on the interplay between quantum spins and superconductivity.
Sample and Tip Preparation
Nb(110) single crystals (Surface Preparation Laboratory) have been prepared in ultra-high vacuum conditions and measured using a Tribus STM head (Scienta Omicron) operated at T = 1.9 K. The samples have been flashed hundreds of times at a temperature T = 2300 K for 12 s using an home-built electron-beam heater.
Small differences in the absolute energy position of the Shiba peaks are related to the use of different Nb-coated tips. As described in Methods, they have been prepared by intending an electrochemically etched tungsten tip into the Nb(110) crystal, and they can show slight differences in their superconducting energy gap.
Authors
Felix Küster, Ana M. Montero, Filipe S. M. Guimarães, Sascha Brinker, Samir Lounis, Stuart S. P. Parkin & Paolo Sessi
Institutes
Max Planck Institute of Microstructure Physics, Halle, Germany
Felix Küster, Stuart S. P. Parkin & Paolo Sessi
Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich & JARA, Jülich, Germany
Ana M. Montero, Filipe S. M. Guimarães, Sascha Brinker & Samir Lounis
Faculty of Physics, University of Duisburg-Essen, Duisburg, Germany
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![All adatoms are characterized by a single and the same adsorption site, which is found to be the hollow site of the Nb(110) surface. The existence of a single adsorption site can be verified in two distinct way: (i) topographic imaging: as shown in the main text, Fig. 1 e, for each element, all adatoms have always the very same appearance on the surface; (ii) spectroscopic measurements: it is well-known that different adsorption sites result in very different hybridization strength, ultimately leading to distinct spectroscopic features. We analyzed several different adatoms for each element, and they all show the very same spectroscopic signatures, which is also found constant across different sample preparation (see Supplementary Note 4). The exact determination of the adsorption sites rely on atomically resolved images. Panel (a) show how adatoms are adsorbed, as expected, in the energetically favourable hollow site, which is the same adsorption site consistently used in our theoretical calculations (see Supplementary Note 9). However, depending on the tip used in the measurements and the set-point parameter, a corrugation reversal effect can be observed, as visualized in panel (b). This is in line with theoretical predictions suggesting strong corrugation reversal at (110) surfaces of bcc metals, with the effect expected to be particularly prominent especially for Nb [2]. | © CC BY 4.0](/Results/ROM/2021/202104/5224/image-thumb__5224__fullImage/Adsorption%20Sites.jpg "© CC BY 4.0")