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Register a new userStudy of Yu-Shiba-Rusinov bound states by tuning the electron density at the Fermi energy
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Magnetic atoms can break the Cooper pairs of superconductors, leading to the formation of Yu-Shiba-Rusinov (YSR) bound states inside superconducting gaps. Theory predicts that the YSR bound states can be controlled by tuning the electron density at the Fermi energy, but it has not been studied deeply. In this work, we studied the nature of YSR bound states in response to the potential scattering U by tuning the electron density at the Fermi energy. By comparing two systems, Mn-phthalocyanine molecules on Pb(111) and Co atoms on PbSe/Pb(111), we demonstrate that the sign of U can be unambiguously determined by varying the electron density at the Fermi energy. We also show that U competes with the exchange interaction JS in the formation of YSR bound states. Our work provides insights into the interactions between magnetic atoms and superconductors at a fundamental level.
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There is presently a tremendous activity around the field of topological superconductivity and Majorana fermions. Among the many questions raised, it has become increasingly important to establish the topological or non-topological origin of features associated with Majorana fermions such as zero-bias peaks. Here, we compare in-gap features associated either with isolated magnetic impurities or with magnetic clusters strongly coupled to the atomically thin superconductor Pb/Si(111). We study this system by means of scanning tunneling microscopy and spectroscopy (STM/STS). We take advantage of the fact that the Pb/Si(111) monolayer can exist either in a crystal-ordered phase or in an incommensurate disordered phase to compare the observed spectroscopic features in both phases. This allows us to demonstrate that the strongly resolved in-gap states we found around the magnetic clusters in the disordered phase of Pb have a clear topological origin.
We study an interacting quantum dot in contact with a small superconducting island described by the interacting pairing model with charging (Coulomb) energy $E_c$. This charge-conserving Hamiltonian admits a compact matrix-product-operator representation and can be accurately solved using the density-matrix renormalization group. We investigate the effects of the $E_c$ term which controls the number of electrons on the superconducting island. Most prominently, the energies of the subgap excited states induced by the impurity are no longer symmetric with respect to the chemical potential and may undergo discontinuous changes as a function of gate voltages. Phase diagrams of spin-singlet and spin-doublet ground states reveal a cross-over from the regime governed by the Yu-Shiba-Rusinov physics to the charge quantization (Coulomb blockade) regime characterized by even-odd electron-number effects. In this regime we find subgap states for both even and odd superconductor occupancy, but with distinctive subgap excitation spectra.
By using scanning tunneling microscopy (STM) we find and characterize dispersive, energy-symmetric in-gap states in the iron-based superconductor $mathrm{FeTe}_{0.55}mathrm{Se}_{0.45}$, a material that exhibits signatures of topological superconductivity, and Majorana bound states at vortex cores or at impurity locations. We use a superconducting STM tip for enhanced energy resolution, which enables us to show that impurity states can be tuned through the Fermi level with varying tip-sample distance. We find that the impurity state is of the Yu-Shiba-Rusinov (YSR) type, and argue that the energy shift is caused by the low superfluid density in $mathrm{FeTe}_{0.55}mathrm{Se}_{0.45}$, which allows the electric field of the tip to slightly penetrate the sample. We model the newly introduced tip-gating scenario within the single-impurity Anderson model and find good agreement to the experimental data.
The coupling of a spin to an underlying substrate is the basis for a plethora of phenomena. In the case of a metallic substrate, Kondo screening of the adatom magnetic moment can occur. As the substrate turns superconducting, an intriguing situation emerges where the pair breaking due to the adatom spins leads to Yu-Shiba-Rusinov bound states, but also intertwines with Kondo phenomena. Through scanning tunneling spectroscopy, we analyze the interdependence of Kondo screening and superconductivity. Our data obtained on single Fe adatoms on Nb(110) show that the coupling and the resulting YSR states are strongly adsorption site-dependent and reveal a quantum phase transition at a Kondo temperature comparable to the superconducting gap. The experimental signatures are rationalized by combined density functional theory and continuous-time quantum Monte-Carlo calculations to rigorously treat magnetic and hybridization effects on equal footing.
Recent studies of mutually interacting magnetic atoms coupled to a superconductor have gained enormous interest due to the potential realization of topological superconductivity. The Kondo exchange coupling J_K of such atoms with the electrons in the superconductor has a pair-breaking effect which produces so-called Yu-Shiba-Rusinov (YSR) states within the superconducting energy gap, whose energetic positions are intimately connected with the requirements for topological superconductivity. Here, using the tip of a scanning tunneling microscope, we artificially craft a multi-impurity Kondo system coupled to a superconducting host consisting of an Fe adatom interacting with an assembly of interstitial Fe atoms on an oxygen-reconstructed Ta(100) surface and we experimentally investigate the signatures of Kondo screening and the YSR states. With the help of numerical renormalization group (NRG) calculations, we show that the observed behavior can be qualitatively reproduced by a two-impurity Kondo system whose inter-impurity antiferromagnetic interaction J is adjusted by the number of interstitial Fe atoms in the assembly. When driving the system from the regime of two decoupled Kondo singlets (small J) to that of an antiferromagnetic dimer (large J), the YSR state shows a characteristic cross-over in its energetic position and particle-hole asymmetry.
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