Spin-Resolved Spectroscopy of the Yu-Shiba-Rusinov States of Individual Atoms

Abstract

A magnetic atom in a superconducting host induces so-called Yu-Shiba-Rusinov (YSR) bound states inside the superconducting energy gap. By combining spin-resolved scanning tunneling spectroscopy with simulations we demonstrate that the pair of peaks associated with the YSR states of an individual Fe atom coupled to an oxygen-reconstructed Ta surface gets spin polarized in an external magnetic field. As theoretically predicted, the electron and hole parts of the YSR states have opposite signs of spin polarizations which keep their spin character when crossing the Fermi level through the quantum phase transition. The simulation of a YSR state right at the Fermi level reveals zero spin polarization which can be used to distinguish such states from Majorana zero modes in chains of YSR atoms. DOI: 10.1103/PhysRevLett.119.197002 Recent investigations of chains [1][2][3] and arrays [4,5] of magnetic atoms in contact with surfaces of s-wave superconductors in view of Majorana zero modes and topological superconductivity triggered renewed interest in the properties of the basic constituent of such systems, the individual magnetic adatom. Typically, such adatoms induce quasiparticle excitations inside the superconducting energy gap [6][7][8], referred to as Yu-Shiba-Rusinov (YSR) states, which hamper the identification of topologically nontrivial edge states [3,9], calling for a thorough characterization of all experimentally accessible properties of YSR states.Following the first experimental verification of YSR states of individual atoms using scanning tunneling spectroscopy (STS) [10], there were numerous experimental studies, focusing on orbital effects [11][12][13], magnetic anisotropy [14], effects of reduced dimensionality of the superconductor [15], effects of coupling [11,16], and the competition between Kondo screening and superconducting pairing [17,18]. However, the investigation of the spin polarization of the YSR state of individual atoms, which could serve as a fingerprint for the distinction from topological states [19], was so far restricted to theoretical predictions [6][7][8][20][21][22].Neglecting orbital effects or magnetic anisotropy, theory predicts one pair of spin-polarized states of a YSR atom, an electron (e − )-and a hole (h)-like part with opposite spin character, which are located at AEjεj from the Fermi level E F [20]. There is a pronounced asymmetry in spectral weight between the e − and h parts due to electron-hole asymmetry of the band structure [21,22] and/or a nonmagnetic scattering potential of the impurity [20,23]. With increasing exchange coupling J between the adatom and substrate, the binding energy ε decreases until the YSR states eventually cross E F through a quantum phase transition (QPT), accompanied by an inversion of the asymmetry in the e − − h spectral weight [17]. Most notably, since both parts of the YSR states keep their spin character, the spin polarization above and below E F is expected to invert [20] through the QPT. Here, we experimentally verify these prediction...

Yu–Shiba–Rusinov States in the Charge-Density Modulated Superconductor NbSe2

“…This gives us full control over the adsorption site and allows us to compare adatoms which differ only in their locations relative to the CDW. Indeed, numerous previous studies [35,40,[43][44][45][46][47] on other substrates have shown that in the absence of CDW ordering, YSR spectra and wavefunctions are fully reproducible for a particular adsorption configuration of the adatom on the substrate.…”

Magnetism and in-gap states of 3d transition metal atoms on superconducting Re

“…Recently, theoretical predictions and experimental indications of topological superconductivity in one- [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] and two-dimensional [16][17][18][19][20] arrays of magnetic atoms coupled to bulk superconductors have triggered renewed interest in the building blocks of such systems, i.e. single magnetic atoms coupled to bulk superconductors [21][22][23][24][25][26][27][28][29][30] . Magnetic atoms locally induce pairs of bound state resonances inside the gap of their host superconductor, known as Yu-Shiba-Rusinov (YSR) states [31][32][33][34][35] .…”

“…However, different effects complicate this picture calling for detailed experimental investigations. The YSR state is accompanied by the formation of the Kondo state 23,30 . Orbital effects 22,27,29 and magnetic anisotropy 25 can lead to its multiplicity, and exchange interactions between the atoms may further split or shift the YSR states 22,[38][39][40][41] .…”

Learning pairing symmetries in disordered superconductors using spin-polarized local density of states

“…Experimentally, there are various methods which can be used to detect the pairing symmetry, e.g., phase-sensitive tetracrystal measurements [4,5], quasiparticle interference spectrum [6][7][8][9], local density of states (LDOS) near magnetic (and nonmagnetic) impurities [10][11][12], nuclear magnetic resonance spectroscopy [13,14], etc. Among them, the local density distribution of Yu-Shiba-Rusinov [15][16][17] resonance state [18][19][20][21][22] and spin-polarized (SP) LDOS [23,24] near a magnetic impurity visualize the pairing symmetry intuitively. However, the pairing symmetry implicated in these LDOSs are fragile against disorder, which makes the identification of pairing symmetries to be poor performed and scarcely used in a variety of interesting materials, which may host unconventional superconductivity and topological superconductivity, such as Cu x Bi 2 Se 3 [25][26][27] and its variants [28][29][30][31][32][33][34], FeTe 0.45 Se 0.55 [35], etc.…”