Magnetic and superconducting interactions couple electrons together to form complex states of matter. We show that, at the atomic scale, both types of interactions can coexist and compete to influence the ground state of a localized magnetic moment. Local spectroscopy at 4.5 kelvin shows that the spin-1 system formed by manganese-phthalocyanine (MnPc) adsorbed on Pb(111) can lie in two different magnetic ground states. These are determined by the balance between Kondo screening and superconducting pair-breaking interactions. Both ground states alternate at nanometer length scales to form a Moiré-like superstructure. The quantum phase transition connecting the two (singlet and doublet) ground states is thus tuned by small changes in the molecule-lead interaction.
We combine scanning-tunneling-spectroscopy experiments probing magnetic impurities on a superconducting surface with a theoretical analysis of the tunneling processes between (superconducting) tip and substrate. We show that the current through impurity-induced Shiba bound states is carried by single-electron tunneling at large tip-substrate distances and Andreev reflections at smaller distances. The single-electron current requires relaxation processes, allowing us to extract information on quasiparticle transitions and lifetimes.
Large π-conjugated molecules, when in contact with a metal surface, usually retain a finite electronic gap and, in this sense, stay semiconducting. In some cases, however, the metallic character of the underlying substrate is seen to extend onto the first molecular layer. Here, we develop a chemical rationale for this intriguing phenomenon. In many reported instances, we find that the conjugation length of the organic semiconductors increases significantly through the bonding of specific substituents to the metal surface and through the concomitant rehybridization of the entire backbone structure. The molecules at the interface are thus converted into different chemical species with a strongly reduced electronic gap. This mechanism of surface-induced aromatic stabilization helps molecules to overcome competing phenomena that tend to keep the metal Fermi level between their frontier orbitals. Our findings aid in the design of stable precursors for metallic molecular monolayers, and thus enable new routes for the chemical engineering of metal surfaces.
A recent experiment [Nadj-Perge et al., Science 346, 602 (2014)] provides evidence for Majorana zero modes in iron (Fe) chains on the superconducting Pb(110) surface. Here, we study this system by scanning tunneling microscopy using superconducting tips. This high-resolution technique resolves a rich subgap structure, including zero-energy excitations in some chains. We compare the symmetry properties of the data under voltage reversal against theoretical expectations and provide evidence that the putative Majorana signature overlaps with a previously unresolved low-energy resonance. Interpreting the data within a Majorana framework suggests that the topological gap is significantly smaller than previously believed. Aided by model calculations, we also analyze higherenergy features of the subgap spectrum and their relation to high-bias peaks which we associate with the Fe d-bands.Building on advances in nanofabrication [1], engineering topological phases by proximity in superconducting hybrid structures has come within reach of current experiments. A major motivation for realizing such phases are their non-abelian Majorana quasiparticles [2][3][4], and their subsequent applications. The underlying topological superconducting phases can be realized in onedimensional (1d) helical liquids contacted by conventional s-wave superconductors [5][6][7][8][9]. Among the most promising platforms studied in experiment are semiconductor nanowires [10][11][12][13][14][15], edges of two-dimensional topological insulators [16,17], and chains of magnetic adatoms [18,19]. While the proximity coupling to a superconductor is needed to induce a gap protecting the topological phase, it also has more subtle consequences. Magnetic interactions mediated by the superconductor can stabilize magnetic order in the 1d system [20][21][22][23]. Conversely, the spin structure may affect the superconductor. This is particularly apparent for adatom chains, where a band of subgap Shiba states [24][25][26][27] may strongly modify the low-energy properties of the system [8,[28][29][30][31][32] and possibly induce trivial zero-energy features at the chain end [33]. At strong coupling, the 1d states bleed substantially into the superconductor, reducing the effective coherence length at low energies [34]. Nadj-Perge et al.[18] recently provided intriguing evidence for Majorana states in Fe chains on Pb(110). Here, we present data on the same system employing scanning tunneling microscopy/spectroscopy (STM/STS) with superconducting tips (see also [18]). We show that the use of superconducting tips not only provides enhanced resolution of the subgap structure, but also allows for additional consistency checks on the interpretation of the data in terms of Majorana quasiparticles. Our observations indicate a subgap spectrum comprising a flat Shiba band and strongly dispersing Fe states. An interpretation in terms of Majorana states suggests that the induced gap is considerably smaller than previously believed.We carried out the experiments in a Specs JT-ST...
The latest concepts for quantum computing and data storage envision to address and manipulate single spins. A limitation for single atoms or molecules in contact to a metal surface are the short lifetime of excited spin states, typically picoseconds, due to the exchange of energy and angular momentum with the itinerant electrons of the substrate [1-4]. Here we show that paramagnetic molecules on a superconducting substrate exhibit excited spin states with a lifetime of approximately 10 ns. We ascribe this increase in lifetime by orders of magnitude to the depletion of electronic states within the energy gap at the Fermi level. This prohibits pathways of energy relaxation into the substrate and allows for electrically pumping the magnetic molecule into higher spin states, making superconducting substrates premium candidates for spin manipulation. We further show that the proximity of the scanning tunneling microscope tip modifies the magnetic anisotropy
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