Yoav Kalcheim

Long-range proximity effect in La2/3Ca1/3MnO3/(100)YBa2Cu3O7−δferromagnet/superconductor bilayers: Evidence for induced triplet superconductivity in the ferromagnet

Scanning tunneling spectroscopy measurements conducted on epitaxially grown bilayers of half-metallic ferromagnetic La2/3Ca1/3MnO3 (LCMO) on superconducting (SC) (100)YBa2Cu3O7-delta (YBCO) reveal long-range penetration of superconducting order into the LCMO. This anomalous proximity effect manifests itself in the tunneling spectra measured on the LCMO layer as gaps and zero bias conductance peaks. Remarkably, these proximity-induced spectral features were observed for bilayers with LCMO thickness of up to 30 nm, an order of magnitude larger than the expected ferromagnetic coherence length in LCMO. We argue that this long-range proximity effect can be accounted for by the formation of spin triplet pairing at the LCMO side of the bilayer due to magnetic inhomogeneity at the interface or at domain walls. Possible symmetries of the induced order parameter are discussed.

Inverse proximity effect at superconductor-ferromagnet interfaces: Evidence for induced triplet pairing in the superconductor

Considerable evidence for proximity-induced triplet superconductivity on the ferromagnetic side of a superconductor-ferromagnet (S-F) interface now exists; however, the corresponding effect on the superconductor side has hardly been addressed. We have performed scanning tunneling spectroscopy measurements on NbN superconducting thin films proximity coupled to the half-metallic ferromagnet La2/3Ca1/3MnO3 (LCMO) as a function of magnetic field. We have found that at zero and low applied magnetic fields the tunneling spectra on NbN typically show an anomalous gap structure with suppressed coherence peaks and, in some cases, a zero-bias conductance peak. As the field increases to the magnetic saturation of LCMO where the magnetization is homogeneous, the spectra become more BCS-like and the critical temperature of the NbN increases, implying a reduced proximity effect. Our results therefore suggest that triplet-pairing correlations are also induced in the S side of an S-F bilayer.

p-wave triggered superconductivity in single layer graphene on an electron-doped oxide superconductor

Electron pairing in the vast majority of superconductors follows the Bardeen–Cooper–Schrieffer theory of superconductivity, which describes the condensation of electrons into pairs with antiparallel spins in a singlet state with an s-wave symmetry. Unconventional superconductivity was predicted in single-layer graphene (SLG), with the electrons pairing with a p-wave or chiral d-wave symmetry, depending on the position of the Fermi energy with respect to the Dirac point. By placing SLG on an electron-doped (non-chiral) d-wave superconductor and performing local scanning tunnelling microscopy and spectroscopy, here we show evidence for a p-wave triggered superconducting density of states in SLG. The realization of unconventional superconductivity in SLG offers an exciting new route for the development of p-wave superconductivity using two-dimensional materials with transition temperatures above 4.2 K.

Signature of proximity-induced px + ipy triplet pairing in the doped topological insulator Bi2Se3 by the s-wave superconductor NbN

In the search for Majorana fermions in proximity-induced topological superconducting junctions, we happened to find a signature of same-spin triplet superconductivity which appears to dominate these elusive elementary excitations. Thin-film junctions and bilayers of the doped topological insulator and the s-wave superconductor NbN exhibit conductance spectra with coexisting prominent zero-bias and coherence peaks. Various tunneling models with different pair potentials have failed to fit our data, except for the triplet pair potential, which breaks time-reversal symmetry, that yielded reasonably good fits. This provides supporting evidence for proximity-induced triplet superconductivity in the layer near the interface with the NbN film.

Increased superconducting transition temperature of a niobium thin film proximity coupled to gold nanoparticles using linking organic molecules.

The superconducting critical temperature, T(C), of thin Nb films is significantly modified when gold nanoparticles (NPs) are chemically linked to the Nb film, with a consistent enhancement when using 3 nm long disilane linker molecules. The T(C) increases by up to 10% for certain linker length and NP size. No change is observed when the nanoparticles are physisorbed with nonlinking molecules. Electron tunneling spectra acquired on the linked NPs below T(C) typically exhibit zero-bias peaks. We attribute these results to a pairing mechanism coupling electrons in the Nb and the NPs, mediated by the organic linkers.

Formation of Au-Silane Bonds

Many intriguing aspects of molecular electronics are attributed to organic-inorganic interactions, yet charge transfer through such junctions still requires fundamental study. Recently, there is a growing interest in anchoring groups, which considered dominating the charge transport. With this respect, we choose to investigate self-assembly of disilane molecules sandwiched between gold surface and gold nanoparticles. These assemblies are found to exhibit covalent bonds not only between the anchoring Si groups and the gold surfaces but also in plane crosslinks that increase the monolayer stability. Finally, using scanning tunneling spectroscopy we demonstrate that the disilane molecules provide strong electrical coupling between the Au nanoparticles and a superconductor substrate.

Increasing the critical temperature of Nb films by chemically linking magnetic nanoparticles using organic molecules

In type-II superconductors vortex pinning enhances the critical current density. One known method to induce pinning sites is the use of magnetic nanostructures that locally degrade the superconductivity via stray fields. In recent studies, we showed that both the critical temperature and critical current of Nb thin films can be enhanced by coupling Au nanoparticles via organic molecules and, concomitantly, a zero-bias peak appeared in the density of states. One suggested mechanism to explain these effects was the interaction of the induced pinning potential landscape with the Cooper pairs and vortices. To further examine this mechanism we study in the present work the effects of chemically linking magnetic nanoparticles to Nb films. Two types of magnetic nanoparticles are investigated, half-metal (Fe3O4) and metallic (Co). For high nanoparticle density, resulting in an effective continuous magnetic film, the critical temperature is reduced, as expected. However, for intermediate density, where the magnetic nanoparticles are well separated and a distinct pinning landscape is formed above the Nb film, critical temperature and current density enhancements are observed for both types of particles. Moreover, the tunnelling spectra acquired on the (metallic) Co nanoparticles exhibit a zero-bias conducting peak. The magnetic nanoparticles proximity through organic molecules presents similar behaviour to the non-magnetic Au nanoparticles inverse proximity results. This may suggest that pinning mechanisms play a role in the critical temperature enhancement.

Corrigendum: p -wave triggered superconductivity in single-layer graphene on an electron-doped oxide superconductor

Nature Communications 8: Article number: 14024 (2017); Published: 19 January 2017; Updated: 1 March 2017 The present address for U. Sassi is incorrect in this Article. This author does not have a present address. The correct full affiliation details for this author are given below: Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, UK.

Dynamic Control of the Vortex Pinning Potential in a Superconductor Using Current Injection through Nanoscale Patterns

Control over the vortex potential at the nanoscale in a superconductor is a subject of great interest for both fundamental and technological reasons. Many methods for achieving artificial pinning centers have been demonstrated, for example, with magnetic nanostructures or engineered imperfections, yielding many intriguing effects. However, these pinning mechanisms do not offer dynamic control over the strength of the patterned vortex potential because they involve static nanostructures created in or near the superconductor. Dynamic control has been achieved with scanning probe methods on the single vortex level but these are difficult so scale up. Here, we show that by applying controllable nanopatterned current injection, the superconductor can be locally driven out of equilibrium, creating an artificial vortex potential that can be tuned by the magnitude of the injected current, yielding a unique vortex channeling effect.

Research data supporting “p-wave triggered superconductivity in single layer graphene on an electron-doped oxide superconductor”

Measurements data collected in several institutions including the Racah Institute of Physics (STM data), at the Cambridge Graphene Centre (Raman spectroscopy data) and Department of Materials Science and Metallurgy (XRD and electronic transport data).