Unconventional order parameter induced by helical chiral molecules adsorbed on a metal proximity coupled to a superconductor (original) (raw)
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Unconventional superconductivity induced in Nb films by adsorbed chiral molecules
New Journal of Physics
Motivated by recent observations of chiral-induced magnetization and spin-selective transport we studied the effect of chiral molecules on conventional BCS superconductors. By applying scanning tunneling spectroscopy, we demonstrate that the singlet-pairing s-wave order parameter of Nb is significantly altered upon adsorption of chiral polyalanine alpha-helix molecules on its surface. The tunneling spectra exhibit zero-bias conductance peaks embedded inside gaps or gap-like features, suggesting the emergence of unconventional triplet-pairing components with either d-wave or p-wave symmetry, as corroborated by simulations. These results may open a way for realizing simple superconducting spintroinics devices.
Nano Letters
Hybrid ferromagnetic/superconducting systems are well known for hosting intriguing phenomena such as emergent triplet superconductivity at their interfaces and the appearance of in-gap, spin polarized Yu-Shiba-Rusinov (YSR) states bound to magnetic impurities on a superconducting surface. In this work we demonstrate that similar phenomena can be induced on a surface of a conventional superconductor by chemisorbing non-magnetic chiral molecules. Conductance spectra measured on NbSe2 flakes over which chiral alpha helix polyalanine molecules were adsorbed, exhibit, in some cases, in-gap states nearly symmetrically positioned around zero bias that shift with magnetic field, akin to YSR states, as corroborated by theoretical simulations. Other samples show evidence for a collective phenomenon of hybridized YSR-like states giving rise to unconventional, possibly triplet superconductivity, manifested in the conductance spectra by the appearance of a zero bias conductance that diminishes, but does not split, with magnetic field. The transition between these two scenarios appears to be governed by the density of adsorbed molecules.
Proximity Effect through Chiral Molecules in Nb-Graphene-Based Devices
Advanced Materials Technologies, 2018
coupling interaction when a charge moves through a chiral-molecule (ChM) potential. [3] This interaction creates an energetic preference for one type of spin to pass, essentially acting as spin filters. Utilizing the CISS effect, simple power-efficient molecular spintronic nanoscale devices were fabricated. For example, a simple magnet-less magnetic memory device and opto-magnetic memory device were produced. [4-7] Moreover, ChMs were able to induce magnetization reversal in a ferromagnetic thin film with perpendicular anisotropy when adsorbed on its surface without the use of electrical current. [8] Nevertheless, the basic properties such as phase coherence memory of the electron and spin transfer through the molecules were not yet studied experimentally. A new approach to study charge transfer properties through organic molecules was recently suggested. [9] This approach combined superconductivity with organic molecules and normal metals (N), thus measuring the proximity-induced superconductivity in N through the organic molecules. So far, the density of states in N has been measured using scanning tunneling spectroscopy (STS). [10] Realizing a vertical device for such a junction with minimal inelastic scattering, which affects the electron phase, is not easy. Additionally, the combination of the CISS effect with superconductivity has not been studied in such a junction. Since the ChMs filter spins, [2,3] this configuration could resemble a junction between a ferromagnet and a superconductor (SC). In the vast majority of SCs (s-wave and d-wave), the electrons pair in a singlet state with spins in opposite directions. In ferromagnets, the spins are aligned; and therefore, in a junction between an SC and a ferromagnet, the proximity effect (PE) is expected to be very short range. Additionally, superconductivity decays inside the ferromagnet on a nanometer scale. In special conditions, [11-14] however, a longrange PE (comparable to that in SC-N junctions) becomes possible due to the transformation of Cooper pairs from a singlet to a parallel-spin triplet state at the interface. Since the CISS effect permits the transfer of mostly one type of spin, a similar result is expected when combining an SC with ChMs. Indeed, evidence for the emergence of triplet chiral p-wave superconductivity in Nb (a conventional s-wave SC) upon the adsorption of ChMs was recently provided by STS. [15] This was manifested in the tunneling spectra as zero-bias conductance peaks embedded in gaps or gap-like structures, spectra that could be
Interlayer pair hopping: Superconductivity from the resonating-valence-bond state
Physical Review B, 1988
Superconductivity in the high-T, materials is interpreted as (eye-k& condensation of hole bosons. Coherent interlayer tunneling of boson pairs results in order-parameter equations &which are quite Nkrent from those of the 8ardeen-Cooper-Schriefer theory. A model is presented which suggests ho~this condensation is aN'ected by chargewarrying boson interactions.
Evidence for chiral superconductivity on a silicon surface
Nature Physics
Sn adatoms on a Si(111) substrate with 1/3 monolayer coverage form a two-dimensional triangular adatom lattice with one unpaired electron per site and an antiferromagnetic Mott insulating state. The Sn layers can be modulation hole-doped and metallized using heavily-doped p-type Si(111) substrates, and become superconducting at low temperatures. While the pairing symmetry of the superconducting state is currently unknown, the combination of repulsive interactions and frustration inherent to the triangular adatom lattice opens up the possibility for a chiral order parameter. Here, we study the superconducting state of Sn/Si(111) using scanning tunneling microscopy/spectroscopy and quasi-particle interference imaging. We find evidence for a dopingdependent Tc with a fully gapped order parameter, the presence of time-reversal symmetry breaking, and a strong enhancement of the zero-bias conductance near the edges of the superconducting domains. While each individual piece of evidence could have a more mundane interpretation, our combined results suggest the tantalizing possibility that Sn/Si(111) is an unconventional chiral dwave superconductor.
Enhancement of superconducting correlation due to interlayer tunneling
Physica C: Superconductivity, 1995
Interlayer single particle tunneling between the Cu − O layers suppress the inplane short range magnetic order (which is modeled as spin density wave (SDW) insulator). Doping over the SDW state kills perfect nesting of the Fermi surface (FS) in certain directions and hence SDW gap reduces to zero in those directions of the FS. Coupling between the planes through interlayer tunneling (t ⊥ ) further suppresses the in-plane magnetic SDW-gap and hence becomes anisotropic. Superconductivity arises in the gapless regions of the FS under the 'modified spin bag' mechanism. We show that the highest T c can only be obtained for non-zero t ⊥ based on this mechanism.
Superconducting nanofilms: molecule-like pairing induced by quantum confinement
Journal of Physics: Condensed Matter, 2012
Quantum confinement of the perpendicular motion of electrons in single-crystalline metallic superconducting nanofilms splits the conduction band into a series of single-electron subbands. A distinctive feature of such a nanoscale multi-band superconductor is that the energetic position of each subband can vary significantly with changing nanofilm thickness, substrate material, protective cover and other details of the fabrication process. It can occur that the bottom of one of the available subbands is situated in the vicinity of the Fermi level. We demonstrate that the character of the superconducting pairing in such a subband changes dramatically and exhibits a clear molecule-like trend, which is very similar to the well-known crossover from the Bardeen-Cooper-Schrieffer regime to Bose-Einstein condensation (BCS-BEC) observed in trapped ultracold fermions. For Pb nanofilms with thicknesses of 4 and 5 monolayers (MLs) this will lead to a spectacular scenario: up to half of all the Cooper pairs nearly collapse, shrinking in the lateral size (parallel to the nanofilm) down to a few nanometers. As a result, the superconducting condensate will be a coherent mixture of almost molecule-like fermionic pairs with ordinary, extended Cooper pairs.
Tunneling spectroscopy of sπ pairing state as a model for FeAs superconductors
2008
We present the self-consistent Bogoliubov-de Gennes calculations of an sπ pairing state of two band superconductivity as a model for the FeAs superconductors. The sπ state is an s-wave pairing state with an internal π phase, that is, nodeless gaps on each band but with the opposite sign. The novel features of this state are investigated by calculating the local density of states of the π phase superconductor/normal metal bilayers. Because of the sign reversal between the two condensates, the zero bias conductance peak appears as observed in tunneling spectroscopy experiments on FeAs superconductors. This eliminates the major obstacle to establish the sπ state as the pairing symmetry of the FeAs superconductors.
Measurement of the Energy Gap in an Organic Superconductor: Evidence for Extremely Strong Coupling
Physical Review Letters, 1986
Point-contact tunneling is used to make what we believe to be the first measurement of a superconducting energy gap (A) in an organic superconductor. For a /3-[6/s(ethylenedithio)tetra-thiafulvalenel2Aul2 single crystal, A shows consistent field and temperature dependences, but is more than 4 times larger than weak-coupling BCS value, implying extremely strong coupling. We speculate that very low-frequency modes may be responsible.