2020 Abstracts TS

Ron Naaman

Spin based properties, applications, and devices are commonly related to magnetic effects and to magnetic materials or materials with large spin orbit coupling. However, we found that chiral molecules act as spin filters for photoelectrons transmission, in electron transfer, and in electron transport.
The new effect, termed Chiral Induced Spin Selectivity(CISS) [1], was found, among others, in bio-molecules and in bio-systems as well as in inorganic chiral crystals. It has interesting implications for the production of new types of spintronics devices [2], in controlling magnetization [3], and on electron transfer and conduction. Recently we also found that charge polarization in chiral molecules is accompanied by spin polarization. This finding shed new light on spin dependent interaction between chiral molecules and between them and magnetic surfaces [4].

[1] R. Naaman, Y. Paltiel, D,H, Waldeck, J. Phys. Chem. Lett., 11 (2020) 3660
[2] K. Michaeli, V. Varade, R. Naaman, D. A Waldeck, J. of Physics: Condensed Matter. 29 (2017) 103002
[3] E. Z. B. Smolinsky et al. J. Phys. Chem. Lett. 10 (2019) 1139
[4] K. Banerjee-Ghosh, et. al., Science 360 (2018) 1331

Takasada Shibauchi

The FeSe1-xSx superconductors involving non-magnetic nematic phase and its quantum criticality provide a unique platform to investigate the relationship between nematicity and superconductivity [1]. It has been shown that across the nematic quantum critical point, the superconducting properties change drastically [2,3], and the non-nematic tetragonal FeSe1-xSx (x>0.17) exhibits substantial low-energy states despite the high-quality of crystals. Here we have perform the muon spin rotation (μSR) measurements on FeSe1-xSx (x=0, 0.20, 0.22) and observed the spontaneous internal field below the superconducting transition temperature Tc, providing strong evidence for time-reversal breaking (TRSB) state in bulk FeSe1-xSx [4]. We also find that the superfluid density in the tetragonal crystals is suppressed from the expected value, indicating the presence of non-superconducting carriers. These results in FeSe1-xSx are consistent with the recently proposed topological phase transition to a novel ultranodal pair state with Bogoliubov Fermi surface [5].

[1] See, for a review, T. Shibauchi, T. Hanaguri, and Y. Matsuda, J. Phys. Soc. Jpn. (in press); arXiv:2005.07315 (2020).
[2] Y. Sato et al., Proc. Natl. Acad. Sci. USA 115, 1227-1231 (2018).
[3] T. Hanaguri et al., Sci. Adv. 4, eaar6419 (2018).
[4] K. Matsuura et al., (unpublished).
[5] C. Setty, S. Bhattacharyya, Y. Cao, A. Kreisel, and P. J. Hirschfeld, Nat. Commun.11, 523 (2020).

Ramón Aguado

Recent experimental efforts have focused on replacing the weak link in the Josephson Junction (JJ) of a superconducting qubit by electrostatically-gateable technologies compatible with high magnetic fields [1]. Such alternatives are crucial in order to reach a regime relevant for readout of topological qubits based on Majorana zero modes (MZMs) [2]. In my talk, I will focus on JJs based on semiconducting nanowires that can be driven to a topological superconductor phase with MZMs. A fully microscopic theoretical description of such hybrid semiconductor-superconducting qubit allows to unveil new physics originated from the coherent interaction between the MZMs and the superconducting qubit degrees of freedom [3]. The corresponding microwave spectroscopy presents nontrivial features, including a full mapping of zero energy crossings and fermionic parity switches in the nanowire owing to Majorana oscillations [4].

[1]Superconducting gatemon qubit based on a proximitized two-dimensional electron gas, Casparis et al, Nature Nanotechnology, 13, 915, (2018); Semiconductor-Nanowire-Based Superconducting Qubit, T. W. Larsen et al. Phys. Rev. Lett. 115, 127001 (2015); Realization of Microwave Quantum Circuits Using Hybrid Superconducting-Semiconducting Nanowire Josephson Elements, G. de Lange et al. Phys. Rev. Lett. 115, 127002 (2015)
[2] Majorana qubits for topological quantum computing, R. Aguado and Leo Kouwenhoven, Physics Today 73, 6, 44 (2020)
[3]Superconducting islands with semiconductor-nanowire-based topological Josephson junctions, J. Avila, E. Prada, P. San-Jose and R. Aguado, arXiv:2003.02852 (Physical Review B, in press)
[4] Majorana oscillations and parity crossings in semiconductor nanowire-based transmon qubits, J. Avila, E. Prada, P. San-Jose and R. Aguado, arXiv:2003.02858 (Physical Review Research, in press)

Bohm-Jung Yang

I am going to talk about the topological properties of the superconductivity that coexists with stable magnetism. In the first part of this talk, we propose a route to achieve odd-parity spin- triplet superconductivity in metallic collinear antiferromagnets with inversion symmetry. Owing to the existence of hidden antiunitary symmetry, which we call the effective time- reversal symmetry (eTRS), the Fermi surfaces of ordinary antiferromagnetic metals are generally spin-degenerate, and spin-singlet pairing is favored. However, by introducing a local inversion symmetry breaking perturbation that also breaks the eTRS, we can lift the degeneracy to obtain spin-polarized Fermi surfaces. In the weak-coupling limit, the spin- polarized Fermi surfaces constrain the electrons to form spin-triplet Cooper pairs with odd- parity. Furthermore, we find that the odd-parity superconducting states host nontrivial band topologies manifested as chiral topological superconductors, second-order topological superconductors, and nodal superconductors. In the second part, I am going to talk about topological superconductivity of spin-polarized fermions in ferromagnets. By generalizing the Fu-Berg-Sato criterion to account for higher order band topology, we show that doped nodal semimetals of spin-polarized fermions can host various types of magnetic higher-order topological superconductivity.

[1] S. H. Lee and B. -J. Yang, "Odd-parity spin-triplet superconductivity in centrosymmetric antiferromagnetic metals", arxiv:2006.15775
[2] J. Ahn and B. –J. Yang, “Higher-order topological superconductivity of spin-polarized fermions”, arXiv:1906.02709; Physical Review Research 2, 012060(R) (2020)

Katharina Franke

Magnetic impurities in conventional superconductors induce a pair-breaking potential, which leads to bound states inside the superconducting energy gap. These states are called Yu-Shiba-Rusinov (YSR) states, and can be probed by scanning tunneling spectroscopy at the atomic scale. The energy of these states depends on the strength of both exchange and potential scattering. The individual YSR states can be regarded as the building blocks for topological superconductivity in adatom chains on conventional superconductors.
Here, we explore different strategies to tune the energy of the YSR states. In the first case, we tune the strength of the magnetic exchange scattering to the Cooper pairs. Upon tip approach we are able to continuously vary the energy of YSR states induced by Fe-porphin molecules on Pb(111) across the Fermi energy. This model system further allows to study the quantum phase transition between a screened and unscreened spin [1].
In the second case, we make use of the charge-density wave (CDW) of NbSe2, which coexists with superconductivity, to tune the energy of YSR states of individual Fe atoms. All atoms are placed in the same atomic adsorption site, but at different positions with respect to the CDW. The YSR states exhibit different energies and different oscillatory patterns. We ascribe the shift in energies to the variation of the density of states as well as to changes in the potential scattering strength [2]. These results are important for designing topological nanostructures.

[1] L. Farinacci, G. Ahmadi, G. Reecht, M. Ruby, N. Bogdanoff, O. Peters, B. W. Heinrich, F. von Oppen, K. J. Franke, Phys. Rev. Lett. 121, 196803 (2018).
[2] E. Liebhaber, S. Acero Gonzalez, R. Baba, G. Reecht. B. W. Heinrich, S. Rohlf, K. Rossnagel, F. von Oppen, K. J. Franke, Nano Lett. 20, 339 (2020).

Sachio Komori

At a ferromagnet / superconductor interface, a magnetic exchange field can couple with the superconducting state. For the case of an s-wave (isotropic) superconductor, the coupling manifests as a spin-splitting of the superconducting density of states which decays in the superconductor over the Cooper pair coherence length which is tens of nanometers in Nb. In a d-wave (anisotropic) superconductor, the Cooper pair coherence length is spatially anisotropic and sub-nanometre in all directions meaning magnetic coupling is equivalently short ranged. In this talk I will present our recent experiments on investigating magnetic coupling at ferromagnet / superconductor interfaces with s-wave (Nb) and d-wave (YBa2Cu3O7) superconductors. For Nb, superconducting spin-transport based on triplet Cooper pairs is blocked in the singlet superconducting state and rapidly-suppressed in the normal state due to spin-orbit scattering [1]. In YBCO, magnetic exchange field are found to be long-ranged, penetrating tens of coherence lengths due to the quasiparticle nodal states [2]. The results demonstrate dynamic coupling between unconventional superconductivity and magnetism.

[1] S. Komori et al., arXiv:2006.16654
[2] A. Di Bernardo et al., Nat. Mater. 18, 1194 (2019)

Oded Millo

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) surface-states bound to magnetic impurities. In this work we demonstrate that similar phenomena can be induced on a surface of a conventional superconductor upon chemisorbing non-magnetic chiral molecules. By applying scanning tunneling spectroscopy, we show that the singlet-pairing s-wave order parameter of Nb, NbN and NbSe2 is significantly altered upon the adsorption of chiral polyalanine alpha-helix molecules on the surface. The tunneling spectra exhibit zero-bias conductance peaks embedded inside gaps or gap-like features, suggesting the emergence of a triplet-pairing component, corroborated by fits to theoretical spectra. Conductance spectra measured on devices comprising NbSe2 flakes over which these chiral molecules were adsorbed, exhibit, in some cases, in-gap states nearly symmetrically positioned around zero bias. These states shift apart 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 peak 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. Chiral molecules were also found to have a unique signature on the TC of Nb and NbRe films when linking Au nanoparticles to them. Finally, low-energy muon spin rotation (LE-μSR) data demonstrate clear evidence for a strong modification of the screening supercurrent distribution deep inside a Nb film upon adsorption of chiral molecules, providing evidence for unconventional chiralinduced superconductivity. The adsorption-modified local magnetic field profile inside the (65 nm thick) Nb film monitored by LE-μSR, a measure of the screening modification, is well fitted to a model calculation where the chiral molecules layer is considered as an insulating spin-active interface that is proximity-coupled to the Nb film.

The work was done in collaboration with the following groups: Yossi Paltiel, Hen Alpern, Nir Sukenik, Tamar Shapira, Shira Yochelis (The Hebrew University of Jerusalem), Jason Robinson, Harry Bradshaw (University of Cambridge), Angelo Di Bernardo, Elke Scheer (Konstanz University), Jacob Linder (Norwegian University of Science and Technology).