2020 Abstracts Coherent Order

Observation of perfect Andreev reflection due to Klein paradox in a topological superconducting state

Victor Galitski

In 1928, P. Dirac proposed a new wave equation to describe relativistic electrons. Shortly afterwards, O. Klein solved a simple potential step problem for the Dirac equation and stumbled upon an apparent paradox - the potential becomes transparent when the height is larger than the electron energy. For massless particles, backscattering is completely forbidden in Klein tunneling, leading to perfect transmission through any potential barrier. Recent advent of condensed matter systems with Dirac-like excitations, such as graphene and topological insulators (TIs), has opened the possibility of observing the Klein tunneling experimentally. In the surface states of TIs, fermions are bound by spin-momentum locking, and are thus immune to backscattering due to time-reversal symmetry. Here we report the observation of perfect Andreev reflection in point contact spectroscopy - a clear signature of Klein tunneling and a manifestation of the underlying relativistic physics of a proximity-induced superconducting state in a topological Kondo insulator.
[1] S. Lee et al., Nature 570, 344-348 (2019)
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Visualizing Electrons in Moiré Superlattices: A playground for correlation and topology

Ali Yazdani

Interactions among electrons and the topology of their energy bands can create novel quantum phases of matter. The discovery of electronic bands with flat energy dispersion in magic-angle twisted bilayer graphene (MATBG) has created a unique opportunity to search for new correlated and topological electronic phases. We have developed new scanning tunneling microscopy (STM) and spectroscopy (STS) techniques to probe the nature of electronic correlations and to detect novel topological phases in two-dimensional systems, such as MATBG. Density-tuned STS studies have enabled us to study the properties of MATBG as function of carrier concentration revealing key and new properties of this novel material. These measurements establish that MATBG is a strong correlated system at all partial filling of its flat bands. [1] The strength of the interactions, which can be measured in our experiments, is found to be larger than the flat bandwidth in the non-interacting limit. We demonstrate that these interactions drive a cascade of transitions at each integer filling of these bands, creating likely the insulating states at low temperatures that are spin or valley polarized.[2] Most recently, we developed a new STS technique to detect topological phases and their associated Chern numbers and used it to show that strong interactions drive the formation of unexpected topological insulating phases in MATBG [3]. These phases, which are stabilized by a weak magnetic field, are rare examples of when topology emerges from interaction between electrons. I will describe these experiments, and other ongoing efforts, that illustrate the power of atomic scale experiments in revealing novel physics of electrons in moiré superlattices.

[1] Y. Xie. et al. Nature 572, 101 (2019).
[2] D. Wong, et al. Nature 582, 198 (2020).
[3] K. Nuckolls et al. arXiv:2007.03810, to appear in Nature.

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Poster Session

 

Poster 16 Aksel Kobialka University of Maria Sklodowska-Curie Dimerization-induced topological superconductivity in a Rashba nanowire
Poster 17 Alexey Kovalev University of Nebraska - Lincoln Magnon Landau Levels and Spin Responses in Antiferromagnets
Poster 18 Ella Lachman UC Berkeley Exchange biased Anomalous Hall Effect driven by frustration in the magnetic Kagome material Co3Sn2S2
Poster 19 Kaveh Lahabi Leiden University Generating Long-Range Spin-Triplet Supercurrents with a Single Ferromagnet
Poster 20 Piotr Majek Adam Mickiewicz University, Poznan, Poland Signatures of Majorana modes leaking into double quantum dots
Poster 21 Lev Mazov FRC "Institute of Appled Physics RAS": IPM RAS Interplay between magnetism and superconductivity in cuprates and pnictides
Poster 22 Archana Mishra MagTop, IFPAN, Warsaw Dynamical torques from Shiba states in s-wave superconductors
Poster 23 Manuel Mueller Walther Meissner Institut Temperature-dependent spin-transport and current-induced torques in superconductor/ferromagnet heterostructures
Poster 24 Risto Ojajarvi University of Jyväskylä Spin and charge currents driven by the Higgs mode in high-field superconductors
Poster 25 Andrzej Ptok Polish Academy of Sciences First-principles study of the nontrivial topological phase in chains of 3d transition metals deposited at superconducting surface
Poster 26 Quentin Remy Insitut Jean Lamour, Université de Lorraine, Nancy Control of Single Pulse All Optical Magnetization Switching of Ferromagnets
Poster 27 Peter Shen Tsinghua University Theory of topological spin Josephson junctions
Poster 28 Mikhail Silaev Jyväskylä University Large enhancement of spin pumping due to the surface bound states in normal metal/superconductor structures
Poster 29 Annika Stellhorn Forschungszentrum Jülich GmbH, JCNS-2 Interplay of proximity effects in Nb/FePd heterostructures: domain-superconductivity, spin-triplet Cooper pair generation, and the impact on the ferromagnet
Poster 30 Gaomin Tang University of Basel Magnetic field-induced “mirage” gap in an Ising superconductor
Poster 31 Dennis Wuhrer University of Konstanz Representation of antiferromagnetic eigenstates as squeezed sublattice magnon states in the presence of a static, external magnetic field

 

In Search of Majorana Pair Along The Golden Path

Jagadeesh Moodera

One of the excellent examples for the topologically driven nontrivial quantum phenomena is the prediction of Majorana zero modes (MZMs or the Majorana pair) to occur in a topological superconductor (TSC) – viz., superconducting surface state of gold. [1] MZMs form when a fermion splits in a TSC into two parts well separated in space, and thus always appear in pair together with its partner. Each of the Majorana pair is an antiparticle of itself. Potter and Lee, [1] predicted that under the right conditions, a superconducting gold nanowire with (111) crystalline surface with its large Rashba spin-orbit (S-O) splitting could host the Majorana pair. In search of this golden pair, creating the needed topological superconductivity and Zeeman field in carefully optimized Au (111) surface we laid the foundation to realize MZM. [2] We experimentally achieved the needed novel stable heterostructures, to directly observe the MZM pair using a low temperature with high vector field scanning tunneling microscope - probing the superconducting gold surface having ferromagnetic EuS nano islands (that provides the crucial internal exchange field). [3] This success opens many questions about the Majorana properties enabling plenty opportunities for future investigations. Through this two-dimensional stable metal platform, with induced superconductivity of the Shockley surface states (SS) of (111)-gold (Au), we can envision a scalable system for building non-local qubits that are intrinsically fault-tolerant. In this talk I will be presenting our path towards the observation of MZMs.

[1] A. C. Potter & P. A. Lee PRL 105, 227003 (2010); PRB 85, 094516 (2012)
[2] Peng Wei, Sujit Manna, Marius Eich, Patrick Lee and J. S. Moodera, Phys. Rev. Lett. 122, 247002 (2019)
[3] Sujit Manna, Peng Wei, Yingming Xie, Kam Tuen Law, Patrick A. Lee and Jagadeesh S. Moodera, Proc. Natl. Acad. Sci. 117 (16) 8775-8782 (Apr. 21, 2020)

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Nonlocal Spin Transport Experiments

Sebastian Goennenwein

The coupling between charge and spin transport channels provided by the spin Hall effect enables a variety of charge-driven and/or charge-detected spin transport experiments. Magnetic insulators hereby are a very attractive class of materials, since they allow for pure spin current transport, while charge currents are prohibited.
The presentation shall give an overview over non-local spin transport experiments in magnetic insulator/metal heterostructures. More specifically, I will introduce the socalled magnon mediated magnetoresistance (MMR) detected in non-local charge transport experiments in yttrium iron garnet/Pt bilayers [1,2]. I will then critically discuss the importance of magnons for the MMR, and show that the MMR concept enables studying the incoherent superposition of magnons generated by several different spin Hall active Pt metal strips. Last but not least, I will address recent MMR type experiments aimed at detecting magnon condensates.

[1] Cornelissen et al., Nat. Phys. 11, 1022 (2015)
[2] Goennenwein et al., Appl. Phys. Lett. 107, 172405 (2015)

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Topological superconductivity in van der Waals heterostructures

Peter Liljeroth

Quantum designer materials that realize electronic responses not found in naturally occurring materials have recently attracted intense interest. For example, topological superconductivity [1] - a key ingredient in topological quantum computing – may not exist in any single material. However, using designer van der Waals (vdW) heterostructures, it is possible to realize the desired physics through the engineered interactions between the different components.
We use molecular-beam epitaxy to grow islands of ferromagnetic CrBr3 [2] on a superconducting NbSe2 substrate [3]. This combines out of plane ferromagnetism with Rashba spin-orbit interactions and s-wave superconductivity and allows us to realize topological superconductivity in a van der Waals heterostructure [4]. We characterize the resulting one-dimensional edge modes using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Achieving topological superconductivity in a vdW heterostructure facilitates its incorporation in future device structures and potentially allows further control through e.g. electrostatic gating.

[1] M. Sato, and Y. Ando, Topological superconductors: a review. Rep. Prog. Phys. 80, 076501 (2017)
[2] W. Chen, Z. Sun, Z. Wang, L. Gu, X. Xu, S. Wu, C. Gao, Direct observation of van der Waals stacking–dependent interlayer magnetism. Science 366, 983 (2019)
[3] S. Kezilebieke, M.N. Huda, O.J. Silveira, V. Vaňo, J. Lahtinen, R. Mansell, S. van Dijken, A.S. Foster, P. Liljeroth, Electronic and magnetic characterization of epitaxial CrBr3 monolayers, arxiv:2009.13465 (2020)
[4] S. Kezilebieke, M. N. Huda, V. Vaňo, M. Aapro, S.C. Ganguli, O.J. Silveira, S. Głodzik, A.S. Foster, T. Ojanen, P. Liljeroth, Topological superconductivity in a designer ferromagnet-superconductor van der Waals heterostructure, arXiv:2002.02141 (2020)

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What really sets the highest possible superconducting Tc in two-dimensions?

Debanjan Chowdhury

Inspired by the discovery of superconductivity in moiré materials, I will discuss the long-standing problem on whether superconductivity can exist if the electrons’ kinetic energy is completely quenched. This is fundamentally a nonperturbative problem, since the interaction energy scale is the only relevant energy scale, and requires going beyond the traditional Bardeen-Cooper-Schrieffer theory of superconductivity. In the first part of the talk, I will discuss the problem of an interacting two-dimensional system with narrow topological bands using numerically exact quantum Monte Carlo calculations. In the second part of my talk, I will discuss the more realistic problem of magic-angle twisted bilayer graphene with electron-phonon interactions. In particular, I will show that certain umklapp processes, which arise physically as a result of the zone folding due to the moiré superlattice structure, contribute significantly towards enhancing pairing. I will comment on the effect of external screening due to a metallic gate on superconductivity and propose a smoking-gun experiment to detect resonant features associated with the phonon-umklapp processes in the differential conductance.

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Non-Hermitian topology of one-dimensional spin-torque oscillator arrays

Benedetta Flebus

Magnetic systems have been extensively studied both from a fundamental physics perspective and as building blocks for a variety of applications. Their topological properties, in particular those of excitations, remain relatively unexplored due to their inherently dissipative nature. The recent introduction of non-Hermitian topological classifications opens up new opportunities for engineering topological phases in dissipative systems. Here, we propose a magnonic realization of a non-Hermitian topological system. A crucial ingredient of our proposal is the injection of spin current into the magnetic system, which alters and can even change the sign of terms describing dissipation. We show that the magnetic dynamics of an array of spin-torque oscillators can be mapped onto a non-Hermitian Su-Schrieffer-Heeger model exhibiting topologically protected edge states. Our findings have practical utility for memory devices and spintronics neural networks relying on spin-torque oscillators as constituent units.

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Superconductivity in infinite layer nickelates – is magnetism relevant?

Harold Hwang

Since their discovery, superconductivity in cuprates has motivated the search for materials with analogous electronic or atomic structure. We have used soft chemistry approaches to synthesize superconducting infinite layer nickelates from their perovskite precursor phase, using topotactic reactions [1,2]. We will present the properties of the nickelate superconductors, our current understanding of the electronic structure [3,4], observation of a doping-dependent superconducting dome in (Nd,Sr)NiO2 [5], and preliminary evidence for substantial magnetic correlations in this system.

[1] D. F. Li, K. Lee, B. Y. Wang, M. Osada, S. Crossley, H. R. Lee, Y. Cui, Y. Hikita, and H. Y. Hwang, Nature 572, 624 (2019).
[2] K. Lee, B. H. Goodge, D. F. Li, M. Osada, B. Y. Wang, Y. Cui, L. F. Kourkoutis, and H. Y. Hwang, APL Materials 8, 041107 (2020).
[3] M. Hepting, D. Li, C. J. Jia, H. Lu, E. Paris, Y. Tseng, X. Feng, M. Osada, E. Been, Y. Hikita, Y.-D. Chuang, Z. Hussain, K. J. Zhou, A. Nag, M. Garcia-Fernandez, M. Rossi, H. Y. Huang, D. J. Huang, Z. X. Shen, T. Schmitt, H. Y. Hwang, B. Moritz, J. Zaanen, T. P. Devereaux, and W. S. Lee, Nature Materials 19, 381 (2020).
[4] B. H. Goodge, D. F. Li, M. Osada, B. Y. Wang, K. Lee, G. A. Sawatzky, H. Y. Hwang, and L. F. Kourkoutis, arXiv:2005.02847.
[5] D. F. Li, B. Y. Wang, K. Lee, S. P. Harvey, M. Osada, B. H. Goodge, L. F. Kourkoutis, and H. Y. Hwang, Physical Review Letters 125, 027001 (2020).

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Photon-assisted resonant Andreev reflections: Yu-Shiba-Rusinov and Majorana states

Felix von Oppen

Photon-assisted tunneling frequently provides detailed information on the underlying charge-transfer process. In particular, the Tien-Gordon approach and its extensions predict that the sideband spacing in bias voltage is a direct fingerprint of the number of electrons transferred in a single tunneling event. In this talk, I will focus on photon-assisted tunneling into subgap states in superconductors in the limit of small temperatures and bias voltages where tunneling is dominated by resonant Andreev processes. I will review recent experiments [1] on photon-assisted tunneling into YuShiba-Rusinov states which exhibit striking deviations from the predictions of simple Tien-Gordon theory and discuss a systematic Keldysh calculation of the subgap conductance, which provides a detailed analytical understanding of these experiment and is in excellent agreement with experiment [2]. I will conclude by extending the analysis to tunneling into Majorana bound states. Here, photon-assisted Andreev reflections provide a high-accuracy method to measure small but nonzero energies of subgap states which can be important for distinguishing conventional subgap states from Majorana bound states.

[1] O. Peters, N. Bogdanoff, S. Acero González, L. Melischek, J.R. Simon, G. Reecht, C.B. Winkelmann, F. von Oppen, K.J. Franke, Resonant Andreev reflections probed by photon-assisted tunnelling at the atomic scale, Nature Physics https://doi.org/10.1038/s41567-020-0972-z (2020)
[2] S. Acero González, L. Melischek, O. Peters, K. Flensberg, K.J. Franke, F. von Oppen, Photon-assisted resonant Andreev reflections: Yu-Shiba-Rusinov and Majorana states, Phys. Rev. B 102, 045413 (2020)

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