Session A60: Topological Materials: Thin Films and Hybrid Structures

Exploring topological superconductivity in topological insulator – superconductor hybrid devices

Topological insulators (TIs) coupled to s-wave superconductors (SC) are predicted to harbor a topological superconducting phase. In this talk, I will first talk about the SC-TI-SC junctions, where an anomalous enhancement of the critical current at low temperatures and a highly non-sinusoidal current-phase relation were observed. These results suggest superconductivity is induced in the spin-helical topological surface states and point toward nearly ballistic nature of superconducting transport. I will then introduce our recent work about the induced SC in a quantum anomalous Hall (QAH) insulator, as a potential platform for the realization of “chiral Majorana modes”. A recent transport experiment claimed the half-quantized two-terminal conductance plateau in a millimeter-size QAH – Nb hybrid structure as evidence for the chiral Majorana modes. However, there are serious concerns about this interpretation because non-Majorana mechanisms can also generate similar signatures, especially in a disordered QAH system. I will present a systematic study of the superconducting contact transparency and its influence on the two-terminal conductance measurements and provide a non-Majorana explanation for the appearance of the half-quantized plateau in the QAH – Nb hybrid devices.   

Majorana π-junctions in full-shell nanowire SQUIDs

Recent studies of InAs nanowires covered by full-shell Al superconductor showed signatures of Majorana zero modes (MZMs) around one applied axial-flux quantum [1]. Here, we study such wires in dc-SQUID geometry with a naturally-formed normal quantum dots in the Josephson junctions. In the trivial regime around zero axial-flux we find 0-π transition as the occupancy of a quantum dot is changed; the transition vanishes at one axial-flux quantum. In addition, we find that the critical currents of the Josephson junctions increase in the topological regime. Tunneling spectroscopy of a junction in the π-state reveals a discrete zero-energy state at one applied axial-flux quantum. Our observations are consistent with the theoretical models of MZMs coupling through a normal quantum dot [2,3].

[1] S. Vaitiekenas et al. arxiv:1809.05513
[2] C. Schrade and L. Fu arxiv:1809.06370
[3] J. Schulenborg and K. Flensberg. arxiv:1910.04106   

Quantum anomalous Hall effect using interfacial Green's function method for an accurate mass gap

By using a Green’s function method with DFT-based tight-binding parameters, we investigate the quantum anomalous Hall (QAH) effect at the interface between topological insulators (SnTe, SnSe) and magnetic insulators (EuS, EuSe, EuTe). QAH or axionic states, a subject of recent broad interest, are achieved by introducing an effective Zeeman field to a topological insulator whose surface Dirac cone then acquires a mass gap, resulting in exotic electromagnetic responses within the mass gap. A number of studies have demonstrated the appearance of such states by using diverse interfacial, magnetic-element-doped, and magnetic-topological systems in agreement with predictions. Although achieving a large mass gap is critical for further investigations and room temperature devices, the microscopic mechanisms determining the size of the mass gap have not been clearly addressed. In this study, we enumerate several combinations of topological crystalline insulators and magnetic insulators in a search for an optimal electronic structure, where a large mass gap is isolated inside a bulk insulating gap. The underlying mechanisms and their dependence on factors such as an external field will be discussed.   

Topological phase transition in epitaxial single layer FeTe1-xSex/SrTiO3(001)

Recent work shows evidence of topological superconductivity on the (001) surface of FeTeSe bulk crystals. In this work, we demonstrate both superconducting and topological phase transitions at higher temperature in FeTe_1-xSe_x thin films, by reducing its thickness down to one atomic layer. High quality single layer FeTe_1-xSe_x films are grown on SrTiO₃(001) substrate by molecular beam epitaxy and characterized by in situ scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy. We find the topological and superconducting properties to be strongly dependent on the Te concentration, which controls spin-orbit coupling. With increasing Te, the top of the valence band at the Γ point moves towards and then crosses the Fermi level, accompanied by a change of the band profile from parabolic to linear. Above a critical Te concentration of 80%, the linear valence band reverts back to parabolic and shifts back down below the Fermi level, characteristic of a topological phase transition. These findings and their implications for the emergence of topological phases in Fe-based superconductors at reduced dimensions will be presented at the meeting.   

Tuning the electrical properties of GdSb thin films by epitaxial strain

Early studies of rare-earth monopnictide (RE-V) thin films have focused mainly on their applications as buried ohmic contacts for III-V semiconductors, THz emitters and detectors, thermoelectrics, diffusion barriers, and plasmonic heterostructures. Recent predictions of topological semimetallic states and observations of extremely large magnetoresistance (XMR) in RE-Vs, and specifically GdSb, have opened up a new research front aimed at studying the interplay between magnetic ordering and XMR. Here we demonstrate the epitaxial growth and characterization of GdSb thin films with thickness varied from 3-60 nm and biaxial strains ranging from -2% to +2% lattice-mismatch. Utilizing x-ray diffraction, in-vacuo angle-resolved photoemission spectroscopy, SQUID magnetometry and magnetotransport measurements we map out shifts in energy bands and trends in exchange interaction parameters due to dimensional confinement and biaxial strain.   

Quantum transport in in-plane selective area InSb-Al nanowire quantum networks

Strong spin-orbit semiconductor nanowires coupled to a superconductor are predicted to host Majorana zero modes. Exchange (braiding) operations of the Majorana modes form the logical gates of a topological quantum computer and require a network of nanowires. Here, we implement an in-plane selective-area growth technique for InSb-Al allowing complex semiconductor-superconductor nanowire networks with excellent quantum transport properties. Essential quantum transport phenomena for topological quantum computing are demonstrated in these structures including phase-coherent transport up to 5 harmonics of Aharonov-Bohm oscillations with a phase coherence length of ~10 μm. Tunneling spectroscopy on hybrid InSb-Al nanowires demonstrates a hard superconducting gap, accompanied by 2e-periodic Coulomb oscillations with an Al-based Cooper pair island integrated in the nanowire network. The results, together with possible Majorana signatures, confirm the high quality of the InSb nanowire networks, holding great promise for this platform for scalable topological networks.   

Microwave spectroscopy reveals the quantum geometric tensor of topological Josephson matter

Quantization effects due to topological invariants such as Chern numbers have become very relevant in many systems, yet, key quantities as the quantum geometric tensor [1] providing local information about quantum states remain experimentally difficult to access. Recently, it has been shown that multiterminal Josephson junctions constitute an ideal platform to synthesize topological systems in a controlled manner [2]. We theoretically study properties of Andreev states in topological Josephson matter and demonstrate that the quantum geometric tensor of Andreev states can be extracted by synthetically polarized microwaves [3]. The oscillator strength of the absorption rates provides direct evidence of topological quantum properties of the Andreev states.

[1] M. Kolodrubetz et al., Phys. Rep. 697, 1 (2017).
[2] R.-P. Riwar et al., Nat. Commun. 7, 11167 (2016); J. S. Meyer and M. Houzet, Phys. Rev. Lett. 119, 136807 (2017); H.-Y. Xie et al., Phys. Rev. B 96, 161406 (2017); Phys. Rev. B 97, 035443 (2018).
[3] R. L. Klees et al., arXiv:1810.11277.   

Observation of the non-Hermitian Skin effect in topolectric circuits

A contemporary frontier of metamaterial research is the challenge non-Hermitian systems pose to the established characterization of topological matter. There, one of the most relied upon principles is the bulk-boundary correspondence (BBC): energy eigenvalues and eigenstates exhibit a perturbatively small change as the boundary conditions are modified from periodic to open by changing a single bond. The framework of BBC captures the emergence of protected surface states at the boundary of a system with non-trivial bulk topology. In this talk, we present a periodic circuit network, where gain and loss conspire with the violation of reciprocity to affect this principle dramatically. Switching from periodic to open boundary conditions results in a non-perturbative change of all eigenmodes in this system. We experimentally observe the non-Hermitian Skin effect with extensive mode localization at open boundaries for the first time.

Reference: T. Helbig, et al. arXiv:1907.11562 (2019).   

Reciprocal skin effect and its realization in a topolectrical circuit

The non-Hermitian Skin effect constitutes a new paradigm in synthetic metamaterial research. The established notion of the Skin effect requires specifically tailored gain and loss conspiring with the breaking of reciprocity. In this talk we present a model which shows extensive eigenmode localization at its boundaries while preserving reciprocity. In contrast to non-reciprocal implementations of the Skin effect requiring external energy supply, this model can be realized with exclusively passive components. We demonstrate this novel phenomenon in a passive RLC circuit network. The reciprocal Skin effect suggests itself for realizations in further metamaterial platforms with limited availability of active components.

[1] T. Hofmann, et al., arxiv:1908.02759 (2019)
[2] T. Helbig, et al., arxiv:1907.11619 (2019)
[3] T. Hofmann, et al., Phys. Rev. Lett. 122, 247702 (2019)
[4] T. Helbig, et al., Phys. Rev. B 99, 161114 (2019)
[5] S. Imhof, et al., Nature Physics 14, 925 (2018)
[6] C. H. Lee, et al., Commun. Phys. 1, 39 (2018)   

Tuning Electronic Band Properties in Binary Chalcogenides with Extrinsic and Intrinsic Strain: A First-Principles Study

We assess the relationship of the electronic band structure and atomic relaxation in Bi₂Se₃ and Bi₂Te₃. We use first-principles Density Functional theory (DFT) method to control for the influence of the exchange-correlation functional (XCF) and slab thickness. After a thorough analysis, in which we modulate the input parameters over a broad swathe of the computational space, we conclude that the GGA+vdW functional reproduces experimental results with best relative computational efficiency. In addition, we conclude that the use of experimental lattice parameters (ELP)—instead of ab initio ones—is sufficient to generate accurate descriptions of the electronic properties. After making this determination, we study the effect of doping on the electronic structure of As₂Te₃ and Bi₂Se₃, in the context of another study on the effect of biaxial strain on the same. In accordance with the first set of results, we use GGA+vdW as the XCF, and we use the ELP as the basis of both materials’ unrelaxed structure. Our results are valuable for the development of mechanisms for tuning the electronic properties of topological and trivial insulators.   

Predicting two-dimensional topological phases in Janus materials by substitutional doping in transition metal dichalcogenide monolayers

Ultrathin Janus two-dimensional (2D) materials are attracting intense interest currently. Substitutional doping of 2D transition metal dichalcogenides (TMDs) is of importance for tuning and possible enhancement of their electronic, physical and chemical properties toward industrial applications. Using systematic first-principles computations, we propose a class of Janus 2D materials based on the monolayers MX₂ (M=V, Nb, Ta, Tc, or Re; X=S, Se, or Te) with halogen (F, Cl, Br, or I) or pnictogen (N, P, As, Sb, or Bi) substitution. Nontrivial phases are obtained on pnictogen substitution of group VB (V, Nb, or Ta), whereas for group VIIB (Tc or Re), the nontrivial phases are obtained for halogen substitution. Orbital analysis shows that the non-trivial phase is driven by the splitting of M-d_yz and M-d_xz orbitals. Our study demonstrates that the Janus 2D materials have the tunability and suitability for synthesis under various conditions.   

Controlling in-gap states in graphene nanoribbons via tunable topological phases

Graphene nanoribbons (GNRs) possess distinct symmetry-protected electron topological phases that depend on structure and termination. We show, through first-principles calculations, that by applying an experimentally accessible transverse electric field, certain designer GNRs may be tuned from a topologically trivial to a nontrivial phase or vice versa. With a spatially varying field, junctions of GNRs with distinct topological phases can be created, with localized topological in-gap interface states emerging at these junctions. We further study the formation of in-gap end states inside different energy gaps around the Fermi level for a finite GNR segment, including the conditions for end state to emerge in energy gaps other than the charge-neutrality gap. This work is supported by the National Science Foundation and the Office of Naval Research under the Muri Program. Computational resources have been provided by DOE at Lawrence Berkeley National Laboratory's NERSC facility.   

Dynamic nuclear spin polarization induced by Edelstein effect at Bi(111) surfaces

Nuclear spin polarization and its Overhauser field, induced by hyperfine interaction and the Edelstein effect, were investigated by quantum transport in individual micrometer-sized Bi(111) thin film samples. A high current density was applied at low temperatures to generate a non-equilibrium carrier spin polarization in the Bi(111) surface states by the Edelstein effect and strong spin-orbit interaction, which then induced dynamic nuclear polarization by hyperfine interaction. The antilocalization magnetotransport measurements showed that as the polarization duration or the polarization current increased, the carrier quantum phase decoherence times decreased while the spin-orbit decoherence times increased, which allowed a quantification of the Overhauser field from the nuclear polarization. By using delay times between polarization and measurement, an exponential decay of the Overhauser field was observed, driven by a nuclear spin relaxation time. Application of an external magnetic field during polarization showed that the carrier spin polarization itself was sufficient to overcome dipolar interactions between nuclear spins. Comparative studies of the transport properties of Bi(111)-on-mica and Bi(111)-on-Si(111) will also be discussed.