Session Y65: Topological Superconductivity II

STM studies of iron-based topological superconductors

FeSe_1-xTe_x (x ~ 0.5) is a unique superconductor that possesses a topological surface state. Self-proximity-induced superconductivity at the surface may effectively be of chiral p-wave where the Majorana quasiparticle is expected in the vortex core. In principle, the Majorana quasiparticle appears as a vortex bound state at precisely zero energy, so scanning tunneling microscopy/spectroscopy (STM/STS) can detect it. In reality, however, low-lying trivial vortex bound states at finite energies (~ 100 μeV) may overlap with the putative zero-energy state, making it difficult to obtain a convincing result. We developed an ultra-low temperature STM that enabled us to achieve high enough energy resolution (~ 20 μeV) to distinguish the Majorana bound state from the trivial ones [1]. We performed experiments on FeSe_0.4Te_0.6 and observed the zero-energy vortex bound state below 20 μeV, which indicates its Majorana-quasiparticle origin [2]. We found that some vortices do not host the zero-energy vortex bound state and argue that this is due to the Majorana-Majorana interaction in a disordered vortex lattice [3]. We also investigated the changes in the superconducting gap and the band structure upon Te substitution using high-resolution STS and quasiparticle-interference imaging on the single crystals in the low Te substitution regime. We found that the superconducting gap and the band structure change concomitantly inside the electronic nematic phase, suggesting that the electronic nematicity plays a minor role in the topological nature.   

Microscopic imaging of UTe2 by scanning SQUID microscopy

Unconventional superconductor UTe₂ is a newly-discovered candidate spin-triplet superconductor. A variety of exotic properties have been reported in UTe₂, such as areentrant superconducting phase at high magnetic field, multiple phase transitions in the superconducting state, chiral edge modes, and time-reversal symmetry breaking. In addition, this material is drawing attention as a candidate for a chiral superconductor.  In order to explore multiple superconducting transitions,chiral superconductivity, and particularly the possible chiral edge current in this material, we measured the local magnetic susceptibility and local magnetic flux on single crystals of UTe₂. In this talk, we discuss our results on the inhomogeneity of T_c, the superfluid density at low temperature, and the observation of spontaneous magnetic flux near the edge.   

The Effects of Annealing in UTe2

The spin-triplet candidate superconductor UTe­₂ [1] crystallizes in the Immm space group, a = ~4.16 Å, b = ~6.14 Å, and c = ~13.98 Å, and contains three crystallographic sites, one U site which occupies the 4i position and two Te sites, where Te1 occupies the 4j position and Te2 occupies the 4h position [2]. An intrinsic multicomponent superconducting order parameter has been suggested to exist in UTe₂ [3] although contrasting reports indicate that the double superconducting transitions arise due to inhomogeneity within the crystal [4-6]. Recent work has optimized T_c by lowering growth temperatures, reaching the highest reported T_c of 2.0 K. In this talk, I will discuss the evolution of superconducting transition(s) in UTe₂ as a function of annealing via thermodynamic measurements. In addition, detailed single crystal X-ray diffraction studies will highlight crystallographic differences in samples with double or single superconducting transitions to elucidate their origin.

[1] S. Ran et al. Science 365, 684 (2019)

[2] A. J. K. Haneveld et al. J. Less-Common Met. 21, 45 (1970)

[3] I. M. Hayes et al. Science 373, 797 (2021)

[4] L. P. Cairns et al. J. Phys.: Condens. Matter 32, 415602 (2020)

[5] S. M. Thomas et al. arXiv:2103.09194 (2021)

[6] P.F.S. Rosa et al. arXiv:2110.06200 (2021)   

Chiral charge order in the topological kagome superconductor RbV3Sb5

Superconductors with kagome lattices have been identified for over 40 years, with a superconducting transition temperature Tc up to 7 K. Recently, certain kagome superconductors have been found to exhibit an exotic charge order, which intertwines with superconductivity and persists to a temperature being one order of magnitude higher than Tc. In this work, we use scanning tunneling microscopy to study the charge order in kagome superconductor RbV₃Sb₅. We observe both a 2 × 2 chiral charge order and nematic surface superlattices (predominantly 1 × 4). We find that the 2 × 2 charge order exhibits intrinsic chirality with magnetic field tunability. Defects can scatter electrons to introduce standing waves, which couple with the charge order to cause extrinsic effects. While the chiral charge order resembles that discovered in KV₃Sb₅, it further interacts with the nematic surface superlattices that are absent in KV₃Sb₅ but exist in CsV₃Sb₅.   

Unusual competition between charge density wave and superconductivity in Kagome lattice CsV3Sb5

The Kagome lattice series AV₃Sb₅ (A = K, Rb, Cs) presents a variety of intriguing properties such as superconductivity, charge density wave (CDW), and a non-trivial electronic topology. Recently, a non-monotonous pressure dependence of T_c in CsV₃Sb₅ suggested an unusual competition between CDW order and superconductivity. On high quality single crystal samples, we have explored the effect of disorder induced by proton irradiation on the CDW and superconducting states of CsV₃Sb₅. Resistivity and magnetization on pristine sample exhibit two clear signatures at T_CDW ~94 K and at T_C∼3.5 K. With increasing disorder both the CDW transition and superconducting transition are suppressed. Interestingly, with complete suppression of the CDW, T_c is reduced to ~1.8 K. This is unexpected in a scenario where CDW order and superconductivity compete for the same electronic density of states as arises for instance in cuprate and dichalcogenide superconductors or in Lu₅Ir₄Si₁₀. Our experimental findings indicate significant anisotropy of the˜ superconducting gap and that the detrimental effect of CDW order on superconductivity is not as strong as previously seen in other systems.   

Locally enhanced Zeeman field in Au (111) for the creation of a topological superconductor

Recent theory modeling has shown that applying a local Zeeman field on spin-helical surface states leads to topological superconductivity and Majorana zero modes. In this talk, we experimentally demonstrate how such a local Zeeman field can be generated in Au(111)-superconductor heterostructures in which the surface Rashaba band consists of two copies of spin-helical states. Our results consist of tunneling studies of epitaxial Nb/Au(111) heterostructures using a unique tunnel barrier material at milli-Kelvin temperatures. We have observed three features of interest. First, we have achieved an enhanced surface superconducting gap - much larger than that being reported before. Second, we are able to obtain a highly transparent tunnel barrier, which allows us to observe fine tunneling features within the superconducting gap such as the Andreev bound states. Third, the tunnel barrier is shown to have a large Lande g-factor which independently tunes the surface superconducting gap with respect to the bulk gap.

Acknowledgements: NSF CAREER DMR-2046648, NSF C-ACCL 2040620, MIT-Lincoln Lab Line fund; NSF C-Accel Track C grant 2040620, MIT-Lincoln Lab Line Proposal Fund, NSF CAREER grant 2046648   

Magnetic Substitution in j=3/2 Superconductor YPtBi

The superconducting state of half-Heusler topological semimetal YPtBi promises exciting possibilities. Due to the strong spin-orbit coupling leading to band inversion and j=3/2 quasiparticles, Cooper pairs may support quintet (j=2) and septet (j=3) pairing states, beyond the singlet (j=0) and triplet (j=1). Penetration depth experiments have established nodal superconductivity in YPtBi. Here, we present a study of the effect of magnetic-ion Nd substitution on the superconductivity of YPtBi in Y1−xNdxPtBi samples. Scattering in nodal superconductors can destroy the superconducting gap. Magnetic scattering is time reversal symmetry breaking and can destroy even non-nodal superconducting gaps. Our experiments show that, despite magnetic scattering due to the Nd substitution (up to x = 0.48), the superconductivity in YPtBi survives with less than 5% reduction in the transition temperature in multiple samples. These surprising results indicate that the nodal superconducting state in YPtBi is robust under magnetic scattering and suggest an exotic nature of superconductivity in YPtBi. Contemporary proposals for a superconducting state for YPtBi discuss more than one pairing channel in the form ∆(k) = F1(k) + iF2(k) which may offer protection against magnetic pair-breaking, as observed in our experiments.   

Competing Vortex Topologies in Iron-based Superconductors

We establish a new theoretical paradigm for vortex Majorana physics in the topological iron-based superconductors (tFeSCs). While tFeSCs are widely accepted as an exemplar of topological insulators (TIs) with intrinsic superconductivity, our theory implies that such common belief could be oversimplified. Our main finding is that the normal-state bulk Dirac nodes, usually ignored in TI-based Majorana theories for tFeSCs, will play a key role of determining the vortex state topology. In particular, the interplay between TI and Dirac nodal bands will lead to multiple competing topological phases for a superconducting vortex line in tFeSCs, including a new hybrid topological vortex state that carries both Majorana bound states and a gapless dispersion. Remarkably, this hybrid vortex phase generally exists in the vortex phase diagram for our minimal model for tFeSCs and is directly relevant to tFeSC candidates such as LiFeAs. In the presence of lattice symmetry breaking, the hybrid vortex gets topologically trivialized and becomes Majorana-free, which naturally explains the puzzle of ubiquitous trivial vortices observed in LiFeAs. Our theory sheds new light on theoretically understanding and experimentally engineering Majorana physics in high-temperature iron-based systems.   

Local tunneling noise spectroscopy of iron-based superconductor FeTe0.55Se0.45: a test for Majorana character of the vortex zero modes

Majorana zero modes are intriguing, exotic quasiparticles in condensed matter physics and are predicted to enable fault-tolerant quantum computation. Recently, quite a few experimental platforms, including nanowires, atom chains, and superconducting vortices, have shown promising signatures of Majorana zero modes, but the evidence generally remains phenomenological: a peak in differential conductance exactly at the zero-bias voltage. In the putative Majorana platform, the iron-based superconductor FeTe_0.55Se_0.45, additional evidence^[1] stems from the saturation of differential conductance but remains controversial. Theoretical work^[2] has shown that an unambiguous experimental test for the Majorana character is possible using local noise spectroscopy. In this talk, I will present scanning tunneling noise spectroscopy experiments^[3] in the vortex cores of FeTe_0.55Se_0.45 and answer the question of whether the zero-energy modes have the Majorana character. 

[1] S. Zhu, et al., Science, 367, 189 (2020).

[2] C. J. Bolech and E. Demler, PRL 98, 237002 (2007); J. Nilsson, A. R. Akhmerov, and C. W. J. Beenakker, PRL 101, 120403 (2008); A. Golub and B. Horovitz, PRB 83, 153415 (2011).

[3] K. M. Bastiaans, et al., RSI 89, 093709 (2018); K. M. Bastiaans, et al., PRB 100, 104506 (2019).   

Interplay between electronic correlations and spin-orbit coupling for topological superconductivity of iron chalcogenide

We investigate the bulk electronic structure and corresponding topological surface Dirac cone of an iron chalcogenide, FeSe_0.5Te_0.5, in the framework of the combination of linearized quasiparticle self-consistent GW and dynamical mean field theory (LQSGW+DMFT) with spin-orbit coupling. It is shown that the present method predicts the bulk electronic structure and the surface Dirac cone of FeSe_0.5Te_0.5 in good agreement with angle resolved photoemission experiments. [1,2,3,4] We confirmed that the interplay between electronic correlations and spin-orbit coupling are of great importance for the correct description of non-trivial Z₂ and the excitation spectrum of FeSe_0.5Te_0.5. In particular, the Hund-coupling induced band renormalization is a decisive factor for the surface topological superconductivity in the un-doped system by locating the surface Dirac cone at the chemical potential. [4]

 

[1] H. Lohani et al., Phys. Rev. B 101, 245146 (2020)

[2] H. Miao et al., Phys. Rev. B 98, 020502 (2018)

[3] P. Johnson et al., Phys. Rev. Lett. 114, 167001 (2015)

[4] P. Zhang et al., Science 360, 182 (2018)   

Josephson Effects of the Monopole Superconductor

As an exotic type of three-dimensional topological superconductors, monopole superconductors arise from the non-trivial Berry phase of Cooper pairs. Its unconventional pairing symmetry is characterized by monopole harmonic functions and can be potentially realized in doped magnetic Weyl semimetals. To have clear understanding of its pairing properties, the phase sensitive transport measurement is crucial. We study the Josephson effects between monopole superconductors and between a monopole superconductor and an s-wave superconductor. For a Josephson junction coupling two identical monopole superconductors, we find that its behavior is similar with that in a junction coupling two identical chiral p-wave superconductors. In sharp contrast, the Josephson junction that couples two monopole superconductors related by time reversal symmetry exhibits an additional π-shift contribution from a part of its Fermi surfaces in its current phase relation. For a Josephson junction coupling a monopole superconductor and an s-wave superconductor, its current phase relation exhibits 2π-periodicity, which is apparently different from that with π-periodicity for the junction between chiral p-wave and s-wave superconductors. Combining the results above, it provides a distinguishing feature to identify monopole superconductors in experiments.

This work was supported as part of the Institute for Quantum Matter, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0019331.