Session N61: Superconductivity: Strontium Ruthenate, Majorana States, and More

Extrinsic nature of the "superconducting diode effect" in hybrid superconductors

We report observation of non-reciprocal critical current in Al/InAs nanowires in the presence of magnetic fields. Non-reciprocal component does not scale with the wire cross section pointing to the extrinsic nature of the effect. Moreover, field dependence of the non-reciprocal correction resembles variation of a critical current in asymmetric rings due to formation of persistent currents. Theoretical analysis shows that a conventional assumption of constant (zero) phase difference between Al and proximatized InAs layer cannot explain experimental observations. Spatial variation of phase difference results in inter-layer Josephson currents and formation of fractional Josephson vortices. In this setting non-reciprocity emerges in the presence of magnetic field even in the absence of spin-orbit interaction. This work can be relevant to the understanding of the recently reported “superconducting diode effect” in hybrid superconductors. It is also important to a search for non-abelian excitations because Al/InAs system is one of the leading candidates to realize a synthetic topological superconductor.   

Strong Increase in Ultrasound Attenuation below Tc in Sr2RuO4

Recent experiments suggest that the superconducting order parameter of Sr₂RuO₄ has two components. A two-component order parameter has multiple degrees of freedom that can result in low-energy collective modes or the formation of domain walls, the dynamics of which can show up in sound attenuation experiments. We measured ultrasound attenuation across the superconducting transition of Sr₂RuO₄ and find that the attenuation for compressional sound increases by a factor of seven immediately below T_c, in sharp contrast with what is found in both conventional (s-wave) and high-T_c (d-wave) superconductors. Our observations appear to be most consistent with the presence of domain walls between different configurations of the superconducting state. The formation of domains naturally explains observations such as the smallness of time reversal symmetry breaking signal at T_c, and telegraph noise in critical current experiments. The fact that we observe an increase in sound attenuation only for compressional strains further suggests an inhomogeneous superconducting state formed of two distinct, accidentally-degenerate superconducting order parameters that are not symmetry-related.    

Structural and electronic behavior of quantum confined ruthenate heterostructures

Unconventional superconductivity in Sr₂RuO₄ has been widely studied over the past decades. After the prediction of majorana fermions, potential quantum information science applications in odd parity superconductors is of great interest. However, the assumption of little or no interaction between isolated RuO₂ layers has not been experimentally verified due to the lack of bulk Ruddlesden-Popper phases with varied separation between electronically active sheets. To solve this outstanding question, epitaxial heterostructures of SrRuO₃ and SrTiO₃ as a means to create artificial analogues to bulk Sr₂RuO₄ were synthesized using pulsed laser deposition. Here, single unit cells of SrRuO₃ were isolated by varying numbers of inert SrTiO₃ layers. In this talk, we report the structural and electronic properties of 1SrRuO₃/nSrTiO₃ superlattices based on the relative thickness of inert layers. These findings highlight the potential importance of interlayer coupling for the observed behavior in 2D layered superconductors.   

Optical evidence for time reversal symmetry breaking of the eutectic Sr2RuO4-Ru (3-Kelvin) phase in Sr2RuO4 crystals

Sr₂RuO₄ (SRO) is a strongly correlated metal with possible p-wave superconductivity below 1.5 K. Besides its intriguing bulk superconductivity, a filamentary superconducting phase nucleating at Tc≈3K in eutectic SRO-Ru at SRO-Ru interface, known as the 3K phase of SRO, has also attracted attention due to its interesting features. Tunneling spectroscopy studies have observed sharp zero bias conductance peak (ZBCP) in SRO-Ru below 3K, indicating possible Andreev surface bound states and unconventional pairing in the 3K phase. Recent theoretical studies suggested the onset of ZBCP is related to a second superconducting phase transition into a time-reversal symmetry breaking phase, with additional component introduced into the order parameter at a temperature below Tc. With a low temperature Sagnac scanning microscope based on a zero-loop fiber-optics Sagnac interferometer, we have performed high resolution magneto-optic Kerr effect (MOKE) measurements on SRO-Ru interface in the eutectic SRO-Ru system, at various locations away from the Ru inclusions. Evidence of time-reversal symmetry breaking below the Tc of the 3K phase and its location dependence will be presented.   

Pairing symmetry of the bulk and eutectic phases of Sr2RuO4 probed by single-particle and Josephson tunneling measurements

In addition to the ultrapure bulk phase, the growth of single crystals of Sr₂RuO₄ by the floating zone technique will result in a eutectic phase featuring islands of single crystal Ru embedded in the bulk Sr₂RuO₄. The size of the Ru island varies - ranging from a submicron to tens of microns – and the interface between Ru and Sr₂RuO₄ is atomically sharp. The interface region on the Sr₂RuO₄ side features an enhanced superconducting transition temperature (T_c) of roughly 3 K, which is larger than those of Ru and Sr₂RuO₄. Single-particle and Josephson tunneling measurements on the bulk and 3-K phases were carried out using junctions prepared by pressing thin indium wire onto the cleaved ab surface of single Sr₂RuO₄ crystals containing sparsely distributed Ru islands. Analysis of features in the tunneling spectrum and evidence of a possible mixed pairing state within the Ru island stabilized by the Joesphson effect will be presented.   

A unified theory of superconductivity and spin-density wave order in layered ruthenates

Significant interest has been paid to the ruthenate family of layered perovskites, with many recent works dedicated to understanding the superconducting (SC) state of the single layer Sr₂RuO₄, while other works have investigated the quantum criticality and spin-density wave (SDW) order observed in its sister material, the bilayer Sr₃Ru₂O₇.  In this work, we address a long-standing puzzle about the lack of superconductivity in Sr₃Ru₂O₇ despite their similar structures, and how to unify competing SC and SDW orders in this family. In previous works, we showed how interorbital spin-triplet pairings arise and their relevance to recent experimental observations in Sr₂RuO₄ [1-4].  We extend our model including both bilayer coupling and staggered octahedron rotations, and present a unified framework for these layered ruthenates. We also predict that the ideal Sr₃Ru₂O₇ without staggered octahedron rotations exhibits interorbital spin-triplet SC pairing.

 

[1] C. M. Puetter and H.-Y. Kee, EPL 98, 27010 (2012).

[2] A. W. Lindquist and H.-Y. Kee, PRR 2, 032055 (2020).

[3] J. Clepkens, A. W. Lindquist, and H.-Y. Kee, PRR 3, 013001 (2021).

[4] J. Clepkens, A. W. Lindquist, X. Liu, and H.-Y. Kee, PRB 104, 104512 (2021).   

Probing nodeless superconductivity in LaMSi (M=Ni, Pt) using muon-spin rotation and relaxation

Systems with strong spin-orbit coupling have been a topic of fundamental interest in condensed matter physics due to the exotic topological phases and the unconventional phenomenon they exhibit. In this particular talk, I will present the results of my investigation on superconductivity in the transition-metal ternary noncentrosymmetric compounds LaMSi (M=Ni, Pt) with different spin-orbit coupling strength, using muon-spin rotation and relaxation measurements. Transverse-field measurements made in the vortex state indicate that the superconductivity in both materials is fully gapped, with a conventional s wave pairing symmetry and BCS-like magnitudes for the zero-temperature gap energies. Zero-field measurements suggest a time-reversal symmetry preserved superconductivity in both the systems, though a small increase in muon depolarization is observed upon decreasing temperature. However, this has been attributed to quasistatic electronic fluctuations.   

A topological flux trap: Majorana bound states at screw dislocations

The engineering of non-trivial topology in superconducting heterostructures is a very challenging task. Reducing the number of components in the system would facilitate the creation of the long-sought Majorana bound states. Here, we uncover a route towards emergent topology in a trivial superconductor without the need for other proximitized materials. Specifically, we show that a vortex hosting an even number of flux quanta is capable of forming a quasi-one-dimensional topological sub-system that can be mapped to the Kitaev wire, as the vortex is trapped at a screw dislocation. This crystallographic defect breaks inversion symmetry and thereby threads a local spin-orbit coupling through the superconductor. The vortex-dislocation pair in the otherwise trivial bulk can harbor a pair of Majorana bound states located at the two surface terminations. We explain the topological transition in terms of a band inversion in the Caroli-de Gennes-Matricon vortex bound states and discuss favorable material parameters.   

Microwave probing of flux-tunable topological insulator-based Josephson quantum circuits

Topological superconductivity and Majorana zero modes (MZMs) are predicted to emerge in hybrid devices comprising s-wave superconductors (S) and topological insulators (TIs). Signatures of MZMs include zero-bias conductance peaks in tunnelling spectroscopy of vortex cores and the fractional Josephson effect in S-TI-S Josephson junctions. Here we explore how S-TI-S junctions integrated with superconducting microwave circuits can be used to detect MZMs in the microwave absorption spectrum probed using circuit quantum electrodynamics (cQED). We perform tight-binding simulations to study how the energy-phase relation of MZMs impacts the cQED response of flux-tuneable circuits and report the latest experimental progress towards realising such devices using V-VI semiconductor (Bi_xSb_1-x)₂Te₃ topological insulators.   

Quantum anomalous Hall effect and topological superconductivity in nanoscale magnetic topological insulators with spatially-correlated disorder

Magnetically-doped 3D topological insulators in the quantum anomalous Hall phase can host ballistic chiral edge channels. When proximitised by an s-wave superconductor, these edge states realise a topological superconductor with Majorana zero modes (MZMs), in principle without the need for large externally-applied magnetic fields. Realising MZMs, however, requires nanoscale structures of the order of the superconducting coherence length where edge states can percolate via the bulk. In this work we use tight-binding simulations to study this trade off between the breakdown of the quantum anomalous Hall phase and the Andreev coupling required for MZMs in the presence of realistic spatially-correlated electrostatic and magnetic disorder.   

Electronic instabilities at higher-order Van Hove singularity filling

Motived by the experimental discovery of higher-order Van Hove (HOVH) singularities in AV3Sb5 kagome metals, we study the instabilities in 2D systems with HOVH singularities and orbital degeneracy. In contrast to conventional Van Hove singularities, larger power-law divergent density of states, together with weaker nesting feature in higher-order Van Hove singularities, conspire to generate distinct competing instabilities. We performed unbiased renormalization group calculations and find a competition between anti-ferromagnetic and chiral superconductivity or Pomeranchuk order with initial repulsive interactions. In the square lattice with two HOVH singularities, our calculations reveal a phase transition from the $d$-wave superconductivity to the uniform charge order or$d$-wave Pomeranchuk order. In the hexagonal lattice with three HOVH singularities, a negative intra-patch interaction will yield superconductivity or Pomeranchuk instability. Implications for correlated states in AV3Sb5 kagome metals are also discussed.   

Topological Kondo Device for distinguishing Quasi-Majorana and Majorana signatures

The hybridized superconductor-semiconductor nanowire device has been predicted to host Majorana zero modes (MZMs) in it.  To confirm the Majorana signatures, significant efforts have been devoted to distinguishing between MZMs and spatially separated quasi-Majorana modes (QMMs). Because both MZMs and QMMs cause a quantized zero-bias peak in the conductance measurement, their verification task is thought to be very difficult. In this work, we proposed a simple device with a single nanowire, where the device could develop clear evidence of the topological Kondo effect in the topologically trivial phase with four QMMs. On the other hand, in the topological superconducting phase with MZMs, the transport signatures are significantly different. Therefore, our scheme provides a simple way to distinguish Majorana and quasi-Majorana modes. Besides, we believe that our device is much easier to realize the topological Kondo effect than the standard TKE devices that require at least two nanowires.