Session N21: Unconventional Superconductivity

Magnetic, superconducting, and topological surface states on FeTeSe

The idea of employing non-Abelian statistics for error-free quantum computing ignited interest in reports of topological surface superconductivity. Recently, topological surface state and Majorana zero modes (MZMs) were detected on "11" Fe-based superconductor FeTe_0.55Se_0.45. However, these properties are not observed uniformly across the sample surface. Scanning tunneling spectroscopy experiments detect coexisting vortices with and without MZM, as well as an inhomogeneous bulk superconductivity. The understanding and practical control of these electronic inhomogeneities present a prominent challenge for potential applications of topological superconductivity. 

Here, we present neutron scattering, scanning angle-resolved photoemission spectroscopy, and microprobe composition and resistivity measurements that characterize the electronic state of Fe_1+yTe_1−xSe_x. Our results establish  a phase diagram in which the superconductivity is observed only at sufficiently low Fe concentration, in association with distinct antiferromagnetic correlations, whereas the coexisting topological surface state occurs only at sufficiently high Te concentration. We find that FeTe_0.55Se_0.45 is located very close to both phase boundaries, which explains the inhomogeneity of superconducting and topological states. We thus demonstrate the degree of compositional control that is required for use of topological MZMs in practical applications.    

Signatures of emergent interfacial superconductivity in 45-degree twisted van der Waals Cuprate Superconductors

We developed a cryogenic assembly technique to fabricate Josephson Junctions with an atomically sharp twisted interface between Bi2Sr2CaCu2O8+x crystals. Transport measurements probing the interface revealed a suppressed but finite critical current close to 45 degrees. Fraunhofer patterns revealed anomalous junction effective thickness, which can be explained by a theoretical analysis that incorporates the second harmonic in the Josephson Energy versus phase relation.  We study the change of Fraunhofer patterns as we vary applied in-plane magnetic field direction, probing the current distribution in the Josephson junction. Our theoretical analysis supports the dominant second harmonic in our Josephson Junctions, indicating time-reversal broken interfacial superconductivity.   

Probing the Josephson effect in twisted nodal superconductors

Motivated by the recent proposals for unconventional emergent physics in twisted bilayers of nodal superconductors, we study the Josephson effect at the twisted interface between d-wave superconductors. We demonstrate that the critical current can exhibit a nonmonotonic temperature dependence with a maximum at a nonzero temperature as well as a complex dependence on the twist angle at low temperatures. Effects of interface inhomogeneity are also discussed and demonstrated that they can drive topological to trivial superconducting transitions. Close to 45 degree twist we find that the critical current does not vanish due to Cooper pair cotunneling, which leads to a strong second harmonic in the current-phase relation and a putative transition to a time-reversal breaking topological superconducting phase. We show how the behavior of critical current in a magnetic field and under microwave drive can yield unambiguous evidence of Cooper pair cotunneling.   

Quantum phase diagram and topological superconductivity by doping Mott insulators on the triangular lattice

Understanding the emergence of unconventional superconductivity by doping Mott insulators and its interplay with conventional orders remains a major challenge in condensed matter physics. In this work we study the extended t-J model with three-spin chiral interactions and establish the quantum phase diagram based on large scale Density Matrix Renormalization Group simulations. With increasing next nearest neighbor hopping (t') and Heisenberg spin exchange (J'), we identify both $d+id$-wave topological superconducting state and extended d-wave topologically-trivial superconducting state, by the topological spin Chern number and pairing symmetry. Through proper bond-dimension scaling, we find that the superconducting correlations demonstrate dominant quasi-long-ranged order on wider cylinders, which may lead to different superconducting states in the 2D limit. Our results suggest that the $d+id$-wave topological superconductivity can be induced by either doping a chiral spin liquid or magnetic ordered state as the hole dynamics plays an essential role in the doping-induced topological phase transition, which provides new insights for the experimental discovery of unconventional topological superconductivity.   

Quantum Geometry And Off-Diagonal Long-Range Order In Multi-band Superconductor

In addition to a conventional contribution, the superfluid density in mutli-band superconductors, has a geometric contribution associated with the inter-band matrix element of the current operator. In the weakly dispersing or flat-band limit, this geometric contribution, which is proportional to the quantum metric of the band, dominates and determines the superfluid weight. In this talk, we will discuss the geometric contribution to the off-diagonal long-range order in multi-band superconductors. Off-diagonal long-range order of the density correlation functions is a characteristic property of superfluidity of helium-3 and superconductivity of BCS wavefunctions. We calculate the density correlation function and the large eigenvalue of density matrix of multiband systems, and isolate the geometric contribution to the off-diagonal long-range order.   

A Universal Way to Engineer the Electronic Properties of Hybrid Devices.

The hybrid devices of superconductor and semiconductor nanowire (NW) with strong spin-orbit interaction (SOI) can be driven to topological regime and host Majorana zero modes (MZMs), which are the building blocks of a topological computer. Here we fabricate hybrid devices with thick/thin Al/Pb film. By well controlling the etching time, we can modify the band bending strength of interface and control the electronic properties. This is demonstrated by that the Al-InSb devices with different etching time shows quantized plateaus with varying number of modes. Moreover, high transmission of channels got by fitting the multiple Andreev reflection also proves the high quality of interface. In the thin Pb-NW devices, we observed gate tunable induced hard superconducting gap from 0 to ~1.4meV, with a suppression of conductance by three orders of magnitude. The effective g-factors, calculated through the evolution of states as a function of magnetic field, are remarkably large, e.g. several tens or even more than one hundred. This indicates the orbital effect and the strong SOI of Pb film. With the combination of pronounced enhancement of induced superconducting gap and g-factor, Pb-NW provides a better platform than Al-NW for searching for MZMs, as well as constructing topological qubit.   

Bound quasiparticle transport along the edges of superfluid 3He

Superfluid ³He-B in a container naturally consists of two virtually isolated systems: the three-dimensional bulk of the superfluid, and a two-dimensional surface layer of Andreev-bound fermions. We discovered how to drive the bound fermions out of equilibrium and expel them into the bulk, where they can be observed [1].  Here we show that the quasiparticles not energetic enough to escape to bulk flow diffusively across macroscopic distances along the surface, demonstrating non-local dynamics that conserve energy and momentum. Similar two-dimensional confinement of electrons at a low temperature has led to the discovery of a variety of quantum Hall phases. Our work thus opens a research outlook ranging from Majorana fermions to composite objects between bound quasiparticles and topological defects or bosonic bulk excitations.