Session W42: Superconductivity in Topological Systems

Superconducting proximity effect in topological Dirac materials

Inducing superconductivity in topological materials stimulates the formation of novel quantum states of matter, including the Majorana zero mode. Besides the original prediction in 3D topological insulators, the notion of topological phases has been generalized to different dimensions and extended to the higher-order states. 

In the last few years, our research has demonstrated the possibility of realizing the topological superconductivity in engineered 3D topological insulators [1], 3D Dirac semimetals [2,3], and their 1D hinge states. Particularly, Cd₃As₂ is predicted to be a higher-order topological semimetal, possessing three-dimensional bulk Dirac fermions, two-dimensional Fermi arcs [4], and one-dimensional hinge states [5] or non-Hermitian states [6]. These topological states have different characteristic length scales in electronic transport. We show that the superconducting proximity effect can also be a sensitive probe for distinguishing these states.   

Measuring Induced gap of InAs/NbTiN using quantum point contacts in Integer Quantum Hall regime

Indium Arsenide (InAs) near surface quantum wells are ideal for the fabrication of semiconductor-superconductor heterostructures given that they allow for a strong hybridization between the two-dimensional electron gas (2DEG) in the quantum well and the states in the superconductor. The possibility to drive the 2DEG into different quantum Hall (QH) states, combined with the strong hybridization between 2DEG and superconductor, makes possible, in principle, the realization of non-Abelian anyons with richer statistics than Majorana modes.  We introduce NbTiN-InAs in a strong magnetic field as a potential candidate platform to realize non-Abelian anyons, and study the gap induced via the proximity effect, in strong magnetic fields. To quantify the QH-edge-states’ pairing correlations induced by proximity, we  fabricate a quantum point contact (QPC) at the interface between the superconductor and the 2DEG. By varying the gate voltage of the QPC we control the number of QH edge modes which can get in close proximity of the superconductor, and by measuring the QPC conductance we extract the strength of the edge modes’ pairing correlation.   

Data-driven identification of connate topological superconductor candidates

One way to achieve topological superconductivity is through identifying a bulk s-wave superconductor that also exhibits topological surface states. The resulting “connate” topological superconductor functions through the self-proximity effect similar to the interfacial proximity effect within topological superconductor heterostructures. While non-trivial electronic structure topology can be predicted through first-principles calculations, superconductivity is difficult to predict a priori, limiting broad screening for materials that combine both phenomena. Here, we present a data-driven approach to compile a catalog of potential connate topological superconductor materials, starting with experimentally-confirmed superconductors and cross-referencing their topological nature via high-throughput computational data. Additionally, subtle pitfalls in the calculation of Z₂ topological invariants for materials without well-defined band gaps (such as superconductors) will be discussed.   

Monolayer WTe2 superconductivity from repulsive interactions

Superconductivity has been observed in WTe₂. However the exact nature of the order parameter is yet unclear. Furthermore, it has been proposed that with a p-wave pairing, the material is a second-order topological superconductor with a pair of Majorana zero modes at the corners of a finite sample. We show that the desired p-wave state arises naturally as a consequence of the Kohn-Luttinger mechanism. We further study the behaviour of the system in response to external in-plane magnetic fields. We find an enhancement to the critical temperature that depends on the direction of the magnetic field.   

Understanding disorder in InAs quantum wells on metamorphic III-V buffer layers for topological Josephson junction

Semiconductor and superconductor heterostructures are prime candidates for quantum devices based on mesoscopic and topological superconductivity. In these structures, the semiconductor and the superconductor should be placed near each other in the lattice to form an ohmic contact. InAs heterostructures that have a narrow bandgap with the Fermi level close to the conduction band are a prime candidate. Here, we present the study of the structural properties of InAs quantum well – Al superconducting layer heterostructures grown by molecular beam epitaxy. We report the modification of the growth conditions of the metamorphic buffer layer and quantum well active region and study disorder using various techniques. We extract background impurity, surface impurities, alloy scattering and surface roughness as a function of growth conditions. We find samples that have high indium content in their graded buffer layer show that the surface roughness decreases with the layer growth temperature. In addition, roughness along certain directions shows a higher mobility than those with anisotropic roughnesses. The growth temperature of the quantum wells was also studied at the range of 445°C to 465°C and its effect on the 2DEG mobility will be presented. A thin capping layer of 1 - 6 nm of In0.81Al0.19As was used to study the effect of surface scattering on the quantum well and the mobility as a function of its thickness.

    

α- and β-phases of Sn thin films grown on GaSb(001) surface by molecular beam epitaxy

Tin (Sn) is known to show distinct phases depending on temperature and pressure. The cubic α-Sn is stable below 13.7 C for bulk (higher for thin films), while tetragonal β-Sn is well known metallic phase present at room temperature and shows superconductivity at 3.7 K. Recently, Sn is gaining interest as a unique and promising candidate because of various topological phases of α-Sn by tuning strain, orientation, and thickness configurations as well as excellent superconducting features of β-Sn for quantum devices. We investigate Sn thin films with different α- and β-phases on GaSb(001) surface using molecular beam epitaxy with active liquid Nitrogen cooling as well as by first-principles calculations. We observed β-Sn(110), β-Sn(001), and α-Sn(110) phases with varying growth temperatures for the same film thickness (40 nm). Transport measurement revealed superconductivity for β-Sn(001) phase whereas no superconductivity and large weak localization-like features were seen down to 0.04 K for β-Sn(110) phase. We also study the crystal structure with temperature-dependent Raman spectroscopy. This work highlights a strong connection among novel electronic phases and structure metastability that is critical for controlling the topological and superconducting nature of Sn.   

Superconducting properties of Ta3Sb and Ti3Sb, A15 compounds which are candidate topological superconductors

We present a study of the superconducting properties of polycrystalline Ti₃Sb (T_c ~ 5.9 K) and Ta₃Sb (T_c ~ 0.67 K), candidate topological superconductors of the A15 family. Via magneto-transport, magnetization and calorimetry measurements, we determine the superconducting phase diagram for both compounds and find large Ginsburg-Landau parameters for both Ti₃Sb (κ = 55) and Ta₃Sb (κ = 90) , identifying them as extremely type-II superconductors. Calorimetry and London penetration depth measurements at ³He temperatures are consistent with a full BCS-like superconducting gap in both compounds.   

Probing for topological signatures in phase-controlled multiterminal Josephson junctions

Andreev bound states in SNS junctions depend periodically on the Josephson phase φ between two superconductors, resembling the periodic dispersion of electron energy levels in a crystal lattice. A normal metal connected to n superconductors corresponds to a n-1 dimensional crystal. It has been predicted that the resulting “band structure” of such multiterminal Josephson junction devices may be topological [1]. In the case where the normal metal is diffusive, the quasiparticle density of states at the Fermi energy varies periodically with the phases, with gapped regions separated by regions where the gap completely closes.  The dependence of the density of states can be probed by measuring the resistance of suitably designed diffusive devices [2].  We report on the fabrication and measurement of 3 terminal diffusive and ballistic devices, using a sample design that permits us to vary the two distinct phases independently.   

Topological Aspects of Electron-Phonon Coupling

Electron-phonon coupling is crucial for various quantum phases, including superconductivity and charge-density wave. We revealed the topological aspects of the electron-phonon coupling strength. In particular, we derived topological lower bounds for the total electron-phonon coupling strength in certain systems with Fermi surfaces. Implications on other systems will also be discussed.   

Pair Density Waves and Superconductivity in the π-flux Hofstadter Model using Mean-Field and Renormalization Group Analyses

The π-flux lattice, or equivalently the Hofstadter model with magnetic flux of half of the flux quantum per unit cell, has historically attracted attention in the context of the pseudogap phase of cuprate superconductors and the study of systems with Dirac fermions, located at half filling in the π-flux lattice. Recently it has become possible to realize the π-flux lattice in cold atomic systems and moiré materials. In our work we consider the model away from half-filling and study the possibility of weak coupling instabilities in the presence of extended Hubbard interactions occuring around the less-studied fillings corresponding to Van Hove singularities. Using both mean-field and renormalization group calculations, we find that paring instabilities, including s-wave and d-wave uniform superconductors (SC) and a triplet pair density wave (PDW), in general win over competing spin density wave instabilities. Of particular interest is the PDW phase, for which some evidence has been seen in cuprates. This phase is characterized by spontaneous formation of Cooper pairs with total non-zero momentum without an explicit breaking of time reversal symmetry, as opposed to the FFLO state. We find that the triplet PDW phase is realized in the π-flux lattice with repulsive on-site interactions together with a moderately small nearest-neighbor attractive interactions, making it the first model in which a PDW phase is realized at weak coupling as a symmetry-enforced logarithmic instability.