Abstract
Topological materials have been one of the main research topics in condensed matter physics over the past decades. Their robust boundary states not only can produce important quantum phenomena, e.g., quantum spin Hall and quantum anomalous Hall effects, but also have promising application potential in high-performance electronics and quantum computing, etc. Various topological materials have been predicted and identified, such as topological insulators, topological semimetals and topological superconductors. Among them, topological uperconductors are highly desired due to their potential for fault-tolerant topological quantum computation. The combination of the boundary states in topological materials and superconducting pairing is a major way to search for topological superconductors. PdTe2 has been reported to be a type-II Dirac semimetal which holds multiple topological boundarystates and exhibits superconductivity at low temperatures. But up to now, no evidence of topological superconductivity in PdTe2 have been reported. In this thesis, we have performed systematic low-temperature transport study of PdTe2, aiming to search for any transport signatures of unconventional superconductivity in PdTe2.
Although magnetic properties of PdTe2 have been explored intensively, the transport study is still not sufficient, especially in the investigation of the transport properties of topologically nontrivial bands. We first study the SdH oscillations in PdTe2, revealing the contributions from two topological bands. In addition, the measured 𝑇4 power law of resistivity suggests the dominant role of longitudinal electron-phonon scattering at low temperatures, which might be essential to the emergence of superconductivity in PdTe2. Together with a two-band model analysis of the Hall effect, our experiment greatly deepens our understanding of the transport properties of PdTe2.
Till now, almost all previous studies point to the existence of conventional superconductivity in PdTe2 below its superconducting transition temperatures (𝑇𝑐), which is at odd with the coexistence of topologically nontrivial states and superconductivity in PdTe2. But in our work, we have observed obvious critical current oscillations in perpendicular magnetic fields due to the fluxoid quantization, suggesting the existence of edge superconductivity in superconducting PdTe2. A comparative study of the superconducting properties of the edge and bulk of PdTe2 further provides direct evidence for the intriguing edge superconductivity. More interestingly, the analysis of the flux penetration area reveals that the edge superconductivity appears not only at the edge of the PdTe2 flake, but also along the interface between PdTe2 and the Au electrode, highlighting the highly nontrivial character of the edge superconductivity. All these results are believed to imply the presence of unconventional superconductivity in PdTe2 below 𝑇𝑐.
Besides the intrinsic superconducting properties, we have also studied the transport properties of Nb-PdTe2-Nb Josephson junctions above 𝑇𝑐. It’s found that the superconducting proximity effect can induce superconductivities in the surface and bulk of PdTe2. By analyzing the obtained supercurrent interference patterns of the edge-touched and -untouched Josephson junctions, we reveal the flowing of dissipationless supercurrents through the left and right edges of the flake. These two edge supercurrents dominate over the bulk ones at low temperatures and are not symmetric with each other. But with increasing temperatures, the edge contribution to the supercurrents diminishes gradually and merge eventually with the bulk supercurrent. The emergence of edge Josephson supercurrents above 𝑇𝑐, together with their peculiar evolution with temperatures, demonstrates the exotic physics of the boundary states of PdTe2, which might be important to the realization of artificial topological superconductivity based on Josephson junctions.
In summary, we have performed a systematic transport study of PdTe2, a superconducting type-II Dirac semimetal, revealing various interesting phenomena relevant to the physics of unconventional superconductivity. All the results collectively indicate that PdTe2 can be a promising platform to search for topological superconductivity.
