Point Contact Spectroscopy On Some Super-Conducting Topological Materials

With the discovery of topological materials,more researches have focused on top-ological superconductors(TSCs)due to the existence of Majorana fermion,which is expected to play an important role in fault tolerant quantum computation.Up to now,several great achievements on TSCs have been reported based on heterostructures with the aid of superconducting proximity effect on topological materials or strong spin-orbital coupling semiconductors.Meanwhile,it is also desirable to search for intrinsic TSCs among superconducting topological materials.In this thesis,we mainly report our point-contact spectroscopy(PCS)studies on several superconducting topological materials in order to provide some evidences of possible topological superconductivity.1.For type-? Dirac semimetal PdTe2 superconductor with superconducting Tc?1.7 K,both mechanical-and soft-PCS differential conductance curves at 0.3 K show a consistent double-peak structure,which can be perfectly fitted by a single s-wave gap based on the Blonder-Tinkham-Klapwijk model.The gap follows a typical Bardeen-Cooper-Schrieffer temperature behavior,yielding ?0?0.29 meV and 2?0/kBTC=4.15 in the strong-coupling regime.A sudden suppression of the superconducting gap in a magnetic field around Hc??130 Oe is observed for most point contacts on PdTe2,characteristic of a first-order transition for type-? superconductors in field.However,for other contacts,a smooth evolution of the PCS conductance persists up to Hc2??600 Oe,signaling a local type-? superconductivity.The observed admixture of type-?and type-? superconductivity can possibly arise from an inhomogeneous electron mean free path on the surface of PdTe2 due to its topological surface states.2.In superconduting topological material PbTaSe2 with Dirac nodal line structure,we have applied the soft-PCS,mechanical-PCS,field rotational-PCS and pressure-PCS to systematically study its superconducting gap.Based on the comparison between soft-and mechanical-PCS,the existence of surface superconductivity is proposed probably from topological surface state.In rotational-PCS,we reported a characteristic in-plane 2-fold behaviour of the resistive upper critical field and point-contact spectra on PbTaSe2,despite its hexagonal lattice symmetry.The 2-fold behaviour persists up to its surface upper critical field Hc2R,even though bulk superconductivity has been sup-pressed at its bulk upper critical field Hc2HC<<Hc2R,signaling its probable surface-only electronic nematicity.In addition,we do not observe any lattice rotational symmetry breaking signal from field-angle-dependent specific heat within the resolution.It is worth noting that such surface-only electronic nematicity is in sharp contrast to the observation in the topological superconductor candidate,CuxBi2Se3,where the ne-maticity occurs in various bulk measurements.In combination with theory,supercon-ducting nematicity is likely to emerge from the topological surface states of PbTaSe2,rather than the proximity effect.The issue of time reversal symmetry breaking is also addressed.Thus,our results on PbTaSe2 shed new light on possible routes to realize nematic superconductivity with nontrivial topology.3.In topological insulator Bi2Se3,we observed the emergence of an exotic local superconductivity by applying voltage pulse on the mesoscopic point contacts.Super-conductivity is evidenced by the observation of Andreev reflection peak in point-con-tact spectra and a sudden drop of resistance in the mesoscopic four-probe resistance measurement.Nematic behavior is also present in the superconducting states,similar to the topological superconductor candidate CuxBi2Se3.More intriguingly,this kind of superconductivity can be erased by thermal cycles,achieved by warming up to high temperatures and cooling down to low temperatures again.These phenomena are only observed in Ag-Bi2Se3 type point-contacts,whereas superconductivity is absent in Au-Bi2Se3,Cu-Bi2Se3 and Ti-Bi2Se3 type point-contacts,suggesting the mechanism of emergent superconductivity in Bi2Se3 is intimately related to the intercalation of Ag atoms into the van der Waals gap of Bi2Se3.