Ultrafast dynamics of SrCd₂Sb_2-xAs_x single crystals studied by optical pump-probe spectroscopy

Topological materials exhibiting symmetry-protected surface states such as topological insulators have recently gained attention due to its unique physical properties and potential applications. In this context, the present study utilizes AB₂X₂ type SrCd₂Sb_2-xAs_x single crystals to study the ultrafast dynamics of carriers and phonons to gain insight into critical changes in crystal structure through chemical substitution. Here, we use optical pump-probe (OP-OP) spectroscopy on the as-synthesized SrCd₂Sb₂, SrCd₂As₂, and As (x=0.1) doped SrCd₂As_0.1Sb_1.9 single crystals. The transient reflectivity (ΔR/R) curve shows similar fitting behavior and paramters for SrCd₂Sb₂ and SrCd₂As_0.1Sb_1.9 which is in contrast to SrCd₂As₂. The significant observations from the OP-OP measurements include, (i) absence of sub-ps decay component in SrCd₂Sb₂, (ii) appearance of ∼10 ps decay term (τ₃) for SrCd₂Sb₂, and (iii) the change of sign for the amplitude (A₂) of several ps τ₂ decay term. The results revealed contrast ultrafast dynamics between SrCd₂As₂ and SrCd₂Sb₂ thereby implying that SrCd₂As₂ and SrCd₂Sb₂ have distinct electronic structures near the band edge. Further, we will carry out OP-OP measurements for the whole doping range (x=0.1 to 2.0) in SrCd₂Sb_2-xAs_x to elucidate the evolving electronic band structure and to observe the desired transition in ultrafast spectroscopy around x = 1.0.