Observation of a giant mass enhancement in the ultrafast electron dynamics of a topological semimetal

Abstract

Topological phases of matter offer exciting possibilities to realize lossless charge and spin information transport on ultrafast time scales. However, this requires detailed knowledge of their nonequilibrium properties. Here, we employ time-, spin- and angle-resolved photoemission to investigate the ultrafast response of the Sb(111) spin-polarized surface state to femtosecond-laser excitation. The surface state exhibits a giant mass enhancement which is observed as a kink structure in its energy-momentum dispersion above the Fermi level. The kink structure, originating from the direct coupling of the surface state to the bulk continuum, is characterized by an abrupt change in the group velocity by ~70%, in agreement with our GW-based band structure calculations. Our observation of this connectivity in the transiently occupied band structure enables the unambiguous experimental verification of the topological nature of the surface state. The influence of bulk-surface coupling is further confirmed by our measurements of the electron dynamics, which show that bulk and surface states behave as a single thermalizing electronic population with distinct contributions from low-k electron-electron and high-k electron-phonon scatterings. These findings are important for future applications of topological semimetals and their excitations in ultrafast spintronics.

Population Inversion and Dirac Fermion Cooling in 3D Dirac Semimetal Cd3As2

“…Considering that the coupling between electron and acoustic phonon typically leads to a much longer lifetime in Cd 3 As 2 than what is observed here, the main path for Dirac fermion cooling is through coupling to optical phonons. The electron–phonon (el–ph) coupling has been shown to play an important role for the carrier dynamics in topological materials. , It is worth noting that such carrier lifetime is much longer than the lifetime reported in 2D Dirac fermion in graphene (0.25 ps) because of the lower optical phonon energy in Cd 3 As 2…”

“…The electron−phonon (el−ph) coupling has been shown to play an important role for the carrier dynamics in topological materials. 43,44 It is worth noting that such carrier , 20,47 where C e is the electron heat capacity, T e is the transient electronic temperature, and T L is the lattice temperature. The slope of 2.1 ± 0.1 ps −1 eV −1 gives an el−ph coupling strength g = (6.2 ± 0.3) × 10 15 W K −1 m −3 , where C e = 70 J K −2 m −3 × T e , 20 T e = 450 K (extracted from the Fermi−Dirac distribution), and T L = 80 K (sample temperature).…”