Electrodynamics of MnBi2Te4 intrinsic magnetic topological insulators

Abstract

We report on the electrodynamics of MnBi2Te4 thin films, an intrinsic magnetic topological material. We study its optical conductivity from terahertz (THz) to ultraviolet (UV) frequencies as a function of the film thickness, highlighting the presence of surface topological states superimposed on the bulk electrodynamics response. For the thinnest film, where the charge transport is dominated by Dirac surface states, we investigate the effect of the phase transition from the high-temperature topological protected state to the low-temperature magnetic (time-reversal broken) state by measuring the optical conductivity across the Néel temperature. At low temperatures, the breaking of the time reversal symmetry affects the optical conductivity, indicating that a magnetic-induced gap opens below TN.

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“…This phenomenon has been further corroborated through the previous IR studies of 7 SL MBT film [17], where the predominance of surface electrons is significantly higher. For the temperatures in the region above T N , ω Drude mostly remains unaltered with increasing temperature, similar to what has been observed in 7SL MBT film [17]. In the case of MBST, the change in ω Drude around T N looks considerably sharper compared to that of MBT (see Fig.…”

“…The addition of effective mass and lifting of the topological protection results in reduced mobility, which eventually alters their conductivity contribution into a nonmetallic nature [17,33]. This phenomenon has been further corroborated through the previous IR studies of 7 SL MBT film [17], where the predominance of surface electrons is significantly higher. For the temperatures in the region above T N , ω Drude mostly remains unaltered with increasing temperature, similar to what has been observed in 7SL MBT film [17].…”

“…The exchange gap (of an amount ∆) modifies the energy-momentum dispersion relation, which causes the massless surface electrons (given by E= ±ℏv F k) to accrue an effective mass (given by E=± (ℏv F k) 2 + (∆/2) 2 ) [5,17,33]. The addition of effective mass and lifting of the topological protection results in reduced mobility, which eventually alters their conductivity contribution into a nonmetallic nature [17,33]. This phenomenon has been further corroborated through the previous IR studies of 7 SL MBT film [17], where the predominance of surface electrons is significantly higher.…”

“…The inherent topological characteristic of MnBi 2 Te 4 enables the coexistence of gapless topological surface states (TSS) and a gapped bulk electronic band structure [10,16]. In the AFM phase, the intrinsic magnetic ordering disrupts the time-reversal symmetry, which is anticipated to trigger the opening of an exchange gap at the Dirac node [17,18]. However, this opening of the exchange gap is highly sample dependent [19], and has elicited divergent views based on their ARPES measurements [10,16].…”