Department of Magnetic Reseach Seminar
Microsoft Teams
Quantum anomalous Hall effect in V-doped (Bi,Sb)2Te3
Kajetan Fijałkowski MSc
University of Würzburg
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Streszczenie:
When the structural parameters of a magnetic topological insulator V (or Cr)-doped (Bi,Sb)2Te3 material are carefully optimized, at sufficiently low temperatures this ferromagnetic material is known to exhibit the quantum anomalous Hall effect [1], characterized by conduction through a single dissipationless chiral edge channel, even at zero external magnetic field. This perfect electronic transport quantization was quickly recognized as a promising platform for quantum metrology, as a zero-field quantum resistance standard. Metrologically comprehensive experiments reveal a great precision of the anomalous Hall resistance quantization in our films [2].
Metrological level precision of quantum anomalous Hall resistance quantization at zero magnetic field so far remains limited to temperature of the order of 20 mK, while the Curie temperature in V/Cr-doped (Bi,Sb)2Te3 is of the order of 20 K. The reason for this large discrepancy in temperature scales is one of the biggest open questions surrounding the effect in the community. By carefully analyzing the non-local signals on a novel multi-terminal Corbino geometry, we show that the chiral edge channel transport persists in the material at zero magnetic field up to the Curie temperature of bulk ferromagnetism in the material. Our analysis shows that the thermally activated bulk conductance is responsible for this ultra-low temperature transport quantization breakdown [3].
Finally, careful analysis of the underlying fundamental physics reveals the existence of two distinct types of quantum anomalous Hall states, related to the systems' dimensionality. Both regimes are experimentally accessible by changing the layer thickness. Thinner films exhibit a conductivity tensor flow diagram equivalent to that of a two-dimensional electron gas, implying a fundamentally two-dimensional origin of the effect. When the film thickness is increased, a transition to the three-dimensional regime is observed. In the three-dimensional limit, the conductivity scaling changes to the one expected for electronic transport on two parallel topological interfaces, encapsulating a volume of distinct topology. This three-dimensional bulk supports axion electrodynamics, revealing the existence of an additional term in the Maxwell's equations, and a quantum state called an "axion insulator" [4,5].
[1] C.-Z. Chang, J. Zhang et al., Science 340, 6129 (2013)
[2] M. Goetz, K. M. Fijalkowski et al., Applied Physics Letters 112, 072102 (2018)
[3] K. M. Fijalkowski, N. liu et al., Nature Communications 12, 5599 (2021)
[4] S. Grauer, K. M. Fijalkowski et al., Physical Review Letters 118, 246801 (2017)
[5] K. M. Fijalkowski, N. liu et al., Physical Review B 103, 235111 (2021)
