Observation of the Quantum Anomalous Hall Insulator to Anderson Insulator Quantum Phase Transition and its Scaling Behavior

Abstract

Fundamental insight into the nature of the quantum phase transition from a superconductor to an insulator in two dimensions, or from one plateau to the next or to an insulator in the quantum Hall effect, has been revealed through the study of its scaling behavior. Here, we report on the experimental observation of a quantum phase transition from a quantum-anomalous-Hall insulator to an Anderson insulator in a magnetic topological insulator by tuning the chemical potential. Our experiment demonstrates the existence of scaling behavior from which we extract the critical exponent for this quantum phase transition. We expect that our work will motivate much further investigation of many properties of quantum phase transition in this new context. DOI: 10.1103/PhysRevLett.117.126802 The quantum anomalous Hall (QAH) state is a topological quantum state displaying quantized Hall resistance and zero longitudinal resistance, similar to the quantum Hall (QH) state. However, while the QH effect requires a large external magnetic field, the origin of the QAH effect is the exchange interaction between electron spin and magnetic moments in ferromagnetic materials, and thus can occur even in the absence of an external magnetic field [1][2][3]. Ever since the discovery of the QAH effect in magnetically doped ðBi; SbÞ 2 Te 3 films [2][3][4][5][6][7], intensive research effort has focused on the global phase diagram and the novel transport properties of this effect, including nonlocal transport [6,8], zero Hall conductance plateau [9][10][11], delocalization behavior [7], and giant anisotropic magnetoresistance [12]. These experimental observations reveal an equivalence between the QAH effect and QH effect in terms of their topological nature.In the QH effect, the appearance of stable quantized Hall plateaus is intimately tied to the Anderson localization of bulk carriers of a two dimensional electron gas (2DEG) under a strong external magnetic field. The quantum phase transition (QPT) point between two Hall plateaus (known as plateau to plateau transition) is marked by the emergence of an extended state, which can be viewed, for a smoothly varying disorder, as the transition where the QH droplets of a state with one Chern number in the background of a QH state of another Chern number begin to percolate. Since this percolation process involves quantum tunneling between nearby chiral edge modes encircling individual droplets, it leads to a different critical exponent from that describing a classical percolation transition [13][14][15]. The QPT between an insulator and ν ¼ 1 plateau belongs to the same universality class as the plateau to plateau transition, and serves as the prototype for the QPT studied in the present work. In the QAH effect, one may expect a similar percolation picture for chiral edge states around magnetic domains at the quantum critical point. In the experiment by Checkelsky et al. [7], evidence for quantum criticality and delocalization was shown while approaching the QAH state, but the scaling behavior was...

Systematics of electronic and magnetic properties in the transition metal doped Sb2Te3 quantum anomalous Hall platform

“…However, several outstanding problems remain and make the QAHE one of the presently most investigated topics of TI research [9][10][11][12]. For example, the nature of magnetic order in these systems is not well understood.…”

“…For example, the nature of magnetic order in these systems is not well understood. Furthermore, although a precise quantization of the Hall conductivity is achieved, a small but nonzero and unexplained dissipative longitudinal conductivity remains detectable [9,10,[13][14][15]. Most importantly, the effect has so far been observed only at ultralow temperatures (typically <100 mK), i.e., well below the Curie temperature of the respective magnetic TI material.…”

Effects of Random Domains on the Zero Hall Plateau in the Quantum Anomalous Hall Effect

“…Plateau transitions between different quantized Hall conductances feature topological properties of QAH effect and have attracted a lot of attention recently [2,[8][9][10][11][12][13]. It was experimentally observed that the critical behaviors of plateau transitions between quantized Hall conductance (±e 2 /h) and zero Hall conductance in the QAH effect is qualitatively consistent with those of quantum Hall effect [10][11][12][13]. However, its critical exponent [11][12][13] is deviated from the universal value κ = 0.42 in the quantum Hall effect [14,15].…”

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