The quantum anomalous Hall (QAH) effect, which has been realized in magnetic topological insulators (TIs), is the key to applications of dissipationless quantum Hall edge states in electronic ...devices. However, investigations and utilizations of the QAH effect are limited by the ultralow temperatures needed to reach full quantization—usually below 100 mK in either Cr‐ or V‐doped (Bi,Sb)2Te3 of the two experimentally confirmed QAH materials. Here it is shown that by codoping Cr and V magnetic elements in (Bi,Sb)2Te3 TI, the temperature of the QAH effect can be significantly increased such that full quantization is achieved at 300 mK, and zero‐field Hall resistance of 0.97 h/e2 is observed at 1.5 K. A systematic transport study of the codoped (Bi,Sb)2Te3 films with varied Cr/V ratios reveals that magnetic codoping improves the homogeneity of ferromagnetism and modulates the surface band structure. This work demonstrates magnetic codoping to be an effective strategy for achieving high‐temperature QAH effect in TIs.
In Cr and V codoped (Bi,Sb)2Te3 topological insulator films, the quantum anomalous Hall (QAH) effect is achieved at 300 mK, about one order of magnitude higher than that for singly Cr‐ or V‐doped ones, and the energy scale of QAH state reaches 1.4 K. The transport study of the codoped films with varied Cr/V ratios reveals the origins of enhancement.
We report experimental investigations on the quantum phase transition between the two opposite Hall plateaus of a quantum anomalous Hall insulator. We observe a well-defined plateau with zero Hall ...conductivity over a range of magnetic field around coercivity when the magnetization reverses. The features of the zero Hall plateau are shown to be closely related to that of the quantum anomalous Hall effect, but its temperature evolution exhibits a significant difference from the network model for a conventional quantum Hall plateau transition. We propose that the chiral edge states residing at the magnetic domain boundaries, which are unique to a quantum anomalous Hall insulator, are responsible for the novel features of the zero Hall plateau.
The quantum anomalous Hall (QAH) effect in a magnetic topological insulator (TI) represents a new state of matter originating from the interplay between topology and magnetism. The defining ...characteristics of the QAH ground state are the quantized Hall resistivity (ρyx) and vanishing longitudinal resistivity (ρxx) in the absence of an external magnetic field. A fundamental question concerning the QAH effect is whether it is merely a zero-magnetic-field quantum Hall (QH) effect or if it can host unique quantum phases and phase transitions that are unavailable elsewhere. The most dramatic departure of the QAH systems from other QH systems lies in the magnetic disorders that induce spatially random magnetization. Because disorder and magnetism play pivotal roles in the phase diagram of two-dimensional electron systems, the high degree of magnetic disorders in QAH systems may create novel phases and quantum critical phenomena. In this work, we perform systematic transport studies of a series of magnetic TIs with varied strength of magnetic disorders. We find that the ground state can be categorized into two distinct classes: the QAH phase and the anomalous Hall (AH) insulator phase, as the zero-magnetic-field counterparts of the QH liquid and Hall insulator phases in the QH systems. In the low-disorder limit of the QAH phase, we observe a universal quantized longitudinal resistanceρxx=h/e2at the coercive field. In the AH insulator regime, we find that a magnetic field can drive it to the QAH phase through a quantum critical point with scaling behaviors that are distinct from those in the QH phase transition. We propose that the transmission between chiral edge states at domain boundaries, tunable by magnetic disorder and magnetic fields, is the key for determining the QAH ground state.
With molecular beam epitaxy, we grew uniformly vanadium-doped Bi2Se3 films which exhibit ferromagnetism with perpendicular magnetic anisotropy. A systematic study on the magneto-transport properties ...of the films revealed the crucial role of topological surface states in ferromagnetic coupling. The enhanced ferromagnetism with reduced carrier density can support quantum anomalous Hall phase in the films, though the anomalous Hall resistance is far from quantization due to high carrier density. The topological surface states of films exhibit a gap of ∼180 meV which is unlikely to be magnetically induced but may significantly influence the quantum anomalous Hall effect in the system.
We report electromagnetic and thermomagnetic transport studies on a magnetic topological insulator thin film Cr0.15(Bi0.1Sb0.9)1.85Te3 grown by molecular beam epitaxy. The temperature and gate ...voltage dependence of the anomalous Hall effect exhibits the typical behavior of a quantum anomalous Hall insulator. The anomalous Nernst effect (ANE) shows a sign reversal when the Fermi level is tuned across the charge neutrality point of the surface Dirac cone. We show that the ambi-polar behavior of the ANE can be explained by the semiclassical Mott relation, in conjunction with the ambi-polar Dirac band structure.
With molecular beam epitaxy we have grown Cry(BixSb1-x)2-yTe3 thin films with homogeneous distribution of Cr dopants and Curie temperature up to 77 K. The films with Cr concentration y ≥ 0.39 are ...found to be topologically trivial, highly insulating ferromagnets, whose conductivity can be tuned over two orders of magnitude by gate voltage. The ferromagnetic insulators with electrically tunable conductivity can be used to realize the quantum anomalous Hall effect at higher temperature in topological insulator heterostructures and to develop field effect devices for spintronic applications.
Abstract
The quantum anomalous Hall (QAH) effect, which has been realized in magnetic topological insulators (TIs), is the key to applications of dissipationless quantum Hall edge states in ...electronic devices. However, investigations and utilizations of the QAH effect are limited by the ultralow temperatures needed to reach full quantization—usually below 100 mK in either Cr‐ or V‐doped (Bi,Sb)
2
Te
3
of the two experimentally confirmed QAH materials. Here it is shown that by codoping Cr and V magnetic elements in (Bi,Sb)
2
Te
3
TI, the temperature of the QAH effect can be significantly increased such that full quantization is achieved at 300 mK, and zero‐field Hall resistance of 0.97
h
/
e
2
is observed at 1.5 K. A systematic transport study of the codoped (Bi,Sb)
2
Te
3
films with varied Cr/V ratios reveals that magnetic codoping improves the homogeneity of ferromagnetism and modulates the surface band structure. This work demonstrates magnetic codoping to be an effective strategy for achieving high‐temperature QAH effect in TIs.
We report experimental investigations on the quantum phase transition between the two opposite Hall plateaus of a quantum anomalous Hall insulator. We observe a well-defined plateau with zero Hall ...conductivity over a range of magnetic field around coercivity when the magnetization reverses. The features of the zero Hall plateau are shown to be closely related to that of the quantum anomalous Hall effect, but its temperature evolution exhibits a significant difference from the network model for a conventional quantum Hall plateau transition. We propose that the chiral edge states residing at the magnetic domain boundaries, which are unique to a quantum anomalous Hall insulator, are responsible for the novel features of the zero Hall plateau.
The quantum anomalous Hall effect in magnetic topological insulators provides an ideal platform for building topological phases and devices. Here we construct a bilayer structure consisting of two ...magnetic topological insulator films with drastically different coercive field. In the intermediate field regime, the two quantum anomalous insulators have opposite spin orientation and counter-propagating edge states, thus realizing a synthetic quantum spin Hall phase. Multi-terminal transport measurements show that a moderate magnetic field can tune the system between chiral and helical edge state transport regimes. The interlayer transport realizes quantized spin-biased resistance, and the coupling between the two edges can be tuned by an epitaxially grown spacer layer in a sandwich structure. The tunable chiral and helical edge states in the quantum anomalous Hall bilayer may find unique applications in spintronics and topological quantum computation.
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