Topological insulators doped with transition metals have recently been found to host a strong ferromagnetic state with perpendicular to plane anisotropy as well as support a quantum Hall state with edge channel transport, even in the absence of an external magnetic field. It remains unclear however why a robust magnetic state should emerge in materials of relatively low crystalline quality and dilute magnetic doping. Indeed, recent experiments suggest that the ferromagnetism exhibits at least some superparamagnetic character. We report on transport measurements in a sample that shows perfect quantum anomalous Hall quantization, while at the same time exhibits traits in its transport data which suggest inhomogeneities. We speculate that this may be evidence that the percolation path interpretation used to explain the transport during the magnetic reversal may actually have relevance over the entire field range. 73.43.Fj, 75.45.+j, 75.50.Pp The recent report on the experimental observation of a quantum anomalous Hall effect (QAHE) in Cr-doped (Bi,Sb) 2 Te 3 [1] generated significant interest in this material system for its potential as a magnetic topological insulator and as a test bed for the study of the Quantum Hall effect without the need for an external magnetic field [2][3][4][5][6]. This original report showed that the anomalous Hall contribution [1] appeared to saturate to a value of one conduction quantum as the sample was cooled to mK temperatures, but did not yet provide evidence that the transport takes place in edge states. In order to convincingly verify that the transport takes place in quantum Hall like edge states, non-local geometries are required. Such measurements were first reported in [3], albeit in configurations where the signals were very small, and where their interpretation required invoking some loss mechanism in the edge channels, and subsequently in [4], where convincing evidence of edge state transport was reported. This last paper also observed some unusual temperature and sweep rate related features in their data, which were at least in part interpreted as additional cooling through adiabatic demagnetization mechanisms. Shortly after the first reports on Cr-doped layers, it was discovered by the Moodera group [5-7] that using V instead of Cr appears to lead to more reproducible samples with a more robust magnetic and quantum anomalous Hall state. Using this material system, the authors were able to reproduce both precise quantization of the Quantum Hall state [5], and unequivocal evidence of edge state transport [6]. While the described quantum anomalous Hall phenomenology is now well established, its microscopic origin remains much less clear. The proposed mechanism for the QAHE is the breaking of time reversal symmetry by a perpendicular to plane internal magnetic field which leads to the reversal of the band inversion of one of the two spin species in a ferromagnetic two dimensional topological insulator [1,8]. The origin of the ferromagnetic state in the original paper [1] w...
A quantum anomalous Hall (QAH) insulator coupled to an s-wave superconductor is predicted to harbor chiral Majorana modes. A recent experiment interprets the half-quantized two-terminal conductance plateau as evidence for these modes in a millimeter-size QAH-niobium hybrid device. However, non-Majorana mechanisms can also generate similar signatures, especially in disordered samples. Here, we studied similar hybrid devices with a well-controlled and transparent interface between the superconductor and the QAH insulator. When the devices are in the QAH state with well-aligned magnetization, the two-terminal conductance is always half-quantized. Our experiment provides a comprehensive understanding of the superconducting proximity effect observed in QAH-superconductor hybrid devices and shows that the half-quantized conductance plateau is unlikely to be induced by chiral Majorana fermions in samples with a highly transparent interface.
We report on the scaling behavior of V-doped (Bi,Sb)_{2}Te_{3} samples in the quantum anomalous Hall regime for samples of various thickness. While previous quantum anomalous Hall measurements showed the same scaling as expected from a two-dimensional integer quantum Hall state, we observe a dimensional crossover to three spatial dimensions as a function of layer thickness. In the limit of a sufficiently thick layer, we find scaling behavior matching the flow diagram of two parallel conducting topological surface states of a three-dimensional topological insulator each featuring a fractional shift of 1/2e^{2}/h in the flow diagram Hall conductivity, while we recover the expected integer quantum Hall behavior for thinner layers. This constitutes the observation of a distinct type of quantum anomalous Hall effect, resulting from 1/2e^{2}/h Hall conductance quantization of three-dimensional topological insulator surface states, in an experiment which does not require decomposition of the signal to separate the contribution of two surfaces. This provides a possible experimental link between quantum Hall physics and axion electrodynamics.
In the quantum anomalous Hall effect, the edge states of a ferromagnetically doped topological insulator exhibit quantized Hall resistance and dissipationless transport at zero magnetic field. Up to now, however, the resistance was experimentally assessed with standard transport measurement techniques which are difficult to trace to the von-Klitzing constant RK with high precision. Here, we present a metrologically comprehensive measurement, including a full uncertainty budget, of the resistance quantization of V-doped (Bi,Sb)2Te3 devices without external magnetic field. We established as a new upper limit for a potential deviation of the quantized anomalous Hall resistance from RK a value of 0.26 ± 0.22 ppm, the smallest and most precise value reported to date. This provides another major step towards realization of the zero-field quantum resistance standard which in combination with Josephson effect will provide the universal quantum units standard in the future.Quantum standards are the backbone of the system of measurement units. Already since 1990 all electrical units are based on flux quantization in units of ℎ 2 ⁄ , realized with the Josephson effect [1,2], and conductance quantization in units of 2 ℎ ⁄ , realized with the quantized Hall effect (QHE) [3,4]. With the revision of the international system of units, SI, in near future [5,6] also the realizations of the units of mass [7,8], the kilogram, and of temperature [9,10], the Kelvin, will utilize and rely on practical electric quantum standards, realizing the vision of Maxwell [11] and Planck [12] of a truly universal system of units. Both electrical quantum standards require temperatures of 4 K or lower for their operation, but since in addition the QHE only works in a magnetic field, it is practically impossible to combine both in one system. However, in ferromagnetic topological insulators like e.g. Cr-or V-doped (Bi,Sb)2Te3, the quantum anomalous Hall effect (QAHE) provides conductance quantization without a magnetic field [13][14][15][16], giving legitimate hope for a future quantum standard where all units based on ℎ and can be realized in one measurement setup.Yet, up to now the precision of the QAHE has not been tested with precision metrology methods, and in particular no uncertainty budgets were presented with the data published [17,18]. Indeed, the fact that very low measurement currents are required makes it difficult to reach uncertainties in the parts in 10 9 range as are routinely obtained in calibrations based on GaAs or graphene QHE devices. A main reason for the limitation of current is the robustness of the ferromagnetic state, which at this stage of development still requires temperatures in the mK-regime and does not tolerate current levels
The ferromagnetic topological insulator V:(Bi,Sb) 2 Te 3 has been recently reported as a quantum anomalous Hall (QAH) system. Yet the microscopic origins of the QAH effect and the ferromagnetism remain unclear. One key aspect is the contribution of the V atoms to the electronic structure. Here the valence band of V:(Bi,Sb) 2 Te 3 thin films was probed in an element-specific way by resonant photoemission spectroscopy. The signature of the V 3d impurity band was extracted and exhibits a high density of states near Fermi level, in agreement with spin-polarized first-principles calculations. Our results indicate the occurrence of a ferromagnetic superexchange interaction mediated by the observed impurity band, contributing to the ferromagnetism in this system.
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