A propagating Majorana mode Although Majorana fermions remain elusive as elementary particles, their solid-state analogs have been observed in hybrid semiconductor-superconductor nanowires. In a nanowire setting, the Majorana states are localized at the ends of the wire. He et al. built a two-dimensional heterostructure in which a one-dimensional Majorana mode is predicted to run along the sample edge (see the Perspective by Pribiag). The heterostructure consisted of a quantum anomalous Hall insulator (QAHI) bar contacted by a superconductor. The authors used an external magnetic field as a “knob” to tune into a regime where a Majorana mode was propagating along the edge of the QAHI bar covered by the superconductor. A signature of this propagation—half-quantized conductance—was then observed in transport experiments. Science , this issue p. 294 ; see also p. 252
We investigate the quantum anomalous Hall Effect (QAHE) and related chiral transport in the millimetersize (Cr 0.12 Bi 0.26 Sb 0.62 ) 2 Te 3 films. With high sample quality and robust magnetism at low temperatures, the quantized Hall conductance of e 2 /h is found to persist even when the film thickness is beyond the twodimensional (2D) hybridization limit. Meanwhile, the Chern insulator-featured chiral edge conduction is manifested by the non-local transport measurements. In contrast to the 2D hybridized thin film, an additional weakly field-dependent longitudinal resistance is observed in the 10 quintuple-layer film, suggesting the influence of the film thickness on the dissipative edge channel in the QAHE regime. The extension of QAHE into the three-dimensional thickness region addresses the universality of this quantum transport phenomenon and motivates the exploration of new QAHE phases with tunable Chern numbers.In addition, the observation of the scale-invariant dissipationless chiral propagation on a macroscopic scale makes a major stride towards ideal low-power interconnect applications.
We report a nearly ideal quantum anomalous Hall effect in a three-dimensional topological insulator thin film with ferromagnetic doping. Near zero applied magnetic field we measure exact quantization in Hall resistance to within a part per 10,000 and longitudinal resistivity under 1 Ω per square, with chiral edge transport explicitly confirmed by non-local measurements. Deviations from this behavior are found to be caused by thermally-activated carriers, which can be eliminated by taking advantage of an unexpected magnetocaloric effect.PACS numbers: 73.43.Fj, 75.45.+j, 75.50.Pp The discovery of the quantum Hall effect (QHE) [1,2] led to a new understanding of electronic behavior in which topology plays a central role [3,4]. Initially, the critical experimental observation was the precise quantization of the Hall resistance to integer divisions of h/e 2 , where h is Planck's constant and e is the electron charge. This quantization, immune to sample-specific disorder, now forms the basis for a metrological standard [5]. A complementary feature-zero longitudinal resistance, reflecting resistanceless transport along sample edges-could also have technological applications, were it not for the demanding environmental requirements for achieving the QHE: a large magnetic field to break timereversal symmetry (TRS) and, in most cases, cryogenic temperatures. Ideas for producing a similar phenomenology without an external magnetic field have long been considered [6], often involving the interplay of symmetry and topology in new material systems.In the past decade, topological insulators (TIs) have emerged as a promising approach.In both twodimensional [7][8][9] and three-dimensional [10][11][12][13][14] forms, conduction in TIs is restricted to topologically-protected boundary states. In the 3D case, the presence of ferromagnetic exchange can break TRS, opening a gap in the otherwise Dirac-like surface states [15][16][17]. But topology adds a twist: even a uniformly magnetized sample will have, relative to the normal vector of the surface, a domain boundary where the magnetization switches from inward to outward. Along this line the gap should close, restoring conduction [16]. In a thin film geometry in which the easy axis of the magnetism is out-of-plane, confinement along the sample side wall should ensure conduction is one-dimensional while the surface gradient of the magnetism restricts it to only one direction, leading to ballistic, chiral transport. In a Hall bar geometry, this would be observed as the quantum anomalous Hall effect (QAHE), with a zero longitudinal resistance and a transverse resistance quantized to h/ne 2 , where n is typically ±1 but can in principle be a higher integer given sufficiently strong exchange [18].Experimental realization of the QAHE has been swift. Doping films of the ternary TI family (Bi,Sb) 2 Te 3 with Mn or Cr was found to produce robust out-of-plane ferromagnetism and a large anomalous Hall effect in transport [19][20][21]. Further growth optimization and chemical potential man...
Electric-field manipulation of magnetic order has proved of both fundamental and technological importance in spintronic devices. So far, electric-field control of ferromagnetism, magnetization and magnetic anisotropy has been explored in various magnetic materials, but the efficient electric-field control of spin-orbit torque (SOT) still remains elusive. Here, we report the effective electric-field control of a giant SOT in a Cr-doped topological insulator (TI) thin film using a top-gate field-effect transistor structure. The SOT strength can be modulated by a factor of four within the accessible gate voltage range, and it shows strong correlation with the spin-polarized surface current in the film. Furthermore, we demonstrate the magnetization switching by scanning gate voltage with constant current and in-plane magnetic field applied in the film. The effective electric-field control of SOT and the giant spin-torque efficiency in Cr-doped TI may lead to the development of energy-efficient gate-controlled spin-torque devices compatible with modern field-effect semiconductor technologies.
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