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.

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We demonstrate a polymer-free method that can routinely transfer relatively large-area graphene to any substrate with advanced electrical properties and superior atomic and chemical structures as compared to the graphene sheets transferred with conventional polymer-assisted methods. The graphene films that are transferred with polymer-free method show high electrical conductance and excellent optical transmittance. Raman spectroscopy and X-ray/ultraviolet photoelectron spectroscopy also confirm the presence of high quality graphene sheets with little contamination after transfer. Atom-resolved images can be obtained using scanning tunneling microscope on as-transferred graphene sheets without additional cleaning process. The mobility of the polymer-free graphene monolayer is as high as 63,000 cm(2) V(-1) s(-1), which is 50% higher than the similar sample transferred with the conventional method. More importantly, this method allows us to place graphene directly on top of devices made of soft materials, such as organic and polymeric thin films, which widens the applications of graphene in soft electronics.

In a ferromagnet, an applied electric field E invariably produces an anomalous Hall current JH that flows perpendicular to the plane defined by E and M (the magnetization). For decades, the question whether JH is dissipationless (independent of the scattering rate), has been keenly debated without experimental resolution. In the ferromagnetic spinel CuCr2Se4−xBrx, the resistivity ρ (at low temperature) may be increased 1000 fold by varying x(Br), without degrading the M. We show that JH /E (normalized per carrier, at 5 K) remains unchanged throughout. In addition to resolving the controversy experimentally, our finding has strong bearing on the generation and study of spin-Hall currents in bulk samples.A major unsettled question in the study of electron transport in a ferromagnet is whether the anomalous Hall current is dissipationless. In non-magnetic metals, the familiar Hall current arises when electrons moving in crossed electric (E) and magnetic (H) fields are deflected by the Lorentz force. However, in a ferromagnet subject to E alone, a large, spontaneous (anomalous) Hall current J H appears transverse to E (in practice, a weak H serves to align the magnetic domains) (1,2). Questions regarding the origin of J H , and whether it is dissipationless, have been keenly debated for decades. They have emerged anew because of fresh theoretical insights and strong interest in spin currents for spin-based electronics. Here we report measurements in the ferromagnet CuCr 2 Se 4−x Br x which establish that, despite a 100-fold increase in the scattering rate from impurities, J H (per carrier) remains constant, implying that it is indeed dissipationless.In 1954, Karplus and Luttinger (KL)(3,4) proposed a purely quantum-mechanical origin for J H . An electron in the conduction band of a crystal lattice spends part of its time in nearby bands because of admixing caused by the (intracell) position operator X. In the process, it acquires a spin-dependent 'anomalous velocity' (5). KL predicted that the Hall current is dissipationless: J H remains constant even as the longitudinal current (J||E) is degraded by scattering from added impurities. A conventional mechanism was later proposed (6) whereby the anomalous Hall effect (AHE) is caused instead by asymmetric scattering of electrons by impurities (skew scattering). Several authors (7,8,9) investigated the theoretical ramifications of these competing models. The role of impurities in the anomalous-velocity theory was clarified by Berger's side-jump model (7 AHE in a semiconductor has been given by Nozières and Lewiner (NL) who derive X = λk × S, with λ the enhanced spin-orbit parameter, k the carrier wavevector and S its spin (9). In the dc limit, NL obtain the AHE currentwhere n is the carrier density and e the charge. As noted, J H is linear in S but independent of the electron lifetime τ . In modern terms, the anomalous velocity term of KL is related to the Berry phase (10), and has been applied (11) to explain the AHE in Mn-doped GaAs (12). The close connection of the AH...

After decades of searching for the dissipationless transport in the absence of any external magnetic field, quantum anomalous Hall effect (QAHE) was recently achieved in magnetic topological insulator films. However, the universal phase diagram of QAHE and its relation with quantum Hall effect (QHE) remain to be investigated. Here, we report the experimental observation of the giant longitudinal resistance peak and zero Hall conductance plateau at the coercive field in the six quintuple-layer (Cr0.12Bi0.26Sb0.62)2Te3 film, and demonstrate the metal-to-insulator switching between two opposite QAHE plateau states up to 0.3 K. Moreover, the universal QAHE phase diagram is confirmed through the angle-dependent measurements. Our results address that the quantum phase transitions in both QAHE and QHE regimes are in the same universality class, yet the microscopic details are different. In addition, the realization of the QAHE insulating state unveils new ways to explore quantum phase-related physics and applications.