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(a) RHEED pattern along  direction of an as-grown surface of Bi2−xCrxSe3 with a thickness of 7 QLs. (b) RHEED oscillations of intensity of the specular beam. The oscillation period is found to be 108 s, corresponding to a growth rate of ∼0.56 QL/min. (c) AFM image of the Bi1.9Cr0.1Se3 thin film with the size of 0.3 μm × 0.3 μm. (d) HRTEM of a Bi1.8Cr0.2Se3 grown on a Si (111) substrate.
ARPES intensity maps of un-capped 50 QL Bi2−xCrxSe3 thin films with (a) x = 0, (b) x = 0.02, (c) x = 0.1, and (d) x = 0.2 on Si (111) along the Γ-K direction. With the introduction of magnetic Cr doping, the surface will deviate from the original linear massless Dirac fermion state and becomes broaden. The surface will also open a larger band gap with higher Cr concentration as illustrated by the solid yellow lines. All data are taken using 52 eV photons under the temperature of 10 K.
(a) The dependence of the longitudinal resistance Rxx on temperature for a Hall bar device fabricated on the 50 QL Bi1.9Cr0.1Se3 thin film sample on Si (111). The monotonically increase in Rxx with temperature decreasing shows the semiconductor behavior. Inset: modified Arrhenius law of the R-T curve. The polaron formation energy Ea is found to be 35.5 meV and α = 3/2 for non-adiabatic small polaron hopping. (b) Magnetization versus temperature for the sample under zero-field cooling (red circles) and field cooling (blue squires) conditions. The external field is 50 Oe perpendicular to the sample surface. Inset: plot of (χ − χ 0)−1 versus temperature T. This relation does not follow the linear Curie-Weiss law above Tc .
(a) M-H loops of the 50 QL Bi1.9Cr0.1Se3 thin film at 5 K, 10 K, 15 K, 20 K, 30 K, and 35 K. Inset: zoom-in hysteresis loop at 5 K, the coercivity field is started to be around 150 Oe. (b) Arrott plots of H/M versus M 2 for magnetic isotherms from 14 K to 26 K. (c) Modified Arrott-Noakes plots of (H/M)1/1.39 versus M 1/0.37 for data in (b). The applied magnetic field is always perpendicular to the sample surface, and the field range which covers from −8000 Oe to 8000 Oe is high enough to saturate the magnetic moment.
Magneto-transport measurements of 50 QL Bi2−xCrxSe3 thin films on Si (111) substrate. (a) The dependence of 2D carrier density (red-open circle) and mobility (blue-solid square) on the Cr doping concentration at 1.9 K. (b) B-field dependent anomalous Hall effect (RA·M) with Cr doping concentration (x = 0.02, 0.1, 0.2, and 0.3). Linear ordinary Hall component of Ro·H has been subtracted from the Hall resistance. Inset: the evolution of coercivity field HC under different Cr compositions at 1.9 K.
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