The magneto‐thermoelectric figure of merit (ZT) in crystals of the topological Dirac semimetal Cd3As2 with different carrier concentrations is studied. The ZTs for all the crystals increase with the temperature and show maxima at high temperatures. Meanwhile, the temperatures corresponding to the ZT maxima increase with the carrier concentration. The limit to the improvement in ZT(T) at high temperature could be related to the unusual large enhancement in thermal conductivity at elevated temperatures. The bipolar effect and Dirac liquid behavior are presented as processes possibly responsible for the peculiar behavior of the thermal conductivity. Applying a transverse magnetic field initially leads to a dramatic enhancement and, subsequently, to a slight reduction in ZT for all the crystals. The maximum ZT achieved in a magnetic field increases with the carrier concentration and reaches 1.24 at 450 K in a magnetic field of 9 T for the crystal with the highest carrier concentration. It is expected that this work will be beneficial to the current interests in optimizing the thermoelectric properties of quantum topological materials.
The discovery of magnetism in 2D materials offers new opportunities for exploring novel quantum states and developing spintronic devices. In this work, using field‐effect transistors with solid ion conductors as the gate dielectric (SIC‐FETs), we have observed a significant enhancement of ferromagnetism associated with magnetic easy‐axis switching in few‐layered Cr2Ge2Te6. The easy axis of the magnetization, inferred from the anisotropic magnetoresistance, can be uniformly tuned from the out‐of‐plane direction to an in‐plane direction by electric field in the few‐layered Cr2Ge2Te6. Additionally, the Curie temperature, obtained from both the Hall resistance and magnetoresistance measurements, increases from 65 to 180 K in the few‐layered sample by electric gating. Moreover, the surface of the sample is fully exposed in the SIC‐FET device configuration, making further heterostructure‐engineering possible. This work offers an excellent platform for realizing electrically controlled quantum phenomena in a single device.
Berry phase effects have significant influences on the electronic properties of condensed matter. In particular, the anomalous Hall conductivity has been recognized as an intrinsic property of the systems with non-zero Berry curvature. Here, we present the anomalous Hall effect observed in the non-magnetic material ZrTe5, which hosts a large Zeeman splitting with Landé g-factor of 26.49. The quantum oscillation analysis reveals non-linear band dispersion near the top of valence band in bulk band structure, and no Weyl node forms with applied magnetic field. The anomalous Hall conductivity reaches 129 Ω−1 cm−1 at 2 K, and shows weak temperature dependence. All these combined with theoretical analysis suggest that the anomalous Hall effect observed in ZrTe5 originates from the non-vanishing Berry curvature induced by combining large Zeeman splitting and strong spin–orbit coupling. Remarkably, the anomalous Hall resistivity reverses its sign from negative to positive at a hydrostatic pressure P = 1.3 GPa, which confirms that the anomalous Hall effect in ZrTe5 is highly related to the band structure-dependent Berry curvature. Our results have verified the anomalous Hall mechanism in ZrTe5 and offer a new platform to study the anomalous Hall effect.
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