Magnetic van der Waals (vdW) materials have been heavily pursued for fundamental physics as well as for device design. Despite the rapid advances, so far magnetic vdW materials are mainly insulating or semiconducting, and none of them possesses a high electronic mobilitya property that is rare in layered vdW materials in general. The realization of a magnetic high-mobility vdW material would open the possibility for novel magnetic twistronic or spintronic devices.Here we report very high carrier mobility in the layered vdW antiferromagnet GdTe 3. The electron mobility is beyond 60,000 cm 2 V -1 s -1 , which is the highest among all known layered magnetic materials, to the best of our knowledge. Among all known vdW materials, the mobility of bulk GdTe 3 is comparable to that of black phosphorus, and is only surpassed by graphite. By mechanical exfoliation, we further demonstrate that GdTe 3 can be exfoliated to ultrathin flakes of three monolayers, and that the magnetic order and relatively high mobility is retained in ~20-nm-thin flakes.VdW materials are the parent compounds of two-dimensional (2D) materials, which are currently actively studied for new device fabrications (1) involving the creation of heterostructure stacks (2) or twisted bilayers (3) of 2D building blocks. Magnetic vdW materials have recently led to the observation of intrinsic magnetic order in atomically thin layers (4-12), which was followed by exciting discoveries of giant tunneling magnetoresistance (13-16) and tunable magnetism (17)(18)(19) in such materials.So far, the known magnetic vdW materials (ferro-or antiferromagnetic) that can be exfoliated are limited to a few examples, such as: CrI3 (4), Cr2Ge2Te6 (5), FePS3 (6,7), CrBr3 (8, 9), CrCl3 (10-12), Fe3GeTe2 (17,20), and RuCl3 (21-23). Out of these, only Fe3GeTe2 is a metallic ferromagnet and there is no known vdW-based 2D antiferromagnetic metal. Moreover, no evidence of high carrier mobilities has been reported in any of these exfoliated thin materials or even in their bulk vdW crystals. In general, high mobility is limited to very few vdW materials, such as graphite (24) and black phosphorus (25). A material with high electronic mobility and a corresponding high mean-free-path (MFP), might be critical for potential magnetic "twistronic" devices (3) where a large MFP could enable interesting phenomena in a Moiré-supercell induced flat band. In addition, conducting antiferromagnetic materials are the prime candidates for high-speed antiferromagnetic spintronic devices (26). Here we report the realization of a very high electronic mobility in a vdW layered antiferromagnet, GdTe3, both in bulk and exfoliated thin flakes.We chose to study GdTe3, since rare-earth tritellurides (RTe3, R = La-Nd, Sm, and Gd-Tm) are structurally related to topological semimetal ZrSiS (27,28), while being known to exhibit an incommensurate charge density wave (CDW) (29-31), rich magnetic properties (32), and becoming superconducting under high-pressure (R = Gd, Tb and Dy) (33). Combined, these properties ...

Dirac and Weyl semimetals display a host of novel properties. In Cd3As2, the Dirac nodes lead to a protection mechanism that strongly suppresses backscattering in zero magnetic field, resulting in ultrahigh mobility (∼ 10 7 cm 2 V −1 s −1 ). In applied magnetic field, an anomalous Nernst effect is predicted to arise from the Berry curvature associated with the Weyl nodes. We report observation of a large anomalous Nernst effect in Cd3As2. Both the anomalous Nernst signal and transport relaxation time τtr begin to increase rapidly at ∼ 50 K. This suggests a close relation between the protection mechanism and the anomalous Nernst effect. In a field, the quantum oscillations of bulk states display a beating effect, suggesting that the Dirac nodes split into Weyl states, allowing the Berry curvature to be observed as an anomalous Nernst effect.The field of topological quantum materials has recently expanded to include the Dirac (and Weyl) semimetals, which feature 3D bulk Dirac states with nodes that are protected by symmetry [1][2][3][4]. In Dirac semimetals, each Dirac cone is the superposition of two Weyl nodes which have opposite chiralities (χ = ±1). The Weyl nodes are prevented from hybridizing by the combination of point group symmetry, inversion symmetry and time-reversal symmetry (TRS) [4]. In the presence of a magnetic field B, the breaking of TRS leads to separation of the Weyl nodes and the appearance of a Berry curvature Ω(k).Because Ω(k) acts like an intense magnetic field, it exerts a strong force on charge carriers [5,6]. The first examples of Dirac semimetals, Na 3 Bi and Cd 3 As 2 , were identified by Wang et al. [7,8] Quite distinct from the chiral anomaly, the Berry curvature arising from separation of the Weyl nodes leads to other unusual transport effects, particularly the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) [24,25]. Unlike conventional system, no ferromagnetism is required for the AHE and ANE in Dirac semimetals because of the strong Berry curvature emanated by Weyl nodes. The anomalous Hall conductivity is expressed as [3,26],where ∆k i is the distance between the i th pair of Weyl nodes. The thermopower and Nernst effect in Weyl semimetals has been calculated in the Boltzmann equation approach [27][28][29][30].We report measurements of the thermoelectric tensor S ij of Cd 3 As 2 in two samples (A4, A5) in "set A" and two samples (B10, B20) in "set B" with the applied thermal gradient −∇T ||x and magnetic field B||ẑ (see Ref. [20] for details of the electrical transport measurements in set A and set B samples). We obtain S xx and S xy aswhere α ij is the thermoelectric linear response tensor, and ρ ij is the resistivity tensor (see Supplement for the details). In Dirac semimetals, the AHE and ANE arise because the Berry curvature Ω(k) imparts to the carriers an anomalous velocity v A = Ω(k) × k , i.e. Ω(k) acts like an effective magnetic field in k space (k is the rate of change of the wavevector k) [25]. Previously, the AHE was observed in Cd 3 As 2 as a wea...

Two-dimensional materials have significant potential for the development of new devices.Here we report the electronic and structural properties of β -GeSe, a previously unreported polymorph of GeSe, with a unique crystal structure that displays strong two-dimensional structural features. β -GeSe is made at high pressure and temperature and is stable under ambient conditions. We compare it to its structural and electronic relatives α-GeSe and black phosphorus. The β form of GeSe displays a boat conformation for its Ge-Se six-ring, while the previously known α form, and black phosphorus, display the more common chair conformation for their six-rings. Electronic structure calculations indicate that β -GeSe is a semiconductor, with an approximate bulk band gap of ∆ ≈ 0.5 eV, and, in its monolayer form, ∆ ≈ 0.9 eV. These * To whom correspondence should be addressed values fall between those of α-GeSe and black phosphorus, making β -GeSe a promising candidate for future applications. The resistivity of our β -GeSe crystals measured in-plane is on the order of ρ ≈ 1 Ωcm, while being essentially temperature independent.

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Layered honeycomb magnets are of interest as potential realizations of the Kitaev quantum spin liquid (KQSL), a quantum state with long-range spin entanglement and an exactly solvable Hamiltonian. Conventional magnetically ordered states are present for all currently known candidate materials, however, because non-Kitaev terms in the Hamiltonians obscure the Kitaev physics. Current experimental studies of the KQSL are focused on 4d-or 5d-transition-metal-based honeycombs, in which strong spin-orbit coupling can be expected, yielding Kitaev interaction that dominate in an applied magnetic field. In contrast, for 3d-based layered honeycomb magnets, spin orbit coupling is weak and thus Kitaev-physics should be substantially less accessible. Here we report our studies on BaCo 2 (AsO 4 ) 2 , for which we find that the magnetic order associated with the non-Kitaev interactions can be fully suppressed by a relatively low magnetic field, yielding a non-magnetic material and implying the presence of strong magnetic frustration and weak non-Kitaev interactions. 1 arXiv:1910.08577v1 [cond-mat.str-el] 18 Oct 2019 Unlike the quantum spin liquids (QSL) found in geometrically frustrated quantum magnets, the Kitaev QSL arises from strong anisotropy and bond-dependent interactions that frustrate the spin configuration on a single site of a honeycomb lattice (1). The Kitaev model, which is an exactly solvable model of honeycomb lattice magnetism, has attracted considerable recent attention as it gives rise to quantum and topological spin liquids and emergent Majorana quasiparticles (1). In real materials, the spin Hamiltonian for such systems can be expressed by the sum of three terms, with J, K and Γ representing Heisenberg (J), Kitaev (K) and bond-dependent off-diagonal exchange interactions (Γ), respectively. This is known as the extended Kitaev-Heisenberg (EKH) quantum spin model (2, 3).In order to approach the ideal KQSL state, Kitaev interaction are required to dominate the spin Hamiltonian (4, 5). Such bond-dependent anisotropic Kitaev-type interactions are believed to dominate in materials with strong entanglement due to spin-orbit coupling (SOC) (6), and thus so far most theoretical and experimental investigations of the Kitaev quantum spin liquid state have been devoted to candidates with 4d-and 5d-transition-metal-based honeycomb lattices, including α-RuCl 3 , A 2 IrO 3 (A=Li, Na), and H 3 LiIr 2 O 6 (7-10). Even though Kitaev interaction are supposed to be strong for these materials, they are nonetheless not strong enough to stabilize the QSL state.Instead, the inevitable non-Kitaev interactions present in all these systems induce conventional magnetic order at finite temperatures (11-13) obscuring the signature (e.g. a half-integer quantized thermal Hall conductivity) of the Kitaev spin liquid state that is potentially present. Theoretical and experimental efforts have shown that the non-Kitaev terms can be suppressed by applying tuning parameters, such as a magnetic field (13)(14)(15), and that the ground state in th...