Dirac semimetals have attracted extensive attentions in recent years. It has been theoretically suggested that many-body interactions may drive exotic phase transitions, spontaneously generating a Dirac mass for the nominally massless Dirac electrons. So far, signature of interaction-driven transition has been lacking. In this work, we report high-magnetic-field transport measurements of the Dirac semimetal candidate ZrTe5. Owing to the large g factor in ZrTe5, the Zeeman splitting can be observed at magnetic field as low as 3 T. Most prominently, high pulsed magnetic field up to 60 T drives the system into the ultra-quantum limit, where we observe abrupt changes in the magnetoresistance, indicating field-induced phase transitions. This is interpreted as an interaction-induced spontaneous mass generation of the Dirac fermions, which bears resemblance to the dynamical mass generation of nucleons in high-energy physics. Our work establishes Dirac semimetals as ideal platforms for investigating emerging correlation effects in topological matters.

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have been recently proposed as appealing candidate materials for spintronic applications owing to their distinctive atomic crystal structure and exotic physical properties arising from the large bonding anisotropy. Here we introduce the first MoS2-based spin-valves that employ monolayer MoS2 as the nonmagnetic spacer. In contrast with what is expected from the semiconducting band-structure of MoS2, the vertically sandwiched-MoS2 layers exhibit metallic behavior. This originates from their strong hybridization with the Ni and Fe atoms of the Permalloy (Py) electrode. The spin-valve effect is observed up to 240 K, with the highest magnetoresistance (MR) up to 0.73% at low temperatures. The experimental work is accompanied by the first principle electron transport calculations, which reveal an MR of ∼9% for an ideal Py/MoS2/Py junction. Our results clearly identify TMDs as a promising spacer compound in magnetic tunnel junctions and may open a new avenue for the TMDs-based spintronic applications.

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Owing to the coupling between open Fermi arcs on opposite surfaces, topological Dirac semimetals exhibit a new type of cyclotron orbit in the surface states known as Weyl orbit. Here, by lowering the carrier density in Cd3As2 nanoplates, we observe a crossover from multiple-frequency to single-frequency Shubnikov–de Haas (SdH) oscillations when subjected to out-of-plane magnetic field, indicating the dominant role of surface transport. With the increase of magnetic field, the SdH oscillations further develop into quantum Hall state with non-vanishing longitudinal resistance. By tracking the oscillation frequency and Hall plateau, we observe a Zeeman-related splitting and extract the Landau level index as well as sub-band number. Different from conventional two-dimensional systems, this unique quantum Hall effect may be related to the quantized version of Weyl orbits. Our results call for further investigations into the exotic quantum Hall states in the low-dimensional structure of topological semimetals.

Since the discovery of graphene, layered materials have attracted extensive interest owing to their unique electronic and optical characteristics. Among them, Dirac semimetals, one of the most appealing categories, have been a long-sought objective in layered systems beyond graphene. Recently, layered pentatelluride ZrTe 5 was found to host signatures of a Dirac semimetal. However, the low Fermi level in ZrTe 5 strongly hinders a comprehensive understanding of the whole picture of electronic states through photoemission measurements, especially in the conduction band. Here, we report the observation of Dirac fermions in ZrTe 5 through magneto-optics and magneto-transport. By applying a magnetic field, we observe a ffiffiffiffi B p dependence of the inter-Landau-level resonance and Shubnikov-de Haas (SdH) oscillations with a nontrivial Berry phase, both of which are hallmarks of Dirac fermions. The angle-dependent SdH oscillations show a clear quasi-two-dimensional feature with a highly anisotropic Fermi surface and band topology, in stark contrast to the three-dimensional (3D) Dirac semimetal such as Cd 3 As 2 . This is further confirmed by the angle-dependent Berry phase measurements and the observation of bulk quantum Hall effect (QHE) plateaus. The unique band dispersion is theoretically understood: the system is at the critical point between a 3D Dirac semimetal and a topological insulator phase. With the confined interlayer dispersion and reducible dimensionality, our work establishes ZrTe 5 as an ideal platform for exploring the exotic physical phenomena of Dirac fermions. NPG Asia Materials (2016) 8, e325; doi:10.1038/am.2016.166; published online 11 November 2016 INTRODUCTION Layered materials, formed by stacking strongly bonded layers with weak interlayer coupling, 1-10 have drawn immense attention in fundamental studies and device applications owing to their tunability in band structures and Fermi energy. 3,4,[11][12][13] Unlike other layered materials such as MoS 2 and BN, graphene stands out as an appealing candidate, as it is featured with a linear energy dispersion and low-energy relativistic quasi-particles. 9,14,15 Many exotic phenomena, such as a half-integer quantum Hall effect (QHE) 1,2 and Klein tunneling, 16 have been realized in graphene. Along this line, extensive efforts were also devoted to exploring new Dirac semimetal states in other layered systems beyond graphene. 5,6 Pentatelluride ZrTe 5 with a layered orthorhombic structure has been widely studied since the 1980s for its resistivity anomaly [17][18][19] and large thermopower. 20,21 For a long time, ZrTe 5 was considered to be a semimetal or degenerated semiconductor with a parabolic energy dispersion. 10,22 However, a recent study 7 revealed a linear dispersion in ZrTe 5 bulk states along with a chiral magnetic effect, hosting the signatures of a Dirac semimetal. Nevertheless, owing to the relatively

We study with first-principles methods the interplay between bulk and surface Dirac fermions in three dimensional Dirac semimetals. By combining density functional theory with the coherent potential approximation, we reveal a topological phase transition in Na 3 Bi 1−x Sb x and Cd 3 ½As 1−x P x 2 alloys, where the material goes from a Dirac semimetal to a trivial insulator upon changing Sb or P concentrations. Tuning the composition allows us to engineer the position of the bulk Dirac points in reciprocal space. Interestingly, the phase transition coincides with the reversal of the band ordering between the conduction and valence bands.

Electrostatic doping in materials can lead to various exciting electronic properties, such as metal-insulator transition and superconductivity, by altering the Fermi level position or introducing exotic phases. Cd 3 As 2 , a three-dimensional (3D) analog of graphene with extraordinary carrier mobility, was predicted to be a 3D Dirac semimetal, a feature confirmed by recent experiments. However, most research so far has been focused on metallic bulk materials that are known to possess ultra-high mobility and giant magneto-resistance but limited carrier transport tunability. Here we report on the first observation of a gate-induced transition from band conduction to hopping conduction in single-crystalline Cd 3 As 2 thin films via electrostatic doping by solid electrolyte gating. The extreme charge doping enables the unexpected observation of p-type conductivity in a ∼50-nm-thick Cd 3 As 2 thin film grown by molecular beam epitaxy. More importantly, the gate-tunable Shubnikov-de Haas oscillations and the temperature-dependent resistance reveal a unique band structure and bandgap opening when the dimensionality of Cd 3 As 2 is reduced. This is also confirmed by our first-principle calculations. The present results offer new insights toward nanoelectronic and optoelectronic applications of Dirac semimetals in general and provide new routes in the search for the intriguing quantum spin Hall effect in low-dimension Dirac semimetals, an effect that is theoretically predicted but not yet experimentally realized.

The search for proximity-induced superconductivity in topological materials has generated widespread interest in the condensed matter physics community. The superconducting states inheriting nontrivial topology at interfaces are expected to exhibit exotic phenomena such as topological superconductivity and Majorana zero modes, which hold promise for applications in quantum computation. However, a practical realization of such hybrid structures based on topological semimetals and superconductors has hitherto been limited. Here, we report the strong proximity-induced superconductivity in type-II Weyl semimetal WTe, in a van der Waals hybrid structure obtained by mechanically transferring NbSe onto various thicknesses of WTe. When the WTe thickness ( t) reaches 21 nm, the superconducting transition occurs around the critical temperature ( T) of NbSe with a gap amplitude (Δ) of 0.38 meV and an unexpected ultralong proximity length ( l) up to 7 μm. With the thicker 42 nm WTe layer, however, the proximity effect yields T ≈ 1.2 K, Δ = 0.07 meV, and a short l of less than 1 μm. Our theoretical calculations, based on the Bogoliubov-de Gennes equations in the clean limit, predict that the induced superconducting gap is a sizable fraction of the NbSe superconducting one when t is less than 30 nm and then decreases quickly as t increases. This agrees qualitatively well with the experiments. Such observations form a basis in the search for superconducting phases in topological semimetals.