We measure the magnetotransport properties of the three-dimensional Dirac semimetal Cd_{3}As_{2} single crystal under magnetic fields up to 36 T. Shubnikov-de Haas (SdH) oscillations are clearly resolved and the n=1 Landau level is reached. A detailed analysis on the intercept of the Landau index plot reveals a significant dependence of the SdH phase factor on the orientation of the applied magnetic field. When the magnetic field is applied in the [001] direction, i.e., along the fourfold screw axis of the tetragonal crystal structure, a nontrivial π Berry phase, as predicted for the Dirac fermions, is observed. However, in a magnetic field tilted away from the [001] direction, the π Berry phase is evidently reduced, and a considerable enhancement of the effective mass is also revealed. Our observations demonstrate that the Dirac dispersion in Cd_{3}As_{2} is effectively modified in a tilted magnetic field, whereas the preserved π Berry phase in a magnetic field along the [001] direction can be related to the realization of the Weyl fermions. The sudden change of the SdH phase also indicates a possible topological phase transition induced by the symmetry-breaking effect.

We develop a novel field effect transistor (FET) device using solid ion conductor (SIC) as a gate dielectric, and we can tune the carrier density of FeSe by driving lithium ions in and out of the FeSe thin flakes, and consequently control the material properties and its phase transitions. A dome-shaped superconducting phase diagram was mapped out with increasing Li content, with Tc ∼ 46.6 K for the optimal doping, and an insulating phase was reached at the extremely overdoped regime. Our study suggests that, using solid ion conductor as a gate dielectric, the SIC-FET device can achieve much higher carrier doping in the bulk, and suit many surface sensitive experimental probes, and can stabilize novel structural phases that are inaccessible in ordinary conditions. PACS numbers: 74.25.F-, 74.70. Xa, Chemical doping is a conventional way to introduce charge carriers into solids by replacing one of the constituent elements with another element of a different valence state. For instance, high temperature superconductivity is realized by suppressing the antiferromagnetism or spin density wave with chemical doping of 10% or 10 21 dopant atoms per cm 3 in copper oxides and iron-based superconductors [1-3]. However, the chemical doping is incapable in many cases, because the element replacement and the variation of carrier density cannot practically cover a large regime and leave many phases unexplored. As a complementary method, the application of field effect transistors (FET) in two-dimensional systems is an effective way to control electronic properties via reversible changes of charge carrier density [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Such an electrostatic doping is desirable to study novel phases that cannot be achieved by material synthetic methods [7, 9-11, 13, 15, 16, 18]. For instance, we have utilized tunable ion intercalation with an ionic liquid to alter charge-ordered states in 1T-TaS 2 and induce phase transitions in thin flakes with reduced dimensionality [15]. The FET devices have been widely applied in the exploration of new superconductors [10,11], the preparation for new devices [19,20] as well as many applications in semiconductor industry [7].So far, only two types of field effect transistor (FET) devices, metal-insulator-semiconductor (MIS) FET ( Fig.

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Transition-metal dichalcogenides open novel opportunities for the exploration of exciting new physics and devices. As a representative system, 2H-MoS2 has been extensively investigated owing to its unique band structure with a large band gap, degenerate valleys and non-zero Berry curvature. However, experimental studies of metastable 1T polytypes have been a challenge for a long time, and electronic properties are obscure due to the inaccessibility of single phase without the coexistence of 1T', 1T'' and 1T''' lattice structures, which hinder the broad applications of MoS2 in future nanodevices and optoelectronic devices. Using Kx(H2O)yMoS2 as the precursor, we have successfully obtained high-quality layered crystals of the metastable 1T'''-MoS2 with √3𝑎𝑎 × √3𝑎𝑎 superstructure and metastable 1T'-MoS2 with 𝑎𝑎 × 2𝑎𝑎 superstructure, as evidenced by structural characterizations through scanning tunneling microscopy, Raman spectroscopy and X-ray diffraction. It is found that the metastable 1T'-MoS2 is a superconductor with onset transition temperature (Tc) of 4.2 K, while the metastable 1 T'''-MoS2 shows either superconductivity with Tc of 5.3 K or insulating behavior, which strongly depends on the synthesis procedure. Both of the metastable polytypes of MoS2 crystals can be transformed to the stable 2H phase with mild annealing at about 70°C in He atmosphere. These findings provide pivotal information on the atomic configurations and physical properties of 1T polytypes of MoS2.