Zhongxia Wei

Pressure-driven dome-shaped superconductivity and electronic structural evolution in tungsten ditelluride

Tungsten ditelluride has attracted intense research interest due to the recent discovery of its large unsaturated magnetoresistance up to 60 T. Motivated by the presence of a small, sensitive Fermi surface of 5d electronic orbitals, we boost the electronic properties by applying a high pressure, and introduce superconductivity successfully. Superconductivity sharply appears at a pressure of 2.5 GPa, rapidly reaching a maximum critical temperature (Tc) of 7 K at around 16.8 GPa, followed by a monotonic decrease in Tc with increasing pressure, thereby exhibiting the typical dome-shaped superconducting phase. From theoretical calculations, we interpret the low-pressure region of the superconducting dome to an enrichment of the density of states at the Fermi level and attribute the high-pressure decrease in Tc to possible structural instability. Thus, tungsten ditelluride may provide a new platform for our understanding of superconductivity phenomena in transition metal dichalcogenides.

Solvothermal Synthesis of Lateral Heterojunction Sb2Te3/Bi2Te3 Nanoplates

A lateral heterojunction of topological insulator Sb2Te3/Bi2Te3 was successfully synthesized using a two-step solvothermal method. The two crystalline components were separated well by a sharp lattice-matched interface when the optimized procedure was used. Inspecting the heterojunction using high-resolution transmission electron microscopy showed that epitaxial growth occurred along the horizontal plane. The semiconducting temperature-resistance curve and crossjunction rectification were observed, which reveal a staggered-gap lateral heterojunction with a small junction voltage. Quantum correction from the weak antilocalization reveals the well-maintained transport of the topological surface state. This is appealing for a platform for spin filters and one-dimensional topological interface states.

Carrier balance and linear magnetoresistance in type-II Weyl semimetal WTe2

Unsaturated magnetoresistance (MR) has been reported in WTe2, and remains irrepressible up to very high field. Intense optimization of the crystalline quality causes a squarely-increasing MR, as interpreted by perfect compensation of opposite carriers. Herein we report our observation of linear MR (LMR) in WTe2 crystals, the onset of which is first identified by constructing the mobility spectra of the MR at low fields. The LMR further intensifies and predominates at fields higher than 20 Tesla while the parabolic MR gradually decays. The LMR remains unsaturated up to a high field of 60 Tesla and persists, even at a high pressure of 6.2 GPa. Assisted by density functional theory calculations and detailed mobility spectra, we find the LMR to be robust against the applications of high field, broken carrier balance, and mobility suppression. Angle-resolved photoemission spectroscopy reveals a unique quasilinear energy dispersion near the Fermi level. Our results suggest that the robust LMR is the low bound of the unsaturated MR in WTe2.

Anomalous quantization trajectory and parity anomaly in Co cluster decorated BiSbTeSe 2 nanodevices

Dirac Fermions with different helicities exist on the top and bottom surfaces of topological insulators, offering a rare opportunity to break the degeneracy protected by the no-go theorem. Through the application of Co clusters, quantum Hall plateaus were modulated for the topological insulator BiSbTeSe2, allowing an optimized surface transport. Here, using renormalization group flow diagrams, we show the extraction of two sets of converging points in the conductivity tensor space, revealing that the top surface exhibits an anomalous quantization trajectory, while the bottom surface retains the 1/2 quantization. Co clusters are believed to induce a sizeable Zeeman gap ( > 4.8 meV) through antiferromagnetic exchange coupling, which delays the Landau level hybridization on the top surface for a moderate magnetic field. A quasi-half-integer plateau also appears at −7.2 Tesla. This allows us to study the interesting physics of parity anomaly, and paves the way for further studies simulating exotic particles in condensed matter physics.The topological surface states usually appear in pairs in a topological insulator, with one on the top surface and the other on the bottom surface. Here, Zhang et al. utilize Co cluster to induce a Zeeman gap on one surface through antiferromagnetic exchange coupling, and observe a quasi-half-integer plateau, suggesting the parity anomaly of Dirac fermions.In three-dimensional topological insulators (TIs), the nontrivial topology in their electronic bands casts a gapless state on their solid surfaces, using which dissipationless TI edge devices based on the quantum anomalous Hall (QAH) effect and quantum Hall (QH) effect have been demonstrated. Practical TI devices present a pair of parallel-transport topological surface states (TSSs) on their top and bottom surfaces. However, due to the no-go theorem, the two TSSs always appear as a pair and are expected to quantize synchronously. Quantized transport of a separate Dirac channel is still desirable, but has never been observed in graphene even after intense investigation over a period of 13 years, with the potential aim of half-QHE. By depositing Co atomic clusters, we achieved stepwise quantization of the top and bottom surfaces in BiSbTeSe2 (BSTS) TI devices. Renormalization group flow diagrams13, 22 (RGFDs) reveal two sets of converging points (CVPs) in the (Gxy, Gxx) space, where the top surface travels along an anomalous quantization trajectory while the bottom surface retains 1/2 e2/h. This results from delayed Landau-level (LL) hybridization (DLLH) due to coupling between Co clusters and TSS Fermions.

Quantum oscillation and nontrivial transport in the Dirac semimetal Cd3As2 nanodevice

Here, we report on the Shubnikov-de Haas oscillation in high-quality Cd3As2 nanowires grown by a chemical vapor deposition approach. The dominant transport of topological Dirac fermions is evident by the nontrivial Berry phase in the Landau Fan diagram. The quantum oscillations rise at a small field of 2 T and preserves up to 100 K, revealing a sizeable Landau level gap and a device mobility of 2138 cm2 V−1 s−1. The angle-variable oscillations indicate the isotropy of the bulk Dirac transport. The large estimated mean free path makes the Cd3As2 nanowire a promising platform for the one-dimensional transport of Dirac semimetals.

Nontrivial surface state transport in Bi2Se3 topological insulator nanoribbons

Topological insulator nanostructures have the larger surface-to-volume ratios than the bulk materials, which enhances the surface state contribution to the electrical transport. Here, we report on the single-crystalline Bi2Se3 narrow nanoribbons synthesized by the chemical vapor deposition method. The surface state induced Aharonov-Bohm effect was observed in the parallel magnetic field. The weak antilocalization (WAL) at various temperatures can be well fitted by the 1D localization theory, and the fitting coherence length is larger than the cross section size of the nanoribbon. The amplitude of WAL after subtracting the bulk background is only dependent on the vertical component of the magnetic field at various angles, revealing the surface nature of WAL. All these signatures indicate the nontrivial surface state transport in our Bi2Se3 narrow nanoribbons.

Probing plasmon resonances of individual aluminum nanoparticles

The plasmon resonances of individual aluminum nanoparticles are investigated by electron energy-loss spectroscopy (EELS) in scanning transmission electron microscope (STEM). Surface plasmon mode an...

Controllable synthesis and magnetotransport properties of Cd3As2 Dirac semimetal nanostructures

Cd3As2, known as the three-dimensional (3D) analogue of graphene, is a Dirac semimetal with a linear dispersion relation along all three directions in momentum space. Here, Cd3As2 nanostructures with various morphologies, including nanowires, nanobelts, nanoplates and nano-octahedra, were synthesized by a facile chemical vapour deposition method. All these kinds of morphologies can be synthesised by carefully adjusting the pressure and argon flow rate. Further, we systematically investigated the magnetotransport properties of the as-grown nanostructures. The temperature dependences of resistance all displayed insulating behaviour, indicating the low carrier density and the Fermi level close to the Dirac point in our Cd3As2 nanostructures. All nanodevices hosted the unsaturated magnetoresistance even up to 14 T. The linear magnetoresistance was observed in nanodevices based on nanoribbons and nanowires. Our detailed study on the morphology regulation and magnetotransport properties of Cd3As2 nanostructures is valuable for the understanding of the growth process and the future nanoelectronic applications of 3D Dirac semimetals.

Synthesis and magnetotransport properties of Bi2Se3 nanowires

Bi2Se3, as a three-dimensional topological insulator, has attracted worldwide attention for its unique surface states which are protected by time-reversal symmetry. Here we report the synthesis and characterization of high-quality single-crystalline Bi2 Se3 nanowires. Bi2Se3 nanowires were synthesized by chemical vapor deposition (CVD) method via gold-catalyzed vapor-liquid-solid (VLS) mechanism. The structure and morphology were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. In magnetotransport measurements, the Aharonov—Bohm (AB) effect was observed in a nanowire-based nanodevice, suggesting the existence of surface states in Bi2Se3 nanowires.