Xiangang Wan

Topological semimetal and Fermi-arc surface states in the electronic structure of pyrochlore iridates

Xiangang Wan, Ari Turner, Ashvin Vishwanath, Sergey Y. Savrasov National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China Department of Physics, University of California, Berkeley, CA 94720 3 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley CA 94720. Department of Physics, University of California, Davis, One Shields Avenue, Davis, CA 95616.

Integrated digital inverters based on two-dimensional anisotropic ReS2 field-effect transistors

Semiconducting two-dimensional transition metal dichalcogenides are emerging as top candidates for post-silicon electronics. While most of them exhibit isotropic behaviour, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. Here we present atomically thin rhenium disulfide (ReS2) flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We fabricated monolayer and few-layer ReS2 field-effect transistors, which exhibit competitive performance with large current on/off ratios (∼107) and low subthreshold swings (100 mV per decade). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known two-dimensional semiconducting materials. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS2 anisotropic field-effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits. Our results underscore the unique properties of two-dimensional semiconducting materials with low crystal symmetry for future electronic applications.

Electronic and magnetic properties of zigzag graphene nanoribbon with one edge saturated

We investigated the energetic stability, electronic, and magnetic properties of the zigzag graphene nanoribbons with one edge saturated by two hydrogen atoms, the other edge saturated by one hydrogen atom by using density-functional theory (DFT). The energy of the ferromagnetic semiconductor state is the lowest state for these nanoribbons. The energy difference between the antiferromagnetic states and the ferromagnetic states varies inversely with the nanoribbon width. Both the band gaps and the magnetic moments in the zigzag graphene nanoribbons with one edge saturated are larger than those of zigzag graphene nanoribbons.

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.

Signature of Strong Spin-Orbital Coupling in the Large Nonsaturating Magnetoresistance Material WTe2.

We report the detailed electronic structure of WTe2 by high resolution angle-resolved photoemission spectroscopy. We resolved a rather complicated Fermi surface of WTe2. Specifically, there are in total nine Fermi pockets, including one hole pocket at the Brillouin zone center Γ, and two hole pockets and two electron pockets on each side of Γ along the Γ-X direction. Remarkably, we have observed circular dichroism in our photoemission spectra, which suggests that the orbital angular momentum exhibits a rich texture at various sections of the Fermi surface. This is further confirmed by our density-functional-theory calculations, where the spin texture is qualitatively reproduced as the conjugate consequence of spin-orbital coupling. Since the spin texture would forbid backscatterings that are directly involved in the resistivity, our data suggest that the spin-orbit coupling and the related spin and orbital angular momentum textures may play an important role in the anomalously large magnetoresistance of WTe2. Furthermore, the large differences among spin textures calculated for magnetic fields along the in-plane and out-of-plane directions also provide a natural explanation of the large field-direction dependence on the magnetoresistance.

Electron-phonon superconductivity near charge-density-wave instability in LaO0.5F0.5BiS2: Density-functional calculations

We discuss the electronic structure, lattice dynamics and electron-phonon interaction of newly discovered superconductor LaO

Raman vibrational spectra of bulk to monolayer ReS2 with lower symmetry

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Strain effects on borophene: ideal strength, negative Possion’s ratio and phonon instability

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Gate-tunable negative longitudinal magnetoresistance in the predicted type-II Weyl semimetal WTe2

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Unexpected Magnetic Semiconductor Behavior in Zigzag Phosphorene Nanoribbons Driven by Half-Filled One Dimensional Band

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