Yan Sun

Bio: Yan Sun is an academic researcher from Max Planck Society. The author has contributed to research in topics: Weyl semimetal & Medicine. The author has an hindex of 62, co-authored 386 publications receiving 20105 citations. Previous affiliations of Yan Sun include University of Vienna & University of Science and Technology of China.

Tet-Mediated Formation of 5-Carboxylcytosine and Its Excision by TDG in Mammalian DNA

Abstract: The prevalent DNA modification in higher organisms is the methylation of cytosine to 5-methylcytosine (5mC), which is partially converted to 5-hydroxymethylcytosine (5hmC) by the Tet (ten eleven translocation) family of dioxygenases. Despite their importance in epigenetic regulation, it is unclear how these cytosine modifications are reversed. Here, we demonstrate that 5mC and 5hmC in DNA are oxidized to 5-carboxylcytosine (5caC) by Tet dioxygenases in vitro and in cultured cells. 5caC is specifically recognized and excised by thymine-DNA glycosylase (TDG). Depletion of TDG in mouse embyronic stem cells leads to accumulation of 5caC to a readily detectable level. These data suggest that oxidation of 5mC by Tet proteins followed by TDG-mediated base excision of 5caC constitutes a pathway for active DNA demethylation.

Dirac semimetal and topological phase transitions in A 3 Bi ( A = Na , K, Rb)

Abstract: Three-dimensional (3D) Dirac point, where two Weyl points overlap in momentum space, is usually unstable and hard to realize. Here we show, based on the first-principles calculations and effective model analysis, that crystalline A(3)Bi (A = Na, K, Rb) are Dirac semimetals with bulk 3D Dirac points protected by crystal symmetry. They possess nontrivial Fermi arcs on the surfaces and can be driven into various topologically distinct phases by explicit breaking of symmetries. Giant diamagnetism, linear quantum magnetoresistance, and quantum spin Hall effect will be expected for such compounds.

Negative magnetoresistance without well-defined chirality in the Weyl semimetal TaP

Abstract: Weyl semimetals (WSMs) are topological quantum states wherein the electronic bands disperse linearly around pairs of nodes with fixed chirality, the Weyl points In WSMs, nonorthogonal electric and magnetic fields induce an exotic phenomenon known as the chiral anomaly, resulting in an unconventional negative longitudinal magnetoresistance, the chiral-magnetic effect However, it remains an open question to which extent this effect survives when chirality is not well-defined Here, we establish the detailed Fermi-surface topology of the recently identified WSM TaP via combined angle-resolved quantum-oscillation spectra and band-structure calculations The Fermi surface forms banana-shaped electron and hole pockets surrounding pairs of Weyl points Although this means that chirality is ill-defined in TaP, we observe a large negative longitudinal magnetoresistance We show that the magnetoresistance can be affected by a magnetic field-induced inhomogeneous current distribution inside the sample

Weyl Semimetals as Hydrogen Evolution Catalysts

Abstract: The search for highly efficient and low-cost catalysts is one of the main driving forces in catalytic chemistry. Current strategies for the catalyst design focus on increasing the number and activity of local catalytic sites, such as the edge sites of molybdenum disulfides in the hydrogen evolution reaction (HER). Here, the study proposes and demonstrates a different principle that goes beyond local site optimization by utilizing topological electronic states to spur catalytic activity. For HER, excellent catalysts have been found among the transition-metal monopnictides-NbP, TaP, NbAs, and TaAs-which are recently discovered to be topological Weyl semimetals. Here the study shows that the combination of robust topological surface states and large room temperature carrier mobility, both of which originate from bulk Dirac bands of the Weyl semimetal, is a recipe for high activity HER catalysts. This approach has the potential to go beyond graphene based composite photocatalysts where graphene simply provides a high mobility medium without any active catalytic sites that have been found in these topological materials. Thus, the work provides a guiding principle for the discovery of novel catalysts from the emerging field of topological materials.

Extremely large magnetoresistance and ultrahigh mobility in the topological Weyl semimetal candidate NbP

Abstract: Weyl semimetals are predicted to exhibit a host of unusual transport properties. NbP, a system predicted to share characteristics of both normal and Weyl semimetals, is now shown to have a very large, non-saturating magnetoresistance. Recent experiments have revealed spectacular transport properties in semimetals, such as the large, non-saturating magnetoresistance exhibited by WTe2 (ref. 1). Topological semimetals with massless relativistic electrons have also been predicted2 as three-dimensional analogues of graphene3. These systems are known as Weyl semimetals, and are predicted to have a range of exotic transport properties and surface states4,5,6,7, distinct from those of topological insulators8,9. Here we examine the magneto-transport properties of NbP, a material the band structure of which has been predicted to combine the hallmarks of a Weyl semimetal10,11 with those of a normal semimetal. We observe an extremely large magnetoresistance of 850,000% at 1.85 K (250% at room temperature) in a magnetic field of up to 9 T, without any signs of saturation, and an ultrahigh carrier mobility of 5 × 106 cm2 V−1 s−1 that accompanied by strong Shubnikov–de Haas (SdH) oscillations. NbP therefore presents a unique example of a material combining topological and conventional electronic phases, with intriguing physical properties resulting from their interplay.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

2D transition metal dichalcogenides

Abstract: Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of atomic-scale thickness, direct bandgap, strong spin–orbit coupling and favourable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examined and their properties are discussed, with particular attention to their charge density wave, superconductive and topological phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties. Two-dimensional transition metal dichalcogenides (TMDCs) exhibit attractive electronic and mechanical properties. In this Review, the charge density wave, superconductive and topological phases of TMCDs are discussed, along with their synthesis and applications in devices with enhanced mobility and with the use of strain engineering to improve their properties.