다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

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.

2D transition metal dichalcogenides

Weyl and Dirac semimetals are three-dimensional phases of matter with gapless electronic excitations that are protected by topology and symmetry. As three-dimensional analogs of graphene, they have generated much recent interest. Deep connections exist with particle physics models of relativistic chiral fermions, and, despite their gaplessness, to solid-state topological and Chern insulators. Their characteristic electronic properties lead to protected surface states and novel responses to applied electric and magnetic fields. The theoretical foundations of these phases, their proposed realizations in solid-state systems, and recent experiments on candidate materials as well as their relation to other states of matter are reviewed.

Weyl and Dirac semimetals in three-dimensional solids

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

The magnetoresistance effect in WTe2, a layered semimetal, is extremely large: the electrical resistance can be changed by more than 13 million per cent at very high magnetic fields and low temperatures. Apply a magnetic field to a magnetoresistive material and its electrical resistance changes — a technologically useful phenomenon that is harnessed, for example, in the data-reading sensors of hard drives. Mazhar Ali and colleagues have now identified a material (tungsten ditelluride or WTe2) in which the magnetoresistance effect is unusually large: the electrical resistance can be changed by more than 13 million per cent. Its remarkable magnetoresitance is evident at very high magnetic fields and at extremely low temperatures, so practical applications are not yet in prospect. But this finding suggests new directions in the study of magnetoresistivity that could ultimately lead to new uses of this effect. Magnetoresistance is the change in a material’s electrical resistance in response to an applied magnetic field. Materials with large magnetoresistance have found use as magnetic sensors1, in magnetic memory2, and in hard drives3 at room temperature, and their rarity has motivated many fundamental studies in materials physics at low temperatures4. Here we report the observation of an extremely large positive magnetoresistance at low temperatures in the non-magnetic layered transition-metal dichalcogenide WTe2: 452,700 per cent at 4.5 kelvins in a magnetic field of 14.7 teslas, and 13 million per cent at 0.53 kelvins in a magnetic field of 60 teslas. In contrast with other materials, there is no saturation of the magnetoresistance value even at very high applied fields. Determination of the origin and consequences of this effect, and the fabrication of thin films, nanostructures and devices based on the extremely large positive magnetoresistance of WTe2, will represent a significant new direction in the study of magnetoresistivity.

Large, non-saturating magnetoresistance in WTe2.

Dirac and Weyl semimetals are 3D analogues of graphene in which crystalline symmetry protects the nodes against gap formation Na3Bi and Cd3As2 were predicted to be Dirac semimetals, and recently confirmed to be so by photoemission experiments Several novel transport properties in a magnetic field have been proposed for Dirac semimetals Here, we report a property of Cd3As2 that was unpredicted, namely a remarkable protection mechanism that strongly suppresses backscattering in zero magnetic field In single crystals, the protection results in ultrahigh mobility, 9 × 10(6) cm(2) V(-1) s(-1) at 5 K Suppression of backscattering results in a transport lifetime 10(4) times longer than the quantum lifetime The lifting of this protection by the applied magnetic field leads to a very large magnetoresistance We discuss how this may relate to changes to the Fermi surface induced by the applied magnetic field

Ultrahigh mobility and giant magnetoresistance in the Dirac semimetal Cd3As2

Dirac semimetals and Weyl semimetals are 3D analogs of graphene in which crystalline symmetry protects the nodes against gap formation [1-3]. Na$_3$Bi and Cd$_3$As$_2$ were predicted to be Dirac semimetals [4,5], and recently confirmed to be so by photoemission [6-8]. Several novel transport properties in a magnetic field $\bf H$ have been proposed for Dirac semimetals [2,9-11]. Here we report an interesting property in Cd$_3$As$_2$ that was unpredicted, namely a remarkable protection mechanism that strongly suppresses back-scattering in zero $\bf H$. In single crystals, the protection results in a very high mobility that exceeds $>10^7$ cm$^2$/Vs below 4 K. Suppression of backscattering results in a transport lifetime 10$^4\times$ longer than the quantum lifetime. The lifting of this protection by $\bf H$ leads to an unusual giant $\bf H$-linear magnetoresistance that violates Kohler's rule. We discuss how this may relate to changes to the Fermi surface induced by $\bf H$.

Ultrahigh mobility and giant magnetoresistance in the Dirac semimetal Cd$_3$As$_2$

Using angle-resolved photoemission spectroscopy, a 3D Dirac semimetal phase has been observed in Cadmium Arsenide for the first time.

Experimental realization of a three-dimensional Dirac semimetal.

A variety of monolayer crystals have been proposed to be two-dimensional topological insulators exhibiting the quantum spin Hall effect (QSHE), possibly even at high temperatures. Here we report the observation of the QSHE in monolayer tungsten ditelluride (WTe 2 ) at temperatures up to 100 kelvin. In the short-edge limit, the monolayer exhibits the hallmark transport conductance, ~ e 2 / h per edge, where e is the electron charge and h is Planck’s constant. Moreover, a magnetic field suppresses the conductance, and the observed Zeeman-type gap indicates the existence of a Kramers degenerate point and the importance of time-reversal symmetry for protection from elastic backscattering. Our results establish the QSHE at temperatures much higher than in semiconductor heterostructures and allow for exploring topological phases in atomically thin crystals.

http://science.sciencemag.org/content/sci/359/6371/76.full.pdf

Quinn Gibson

Papers

Large, non-saturating magnetoresistance in WTe2.

Ultrahigh mobility and giant magnetoresistance in the Dirac semimetal Cd3As2

Ultrahigh mobility and giant magnetoresistance in the Dirac semimetal Cd$_3$As$_2$

Experimental realization of a three-dimensional Dirac semimetal.

Observation of the quantum spin Hall effect up to 100 kelvin in a monolayer crystal