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

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

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

We study a class of photonic crystals in which both the dielectric constant e d the magnetic permeability m vary within the crystal. We find that in the case where both e andm have their maximum values in the same material, the photonic band gaps tend to disappear. However, in the opposite case where e andm have their maximum values in different materials, the gaps become wider with maximum gap over midgap frequency as high as 0.8. We also discuss the possibility of designing such photonic crystals with ferrites. @S0163-1829 ~97!06227-9# BRIEF REPORTS

Physical Review B

Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

Superconductivity in iron compounds

The past three years have witnessed the discovery of a series of novel high-temperature superconductors. Trailing behind the cuprates, these iron-based compounds are the second-highest-temperature superconducting material family known to date. Despite the marked differences in the chemical composition, these materials share many properties with the cuprates and offer the hope of finally unveiling the secret of high-temperature superconductivity. The main theme of this review is the electron-pairing mechanism responsible for their superconductivity. We discuss the progress in this young field and point out the open issues.

https://science.sciencemag.org/content/sci/332/6026/200.full.pdf

The Electron-Pairing Mechanism of Iron-Based Superconductors

The recently discovered FeAs-based materials exhibit a $(\ensuremath{\pi},0)$ spin density wave (SDW) in the undoped state, which gives way to superconductivity upon doping. Here we show that due to an interesting topological feature of the band structure, the SDW state cannot acquire a full gap. This is demonstrated within the SDW mean-field theory of both a simplified two-band model and a more realistic five-band model. The positions of the nodes are different in the two models and can be used to detect the validity of each model.

Nodal Spin Density Wave and band topology of the FeAs based materials

We point out the possibility of a nearly flat band with Chern number $C=2$ on the dice lattice in a simple nearest-neighbor tight-binding model. This lattice can be naturally formed by three adjacent $(111)$ layers of cubic lattice, which may be realized in certain thin films or artificial heterostructures, such as the SrTiO${}_{3}$/SrIrO${}_{3}$/SrTiO${}_{3}$ trilayer heterostructure grown along the $(111)$ direction. The flatness of two bands is protected by the bipartite nature of the lattice. Including the Rashba spin-orbit coupling on nearest-neighbor bonds causes the flat bands to separate from the others but maintain their flatness. Repulsive interaction will drive spontaneous ferromagnetism on the Kramer pair of the flat bands and split them into two nearly flat bands with Chern number $C=\ifmmode\pm\else\textpm\fi{}2$. We thus propose that this may be a route to the quantum anomalous Hall effect and further conjecture that the partial filling of the $C=2$ band may realize exotic fractional quantum Hall effects.

/pdf/nearly-flat-band-with-chern-number-c-2-on-the-dice-lattice-3vb9e76cz5.pdf

Nearly flat band with Chern number C=2 on the dice lattice

In common with other iron-based high-temperature superconductors, FeSe exhibits a transition to a ‘nematic’ phase below 90 K in which the crystal rotation symmetry is spontaneously broken. However, the absence of strong low-frequency magnetic fluctuations near or above the transition has been interpreted as implying the primacy of orbital ordering. In contrast, we establish that quantum fluctuations of spin-1 local moments with strongly frustrated exchange interactions can lead to a nematic quantum paramagnetic phase consistent with the observations in FeSe. We show that this phase is a fundamental expression of the existence of a Berry’s phase associated with the topological defects of a Neel antiferromagnet, in a manner analogous to that which gives rise to valence bond crystal order for spin-1/2 systems. We present an exactly solvable model realizing the nematic quantum paramagnetic phase, discuss its relation with the spin-1 J1–J2 model, and construct a field theory of the Landau-forbidden transition between the Neel state and this nematic quantum paramagnet. Nematic phases with broken crystal rotation symmetry are as ubiquitous in superconductors as they are puzzling. One model shows that frustrated magnetism alone can account for the nematicity in FeSe, which shows no measurable magnetic order.

Nematicity and quantum paramagnetism in FeSe

The Griffiths singularity in a phase transition, caused by disorder effects, was predicted more than 40 years ago. Its signature, the divergence of the dynamical critical exponent, is challenging to observe experimentally. We report the experimental observation of the quantum Griffiths singularity in a two-dimensional superconducting system. We measured the transport properties of atomically thin gallium films and found that the films undergo superconductor-metal transitions with increasing magnetic field. Approaching the zero-temperature quantum critical point, we observed divergence of the dynamical critical exponent, which is consistent with the Griffiths singularity behavior. We interpret the observed superconductor-metal quantum phase transition as the infinite-randomness critical point, where the properties of the system are controlled by rare large superconducting regions.

https://science.sciencemag.org/content/sci/early/2015/10/14/science.aaa7154.full.pdf

Quantum Griffiths singularity of superconductor-metal transition in Ga thin films

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