Topological semimetals (TSMs) are characterized by bulk band crossings in their electronic structures, which are expected to give rise to gapless electronic excitations and topological features that ...underlie exotic physical properties. The most famous examples are Dirac and Weyl semimetals, in which the corresponding low-energy fermionic excitations, i.e., the Dirac and Weyl fermions, are direct analogs of elementary particles in quantum field theory. The last decade has witnessed an explosion of research activities in the field of TSMs thanks to precise theoretical predictions, well-controlled material synthesis, and advanced characterization techniques including angle-resolved photoemission spectroscopy, scanning tunneling microscopy, magnetotransport measurements, optical spectroscopy, etc. Here recent progress in three-dimensional TSMs is reviewed with an emphasis on their characteristic bulk electronic structures, including dimensionality (such as zero-dimensional nodal points, one-dimensional nodal lines, and two-dimensional nodal surfaces), degeneracy (twofold, threefold, fourfold, sixfold, or eightfold) of the band crossing, the slope (type I and type II) and order (linear, quadratic, or cubic) of the band dispersion near the crossing, the characteristic topological invariants (such as monopole charges), and the crystallographic symmetries that stabilize the band crossings. The distinct signatures of the various topological semimetal phases, such as the nontrivial surface states (including Fermi arcs of Dirac and Weyl semimetals) and the unique transport and optical responses (such as chiral anomaly-induced negative magnetoresistance in Dirac and Weyl semimetals), are also reviewed.
Weyl semimetals are a class of materials that can be regarded as three-dimensional analogs of graphene upon breaking time-reversal or inversion symmetry. Electrons in a Weyl semimetal behave as Weyl ...fermions, which have many exotic properties, such as chiral anomaly and magnetic monopoles in the crystal momentum space. The surface state of a Weyl semimetal displays pairs of entangled Fermi arcs at two opposite surfaces. However, the existence of Weyl semimetals has not yet been proved experimentally. Here, we report the experimental realization of a Weyl semimetal in TaAs by observing Fermi arcs formed by its surface states using angle-resolved photoemission spectroscopy. Our first-principles calculations, which match remarkably well with the experimental results, further confirm that TaAs is a Weyl semimetal.
A Weyl semimetal possesses spin-polarized band-crossings, called Weyl nodes, connected by topological surface arcs. The low-energy excitations near the crossing points behave the same as massless ...Weyl fermions, leading to exotic properties like chiral anomaly. To have the transport properties dominated by Weyl fermions, Weyl nodes need to locate nearly at the chemical potential and enclosed by pairs of individual Fermi surfaces with non-zero Fermi Chern numbers. Combining angle-resolved photoemission spectroscopy and first-principles calculation, here we show that TaP is a Weyl semimetal with only a single type of Weyl fermions, topologically distinguished from TaAs where two types of Weyl fermions contribute to the low-energy physical properties. The simple Weyl fermions in TaP are not only of fundamental interests but also of great potential for future applications. Fermi arcs on the Ta-terminated surface are observed, which appear in a different pattern from that on the As-termination in TaAs and NbAs.
The origin of enhanced superconductivity over 50 K in the recently discovered FeSe monolayer films grown on SrTiO
(STO), as compared to 8 K in bulk FeSe, is intensely debated. As with the ...ferrochalcogenides A
Fe
Se
and potassium-doped FeSe, which also have a relatively high-superconducting critical temperature (T
), the Fermi surface (FS) of the FeSe/STO monolayer films is free of hole-like FS, suggesting that a Lifshitz transition by which these hole FSs vanish may help increasing T
. However, the fundamental reasons explaining this increase of T
remain unclear. Here we report a 15 K jump of T
accompanying a second Lifshitz transition characterized by the emergence of an electron pocket at the Brillouin zone centre, which is triggered by high-electron doping following in situ deposition of potassium on FeSe/STO monolayer films. Our results suggest that the pairing interactions are orbital dependent in generating enhanced superconductivity in FeSe.
The matrix proteins (M) of many enveloped RNA viruses mediate virus assembly and budding. However, it remains poorly understood how M are involved in virus budding and how they interact with envelope ...proteins. Here, we show that the expression level of Nipah (NiV) M in particles produced by the host cells deviates from a gamma distribution and does not reflect that of the host cells, indicating assembly of the NiV-M in the process. Our data reveal that NiV-M affects the circularity of the particles while the NiV envelope proteins do not. The organization of NiV envelope proteins on the membrane of the particles is similar to those that do not express NiV-M, suggesting that NiV-M does not directly interact with the envelope proteins during assembly and budding.
Observation of Weyl nodes in TaAs Lv, B. Q.; Xu, N.; Weng, H. M. ...
Nature physics,
09/2015, Letnik:
11, Številka:
9
Journal Article
Recenzirano
Odprti dostop
In 1929, H. Weyl proposed that the massless solution of the Dirac equation represents a pair of a new type of particles, the so-called Weyl fermions1. However, their existence in particle physics ...remains elusive after more than eight decades. Recently, significant advances in both topological insulators and topological semimetals have provided an alternative way to realize Weyl fermions in condensed matter, as an emergent phenomenon: when two non-degenerate bands in the three-dimensional momentum space cross in the vicinity of the Fermi energy (called Weyl nodes), the low-energy excitations behave exactly as Weyl fermions. Here we report the direct observation in TaAs of the long-sought-after Weyl nodes by performing bulk-sensitive soft X-ray angle-resolved photoemission spectroscopy measurements. The projected locations at the nodes on the (001) surface match well to the Fermi arcs, providing undisputable experimental evidence for the existence of Weyl fermionic quasiparticles in TaAs.
We have investigated the spin texture of surface Fermi arcs in the recently discovered Weyl semimetal TaAs using spin- and angle-resolved photoemission spectroscopy. The experimental results ...demonstrate that the Fermi arcs are spin polarized. The measured spin texture fulfills the requirement of mirror and time-reversal symmetries and is well reproduced by our first-principles calculations, which gives strong evidence for the topologically nontrivial Weyl semimetal state in TaAs. The consistency between the experimental and calculated results further confirms the distribution of chirality of the Weyl nodes determined by first-principles calculations.
MoTe2 is an exfoliable transition metal dichalcogenide (TMD) that crystallizes in three symmetries: the semiconducting trigonal-prismatic 2H- or α-phase, the semimetallic and monoclinic 1T′- or ...β-phase, and the semimetallic orthorhombic γ-structure. The 2H-phase displays a band gap of ∼1 eV making it appealing for flexible and transparent optoelectronics. The γ-phase is predicted to possess unique topological properties that might lead to topologically protected nondissipative transport channels. Recently, it was argued that it is possible to locally induce phase-transformations in TMDs, through chemical doping, local heating, or electric-field to achieve ohmic contacts or to induce useful functionalities such as electronic phase-change memory elements. The combination of semiconducting and topological elements based upon the same compound might produce a new generation of high performance, low dissipation optoelectronic elements. Here, we show that it is possible to engineer the phases of MoTe2 through W substitution by unveiling the phase-diagram of the Mo1–x W x Te2 solid solution, which displays a semiconducting to semimetallic transition as a function of x. We find that a small critical W concentration x c ∼ 8% stabilizes the γ-phase at room temperature. This suggests that crystals with x close to x c might be particularly susceptible to phase transformations induced by an external perturbation, for example, an electric field. Photoemission spectroscopy, indicates that the γ-phase possesses a Fermi surface akin to that of WTe2.
The aim of this study was to develop a widely accepted prognostic nomogram for extranodal NK/T-cell lymphoma, nasal-type (NKTCL). The clinical data from 1383 patients with NKTCL treated at 10 ...participating institutions between 2000 and 2011 were reviewed. A nomogram was developed that predicted overall survival (OS) based on the Cox proportional hazards model. To contrast the utility of the nomogram against the widely used Ann Arbor staging system, the International Prognostic Index (IPI) and the Korean Prognostic Index (KPI), we used the concordance index (C-index) and a calibration curve to determine its predictive and discriminatory capacity. The 5-year OS rate was 60.3% for the entire group. The nomogram included five important variables based on a multivariate analysis of the primary cohort: stage; age; Eastern Cooperative Oncology Group performance status; lactate dehydrogenase; and primary tumor invasion. The calibration curve showed that the nomogram was able to predict 5-year OS accurately. The C-index of the nomogram for OS prediction was 0.72 for both cohorts, which was superior to the predictive power (range, 0.56-0.64) of the Ann Arbor stage, IPI and KPI in the primary and validation cohorts. The proposed nomogram provides an individualized risk estimate of OS in patients with NKTCL.
Simple structural modifications using oxidation and methylation of a quinoline-containing diarylethene result in dramatic variation of photophysical properties. Turn-on fluorescence, room temperature ...phosphorescence (RTP) and red-green-blue (RGB) switching were achieved in three different related compounds. Photoswitchable diarylethenes (DAEs) that exhibit turn-on fluorescence are in high demand for super-resolution microscopy, and the development of purely organic phosphorescent materials in the amorphous state is attractive but challenging. The findings reported here provide a novel toolkit for designing turn-on fluorescence DAEs for super-resolution microscopy and extending the scope of amorphous RTP materials. More importantly, we bridge between these two fundamentally significant photochemical and photophysical phenomena, and reveal structure-property relationships between DAE photochromism and RTP.
Simple structural modifications using oxidation and methylation of a quinoline-containing diarylethene result in dramatic variation of photophysical properties.
PDF
