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We performed systematic angle-resolved photoemission spectroscopy measurements in situ on ${T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}\mathrm{Cu}{\mathrm{O}}_{4\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}}$ (LCCO) thin films over the extended doping range prepared by the refined ozone/vacuum annealing method. Electron doping level ($n$), estimated from the measured Fermi surface (FS) volume, varied from 0.05 to 0.23, fully encompassing the superconducting dome. We observed an absence of the insulating behavior around $n\ensuremath{\sim}0.05$ and the shift of FS reconstruction to $n\ensuremath{\sim}0.11$ in LCCO from $n\ensuremath{\sim}0.15$ in other electron-doped cuprates, suggesting that the antiferromagnetic order is strongly suppressed in this material. The possible explanation may lie in the enhanced next-nearest-neighbor hopping in LCCO because of the largest ${\mathrm{La}}^{3+}$ ionic radius among all the lanthanide elements.

In condensed matter systems, chiral topological nodes are robust band crossing points in momentum space that carry nonzero Chern numbers. The chirality is manifested by the presence of surface Fermi arcs connecting the projections of nodes with opposite Chern numbers. In addition to the well-known Weyl nodes, theorists have proposed several other types of chiral topological nodes in condensed matter systems, but the direct experimental evidence of their existence is still lacking. Here, using angle-resolved photoemission spectroscopy, we reveal two types of new chiral nodes, namely the spin-1 nodes and charge-2 Dirac nodes, at the band crossing points near the Fermi level in CoSi, the projections of which on the (001) surface are connected by topologically protected surface Fermi arcs. As these chiral nodes in CoSi are enforced at the Brillouin zone (BZ) center and corner by the crystalline symmetries, the surface Fermi arcs connecting their projections form a non-contractible path traversing the entire (001) surface BZ, in sharp contrast to pairs of Weyl nodes with small separation. Our work marks the first experimental observation of chiral topological nodes beyond the Weyl nodes both in the bulk and on the surface in condensed matter systems.

Topological crystalline insulators (TCIs) are insulating electronic states with nontrivial topology protected by crystalline symmetries. Recently, theory has proposed new classes of TCIs protected by rotation symmetries $$\hat C_n$$ C ̂ n , which have surface rotation anomaly evading the fermion doubling theorem, i.e., n instead of 2n Dirac cones on the surface preserving the rotation symmetry. Here, we report the first realization of the $$\hat C_2$$ C ̂ 2 rotation anomaly in a binary compound SrPb. Our first-principles calculations reveal two massless Dirac fermions protected by the combination of time-reversal symmetry $$\hat T$$ T ̂ and $$\hat C_{2y}$$ C ̂ 2 y on the (010) surface. Using angle-resolved photoemission spectroscopy, we identify two Dirac surface states inside the bulk band gap of SrPb, confirming the $$\hat C_2$$ C ̂ 2 rotation anomaly in the new classes of TCIs. The findings enrich the classification of topological phases, which pave the way for exploring exotic behavior of the new classes of TCIs.

The interplay between time-reversal symmetry (TRS) and band topology plays a crucial role in topological states of quantum matter. In time-reversal-invariant (TRI) systems, the inversion of spin-degenerate bands with opposite parity leads to nontrivial topological states, such as topological insulators and Dirac semimetals. When the TRS is broken, the exchange field induces spin splitting of the bands. The inversion of a pair of spin-splitting subbands can generate more exotic topological states, such as quantum anomalous Hall insulators and magnetic Weyl semimetals. So far, such topological phase transitions driven by the TRS breaking have not been visualized. In this work, using angle-resolved photoemission spectroscopy, we have demonstrated that the TRS breaking induces a band inversion of a pair of spin-splitting subbands at the TRI points of Brillouin zone in EuB$_6$, when a long-range ferromagnetic order is developed. The dramatic changes in the electronic structure result in a topological phase transition from a TRI ordinary insulator state to a TRS-broken topological semimetal (TSM) state. Remarkably, the magnetic TSM state has an ideal electronic structure, in which the band crossings are located at the Fermi level without any interference from other bands. Our findings not only reveal the topological phase transition driven by the TRS breaking, but also provide an excellent platform to explore novel physical behavior in the magnetic topological states of quantum matter., Comment: 22 pages, 7 figures, accepted by Phys. Rev. X

Coxsackievirus A10 (CV-A10) is responsible for an escalating number of severe infections in children, but no prophylactics or therapeutics are currently available. KREMEN1 (KRM1) is the entry receptor for the largest receptor-group of hand-foot-and-mouth disease causing viruses, which includes CV-A10. We report here structures of CV-A10 mature virus alone and in complex with KRM1 as well as of the CV-A10 A-particle. The receptor spans the viral canyon with a large footprint on the virus surface. The footprint has some overlap with that seen for the neonatal Fc receptor complexed with enterovirus E6 but is larger and distinct from that of another enterovirus receptor SCARB2. Reduced occupancy of a particle-stabilising pocket factor in the complexed virus and the presence of both unbound and expanded virus particles suggests receptor binding initiates a cascade of conformational changes that produces expanded particles primed for viral uncoating., Here, the authors provide the structure of mature Coxsackie Virus A10 alone and in complex with its receptor KREMEN1, and of A-particles. This shows how the receptor spans the viral canyon and suggests that receptor binding triggers pocket factor release and conformational changes resulting in expanded particles.

Chiral fermions in solid state feature "Fermi arc" states, connecting the surface projections of the bulk chiral nodes. The surface Fermi arc is a signature of nontrivial bulk topology. Unconventional chiral fermions with an extensive Fermi arc traversing the whole Brillouin zone have been theoretically proposed in CoSi. Here, we use scanning tunneling microscopy / spectroscopy to investigate quasiparticle interference at various terminations of a CoSi single crystal. The observed surface states exhibit chiral fermion-originated characteristics. These reside on (001) and (011) but not (111) surfaces with pi-rotation symmetry, spiral with energy, and disperse in a wide energy range from ~-200 to ~+400 mV. Owing to the high-energy and high-space resolution, a spin-orbit coupling-induced splitting of up to ~80 mV is identified. Our observations are corroborated by density functional theory and provide strong evidence that CoSi hosts the unconventional chiral fermions and the extensive Fermi arc states., Comment: 20 pages, 4 figures

In materials chiral fermions such as Weyl fermions are characterized by nonzero chiral charges, which are singular points of Berry curvature in momentum space. Recently, new types of chiral fermions beyond Weyl fermions have been discovered in structurally chiral crystals CoSi, RhSi and PtAl. Here, we have synthesized RhSn single crystals, which have opposite structural chirality to the CoSi crystals we previously studied. Using angle-resolved photoemission spectroscopy, we show that the bulk electronic structures of RhSn are consistent with the band calculations and observe evident surface Fermi arcs and helical surface bands, confirming the existence of chiral fermions in RhSn. It is noteworthy that the helical surface bands of the RhSn and CoSi crystals have opposite handedness, meaning that the chiral fermions are reversed in the crystals of opposite structural chirality. Our discovery establishes a direct connection between chiral fermions in momentum space and chiral lattices in real space., Exotic chiral fermions beyond Weyl fermions have recently been discovered in a series of chiral crystals such as CoSi. Here, the authors report the evidences of chiral fermions in RhSn with opposite handedness compared to those observed in CoSi, where the structural chirality is also opposite.

In magnetic topological insulators (TIs), the interplay between magnetic order and nontrivial topology can induce fascinating topological quantum phenomena, such as the quantum anomalous Hall effect, chiral Majorana fermions, and axion electrodynamics. Recently, a great deal of attention has been focused on the intrinsic magnetic TIs, where disorder effects can be eliminated to a large extent, which is expected to facilitate the emergence of topological quantum phenomena. Despite intensive efforts, experimental evidence of the topological surface states (SSs) remains elusive. Here, by combining first-principles calculations and angle-resolved photoemission spectroscopy (ARPES) experiments, we reveal that EuSn2As2 is an antiferromagnetic TI with the observation of Dirac SSs consistent with our prediction. We also observe nearly gapless Dirac SSs in antiferromagnetic TIs MnBi2nTe3n+1 (n=1 and 2), which are absent in previous ARPES results. These results provide clear evidence for nontrivial topology of these intrinsic magnetic TIs. Furthermore, we find that the topological SSs show no observable changes across the magnetic transition within the experimental resolution, indicating that the magnetic order has a quite small effect on the topological SSs, which can be attributed to weak hybridization between the localized magnetic moments, from either 4f or 3d orbitals, and the topological electronic states. This finding provides insights for further research that the correlations between magnetism and topological states need to be strengthened to induce larger gaps in the topological SSs, which will facilitate the realization of topological quantum phenomena at higher temperatures.

We study the systematic doping evolution of nodal dispersions by in situ angle-resolved photoemission spectroscopy on the continuously doped surface of a high-temperature superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+x}$ and reveal that the nodal dispersion has three fundamentally different segments separated by two kinks, located at \ensuremath{\sim}10 meV and roughly 70 meV, respectively. These three segments have different band velocities and different doping dependence. In particular, in the underdoped region the velocity of the high-energy segment increases monotonically as the doping level decreases and can even surpass the bare band velocity. We propose that electron fractionalization is a possible cause for this anomalous nodal dispersion and may even play a key role in the understanding of exotic properties of cuprates.

Surface-sensitive measurements are crucial to many types of researches in condensed matter physics. However, it is difficult to obtain atomically flat surfaces of many single crystals by the commonly used mechanical cleavage. We demonstrate that the grind-polish-sputter-anneal method can be used to obtain atomically flat surfaces on topological materials. Three types of surface-sensitive measurements are performed on CoSi (001) surface with dramatically improved quality of data. This method extends the research area of surface-sensitive measurements to hard-to-cleave alloys, and can be applied to irregular single crystals with selective crystalline planes. It may become a routine process of preparing atomically flat surfaces for surface-sensitive technologies.

We report a technique of a continuously doped surface of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+x}$ through ozone/vacuum annealing and a systematic measurement over the nearly whole superconducting dome on the same sample surface by in situ angle-resolved photoemission spectroscopy. We find that the quasiparticle weight on the antinode is proportional to the doped carrier concentration $x$ within the entire superconducting dome, while the nodal quasiparticle weight changes more mildly. More significantly, we discover that a $d$-wave pairing energy gap extracted from the nodal region scales well with the onset temperature of the Nernst signal. These findings suggest that the emergence of superconducting pairing is concomitant with the onset of free vortices.

Recently we developed a technique of ozone/vacuum annealing to continuously change the doping level of the surface of Bi2Sr2CaCu2O8+ x and measured a nearly whole superconducting dome on one surface by in-situ angle-resolved photoemission spectroscopy [arXiv: 1805.06450]. Here we study the evolution of the electronic structures of Bi2Sr2CaCu2O8+ x using this technique together with tight binding fits. The tight binding parameters are extracted to study their evolution with doping.