Topological semimetals (TSs) in structurally chiral crystals (which possess a handedness due to a lack of mirror and inversion symmetries) are expected to display numerous exotic physical phenomena, such as new fermionic excitations with large topological charge 1 , long Fermi-arc surface states 2,3 , unusual magnetotransport 4 and lattice dynamics 5 , as well as a quantized response to circularly polarized light 6 . To date, however, all experimentally confirmed TSs crystallize in space groups that contain mirror operations, which forces the aforementioned properties to vanish. Here, by employing angle-resolved photoelectron spectroscopy and ab-initio calculations, we show that AlPt is a structurally chiral TS that hosts new fourfold and sixfold fermions, which can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. Remarkably, these multifold fermions are located at high symmetry points with Chern numbers larger than those in Weyl-semimetals, thus resulting in multiple Fermi-arcs that thread through the full diagonal of the surface Brillouin zone (BZ), spanning the largest portion of the BZ of any material. By imaging these long Fermi-arcs, we can experimentally determine the magnitude and sign of their Chern number, which allows us to relate their dispersion to the handedness of their host crystal.Revised manuscript 2 An object that cannot be superimposed with its mirror image is said to be chiral, a concept first proposed by Lord Kelvin 7 that has found widespread applications across the modern sciences, from high energy physics to biology 8,9 . In condensed matter physics (where the properties of crystalline materials are tightly constrained by spatial lattice symmetries), chiral crystals can only be found in the 65 Sohncke space groups, which contain only orientation-preserving operations, and can therefore be assigned a handedness. The structural chirality of these systems can endow them with fascinating properties, such as natural optical activity 10 , negative refraction in metamaterials 11 , non-reciprocal effects such as magnetochiral birefringence of light or electronic magnetochiral anisotropy 12,13 , chiral magnetic textures such as helices and skyrmions 14 , or unusual superconductivity 15 . In the recently discovered Weyl-semimetals, not just the crystal structure itself, but also the electronic wavefunctions can exhibit a chirality at point-like twoband crossings of the quasiparticle dispersion, which are known as Weyl fermions [16][17][18][19] . These crossings are topologically protected because they carry a topological charge, a quantized Berry flux through any surface enclosing them in momentum space. This charge has integer magnitude C= ±1, which is known as the Chern number and gives the electronic wavefunction a handedness. This topological property results in a plethora of exotic phenomena, such as Fermiarc surface states 20 , unconventional magnetoresistance 16,21,22 , nonlocal transport 23 , and many

A new layered oxide semiconductor (Bi2O2Se) is found with excellent electronic properties for promising applications.

Topological Dirac semimetals (TDSs) represent a new state of quantum matter recently discovered that offers a platform for realizing many exotic physical phenomena. A TDS is characterized by the linear touching of bulk (conduction and valance) bands at discrete points in the momentum space (i.e. 3D Dirac points), such as in Na3Bi and Cd3As2. More recently, new types of Dirac semimetals with robust Dirac line-nodes (with non-trivial topology or near the critical point between topological phase transitions) have been proposed that extends the bulk linear touching from discrete points to 1D lines. In this work, using angle-resolved photoemission spectroscopy (ARPES), we explored the electronic structure of the non-symmorphic crystals MSiS (M=Hf, Zr). Remarkably, by mapping out the band structure in the full 3D Brillouin Zone (BZ), we observed two sets of Dirac line-nodes in parallel with the kz-axis and their dispersions. Interestingly, along directions other than the line-nodes in the 3D BZ, the bulk degeneracy is lifted by spinorbit coupling (SOC) in both compounds with larger magnitude in HfSiS. Our work not only experimentally confirms a new Dirac line-node semimetal family protected by nonsymmorphic symmetry, but also helps understanding and further exploring the exotic properties as well as practical applications of the MSiS family of compounds.

Topological semimetals feature protected nodal band degeneracies characterized by a topological invariant known as the Chern number (C). Nodal band crossings with linear dispersion are expected to have at most |C|=4, which sets an upper limit to the magnitude of many topological phenomena in these materials. Here, we show that the chiral crystal palladium gallium (PdGa) displays multifold band crossings, which are connected by exactly four surface Fermi arcs, thus proving that they carry the maximal Chern number magnitude of 4. By comparing two enantiomers, we observe a reversal of their Fermi-arc velocities, which demonstrates that the handedness of chiral crystals can be used to control the sign of their Chern numbers.