Condensed matter systems can host quasiparticle excitations that are analogues to elementary particles such as Majorana, Weyl, and Dirac fermions. Recent advances in band theory have expanded the classification of fermions in crystals, and revealed crystal symmetry-protected electron excitations that have no high-energy counterparts.Here, using angle-resolved photoemission spectroscopy, we demonstrate the existence of a triply degenerate point in the electronic structure of MoP crystal, where the quasiparticle excitations are beyond the Majorana-Weyl-Dirac classification.Furthermore, we observe pairs of Weyl points in the bulk electronic structure coexisting with the 'new fermions', thus introducing a platform for studying the interplay between different types of fermions.In quantum field theory, Lorentz invariance gives three types of fermions, namely, the Dirac, Weyl and Majorana fermions (1,2). While it is still under debate whether any elementary particle of Weyl or Majorana types exists, all three types of fermions have been proposed to exist as low-energy and long-wavelength quasiparticle excitations in condensed matter systems (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). The existence of Dirac and Weyl fermions has been experimentally confirmed (15)(16)(17)(18)(19)(20) and that of Majorana fermions has been supported by various experiments (21,22). Recently, it has been shown theoretically that as the Poincare group (Lorentz group plus 4-translation) in the continuum space-time is reduced to the 230 space groups in lattices, more types of fermions (dubbed 'new fermions') are allowed to appear as quasiparticle excitations near certain band crossing points (23-29).Specially, it is well known that fermion statistics is incompatible with three-fold degeneracy in the continuum due to the half-integer spin; yet, three-fold degeneracy (triply degenerate point (TP)) can be protected in a lattice either by rotation symmetries (25-29) or nonsymmorphic symmetries (23,24). In either case, the three-component fermions conceptually lie between Weyl fermions (two-component) and Dirac fermions (four-component) (Fig. 1A), and carry characteristic properties distinct from the other two, including unique surface states and transport features. The crossing point is triply degenerate and protected by the C 3 symmetry along Γ-A, which is similar to the case of the Dirac semimetals Na 3 Bi (7) and Cd 3 As 2 (9). With SOC considered, the bands along Γ-A are reconstructed into two doubly-degenerate |J z | = 1/2 bands and two non-degenerate |J z | = 3/2 bands due to the M z mirror symmetry. The crossing points of the bands with different |J z | are protected by the C 3 symmetry, forming four TPs along the Γ-A line (Fig. 1F).We first perform core level photoemission measurements, which confirms the chemical composition of MoP ( Fig. 2A). respectively. We observe one hexagonal hole pocket around Γ and one small hole pocket at K at k z = 0, as well as one almost circular electron pocket around Α at k z = π.These experimental...

Monolayer antimonene is fabricated on PdTe by an epitaxial method. Monolayer antimonene is theoretically predicted to have a large bandgap for nanoelectronic devices. Air-exposure experiments indicate amazing chemical stability, which is great for device fabrication. A method to fabricate high-quality monolayer antimonene with several great properties for novel electronic and optoelectronic applications is provided.

We report a new kagome quantum spin liquid candidate Cu3Zn(OH)6FBr, which does not experience any phase transition down to 50 mK, more than three orders lower than the antiferromagnetic Curie-Weiss temperature (∼ 200 K). A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from 19 F nuclear magnetic resonance measurements. We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations. Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system, resembling charge fractionalization in the fractional quantum Hall state.

Barlowite Cu4(OH)6FBr shows three-dimensional (3D) long-range antiferromagnetism, which is fully suppressed in Cu3Zn(OH)6FBr with a kagome quantum spin liquid ground state. Here we report systematic studies on the evolution of magnetism in the Cu4−xZnx(OH)6FBr system as a function of x to bridge the two limits of Cu4(OH)6FBr (x=0) and Cu3Zn(OH)6FBr (x=1). Neutrondiffraction measurements reveal a hexagonal-to-orthorhombic structural change with decreasing temperature in the x = 0 sample. While confirming the 3D antiferromagnetic nature of low-temperature magnetism, the magnetic moments on some Cu 2+ sites on the kagome planes are found to be vanishingly small, suggesting strong frustration already exists in barlowite. Substitution of interlayer Cu 2+ with Zn 2+ with gradually increasing x completely suppresses the bulk magnetic order at around x = 0.4, but leaves a local secondary magnetic order up to x ∼ 0.8 with a slight decrease in its transition temperature. The high-temperature magnetic susceptibility and specific heat measurements further suggest that the intrinsic magnetic properties of kagome spin liquid planes may already appear from x > 0.3 samples. Our results reveal that the Cu4−xZnx(OH)6FBr may be the long-thought experimental playground for the systematic investigations of the quantum phase transition from a long-range antiferromagnet to a topologically ordered quantum spin liquid.

We report our point-contact spectroscopy (PCS) study on the superconducting state of the type-II Dirac semimetal PdTe2 with a superconducting transition temperature Tc ∼ 1.65 K. Both mechanical-and soft-PCS differential conductance curves at 0.3 K show a consistent double-peak structure and they can be perfectly fitted by a single s-wave gap based on the Blonder-Tinkham-Klapwijk model. The gap follows a typical Bardeen-Cooper-Schrieffer temperature behavior, yielding ∆0 ∼ 0.29 meV and 2∆0/kBTc = 4.15 in the strong coupling regime. A sudden suppression of the superconducting gap in magnetic field around Hc1 ∼ 130 Oe is observed for most point-contacts on PdTe2, characteristic of a first-order transition for type-I superconductor in field. However, for other contacts, a smooth evolution of the PCS conductance persists up to Hc2 ∼ 600 Oe, signaling a local type-II superconductivity. The observed admixture of type-I and type-II superconductivity can possibly arise from an inhomogeneous electron mean free path on the surface of PdTe2 due to its topological surface states.

A quantum spin liquid with a Z2 topological order has long been thought to be important for the application of quantum computing and may be related to high-temperature superconductivity [1][2][3]. While a two-dimensional kagome antiferromagnet may host such a state, strong experimental evidences are still lacking [4][5][6][7][8][9]. Here we show that the spin excitations from the kagome planes in magnetically ordered Cu4(OD)6FBr and non-magnetically ordered Cu3Zn(OD)6FBr are similarly gapped although the content of inter-kagome-layer Cu 2+ ions changes dramatically. This suggests that the spin triplet gap and continuum of the intrinsic kagome antiferromagnet are robust against the interlayer magnetic impurities. Our results show that the ground state of Cu3Zn(OD)6FBr is a gapped quantum spin liquid with Z2 topological order.

We investigated the magnetic structure and dynamics of YbMnBi2, with elastic and inelastic neutron scattering, to shed light on the topological nature of the charge carriers in the antiferromagnetic phase. We confirm C-type antiferromagnetic ordering of the Mn spins below TN = 290 K, and determine that the spins point along the c-axis to within about 3 • . The observed magnon spectrum can be described very well by the same effective spin Hamiltonian as was used previously to model the magnon spectrum of CaMnBi2. Our results show conclusively that the creation of Weyl nodes in YbMnBi2 by the time-reversal-symmetry breaking mechanism can be excluded in the bulk.

Topological semimetals with different types of band crossings provide a rich platform to realize novel fermionic excitations, known as topological fermions. In particular, some fermionic excitations can be direct analogues of elementary particles in quantum field theory when both obey the same laws of physics in the low-energy limit. Examples include Dirac and Weyl fermions, whose solid-state realizations have provided new insights into long-sought phenomena in highenergy physics. Recently, theorists predicted new types of fermionic excitations in condensed-matter systems without any high-energy counterpart, and their existence is protected by crystalline symmetries. By studying the topology of the electronic structure in PdBiSe using density functional theory calculations and bulk-sensitive soft X-ray angle-resolved photoemission spectroscopy, we demonstrate a coexistence of four different types of topological fermions: Weyl, Rarita-Schwinger-Weyl, double class-II three-component, and charge-2 fourfold fermions. Our discovery provides a remarkable platform to realize multiple novel fermions in a single solid, charting the way forward to studies of their potentially exotic properties as well as their interplay.