Superconductivity often emerges in the proximity of, or in competition with, symmetry breaking ground states such as antiferromagnetism or charge density waves (CDW) 1--5 . A number of materials in the cuprate family, which includes the high--transition--temperature (high--T c ) superconductors, show spin and charge density wave order 5--7 . Thus a fundamental question is to what extent these ordered states exist for compositions close to optimal for superconductivity. Here we use high--energy x--ray diffraction to show that a CDW develops at zero field in the normal state of superconducting YBa 2 Cu 3 O 6.67 (T c = 67 K). Below T c , the application of a magnetic field suppresses superconductivity and enhances the CDW. Hence, the CDW and superconductivity are competing orders in this typical high--T c superconductor, and high--T c superconductivity can form from a pre--existing CDW state. Our results explain observations of small Fermi surface pockets 8 , negative Hall and Seebeck effect 9,10 and the "T c plateau" 11 in this material when underdoped.Charge density waves in solids are periodic modulations of conduction electron density. They are often present in low dimensional systems such as NbSe 2 4 . Certain cuprate materials such as La 2--x--y Nd y Sr x CuO 4 (Nd--LSCO) and La 2--x Ba x CuO 4 , (LBCO) also show charge modulations that suppress superconductivity near x=1/8 6,7 . In some cases, these are believed to be unidirectional in the CuO 2 plane, and have been dubbed 'stripes' 2,3 . There is now a mounting body of indirect evidence that charge and/or spin density waves may be present at high magnetic fields in samples with high T c : quantum oscillation experiments on underdoped YBa 2 Cu 3 O y (YBCO) have revealed the existence of at least one small Fermi surface pocket 8,9 which may be created by a charge modulation 10 . More recently, NMR studies have shown a magnetic--field--induced splitting of the Cu2F lines of YBCO 12 . An important issue is the extent to which the tendency towards charge order exists in high--T c superconductors 2,3 .Here we report a hard (100 keV) X--ray diffraction study, in magnetic fields up to 17 T, of a de-twinned single crystal of YBa 2 Cu 3 O 6.67 (with ortho--VIII oxygen ordering 11,13 , T c = 67 K and p = 0.12 where p is the hole concentration per planar Cu). We find that a CDW forms in the normal state below T CDW ≈ 135 K. The charge modulation has two fundamental wavevectors q CDW = q 1 = (δ 1 , 0, 0.5) and q 2 = (0, δ 2 , 0.5), where δ 1 ≈ 0.3045(2) and δ 2 ≈0.3146(7). These give satellites of the parent crystal Bragg peaks at positions such as Q=(2±δ 1 , 0, 0.5). Although the satellite intensities have a strong temperature and magnetic field dependence, the CDW is not field-induced and is unaffected by field in the normal state. Below T c it competes with superconductivity, and a decrease of the CDW amplitude in zero field becomes an increase when superconductivity is suppressed by field. Figure 1a,g shows scans through the (2--δ 1 , 0, 0.5) and (0, 2--δ 2 , 0.5) p...

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 fermions 1 . 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.The massless Dirac equation in the three-dimensional (3D) momentum space can be regarded as the overlap of two Weyl fermions with opposite chirality 1,2 . The Dirac fermionic quasiparticle is stable under the protection of some crystal symmetry in topological Dirac semimetals such as Na 3 Bi (ref. 3) and Cd 3 As 2 (ref. 4). However, a separated single Weyl node is much more robust and requires no protection of crystal symmetry. An isolated Weyl node is a sink or source of gauge field of Berry curvature, like a monopole in momentum space, and the chirality corresponds to its topological charge [5][6][7] . Weyl nodes appear in pairs of opposite chirality in a real material due to the 'No-go theorem' 8,9 . To obtain isolated Weyl nodes, the spin degeneracy of the electronic bands has to be removed by breaking either inversion symmetry or time-reversal symmetry. Although non-degenerate band crossing is not rare, finding a material with only Weyl nodes near the Fermi energy (E F ) is a big challenge. Recently, the noncentrosymmetric and non-magnetic transition-metal monoarsenide TaAs has been predicted to be a Weyl semimetal (WSM), and twelve pairs of Weyl nodes are expected in its 3D Brillouin zone (BZ; refs 10,11). Compared with other proposals 6,7,12-22 for realizing a Weyl state, the TaAs family features easy sample fabrication, a non-magnetic state and no fine-tuning of the electronic states is necessary, making experimental studies of Weyl semimetals possible. Many exotic properties induced by the Weyl nodes have been predicted and observed recently, such as surface states with Fermi arcs 23,24 and a negative magneto-resistivity 25,26 due to the chiral anomaly 27-29 . However, crucial evidence for Weyl nodes in the bulk states has not been observed. In this paper, by using soft X-ray angle-resolved photoemission spectroscopy (ARPES), which is sensitive to the bulk states, we report the first experimental observation of Weyl nodes in TaAs.TaAs crystallizes in a body-centred-tetragonal structure with the nonsymmorphic space group...