In condensed matter physics, spontaneous symmetry breaking has been a key concept, and discoveries of new types of broken symmetries have greatly increased our understanding of matter 1,2 . Recently, electronic nematicity, novel spontaneous rotational-symmetry breaking leading to an emergence of a special direction in electron liquids, has been attracting significant attention 3-6 . Here, we show bulk thermodynamic evidence for nematic superconductivity, in which the nematicity emerges in the superconducting gap amplitude, in Cu x Bi 2 Se 3 . Based on high-resolution calorimetry of single-crystalline samples under accurate two-axis control of the magnetic field direction, we discovered clear two-fold symmetry in the specific heat and in the upper critical field despite the trigonal symmetry of the lattice. Nematic superconductivity for this material should possess a unique topological nature associated with odd parity 7-9 . Thus, our findings establish a new class of spontaneously symmetry-broken states of matter-namely, odd-parity nematic superconductivity.

We have performed angle-resolved photoemission spectroscopy on HfSiS, which has been predicted to be a topological line-node semimetal with square Si lattice. We found a quasi-two-dimensional Fermi surface hosting bulk nodal lines, alongside the surface states at the Brillouin-zone corner exhibiting a sizable Rashba splitting and band-mass renormalization due to many-body interactions. Most notably, we discovered an unexpected Dirac-like dispersion extending one-dimensionally in k space -the Dirac-node arc -near the bulk node at the zone diagonal. These novel Dirac states reside on the surface and could be related to hybridizations of bulk states, but currently we have no explanation for its origin. This discovery poses an intriguing challenge to the theoretical understanding of topological line-node semimetals. In contrast to conventional semimetals with a finite band-overlap between valence band (VB) and conduction band (CB), topological semimetals are categorized by the band-contacting nature between VB and CB in the Brillouin zone (BZ); point-contact (Dirac/Weyl semimetals) or line-contact (line-node semimetals; LNSMs). The existence of three-dimensional (3D) Dirac semimetals was first confirmed by angle-resolved photoemission spectroscopy (ARPES) of Cd 3 As 2 [9, 10] and Na 3 Bi [11], where the VB and CB contact each other at the point (Dirac point) protected by rotational symmetry of the crystal [12,13]. Recent ARPES studies on noncentrosymmetric transition-metal monopnictides [14][15][16][17] have clarified pairs of bulk Dirac-cone bands and Fermi-arc SSs, supporting their Weyl-semimetallic nature [18,19]. While the existence of Weyl semimetals with point nodes has been confirmed experimentally, the experimental studies of LNSMs with line nodes are relatively scarce [20][21][22][23] despite many theoretical predictions [24][25][26][27][28][29][30].Recently, it was theoretically proposed by Xu et al. that ZrSiO with PbFCl-type crystal structure (space group P 4/nmm) and its isostructural family WHM (W = Zr, Hf, or La; H = Si, Ge, Sn, or Sb; M = O, S, Se and Te; see Fig. 1 . These studies demonstrated the realization of LNSM phase as well as an appearance of nearly-flat SSs around theX point, both were explained on the basis of band calculations.In this Letter, we report the ARPES results on HfSiS. In addition to the overall VB structure which is in support of the LNSM nature of HfSiS, we found new spectral features, such as a large Rashba splitting of SSs atX, a dispersion kink at ∼0.05 eV, and most importantly, unexpected Dirac-like SSs forming a "Dirac-node arc". This is a rare case in the research of topological materials that experiment finds novel SSs that were not predicted by theory.Figure 1(c) shows the ARPES-intensity plot in the VB region as a function of wave vector and binding energy (E B ) measured along theΓX cut at hν = 80 eV (see Supplemental Materials for details of sample preparation [33] and ARPES measurements). One can notice several dispersive bands; holelike bands atΓ (h) with the to...

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on noncentrosymmetric Weyl semimetal candidate NbP, and determined the electronic states of both Nband P-terminated surfaces corresponding to the "opposite" surfaces of a polar crystal. We revealed a drastic difference in the Fermi-surface topology between the opposite surfaces, whereas the Fermi arcs on both surfaces are likely terminated at the surface projection of the same bulk Weyl nodes. Comparison of the ARPES data with our first-principles band calculations suggests notable difference in electronic structure at the Nb-terminated surface between theory and experiment. The present result opens a platform for realizing exotic quantum phenomena arising from unusual surface properties of Weyl semimetals.