arXiv:0708.2333v2 [cond-mat.supr-con] 27 Jul 2008 The coherent d-wave superconducting gap in underdoped La 2-x Sr x CuO 4 as studied by angle-resolved photoemission M. Shi, 1 J. Chang, 2 S. Pailh´ es, 2 M. R. Norman, 3 J. C. Campuzano, 4, 3 M. M˚ ansson, 5 T. Claesson, 5 O. Tjernberg, 5 A. Bendounan, 2 L. Patthey, 1 N. Momono, 6 M. Oda, 6 M. Ido, 6 C. Mudry, 7 and J. Mesot 2 1 Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland 2 Laboratory for Neutron Scattering, ETH Zurich and Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland 3 Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 USA 4 Department of Physics, University of Illinois at Chicago, Chicago, IL 60607 USA 5 Materials Physics, Royal Institute of Technology KTH, S-164 40 Kista, Sweden 6 Department of Physics, Hokkaido University Sapporo 060-0810, Japan 7 Condensed Matter Theory Group, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland (Dated: July 27, 2008) We present angle-resolved photoemission spectroscopy (ARPES) data on moderately underdoped La1.855 Sr0.145CuO4 at temperatures below and above the superconducting transition temperature. Unlike previous studies of this material, we observe sharp spectral peaks along the entire underlying Fermi surface in the superconducting state. These peaks trace out an energy gap that follows a simple d-wave form, with a maximum superconducting gap of 14 meV. Our results are consistent with a single gap picture for the cuprates. Furthermore our data on the even more underdoped sample La1.895 Sr0.105 CuO4 also show sharp spectral peaks, even at the antinode, with a maximum superconducting gap of 26 meV. PACS numbers: 74.72.Dn, 74.25.Jb, 79.60.Bm The energy gap is a fundamental property of super- conductors [1]. The nature of its anisotropy has played a key role in the testing and building of microscopic theo- ries of the superconductivity discovered in layered copper oxides [2]. This anisotropy can be measured by angle- resolved photoemission spectroscopy (ARPES), which is a unique probe of electronic excitations and their mo- mentum dependence [3, 4]. For underdoped samples, an energy gap persists above T c [5, 6]. This pseudogap is most prominent in the (π, 0) region of the Brillouin zone, giving rise to a gapless arc of states centered about the zone diagonal known as a Fermi arc [7]. How the gap evolves from the superconducting state to the pseudogap phase remains one of the most important questions be- ing debated in the cuprates [8]. It has been suggested that the superconducting gap only exists on the Fermi arcs, and thus is distinct from the pseudogap [9]. This “two gap” scenario has been supported by recent ARPES studies on optimally doped La 2−x Sr x CuO 4 (LSCO) [10] and Bi 2 Sr 2 CuO 6 (Bi2201) [11], as well as on heavily un- derdoped Bi2212 [12]. In these studies, sharp spectral peaks are observed below T c only along the arc, whereas the states in the region around the (π, 0) - (π,π), Fermi crossing (the antinode) remain incoherent, as they were above T c . Moreover, the size of the energy gap in the antinodal region is significantly larger than that expected from a simple extrapolation of the gap from the arc. Our ARPES results on moderately underdoped LSCO (x =0.145) reveal a very different picture. Similar to previous studies of underdoped Bi2212 [13], below T c we observe sharp spectral peaks along the entire underly- ing Fermi surface (FS), which trace out a simple d-wave gap with a maximum amplitude of 14 meV. For more underdoped LSCO (x =0.105) the spectra are still char- acterized by coherent peaks in the (π, 0) region with a maximal gap amplitude of 26 meV. We see no evidence for a much larger gap in the antinodal region as reported in other studies [10, 11, 12]. More significantly, we find that the superconducting and pseudogaps have similar maximum amplitudes. ARPES results were obtained on very high quality sin- gle crystals of LSCO, grown using the travelling solvent floating zone method [14], with a transition temperature T c = 36 K and 30 K for x =0.145 and x =0.105, respec- tively. The transition width for both dopings are ΔT c ≈ 1.5 K. ARPES experiments were carried out at the Sur- face and Interface Spectroscopy beamline at the Swiss Light Source. During the measurements, the base pres- sure always remained less than 5 × 10 −11 mbar. The ARPES spectra were recorded with a Scienta SES2002 electron analyzer with an angular resolution of better than 0.15 ◦ . Circularly polarized light with hν = 55 eV and linearly polarized light with hν = 25 eV were used. The energy resolutions were 17 meV and 12 meV for hν = 55 eV and hν = 25 eV, respectively. The Fermi level was determined by recording the photoemis- sion spectra from polycrystalline copper on the sample holder. The samples were cleaved in situ by using a specially designed cleaving tool [15]. Clear (1×1) low- energy electron-diffraction patterns obtained after the ARPES measurements indicate that the cleaved surfaces had good quality. Figure 1 shows typical ARPES intensity maps of LSCO (x =0.145) in the superconducting phase as a function