The electronic structure of the La2−xSrxCuO4 (LSCO) system has been studied by angle-resolved photoemission spectroscopy (ARPES). We report on the evolution of the Fermi surface, the superconducting gap and the band dispersion around the extended saddle point k = (π, 0) with hole doping in the superconducting and metallic phases. As hole concentration x decreases, the flat band at (π, 0) moves from above the Fermi level (EF) for x > 0.2 to below EF for x < 0.2, and is further lowered down to x = 0.05. From the leading-edge shift of ARPES spectra, the magnitude of the superconducting gap around (π, 0) is found to monotonically increase as x decreases from x = 0.30 down to x = 0.05 even though Tc decreases in the underdoped region, and the superconducting gap appears to smoothly evolve into the normal-state gap at x = 0.05. It is shown that the energy scales characterizing these low-energy structures have similar doping dependences. For the heavily overdoped sample (x = 0.30), the band dispersion and the ARPES spectral lineshape are analyzed using a simple phenomenological self-energy form, and the electronic effective mass enhancement factor m * /m b ≃ 2 has been found. As the hole concentration decreases, an incoherent component that cannot be described within the simple self-energy analysis grows intense in the high-energy tail of the ARPES peak. Some unusual features of the electronic structure observed for the underdoped region (x < ∼ 0.10) are consistent with the numerical works on the stripe model.

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We report on the result of angle-resolved photoemission (ARPES) study of La2−xSrxCuO4 (LSCO) from an optimally doped superconductor (x = 0.15) to an antiferromagnetic insulator (x = 0). Near the superconductor-insulator transition (SIT) x ∼ 0.05, spectral weight is transferred with hole doping between two coexisting components, suggesting a microscopic inhomogeneity of the dopedhole distribution. For the underdoped LSCO (x ≤ 0.12), the dispersive band crossing the Fermi level becomes invisible in the (0, 0)−(π, π) direction unlike Bi2Sr2CaCu2O8+y. These observations may be reconciled with the evolution of holes in the insulator into fluctuating stripes in the superconductor.PACS numbers: 74.25.Jb, 71.30.+h, 74.72.Dn, 74.62.Dh The key issue to clarify the nature of high-temperature superconductivity in the cuprates is how the electronic structure evolves from the antiferromagnetic insulator (AFI) to the superconductor (SC) with hole doping. For the hole-doped CuO 2 planes in the superconductors, band dispersions and Fermi surfaces have been extensively studied by angle-resolved photoemission spectroscopy (ARPES) primarily on Bi 2 Sr 2 CaCu 2 O 8+y (BSCCO) 1-4 . Also for the undoped AFI, band dispersions have been observed for Sr 2 CuO 2 Cl 2 5,6 . However, the band structures of the AFI and the SC are distinctly different and ARPES data have been lacking around the boundary between the AFI and the SC. In order to reveal the missing link, the present ARPES study has been performed on La 2−x Sr x CuO 4 (LSCO), which covers continuously from the SC to the AFI in a single system.In addition, the family of LSCO systems show a suppression of T c at a hole concentration δ ≃ 1/8, while the BSCCO system does not. As the origin of the anomaly at δ ≃ 1/8, the instability towards the spin-charge order in a stripe form has been extensively discussed on the basis of the incommensurate peaks in inelastic neutron scattering (INS) 7-9 . Comparing the ARPES spectra of LSCO and BSCCO will help us to clarify the impact of the stripe fluctuations.In the present paper, we discuss the novel observation of two spectral components coexisting around the SIT (x ∼ 0.05), the unusual disappearance of the Fermi surface near (π/2, π/2) in the underdoped LSCO (x ≤ 0.12) 4 , and their relevance to the stripe fluctuations.The ARPES measurements were carried out at beamline 5-3 of Stanford Synchrotron Radiation Laboratory (SSRL). Incident photons had an energy of 29 eV and were linearly polarized. The total energy resolution was approximately 45 meV and the angular resolution was ±1 degree. Single crystals of LSCO were grown by the traveling-solvent floating-zone method and then annealed so that the oxygen content became stoichiometric. The accuracy of the hole concentration δ was ±0.01. The x = 0 samples were slightly hole doped by excess oxygen so that δ ≃ 0.005 was deduced from its Néel temperature T N = 220 K 10 . The spectrometer was kept in an ultra high vacuum better than 5 × 10 −11 Torr during the measurements. The samples were cleaved ...

Using angle-resolved photoemission spectroscopy (ARPES), we observe the band structure, the Fermi surface and their doping dependences in La2−xSrxCuO4. The results reveal that the Fermi surface undergoes a dramatic change: it is holelike and centered at (π,π) in underdoped (x = 0.1) and optimally doped (x = 0.15) samples as in other cuprates, while it is electronlike and centered at (0,0) in heavily overdoped (x = 0.3) ones. The peak in the ARPES spectra near (π/2,π/2) is broad and weak unlike that in other cuprates. In the underdoped and optimally doped samples, a superconducting gap (∆ = 10 − 15 meV) is observed near (π,0). KEYWORDS: La 2−x SrxCuO 4 , ARPES, Fermi surface, band dispersion, superconducting gap, doping dependence Among the family of high-T c cuprate superconductors, La 2−x Sr x CuO 4 (LSCO) provides unique opportunities to study the systematic evolution of the electronic structure with hole doping. First, LSCO has a simple crystal structure with single CuO 2 layers. It has neither Cu-O chains as in YBa 2 Cu 3 O 7−δ (YBCO) nor complicated structural modulation of the block layers as in Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212). Second, the hole concentration in the CuO 2 plane can be controlled over a wide range and uniquely determined by the Sr concentration x (and the small oxygen non-stoichiometry). One can therefore investigate the doping dependence of the electronic structure continuously from the heavily overdoped limit (x ∼ 0.35) to the undoped insulator (x = 0) in the same system. This information would be highly useful to critically check existing theories of electron correlations and superconductivity in the CuO 2 plane. Indeed, the doping dependences of thermodynamic and transport properties have been extensively studied for the LSCO system. 1-5)Angle-resolved photoemission spectroscopy (ARPES) is a powerful method to probe the electronic structure of low-dimensional systems. In particular, band structures, Fermi surfaces 6-12) and superconducting and normalstate gaps in the high-T c cuprates [13][14][15][16][17][18][19] have been observed by ARPES. However, most of the ARPES experiments have focused on the Bi2212 system and its family compounds; ARPES studies of LSCO have been hindered probably because LSCO is difficult to cleave and its surface is not as stable as that of Bi2212 under an ultrahigh vacuum.We have recently focused on the LSCO system and carried out a series of photoemission studies. 20, 21)The angle-integrated photoemission (AIPES) spectra of * E-mail: ino@wyvern.phys.s.u-tokyo.ac.jp * * Present address: The Institute of Physical and Chemical Research (RIKEN), SPring-8, Kamigori-cho, Hyogo 678-12, Japan.LSCO show a broad feature (at ∼ −100 meV) and a suppression of the density of states at the Fermi level (E F ) in the underdoped region.21) In the present study, we have overcome the experimental difficulties in the ARPES of LSCO using high-quality single crystals, and have measured ARPES spectra of underdoped (x = 0.1), optimally doped (x = 0.15) and heavily overdoped (x = 0.3) ...

We have made a high-resolution photoemission study of La 22x Sr x CuO 4 in a wide hole concentration (x) range from a heavily overdoped metal to an undoped insulator. As x decreases, the spectral density of states at the chemical potential (m) is suppressed with an x dependence similar to the suppression of the electronic specific heat coefficient. In the underdoped region, the spectra show a pseudogap structure on the energy scale of 0.1 eV. The width of the pseudogap increases with decreasing x following the x dependence of the characteristic temperatures of the magnetic susceptibility and the Hall coefficient. [S0031-9007(98)06985-3] PACS numbers: 71.30. + h, 74.72.Dn, 74.25.Jb, 79.60.Bm In order to understand the mechanism of hightemperature superconductivity in doped cuprates, a central issue has been the evolution of the electronic structure with hole doping near the filling-control metal-insulator transition (MIT). In spite of extensive photoemission studies, it still remains unclear how the electronic structure evolves, especially between underdoped metal and antiferromagnetic insulator. For a systematic study of the doping dependence near the MIT, La 22x Sr x CuO 4 (LSCO) is a suitable system. It has the simplest crystal structure with single CuO 2 layers and the hole concentration in the CuO 2 plane is well controlled over a wide range and uniquely determined by the Sr concentration x (and small oxygen nonstoichiometry). So far photoemission studies of high-T c cuprates were concentrated on Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) and YBa 2 Cu 3 O 72y (YBCO) systems. With the LSCO system, one can investigate the electronic structure of the CuO 2 plane continuously from the heavily overdoped limit (x ϳ 0.35) to the undoped insulator (x 0) in a single system.Recently, in underdoped cuprates a "normal-state gap" behavior above T c has been observed by angle-resolved photoemission spectroscopy (ARPES) in BSCCO [1] and a "spin-gap behavior" by NMR in YBCO [2]. The magnitude of the normal-state gap is of the same order as the superconducting gap at optimal doping. Meanwhile, underdoped cuprates have characteristic temperatures which are considerably higher than T c in the uniform magnetic susceptibility [3], the electronic specific heat [4], the Hall coefficient [5], and the electrical resistivity [3]. All these characteristic temperatures show similar behaviors in LSCO: they increase from ϳ300 K at optimal doping x ϳ 0.15 to ϳ600 K at x ϳ 0.1 for the LSCO system, suggesting a pseudogap-type electronic structure. In addition, it can be reconciled only if a pseudogap is opened at the chemical potential that both the electronic specific heat coefficient g [4,6] and the chemical potential shift with doping [7] are suppressed towards the MIT. To obtain a full picture of the evolution of those "gaps," it is also necessary to know the total density of state (DOS), which is most directly observed by angle-integrated photoemission spectroscopy (AIPES). In the present study, we have performed high-resolution AIPES measurements...