Fermi Surface and Band Dispersion in La_2-xSr_xCuO₄

Abstract

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) ...

Proposal of an extendedt−JHamiltonian for high-Tccuprates fromab initiocalculations on embedded clusters

“…Recent analysis of angle-resolved photoemission spectroscopy data has shown that both t N N N and t nN N N are necessary for understanding the dispersion and line shape of the spectral function in the t-J model [26]. Tohyama et al [25] have estimated the ratio t N N N /t N N and t nN N N /t N N to be -0.12 and 0.08, respectively, by fitting the tight-binding Fermi surface to the experimental one in the overdoped sample [47] on the assumption that in the overdoped region the Fermi surface of the tight-binding band is the same as that of the t-t'-t"-J model. The values here-proposed are in good agreement with these ratio: (ii) the singlet displacement operator h SD has to be taken into account, at least for the colinear displacement, since h SD ≃ -80 meV.…”

Temperature dependence of spin correlation and charge dynamics in the stripe phase of high-Tcsuperconductors

“…Anomalous behaviors that are supposed to be related to the presence of the stripes have been observed in the angle-resolved photoemission (ARPES) spectrum as suppressed spectral weight along the (0,0)-(π,π) direction 9 and in the optical conductivity as the enhancement of intensity in the mid-infrared region. 10 These features have been explained by the present authors, by using the exact diagonalization calculation at zero-temperature for a model that includes both the strong electron correlation and the stripes, i.e., a t-J model with an additional potential introduced to stabilize vertical charge stripes.…”

Electronlike Fermi surface and remnant(π,0)feature in overdopedLa1.78

“…Presumably the (π, 0) flat band feature which exists in the optimum and underdoped regions does not completely lose its spectral weight even for x = 0.22 (and probably for x = 0.3, see data in Ref. 8 ) where the saddle point is located above E F . Therefore, the spectral weight around (π, 0) appears as a remnant of the "flat band".…”

“…La 2−x Sr x CuO 4 (LSCO), by virtue of the absence of these effects and the availability of high quality single crystal samples over the entire doping range, provides the opportunity to advance our understanding of the high temperature superconductors. In the underdoped regime, earlier studies have uncovered the presence of two electronic components [8][9][10] , a systematic suppression of the spectral weight near (π/2, π/2) (when compared with that of overdoped samples or BSCCO for data taken under the same conditions), and straight Fermi surface segments near (π,0) of width ∼ π/2 11,12 , which have been interpreted as evidence for electronic inhomogeneities. In the overdoped regime, an electronlike Fermi surface was observed in the high-T c superconductors for the first time 8 , but despite this progress, important problems remain.…”