ALS BL10.01, PF BL-28A

Two-gap behaviors of the high-Tc cuprate superconductors: Universal versus
material-dependent properties
T. Yoshida1, M. Hashimoto1, K. Tanaka2, N. Mannnella2, Z. Hussain3, Z.-X. Shen2,A. Fujimori1, M. Kubota4, K. Ono4, S. Komiya5, Y. Ando6, H. Eisaki7, S. Uchida11Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan2
Department of Applied Physics and Stanford Synchrotron Radiation Laboratory, Stanford
University, Stanford, CA94305
3
Advanced Light Source, Lawrence Berkeley National Lab, Berkeley, CA 94720, USA
4
Photon Factory, Institute of Materials Structure Science, KEK, Tsukuba, Ibaraki 305-0801,Japan
5Central Research Institute of Electric Power Industry, Komae, Tokyo 201-8511, Japan
6
The Institute of Scientific and IndustrialResearch, Osaka University, Ibaraki, Osaka
567-0047, Japan
7
National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8568, JapanOne of the central issues in the research of thehigh-Tc cuprates superconductors is whether thepseudogap is a distinct phenomenon fromsuperconuctivity or a gap due to local pairing orincoherent superconducting fluctuations above Tc. Inthe former scenario, a possible origin of thepseudogap is preformed Cooper pairs lacking phasecoherence. In the latter scenario, the pseudogap is due
to a competing order such as spin density wave,
charge density wave, d-density wave, etc. It has been
well known that the pseudogap in the anti-nodal (,0)region increases with underdoping as observed byangle-resolved photoemission spectroscopy (ARPES).
However, the energy gap measured by otherexperimental methods such as Andreev reflection,
whichismore
directlyassociatedwithsuperconductivity, exhibits opposite trend, that is, thegap decreases with underdoping, suggesting adifferent origin of the superconducting gap from theantinodal gap.
A recent ARPES study of Bi2Sr2Ca1-xYxCu2O8(Bi2212) has revealed the presence of two distinct
energy gaps in different regions of momentum space[1,2]. One is the antinodal region as mentioned above,and increases with underdoping. The other opens in
the near-nodal region showing a coherent peak, anddoes not increase with underdoping. On the otherhand, attempts have been made to understand thepseudogap within a single d-wave energy gap [3]. Insuch a single gap picture, preformed Cooper pairs are
the most likely origin of the pseudogap.
Since the doping and temperature dependences ofthe energy gap would reveal the entangled two-gap
behavior, we have investigated the energy gap oflightly- to optimally-doped LSCO by ARPES as afunction of doping and temperature. In the presentwork, the momentum dependence of the gap clearlyexhibits two-gap behavior as in the case of heavily
underdoped Bi2212: the pseudogap * in theantinodal region and the d-wave like gap 0 aroundthe node (Fig. 1). The doping dependence of theobtained parameter 0 qualitatively explains thereduction of the Tc with underdoping. Furthermore,from comparison of the present results with those on
Bi2212 and other cuprates with higher Tc's, we havefound that the magnitude of the * and the pseudogap
temperature T* is not appreciably material-dependent,
suggesting that the pseudogap is properties of a single
CuO2 plane. On the other hand, the magnitude of the0, which is proportional to the superconducting gap,
is strongly material-dependent (CuO2 layer numberdependent) like Tc, suggesting that they areinfluenced by the effect of neighboring CuO2 planesand block layers.
References[1] K. Tanaka et al., Science 314, 1910 (2006).
[2] W. S. Lee et al., Nature 450, 81 (2007).
[3] A. Kanigel et al., Nature Physics 2, 447 (2006)

Fig. 1. Angular dependence of the gap of LSCO
observed by ARPES. The definition of * and 0 isshown for x=0.15.