We use angle-resolved photoemission spectroscopy (ARPES) to investigate the properties of the energy gap(s) in optimally doped (Bi,Pb)2(Sr,La)2CuO 6+δ (Bi2201). We find that the spectral gap has two components in the superconducting state: a superconducting gap and pseudogap. Significant differences in their momentum and temperature dependence suggest that they represent two separate energy scales. Spectra near the node reveal a sharp peak with a small gap below Tc that closes at Tc. Near the antinode, the spectra are broad with a large energy gap of ∼40meV above and below Tc. The spectral shape and gap magnitude around the antinode are almost constant across Tc, which indicates that the pseudogap state coexists with the superconducting state below Tc, and it dominates the character of the spectra around the antinode. We speculate that the pseudogap state competes with the superconductivity by diminishing spectral weight in the antinodal regions, where the magnitude of the superconducting gap is largest. PACS numbers:The pseudogap is one of the most fascinating properties of the high temperature superconductors 1 . It gives rise to a strange state of matter above T c where parts of the Fermi surface consist of disconnected "arcs" 2 , while the remainder is gapped. Recent Angle Resolved Photoemission Spectroscopy (ARPES) measurements show that the pseudogap state extrapolates at absolute zero to a nodal liquid where the Fermi surface consists of just four points 3 . Since the pseudogap is often closely linked to the mechanism of high temperature superconductivity, it is very important to understand its properties and relationship to the superconducting gap. According to one class of theories 4 , the pseudogap opens because electrons are paired at temperatures much higher than the critical temperature (T c ) with the same pairing mechanism as the superconducting gap. The pairs only condense when the sample is cooled to T c . This scenario is supported by a number of ARPES studies on mostly Bi 2 Sr 2 CaC 2 O 8+δ (Bi2212) suggesting that the behavior and symmetry of the pseudogap above T c are similar to those of the superconducting gap below T c 5,6,7 . Another class of theories 8,9,10 link the pseudogap to an ordered state with possibly a separate energy scale. The first ARPES experiment designed to detect an ordered state below the pseudogap temperature gave a positive result 11 . However, a later ARPES experiment was unable to detect the same small signatures in the data 12 . More recently, a high precision neutron scattering experiment provided direct evidence for the existence of an ordered state of particular symmetry below the pseudogap temperature 13 . This result confirmed the predictions of Varma and was in agreement with the first ARPES study. Recent scanning tunneling microscopy and scanning tunneling spectroscopy (STM/STS) experiments on Bi2212 show that even below T c , a pseudogap state, characterized by a large gap and broad spectral peaks, coexists with the superconducting state, which has a smalle...

With a strong help of high-resolution photoemission spectroscopy we demonstrate in this paper that the large thermoelectric power observed in the layered cobalt oxides, such as Ca 3 Co 4 O 9 , Na 0.6 CoO 2 , and Bi 2 Sr 2 Co 2 O 9 , can be well accounted for with the Boltzmann-type metallic electrical conduction. An intense peak with 1.5-2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level E F in these compounds. The density of states at E F is finite but negligibly small at room temperature, because E F is located near the high-energy edge of this narrow band. We calculated thermoelectric power S using the Boltzmann transport equation with the electronic structure near E F determined by the photoemission measurement. The calculated S shows fairly good consistency with the measured value both in its magnitude and the temperature dependence.

We report the polarized optical absorption spectra of single-walled 4 Å carbon nanotubes arrayed in the channels of an AlPO 4 -5 single crystal. When the light electric field ͑E͒ is polarized parallel to the tube direction ͑c͒, the spectra display a sharp peak at 1.37 eV, with two broadbands at 2.1 and 3.1 eV. In the EЌc configuration, the tube is nearly transparent in the measured energy region 0.5-4.1 eV. The optical dipole selection rules are discussed, and the absorption bands are assigned to the dipole transitions between the Van Hove singularities. The measured absorption spectra agreed well with the ab initio calculations of band structure based on the local density function approximation. DOI: 10.1103/PhysRevLett.87.127401 PACS numbers: 78.67.Ch, 61.46. +w, 73.22.-f, 82.75.Mj In 1993, Iijima's group as well as Bethune's group found that the use of transition-metal catalysts leads to nanotubes with only a single wall [1]. A single-walled carbon nanotube (SWNT) is wrapped from a two-dimensional graphite sheet. The diameter of each freestanding SWNT ranges from 0.7 to a few tens of nanometers with a maximum length of about 1 mm. Within the band-folding scheme, the diameter and the chirality of a SWNT are believed to determine whether the nanotube is metallic or semiconducting [2]. Their electronic densities of states have Van Hove singularities, which have been directly observed by scanning tunneling spectroscopy [3]. Optical absorption spectra have been measured for bundles of SWNTs and nanotube thin films [4]. The absorption bands can be attributed to optical transitions between the Van Hove singularities. More controlled experimental studies on the optical properties for SWNTs are, however, not easy to carry out because of the technical difficulty in fabricating monosized and well-aligned nanotubes. Recently, SWNTs with a diameter as small as 4 Å have been produced in the 1 nm-sized channels of the AlPO 4 -5 single crystals (AFI in the zeolite terminology) [5,6]. Very recently, one-dimensional superconducting fluctuation with a mean-field superconducting temperature of 15 K has been observed in the 4 Å SWNTs [7]. A 4 Å nanotube has also been synthesized by the mass-selected carbon ion beam deposition as the inner shell of an eighteen-layered multiwalled nanotube [8]. The system of the monosized SWNTs stabilized in the zeolite channels brings the experimental results much closer to the reality of theoretical predictability.In this Letter, we report polarized absorption spectra for the 4 Å SWNTs arrayed in the one-dimensional channels of an AFI single crystal. AFI is a type of porous aluminophosphate single crystal. Its framework consists of alternate tetrahedra of ͑AlO 4 ͒ 2 and ͑PO 4 ͒ 1 which form open one-dimensional channels packed in the hexagonal structure. Figure 1a schematically shows the framework structure of the AFI single crystal viewed along the [001] direction. The typical dimension of the AFI crystals used in our experiment is about 100 mm in diameter and about 300 mm in length. The A...

The energy-momentum ͑ − k͒ dispersion and the shape of the Fermi surface in the ͑Bi, Pb͒ 2 ͑Sr, La͒ 2 CuO 6+␦ ͑Bi2201͒ superconductors with various hole concentrations were determined by the high resolution angleresolved photoemission spectroscopy. On the basis of the − k dispersion thus obtained, temperature and hole-concentration dependence of the thermoelectric power S͑T͒ was calculated within the framework of the Boltzmann transport theory. S͑T͒ of the optimally and overdoped samples was quantitatively reproduced by the present calculation, strongly indicating that characteristics in S͑T͒ of the high-T c cuprates under the optimally to overdoped conditions is unambiguously caused by the band structure characterized by the van Hove singularity near the Fermi level.

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