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We performed low-bias STM measurements on underdoped Bi2212 crystals, and confirmed that a two-dimensional (2D) superstructure with a periodicity of four lattice constants (4a) is formed within the Cu-O plane at T < Tc. This 4a×4a superstructure, oriented along the Cu-O bonding direction, is nondispersive and more intense in lightly doped samples with a zero temperature pseudogap (ZTPG) than in samples with a d-wave gap. The nondispersive 4a×4a superstructure was clearly observed within the ZTPG or d-wave gap, while it tended to fade out outside the gaps. The present results provide a useful test for various models proposed for an electronic order hidden in the underdoped region of high-Tc cuprates.KEYWORDS: STM/STS, pseudogap, electronic charge ordering, superstructure, cuprateRecently STM/STS studies on the pseudogap state of Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) at T > T c have revealed a nondispersive two-dimensional (2D) superstructure with a periodicity of about four lattice constants (∼4a×4a superstructure) in the map of energy-resolved differential tunneling conductance dI/dV , proportional to the local density of states (LDOS).1) The nondispersive 4a×4a structure, electronic in origin, was also reported in the LDOS maps taken on the zero temperature pseudogap (ZTPG) regions of lightly doped Ca 2−x Na x CuO 2 Cl 2 (Na-CCOC) and Bi2212 samples.2,3) A similar spatial structure was first observed around the vortex cores of Bi2212 exhibiting a pseudogap-like V-shaped spectrum with no coherence peak.4) Such 2D spatial structures have attracted much attention because they can be a possible electronic order hidden in the pseudogap state.From the LDOS maps taken for the superconducting (SC) state of Bi2212, Hoffman et al. and McElroy et al. reported a strongly dispersive 2D superstructure. 5,6)This superstructure has been successfully explained in terms of a quasiparticle scattering interference. Furthermore, Howald et al. found a nondispersive ∼4a×4a superstructure with anisotropy in addition to the weakly dispersive one, and claimed that the nondispersive ∼4a×4a superstructure will be due to the so-called stripe order and coexist with superconductivity.7−10) However, the nondispersive ∼4a×4a superstructure was not confirmed in later LDOS measurements on Bi2212 at T < T c . 1,5)This inconsistency mainly originates from the difficulty in distinguishing the nondispersive ∼4a×4a superstructure from the weakly dispersive ∼4a×4a one resulting from quasiparticle scattering interference. Since the dispersive features in the LDOS are reduced by integrating the LDOS over a wide range of energy, it is desirable to investigate the nondispersive features of the LDOS by using a conventional STM technique which actually maps the LDOS integrated between E F and E F +eV s , where V s is the sample bias voltage. To clarify whether the nondispersive 4a×4a electronic superstructure persists in the SC state (T < T c ) will promote our understanding of the physics in the underdoped regionIn the present study, we performed low-bias STM me...

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This study examines the effects of the Si/Al ratio and metal cation exchange of a zeolite on the ability to adsorb and reform unburned hydrocarbons (HCs) in emissions from gasoline vehicles. Iron ion‐exchanged β‐zeolites with a low Si/Al ratio not only trap but also oxidize HCs efficiently, which spurred on the development of a “super HC reformer trap”. Toluene temperature‐programmed desorption experiments in an O2‐containing stream revealed that >90 % of the adsorbed toluene was converted into CO2, and other oxygenates were not observed. This material could be reused and it retained its performance. The strong adsorption of toluene on the zeolites was derived from two synergistic effects: (1) electrostatic interactions between the π orbital of the aromatic ring and the p orbital of the metal cation and (2) interactions between the positively polarized methyl group of toluene and a basic oxygen atom in the zeolite framework. This new material will pave the way for low‐emission, Pt‐free vehicles.

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Spatial variation of low temperature tunneling spectrum on atomic scale was examined on the slightly underdoped Bi 2212 sample (Tc~74 K , p~0.12) by using a STM/STS technique. We observed a V-shaped gap with sharp coherence peaks, which is expected for d-wave superconductors, over the distance of ~18 nm and no other kind of gap. The spatial variation of gap size Δ0 is small, ranging from 41 to 53 meV . These results indicate that the superconductivity takes place rather homogeneously in the slightly underdoped sample.

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