We have studied by NMR the electronic properties of the stable quasicrystalline phases AI57-Cuio.8Li32.2 and Al62Cu25.sFei2.5 and of two crystalline approximants, the bcc R phase AI56C1112L132 and the rhombohedral Al62.8Cu26Fen.2 phase. Low values of the density of states at the Fermi level are observed in the crystalline as well as in the quasicrystalline phases. The existence of a pseudogap at the Fermi level is therefore not a consequence of the quasiperiodicity. These results renew the debate on the stability of quasicrystals.PACS numbers: 61.42.+h, 76.60.Es Much work has been devoted to the electronic properties of newly discovered stable quasicrystalline (hereafter qc) phases with icosahedral symmetry, such as Al-Cu-Li [1], Al-Cu-Fe [2,3], or Al-Cu-Ru [4], to decide whether or not the quasiperiodicity induces specific properties. A well-established common feature of all these qc phases is a high room-temperature resistivity which moreover increases as the temperature decreases. Such effects have been ascribed to localization effects, and the proximity of a metal-insulator transition has been evoked [3]. Low values of the electronic density of states at the Fermi level, 7V(£>), deduced from specific-heat measurements, have been reported. The reduction of /V(£>) with respect to estimated free-electron values is about one-third in Al-Li-Cu [1] and Al-Cu-Fe [2,3]. It reaches one-tenth in Al-Cu- Ru [4]. This observation of a pseudogap at the Fermi level in qc materials is in agreement with theoretical predictions [5]. It has been suggested that this pseudogap could explain the thermodynamical stability of these qc phases, following a Hume-Rothery scheme [5-71.The qc phases are found in a very narrow composition range of complex ternary phase diagrams. Several different crystalline phases usually exist with close compositions. Among these, the so-called approximant phases are especially interesting. The qc and approximant phases have very similar local order and exhibit closely related diffraction spectra. Within a theoretical point of view, the existence of approximants is quite natural [8]. Whereas the 3D quasiperiodic lattice is obtained from a 6D hypercubic lattice by a cut and projection along irrational directions of the 6D space, periodic approximants are obtained by projection along rational close directions. The period of such an approximant, of course, increases when the rational directions get closer to the irrational ones. Although the existence of approximants should be quite general, their identification in real phase diagrams is not alway easy. The crystalline bcc phase AI56CU12L132 (the so-called R phase) has been recognized early as an approximant of the icosahedral qc AI57CU 10.8^32.2 phase [9]. The existence of an approximant in the Al-Cu-Fe system was suggested by the observation of a microcrystalline structure with a rhombohedral symmetry in small dodecahedral particles extracted from an ingot [10]. However, this approximant phase has been only very recently prepared in appreciable quantit...
A barium deficiency in the"initia1 composition for the synthesis of high-temperature supercondutors (HTSC), i.e. a nominal (RE Ba,-xCu,O,) composition with 0 $ x $ 1 leads to different materials depending on the nature of the rare earth. If the rare earth is 'yltric' (Y, Er, Yb), the resulting compound is a composite YBa,Cu,O, + Y,BaCuO,, whose critical current density depends on the x-value. The determination of the irreversible behaviour by means of AC susceptibility or magnetization measurements shows that in the presence of the 21 1-phase, the intergranular current-density decreases whereas the intragranular current density varies very little. If the rare earth is 'ceric' (Nd, Eu), the lack of barium leads to a quasi single-phased compound. In t h e Nd-case, the critical current density decreases strongly with increasingx. At highx (0.5 < x < 1) and low temperature, t h e magnetization-against-fieid data show paramagnetic behaviour due to the neodymium ions, with an effective momentp.,,,= 1.80 p a . The domain of the reversible behaviour in the (H, r) plane is more extended forNbBaCu0 and EuBaCuO than for YBaCuO.
Mossbauer spectroscopy of the three magnetic phases of single crystal FeCI 2 · 2H2 0 in ex ternal magnetic fields at 4.2 K is reported.
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