Superconductor

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4 <strong>Superconductor</strong> superconductor (4) Pb may have the oxidation states: Pb(+II) and Pb(+IV) and Bi may have the oxidation states Bi(+III) and Bi(+V); in the copper oxide superconductors (5), (6), (8), (9) and (10) Cu may have the oxidation states Cu(+I) and Cu(+III). Metal Electron configurations Oxidation states As [Ar]3d 104s 24p 3 +III, +V Bi [Xe]4f 145d 106s 26p 3 +III, +V Cu [Ar]3d 104s 1 +I, +II, +III Fe [Ar]3d 64s 2 +II, +III, +IV, +V Nb [Kr]4d 35s 2 +III, +V Pb [Xe]4f 145d 106s 26p 2 +II, +IV Ti [Ar]3d 24s 2 +II, +III, +IV Tl [Xe]4f 145d 106s 26p 1 +I, +III W [Xe]4f 145d 46s 2 +IV, +V, +VI Table 2. Electron configurations and oxidation states of some metals Note also that in the superconductor (7) (without copper), Bi may have the oxidation states Bi(+III) and Bi(+V). In the superconductor (11), an example of the recent discovery of ironbased superconductors (Yang et al., 2008), we can verify that Fe may have the oxidation states Fe(+II) and Fe(+IV) and As may have the oxidation states As(+III) and As(+V). Observe that in most high-Tc superconductors there are alkaline earth metals (such as Ca, Sr, and Ba). We know that the electron configuration of an alkaline earth metal is given by [noble gas] ns2, where n is the number of the row in the periodic table. Thus, an alkaline earth atom may lose two paired external electrons (ns2). According to Table 1, among the high-Tc oxide superconductors, HgBa2Ca2Cu3O8 + x (Hg-1223) has the highest critical temperature (Tc = 135 K) at 1 atm (Schilling & Cantoni, 1993). According to the tables in the textbook (Lee, 1991), the electron configuration of Hg is given by: [Xe] 4f14 5d10 6s2. Because all electrons are paired in a Hg atom, it is possible that an Hg atom may lose two paired electrons at the external level (6s2). Therefore, alkaline earth metals atoms (such as Ca, Sr, and Ba) as well as Hg atoms may lose two paired electrons at the external level. According to a number of authors the probable existence of double charge fluctuations in oxide superconductors is very likely (Callaway et al., 1987; Foltin, 1988; Ganguly & Hegde, 1988; Varma, 1988). Spectroscopic experiments (Ganguly & Hegde, 1988), indicate that double charge fluctuations is a necessary, but not sufficient, criterion for superconductivity. We argue that these charge fluctuations should involve paired electrons hoping from ions (or atoms) in order to occupy empty levels. That is, our basic phenomenological hypothesis is that the electrons involved in the hopping mechanisms might be paired electrons coming from neighboring ions or neighboring atoms. Possible microscopic mechanisms for double charge fluctuations are: (1) hopping mechanisms (Foltin, 1989; Wheatley et al., 1988), (2) tunneling mechanisms (Kamimura, 1987), and (3) bipolaronic mechanisms (Alexandrov, 1999). The discovery of Fe-based high-Tc superconductors (Yang et al., 2008) has reopened the hypothesis of spin fluctuations for the microscopic mechanisms of high-Tc superconductivity. However, it is interesting to note that Fe may have the oxidation states Fe(+II) and Fe(+IV). Thus, the conjecture of double charge fluctuations cannot be ruled out
A Model to Study Microscopic Mechanisms in High-T c <strong>Superconductor</strong>s in the study of the microscopic mechanisms in all Fe-based high-Tc superconductors. It is worthwhile to study the competition between double charge fluctuations and spin fluctuations in order to identify which phenomenon is more appropriate to investigate the microscopic mechanisms in the condensation of the superconducting state of Fe-based materials. 4. Valence skip What is valence skip? About fifteen elements in the periodic table skip certain valences in all components they form. For example, according to Table 2, the stable oxidation states of bismuth are Bi(+III) and Bi(+V). The oxidation state Bi(+IV) is not stable. If the state Bi(+IV) is formed, occurs immediately a disproportionation, according to the reaction: 2Bi(+IV) = Bi(+III) + Bi(+V). In the compound BaBiO3, the formal valence Bi(+IV) is understood as an equilibrium situation involving a mixture of equal amounts of the ions Bi(+III) and Bi(+V). Observing Table 2, other important examples of elements with valence skip are As, Pb and Tl. In (Varma, 1988) there is a discussion about the microscopic physics behind the phenomenon of valence skip. Elements with valence skip, like Bi and Pb, are the most appropriate elements to study the hypothesis of double charge fluctuations. It has been stressed that all elements with valence skip may be used in the synthesis of superconductors (Varma, 1988). 5. D-wave superconductivity Let us consider copper oxide high-Tc superconductors. We assume that the copper-oxygen planes are parallel to the plane x, y and that the z-axis is parallel to the crystallographic axis (c-axis). In a weak crystal field, according to Hund’s rule, the ion Cu(+III) is paramagnetic because the levels 3d(x 2 – y 2) and 3d(z 2) are half-filled. However, it has been shown that in a strong crystal field this ion becomes diamagnetic (McMurry & Fay, 1998). We know that the electron configuration of the ion Cu(+III) is [Ar]3d 8. Considering a strong crystal field, the ion Cu(+III) may give rise to a square planar complex. On the other hand, considering a strong crystal field, the spin-pairing energy P is smaller than the splitting energy Δ (McMurry & Fay, 1998). Thus, in this case, all electrons in a Cu(+III) ion should be paired and this ion should be diamagnetic. This hypothesis is consistent with a correlation between crystal field splitting and high transition temperatures (Zuotao, 1991). If there are paired electrons in the nearest neighbors of the Cu(+III) ions, two neighboring paired electrons can be attracted by Coulomb interactions and eventually may occupy double empty energy levels. Because the orbital 3d(z 2) becomes filled, the electrons coming from +z and -z directions are strongly repelled. On the other side, electrons coming from the directions +x, -x, +y, -y are not repelled and, eventually, may jump to occupy the empty levels. These electrons are obviously d-electrons and these jumps may give rise to a collective wave function of d-electrons. This hypothesis is consistent with the so-called assumption of d-wave superconductivity. The probable existence of d-wave superconductivity in oxide superconductors is supported by a great number of experiments (Leggett, 1994; Scalapino, 1995; Shen & Dessau, 1995; Tanaka, 1994). This picture leads to the conclusion that the microscopic mechanism of the condensation of the superconducting state should be a Bose-Einstein condensation. In the next section we discuss the possibility of a direct Bose-Einstein condensation in oxide superconductors. 5
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4 Superconductor Properties for Sil
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Superconductor Properties for Silic
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MgB2-MgO Compound Superconductor Yi
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MgB 2-MgO Compound Superconductor t
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MgB 2-MgO Compound Superconductor S
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MgB 2-MgO Compound Superconductor F
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MgB 2-MgO Compound Superconductor
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MgB 2-MgO Compound Superconductor F
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MgB 2-MgO Compound Superconductor T
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MgB 2-MgO Compound Superconductor 4
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MgB 2-MgO Compound Superconductor G
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1. Introduction Superconducting Pro
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Superconducting Properties of Carbo
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Development of Large Scale YBa 2Cu
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1. Introduction 15 Superconductors
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Superconductors and Quantum Gravity
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Superconductor

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