P-type superconductors

In terms of formal charges on the ions, p-type (or hole-doped) YBCO may be written as Y +(Ba +)2(Cu +)2Cu (0 )7 5. YBCO becomes superconductive at < 0.4, with its most pronounced superconductivity at 8 = 0.05. It should be noted that there are other examples of p-type superconductors that involve metal doping rather than varying oxygen content, such as La2 xStxCu04 (Tc = 34 K at X = 0.15). Similarly, electron-doped (n-type) superconductors may be synthesized such as Nd2 xCcxCu04 (Tc = 20 K), written formally as Nd2 x -"Ce, (e )xCu2+04. 

Note there are two ways to account for charge neutrality in p-type superconductors. First, La2 xSrxCu04 may be formally written as La2 xSrxCui x CUx +04 where one Cu + (or a Cu + with a trapped hole, Cu +(h+)) forms for each added. Alternatively, the formula may be written as La2 xSrxCu 04 (x/ 2), where one oxygen vacancy is formed for every two Sr ions added to the lattice. 

Plots of Tc vs in-plane rCu 0 for the series of p-type cuprate superconductors are grouped into three classes distinguished by the size of the 9-coordinate site cations (that is, La-, Sr- and Ba-classes) because of the combined electronic and nonelectronic effects. Every class of the Tc vs in-plane rCu 0 plot shows a maximum, so that every class of the p-type cuprate superconductors possesses an optimum hole density for which the Tc is maximum (40). 

Until 1988, all the high temperature superconductors that had been found were p-type, and it was assumed by many that this would be a feature of high temperature superconductors. However, some n-type superconductors have also been discovered, where the charge carriers are electrons the first to be found was based on the compound Nd2Cu04 with small amounts of the three-valent neodymium substituted by four-valent cerium—Nd2-/le/lu04-j where v 0.17 (samarium, europium, or praeseodymium can... 

H. Zhang, H. Sato, Universal Relationship Between Tc and the Hole Content in p-Type Cuprate Superconductors, Phys. Rev. Lett. 70 (1993) 1697. 

This behavior is easy to understand, because O 4 can be doped p-type to, say O 1 but it cannot be doped n-type to, say O ", because the maximum negative charge on oxygen is approximately —2. Hence the claimed n-type simple doping cannot occur the doping must be more complex and also must be p-type. There are no n-type high-temperature superconductors. 

The superconducting hole condensate resides in the SrO or BaO planes of most high-temperature superconductors, and in the interstitial oxygen regions of many of the rest, such as Nd Xt- Xu04. All of these superconductors are p-type, s-wave superconductors. 

So far all TMTSF and almost all ET-type superconductors have mixed valence and p=. The TMTSF salts and some ET salts are quasi-ID, but with enough warping of the Fermi surface to make them pseudo-2D and to defeat the Peierls transition. The k-phase (BEDT-TTF) salts are really 2-D systems this phase, shown in Fig. 12.7, has isolated dimers connected by dispersion interactions to form, roughly, two-dimensional sheets. Figure 12.8 shows the superconductivity of the salt k-(BEDT-TTF)2Cu(NCS)2 at 10.4 K [34]. 

So far all TMTSF and ET-type superconductors have mixed valence and p = 1/2 (except p = 2/3 for (ET)3Cl2.2H20). The TMTSF salts and some ET salts are quasi-ID, but with enough warping of the Fermi surface to make them pseudo-... 

Besides this large family of layered cuprates there exist two other structural types which are closely related to these oxides. The first one, YBa2Cu40g is a p-typt superconductor with a critical temperature of 70 K, whose structure derives from that of YBa2Cu307 and will be described later. 

One different way of considering superconductors is related to their compliance with the classic BCS theory. Hence conventional (i.e., BCS-compatible) and unconventional (i.e., BCS-incompatible) materials exist. The common scientific language therefore distinguishes also between ITS (low-temperature superconductors) and HTS (high-temperature superconductors). In general, ITS are electron-doped (n-type), while HTS are hole-doped (p-type) phases. 

In the fully-tetravalent state CeN and CeP(p) have the same valence-electron configuration as the NaCl-type superconductors ZrN, ZrP(h), HfN, ThN, ThP, YS, LaS, LuS, etc. By analogy we argued that CeN (Hulliger, 1968) and CeP(p) (Hulliger and Hull, 1970) might also show a transition to the superconductive state near 1 K. On our CeN samples, however, we were not able to detect a transition neither under a pressure of 25 kbar down to 1.2 K nor at normal... 

High temperature superconductors (HTS), 23 814, 826, 829. See also Anisotropic HTS HTS entries applications of, 23 852-872 layered, 23 827, 840 magnetic phase diagram of, 23 838-842 p- and n-type, 23 838 structural anisotropy and fluxon line fragmentation in, 23 841 thallium- and mercury-based, 23 848-850... 

It is clear that the decrease of the rate of the electron transfer operated by the temperature makes the oxidation of ferrocene become quasi-reversible for both the electrode materials. Moreover, it is noted that for both types of electrode the faradaic current increases with temperature. For both the electrodes the oxidation process is governed by diffusion, since in both cases the plot of log(/p) vs. 1/T is linear. Furthermore, one should note in particular that, contrary to the naive expectation, for the superconducting electrode one does not observe any abrupt change in the response upon crossing the barrier from superconductor (that should exchange pairs of electrons) to simple conductor (that should exchange single electrons). 

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