Superconductivity pressure dependence

Uranium metal is weaMy paramagnetic, with a magnetic susceptibility of 1.740 X 10 A/g at 20°C, and 1.804 x 10 A/g (A = 10 emu) at 350°C (51). Uranium is a relatively poor electrical conductor. Superconductivity has been observed in a-uranium, with the value of the superconducting temperature, being pressure-dependent. This was shown to be a result of the fact that there are actually three transformations within a-uranium (37,52). 

The pressure dependence of N(EF) has been measured recently through the pressure dependence of the 13C-NMR Knight shift in K3Q0 [94]. In Fig. 25, a plot of In T P) versus K(P) is presented. As shown by this plot, linear behavior is effectively observed, which intersects the y axis at flph = 600 K and = N(EF)V = 0.3 at ambient pressure [94]. Thus the value of Tc appears to be governed by N(EF) and the pressure data suggest that high-frequency intraball phonons are likely to be involved in the superconductivity of fullerenes [20,94]. 

The common pressure dependence of the (TMTSF)2X salts is illustrated in Fig. 20. Below the critical pressure Pc the electronic ground state is an insulator of either the spin density wave type or the anion-assisted Peierls type. Above Pc the ground state is superconducting. The critical pressure Pc varies monotonically with the anion volume discussed above but the transition temperature at ambient pressure and the type of low-pressure ground state depend specifically on the... 

Sp91 G. Spam ei al, Pressure Dependence of Superconductivity in Single-Phase KaCso, Science 252, 1829-1831 (1991). 

Lu2Fe3Si5 is isotypic with the crystal structure of Sc2Fc3Si5 P4/mnc, a = 10.34(1), c = 5.375(8) (Braun, 1980 X-ray powder diffraction data). For sample preparation, see Dy2Fe3Sij. Mossbauer data confirm the absence of a magnetic moment oh the iron site (Braun et al., 1981). Lu2Fe3Si5 is superconducting below 7 = 6.1-5.8 K. The pressure dependence was measured by Segre (1981) as dT dp... 

The light lanthanides also exhibit some unusual electrical and magnetic properties. The superconductivity of lanthanum has already been mentioned. But when its behavior is compared to the superconductivity in scandium, yttrium and lutetium, we find lanthanum is unusual. The ordering temperature T, and pressure dependence of T are both larger in lanthanum than in the other three superconductors (see section 3.7.3 and fig. 12a). 

Of the seventeen rare earth metals only lanthanum is superconducting at atmospheric pressure. Lanthanum is one of the better elemental superconductors - its 6.1 K transformation ()3-La, fee) is only exceeded by those of lead (7.2 K) and niobium (9.2 K), and its thermodynamic critical field, H 0) = 1600 Oe is second to that of niobium s 1980 Oe. When the pressure dependence of the superconducting transition temperatures is taken into account lanthanum has the highest known elemental transition temperature, 13K, see fig. 12a. 

Fig. 12. The pressure dependences of the superconductivity transition temperatures of several rare earth elements (a) lanthanum, lutetium, scandium and yttrium (after Wittig et al. 1979) and (b) cerium (after Probst and Wittig 1978).

Below we will describe briefly the major early contributions which pointed the way to our present state of knowledge (sections 6.1 and 6.2). In addition to these two topics, more recent supporting information is found in the pressure dependence of the superconducting temperatures (section 3.7.3) and the solid solubilities of Sc, Y and Lu relative to those of lanthanides which have unpaired 4f electrons (section 6.3). 

The purpose of this chapter is to review the superconductivity of the rare earth metals, alloys and compounds. We place emphasis upon the pure elements because of their principal importance for an understanding. In section 2 we present the existing data for the pressure dependence of Tc for La, Ce, Y and Lu. The data for Lu have not been published elsewhere (Probst, 1974). The resistivity of La shows highly anomalous features under pressure. The data are included, since they are probably related to the superconductivity of La. In addition, electron tunneling experiments have provided interesting information about the phonon spectrum of La and its changes under pressure. 

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