Phosphorus, the element itself, becomes superconducting near 4.7 degrees K and at pressures exceeding 100 kilobars. This constitutes one of the four last missing links in the proof that superconductivity is normal behavior for every truly metallic sp element. The three remaining ones are arsenic, sulfur, and iodine.
Superconductivity has been observed in a high-pressure phase of cesium. Yttrium, normal at zero pressure down to millidegree temperatures, shows pressure-induced superconductivity above 110 kbar, apparently without change in crystal structure. Superconductivity, once restricted to the puzzling case of lanthanum in this part of the periodic system, turns out to be the general phenomenon when we apply pressure.This Letter introduces two new superconducting elements, cesium and yttrium, which show superconductivity only at high pressure. Previous work had led to the discovery of pressure-induced superconductivity in the cerium 1 and barium. The transition temperature of lanthanum increases dramatically up to 12°K at very high pressure (P = 140 kbar). 3 A crude extrapolation of the monotonic T C -P relation to "negative pressures" reveals T c = Q°Kfor approximately P = -30 kbar. Thus the pressure experiments may lead to the notion that the superconductivity of lanthanum at zero pressure is more or less accidental. Adopting this viewpoint, there is now a sequence of three pressure-induced superconducting elements (Ba, La, Ce; cf. Table I) preceding the magnetic rare-earth series. 4 In the early stage, Hamilton and Jensen 5 suggested that a nonmagnetic virtual bound state, connected with the open 4/ shell, could cause superconductivity in La. Kondo's two-band model 6 was another attempt to explain the singular behavior of La through a cooperation of/ states. With the discovery of pressure-induced superconductivity in lanthanum's left-hand neighbor, barium, we seem to run into difficulties with any model based on 4/ states at the Fermi surface in promoting superconductivity. Spectroscopic term values of the free Ba atom do not indicate that the / electron collapses into an inner orbit as it does in La. 7 On the other hand, an interesting calculation by Goeppert Mayer, 8 using the Thomas-FerTable I. Left side of the periodic system featuring the presently known pressure-induced superconductorspreceding the classical transition-metal superconductors. The superconducting elements are underlined. H Li Na K Rb Cs Be Mg Ca Sr Ba Sc Y La Ce Pr Ti Zr Hf V Nb Tami approximation, shows that there is an additional attractive trough in the effective potential inside the ion core for an/ electron for a couple of elements before the magnetic rare-earths begin. This trough gradually deepens with increasing nuclear charge, finally leading to a bound state. Taking this behavior of an/ function into account and postulating that it is reflected in solid-state properties at high pressure, the situation indeed could provide allowance for a virtual bound state in a number of elements preceding the rare earths, if there is one at all. Cesium was studied as the next element in order to investigate the obvious "horizontal" similarity of elements around lanthanum (Table I) with respect to pressure-induced superconductivity.The apparatus consists essentially of Bridgman anvils, a six-lead resistance cell, and a mechanical press for operation at...
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