High pressure resistance anomalies

Cerium metal is discussed in ch. 4 and only a brief mention of its high pressure behavior will be made here (for references see the list in ch. 4). Cerium can exist at atmospheric pressure in the fee (y) or dhep (iS) form and undergoes an isostructural transition near 100 K to another fcc-form referred to as o-Ce. The y-a Ce transition occurs at 7 kbar at room temperature and this transition is accompanied by about 8% volume decrease. This is one of the most widely studied transitions as a function of pressure and temperature and is believed to involve a valence change from 3 towards a higher valence state (3.7 ). The y to a transition line terminates at a critical point the very first example in which a solid - solid transition was shown to exhibit a liquid-vapor-like critical point. A pressure-induced phase transition near 50 kbar, initially reported to be yet another isostructural transition has been shown to be from fee (a-Ce) to an orthorhombic phase with the a-U structure. Stager and Drickamer (1964) have reported a pronounced resistance anomaly near 120 kbar indicative of a phase transition, but the nature of this transition is unknown. The fusion behavior of Ce is again unique in that it exhibits a minimum. 

We would like to draw attention to another anomaly of La which is also not well understood at present. The temperature dependence of the electrical resistivity shows a strong negative curvature, particularly at high pressure (cf. the 108 kbar-isobar in fig. 10.4). On the other hand, the electrical resistivity of Lu increases linearly with T above 200 K at all pressures (Probst, 1974). La seem to be exceptional in this respect among the elemental d-band superconductors. The 5f-band metals U, Np and Pu possess similar R-T characteristics as La (Brodsky et al., 1974). Phenomenologically, this also points to a partially occupied 4f band in La even though the anomaly is not understood at present (Balster and Wittig, 1975). 

Figure 95 shows the electrical resistivity p(T) for a- and c-axes as a function of temperature below 40 K under high pressures. At ambient pressure, the resistivity shows a sudden decrease near 35 K (=Tni). Moreover, the resistivity shows a small anomaly at 24 K (=Tn2)- The anomalies at and Tn2 correspond to the magnetic phase transitions which were mentioned before. The anomaly near Tni becomes less prominent with increasing pressure, corresponding to the pressure-induced decrease of the sublattice magnetization (Kawarazaki et al., 2000). Tni decreases with increasing pressure and disappears above 1.0 GPa ( Pci)-... 

Figure 105 shows the temperature dependence of the electrical resistivity over a wide temperature range at high pressures up to 8 GPa. No anomaly was detected in the p-T above 1.5 GPa, since magnetic ordering was suppressed completely. Instead, an inflection point at is observed in the p curve above 1.5 GPa. shifts rapidly from 44 K at 1.5 GPa to higher temperature with increasing pressure. 

The room-temperature conductivity of [(CH3)4NJ-[Ni(dmit)2]2 is about 50 S cm . A resistivity jump at around 100 K and a sharp rise of resistivity below 20 K are characteristic of this crystal [36a,36c,37]. Hydrostatic pressure reduces both anomalies. In the sample where the low-temperature anomaly was suppressed by high pressure, the superconducting transition was observed under 3.2 kbar at 3.0 K. The transition temperature becomes high with increasing pressure. At 7 kbar, an onset of a superconducting transition occurs at 5.0 K (Figure 5.19). This was the first observation of superconductivity in n acceptor conductors with closed-shell cations. 

Figure 120 depicts the linear thermal expansion along a- and b-axes at high pressiue. (Al/l)a shows a smooth decrease with decreasing temperature and then a sudden decrease is observed near Tc, while Al/l)b begins to increase below Tq- Below 1.5 GPa, the spontaneous MS is clearly observed. The linear thermal expansion coefficient xj, at ambient pressure is shown in the inset of Fig. 120B. An anomaly was observed near T, which also decreases as pressure increases. This result is consistent with the result of the electrical resistivity (Oomi et al., 1998). The anomaly due to ferromagnetic ordering becomes less prominent with increasing pressure, and Tc also decreases.

The crystal shows a V- dependence of resistivity (c xC ) = 50 S cm , Ci, /Q, = 0.6, Oj/q, = 4 X 10 3) similar to that of (TMTSFljX. decreases with increasing pressure up to 0.04 GPa but discontinuously increases to 7 to 8 K at 0.04 GPa. Further increase of pressure decreases monotonic lly — (drydP= -10 K (GPa) ). The depressurized sample consists of the low-T nd high- c phaS - - c2> > nd other physical parameters of the two phases are summarized in Table 10.2. Although the H 2 within the 2D plane for the high-T phase is comparable with that of Nb-Ti, the resistance recovery by the magnetic field is very dull and the temperature dependence of 77j2(-0 shows at lower temperatures an upper curvature instead of the common saturated behavior. The F1 NMR measurements indicate an abnormal enhancement of l/T i far below 7 . These anomalies are commonly noticed in the 10 K class ET SCs and are ascribed to the fluctuation of and/or to the dynamics of the vortex. In such a case, the conventional determination of is not valid. Hence, the 77,-2(0)... 

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