From low-temperature measurements of the specific heat and the magnetic susceptibility, we determined T"y, and g'p'" for both the cubic and the rhombohedral phase of the C15 compoundV2Zr. In addition, neutron-diffraction experiments were performed for a proper identification of these two phases. We found that the martensitic transformation (T~-110K) from the cubic to the rhombohedral phase leads to a substantial reduction in y and g' '": y,"b (y,h) -23 (16) mJ/g-at. K and g~g (gI, "")-2. 1)&10 (1.4)&10 ) emu jg-at. In contrast, T, is only slightly changed: T, ""-8. 7 K and T, '"-8. 0 K. Together with the fact that the "Wilson ratio" E(g)/X(y)-0. 7 in both phases, we take our findings as clear evidence that in V2Zr T, is limited by spin fluctuations.
I. INTRODIJCTIONA high electronic density of states (DOS) at the Fermi level, N(0), favors the onset of superconductivity in two respects. First, since the formation of Cooper pairs is restricted to the immediate neighborhood of the Fermi surface, N(0) is a direct measure for the number of electrons sharing the superconducting quantum state. In 8CS theory, this is reflected by the explicit appearance of N (0) » thc couP»ng s«ength, A, Bcs --N(0) V. Second, the interaction V which is mediated by virtual phonon exchange, is itself a function of N (0). A higher N (0) leads to phonon softening and thus to a higher V, although this effect may partly be compensated by enhanced screening of the electron-phonon vertex. ' On the other hand, this phonon softening can culminate in an electronically driven (Jahn-Tellertype) phase transition into a stabler phase with lower N(0) and thus weaker electron-phonon coupling. Also, changes in N(0) affect the Coulomb repulsion between the electrons, i.e. , p*. In particular, in cooperation with Coulomb exchange and correlation, an increasing N(0) alIoays implies a growing tendency towards itinerant magnetism, thus leading either to a depression of T, by spin fluctuations or even to the total quenching of superconductivity.Combining all these effects, one may imagine situations where for a superconductor, T, can still be raised by raising N(0), i.e. , where dT, /dN(0) &0, as well as situations where dT, /dN(0)~0. In this latter case, the optimum value for N(0) has already been surpassed, and an increase in T, requires a reduction of N(0). (Pd may be an example of this rare happening .) Thus, when looking for high-T, superconductivity within a given class of materials, it is of particular interest to find the point where de /dN (0) 0, I.c. , to IIlcct tllc optlIIllllI1 coIldltloIls fo1 high T, with respect to N (0).In this paper, we present experimental evidence for the fact that in the C15 compound VIZr, such an optimum situation has nearly been achieved. In several respects, VzZr is reminiscent of the high-T, 215 compounds VISi and Nb3Sn. It has a high magnetic susceptibility g(T)with an anomalously large temperature dependence, and it undergoes a martensitic phase transformation from the cubic into a rhombohedral phase below T~-110 K, wh...
