Exponential temperature dependence of the resistivity, thermopower, and susceptibility of Y9Co7: Evidence for local mode (abstract)
Authors: Sarkissian B. V. B. 
Published: July 2002

The magnetic superconductor Y9Co7 is unique in that its magnetism coexists with superconductivity at a temperature (∼5.5 K) before it transforms into the superconducting state at a lower temperature (∼2.7 K) [B. V. B. Sarkissian, J. Appl. Phys. 53, 8070 (1982)]. There is now ample evidence that the compound has a puzzling array of magnetic properties. The magnetism is consistent with the lack of long-range order, and shows some evidence of dominant incipient ferromagnetic ordering. Apart from these striking effects this compound shows anomalous temperature dependence in a variety of physical properties at high temperatures, in particular, the ‘‘saturation’’ of the resistivity and thermopower with increasing temperature and marked departure of the susceptibility from the usual Curie–Weiss behavior. The conventional view of these effects is that the resistivity and the thermopower anomalies are a consequence of the s-d scattering mechanism [E. Gratz et al., J. Magn. Magn. Mater. 21, 191 (1980)], whereas the magnetic susceptibility is viewed as a composite of a Curie–Weiss and a temperature independent part [A. Kolodziejczyk and J. Spalek, J. Phys. F 14, 1277 (1984)]. We have carried out measurements of the variation in resistivity, thermopower, and susceptibility with temperature in Y9Co7. All the results show a temperature dependence dominated by an exponential-type behavior with a well-defined characteristic temperature. The latter defines the crossover from the region dominated by strong T dependence to that dominated by weak T dependence. The presence of similar unusual high T properties in many intermetallic compounds—particularly the A15 compounds—has been evident for many years. Many of them show one or more of the properties of the Y9Co7; however, at lower temperatures their behavior differs considerably from that of Y9Co7. In interpreting the anomalous exponential behavior shown by Y9Co7 a mechanism, closely related to localized phonon mode formation, that suggests itself is one developed by Yu and Anderson [Phys. Rev, B 29, 6165 (1984)] to explain the high-T anomalous physical properties of high T superconductors, such as V3Si. It thereby implies that the unusual high-T features of Y9Co7 are due to dominating effects of the ‘‘hopping’’ mechanism of the Co atoms from one site to another along the c-axis chains.
The phenomenon should occur in Y9Co7 because of the reduced spatial symmetry associated with the potential sites of Co atoms along the chains (chains of less rigidly confined Co atoms). The mechanism we are postulating for the observed high-T anomaly of Y9CO7 is also in agreement with the unusual low-T magnetic and superconducting properties of this system. The emphasis, therefore, is on behavior related to the c-axis Co atom hopping rate, which we expect to become progressively more important on cooling, due to renormalization of the electron-phonon coupling. The interesting point to be made is that this hopping mechanism undoubtedly leads to mode softening, as well as to a broadening in the effective d-level width of magnetic Co atoms. From this point of view it can be inferred that when (in the magnetic region) the hopping rate is low compared to the effective d-level width of the Co atoms then Co will conserve its magnetism. If the hopping rate is high, on the other hand, magnetism of Co may be weakened, and, with a suitable mode softening effect, the system may then condense to a true superconducting state. The magnetic character, as we perceive it, in Y9Co7 is akin to the finite lifetime spin fluctuations (quasi localized), and is the result of the balance of the durability of the Co spin against the fluctuations in configuration space accompanying the hopping process. The latter has obvious attraction as an explanation of the observed hysteresis [B. V. B. Sarkissian and A. K. Grover, J. Phys, F 12, L107 (1982)] and strong pressure effects [B. V. B. Sarkissian and J. Beille, J. Appl. Phys. 55, 2004 (1984)] in Y9Co7.