Marginal metals

It was pointed out in the introduction to this chapter that an experimental criterion for metallicity is the observation of a positive temperature coefficient to the electrical resistivity. The so-called bad or marginal metals are those that meet this criterion, but in which the value for the resistivity is relatively high (p 10 flcm). Many transition metal oxides behave in this manner, while others (e.g. ReOs and RuOa) have very low electrical resistivities, similar in scale to those of conventional metals (p 10 O cm). Consider the Ruddlesden-Popper mthenates. Both strong Ru 4d-0 2p hybridization and weaker intrasite correlation effects compared to the 3d transition metals are 

It is believed that electron correlation plays an important role with the anomalously high resistivity exhibited in marginal metals. Unfortunately, although the Mott-Hubbard model adequately explains behavior on the insulating side of the M-NM transition, on the metallic side, it does so only if the system is far from the transition. Electron dynamics of systems in which U is only slightly less than W (i.e. metallic systems close to the M-NM transition), are not well described by a simple itinerant or localized picture. The study of systems with almost localized electrons is still an area under intense investigation within the condensed matter physics community. A dynamical mean field theory (DMFT) has been developed for the Hubbard model, which enables one to describe both the insulating state and the metallic state, at least for weak correlation. 

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Fig 2 Resistivities of some of the oxides exhibiting marginal metallicity. Data on metallic Nb.Sn. Pd and Cu are shown for comparison. 

Fig. 12 Complexity of the problem of marginal metallicity (adapted from ref. 27). The oxides discussed in this article fall somewhere in the three-dimensional space indicated here. The other factors include electron-lattice interaction, magnetic polaron and finite temperature effects.

What does the term marginal metal signify ... 

Beckman, T.A. Pitrer, K.S. The infrared spectra of marginally metallic systems Sodium-ammonia solutions. 

T. A. Beckman, Infrared Spectra of Marginally Metallic Systems, U. S. At. Energy Comm., UCRL-9330, 1-72, 1960. 

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