Virtues of marginally metallic oxides
Transition-metal oxides at the metal–insulator boundary, especially those belonging to the perovskite family, exhibit fascinating phenomena such as insulator–metal transitions controlled by composition, high-temperature superconductivity and giant magnetoresistance (GMR). Interestingly, many of these marginally metallic oxides obey the established criteria for metallicity and have a finite density of states at the Fermi level. The perovskite manganates exhibiting GMR, on the other hand, are unusual in that they possess very high resistivities in the ‘metallic’ state and show no significant density of states at the Fermi level. Marginal metallicity in oxide systems is a problem of great complexity and contemporary interest and its understanding is of crucial significance to the diverse phenomena exhibited by these materials.