Careful measurements using metallic glasses permit the testing of theories for the anomalous transport properties of disordered systems, notably those involving quantum effects.Low-temperature transport properties per haps demonstrate the most interesting of all the anomalous features found in amorphous metallic alloys because they represent ma croscopic quantum phenomena with a rich variety of behaviours. For example, the electrical resistivity at low temperatures of some metallic glasses shows different tem perature dependences (Fig. 1) originating from quantum effects. The magnetoresis tance (B)/ -the dependence of the electrical resistivity p on the magnetic field B -is also anomalous at low temperatures. For instance, the magnetoresistance at 4.2 K is seen in Fig. 2 to be positive for Cu-La and negative for Cu-Mg.The anomalies in electrical resistivity, magnetoresistivity and other transport pro perties of disordered materials arise since the electron mean path (the distance bet ween electron collision events) becomes very short in disordered materials, of the order of the distance between atoms. In this situation, classical transport theories are no longer valid. The electrons cannot be descri bed as Independent particles and quantum corrections to the transport properties must be considered. These effects are of two types: weak localization (WL) and electronelectron interaction (EEI).Before discussing the status of ap proaches for describing quantum correc tions to the transport properties of metallic glasses at low temperatures, it is useful to briefly review the full range of anomalous properties.
Electrical ResistivityThe electrical properties of amorphous metals have in fact offered repeated sur prises for more than 15 years. Although the temperature coefficient of resistance for a resistive metallic glass (an amorphous metallic alloy with unfilled d-bands having a high electrical resistance) is small, often a few times 10-4, the more physically interest ing quantity, namely the magnitude of the temperature T dependence of the electrical resistivity dp/dT is, in contrast, often large (roughly five times larger than that of copper at room temperature for typical resistivities of amorphous metals of about 200 µ cm). Moreover, the coefficient is negative from helium temperatures up to several hundreds of degrees centigrade, where relaxation effects and/or crystallization of the glass sets in. This unusual temperature depen dence is the oldest reported anomaly in the electrical properties of amorphous metals.
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Hall EffectAnother controversial aspect of the trans port properties of metallic glasses concerns the Hall effect (the creation of a voltage per pendicular to the direction of current flow when the sample is placed in a magnetic field). Electronic properties are expected to be isotropic in amorphous alloys so the Fermi surface would be spherical, albeit somewhat blurred, and the Hall coefficient should be negative. Both are observed and often quantitatively well accounted for in simple ...