Doping a semiconductor to create an unconventional metal

TL;DR

This paper demonstrates that doped small gap semiconductors near a metal-insulator transition can exhibit non-Fermi liquid behavior, which can be reversed by a magnetic field, aligning with the undercompensated Kondo effect theory.

Contribution

It provides experimental evidence of non-Fermi liquid states in doped semiconductors and links this behavior to the undercompensated Kondo effect, a theoretical route.

Findings

Doped small gap semiconductors show non-Fermi liquid behavior.

A magnetic field restores Fermi liquid behavior.

The results support the undercompensated Kondo effect as a physical mechanism.

Abstract

Landau Fermi liquid theory, with its pivotal assertion that electrons in metals can be simply understood as independent particles with effective masses replacing the free electron mass, has been astonishingly successful. This is true despite the Coulomb interactions an electron experiences from the host crystal lattice, its defects, and the other ~1022/cm3 electrons. An important extension to the theory accounts for the behaviour of doped semiconductors1,2. Because little in the vast literature on materials contradicts Fermi liquid theory and its extensions, exceptions have attracted great attention, and they include the high temperature superconductors3, silicon-based field effect transistors which host two-dimensional metals4, and certain rare earth compounds at the threshold of magnetism5-8. The origin of the non-Fermi liquid behaviour in all of these systems remains controversial. Here we report that an entirely different and exceedingly simple class of materials - doped small gap semiconductors near a metal-insulator transition - can also display a non-Fermi liquid state. Remarkably, a modest magnetic field functions as a switch which restores the ordinary disordered Fermi liquid. Our data suggest that we have finally found a physical realization of the only mathematically rigourous route to a non-Fermi liquid, namely the 'undercompensated Kondo effect', where there are too few mobile electrons to compensate for the spins of unpaired electrons localized on impurity atoms9-12.