Theory of temperature-dependent angle-resolved-photoemission spectrum of heavy-fermion semiconductors

The heavy-fermion semiconductors are a class of strongly correlated materials, that at high temperatures show properties similar to those of heavy-fermion materials, but at low temperatures show a crossover into a semiconducting state. The low-temperature insulating state is characterized by an anomalously small energy gap, varying between 10 and 100 K. The smallness of the gap is attributed to the result of a many-body renormalization. We calculate the temperature dependence of the electronic spectral density of states, using the Anderson lattice model at half-filling, together with a 1/N expansion, where N is the degeneracy of the f orbitals. The spectrum is calculated to second order in 1/N using a slave boson technique. We find that to first order the system is an indirect gap semiconductor, with a temperature-dependent renormalized band structure. The indirect gap is subject to a temperature-dependent many-body renormalization, and leads to a temperaturedependent sharp structure in the angle-integrated spectrum at the indirect threshold. To second order in 1/N , one has to include charge fluctuations that are represented by fluctuations in the slave boson field. The effect of emission and absorption of fluctuations in the slave boson field is to broaden the angle-resolved spectrum A(k,v), yielding a low-energy tail to the spectrum that, for general wave vectors k, extends all the way down to the indirect threshold. We show that as the temperature is reduced, the structure in the vicinity of the Fermi energy sharpens up. We apply the theory to experiments on the materials FeSi and Ce3Bi4Pt3. @S0163-1829~98!03948-4#