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

The heavy-fermion semiconductors are a class of strongly correlated materia ls, that at high temperatures show properties similar to those of heavy-fer mion materials, but at low temperatures show a crossover into a semiconduct ing state. The low-temperature insulating state is characterized by an anom alously small energy gap, varying between 10 and 100 K. The smallness of th e gap is attributed to the result of a many-body renormalization. We calcul ate the temperature dependence of the electronic spectral density of states , using the Anderson lattice model at half-filling, together with a 1/N exp ansion, where N is the degeneracy of the f orbitals. The spectrum is calcul ated 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 te mperature-dependent many-body renormalization, and leads to a temperature-d ependent sharp structure in the angle-integrated spectrum at the indirect t hreshold. To second order in 1/N, one has to include charge fluctuations th at are represented by fluctuations in the slave boson field. The effect of emission and absorption of fluctuations in the slave boson field is to broa den the angle-resolved spectrum A(k, omega), 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 st ructure in the vicinity of the Fermi energy sharpens up. We apply the theor y to experiments on the materials FeSi and Ce3Bi4Pt3. [S0163-1829(98)03948- 4].