Theoretical Model for the Semi-Metal Yb 4 As 3

We present a model which can explain semi-quantitatively a number of the unusual properties of Yb 4 As 3 . The structural phase transition at T c ≃ 300 K is described by a band Jahn-Teller effect of correlated electrons and is interpreted as a charge ordering of the Yb ions. The low carrier concentration in the low-temperature phase follows from the strong electron correlations of the 4 f -holes on the Yb sites and can be viewed as self-doping of charge-ordered chains. The observed heavy-fermion behaviour is on a scale of T * ≃ 50 K and is due to spinon-like excitations in the Yb 3+ -chains. The appearance of a second low-energy scale around 0.2 K is due to the Fermi energy of the low-density carriers.