* Corresponding author. Fax: #49-351-463-7029; e-mail: helmut@tmps06.mpg.tu-dresden.de. Physica B 259—261 (1999) 267—268 Electronic structure of low carrier ytterbium pnictides H. Eschrig*, V.N. Antonov, A.N. Yaresko, A.Ya. Perlov, P. Thalmeier, P. Fulde, P.M. Oppeneer Institut fu ( r Theoretische Physik, Technische Universita ( t, D-01062 Dresden, Germany Institute of Metal Physics, 36 Vernadskii street, 252142 Kiev, Ukraine Max-Planck-Institut fu ( r Physik komplexer Systeme, D-01187 Dresden, Germany Abstract The charge carriers of Yb  X  ,X"P, As, Sb, Bi, are Yb-4f electrons and X-p holes. A very low X-p density of states towards the band maximum in connection with a strong Yb-4f onsite Coulomb correlation results in the extremely low carrier concentration found in these compounds. Charge ordering among the Yb and Yb ions is essential in this respect. Electronic structure calculations are reported which reveal this picture, and which suggest that a large part of the strongly enhanced Sommerfeld constant is due to spin excitations in charge ordered chains.  1999 Elsevier Science B.V. All rights reserved. Keywords: Ytterbium pnictide; Charge ordering; Heavy fermion The rare-earth pnictides Yb  X  (X " P, As, Sb, Bi) are semiconductors at the light pnictine end and semimetals at the heavy pnictine end [1]. The transition takes place for Yb  (P  As  )  at about x"0.6. Again at the light pnictine end, but extending much further, to Yb  (As  Sb  )  at about y"0.2, charge ordering on the Yb sites is observed below about 300 K [2], while the heavy pnictine compounds are valence fluctuating semimetals. In the charge-ordered semimetallic state, the carrier concentration is extremely small, about 0.001 carrier per formula unit in Yb  As  [3]. Nevertheless, the Sommerfeld constant of the low temperature specific heat is very large in all charge ordered materials, " 200 mJ mol K for Yb  As  [3] and "250 mJ mol K for Yb  P  , although the latter is semicon- ducting [4]. The high temperature phase of Yb pnictides has the non-symmorphic cubic I43  d space group. It corresponds to Fig. 1 , if one identifies all Yb sites. In the charge ordered state, the Yb ions along one of the diagonal chains are in the 3# state while all the others are in the 2# state, resulting in YbYb  As  (see Fig. 1). This structure has the trigonal space group R3c. To elucidate the electronic structure of this charge ordered state and the nature of the carriers, spin-polarized relativistic LSDA#º electronic structure calculations within the linearized muffin-tin orbital scheme have been per- formed, with an º-value of 9.6 eV to split the unoccupied from the occupied 4f-states of the Yb ions, while the occupied 4f-states of the Yb have been treated as pseudo-core states. A detailed report of these calculation is given elsewhere [5]. The resulting bandstructure of Yb  As  is shown on Fig. 2. The relevant features are an ordinary band gap between the pnictine p-band and the Yb 5d-band and a position of the nearly unoccupied Yb 4f-level and hence also of the Fermi level very close to the top of the pnictine p-band. Of course, both the pnictine p-band and the Yb 5d-band are hybridized due to covalency. However, as usual this hybridization tends to zero towards the band edges on both sides of the covalency gap. Hence, the position of the nearly unoccupied Yb 4f-level is found by a charge balance between the 4f-state and the pnictine 0921-4526/99/$ — see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 4 2 - 9