Electronic structure of rare-earth pnictides
A. G. Petukhov
Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106-7079
and Physics Department, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701-3995*
W. R. L. Lambrecht and B. Segall
Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106-7079
Received 5 June 1995
The results of first-principles calculations of the electronic band structures, equilibrium lattice constants,
cohesive energies, bulk moduli, and magnetic moments are presented for the rare-earth pnictides with the
rocksalt structure and chemical formula R -V , where R =Gd, Er, and the group-V elements N, P, and As. The
linear-muffin-tin-orbital method was used in the atomic sphere approximation. The 4 f states were treated as
localized corelike states with fixed spin occupancies. Justifications for this procedure are presented. The
systems were studied with the 4 f spins on all rare-earth ions aligned ferromagentic phase and with the spins
randomly oriented paramagentic phase. Within the local spin-density approximation, all systems studied were
found to be semimetallic with a hole section of the Fermi surface near and electron section near X . The
nitrides, however, have a nearly zero band-gap overlap. We estimated quasiparticle self-energy corrections
using an approach previously used for semiconductors. With these corrections, GdN is found to be a semicon-
ductor in the paramagnetic phase and a semimetal in the ferromagnetic phase. ErN, on the other hand, is found
to be a semiconductor in both phases. All systems correspond to a trivalent state of the rare-earth element and
are characterized by ionic bonding. The results for the lattice constants and the qualitative conclusion about the
semimetallic nature are in agreement with experimental data and with the previous calculations for Gd-
pnictides. For ErAs, the calculated magnetic exchange splittings, electron and hole concentrations, Fermi-
surface cross-sectional areas, and cyclotron masses are in satisfactory agreement with the available
Shubnikov–de Haas data on Er
x
Sc
1-x
As when account is taken of the differences due to the presence of Sc
and of the self-energy corrections to the local-density approximation.
I. INTRODUCTION
The rare-earth pnictides i.e., group-V compounds, which
we shall denote R -V , form an interesting family of materials,
because of the great variety of their magnetic and electrical
properties,
1
despite their common simple crystal structure,
the rocksalt structure. An interesting aspect of these com-
pounds is the occurence of localized strongly correlated 4 f
electrons, the treatment of which presents a challenge to
band-structure theory. The strong exchange coupling be-
tween the localized rare-earth 4 f spins and the valence and
conduction electrons in these materials which are either
semimetals or semiconductors also leads to interesting mag-
netic properties. For example, the R -As and R -P compounds
have antiferromagnetic ground states, while the correspond-
ing R -N compounds are ferromagnets.
2
The spin ordering
vector is along the unusual 111 direction. The Ne ´el and
Curie temperatures of these materials are extremely low a
few K. These properties are rather intriguing. While they are
perhaps not of great use for usual magnetic applications, the
exchange coupling of the valence bands to the 4 f ’s offers the
possibility of modifying the electronic properties of the R -V
compounds by an external magnetic field. This might in turn
be used to magnetically tune R -V /semiconductor interface
properties, e.g., in spin superlattices. Previous work on spin
superlattices has focused on dilute magnetic impurties in
semiconductors, such as Zn
1 -x
Mn
x
Se/ZnSe.
3
R -V com-
pounds offer the prospect of achieving a much larger concen-
tration of local magnetic moments in such systems and hence
the observation of large enhancements of the Zeeman effects
on the valence and conduction bands.
Recently, the interest in these materials has sharply in-
creased by the demonstration that they can be grown epitaxi-
ally on semiconductors.
4–7
This opens the way to the devel-
opment of electronic devices, such as metal base transistors.
5
Important progress towards this goals was achieved by
Palmstro ” m et al.,
4
by demonstrating heteroepitaxial growth
of rare-earth monoarsenide Er
x
Sc
1 -x
As on GaAs and vice-
versa. Allen et al.
8
explored the band structure of these thin
epitaxial films of Er
x
Sc
1 -x
As buried in GaAs by measure-
ments of the Hall resistance, transverse magnetoresistance,
and Shubnikov–De Haas SdH oscillations. This study
showed that Er
x
Sc
1 -x
As is a semimetal with an electron and
a hole concentration of (3.10.1) 10
20
cm
-3
and has large
exhange splittings induced by the 4 f open shell. Further de-
tails of the Fermi surface were obtained by subsequent SdH
studies at higher magnetic fields by Bogaerts et al.
9–11
The first band-structure investigation from first-principles
of the rare-earth group-V compounds was carried out by Ha-
segawa and Yanase HYRef. 12 and was concerned with
the Gd monopnictides GdSb, GdAs, GdP, and GdN. Later,
calculations were also reported on CeSb,
13
DyBi, and DyP.
14
Closely related are studies of the IIIb - V compounds, such as
LaSb, LaBi,
15
and ScN.
16,17
In the absence of results on Er-V
compounds, the first interpretations of the experimental in-
vestigations of Er
x
Sc
1 -x
As Refs. 8–10 were largely based
PHYSICAL REVIEW B 15 FEBRUARY 1996-II VOLUME 53, NUMBER 8
53 0163-1829/96/538/432416/$06.00 4324 © 1996 The American Physical Society