title: Eu2+-Eu3+valence transition in double, Eu-, and Na-doped PbSe from transport, magnetic, and electronic structure studies.
abstract: The Eu atoms in Pb1-xEuxSe have long been assumed to be divalent. We show that p-type doping of this magnetic semiconductor alloy with Na can modify the effective Eu valence: a mixed, Eu2+-Eu3+state appears in Pb1-x-yEuxNaySe at particular values of y. Magnetization, carrier concentration, resistivity, and thermopower of Pb1-x-yEuxNaySe are reported for a number of samples with different x and y. A pronounced increase in thermopower at a given carrier concentration was identified and attributed to the presence of enhanced ionized impurity scattering. A strong decrease in the hole concentration is observed in Pb1-yNaySe when Eu is added to the system, which we attribute to a Eu2+-Eu3+self-ionization process. This is evidenced by magnetization measurements, which reveal a significant reduction of the magnetic moment of Pb1-xEuxSe upon alloying with Na. Further, a deviation of magnetization from a purely paramagnetic state, described by a Brillouin function, identifies antiferromagnetic interactions between the nearest-neighbor Eu atoms: a value of Jex/kB= -0.35 K was found for the exchange coupling parameter. The conclusion of a Eu2+-Eu3+self-ionization process being in effect is supported further by the electronic structure calculations, which show that an instability of the 4f7configuration of the Eu2+ion appears with Na doping. Schematically, it was found that the Eu 4f levels form states near enough to the Fermi energy that hole doping can lower the Fermi energy and trigger a reconfiguration of a 4f electronic shell.
authors.affiliation: Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Aleja A. Mickiewicza 30, 30-059 Krakow, Poland. wiendlocha@fis.agh.edu.pl and Department of Mechanical & Aerospace Engineering, The Ohio State University, Columbus, Ohio, USA.
authors.forename: Bartlomiej
authors.identifier: http://orcid.org/0000-0001-9536-7216
authors.initials: B
authors.lastname: Wiendlocha
authors.affiliation: Department of Mechanical & Aerospace Engineering, The Ohio State University, Columbus, Ohio, USA.
authors.forename: SunPhil
authors.initials: S
authors.lastname: Kim
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, IL, USA.
authors.forename: Yeseul
authors.initials: Y
authors.lastname: Lee
authors.affiliation: Department of Mechanical & Aerospace Engineering, The Ohio State University, Columbus, Ohio, USA.
authors.forename: Bin
authors.initials: B
authors.lastname: He
authors.affiliation: Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI, USA.
authors.forename: Gloria
authors.initials: G
authors.lastname: Lehr
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, IL, USA.
authors.forename: Mercouri G
authors.initials: MG
authors.lastname: Kanatzidis
authors.affiliation: Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI, USA.
authors.forename: Donald T
authors.initials: DT
authors.lastname: Morelli
authors.affiliation: Department of Mechanical & Aerospace Engineering, The Ohio State University, Columbus, Ohio, USA and Department of Physics, The Ohio State University, Columbus, OH, USA.
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Heremans