Multiple-Structure Optical Spectral Weight Transfer in Ferromagnetic EuB6

Abstract

We present the first wide range (2 meV−5.5 eV) optical conductivity of EuB6 from reflectivity and ellipsometry measurements. Upon the ferromagnetic transition at Tc=15.5 K, interband transition σ1(ω) decreases at three different energy ω=0.5, 1.4, 2.75 eV and the lost spectral weight is transferred to the Drude σ1(ω) at ω <0.33 eV. We succeeded in explaining this unprecedented multiple-energy σ1(ω) change using LDA+U calculated band structure by Kunes and Pickett (Phys. Rev. B, 69, 165111). Our finding supports strongly that (1) EuB6 is a semimetal and (2) the exchange-driven band splitting is the primary source of the drastic ρ(T ) and ω 2 p changes at T < Tc.PACS numbers: 78.20. Ls,78.40.Kc,71.20.Eh,71.70.Gm Interplay of itinerant carrier with localized magnetic moment can lead to novel magneto-resistance effect in solid. When the moments align with long range ferromagnetic order, dc-resistivity exhibits a drastic change as seen in EuO and (Ga,Mn)As. Theories such as the Kondo-lattice model, Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and spin-polaron model propose possible microscopic mechanism of the carrier-moment coupling in magnetic conductors.Europium hexaboride EuB 6 is a simple cubic compound where dilute conducting carrier (n ∼ 10 19 cm −3 ) coexists with localized 4f spin of Eu +2 (S=7/2). The latter undergoes ferromagnetic (FM) transition at T c =15.5 K where simultaneously dc-resistivity ρ drops sharply [1] and the infrared plasma frequency ω 2 p (=4πe 2 n/m) suddenly increases [2]. Such dramatic FM-driven changes were seen in the manganites like (La 0.7 Ca 0.3 )MnO 3 [3] whereas EuB 6 being free from complications like lattice distortion and random substitution offers a cleaner and ideal example of ferromagnetic metal where the carrier-moment interaction can be studied in more fundamental level.In the Kondo-lattice model itinerant electron band is split in FM phase by the s-f exchange interaction into two spin-polarized sub-bands. Kreissl and Nolting applied this theory to EuB 6 to find that the band splitting brings about a significant increase of the carrier density [4]. The ρ(T ) drop and ω 2 p increase are attributed to it. In contrast Hirsch proposed an alternative model that conduction band becomes broader in FM state because the spin-polarization reduces the bond-charge repulsion effect [5,6]. This band width increase or equivalently carrier mass decrease is considered as a source of the enhanced metallicity. Pereira et al. suggested yet another scenario that at T>T c the disordered spin background can localize the carriers via their exchange interaction and it introduces a mobility edge in the conduction band [7]. FM spin alignment reduces the localization effect, lowering the mobility edge position, and the carriers become delocalized. This theoretical diversity shows that EuB 6 is an important prototype of metallic ferromagnetism. At present it is not clear which model is correctly describing it.Probing electronic band structure is an essential step to solve this issue. Accordin...

High Eu 4f low-energy oscillator strength in the isostructural rare-earth Zintl compounds EuIn2X2 (X = P,As)

“…There are several contributions to this structure, where the lower two humps mainly come from transitions between occupied Eu 4f and unoccupied Eu 5d states. 12 These two humps evolve to peaks at 0.5 eV and 1.2 eV when the simple LSDA is employed, see the discussion of In Figs. 4(e) and 4(f), we can see that the Kerr rotation and ellipticity are related to each other: when K crosses zero a peak appears in h K due to the Kramers-Kronig relation. 28 It FIG.…”

“…A similar gain was observed in EuB 6 below the ferromagnetic transition temperature and attributed to the spindependent splitting of the electronic bands in the ferromagnetic phase. 12 The peak at 600 cm À1 (74 meV) was assigned to transitions across the hybridization gap. Interestingly, the authors report on a pronounced peak at 0.6 eV and a weak peak at 1.1 eV, and attribute them to electronic interband transitions mostly involving the Eu 4f and 5 d orbitals.…”