Role of Oxygen Electrons in the Metal-Insulator Transition in the Magnetoresistive OxideLa2−2xSr1+2xMn2

Abstract

We have studied the [100]- [110] anisotropy of the Compton profile in the bilayer manganite. Quantitative agreement is found between theory and experiment with respect to the anisotropy in the two metallic phases (i.e. the low temperature ferromagnetic and the colossal magnetoresistant phase under a magnetic field of 7 T). Robust signatures of the metal-insulator transition are identified in the momentum density for the paramagnetic phase above the Curie temperature. We interpret our results as providing direct evidence for the transition from the metallic-like to the admixed ioniccovalent bonding accompanying the magnetic transition. The number of electrons involved in this phase transition is estimated from the area enclosed by the Compton profile anisotropy differences. Our study demonstrates the sensitivity of the Compton scattering technique for identifying the number and type of electrons involved in the metal-insulator transition. [5,6]. Above T c , the phase diagram displays an insulating paramagnetic (PM) phase. The Mn-3d electronic states, which are responsible for these properties, split into e g and t 2g contributions in the crystal field of the MnO 6 octahedron. The FM phase below T c and its metallic conductivity are usually explained on the basis of the double exchange (DE) mechanism [7], where e g electrons hop between Mn sites through hybridization with the oxygen 2p orbitals and align the localized t 2g spins by the exchange interaction. While the DE mechanism appears to capture the tendency towards ferromagnetism, it still remains unclear if oxygen orbitals should be explicitly included in the electronic degrees of freedom, or whether they can be integrated out as is often assumed in the standard models [8,9].Recent magnetic Compton scattering (MCS) studies [10] of the manganite FM phase have shown how the occupation numbers of the e g states vary with doping [11,12] as well as temperature [13,14]. In addition, they have provided evidence for the coexistence of localized and itinerant e g magnetic electrons [15,16] in the FM phase. MCS has also been used to study other spintronics materials such as magnetite Fe 3 O 4 and its mysterious Verwey transition [17].In this letter, we show that the anisotropy of high resolution Compton profiles (CP) displays a striking difference between the insulating PM and metallic FM case. This is important because this difference originates in the MnO planes which are the seat of the CMR properties. Similar effects have been observed in the metallic YBa 2 Cu 3 O 7 and insulating PrBa 2 Cu 3 O 7 systems [18]. However, this is the first time that this effect has been observed on the same sample under the influence of external parameters such as temperature and magnetic field. We also provide a measure of the number of electrons involved in the CMR effect.Compton scattering, or inelastic scattering with very

Enhancement of Compton scattering by an effective coupling constant

“…Compton scattering is also a unique spectroscopy for condensed-matter physics, which has acquired greater importance with the advent of modern synchrotron sources [2][3][4]. For instance, it has been used to extract information about wave functions of valence electrons in a variety of systems ranging from ice [5,6] and water [7] to alloys [8] and correlated electron systems [9]. Moreover, Compton scattering can potentially help delineate confinements [10] and spin polarization effects [11] in nanoparticles.…”

A Minimal tight-binding model for ferromagnetic canted bilayer manganites

“…While the DE mechanism appears to capture the tendency towards ferromagnetism, the oxygen orbitals must be explicitly included to explain correctly the metal insulator transition at the Curie temperature16.…”

fElectron Contribution to the Change of Electronic Structure inCeRu2Si2with Temperature: A Compton Scattering Study

“…On the other hand, both measured and calculated anisotropies show a similar oscillatory structure. The oscillations are associated with characteristic distances corresponding to the coherence of wave functions in given crystal orientations [24,25]. Above T K , the Ce-4f electron is well localized and regarded as a core state, while below T K , the 4f electron forms a narrow conduction band where the 4f wave function has a shorter coherence length relative to the others.…”

“…Experimental and theoretical anisotropies of Compton profiles (a) which are shown in the percentage of the Compton peak measured in the [100] direction[24,25], and power densities (b) of the anisotropies. All the curves in (b) are normalized to a unit area[25]. In both figures, black solid and (red) open circles denote experimental ones at 5 K and RT, respectively.…”