Role of Oxygen Electrons in the Metal-Insulator Transition in the Magnetoresistive Oxide ${\mathrm{La}}_{2-2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_2{\mathbf{O}}_7$ Probed by Compton Scattering

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 metalliclike to the admixed ionic-covalent bonding accompanying the magnetic transition. The number of electrons involved in this phase transition is estimated. Our study demonstrates the sensitivity of the Compton scattering technique for identifying the number and type of electrons involved in the metal-insulator transition.

Figure 1

Anisotropy $A_{C_{4v}}^{2\mathrm{D}}(p_y,p_z)$ of the 2D projection of the theoretical electron momentum density onto the (001) plane, normalized to the amplitude ${\rho }^{2\mathrm{D}}(0,0)$.

Figure 2

Theoretical anisotropies $A^{1\mathrm{D}}(p_z)$ and $A_{\mathrm{mag}}^{1\mathrm{D}}(p_z)$ [12] (without resolution broadening).

Figure 3

Comparison of the theoretical and experimental $A^{1\mathrm{D}}(p_z)$. In the legend, Low Temp FM is for the ferromagnetic phase at 20 K, $T>T_c$ PM is for the paramagnetic phase at 131 K, and CMR 7 T is for the colossal magnetoresistance phase in an external field of 7 T at 131 K. Theory is convoluted with the experimental resolution. Error bars shown for the FM phase are comparable for other experimental CP anisotropies.

Figure 4

Comparison of the theoretical and experimental $P_D(z)$. All the curves are normalized to a unit area. The notations in the legend are as in Fig. 3.