Observation of semilocalized dispersive states in the strongly correlated electron-doped ferromagnet ${\mathrm{Eu}}_{1-x}{\mathrm{Gd}}_x\mathrm{O}$

Chemical substitution plays a key role in controlling the electronic and magnetic properties of complex materials. For instance, in EuO, carrier doping can induce a spin-polarized metallic state and colossal magnetoresistance, and significantly enhance the Curie temperature. Here, we employ a combination of molecular-beam epitaxy, angle-resolved photoemission spectroscopy, and an effective model calculation to investigate and understand how semilocalized states evolve in lightly electron-doped ${\mathrm{Eu}}_{1-x}{\mathrm{Gd}}_x\mathrm{O}$ above the ferromagnetic Curie temperature. Our studies reveal a characteristic length scale for the spatial extent of the donor wave functions which remains constant as a function of doping, consistent with recent tunneling studies of doped EuO. Our work sheds light on the nature of the semiconductor-to-metal transition in ${\mathrm{Eu}}_{1-x}{\mathrm{Gd}}_x\mathrm{O}$ and should be generally applicable for doped complex oxides.

Figure 1

(a)–(c) ARPES dispersions for ${\mathrm{Eu}}_{1-x}{\mathrm{Gd}}_x\mathrm{O}$ taken at constant temperature $T=140$ K around $\mathrm{\Gamma }$ while varying $x$. The inset in (a) is the Fermi surface for ${\mathrm{Eu}}_{0.95}{\mathrm{Gd}}_{0.05}\mathrm{O}$, reproduced from Ref. [11], illustrating where in $k$ space the cut is taken. (d) Evolution of the Fermi wave vector $k_{\mathrm{F}}$ with $x$.

Figure 2

(a) Simulated spectral function for a one-dimensional system ($N=1000$ sites) consisting only of spatially extended impurity states with the Gaussian envelope removed to highlight the impurity band dispersion. (b)–(d) Isolated impurity band spectral functions calculated for doping values of 1/8, 1/4, and 1/2. The range of the color scales for each panel has been individually normalized to make the impurity bands more visible.

Figure 3

Momentum distribution curves for doped EuO. (a) Illustration of the electrically active (green) and inactive (blue) states above and below $T_{\mathrm{C}}$. (b)–(d) Integrated MDC curves from $-1$ to +0.1 eV, at 140 K (red) and 10 K (blue), and the difference (green) for (b) $x=0.013$, (c) 0.05, and (d) 0.21. Each spectrum has been normalized to the peak in the Eu $4f$ valence band. (e) Summary of difference curves normalized to their peak intensity, illustrating identical widths irrespective of doping value/$k_{\mathrm{F}}$.