Fermi Surface Expansion above Critical Temperature in a Hund Ferromagnet

Using a cluster extension of the dynamical mean-field theory, we show that strongly correlated metals subject to Hund’s physics exhibit significant electronic structure modulations above magnetic transition temperatures. In particular, in a ferromagnet having a large local moment due to Hund’s coupling (Hund’s ferromagnet), the Fermi surface expands even above the Curie temperature ($T_C$) as if a spin polarization occurred. Behind this phenomenon, effective “Hund’s physics” works in momentum space, originating from ferromagnetic fluctuations in the strong-coupling regime. The resulting significantly momentum-dependent (spatially nonlocal) electron correlations induce an electronic structure reconstruction involving a Fermi surface volume change and a redistribution of the momentum-space occupation. Our finding will give a deeper insight into the physics of Hund’s ferromagnets above $T_C$.

Figure 1

Real-space spin-spin correlation function and imaginary part of the self-energy at the momenta (0, 0), $(\pi ,0)$ [or equivalently $(0,\pi )$], and $(\pi ,\pi )$. (a),(c) Results for $n=1.9$ and $U=4J=6$. (b),(d) Results for $n=1.5$ and $U=4J=12$.

Figure 2

Results for $n=1.9$. (a) Imaginary part of the periodized self-energy for various values of $U=4J$. (b) The FS ($-\beta G_{\mathit{k}}(\tau =\beta /2)$ normalized by its maximum value) at $U=4J=6$. The light green curve in (b) shows the FS at $U=J=0$.

Figure 3

Results for $n=1.5$ and $J/U=1/4$. (a),(b) The FS ($-\beta G_{\mathit{k}}(\tau =\beta /2)$ normalized by its maximum value), (c) the imaginary part of the periodized self-energy, and (d) the effective dispersion ${\xi }_{\mathit{k}}^{\mathrm{eff}}={\epsilon }_{\mathit{k}}+\mathrm{Re}{\mathrm{\Sigma }}_{\mathit{k}}(0)-\mu$ [where $\mathrm{Re}{\mathrm{\Sigma }}_{\mathit{k}}(0)$ is approximated by $\mathrm{Re}{\mathrm{\Sigma }}_{\mathit{k}}(i{\omega }_0)$] with the inset showing the contour map of ${\xi }_{\mathit{k}}^{\mathrm{eff}}$ for $U=12$. The light green curves in (a),(b) and the inset of (d) show the FS at $U=J=0$. In (c), for comparison, the result for $U=8J=12$ (dashed curve) is also shown.

Figure 4

(a) Change in the momentum-space occupation $\mathrm{\Delta }{n}_{\mathit{k}}$ per orbital from the noninteracting case, calculated for $n=1.5$, $U=4J=12$, and $T=0.08$. The shaded part indicates the region inside the FS at $U=0$. The result with the density-density-type interaction (dotted curve) is also shown for comparison. (b) Spectral function $A_{\mathit{k}}(\omega )=-(1/\pi )\mathrm{Im}{G}_{\mathit{k}}(\omega )$ at $n=1.5$, $U=4J=12$, and $T=0.06$ for the density-density-type interaction model, where the correlation effect is exaggerated compared to the rotationally invariant case. The solid (dashed) curve indicates the noninteracting (fully polarized ferromagnetic majority-spin) band dispersion. For the analytic continuation, we used the maxent package [40].