Block Magnetic Excitations in the Orbitally Selective Mott Insulator ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$

Iron pnictides and selenides display a variety of unusual magnetic phases originating from the interplay between electronic, orbital, and lattice degrees of freedom. Using powder inelastic neutron scattering on the two-leg ladder ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$, we fully characterize the static and dynamic spin correlations associated with the ${\mathrm{Fe}}_4$ block state, an exotic magnetic ground state observed in this low-dimensional magnet and in ${\mathrm{Rb}}_{0.89}{\mathrm{Fe}}_{1.58}{\mathrm{Se}}_2$. All the magnetic excitations of the ${\mathrm{Fe}}_4$ block state predicted by an effective Heisenberg model with localized spins are observed below 300 meV and quantitatively reproduced. However, the data only account for $16(3){\mu }_B^2$ per ${\mathrm{Fe}}^{2+}$, approximatively $2/3$ of the total spectral weight expected for localized $S=2$ moments. Our results highlight how orbital degrees of freedom in iron-based magnets can conspire to stabilize an exotic magnetic state.

Figure 1

(a) Crystal structure of ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ with $a=11.88\AA$, $b=5.41\AA$, and $c=9.14\AA$. Ba atoms are omitted. The ${\mathrm{Fe}}_4$ block ground state is represented with light (spin-down) and dark (spin-up) bold arrows. (b) Structure of an individual ladder. (c) Values of exchange interactions determined from our data using an effective Heisenberg model.

Figure 2

Low-energy spectrum of ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$. (a) $\tilde{I}(Q,E)$ at $T=5\mathrm{K}$ with $E_i=50\mathrm{meV}$. (b) $E$-integrated inelastic scattering $\tilde{I}(Q)$ (open symbols) and elastic scattering ${\tilde{I}}_0(Q)$ multiplied by a factor $1/150$ (full symbols) compared to the text’s model cross section (solid blue line) and elastic scattering (dashed red line). Inset: Fit ${\chi }^2$ versus ${\xi }_a$ and ${\xi }_c$ and best fit values (blue symbol). (c) $T$ dependence of the integrated magnetic intensity measured with $E_i=20\mathrm{meV}$ (full symbols) and magnetic order parameter as a guide to the eye (blue line). (d),(e) $\tilde{I}(Q,E)$ with $E_i=20\mathrm{meV}$ at $T=5\mathrm{K}$ and $T=300\mathrm{K}$. (f) Momentum-integrated inelastic scattering with different $E_i$’s. (g) SWT prediction ${\tilde{I}}_{\mathrm{SWT}}(Q,E)$ for $E_i=50\mathrm{meV}$ in arbitrary intensity units.

Figure 3

High-energy spectrum of ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$. (a),(b) Intensity plot of $\tilde{I}(Q,E)$ at $T=5\mathrm{K}$ with (a) $E_i=450\mathrm{meV}$ and (b) $E_i=150\mathrm{meV}$. (c) Momentum-integrated inelastic scattering $\tilde{I}(E)$ (open symbols) for various ranges of $Q$ and fits to Lorentzian line shapes (solid blue lines). (d) Energy-integrated inelastic scattering $\tilde{I}(Q)$ for the four modes (open symbols) compared to the nearby background $B(Q)$ (full symbols) and to the ${\mathrm{Fe}}^{2+}$ form factor $I(Q)=A|f(Q){|}^2+B(Q)$ (solid blue lines). (e),(f) SWT prediction ${\tilde{I}}_{\mathrm{SWT}}(Q,E)$ for (e) $E_i=450\mathrm{meV}$ and (f) $E_i=150\mathrm{meV}$ in arbitrary units.