Unusual Temperature Dependence of Band Dispersion in $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Ru}}_x{)}_2{\mathrm{As}}_2$ and its Consequences for Antiferromagnetic Ordering

We have performed detailed studies of the temperature evolution of the electronic structure in $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Ru}}_x{)}_2{\mathrm{As}}_2$ using angle resolved photoemission spectroscopy. Surprisingly, we find that the binding energy of both hole and electron bands changes significantly with temperature in both pure and Ru substituted samples. The hole and electron pockets are well nested at low temperature in unsubstituted (${\mathrm{BaFe}}_2{\mathrm{As}}_2$) samples, which likely drives the spin density wave and resulting antiferromagnetic order. Upon warming, this nesting is degraded as the hole pocket shrinks and the electron pocket expands. Our results demonstrate that the temperature dependent nesting may play an important role in driving the antiferromagnetic-paramagnetic phase transition.

Figure 1

The band dispersion data of the parent ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ compound measured with (a)–(d) $h\nu =35\mathrm{eV}$ (linear horizontal polarization) and (g)–(k) $h\nu =63\mathrm{eV}$ (linear vertical polarization) for various sample temperature. (e), (l) The MDCs at $E_F$. (f), (m) The extracted Fermi surface calipers $\Delta k_F$ and width of the MDC’s plotted as a function of temperature.

Figure 2

(a)–(d) The FS maps of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ measured with $h\nu =35\mathrm{eV}$ (linear horizontal polarization) at various sample temperatures.

Figure 3

The hole Fermi surface maps of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ (at various sample temperatures) along the $k_{\parallel }-k_z$ plane measured around the center of the BZ by changing $h\nu$.

Figure 4

The band dispersion plots for $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Ru}}_x){\mathrm{As}}_2$, $x=0.36$ sample as a function of temperature. (a)–(d) hole pocket at the center of the BZ.a (e)–(h) electron pocket at the planar corner of the BZ. Data measured with photon energy $h\nu =40.8\mathrm{eV}$.

Figure 5

Dependence of the size of the hole and electron pockets on temperature for $x=0.36$ measured with $h\nu =40.8\mathrm{eV}$. (a) Diameter ($\Delta k_F$) of hole and electron pockets as a function of temperature, (b) energy position of the bottom of electron pocket as a function of temperature.

Figure 6

Band structure calculation for ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ (a) the $Z$ hole pocket and (b) the $X$ electron pocket.