Anomalous State Sandwiched between Fermi Liquid and Charge Ordered Mott-Insulating Phases of ${\mathrm{Ti}}_4{\mathbf{O}}_7$

The Magnéli phase ${\mathrm{Ti}}_4{\mathrm{O}}_7$ exhibits two sharp jumps in resistivity with coupled structural transitions as a function of temperature at $T_{c1}\sim 142\mathrm{K}$ and $T_{c2}=154\mathrm{K}$. We have studied electronic structure changes across the two transitions using 7 eV laser, soft x-ray, and hard x-ray (HX) photoemission spectroscopy (PES). Ti $2p-3d$ resonant PES and HX PES show a clear metallic Fermi edge and mixed valency above $T_{c2}$. The low temperature phase below $T_{c1}$ shows a clear insulating gap of $\sim 100\mathrm{meV}$. The intermediate phase between $T_{c1}$ and $T_{c2}$ indicates a pseudogap coexisting with remnant coherent states. HX PES and complementary calculations have confirmed the coherent screening in the strongly correlated intermediate phase. The results suggest the existence of a highly anomalous state sandwiched between the mixed-valent Fermi liquid and charge ordered Mott-insulating phase in ${\mathrm{Ti}}_4{\mathrm{O}}_7$.

Figure 1

$T$-dependent electronic structure near $E_F$. (a) Resonant PES at the Ti $L_3$ edge. The inset shows the on- and off-spectra at the HT phase over a wide energy range. (b) Experimental laser-PES spectra. (c) The spectral function $A(\omega )$. Shaded areas (yellow and green) highlight the difference of $A(\omega )$ to the LT-phase $A(\omega )$ (light blue). (d) Enlarged plot of $A(\omega )$ near $E_F$. (e) The O $1s$ core-level shift of HX PES compared to XAS. Upper panel, O $1s$ XAS. Lower panel, O $1s$ HX PES. Wide-range O $1s$ XAS are shown in the inset.

Figure 2

Schematic pictures illustrating the evolution of the low energy electronic structure with temperature.

Figure 3

(a) Comparison between experimental Ti $2p$ HX PES spectra for the HT, IT, and LT phases. (b),(c),(d) CIM calculations (lower panel) are compared with experiments. The dotted curves are the ${\mathrm{Ti}}^{3+}$ component.