Revealing multiple density wave orders in nonsuperconducting titanium oxypnictide ${\mathrm{Na}}_2$${\mathrm{Ti}}_2$${\mathrm{As}}_2\mathrm{O}$

We report an optical spectroscopy study on single crystals of ${\mathrm{Na}}_2$${\mathrm{Ti}}_2$${\mathrm{As}}_2\mathrm{O}$, a sister compound of the superconductor Ba${\mathrm{Ti}}_2$Sb${}_2\mathrm{O}$. The study unexpectedly reveals two density wave phase transitions. The first transition, at 320 K, results in the formation of a large energy gap and removes the majority of the Fermi surfaces. But the compound remains metallic with residual itinerant carriers. Below 42 K, another density wave phase transition with a smaller energy gap scale occurs and drives the compound into the semiconducting ground state. These experiments thus enable us to shed light on the complex electronic structure in titanium oxypnictides.

Figure 1

Temperature-dependent in-plane resistivity and magnetic susceptibility of ${\mathrm{Na}}_2$${\mathrm{Ti}}_2$${\mathrm{As}}_2\mathrm{O}$.

Figure 2

$R\left(\omega \right)$ spectra for ${\mathrm{Na}}_2$${\mathrm{Ti}}_2$${\mathrm{As}}_2\mathrm{O}$ at several representative temperatures. Left inset: $R\left(\omega \right)$ spectra up to 5000 cm${}^{-1}$ on a linear frequency scale. Right inset: $R\left(\omega \right)$ spectra up to 600 cm${}^{-1}$ at low temperatures.

Figure 3

${\sigma }_1\left(\omega \right)$ spectra for ${\mathrm{Na}}_2$${\mathrm{Ti}}_2$${\mathrm{As}}_2\mathrm{O}$ at low temperatures over a broad range of frequencies. Left inset: Conductivity spectra up to 5000 cm${}^{-1}\mathrm{O}$n a linear frequency scale. Right inset: Plot of energy gap ${\Delta }_1$, extracted from the optical measurement, as a function of temperature. Data points follow the curve of BCS theory for a density-wave phase transition (dashed curve).

Figure 4

Experimental data on ${\sigma }_1\left(\omega \right)$ at 330, 100, and 10 K together with Drude-Lorentz fits. Lower right panel: Variations of $1/\tau$ and ${\omega }_p^2$ normalized to the values at 330 K as a function of temperature.

Figure 5

Frequency-dependent spectral weight at different temperatures. Curvatures indicated by arrows reflect the effect of spectral weight redistributions caused by DW gap openings.