Observation of anomalous nonequilibrium terahertz dynamics and a gapped topological phase in a semimetal $\mathrm{CaIr}{\mathrm{O}}_3$ thin film

Understanding the effects of coexisting electron correlations and spin-orbit coupling on the electronic ground state is the crux of emergent quantum phenomena in complex oxide systems. Here, we explore the critical role of such host interactions on the semimetallic state in $\mathrm{CaIr}{\mathrm{O}}_3$ epitaxial thin film using steady-state and time-resolved terahertz (THz) spectroscopies, and dc magneto transport measurements. We observe a THz optical gap of ∼1.6 meV in the Dirac bands along with a theoretically predicted topological semimetallic phase. The ultrafast dynamics of this gapped semimetallic phase, as studied by an optical-pump THz-probe tool, reveal a gigantic change in transmission that is unusual for a metal but typical of an insulating phase. As observed, this photoinduced change in THz transmission in the semimetallic phase of $\mathrm{CaIr}{\mathrm{O}}_3$ (bandgap of a few meV) is of the same order as in the insulating phase of other correlated perovskites having a bandgap of a few eV. An extremely slow recovery time compared to that of metals and other Dirac semimetallic compounds is another anomalous feature. These observations along with anisotropy in THz optical conductivity and dc magnetoconductance along the in-plane [100] and [010] crystal axes, place $\mathrm{CaIr}{\mathrm{O}}_3$ in a new category. This unprecedented combination of ultrafast THz dynamical properties provides valuable insights into the unified control of electron correlations and strong spin-orbit coupling in $\mathrm{CaIr}{\mathrm{O}}_3$, along with new room-temperature ultrafast THz modulation functionalities.

Figure 1

(a) Shows the XRD θ-2θ scan of CIO thin film deposited on LSAT (100) substrate. The inset shows the zoomed view around the (200) peak. (b) X-ray reflectivity of the CIO thin film along with fitting. (c) and (d) show the reciprocal space maps of CIO thin film along Bragg reflection peaks (103) and (013) of the LSAT (100) substrate, respectively.

Figure 2

Shows the temperature-dependent optical properties of CIO/LSAT (100) thin film along two orthogonal in-plane directions. (a),(c) and (b),(d) show the temperature-dependent optical conductivity and dielectric constant along the [100] and [010] directions at various frequencies, respectively. Inset in (b) represents the temperature-dependent dc electrical resistivity along both in-plane directions.

Figure 3

Frequency-dependent optical properties of CIO/LSAT (100) thin film along orthogonal in-plane directions. (a),(c) and (b),(d) represent the frequency-dependent optical conductivity and dielectric constant along in-plane directions [100] and [010] at various temperatures, respectively. dc conductivity obtained using the four-probe method is shown at zero frequency. The optical conductivity is fitted with a sum of the Drude-Smith+Lorentz model as explained in the main text.

Figure 4

Temperature-dependent fitting parameters of the THz complex optical conductivity along both in-plane directions. (a) and (b) show the strength of the optical excitation mode (${\omega }_{\mathrm{pL}}$) and the linewidth (${\mathrm{\Gamma }}_{\mathrm{L}}$) as a function of temperature, respectively. (c) and (d) show the $\mathrm{S}{\mathrm{W}}_{\mathrm{DS}}$ (${\omega }_{\mathrm{pDS}}^2$) and scattering rate (${\mathrm{\Gamma }}_{\mathrm{DS}}$) of the free carriers as a function of temperature, respectively. Dark yellow and purple colors correspond to the two in-plane orientations [100] and [010], respectively.

Figure 5

(a) and (b) show the transient THz transmission change as a function of pump-probe delay time at various optical-pump fluence at longer and shorter timescales, respectively. Black line in (b) represents the biexponential fits to the experimental data. (c) and (d) show the obtained relaxation time constants by fitting the biexponential function to transient THz transmission change, as a function of optical-pump fluence. The inset in (c) shows the obtained rise time as a function of optical-pump fluence. The lines joining the data points in (c), (d), and the inset are guides to the eyes.

Figure 6

(a) and (b) show the magnetoconductance $\left(\mathrm{\Delta }\sigma \right)$ as a function of applied magnetic field at various temperatures along both in-plane directions [100] and [010], respectively. The solid lines in (a) and (b) are the simplified HLN equation fitting to the experimental data. (c) shows the temperature-dependent phase coherence length ($l_{\varphi }$) and spin-orbit coupling strength (α); (d) shows the correlation between phase coherence length ($l_{\varphi }$) and spin-orbit coupling strength $\left(\alpha \right)$ along both in-plane directions. The black and red colors in (c) and (d) indicate the [100] and [010] directions, respectively.