Tuning the terahertz low-energy charge dynamics by simultaneous effect of epitaxial and anisotropic strain in $\mathrm{PrNi}{\mathrm{O}}_3$ thin films

The interplay of charge, spin, and lattice correlations strongly influence the insulator-metal (I-M) transition and magnetic ordering in rare earth nickelates. In this context, we explored the low-energy charge dynamics in structurally modulated $\mathrm{PrNi}{\mathrm{O}}_3$ (PNO) thin films to unravel the complexity of ground state across I-M transition using terahertz (THz) spectroscopy. The THz optical constants of compressive film on $\mathrm{LaAl}{\mathrm{O}}_3$ (100) substrate and the tensile films on $\mathrm{NdGa}{\mathrm{O}}_3$ (100), (001), (110), and (111) substrates with varying orthorhombic distortion exhibit remarkably distinct features as a function of frequency and temperature. The THz conductivity of compressive film sans any I-M transition follows the Drude model. In contrast, the tensile strained films exhibit non-Drude THz conductivity, a giant positive dielectric permittivity, and negative imaginary conductivity, all of which can be explained by the Drude-Smith model. This rich variety of low-energy dynamics manifests as a function of temperature, strain, and crystal orientation. Such distinct THz spectral features, as induced by a subtle variation in strain while crossing over from tensile to compressive strain and with varying degree of orthorhombicity coupled with oxygen vacancies, reveal a novel facet of structure-property relationship of PNO.

Figure 1

$\theta \text{−}2\theta$ scans of PNO films grown on (a) LAO ${\left(100\right)}_{\mathrm{pc}}$-, (b) NGO ${\left(111\right)}_{\mathrm{o}}$-, (c) NGO ${\left(110\right)}_{\mathrm{o}}$-, (d) NGO ${\left(001\right)}_{\mathrm{o}}$-, and (e) NGO ${\left(100\right)}_{\mathrm{o}}$-oriented substrates, respectively.

Figure 2

(a)–(e) RSMs along asymmetric peaks of (113), (444), (402), (301), and (321) of PNO films grown on NGO ${\left(001\right)}_{\mathrm{o}}$-, ${\left(110\right)}_{\mathrm{o}}$-, ${\left(100\right)}_{\mathrm{o}}$-, $\mathrm{LAO}{\left(100\right)}_{\mathrm{pc}}$-, and ${\left(111\right)}_{\mathrm{o}}$-oriented substrates. Similarly, (f)–(h) show the AFM images of PNO films grown on LAO ${\left(100\right)}_{\mathrm{pc}}$, NGO ${\left(110\right)}_{\mathrm{o}}$, and ${\left(001\right)}_{\mathrm{o}}$ orientations, respectively.

Figure 3

(a) The dc resistivity plots of PNO thin films grown on ${\left(110\right)}_{\mathrm{o}}$-, ${\left(001\right)}_{\mathrm{o}}$-, ${\left(100\right)}_{\mathrm{o}}$-, and ${\left(111\right)}_{\mathrm{o}}$-oriented NGO substrates in addition to the PNO thin film grown on ${\left(100\right)}_{\mathrm{pc}}$-oriented LAO substrate. Here, filled and void symbols denote the heating and cooling cycles. (b) Phase diagram depicts the $T_{\mathrm{IM}}$ of PNO films as a function of bond angle ${\mathrm{\Theta }}_{\mathrm{Ni}\text{−}\mathrm{O}\text{−}\mathrm{Ni}}$. For comparison, PNO, NNO, and LNO bulk data are plotted. Orange (turquoise) color regions represents the tensile (compressive) strained regimes. Dotted line passes through bulk pseudocubic lattice parameter of PNO $(a_{\mathrm{pc}}=3.815)$.

Figure 4

(a) Temperature dependence of the Hall coefficient ($R_{\mathrm{H}}$) and Hall mobility of the charge carriers. (b) The calculated hole ($p$) and electron ($n$) carrier densities of PNO film on LAO ${\left(100\right)}_{\mathrm{pc}}$-oriented substrate.

Figure 5

(a) and (b) Temperature-dependent real parts of THz conductivity (${\sigma }_1$) and dielectric permittivity (${\varepsilon }_1$) spectra of PNO/NGO ${\left(001\right)}_{\mathrm{o}}$ film. Similarly, (c) shows the THz imaginary conductivity (${\sigma }_2$) spectra at various temperatures. Here, symbols correspond to the experimental data, and solid lines denote the fits to Eq. (2). (d) Temperature dependence of the THz conductivity (${\sigma }_1$) replotted for selected frequencies of PNO/NGO ${\left(001\right)}_{\mathrm{o}}$ film.

Figure 6

Temperature-dependent complex conductivity spectra of PNO/NGO ${\left(110\right)}_{\mathrm{o}}$-, PNO/NGO ${\left(100\right)}_{\mathrm{o}}$-, and PNO/NGO ${\left(111\right)}_{\mathrm{o}}$-oriented films. Here, solid lines denote the theoretical fits of experimental data (symbols) to the D-S model.

Figure 7

(a) and (b) Temperature-dependent real and imaginary parts of THz conductivity (${\sigma }_1,{\sigma }_2$) spectra of PNO/LAO ${\left(100\right)}_{\mathrm{pc}}$ film. (c) Temperature dependence of the THz conductivity (${\sigma }_1$) replotted for selected THz frequencies. (d) The normalized THz conductivity plots of all PNO thin films.