First-Order Melting of a Weak Spin-Orbit Mott Insulator into a Correlated Metal

The electronic phase diagram of the weak spin-orbit Mott insulator (${\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{)}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ is determined via an exhaustive experimental study. Upon doping electrons via La substitution, an immediate collapse in resistivity occurs along with a narrow regime of nanoscale phase separation comprised of antiferromagnetic, insulating regions and paramagnetic, metallic puddles persisting until $x\approx 0.04$. Continued electron doping results in an abrupt, first-order phase boundary where the Néel state is suppressed and a homogenous, correlated, metallic state appears with an enhanced spin susceptibility and local moments. As the metallic state is stabilized, a weak structural distortion develops and suggests a competing instability with the parent spin-orbit Mott state.

Figure 1

(a) Resistivity as a function of temperature $\rho (T)$ for $({\mathrm{Sr}}_{1-x}{A}_x{)}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, $A=\mathrm{La}$ and Ca. (b) Magnetization data for La-doped Sr-327. Data plotted are field-cooled (FC) minus zero-field cooled (ZFC) data under 800 Oe applied parallel to the $c$ axis. (c) Powder x-ray data showing the $a$ axis and unit cell volume as a function of La and Ca substitution in Sr-327. (d) Neutron scattering data showing relative shifts in lattice constants for La-doped Sr-327. Values are the fractional change of lattice values from 225 K, e.g., $a$ axis $\mathrm{\Delta }\%=100\{[a(T)-a(225K)]/a(225K)\}$.

Figure 2

(a) Background subtracted magnetic order parameter measurements for La-doped Sr-327. Data were collected at the $\mathbf{Q}=(1,0,2)$ position and normalized to a sample-dependent scale factor. (b) AF-ordered moment and relative weight of forbidden structural peak (1,0,9) (representative of $T_S$ for La-doped Sr-327). Data for $x=0$ were taken from Ref. [17]. (c) Background subtracted neutron scattering data showing select $T_S$ order parameters at the (1,0,9) wave vector. Intensity of the scattering has been normalized via a sample-dependent scale factor. (d) Heat capacity $c_v(T)$ data for $x=0.058$ La. Dashed line is a fit to the form $c_v(T)=\gamma T+\beta T^3$ with $\gamma =19.88\pm 0.30$ ($\mathrm{mJ}{\mathrm{mole}}^{-1}{\mathrm{K}}^{-2}$) and $\beta =0.409\pm 0.007$ ($\mathrm{mJ}{\mathrm{mole}}^{-1}{\mathrm{K}}^{-4}$). Inset shows $\chi (T)$ for this same sample with $H=20\mathrm{kOe}\parallel ab$ plane with dotted line denoting the Curie-Weiss fit discussed in the text.

Figure 3

STM topography of La-doped Sr-327 at 300 mV bias for (a) insulating $x=0.035$ and (b) metallic $x=0.048$ samples. $dI/dV$ spectra obtained on a grid in each topograph are plotted in panels (c) for $x=0.035$ and (d) for $x=0.048$. Representative numbered points are highlighted in each map and the corresponding $dI/dV$ spectra are emphasized as solid lines in spectral histograms. Fully gapped and gapless spectra are observed in the $x=0.035$ sample, while a homogenous, gapless state appears across spectra in $x=0.048$.

Figure 4

Electronic phase diagram of La-doped Sr-327. Open symbols denote neutron scattering measurements and closed symbols denote x-ray data. Black triangles denote scattering measurements of $T_{\mathrm{AF}}$ and red squares denote measurements of $T_S$. The dashed line marks the MIT measured via charge transport and the first-order line where the Néel state vanishes. The hatched region marks the electronically phase separated region where $T_S$ and $T_{\mathrm{AF}}$, AFI denotes antiferromagnetic insulating state, and PMM marks the paramagnetic metallic phase. Data for $x=0$ taken from Ref. [17].