Collapse of the Mott Gap and Emergence of a Nodal Liquid in Lightly Doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$

We report angle resolved photoemission experiments on the electron doped Heisenberg antiferromagnet $({\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{)}_2{\mathrm{IrO}}_4$. For a doping level of $x=0.05$, we find an unusual metallic state with coherent nodal excitations and an antinodal pseudogap bearing strong similarities with underdoped cuprates. This state emerges from a rapid collapse of the Mott gap with doping resulting in a large underlying Fermi surface that is backfolded by a $(\pi ,\pi )$ reciprocal lattice vector which we attribute to the intrinsic structural distortion of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$.

Figure 1

Nodal liquid in lightly doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. (a) Constant energy contour of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ at $-200\mathrm{meV}$ showing the loci of the lowest lying charge excitations of the parent insulator in the large Brillouin zone corresponding to the Ir-Ir nearest neighbor square lattice. The white square illustrates the actual structural Brillouin zone that coincides with the magnetic zone and contains two iridium sites per plane. (b) For $x=0.01$, the lowest lying spectral weight shifts from $(\pi ,0)$ to $(\pi /2,\pi /2)$. (c) At the higher doping of $x=0.05$, we observe the emergence of coherent excitations along the nodal direction with clear spectral weight on the backside of the Fermi arcs. (d) Momentum distribution curves along the nodal (blue) and antinodal (red) direction illustrating the dichotomic behavior of the single particle excitations. The data in (a)–(c) have been fourfold rotationally averaged.

Figure 2

Collapse of the Mott gap. (a),(d) Photoemission intensity in the fully gapped parent insulator along the nodal and antinodal direction, respectively. (b),(e) For $x=0.05$, electronlike metallic states appear at $(\pi /2,\pi /2)$ while the apex of the holelike band at $(\pi ,0)$ moves above the chemical potential resulting in a collapse of the charge gap. (c),(f) Curvature plots of the raw data used to extract band positions. (g) Band dispersion for $x=0$ compared to a $\mathrm{TB}+\mathrm{SO}+U$ calculation. (h) Band dispersion for $x=0.05$ compared to a calculation with $U=0$. Square (circular) symbols indicate data points extracted from two-dimensional (one-dimensional momentum) curvature plots. (i) Large electronlike Fermi surface corresponding to the TB calculation with $U=0$ and comprising $1+x^{\prime }$ carriers. The color scale in (g)–(i) encodes pseudospin $J_{\text{eff}}=1/2$ character.

Figure 3

Anisotropic pseudogap for $x=0.05$. (a),(b) Energy distribution curves along $(0,0)-(\pi ,\pi )$ and $(0,0)-(\pi ,0)$, respectively, showing a suppression of the low-energy spectral weight in the antinodal region [panel (b)]. (c) Symmetrized Energy Distribution Curves (blue dots) and fits to the Dynes formula (red lines) for different angles along the Fermi surface. (d) Fermi surface with the color-coded magnitude of the pseudogap overlaid. Note that the onset of the pseudogap is observed slightly before the apex of the lenslike contour. (e) Pseudogap value for the original (blue markers) and back folded (BF) (red markers) band as a function of Fermi surface angle.