Electronic phase separation in lightly dopedLa2−xSrxCuO4

The hole concentration dependence of the magnetic correlations is studied in very lightly doped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ $\mathrm{}(x<\mathrm{}0.02\mathrm{}),$ which shows coexistence of a three-dimensional antiferromagnetic long-range-ordered phase and a spin-glass phase at low temperatures. It is found that the spin-glass phase in the coexistence region also shows a diagonal spin modulation as in the pure spin-glass phase in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ $(0.02\mathrm{}<~x<~0.055).$ Below $x\sim 0.02$ the diagonal stripe structure is the same as that in $x\sim 0.02$ with a volume fraction almost proportional to hole concentration, suggesting that electronic phase separation of the doped holes occurs so that some regions with hole concentration $c_h\sim 0.02$ and the rest with $c_h\sim 0$ are formed. To our best knowledge, this represents the most direct observation to date of electronic phase separation in lightly doped antiferromagnets. Such phase separation has been predicted by a number of theories.