Experimental observation of pseudogap in a modulation-doped Mott insulator: Sn/Si(111)-($\sqrt{3}\times \sqrt{3}$)$R$30$^\textrm{o}$

Unusual quantum phenomena usually emerge upon doping Mott insulators. Using a combinatorial molecular beam epitaxy system integrated with cryogenic scanning tunneling microscopy, we investigate the electronic structure of modulation-doped insulator Sn/Si(111)-($\sqrt{3}\times \sqrt{3}$)-R30$^\textrm{o}$. In underdoped regions, observe universal pseudogap opening around Fermi level, which changes little applied magnetic field and occurrence Sn vacancies. The gets smeared out at elevated temperatures alters in size spatial confinement insulating phase. Our findings, along previously observed superconductivity higher are highly reminiscent phase diagram doped copper oxide compounds.