The spatial variation of electronic states was imaged in the lightly doped Mott insulator Ca2−xNaxCuO2Cl2 using scanning tunneling microscopy / spectroscopy (STM/STS). We observed nano-scale domains with a high local density of states within an insulating background. The observed domains have a characteristic length scale of 2 nm (∼4-5a, a : lattice constant) with preferred orientations along the tetragonal [100] direction. We argue that such spatially inhomogeneous electronic states are inherent to slightly doped Mott insulators and play an important role for the insulator to metal transition.

The local spectroscopic signatures of metamagnetic criticality in Sr3Ru2O7 were explored using scanning tunneling microscopy (STM). Singular features in the tunneling spectrum were found close to the Fermi level, as would be expected in a Stoner picture of itinerant electron metamagnetism. These features showed a pronounced magnetic field dependence across the metamagnetic critical point, which cannot be understood in terms of a naive Stoner theory. In addition, a pseudo-gap structure was observed over several tens of meV, accompanied by a c(2 × 2) superstructure in STM images. This result represents a new electronic ordering at the surface in the absence of any measurable surface reconstruction.PACS numbers: 68.37. Ef, 71.27.+a, 75.30.Kz The subtle interplay between spin, charge and orbital degrees of freedom in strongly correlated electron systems gives rise to a wide variety of competing electronic phases [1]. A variety of exotic properties have been reported as a result of this competition between different forms of order, and the associated (quantum) critical behavior. Recently, the bi-layer ruthenate Sr 3 Ru 2 O 7 , n = 2 member of the Ruddlesden-Popper series Sr n+1 Ru n O 3n+1 , has attracted considerable interest because of its criticalities. In dimensionality, Sr 3 Ru 2 O 7 is intermediate between the n = 1 compound Sr 2 RuO 4 , a spin triplet superconductor [2], and the n = ∞ system SrRuO 3 , an itinerant ferromagnet [3]. In common with other Sr n+1 Ru n O 3n+1 compounds, four 4d-electrons are accommodated in its almost triply degenerate t 2g orbitals, implying that orbital degrees of freedom are likely to play an important role.The ground state of Sr 3 Ru 2 O 7 is a (paramagnetic) Fermi liquid. However, the effective mass of the conduction electrons is greatly enhanced, and there is strong evidence of underlying criticality towards electronically and magnetically ordered states. The magnetic susceptibility shows a well-defined peak at T ∼ 16 K, and its large value in the limit T → 0 suggests a substantial Stoner enhancement [4]. These results indicate that the system is very close to a ferromagnetic instability. Indeed, a weakly first-order metamagnetic phase transition occurs in an applied magnetic field of 5.5 T for B//ab, and 8 T for B//c [5,6]. Furthermore, at only 5% doping of Mn impurities on Ru sites, the system becomes an antiferromagnetic insulator and is believed to exhibit orbital ordering [7].The criticalities should manifest themselves as anomalies in low energy spin and charge excitations. The existence of low-lying ferromagnetic spin fluctuations near the metamagnetic critical point has been confirmed by NMR [8]. Similarly, quasi-particle mass anomalies have been observed in de Haas van Alphen (dHvA) measurements at the critical point [9]. However, until now, no spectroscopic data spanning the transition have been available. Given that criticality is well defined only for temperatures of a few K, meV (or better) resolution is necessary to capture the spectroscopic signatures of crit...

Sr3(Ru(1-x)Mnx)2O7, in which 4d-Ru is substituted by the more localized 3d-Mn, is studied by x-ray dichroism and spin-resolved density functional theory. We find that Mn impurities do not exhibit the same 4+ valence of Ru, but act as 3+ acceptors; the extra eg electron occupies the in-plane 3d(x2-y2) orbital instead of the expected out-of-plane 3d(3z2-r2). We propose that the 3d-4d interplay, via the ligand oxygen orbitals, is responsible for this crystal-field level inversion and the material's transition to an antiferromagnetic, possibly orbitally ordered, low-temperature state.