Tunneling spectroscopy of ultrathin insulating Cu2N films, and single Co adatoms

Abstract

Scanning tunneling microscopy is used to characterize the electronic structure of 1 ML films of c(2×2)N∕Cu(100) (i.e., Cu2N). By varying nitrogen coverage, a variety of morphologies are prepared, including (1) isolated ∼25nm2 islands, (2) close-packed arrays of islands, and (3) quasicontinuous monolayer films. In all three regimes, the authors find that Cu2N acts as an insulator, with a band gap that exceeds 4eV. The insulating Cu2N films are used to control the coupling of adsorbed Co atoms to the Cu(100) surface electron density. Tunneling spectroscopy of Co on Cu2N reveals an unoccupied atomic resonance, Kondo effect, and spin-flip excitation. These features depend on binding site within the Cu2N film, and are distinctly different than corresponding spectra for Co on Cu(100).

Emergence of surface states in nanoscale Cu2N islands

“…Increased annealing time (~2 min) makes the islands squarer, while increased nitrogen coverage forms islands up to ~5 x 5 nm 2 (11). These islands coalesce into quasicontinuous monolayer films with further coverage (12).…”

Tunneling conductance in a system with strong electron correlation

“…As a result, understanding the observed dI/dV lineshapes for a sample with a strong electron correlation is unsettled. Typical examples that belong to this category are quantum point contacts [3][4][5][6], a single electron transistor with a quantum dot [7][8][9][10], the STS on a magnetized atom adsorbed on a metallic surface [11][12][13][14], doped graphenes [15,16], and high-T c superconductors [17].…”

“…The process shown in figure 1 represents the Kondo dynamics in a two-reservoir Anderson impurity model near zero bias and results in the zero-bias peak in dI/dV lineshape. Therefore, the zero-bias peak shown in a quantum impurity system with two reservoirs [3][4][5][6]14] corresponds to the Kondo peak.…”

“…We show the suppression of the zerobias peak by superimposing two lineshapes, as shown in figure 2. The dI/dV lineshape showing zero-bias peak (black solid line) has been obtained for a Co atom adsorbed on a Cu (100) surface that is covered by an insulating layer Cu 2 N [14]. The insulating layer suppresses fluctuations in the metallic substrate and enhances axial coherence connecting STM tip, Co atom, and Cu atom in Cu 2 N [18].…”

“…Other lineshapes in figure 2 are obtained for a Co atom adsorbed on a Cu (111) surface. The experimental dI/dV lineshapes obtained for a Cu (100) surface [13,14] are highly asymmetric and sample dependent. One can understand that higher fluctuation exists in Cu (100) surface than in Cu (111) surface by comparing the data obtained for Cu (100) substrate [13] and those for Cu (111) [12].…”