Search

We investigate the surface structure and electronic properties of β-Sn islands deposited on a graphitized 6H-SiC (0001) substrate via low temperature scanning tunneling microscopy and spectroscopy. Owing to the confinement of the island geometry, quantum well states (QWSs) are formed, manifesting as equidistant peaks in the tunneling spectra. Furthermore, a distinct strip feature appears on the surfaces of odd-layer Sn islands, ranging from 15–19 layers, which is not present on the surfaces of even-layer Sn islands. The spatial distribution of strips can be modified by applying a bias pulse, using an STM tip. Furthermore, the strip-like structure shows significant impacts on the QWS. An energy splitting of the lowest unoccupied QWSs is observed in strip regions; this may be ascribed to caused the phase shift of the wave functions of the QWSs on the top surface, due to surface distortions created by the aforementioned strips. [ABSTRACT FROM AUTHOR]

We report high transition temperature superconductivity in one unit-cell (UC) thick FeSe films grown on a Seetched SrTiO3(001) substrate by molecular beam epitaxy (MBE). A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy (STM) suggest that the superconductivity of the 1 UC FeSe films could occur around 77 K. The control transport measurement shows that the onset superconductivity temperature is well above 50 K. Our work not only demonstrates a powerful way for finding new superconductors and for raising TC, but also provides a well-defined platform for systematic studies of the mechanism of unconventional superconductivity by using different superconducting materials and substrates. [ABSTRACT FROM AUTHOR]

Submonolayer Bi and Au adsorptions on the GaAs(001)-2 × 4 surface are investigated by scanning tunnelling microscopy, low energy electron diffraction and first-principles calculations. The 1 × 4 and 3 × 4 reconstructed surface induced by Bi and Au, respectively, are revealed and their structural models are proposed based on experiments and first-principles calculations. Moreover, the validity of the recently proposed generalized electron counting (GEC) model [Phys. Rev. Lett. 97 (2006) 126103] is examined in detail by using the two surfaces. The GEC model perfectly explains the structural features, such as the characteristic short double-line structure in the Bi-1 × 4 surface and the 3× arrangement of four-atom Au clusters. [ABSTRACT FROM AUTHOR]

Morphologies of Cu(111) films on Si(111)-7×7 surfaces prepared at low temperature are investigated by scanning tunnelling microscopy (STM) and reflection high-energy electron diffraction (RHEED). At the initial growth stage, Cu films are flat due to the formation of silicide at the interface that decreases the mismatch between Cu films and the Si substrate. Different from the usual multilayer growth of Cu/Cu(111), on the silicide layer a layer-by-layer growth is observed. The two dimensional (2D) growth is explained by the enhanced high island density at low deposition temperature. Increasing deposition rate produces films with different morphologies, which is the result of Ostwald ripening. [ABSTRACT FROM AUTHOR]

The controlled growth of Zn-polar ZnO films on Al-terminated a-Al2O3 (0001) substrates is investigated by the radio-frequency plasma-assisted molecular beam epitaxy method. Prior to the growth, a-Al2O3 (0001) surface is modified by an ultrathin MgO layer, which serves as a uniform template for epitaxy of Zn-polar ZnO films. The microstructures of ZnO/MgO/Al2O3 interface are investigated by in-situ reflection high-energy electron diffraction observations and ex-situ high-resolution transmission electron microscopy characterization. It is found that under Mg-rich condition, the achievement of the wurtzite MgO ultrathin layer plays a key role in the subsequent growth of Zn-polar ZnO. An interfacial atomic model is proposed to explain the mechanism of polarity selection of both MgO and ZnO films. [ABSTRACT FROM AUTHOR]

The Kondo effect in two-dimensional manganese phthalocyanine (MnPc) self-assembled monolayer films on Pb(111) islands is studied by low-temperature scanning tunneling microscopy. Variation of the Kondo temperature from 50 K to 300 K at different molecule adsorption sites is revealed. It is shown that the variation is mainly due to the change in the width of d orbital, rather than the shift of its energy. The two-dimensional dI/dV mapping reveals the periodic modulation of the Kondo resonance in the self-assembled MnPc monolayer. [ABSTRACT FROM AUTHOR]

We demonstrate the importance of interface modification on improving electron confinement by preparing Pb quantum islands on Si(111) substrates with two different surface reconstructions, i.e., Si(111)-7 x 7 and Si(111)-Root3 x Root3-Pb (hereafter, 7 x 7 and R3). Characterization with scanning tunneling microscopy/spectroscopy shows that growing Pb films directly on a 7 x 7 surface will generate many interface defects, which makes the lifetime of quantum well states (QWSs) strongly dependent on surface locations. On the other hand, QWSs in Pb films on an R3 surface are well defined with small variations in linewidth on different surface locations and are much sharper than those on the 7 x 7 surface. We show that the enhancement in quantum confinement is primarily due to the reduced electron-defect scattering at the interface. [ABSTRACT FROM AUTHOR]

Effect of oxygen vacancy on transport property of perovskite microstructures is studied theoretically. Compared with calculated and measured I-V curves, it is revealed that electron conduction plays an important role in the oxygen nonstoichiometry perovskite heterostructures even with hole-doped or un-doped material due to the oxygen vacancies. In addition, a detailed understanding of the influence of oxygen vacancy concentration and temperature on the conduction characteristics of oxide heterojunction with both forward and reverse biases is obtained by calculation. [ABSTRACT FROM AUTHOR]

We report on a two-step method for oxidation of Pb(111) surfaces, which consists of low temperature (90K) adsorption of O2 and subsequent annealing to room temperature. In situ scanning tunnelling microscopy observation reveals that oxidation of Pb(111) can occur effectively by this method, while direct room temperature adsorption results in no oxidation. Temperature-dependent adsorption behaviour suggests the existence of a precursor state for O2 adsorption on Pb(111) surfaces and can explain the oxidation-resistance of clean Pb(111) surface at room temperature. [ABSTRACT FROM AUTHOR]