Quantum Theory of Advanced Materials

n/a

Here{,} by utilizing the special interaction between metal Cu and multi-walled carbon nanotubes (CNTs){,} we have successfully realized the in situ growth of Cu2Se on the surface of CNTs and then fabricated a series of Cu2Se/CNT hybrid materials. Due to the high degree of homogeneously dispersed molecular CNTs inside the Cu2Se matrix{,} a record-high thermoelectric figure of merit zT of 2.4 at 1000 K has been achieved.

In this paper, we present the direct observation of quantum size effects (QSE) on the work function in ultrathin Pb films. By using scanning tunneling microscopy and spectroscopy, we show that the very existence of quantum well states (QWS) in these ultrathin films profoundly affects the measured tunneling decay constant kappa, resulting in a very rich phenomenon of "quantum oscillations" in kappa as a function of thickness, L, and bias voltage, V(s). More specifically, we find that the phase of the quantum oscillations in kappa vs. L depends sensitively upon the bias voltage, which often results in a total phase reversal at different biases. On the other hand, at very low sample bias (vertical bar V(s)vertical bar < 0.03 V) the measurement of kappa vs. L accurately reflects the quantum size effect on the work function. In particular, the minima in the quantum oscillations of kappa vs. L occur at the locations where QWS cross the Fermi energy, thus directly unraveling the QSE on the work function in ultrathin films, which was predicted more than three decades ago. This further clarifies several contradictions regarding the relationship between the QWS locations and the work function.

By using a low temperature scanning tunneling microscope we have probed the superconducting energy gap of epitaxially grown Pb films as a function of the layer thickness in an ultrathin regime (5-18 ML). The layer-dependent energy gap and transition temperature (T-c) show persistent quantum oscillations down to the lowest thickness without any sign of suppression. Moreover, by comparison with the quantum-well states measured above T-c and the theoretical calculations, we found that the T-c oscillation correlates directly with the density of states oscillation at E-F. The oscillation is manifested by the phase matching of the Fermi wavelength and the layer thickness, resulting in a bilayer periodicity modulated by a longer wavelength quantum beat.