Quantum Theory of Advanced Materials

Angle-resolved photoemission is employed to measure the band structure of TiSe2 in order to clarify the nature of the (2x2x2 ) charge density wave transition. The results show a very small indirect gap in the normal phase transforming into a larger indirect gap at a different location in the Brillouin zone. Fermi surface topology is irrelevant in this case. Instead, electron-hole coupling together with a novel indirect Jahn-Teller effect drives the transition.

Angle-resolved photoemission has been utilized to study the surface electronic structure of 1/3 monolayer of Sn on Ge(lll) in both the room-temperature (root3 x root3)R30 degrees phase and the low-temperature (3 x 3) charge-density-wave phase. The results reveal a gap opening around the (3 x 3) Brillouin zone boundary, suggesting a Peierls-like transition despite the well-documented lack of Fermi nesting, a highly sensitive electronic response to doping by intrinsic surface defects is the cause for this unusual behavior, and a detailed calculation illustrates the origin of the (3 x 3) symmetry.

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.

We studied the hydrogen-induced reconstruction of the W(110) surface using the pseudopotential plane wave approach. The calculations for a full monolayer of hydrogen coverage showed that the quasithreefold hollow site (distorted bridge) has the lowest energy, and that for this geometry a surface reconstruction, consisting of a small uniform shift of the W top layer in the [1(1) over bar0$] direction, is energetically favorable. We also studied the surface states for clean and H-covered W(110) and investigated the effect of the reconstruction on electronic structure.