Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 107, Issue 29, Pages 12761-12765Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0915171107
Keywords
STM; quantum well states; Pb
Categories
Funding
- NSF [DMR-0906025, CMMI-0928664]
- Welch Foundation [F-1672]
- Texas Advanced Research Program [003658-0037-2007]
- DOE [DE-FG02-97ER45632]
- Division Of Materials Research
- Direct For Mathematical & Physical Scien [0906025] Funding Source: National Science Foundation
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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.
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