Closed-loop conductance scanning tunneling spectroscopy: demonstrating the equivalence to the open-loop alternative

We demonstrate the validity of using closed-loop z(V) conductance scanning tunneling spectroscopy (STS) measurements for the determination of the effective tunneling barrier by comparing them to more conventional open-loop I(z) measurements. Through the development of a numerical model, the individual contributions to the effective tunneling barrier present in these experiments, such as the work function and the presence of an image charge, are determined quantitatively. This opens up the possibility of determining tunneling barriers of both vacuum and molecular systems in an alternative and more detailed manne

Topologically protected one-dimensional electronic states in group IV two-dimensional Dirac materials

In this report we give a brief introduction on the occurrence of topologically protected one-dimensional electronic states in group IV two-dimensional graphene-like materials. We discuss the effect of spin-orbit coupling on the electronic band structure and show that these materials are potential candidates to exhibit the quantum spin Hall effect. The quantum spin Hall effect is characterized by a gapped interior and metallic counter-propagating spin-polarized topologically protected edges states. We also elaborate on the electric-field induced formation of a hexagonal network of one-dimensional topologically protected electronic states in small-angle twisted bilayer graphene.Comment: These are lecture notes meant for undergraduate and graduate physics students. This manuscript will not be submitted for publication. Pages: 21. Figures: 1

Dead on arrival: the unused cartographic legacy of Carl Friedrich Reimer

After the Fourth Anglo-Dutch War (1780–1784), which ended disastrously for the Dutch East India Company (VOC), the need to reform was strongly felt. The Board of Directors (Heren XVII) asked for state support. This resulted in the formation of an independent Military Commission, with the mandate ofreporting on the (military) state of affairs in the East, surveying the settlements and making plans for their improvement. The Prussian-born Carl Friedrich Reimer was employed as the main surveyor and military engineer. He had already been in the VOC’s service for two decades before he was given this important task and became a confidant of Governor-General Arnold Willem Alting. The Governor-General wasvery skeptical towards the activities of the Military Commission, which operated fully outside the Company’s established chain of command. By maneuvering Reimer into the Commission, Alting had eyes and ears in its affairs. Next to observing, surveying, drawing plans and writing recommendations, Reimer wouldalso inform Alting about the journeys. Every major Dutch settlement from South Africa to the Moluccas was visited by the Military Commission, forming a unique view on the (military) state of affairs of the Dutch presence in Asia in around 1790. Together with the various recommendations that were accompanied by the excellent military maps by Reimer, the Dutch could make a fresh start in their imperial ambitions. However, when the Commission Fleet returned to the Republic in 1793 and all the reports and maps were transferred, the political constellation no longer had an eye for the overseas troubles of the VOC. The young and revolutionary French Republic just declared war. As such, the cartographic legacy of CF Reimerwas ‘dead on arrival’.UBL - Publicaties niet-programma gebonde

Strain-induced shape transition of VSi₂ clusters on Si(111)

We investigated the growth and shape of vanadium silicide clusters on Si(111) using scanning tunneling microscopy, atomic force microscopy and scanning electron microscopy. The deposition of 8 monolayers of vanadium on Si(111) and subsequent annealing at 1200 K leads to the formation of large elongated VSi2 clusters. This is in marked contrast to the deposition of submonolayer amounts of V where the VSi2 islands have a compact shape. The shape transition can be explained by a competition between edge formation and strain relaxation energy terms. The VSi2 clusters, which are aligned along the three high symmetry directions of the Si(111) surface and surrounded by a denuded zone, form a two-dimensional network.</p

Parallel electron-hole bilayer conductivity from electronic interface reconstruction

The perovskite SrTiO$_3$-LaAlO$_3$ structure has advanced to a model system to investigate the rich electronic phenomena arising at polar interfaces. Using first principles calculations and transport measurements we demonstrate that an additional SrTiO$_3$ capping layer prevents structural and chemical reconstruction at the LaAlO$_3$ surface and triggers the electronic reconstruction at a significantly lower LaAlO$_3$ film thickness than for the uncapped systems. Combined theoretical and experimental evidence (from magnetotransport and ultraviolet photoelectron spectroscopy) suggests two spatially separated sheets with electron and hole carriers, that are as close as 1 nm.Comment: Phys. Rev. Lett., in pres

A disastrous project: C. P. Keller and the fortification (plans) of Bimilipatnam

Leiden University Libraries has two remarkable fortification plans in its collections. They depict the village of Bimilipatnam under VOC rule. After a ransack by plundering Maratha troops in 1754, the local Mughal-regent allowed the Dutch to construct a stronghold in the village. VOC-authorities commissioned military engineer Coenraad Pieter Keller to survey the plot of land allocated to them and deliver plans and a proposal how this fortification could be constructed. The eventual fort was poorly constructed and collapsed two times. Keller was scapegoated by his superiors. He did not let the allegations pass and defended himself at Batavia. His written apologia has been preserved and provides unique insights in the career of a hybrid mapmaker in colonial service. By careful archival research we can reconstruct the context in which the plans were produced, interpreted by their intended audience and circulated until it was acquired by Leiden UniversityLibraries.UBL - Publicaties niet-programma gebonde

Split-off dimer defects on the Si(001)2x1 surface

Dimer vacancy (DV) defect complexes in the Si(001)2x1 surface were investigated using high-resolution scanning tunneling microscopy and first principles calculations. We find that under low bias filled-state tunneling conditions, isolated 'split-off' dimers in these defect complexes are imaged as pairs of protrusions while the surrounding Si surface dimers appear as the usual 'bean-shaped' protrusions. We attribute this to the formation of pi-bonds between the two atoms of the split-off dimer and second layer atoms, and present charge density plots to support this assignment. We observe a local brightness enhancement due to strain for different DV complexes and provide the first experimental confirmation of an earlier prediction that the 1+2-DV induces less surface strain than other DV complexes. Finally, we present a previously unreported triangular shaped split-off dimer defect complex that exists at SB-type step edges, and propose a structure for this defect involving a bound Si monomer.Comment: 8 pages, 7 figures, submitted to Phys. Rev.