arXiv subjects

H. J. W. Zandvliet

Publications and source records attributed to H. J. W. Zandvliet.

10 recordsLinked to original sources

arXiv2023

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.

cond-mat.mes-hall↗

arXiv2021

Spectroscopic signature of surface states and bunching of bulk subbands in topological insulator (Bi$_{0.4}$Sb$_{0.6}$)$_2$Te$_3$ thin films

High quality thin films of the topological insulator (Bi$_{0.4}$Sb$_{0.6}$)$_2$Te$_3$ have been deposited on SrTiO$_3$ (111) by molecular beam epitaxy. Their electronic structure was investigated by in situ angle-resolved photoemission spectroscopy and in situ scanning tunneling spectroscopy. The experimental results reveal striking similarities with relativistic ab-initio tight binding calculations. We find that ultrathin slabs of the three-dimensional topological insulator (Bi$_{0.4}$Sb$_{0.6}$)$_2$Te$_3$ display topological surface states, surface states with large weight on the outermost Te atomic layer, and dispersive bulk energy levels that are quantized. We observe that the bandwidth of the bulk levels is strongly reduced. These bunched bulk states as well as the surface states give rise to strong peaks in the local density of states.

cond-mat.mtrl-sci↗

arXiv2021

Valley protected one-dimensional states in small-angle twisted bilayer graphene

Theory predicts that the application of an electric field breaks the inversion symmetry of AB and BA stacked domains in twisted bilayer graphene, resulting in the formation of a triangular network of one-dimensional valley-protected helical states. This two-dimensional network of one-dimensional states has been observed in several studies, but direct experimental evidence that the electronic transport in these one-dimensional states is valley-protected is still lacking. In this study, we report the existence of the network in small-angle twisted bilayer graphene at room temperature. Moreover, by analyzing Fourier transforms of atomically resolved scanning tunnelling microscopy images of minimally twisted bilayer graphene, we provide convincing experimental evidence that the electronic transport in the counter-propagating one-dimensional states is indeed valley protected.

cond-mat.mes-hall↗

arXiv2021

Dual modulation STM: Simultaneous high-resolution mapping of the differential conductivity and local tunnel barrier height demonstrated on Au(111)

We present a scanning tunneling microscopy (STM) technique to simultaneously measure the topography, the local tunnel barrier height (dI/dz) and the differential conductivity (dI/dV). We modulate the voltage and tip piezo with small sinusoidal signals that exceed the cut-off frequency of the STM electronics and feed the tunneling current into two lock-in amplifiers (LIAs). We derive and follow a set of criteria for the modulation frequencies to avoid any interference between the LIA measurements. To validate the technique, we measure Friedel oscillations and the subtle tunnel barrier difference between the hcp and fcc stacked regions of the Au(111) herringbone reconstruction. Finally, we show that our method is also applicable to open feedback loop measurements by performing grid I(V) spectroscopy.

cond-mat.mtrl-sci↗

arXiv2017

Two-dimensional Dirac signature of germanene

The structural and electronic properties of germanene coated Ge$_2$Pt clusters have been determined by scanning tunneling microscopy and spectroscopy at room temperature. The interior of the germanene sheet exhibits a buckled honeycomb structure with a lattice constant of 4.3 Åand a buckling of 0.2 Å. The zigzag edges of germanene are reconstructed and display a 4$\times$ periodicity. The differential conductivity of the interior of the germanene sheet has a V-shape, which is reminiscent of the density of states of a two-dimensional Dirac system. The minimum of the differential conductivity is located close to the Fermi level and has a non-zero value, which we ascribe to the metallic character of the underlying Ge$_2$Pt substrate. Near the reconstructed germanene zigzag edges the shape of the differential conductivity changes from a V-shape to a more parabolic-like shape, revealing that the reconstructed germanene zigzag edges do not exhibit a pronounced metallic edge state.

cond-mat.mtrl-sci↗

arXiv2017

Germanene termination of Ge${_2}$Pt crystals on Ge(110)

We have investigated the growth of Pt on Ge(110) using scanning tunneling microscopy and spectroscopy. The deposition of several monolayers of Pt on Ge(110) followed by annealing at 1100 K results in the formation of three-dimensional metallic Pt-Ge nanocrystals. The outermost layer of these crystals exhibits a honeycomb structure. The honeycomb structure is composed of two hexagonal sub-lattices that are displaced vertically by 0.2 Å with respect to each other. The nearest-neighbor distance of the atoms in the honeycomb lattice is 2.5${\pm}$0.1 Å, i.e. very close to the predicted nearest-neighbor distance in germanene (2.4 Å). Scanning tunneling spectroscopy reveals that the atomic layer underneath the honeycomb layer is more metallic than the honeycomb layer itself. These observations are in line with a model recently proposed for metal di-(silicides/)germanides: a hexagonal crystal with metal layers separated by semiconductor layers with a honeycomb lattice. Based on our observations we propose that the outermost layer of the Ge2Pt nanocrystal is a germanene layer.

physics.app-ph↗

arXiv2017

Structural and electronic properties of germanene on MoS$_2$

To date, germanene has only been synthesized on metallic substrates. A metallic substrate is usually detrimental for the two-dimensional Dirac nature of germanene because the important electronic states near the Fermi level of germanene can hybridize with the electronic states of the metallic substrate. Here we report the successful synthesis of germanene on molybdenum disulfide (MoS$_2$), a band gap material. Pre-existing defects in the MoS$_2$ surface act as preferential nucleation sites for the germanene islands. The lattice constant of the germanene layer (3.8 $\pm$ 0.2 Å) is about 20\% larger than the lattice constant of the MoS$_2$ substrate (3.16 Å). Scanning tunneling spectroscopy measurements and density functional theory calculations reveal that there are, besides the linearly dispersing bands at the $K$ points, two parabolic bands that cross the Fermi level at the $Γ$ point.

cond-mat.mtrl-sci↗

arXiv2015

In-situ spectroscopy of intrinsic Bi2Te3 topological insulator thin films and impact of extrinsic defects

Combined in-situ x-ray photoemission spectroscopy, scanning tunnelling spectroscopy and angle resolved photoemission spectroscopy of molecular beam epitaxy grown Bi2Te3 on lattice mismatched substrates reveal high quality stoichiometric thin films with topological surface states without a contribution from the bulk bands at the Fermi energy. The absence of bulk states at the Fermi energy is achieved without counter doping. We observe that the surface morphology and electronic band structure of Bi2Te3 are not affected by in-vacuo storage and exposure to oxygen, whereas major changes are observed when exposed to ambient conditions. These films help define a pathway towards intrinsic topological devices.

cond-mat.mtrl-sci↗

arXiv2010

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.

cond-mat.mtrl-sci↗

arXiv2002

The difference between Si and Ge(001) surfaces in the initial stages of growth

The initial stages of growth of Ge and Si on the Ge(001) surface are studied and compared to growth on the Si(001) surface. Metastable rows of diluted ad-dimers exist on both surfaces as intermediate stages of epitaxial growth. Unexpectedly, for Ge(001) these rows are found exclusively in the <310> directions, whereas on Si(001) the preferred direction is <110>. This qualitative difference between Si and Ge surfaces reflects the subtle difference in the chemistry of these two elements, which has direct consequences for epitaxial growth on these surfaces.

cond-mat.mtrl-sci↗