arXiv subjects

D. Vanmaekelbergh

Publications and source records attributed to D. Vanmaekelbergh.

13 recordsLinked to original sources

arXiv2026

Decorated electronic kagome lattice in twisted bilayer germanene

Artificial kagome lattices provide a route to electronic flat bands, geometric frustration, and correlation driven phases, but their realization in atomically controlled two-dimensional materials remains scarce. Here, we show that commensurate twisted bilayer germanene on Ge2Pt produces two electronically distinct large-angle moir\'e phases. Scanning tunneling microscopy measurements and density functional theory calculations reveal that commensurate twisted bilayers that are odd under an exchange of sublattices are semiconducting, whereas the twisted bilayers with an even parity are metallic. The twisted bilayers with an even parity host an empty state resonance that exhibits an emergent decorated kagome structure with a C3 symmetry. These results establish large-angle twisted germanene as a platform for engineering kagome-like electronic states in a buckled two-dimensional material.

cond-mat.mes-hall↗

arXiv2021

Spin-orbit gaps in the s and p orbital bands of an artificial honeycomb lattice

Muffin-tin methods have been instrumental in the design of honeycomb lattices that show, in contrast to graphene, separated s and in-plane p bands, a p orbital Dirac cone, and a p orbital flat band. Recently, such lattices have been experimentally realized using the 2D electron gas on Cu(111). A possible next avenue is the introduction of spin-orbit coupling to these systems. Intrinsic spin-orbit coupling is believed to open topological gaps, and create a topological flat band. Although Rashba coupling is straightforwardly incorporated in the muffin-tin approximation, intrinsic spin-orbit coupling has only been included either for a very specific periodic system, or only close to the Dirac point. Here, we introduce general intrinsic and Rashba spin-orbit terms in the Hamiltonian for both periodic and finite-size systems. We observe a strong band opening over the entire Brillouin zone between the p orbital flat band and Dirac cone hosting a pronounced edge state, robust against the effects of Rashba spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2021

Design and characterization of electronic fractals

The dimensionality of an electronic quantum system is decisive for its properties. In 1D electrons form a Luttinger liquid and in 2D they exhibit the quantum Hall effect. However, very little is known about the behavior of electrons in non-integer, i.e. fractional dimensions. Here, we show how arrays of artificial atoms can be defined by controlled positioning of CO molecules on a Cu(111) surface, and how these sites couple to form electronic Sierpinski fractals. We characterize the electron wavefunctions at different energies with scanning tunneling microscopy and spectroscopy and show that they inherit the fractional dimension. Wavefunctions delocalized over the Sierpinski structure decompose into self-similar parts at higher energy, and this scale invariance can also be retrieved in reciprocal space. Our results show that electronic quantum fractals can be man-made by atomic manipulation in a scanning tunneling microscope. The same methodology will allow to address fundamental questions on the effects of spin-orbit interaction and a magnetic field on electrons in non-integer dimensions. Moreover, the rational concept of artificial atoms can readily be transferred to planar semiconductor electronics, allowing for the exploration of electrons in a well-defined fractal geometry, including interactions and external fields.

cond-mat.mes-hall↗

arXiv2019

p-band engineering in artificial electronic lattices

Artificial electronic lattices, created atom by atom in a scanning tunneling microscope, have emerged as a highly tunable platform to realize and characterize the lowest-energy bands of novel lattice geometries. Here, we show that artificial electronic lattices can be tailored to exhibit higher-energy bands. We study p-like bands in four-fold and three-fold rotationally symmetric lattices. In addition, we show how an anisotropic design can be used to lift the degeneracy between p_x- and p_y-like bands. The experimental measurements are corroborated by muffin-tin and tight-binding calculations. The approach to engineer higher-energy electronic bands in artificial quantum systems introduced here enables the realization of complex band structures from the bottom up.

cond-mat.mes-hall↗

arXiv2019

Robust zero-energy modes in an electronic higher-order topological insulator: the dimerized Kagome lattice

Quantum simulators are an essential tool for understanding complex quantum materials. Platforms based on ultracold atoms in optical lattices and photonic devices led the field so far, but electronic quantum simulators are proving to be equally relevant. Simulating topological states of matter is one of the holy grails in the field. Here, we experimentally realize a higher-order electronic topological insulator (HOTI). Specifically, we create a dimerized Kagome lattice by manipulating carbon-monoxide (CO) molecules on a Cu(111) surface using a scanning tunneling microscope (STM). We engineer alternating weak and strong bonds to show that a topological state emerges at the corner of the non-trivial configuration, while it is absent in the trivial one. Contrarily to conventional topological insulators (TIs), the topological state has two dimensions less than the bulk, denoting a HOTI. The corner mode is protected by a generalized chiral symmetry, which leads to a particular robustness against perturbations. Our versatile approach to quantum simulation with artificial lattices holds promises of revealing unexpected quantum phases of matter.

cond-mat.mes-hall↗

arXiv2015

Topological states in multi-orbital HgTe honeycomb lattices

Research on graphene has revealed remarkable phenomena arising in the honeycomb lattice. However, the quantum spin Hall effect predicted at the K point could not be observed in graphene and other honeycomb structures of light elements due to an insufficiently strong spin-orbit coupling. Here we show theoretically that 2D honeycomb lattices of HgTe can combine the effects of the honeycomb geometry and strong spin-orbit coupling. The conduction bands, experimentally accessible via doping, can be described by a tight-binding lattice model as in graphene, but including multi-orbital degrees of freedom and spin-orbit coupling. This results in very large topological gaps (up to 35 meV) and a flattened band detached from the others. Owing to this flat band and the sizable Coulomb interaction, honeycomb structures of HgTe constitute a promising platform for the observation of a fractional Chern insulator or a fractional quantum spin Hall phase.

cond-mat.mes-hall↗

arXiv2015

Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry

We study theoretically two-dimensional single-crystalline sheets of semiconductors that form a honeycomb lattice with a period below 10 nm. These systems could combine the usual semiconductor properties with Dirac bands. Using atomistic tight-binding calculations, we show that both the atomic lattice and the overall geometry influence the band structure, revealing materials with unusual electronic properties. In rocksalt Pb chalcogenides, the expected Dirac-type features are clouded by a complex band structure. However, in the case of zinc-blende Cd-chalcogenide semiconductors, the honeycomb nanogeometry leads to rich band structures, including, in the conduction band, Dirac cones at two distinct energies and nontrivial flat bands and, in the valence band, topological edge states. These edge states are present in several electronic gaps opened in the valence band by the spin-orbit coupling and the quantum confinement in the honeycomb geometry. The lowest Dirac conduction band has S-orbital character and is equivalent to the pi-pi* band of graphene but with renormalized couplings. The conduction bands higher in energy have no counterpart in graphene; they combine a Dirac cone and flat bands because of their P-orbital character. We show that the width of the Dirac bands varies between tens and hundreds of meV. These systems emerge as remarkable platforms for studying complex electronic phases starting from conventional semiconductors. Recent advancements in colloidal chemistry indicate that these materials can be synthesized from semiconductor nanocrystals.

cond-mat.mes-hall↗

arXiv2009

Electronic coupling of colloidal CdSe nanocrystals monitored by thin-film positron-electron momentum density methods

The effect of temperature controlled annealing on the confined valence electron states in CdSe nanocrystal arrays, deposited as thin films, was studied using two-dimensional angular correlation of annihilation radiation (2D-ACAR). A reduction in the intensity by ~35% was observed in a feature of the positron annihilation spectrum upon removal of the pyridine capping molecules above 200 degrees Celsius in a vacuum. This reduction is explained by an increased electronic interaction of the valence orbitals of neighboring nanocrystals, induced by the formation of inorganic interfaces. Partial evaporation of the nanoporous CdSe layer and additional sintering into a polycrystalline thin film was observed at a relatively low temperature of ~486 degrees Celsius.

cond-mat.mtrl-sci↗

arXiv2007

Positron studies of surfaces, structure and electronic properties of nanocrystals

A brief review is given of recent positron studies of metal and semiconductor nanocrystals. The prospects offered by positron annihilation as a sensitive method to access nanocrystal (NC) properties are described and compared with other experimental methods. The tunability of the electronic structure of nanocrystals underlies their great potential for application in many areas. Owing to their large surface-to-volume ratio, the surfaces and interfaces of NCs play a crucial role in determining their properties. Here we focus on positron 2D angular correlation of annihilation radiation (2D-ACAR) and (two-detector) Doppler studies for investigating surfaces and electronic properties of CdSe NCs.

cond-mat.mtrl-sci↗

arXiv2007

Statistical analysis of time-resolved emission from ensembles of semiconductor quantum dots: interpretation of exponential decay models

We present a statistical analysis of time-resolved spontaneous emission decay curves from ensembles of emitters, such as semiconductor quantum dots, with the aim to interpret ubiquitous non-single-exponential decay. Contrary to what is widely assumed, the density of excited emitters and the intensity in an emission decay curve are not proportional, but the density is a time-integral of the intensity. The integral relation is crucial to correctly interpret non-single-exponential decay. We derive the proper normalization for both a discrete, and a continuous distribution of rates, where every decay component is multiplied with its radiative decay rate. A central result of our paper is the derivation of the emission decay curve in case that both radiative and non-radiative decays are independently distributed. In this case, the well-known emission quantum efficiency can not be expressed by a single number anymore, but it is also distributed. We derive a practical description of non-single-exponential emission decay curves in terms of a single distribution of decay rates; the resulting distribution is identified as the distribution of total decay rates weighted with the radiative rates. We apply our analysis to recent examples of colloidal quantum dot emission in suspensions and in photonic crystals, and we find that this important class of emitters is well described by a log-normal distribution of decay rates with a narrow and a broad distribution, respectively. Finally, we briefly discuss the Kohlrausch stretched-exponential model, and find that its normalization is ill-defined for emitters with a realistic quantum efficiency of less than 100 %.

physics.optics↗

arXiv2005

Frequency-dependent spontaneous emission rate from CdSe and CdTe nanocrystals: influence of dark states

We studied the rate of spontaneous emission from colloidal CdSe and CdTe nanocrystals at room temperature. The decay rate, obtained from luminescence decay curves, increases with the emission frequency in a supra-linear way. This dependence is explained by the thermal occupation of dark exciton states at room temperature, giving rise to a strong attenuation of the rate of emission. The supra-linear dependence is in agreement with the results of tight-binding calculations.

cond-mat.mtrl-sci↗

arXiv2004

Planar nanocontacts with atomically controlled separation

We have developed a technology to reproducibly make gaps with distance control on the single atom scale. The gold contacts are flat on the nanometre scale and are fabricated on an oxidized aluminium film that serves as a gate. We show that these contacts are clean and can be stabilized via chemical functionalization. Deposition of conjugated molecules leads to an increase in the gap conductance of several orders of magnitude. Stable current-voltage characteristics at room temperature are slightly nonlinear. At low temperature, they are highly nonlinear and show a clear gate effect.

cond-mat.mtrl-sci↗

arXiv2004

Wide energy-window view on the density of states and hole mobility of poly(p-phenylene vinylene)

Using an electrochemically gated transistor, we achieved controlled and reversible doping of poly(p-phenylene vinylene) in a large concentration range. Our data open a wide energy-window view on the density of states (DOS) and show, for the first time, that the core of the DOS function is Gaussian, while the low-energy tail has a more complex structure. The hole mobility increases by more than four orders of magnitude when the electrochemical potential is scanned through the DOS.

cond-mat.mtrl-sci↗