95 research outputs found
Four-point probe measurements using current probes with voltage feedback to measure electric potentials
We present a four-point probe resistance measurement technique which uses
four equivalent current measuring units, resulting in minimal hardware
requirements and corresponding sources of noise. Local sample potentials are
measured by a software feedback loop which adjusts the corresponding tip
voltage such that no current flows to the sample. The resulting tip voltage is
then equivalent to the sample potential at the tip position. We implement this
measurement method into a multi-tip scanning tunneling microscope setup such
that potentials can also be measured in tunneling contact, allowing in
principle truly non-invasive four-probe measurements. The resulting measurement
capabilities are demonstrated for BiSbTe and Si samples
Lifting the spin-momentum locking in ultra-thin topological insulator films
Three-dimensional (3D) topological insulators (TIs) are known to carry 2D
Dirac-like topological surface states in which spin-momentum locking prohibits
backscattering. When thinned down to a few nanometers, the hybridization
between the topological surface states at the top and bottom surfaces results
in a topological quantum phase transition, which can lead to the emergence of a
quantum spin Hall phase. Here, we study the thickness-dependent transport
properties across the quantum phase transition on the example of
(BiSb)Te films, with a four-tip scanning tunnelling
microscope. Our findings reveal an exponential drop of the conductivity below
the critical thickness. The steepness of this drop indicates the presence of
spin-conserving backscattering between the top and bottom surface states,
effectively lifting the spin-momentum locking and resulting in the opening of a
gap at the Dirac point. Our experiments provide crucial steps towards the
detection of quantum spin Hall states in transport measurements
Probing edge state conductance in ultra-thin topological insulator films
Quantum spin Hall (QSH) insulators have unique electronic properties,
comprising a band gap in their two-dimensional interior and one-dimensional
spin-polarized edge states in which current flows ballistically. In scanning
tunneling microscopy (STM), the edge states manifest themselves as a localized
density of states. However, there is a significant research gap between the
observation of edge states in nanoscale spectroscopy, and the detection of
ballistic transport in edge channels which typically relies on transport
experiments with microscale lithographic contacts. Here, we study few-layer
films of the three-dimensional topological insulator
(BiSbTe, for which a topological transition to a
two-dimensional topological QSH insulator phase has been proposed. Indeed, an
edge state in the local density of states is observed within the band gap. Yet,
in nanoscale transport experiments with a four-tip STM, 2 and 3 quintuple layer
films do not exhibit a ballistic conductance in the edge channels. This
demonstrates that the detection of edge states in spectroscopy can be
misleading with regard to the identification of a QSH phase. In contrast,
nanoscale multi-tip transport experiments are a robust method for effectively
pinpointing ballistic edge channels, as opposed to trivial edge states, in
quantum materials
Suspended dry pick-up and flip-over assembly for van der Waals heterostructures with ultra-clean surfaces
Van der Waals heterostructures are an excellent platform for studying
intriguing interface phenomena, such as moir\'e and proximity effects. Surface
science techniques like scanning tunneling microscopy (STM) have proven a
powerful tool to study such heterostructures but have so far been hampered
because of their high sensitivity to surface contamination. Here, we report a
dry polymer-based assembly technique to fabricate van der Waals
heterostructures with atomically clean surfaces. The key features of our
suspended dry pick-up and flip-over technique are 1) the heterostructure
surface never comes into contact with polymers, 2) it is entirely solvent-free,
3) it is entirely performed in a glovebox, and 4) it only requires temperatures
below 130. By performing ambient atomic force microscopy and
atomically-resolved scanning tunneling microscopy on example heterostructures,
we demonstrate that we can fabricate air-sensitive heterostructures with
ultra-clean interfaces and surfaces. Due to the lack of polymer melting, the
technique is further compatible with heterostructure assembly under ultra-high
vacuum conditions, which promises ultimate heterostructure quality
Literacy and multilingualism in Africa
Literacy and multilingualism in Africa is approached here as a field of practice rather than a unified field of research. This field presents a crucial paradox: African contexts present some of the world’s most diverse and vital multilingual situations but also feature in the world’s poorest literacy rates and are routinely said to lack a literate tradition altogether. By reviewing Africa’s script inventions this chapter offers counter-evidence for this deceptive view. Throughout Africa – from the Maghreb over West and Central Africa to the Horn of Africa – there have been significant indigenous script traditions and inventions, including Tifinagh, N’ko, Vai, Bamum and Ge’ez. In fact, some of the world’s oldest known scripts (e.g. Egyptian hieroglyphs) are African scripts. The chapter further outlines two relatively young fields of practice and research that have begun to make major contributions to literacy and multilingualism in Africa: digital literacy and linguistic landscape. These fields share a common interest in the materiality of real language as opposed to idealized images of language and in local agency and creativity in the site of struggle that is language. Like digital language practices, linguistic landscapes constitute a domain for African written multilingualism that is not generally supported or monitored by African states. Nor does either field present simple continuities from colonially inherited language policies and ideologies, in the way that classrooms do. As spaces for writing par excellence linguistic landscapes and mobile phones promise to contribute in no minor way to the development of African language literacies and multilingualism in Africa
A comparative analysis of foliar chemical composition and leaf construction costs of beech (Fagus sylvatica L.), sycamore maple (Acer pseudoplatanus L.) and ash (Fraxinus excelsior L.) saplings along a light gradient
• Construction cost (g glucose g−1), chemical composition and morphology of
leaves of beech (Fagus sylvatica L.) and two co-occurring valuable
broadleaved species (sycamore maple – Acer pseudoplatanus L. – and ash –
Fraxinus excelsior L.) were investigated along a horizontal light
gradient (3–60% of above canopy radiation) and from top to bottom within the crowns in a
fairly even-aged mixed-species thicket established by natural regeneration beneath a
patchy shelterwood canopy.
• Construction cost and carbon concentration increased with irradiance in ash and
sycamore maple and were independent of irradiance in beech. Leaf traits expressed on an
area basis, like construction cost, nitrogen content and leaf mass (LMA) increased
significantly with irradiance in all three species and decreased from top to bottom within
crowns.
• The shade tolerant beech invested more glucose to produce a unit foliar biomass, but
less to build a unit foliar area due to lower LMA. Thereby beech was able to display a
greater total leaf area, what at least in parts counterbalanced the lower values of
Na as compared to ash and sycamore maple
- …