arXiv subjects

Martina Luysberg

Publications and source records attributed to Martina Luysberg.

8 recordsLinked to original sources

arXiv2026

Kinetics-Driven Selective Stoichiometric Shift and Structural Asymmetry in $Bi_4Te_3$ Nanostructures for Hybrid Quantum Architectures

Advances in hybrid quantum architectures hinge on topological materials that can be synthesized with precise stoichiometric and structural control at the nanoscale. While $Bi_4Te_3$ is a promising candidate due to its dual topological phases, acting as both a strong topological insulator and a topological crystalline insulator, high-quality growth remains challenging due to a narrow stoichiometric window and high sensitivity to surface kinetics. Here, we establish a reproducible molecular beam epitaxy (MBE) process to produce stoichiometric, twin-free $Bi_4Te_3$ thin films with ultra-smooth surfaces and atomically sharp van der Waals stacks. By employing selective area epitaxy (SAE), we realize laterally confined $Bi_4Te_3$ nanostructures that exhibit a feature-dependent stoichiometric deviation. This phenomenon, which we term the selective stoichiometric shift, arises from the unequal lateral diffusion of Bi and Te adatoms, revealing a direct coupling between adatom kinetics and nanoscale compositional stability. Atomic-resolution imaging further uncovers asymmetric van der Waals gaps within the stacking sequence, identifying an intrinsic structural asymmetry between the quintuple and bilayer units. These findings provide fundamental insights into the crystallization of Bi_4Te_3$ and demonstrate a scalable route for integrating functional topological materials into next-generation superconducting hybrid quantum circuits.

cond-mat.mes-hall↗

arXiv2022

Proximity-induced superconductivity in (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ topological-insulator nanowires

When a topological insulator is made into a nanowire, the interplay between topology and size quantization gives rise to peculiar one-dimensional states whose energy dispersion can be manipulated by external fields. In the presence of proximity-induced superconductivity, these 1D states offer a tunable platform for Majorana zero modes. While the existence of such peculiar 1D states has been experimentally confirmed, the realization of robust proximity-induced superconductivity in topological-insulator nanowires remains a challenge. Here, we report the realization of superconducting topological-insulator nanowires based on (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ (BST) thin films. When two rectangular pads of palladium are deposited on a BST thin film with a separation of 100--200 nm, the BST beneath the pads is converted into a superconductor, leaving a nanowire of BST in-between. We found that the interface is epitaxial and has a high electronic transparency, leading to a robust superconductivity induced in the BST nanowire. Due to its suitable geometry for gate-tuning, this platform is promising for future studies of Majorana zero modes.

cond-mat.mes-hall↗

arXiv2020

Flux periodic oscillations and phase-coherent transport in GeTe nanowire-based devices

Despite the fact that GeTe is known to be a very interesting material for applications in thermoelectrics and for phase-change memories, the knowledge on its low-temperature transport properties is only limited. Here, we report on phase-coherent phenomena in the magnetotransport of GeTe nanowires. From universal conductance fluctuations, a phase-coherence length of about 200nm at 0.5K is determined for the hole carriers. The distinct phase-coherence is confirmed by the observation of Aharonov--Bohm type oscillations for magnetic fields applied along the nanowire axis. We interpret the occurrence of these magnetic flux-periodic oscillations by the formation of a tubular hole accumulation layer on the nanowire surface. In addition, for Nb/GeTe-nanowire/Nb Josephson junctions, we obtained a proximity-induced critical current of about 0.2$μ$A at 0.4K. By applying a magnetic field perpendicular to the nanowire axis, the critical current decreases monotonously with increasing magnetic field, which indicates that the structure is in the small-junction-limit. Whereas, by applying a parallel magnetic field the critical current oscillates with a period of the magnetic flux quantum indicating once again the presence of a tubular hole channel.

cond-mat.mes-hall↗

arXiv2020

Novel self-epitaxy for inducing superconductivity in the topological insulator (Bi1-xSbx)2Te3

Using the superconducting proximity effect for engineering a topological superconducting state in a topological insulator (TI) is a promising route to realize Majorana fermions. However, epitaxial growth of a superconductor on the TI surface to achieve a good proximity effect has been a challenge. We discovered that simply depositing Pd on thin films of the TI material (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ leads to an epitaxial self-formation of PdTe$_2$ superconductor having the superconducting transition temperature of ~1 K. This self-formed superconductor proximitizes the TI, which is confirmed by the appearance of a supercurrent in Josephson-junction devices made on (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$. This self-epitaxy phenomenon can be conveniently used for fabricating TI-based superconducting nanodevices to address the superconducting proximity effect in TIs.

cond-mat.mes-hall↗

arXiv2018

Boosting Transparency in Topological Josephson Junctions via Stencil Lithography

Hybrid devices comprised of topological insulator (TI) nanostructures in proximity to s-wave superconductors (SC) are expected to pave the way towards topological quantum computation. Fabrication under ultra-high vacuum conditions is necessary to attain high quality of TI-SC hybrid devices, because the physical surfaces of V-VI three-dimensional TIs suffer from degradation at ambient conditions. Here, we present an in-situ process, which allows to fabricate such hybrids by combining molecular beam epitaxy and stencil lithography. As-prepared Josephson junctions show nearly perfect interface transparency and very large $I_CR_N$ products. The Shapiro response of radio frequency measurements indicates the presence of gapless Andreev bound states, so-called Majorana bound states.

cond-mat.supr-con↗

arXiv2017

Nanoscale X-ray investigation of magnetic metallofullerene peapods

Endohedral lanthanide ions packed inside carbon nanotubes (CNTs) in a one-dimensional assembly have been studied with a combination of high resolution transmission electron microscopy (HRTEM), scanning transmission X-ray microscopy (STXM), and X-ray magnetic circular dichroism (XMCD). By correlating HRTEM and STXM images we show that structures down to 30 nm are resolved with chemical contrast and record X-ray absorption spectra from endohedral lanthanide ions embedded in individual nanoscale CNT bundles. XMCD measurements of an Er$_3$N@C$_{80}$ bulk sample and a macroscopic assembly of filled CNTs indicates that the magnetic properties of the endohedral Er3+ ions are unchanged when encapsulated in CNTs. This study demonstrates the feasibility of local magnetic X-ray characterization of low concentrations of lanthanide ions embedded in molecular nanostructures.

cond-mat.mtrl-sci↗

arXiv2016

$\mbox{Bi}_{1}\mbox{Te}_{1}$: a dual topological insulator

A combined theoretical and experimental study reveals evidence for the dual topological insulating character of the stoichiometric natural superlattice phase $\mathrm{Bi_{1}Te_{1}}=\mathrm{[Bi_{2}]_{1}[Bi_{2}Te_{3}]_{2}}$, being a stack of alternating Bi bilayers and two quintuple layers of $\mathrm{Bi_{2}Te_{3}}$. We identify $\mathrm{Bi_{1}Te_{1}}$ by density functional theory to exhibit a non trivial time-reversal symmetry-driven character of $\mathbb{Z}_{2}=(0;001)$ and additionally a mirror-symmetry induced mirror Chern number of $n_{\cal M}=-2$, which indicates that $\mathrm{Bi_{1}Te_{1}}$ is both a weak topological insulator and a topological crystalline insulator. The coexistence of the two phenomena preordains distinct crystal planes to host topological surface states that are protected by the respective symmetries. The surface perpendicular to the stacking direction is the 'dark' surface of the weak topological insulator, while hosting mirror-symmetry protected surface states along the $\bar{Γ\mathrm{M}}$ direction at non-time-reversal invariant momenta points. We confirm the stacking sequence of our MBE-grown $\mathrm{Bi_{1}Te_{1}}$ thin films by X-ray diffraction and transmission electron microscopy, and find indications of the topological crystalline and weak topological character in the surface electronic spin structure by spin- and angle-resolved photoemission spectroscopy, which nicely match the results from density functional theory.

cond-mat.mtrl-sci↗

arXiv2015

Realization of a vertical topological p-n junction in epitaxial $\mathrm{Sb_2Te_3 / Bi_2Te_3}$ heterostructures

3D topological insulators are a new state of quantum matter which exhibits both a bulk band structure with an insulating energy gap as well as metallic spin-polarized Dirac fermion states when interfaced with a topologically trivial material. There have been various attempts to tune the Dirac point to a desired energetic position for exploring its unusual quantum properties. Here we show a direct experimental proof by angle-resolved photoemission of the realization of a vertical topological p-n junction made of a heterostructure of two different binary 3D TI materials $\mathrm{Bi_2Te_3}$ and $\mathrm{Sb_2Te_3}$ epitaxially grown on Si(111). We demonstrate that the chemical potential is tunable by about 200 meV when decreasing the upper $\mathrm{Sb_2Te_3}$ layer thickness from 25 to 6 quintuple layers without applying any external bias. These results make it realistic to observe the topological exciton condensate and pave the way of exploring other exotic quantum phenomena in the near future.

cond-mat.mes-hall↗