arXiv subjects

Gregor Mussler

Publications and source records attributed to Gregor Mussler.

At least 19 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↗

arXiv2025

Observation of angular momentum transfer among crystal lattice modes

Transfer of energy and linear momentum between lattice vibrations via anharmonic coupling is an important concept in solid-state physics. However, it remains difficult to directly observe how angular momentum is exchanged and conserved among lattice modes, even though these processes are thought to play an important role in achieving magnetization equilibrium and in spin relaxation effects like the Einstein-de Haas effect. Here, we demonstrate and coherently control angular momentum transfer between two lattice modes using the inverse process of anharmonic decay. The observed rotational phonon-phonon Umklapp scattering enforces the conservation of quantized crystal angular momentum, as dictated by the crystal's discrete rotational symmetry. We thereby experimentally confirm the fundamental analogy between linear and angular momentum conservation in solids. Moreover, we establish axial nonlinear phononics towards ultrafast control of material properties.

cond-mat.other↗

arXiv2024

Molecular order induced charge transfer in a C$_{60}$-topological insulator moiré heterostructure

We synthesize and spectroscopically investigate monolayer C$_{60}$ on the topological insulator (TI) Bi$_4$Te$_3$. This C$_{60}$/Bi$_4$Te$_3$ heterostructure is characterized by excellent translational order in a novel (4 x 4) C$_{60}$ superstructure on a (9 x 9) unit of Bi$_4$Te$_3$. We measure the full two-dimensional energy band structure of C$_{60}$/Bi$_4$Te$_3$ using angle-resolved photoemission spectroscopy (ARPES). We find that C$_{60}$ accepts electrons from the TI at room temperature but no charge transfer occurs at low temperatures. We unravel this peculiar behaviour by Raman spectroscopy of C$_{60}$/Bi$_4$Te$_3$ and density functional theory (DFT) calculations of the electronegativity of C$_{60}$. Both methods are sensitive to orientational order of C$_{60}$. At low temperatures, Raman spectroscopy shows a dramatic intensity increase of the C$_{60}$ Raman signal, evidencing a transition to a rotationally ordered state. DFT reveals that the orientational order of C$_{60}$ at low temperatures has a higher electron affinity than at high temperatures. These results neatly explain the temperature-dependent charge transfer observed in ARPES. Our conclusions are supported by the appearance of a strong photoluminescence from C$_{60}$/Bi$_4$Te$_3$ at low temperatures.

cond-mat.mtrl-sci↗

arXiv2023

Single in situ Interface Characterization Composed of Niobium and a Selectively Grown (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ Topological Insulator Nanoribbon

With increasing attention in Majorana physics for possible quantum bit applications, a large interest has been developed to understand the properties of the interface between a $s$-type superconductor and a topological insulator. Up to this point the interface analysis was mainly focused on in situ prepared Josephson junctions, which consist of two coupled single interfaces or to ex-situ fabricated single interface devices. In our work we utilize a novel fabrication process, combining selective area growth and shadow evaporation which allows the characterization of a single in situ fabricated Nb/$\mathrm{(Bi_{0.15}Sb_{0.85})_2Te_3}$ nano interface. The resulting high interface transparency is apparent by a zero bias conductance increase by a factor of 1.7. Furthermore, we present a comprehensive differential conductance analysis of our single in situ interface for various magnetic fields, temperatures and gate voltages. Additionally, density functional theory calculations of the superconductor/topological insulator interface are performed in order to explain the peak-like shape of our differential conductance spectra and the origin of the observed smearing of conductance features.

cond-mat.mes-hall↗

arXiv2023

Aharonov-Bohm interference and phase-coherent surface-state transport in topological insulator rings

We present low-temperature magnetotransport measurements on selectively-grown Sb$_2$Te$_3$-based topological insulator ring structures. These topological insulator ring geometries display clear Aharonov-Bohm oscillations in the conductance originating from phase-coherent transport around the ring. The temperature dependence of the oscillation amplitude indicates that the Aharonov-Bohm oscillations originate from ballistic transport along the ring arms. The oscillations can therefore be attributed to topological surface states, which can maintain a quasi-ballistic transport regime in the presence of disorder. Further insight on the phase coherence is gained by comparing with similar Aharonov-Bohm-type oscillations in topological insulator nanoribbons exposed to an axial magnetic field. Here, quasi-ballistic phase-coherent transport is confirmed for closed-loop topological surface states in transverse direction enclosing the cross-section of the nanoribbon. In contrast, the appearance of universal conductance fluctuations indicates phase-coherent transport in the diffuse regime, which is attributed to bulk carrier transport. Thus, it appears that even in the presence of diffusive $p$-type charge carriers in Aharonov-Bohm ring structures, phase-coherent quasi-ballistic transport of topologically protected surface states is maintained over long distances.

cond-mat.mes-hall↗

arXiv2023

Ballistic surface channels in fully in situ defined Bi$_4$Te$_3$ Josephson junctions with aluminum contacts

In this letter we report on the electrical transport properties of Bi$_4$Te$_3$ in a Josephson junction geometry using superconducting Al electrodes with a Ti interdiffusion barrier. Bi$_4$Te$_3$ is proposed to be a dual topological insulator, for which due to time-reversal and mirror symmetry both a strong topological insulator phase as well as a crystalline topological phase co-exist. The formation of a supercurrent through the Bi$_4$Te$_3$ layer is explained by a two-step process. First, due to the close proximity of the Al/Ti electrodes a superconducting gap is induced within the Bi$_4$Te$_3$ layer right below the electrodes. The size of this gap is determined by analysing multiple Andreev reflections (MARs) identified within the devices differential resistance at low voltage biases. Second, based on the Andreev reflection and reverse Andreev reflection processes a supercurrent establishes in the weak link region in between these two proximity coupled regions. Analyses of the temperature dependency of both the critical current as well as MARs indicate mostly ballistic supercurrent contributions in between the proximitized Bi$_4$Te$_3$ regions even though the material is characterized by a semi-metallic bulk phase. The presence of these ballistic modes gives indications on the topological nature of Bi$_4$Te$_3$.

cond-mat.mes-hall↗

arXiv2023

Supercurrent in Bi$_4$Te$_3$ Topological Material-Based Three-Terminal Junctions

In an in-situ prepared three-terminal Josephson junction based on the topological insulator Bi$_4$Te$_3$ and the superconductor Nb the transport properties are studied. The differential resistance maps as a function of two bias currents reveal extended areas of Josephson supercurrent including coupling effects between adjacent superconducting electrodes. The observed dynamics for the coupling of the junctions is interpreted using a numerical simulation of a similar geometry based on a resistively and capacitively shunted Josephson junction model. The temperature dependency indicates that the device behaves similar to prior experiments with single Josephson junctions comprising topological insulators weak links. Irradiating radio frequencies to the junction we find a spectrum of integer Shapiro steps and an additional fractional step, which is interpreted by a skewed current-phase relationship. In a perpendicular magnetic field we observe Fraunhofer-like interference patterns of the switching currents.

cond-mat.supr-con↗

arXiv2022

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 (Bi$_{x}$Sb$_{1-x})_2$Te$_3$, 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.

cond-mat.mes-hall↗

arXiv2022

Gate-induced decoupling of surface and bulk state properties in selectively-deposited Bi$_2$Te$_3$ nanoribbons

Three-dimensional topological insulators (TIs) host helical Dirac surface states at the interface with a trivial insulator. In quasi-one-dimensional TI nanoribbon structures the wave function of surface charges extends phase-coherently along the perimeter of the nanoribbon, resulting in a quantization of transverse surface modes. Furthermore, as the inherent spin-momentum locking results in a Berry phase offset of $π$ of self-interfering charge carriers an energy gap within the surface state dispersion appears and all states become spin-degenerate. We investigate and compare the magnetic field dependent surface state dispersion in selectively deposited Bi$_2$Te$_3$ TI micro- and nanoribbon structures by analysing the gate voltage dependent magnetoconductance at cryogenic temperatures. While in wide microribbon devices the field effect mainly changes the amount of bulk charges close to the top surface we identify coherent transverse surface states along the perimeter of the nanoribbon devices responding to a change in top gate potential. We quantify the energetic spacing in between these quantized transverse subbands by using an electrostatic model that treats an initial difference in charge carrier densities on the top and bottom surface as well as remaining bulk charges. In the gate voltage dependent transconductance we find oscillations that change their relative phase by $π$ at half-integer values of the magnetic flux quantum applied coaxial to the nanoribbon, which is a signature for a magnetic flux dependent topological phase transition in narrow, selectively deposited TI nanoribbon devices.

cond-mat.mes-hall↗

arXiv2021

In-plane magnetic field-driven symmetry breaking in topological insulator-based three-terminal junctions

Topological surface states of three-dimensional topological insulator nanoribbons and their distinct magnetoconductance properties are promising for topoelectronic applications and topological quantum computation. A crucial building block for nanoribbon-based circuits are three-terminal junctions. While the transport of topological surface states on a planar boundary is not directly affected by an in-plane magnetic field, the orbital effect cannot be neglected when the surface states are confined to the boundary of a nanoribbon geometry. Here, we report on the magnetotransport properties of such three-terminal junctions. We observe a dependence of the current on the in-plane magnetic field, with a distinct steering pattern of the surface state current towards a preferred output terminal for different magnetic field orientations. We demonstrate that this steering effect originates from the orbital effect, trapping the phase-coherent surface states in the different legs of the junction on opposite sides of the nanoribbon and breaking the left-right symmetry of the transmission across the junction. The reported magnetotransport properties demonstrate that an in-plane magnetic field is not only relevant but also very useful for the characterization and manipulation of transport in three-dimensional topological insulator nanoribbon-based junctions and circuits, acting as a topoelectric current switch.

cond-mat.mes-hall↗

arXiv2021

Integration of topological insulator Josephson junctions in superconducting qubit circuits

The integration of semiconductor Josephson junctions (JJs) in superconducting quantum circuits provides a versatile platform for hybrid qubits and offers a powerful way to probe exotic quasiparticle excitations. Recent proposals for using circuit quantum electrodynamics (cQED) to detect topological superconductivity motivate the integration of novel topological materials in such circuits. Here, we report on the realization of superconducting transmon qubits implemented with $(Bi_{0.06}Sb_{0.94})_{2}Te_{3}$ topological insulator (TI) JJs using ultra-high vacuum fabrication techniques. Microwave losses on our substrates with monolithically integrated hardmask, used for selective area growth of TI nanostructures, imply microsecond limits to relaxation times and thus their compatibility with strong-coupling cQED. We use the cavity-qubit interaction to show that the Josephson energy of TI-based transmons scales with their JJ dimensions and demonstrate qubit control as well as temporal quantum coherence. Our results pave the way for advanced investigations of topological materials in both novel Josephson and topological qubits.

cond-mat.mes-hall↗

arXiv2021

Probing embedded topological modes in bulk-like GeTe-Sb$_2$Te$_3$ heterostructures

The interface between topological and normal insulators hosts metallic states that appear due to the change in band topology. While these topological states at a surface, i.e., a topological insulator-air/vacuum interface, have been studied intensely, topological states at a solid-solid interface have been less explored. Here we combine experiment and theory to study such \textit{embedded} topological states (ETSs) in heterostructures of GeTe (normal insulator) and Sb$_2$Te$_3$ (topological insulator). We analyse their dependence on the interface and their confinement characteristics. To characterise the heterostructures, we evaluate the GeTe-Sb$_2$Te$_3$ band offset using X-ray photoemission spectroscopy, and chart the elemental composition using atom probe tomography. We then use first-principles to independently calculate the band offset and also parametrise the band structure within a four-band continuum model. Our analysis reveals, strikingly, that under realistic conditions, the interfacial topological modes are delocalised over many lattice spacings. Interestingly, the first-principles calculations indicate that the ETSs are relatively robust to disorder and this may have practical ramifications. Our study provides insights into how to manipulate topological modes in heterostructures and also provides a basis for recent experimental findings [Nguyen \textit{et al.}, Sci. Rep. \textbf{6}, 27716 (2016)] where ETSs were seen to couple over large distances.

cond-mat.mes-hall↗

arXiv2021

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 (Bi$_{0.16}$Sb$_{0.84}$)$_2$Te$_3$ 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.

cond-mat.mes-hall↗

arXiv2020

Quantum transport in topological surface states of Bi$_2$Te$_3$ nanoribbons

Quasi-1D nanowires of topological insulators are emerging candidate structures in superconductor hybrid architectures for the realization of Majorana fermion based quantum computation schemes. It is however technically difficult to both fabricate as well as identify the 1D limit of topological insulator nanowires. Here, we investigated selectively-grown Bi$_2$Te$_3$ topological insulator nanoribbons and nano Hall bars at cryogenic temperatures for their topological properties. The Hall bars are defined in deep-etched Si$_3$N$_4$/SiO$_2$ nano-trenches on a silicon (111) substrate followed by a selective area growth process via molecular beam epitaxy. The selective area growth is beneficial to the device quality, as no subsequent fabrication needs to be performed to shape the nanoribbons. Transmission line measurements are performed to evaluate contact resistances of Ti/Au contacts applied as well as the specific resistance of the Bi$_2$Te$_3$ binary topological insulator. In the diffusive transport regime of these unintentionally $n$-doped Bi$_2$Te$_3$ topological insulator nano Hall bars, we identify distinguishable electron trajectories by analyzing angle-dependent universal conductance fluctuation spectra. When the sample is tilted from a perpendicular to a parallel magnetic field orientation, these high frequent universal conductance fluctuations merge with low frequent Aharonov-Bohm type oscillations originating from the topologically protected surface states encircling the nanoribbon cross section. For 500 nm wide Hall bars we also identify low frequent Shubnikov-de Haas oscillations in the perpendicular field orientation, that reveal a topological high-mobility 2D transport channel, partially decoupled from the bulk of the material.

cond-mat.mes-hall↗

arXiv2019

Thermally activated diffusion and lattice relaxation in (Si)GeSn materials

Germanium-Tin (GeSn) alloys have emerged as a promising material for future optoelectronics, energy harvesting and nanoelectronics owing to their direct bandgap and compatibility with existing Si-based electronics. Yet, their metastability poses significant challenges calling for in-depth investigations of their thermal behavior. With this perspective, this work addresses the interdiffusion processes throughout thermal annealing of pseudomorphic GeSn binary and SiGeSn ternary alloys. In both systems, the initially pseudomorphic layers are relaxed upon annealing exclusively via thermally induced diffusional mass transfer of Sn. Systematic post-growth annealing experiments reveal enhanced Sn and Si diffusion regimes that manifest at temperatures below 600°C. The amplified low-temperature diffusion and the observation of only subtle differences between binary and ternary hint at the unique metastability of the Si-Ge-Sn material system as the most important driving force for phase separation.

cond-mat.mtrl-sci↗

arXiv2019

Capacitance-voltage measurements of (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ field effect device

Capacitance-voltage ($\textit{C-V}$) traces in n-type-(Bi$_{1-x}$Sb$_x$)$_2$Te$_3$/oxide/metal capacitor structures using an AC capacitance bridge are investigated. By tuning the top gate voltage from positive to negative values, the system at the interface is tuned from accumulation, via depletion into inversion. Our results show the typical low-frequency and high frequency $\textit{C-V}$ traces, depending on measuring frequency, temperature and illumination intensity and reflecting their sensitive dependence on recombination/generation rates. Superimposed a strong hysteresis under inversion is also observed which is ascribed to the presence of conventional localized surface states which coexist with topological surface states.

cond-mat.mes-hall↗

arXiv2019

Phase-coherent loops in selectively-grown topological insulator nanoribbons

Universal conductance fluctuations and the weak antilocalization effect are defect structure specific fingerprints in the magnetoconductance that are caused by electron interference. Experimental evidence is presented that the conductance fluctuations in the present topological insulator (Bi$_{0.57}$Sb$_{0.43}$)$_2$Te$_3$ nanoribbons which are selectively grown by molecular beam epitaxy are caused by well-defined and sharply resolved phase-coherent loops. From measurements at different magnetic field tilt angles we deduced that these loops are preferentially oriented parallel to the quintuple layers of the topological insulator material. Both from a theoretical analysis of universal conductance fluctuations and from weak antilocalization measured at low temperature the electronic phase-coherence lengths $l_ϕ$ are extracted, which is found to be larger in the former case. Possible reasons for this deviation are discussed.

cond-mat.mes-hall↗

arXiv2018

Thickness dependence of electron-electron interactions in topological p-n junctions

Electron-electron interactions in topological p-n junctions consisting of vertically stacked topological insulators are investigated. n-type Bi2Te3 and p-type Sb2Te3 of varying relative thicknesses are deposited using molecular beam epitaxy and their electronic properties measured using low-temperature transport. The screening factor is observed to decrease with increasing sample thickness, a finding which is corroborated by semi-classical Boltzmann theory. The number of two-dimensional states determined from electron-electron interactions is larger compared to the number obtained from weak-antilocalization, in line with earlier experiments using single layers.

cond-mat.mes-hall↗