arXiv subjects

Peter Schüffelgen

Publications and source records attributed to Peter Schüffelgen.

At least 19 recordsLinked to original sources

arXiv2026

Highly crystalline superconducting TiN resonators grown on thermally reconstructed sapphire

High quality crystalline growth of a thin film on sapphire requires sufficient substrate preparation, often achieved via the use of aggressive chemical cleaning. Direct thermal reconstruction of the sapphire substrate via a CO$_2$ laser beam may allow for an alternative way to prepare the substrate for epitaxy without the use of any chemical processing. Within this work, we demonstrate that thermal annealing of sapphire into its ($\sqrt{31}$$\times$$\sqrt{31}$)$R$$\pm$9° reconstruction is a valid alternative preparation technique for sapphire substrates. TiN films grown via plasma-assisted molecular beam epitaxy upon these substrates exhibit greater crystallinity than those grown on chemically cleaned sapphire substrates. Superconducting resonators fabricated from these films exhibit similar performance, with many possessing internal quality factors at single photon levels greater than 10$^6$ for both substrate preparation methods.

cond-mat.supr-con↗

arXiv2026

Proximitized Topological Insulator Charge Island Fabricated via In Situ Multi-Angle Stencil Lithography

Hybrid superconductor-topological insulator (TI) nanostructures constitute a promising materials platform for exploring proximity-induced superconductivity in systems with topologically protected surface states. A key obstacle has been the realization of clean and well-controlled superconductor-TI interfaces, as TI surfaces rapidly degrade under ambient conditions. Here, we introduce a fully in situ, multi-angle stencil lithography technique that enables the fabrication of proximitized charge islands in TIs. The approach combines selective-area growth of (Bi,Sb)$_2$Te$_3$ nanoribbons with angle-controlled deposition of diffusion barriers, superconducting Al, and ultrathin oxide tunnel barriers, allowing scalable fabrication of hybrid nanostructures without post-growth processing. Low-temperature transport measurements reveal robust Coulomb blockade and a pronounced suppression of low-energy conductance which vanishes with magnetic field, consistent with proximity-induced superconductivity in the island. These results establish a versatile nanofabrication platform that enables access to previously unexplored TI-based hybrid quantum devices and opens new routes for investigating superconductivity in topological nanostructures.

cond-mat.mes-hall↗

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

On-chip stencil lithography for superconducting qubits

Improvements in circuit design and more recently in materials and surface cleaning have contributed to a rapid development of coherent superconducting qubits. However, organic resists commonly used for shadow evaporation of Josephson junctions (JJs) pose limitations due to residual contamination, poor thermal stability and compatibility under typical surface-cleaning conditions. To provide an alternative, we developed an inorganic SiO$_2$/Si$_3$N$_4$ on-chip stencil lithography mask for JJ fabrication. The stencil mask is resilient to aggressive cleaning agents and it withstands high temperatures up to 1200°C, thereby opening new avenues for JJ material exploration and interface optimization. To validate the concept, we performed shadow evaporation of Al-based transmon qubits followed by stencil mask lift-off using vapor hydrofluoric acid, which selectively etches SiO$_2$. We demonstrate average $T_1 \approx 75 \pm 11 μ$s over a 200 MHz frequency range in multiple cool-downs for one device, and $T_1 \approx 44\pm 8 μ$s for a second device. These results confirm the compatibility of stencil lithography with state-of-the-art superconducting quantum devices and motivate further investigations into materials engineering, film deposition and surface cleaning techniques.

quant-ph↗

arXiv2025

Controlling the magneto-transport properties of magnetic topological insulator thin films from Cr$_x$(Bi$_y\,$Sb$_{1-y}$)$_{2-x}$Te$_3$ via molecular beam epitaxy

In this work we present a systematic in-depth study of how we can alter the magneto-transport properties of magnetic topological insulator thin films by tuning the parameters of the molecular beam epitaxy. First, we show how a varying substrate temperature changes the surface morphology and when chosen properly leads to a high crystal quality. Next, the effect of the chromium concentration on the film roughness and crystal quality is investigated. Finally, both the substrate temperature and the chromium concentration are investigated with respect to their effect on the magneto-transport properties of the magnetic topological insulator thin films. It becomes apparent that the substrate temperature and the chromium concentration can be used to tune the Fermi level of the film which allows to make the material intrinsically charge neutral. A very low chromium concentration furthermore allows to tune the magnetic topological insulator into a regime where strong superconducting correlations can be expected when combining the material with a superconductor.

cond-mat.mes-hall↗

arXiv2024

Observation of Josephson Harmonics in Tunnel Junctions

Approaches to developing large-scale superconducting quantum processors must cope with the numerous microscopic degrees of freedom that are ubiquitous in solid-state devices. State-of-the-art superconducting qubits employ aluminum oxide (AlO$_x$) tunnel Josephson junctions as the sources of nonlinearity necessary to perform quantum operations. Analyses of these junctions typically assume an idealized, purely sinusoidal current-phase relation. However, this relation is only expected to hold in the limit of vanishingly low-transparency channels in the AlO$_x$ barrier. Here we show that the standard current-phase relation fails to accurately describe the energy spectra of transmon artificial atoms across various samples and laboratories. Instead, a mesoscopic model of tunneling through an inhomogeneous AlO$_x$ barrier predicts percent-level contributions from higher Josephson harmonics. By including these in the transmon Hamiltonian, we obtain orders of magnitude better agreement between the computed and measured energy spectra. The presence and impact of Josephson harmonics has important implications for developing AlO$_x$-based quantum technologies including quantum computers and parametric amplifiers. As an example, we show that engineered Josephson harmonics can reduce the charge dispersion and the associated errors in transmon qubits by an order of magnitude, while preserving their anharmonicity.

quant-ph↗

arXiv2024

In-plane magnetic field driven conductance modulations in topological insulator kinks

We present low-temperature magnetoconductance measurements on Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$ kinks with ribbon-shaped legs. The conductance displays a clear dependence on the in-plane magnetic field orientation. The conductance modulation is consistent with orbital effect-driven trapping of the topological surface states on different side facets of the legs of the kink, which affects their transmission across the kink. This magnetic field-driven trapping and conductance pattern can be explained with a semiclassical picture and is supported by quantum transport simulations. The interpretation is corroborated by varying the angle of the kink and analyzing the temperature dependence of the observed magnetoconductance pattern, indicating the importance of phase coherence along the cross section perimeter of the kink legs.

cond-mat.mes-hall↗

arXiv2024

Topological insulator based axial superconducting quantum interferometer structures

Nanoscale superconducting quantum interference devices (SQUIDs) are fabricated in-situ from a single Bi$_{0.26}$Sb$_{1.74}$Te$_{3}$ nanoribbon that is defined using selective-area growth and contacted with superconducting Nb electrodes via a shadow mask technique. We present $h/(2e)$ magnetic flux periodic interference in both, fully and non-fully proximitized nanoribbons. The pronounced oscillations are explained by interference effects of coherent transport through topological surface states surrounding the cross-section of the nanoribbon.

cond-mat.mes-hall↗

arXiv2024

Robust Majorana bound states in magnetic topological insulator nanoribbons with fragile chiral edge channels

Magnetic topological insulators in the quantum anomalous Hall regime host ballistic chiral edge channels. When proximitized by an $s$-wave superconductor, these edge states offer the potential for realizing topological superconductivity and Majorana bound states without the detrimental effect of large externally-applied magnetic fields on superconductivity. Realizing well-separated unpaired Majorana bound states requires magnetic topological insulator ribbons with a width of the order of the transverse extent of the edge state, however, which is expected to bring the required ribbon width down to around 100 nm. In this regime, it is known to be extremely difficult to retain the ballistic nature of chiral edge channels and realize a quantized Hall conductance. In this paper, we study the impact of disorder in such magnetic topological insulator nanoribbons and compare the fragility of ballistic chiral edge channels with the stability of Majorana bound states when the ribbon is covered by a superconducting film. We find that the Majorana bound states exhibit greater robustness against disorder than the underlying chiral edge channels.

cond-mat.mes-hall↗

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

Current-induced magnetization switching in a magnetic topological insulator heterostructure

We present the current-induced switching of the internal magnetization direction in a magnetic topological insulator/topological insulator heterostructure in the quantum anomalous Hall regime. The switching process is based on the bias current dependence of the coercive field, which is attributed to the effect of the spin-orbit torque provided by the unpolarized bias current. Increasing the bias current leads to a decrease in the magnetic order in the sample. When the applied current is subsequently reduced, the magnetic moments align with an externally applied magnetic field, resulting in repolarization in the opposite direction. This includes a reversal of the spin polarisation and hence a reversal of the chiral edge mode. Possible applications in spintronic devices are discussed.

cond-mat.mes-hall↗

arXiv2023

Fourier transformation based analysis routine for intermixed longitudinal and transversal hysteretic data for the example of a magnetic topological insulator

We present a symmetrization routine that optimizes and eases the analysis of data featuring the anomalous Hall effect. This technique can be transferred to any hysteresis with (point-)symmetric behaviour. The implementation of the method is demonstrated exemplarily using intermixed longitudinal and transversal data obtained from a chromium-doped ternary topological insulator revealing a clear hysteresis. Furthermore, by introducing a mathematical description of the anomalous Hall hysteresis based on the error function precise values of the height and coercive field are determined.

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

Robust and fragile Majorana bound states in proximitized topological insulator nanoribbons

Topological insulator (TI) nanoribbons with proximity-induced superconductivity are a promising platform for Majorana bound states (MBSs). In this work, we consider a detailed modeling approach for a TI nanoribbon in contact with a superconductor via its top surface, which induces a superconducting gap in its surface-state spectrum. The system displays a rich phase diagram with different numbers of end-localized MBSs as a function of chemical potential and magnetic flux piercing the cross section of the ribbon. These MBSs can be robust or fragile upon consideration of electrostatic disorder. We simulate a tunneling spectroscopy setup to probe the different topological phases of top-proximitized TI nanoribbons. Our simulation results indicate that a top-proximitized TI nanoribbon is ideally suited for realizing fully gapped topological superconductivity, in particular when the Fermi level is pinned near the Dirac point. In this regime, the setup yields a single pair of MBSs, well separated at opposite ends of the proximitized ribbon, which gives rise to a robust quantized zero-bias conductance peak.

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

Spin coherence of near-surface ionised $^{125}$Te$^+$ donors in silicon

Impurity spins in crystal matrices are promising components in quantum technologies, particularly if they can maintain their spin properties when close to surfaces and material interfaces. Here, we investigate an attractive candidate for microwave-domain applications, the spins of group-VI Impurity spins in crystal matrices are promising components in quantum technologies, particularly if they can maintain their spin properties when close to surfaces and material interfaces. Here, we investigate an attractive candidate for microwave-domain applications, the spins of group-VI $^{125}$Te$^+$ donors implanted into natural Si at depths as shallow as 20~nm. We show that surface band-bending can be used to ionise such near-surface Te to spin-active Te$^+$ state, and that optical illumination can be used further to control the Te donor charge state. We examine spin activation yield, spin linewidth, relaxation ($T_1$) and coherence times (\ttwo) and show how a zero-field 3.5~GHz `clock transition' extends spin coherence times to over 1~ms, which is about an order of magnitude longer than other near-surface spin systems.

cond-mat.mes-hall↗

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↗