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)_2Te_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.

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 SiO2/Si3N4 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 liftoff using vapor hydrofluoric acid, which selectively etches SiO2. We demonstrate an average T1≈75±11 μs over a 200 MHz frequency range across multiple cooldowns for one device, and T1≈44±8 μs 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.

Stencil mask lithography is an advanced technique for fully in situ fabricating Josephson junctions, which is increasingly being used for multiterminal Josephson junctions. This study provides information on the optimal mask design and mask reliability. For this, 270 mask designs were systematically fabricated and investigated under a scanning electron microscope. Reliable statements are made about mask yield, minimal dimensions, and systematic dependencies on the number of superconducting terminals. We find that stencil mask lithography can be used reliably for fabricating multiterminal Josephson junctions, enabling lateral mask dimensions down to 40 nm on average.

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 (√(31)×√(31))R±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.

The Josephson diode effect, where the switching current magnitude depends on its direction, arises when both time-reversal and inversion symmetries are broken, often achieved by a combination of spin-orbit interaction and applied magnetic fields. Taking advantage of the strong spin-orbit coupling inherent in three-dimensional topological insulators, we study this phenomenon in Nb/Bi0.8Sb1.2Te3/Nb Josephson weak-link junctions. Under an in-plane magnetic field perpendicular to the current direction, we observe a pronounced Josephson diode effect with efficiencies up to 7%. A crucial component of this behavior is the nonsinusoidal current-phase relationship and an anomalous phase shift, which we attribute to the presence of a ballistic supercurrent component due to the surface states. These findings open up new avenues for harnessing and controlling the Josephson diode effect in topological material systems.

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.

The study investigates the Al2O3/GaAs interface formed by in situ atomic layer deposition (ALD) of Al2O3 on pristine (0 0 1)-oriented GaAs substrate, using X-ray photoelectron spectroscopy (XPS) and room-temperature photoluminescence (PL). The Al2O3/GaAs interface is free of high-valence arsenic (As2O3 and As2O5) and gallium (Ga2O3) oxides when the Al2O3 layer thickness exceeds 3 nm. The Al2O3 deposition on an As-terminated GaAs substrate leads to the presence of low-valence arsenic oxides (As2O and AsO) and As-As bonds related to As-As dimers at the interface. These dimers restrict the PL enhancement to approximately twice that of unpassivated GaAs. In contrast, Ga-terminated GaAs exhibits a reduction of roughly an order of magnitude in these interfacial species, as well as a PL intensity that is significantly (x20) higher than that of unpassivated GaAs. When the Al2O3 layer thickness is less than 3 nm, both the low-and high-valence oxides, as well as elemental As, are detected in the near-interface region of Ga-and As-terminated GaAs samples. The PL intensity of GaAs capped with an Al2O3 layer thinner than 3 nm is still considerably (x8) higher than that of unpassivated GaAs and is mainly limited by elemental arsenic. According to XPS data, oxide deposition reduces band bending in GaAs at the Al2O3/GaAs interface by 0.2-0.4 eV, indicating a decrease in interface state density, i.e. the unpinned state of the Fermi level at the interface. The study delivers criteria for the high-quality passivation of (0 0 1) GaAs surface.

Bi 4 Te 3 has been first realized as an element of the adaptive series ( Bi 2 ) m ( Bi 2 Te 3 ) n —deriving from stacking metallic Bi and topological insulator Bi 2 Te 3 blocks—and studied for its promising thermoelectric properties. Recently, Bi 4 Te 3 has also been found as a topological insulator hosting a complex band structure with three Dirac cones in k space. Here we report measurements of the optical conductivity of five Bi 4 Te 3 thin films with thickness varying from 11 to 32 nm. This extensive study, based on terahertz time-domain spectroscopy and Fourier-transform infrared spectroscopy, provides a broadband optical conductivity. By employing a Kramers-Kronig constrained Drude-Lorentz model, the spectral contributions to the optical conductivity of each sample have been singled out. In contrast to an ideal topological insulator—with perfectly insulating bulk—in Bi 4 Te 3 far-infrared and mid-infrared bands indicate the presence of localized massive carriers, which dope the bulk system, as also observed in chemically compensated single crystals of other bismuth-based topological insulators. In addition, the localized bulk charges strongly renormalize the electronic mass. The charge dressing enhances as the thickness increases, indicating a trivial metallic behavior of Bi 4 Te 3 thin films. This study elucidates the inner relationship between the optical properties of a topological insulator like Bi 4 Te 3 and its complex band structure, since the detection of the topological massless carriers by optical measurements is prevented by the occurrence of novel and unusual conductive properties.

The study investigates the Al2O3/GaAs interface formed by in situ atomic layer deposition (ALD) of Al2O3 on pristine (0 0 1)-oriented GaAs substrate, using X-ray photoelectron spectroscopy (XPS) and room-temperature photoluminescence (PL). The Al2O3/GaAs interface is free of high-valence arsenic (As2O3 and As2O5) and gallium (Ga2O3) oxides when the Al2O3 layer thickness exceeds 3 nm. The Al2O3 deposition on an As-terminated GaAs substrate leads to the presence of low-valence arsenic oxides (As2O and AsO) and As–As bonds related to As–As dimers at the interface. These dimers restrict the PL enhancement to approximately twice that of unpassivated GaAs. In contrast, Ga-terminated GaAs exhibits a reduction of roughly an order of magnitude in these interfacial species, as well as a PL intensity that is significantly (x20) higher than that of unpassivated GaAs. When the Al2O3 layer thickness is less than 3 nm, both the low- and high-valence oxides, as well as elemental As, are detected in the near-interface region of Ga- and As-terminated GaAs samples. The PL intensity of GaAs capped with an Al2O3 layer thinner than 3 nm is still considerably (x8) higher than that of unpassivated GaAs and is mainly limited by elemental arsenic. According to XPS data, oxide deposition reduces band bending in GaAs at the Al2O3/GaAs interface by 0.2–0.4 eV, indicating a decrease in interface state density, i.e. the unpinned state of the Fermi level at the interface. The study delivers criteria for the high-quality passivation of (0 0 1) GaAs surface.

The engineering of inversion symmetry breaking in 2D materials, for example via interlayer twist-angle control, has initiated the '2D age' due to its transformation on the landscape of fundamental research and technological advances. The typical paradigms include nonlinear optical response, topological polar vortices, superconductivity, etc. In parallel, intrinsic structural defects induced inversion symmetry breaking also serves as a lever to create emerging physical phenomena. Here, we report on discovery of antisite defects induced exotic polarization waves in bismuth telluride (Bi2)m(Bi2Te3)n topological insulator thin films by using atomic-resolution scanning transmission electron microscopy. Despite weak van der Waals (vdW) bonding, driven by stochastic antisite defects, a quasi-cycloidal polarization order is observed along normal direction of the vdW interface in Bi2Te3 phase. As metallic Bi-Bi bilayer is replaced by Bi-Te bilayer via regular TeBi antisite defect, hyperbolic polarization waves are observed at septuple layers (SLs) of the Bi8Te9 phase. Our first-principles calculations reveal that the face-shared unusual packing of acentric octahedral units and insufficient carrier screening are responsible for the polar metal states. Our findings provide a new degree of freedom to manipulate the physical properties of topological insulators (TIs) and their future device application.

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.

Superconducting electrodes are an integral part of hybrid Josephson junctions used in many applications including quantum technologies. We report on the fabrication and characterization of superconducting hybrid Au/YBa2Cu3O7-x (YBCO) electrodes on vicinal substrates. In these structures, superconducting CuO2-planes face the gold film, resulting in a higher value and smaller variation of the induced energy gap compared to the conventional Au/YBCO electrodes based on films with the c-axis normal to the substrate surface. Using scanning tunneling microscopy, we observe an energy gap of about 10-17 meV at the surface of the 15- nm-thick gold layer deposited in situ atop the YBCO film. To study the origin of this gap, we fabricate nanoconstrictions from the Au/YBCO heterostructure and measure their electrical transport characteristics. The conductance of the nanoconstrictions shows a series of dips due to multiple Andreev reflections in YBCO and gold providing clear evidence of the superconducting nature of the gap in gold. We consider the Au/YBCO electrodes to be a versatile platform for hybrid Josephson devices with a high operating temperature.

The combination of an ordinary s-type superconductor with three-dimensional topological insulators creates a promising platform for fault-tolerant topological quantum computing circuits based on Majorana braiding. The backbone of the braiding mechanism are three-terminal Josephson junctions. It is crucial to understand the transport in these devices for further use in quantum computing applications. We present low-temperature measurements of topological insulator-based three-terminal Josephson junctions fabricated by a combination of selective-area growth of Bi0.8Sb1.2Te3 and shadow mask evaporation of Nb. This approach allows for the in situ fabrication of Josephson junctions with an exceptional interface quality, important for the study of the proximity-effect. We map out the transport properties of the device as a function of bias currents and prove the coupling of the junctions by the observation of a multiterminal geometry-induced diode effect. We find good agreement of our findings with a resistively and capacitively shunted junction network model.

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.

We synthesized and spectroscopically investigated monolayer (ML) C60 on the topological insulator (TI) Bi4Te3. This C60/Bi4Te3 heterostructure is characterized by an excellent translational order in a novel (4 × 4) C60 superstructure on a (9 × 9) cell of Bi4Te3. Angle-resolved photoemission spectroscopy (ARPES) of C60/Bi4Te3 reveals that ML C60 accepts electrons from the TI at room temperature, but no charge transfer occurs at low temperatures. This temperature-dependent doping is further investigated by Raman spectroscopy, photoluminescence (PL), and calculations of C60/Bi4Te3. At low temperatures, Raman spectroscopy and PL show a dramatic intensity increase of the C60-related signal, suggesting a transition to a rotationally ordered state. Calculations explain the charge transfer by C60 adsorption to Bi4Te3 surface defects. The temperature dependence of the charge transfer is attributed to the orientational order of C60. The electron affinity of C60 increases at low temperatures due to the freezing of the rotational motion.

Buckminsterfullerene (C 60 ) has extensively been studied due to its various exotic electronic and magnetic properties which range semiconductor in the pristine phase to metals or Mott insulators and even superconductors when C 60 is doped by alkali atoms [1]. Ultrathin films of endohedral fullerenes encapsulating metal ions on highly ordered substrates further widen the range of complex and exotic electronic states [1]. While the fullerene thin film deposition on metal and insulating substrates has been explored, there are not many studies focusing on fullerene thin film deposition on topologically protected surfaces [2]. Here, we study the electronic structure of a highly ordered ultrathin fullerene film (1ML C 60 ) deposited on the topological insulator Bi 4 Te 3 using ARPES, Raman, and DFT methods [3]. In addition to hexagonal ordering of C 60 film on Bi 4 Te 3 , the LEED analysis confirms a (4X4) reconstruction of the C 60 on a (9X9) supercell of the Bi 4 Te 3 surface. The ARPES and Fermi-surface mapping of the topologically protected surface state confirms a strong hexagonal warping deviating from the typical linearly dispersive Dirac bands [4,5]. While we observe a hole doping to the TI with C 60 deposition at room temperature as rigid shifting of the Dirac point, no charge transfer at low temperature is observed. The estimated hole doping to the TI surface at room temperature is ~ 0.03 holes per C 60 molecule. Due to excellent long-range ordering of C 60 molecules on TI substrate, both HOMO and HOMO-1 molecular bands of C 60 show a clear electron and weakly hole like band dispersions with p- and s-polarized lights, respectively. Clearly, both the molecular bands of C 60 on TI surface are further splitted into at least two degenrate states due to long range hexagonal ordering. Comparison of the momentum distribution curves at C 60 bands shows a rigid shift of the bands towards Fermi level with cooling consistent with observed changes in the TI surface band. Temperature dependence resonance Raman spectroscopy of the C 60 pentagon pinch reveals a molecular ordering of the C 60 thin film below 250 K that is reminiscent to the structural transition in bulk C 60 . Significant change in the Photoluminescence of C 60 film at low temperature further confirms the molecular ordering at low temperature. Ab initio calculations of the reconstructed heterostructure suggest electron-doping of the C 60 molecules due to tri-vacancy of the Te-terminated surface. Simulations performed for different molecular ordering suggest low electron affinity in the ground state comparison to the 300 K affecting the charge transfer at low temperatures. Our work highlights that TI surfaces are excellent substrates for the growth of highly ordered layers of fullerenes. The work shown here also paves the way for further experiments using magnetic fullerenes and superconducting C 60 films grown on TI. [1] László Forró and László Mihály, Rep. Prog. Phys. 64, 649-699 (2021). [2] X.-Q. Shi et. al., J Mater Sci. 47, 7341-7355 (2012); A. N. Mihalyuk et al., J. Chem. Phys. 154, 104703(2021). [3] Pandeya, R. P., et al. arXiv preprint arXiv:2405.09119 (2024). [4] T. Chagas et. al., Electron. Struct. 2, 015002 (2020); T. Chagas et. al., Phys. Rev. B 105, L081409 (2022). [5] L. Fu, Phys. Rev. Lett. 103, 266801 (2009); K. Kuroda et. al., Phys. Rev. Lett. 105, 076802 (2010). [6] Takabayashi, Y., and Kosmas, P. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 374.2076 (2016): 20150320. Figure 1

Phase-change memory (PCM) is a promising candidate for in-memory computation and neuromorphic computing due to its high endurance, low cycle-to-cycle variability, and low read noise. However, among other factors, its performance strongly depends on the post-lithography fabrication steps. This study examines the impact of reactive ion etching (RIE) on PCM device performance by evaluating different etching gas mixtures, CHF3:O2, H2:Ar, and Ar, and determining their impact on key device characteristics, particularly initial resistance and cycling stability. The present study demonstrates that a two-step etching approach in which the capping layer is first removed using H2:Ar and the underlying GST layer is subsequently etched using physical Ar sputtering ensures stable and reliable PCM operation. In contrast, chemically reactive gases negatively impact the initial resistance, cycling stability, and device lifetime, likely due to alterations in the material composition. For the cycling stability evaluation, an advanced measurement algorithm utilizing the aixMATRIX setup by aixACCT Systems is employed. This algorithm enables automated testing, dynamically adjusting biasing parameters based on cell responses, ensuring a stable ON/OFF ratio and high-throughput characterization.

In this work we present a systematic in-depth study of how we can alter the magnetotransport 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 magnetotransport 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.

Phase change memory cells are outstanding candidates for processing-in-memory and neuromorphic computing. The high endurance, low cycle-to-cycle variability, and low read noise especially suit many applications, whereas the high cell-to-cell variability poses a challenge, since each cell serves slightly different results. Therefore, automated characterization is necessary to create a sufficient statistical database to thoroughly study the switching behavior. This paper introduces a sophisticated algorithm for the endurance measurement of phase change memory cells using the aixMATRIX setup by aixACCT Systems. The algorithm is able to perform endurance measurements on devices over an entire sample, adjusting the biasing parameters according to the switching behavior of each cell and sorting out nonfunctional cells. The stepping between the cells is performed automatically.We highlight the benefits of our algorithm by providing an in-depth analysis of all devices on one phase change memory in a bridge geometry sample.

The combination of an ordinary s-type superconductor with three-dimensional topological insulators creates a promising platform for fault-tolerant topological quantum computing circuits based on Majorana braiding. The backbone of the braiding mechanism are three-terminal Josephson junctions. It is crucial to understand the transport in these devices for further use in quantum computing applications. We present low-temperature measurements of topological insulator-based three-terminal Josephson junctions fabricated by a combination of selective-area growth of Bi_0.8Sb_1.2Te_3 and shadow mask evaporation of Nb. This approach allows for the in-situ fabrication of Josephson junctions with an exceptional interface quality, important for the study of the proximity-effect. We map out the transport properties of the device as a function of bias currents and prove the coupling of the junctions by the observation of a multi-terminal geometry induced diode effect. We find good agreement of our findings with a resistively and capacitively shunted junction network model.