Kiryl Niherysh

Here, we report the synthesis and electrical properties of ferrocene-doped Bi2Se3 nanostructures with a large surface-to-volume ratio (nanoribbons and ultrathin films) obtained via a cost-efficient physical vapor deposition (PVD) technique. Comprehensive structural characterization confirms the successful incorporation of the magnetic dopant while preserving the crystalline quality of the Bi2Se3 nanostructures. Low-temperature magnetotransport and thermoelectric measurements reveal systematic changes in the electrical transport induced by magnetic doping. In particular, magnetotransport measurements show the signatures of the anomalous Hall effect together with weak anti-localization suppression. These results demonstrate that ferrocene-assisted PVD provides a simple and effective route for tailoring the electrical properties of Bi2Se3 nanostructures, making them promising candidates for magnetic topological devices.

We investigate phase-coherent quantum transport in nanoscale Al–Bi₂Se₃–Al nanoribbon Josephson junctions by combining normal-state conductance spectroscopy with a junction-length-dependent study of Josephson transport. Differential conductance maps versus bias and gate voltage reveal Fabry–Pérot interference, whose periodicity matches the nanoribbon width, consistent with transverse quantization and quasi-ballistic surface-state trajectories in 430 nm wide devices. A systematic investigation of the characteristic voltage I _c R _n as a function of junction length L exhibits a clear plateau for L ≤ 500 nm, indicative of a short ballistic contribution to the Josephson transport, and decreases for longer junctions as diffusive transport dominates. Together, Fabry–Pérot interference and Josephson transport measurements provide complementary, channel-selective evidence for quasi-ballistic surface-state transport persisting over several hundred nanometers in hybrid topological insulator nanostructures. These results demonstrate the potential of Bi₂Se₃ nanoribbon Josephson junctions as a nanoscale platform for phase-coherent superconducting electronics, topological quantum computing architectures, and topological spintronic devices.

We present an investigation of the thermal damage threshold of passivated Bi₂Se₃ films upon laser illumination, with a focus on their employment in terahertz (THz) spectroscopic applications. Passivation was achieved by depositing a thin 3 nm Al capping layer which, exposed to the ambient, forms a natural oxide. In THz transient emission experiments, the samples were exposed to a train of 100 fs wide laser pulses with 800 nm wavelength at 78 MHz repetition rate and peak power density up to 295 mW/µm². For the sake of comparison, the films were also exposed to continuous wave laser light with a wavelength of 532 nm in the average optical power density range from 5 × 10⁻² mW/µm² to 50 mW/µm². In both cases, changes in film appearance, detected by optical microscopy, or even film removal in a small area close to the center of the illuminated spot could be induced. Raman spectroscopy provided evidence that the crystalline phase of Bi₂Se₃ films is present in areas that have been exposed but not damaged. Conversely, in the film region illuminated with the highest peak power density no Raman signal was detected in the range under investigation which we ascribe to material removal. At the perimeter of this ablated area, we observed a dominant Raman mode at approximately 255 cm⁻¹ that we can attribute to selenium and indicates partial Bi₂Se₃ decomposition. In contrast, we observed Raman spectra corresponding to as-deposited Bi₂Se₃ only a few micrometers away from the laser-damaged area. Hence, the observed THz radiation originates from this illuminated but undamaged region. This detailed knowledge is expected to serve as a guide for designing the emitter’s thermal management and choosing laser parameters for optimal operation.

In this work, we present a method to enhance the longitudinal spin Seebeck effect at platinum/yttrium iron garnet (Pt/YIG) interfaces. The introduction of a partial interlayer of bismuth selenide (Bi2Se3, 2.5% surface coverage) interfaces significantly increases (by ∼380%–690%) the spin Seebeck coefficient over equivalent Pt/YIG control devices. Optimal devices are prepared by transferring Bi2Se3 nanoribbons, prepared under anaerobic conditions, onto the YIG (111) chips followed by rapid over-coating with Pt. The deposited Pt/Bi2Se3 nanoribbon/YIG assembly is characterized by scanning electron microscope. The expected elemental compositions of Bi2Se3 and YIG are confirmed by energy dispersive x-ray analysis. A spin Seebeck coefficient of 0.34–0.62 μV/K for Pt/Bi2Se3/YIG is attained for our devices, compared to just 0.09 μV/K for Pt/YIG controls at a 12 K thermal gradient and a magnetic field swept from −50 to +50 mT. Superconducting quantum interference device magnetometer studies indicate that the magnetic moment of Pt/Bi2Se3/YIG treated chips is increased by ∼4% vs control Pt/YIG chips (i.e., a significant increase vs the ±0.06% chip mass reproducibility). Increased surface magnetization is also detected in magnetic force microscope studies of Pt/Bi2Se3/YIG, suggesting that the enhancement of spin injection is associated with the presence of Bi2Se3 nanoribbons.

In this work, a simple catalyst-free physical vapor deposition method is optimized by adjusting source material pressure and evaporation time for the reliable obtaining of freestanding nanoribbons with thicknesses below 15 nm. The optimum synthesis temperature, time and pressure were determined for an increased yield of ultrathin Bi₂Se₃ nanoribbons with thicknesses of 8–15 nm. Physical and electrical characterization of the synthesized Bi₂Se₃ nanoribbons with thicknesses below 15 nm revealed no degradation of properties of the nanoribbons, as well as the absence of the contribution of trivial bulk charge carriers to the total conductance of the nanoribbons.

We used the asymmetric superconducting quantum interference device (SQUID) technique to extract the current phase relation (CPR) of a Josephson junction with a 3D-topological insulator (3D-TI) Bi₂Se₃ nanobelt as the barrier. The obtained CPR shows deviations from the standard sinusoidal CPR with a pronounced forward skewness. At temperatures below 200 mK, the junction skewness values are above the zero temperature limit for short diffusive junctions. Fitting of the extracted CPR shows that most of the supercurrent is carried by ballistic topological surface states (TSSs), with a small contribution of diffusive channels primarily due to the bulk. These findings are instrumental in engineering devices that can fully exploit the properties of the topologically protected surface states of 3D TIs.

We used the asymmetric superconducting quantum interference device (SQUID) technique to extract the current phase relation (CPR) of a Josephson junction with a 3D-topological insulator (TI) Bi2Se3 nanobelt as the barrier. The obtained CPR shows deviations from the standard sinusoidal CPR with a pronounced forward skewness. At temperatures below 200 mK, the junction skewness values are above the zero temperature limit for short diffusive junctions. Fitting of the extracted CPR shows that most of the supercurrent is carried by ballistic topological surface states (TSSs), with a small contribution of diffusive channels primarily due to the bulk. These findings are instrumental in engineering devices that can fully exploit the properties of the topologically protected surface states of 3D TIs.

The majority of proposed exotic applications employing 3D topological insulators require high-quality materials with reduced dimensions. Catalyst-free, PVD-grown Bi₂Se₃ nanoribbons are particularly promising for these applications due to the extraordinarily high mobility of their surface Dirac states, and low bulk carrier densities. However, these materials are prone to the formation of surface accumulation layers; therefore, the implementation of surface encapsulation layers and the choice of appropriate dielectrics for building gate-tunable devices are important. In this work, all-around ZnO-encapsulated nanoribbons are investigated. Gate-dependent magnetotransport measurements show improved charge transport characteristics as reduced nanoribbon/substrate interface carrier densities compared to the values obtained for the as-grown nanoribbons on SiO₂ substrates.