arXiv subjects

Roozbeh Yazdanpanah

Publications and source records attributed to Roozbeh Yazdanpanah.

3 recordsLinked to original sources

arXiv2025

Ex Situ Fabrication of Superconducting Nanostructures for Low-Temperature STM

Nanofabrication of topological insulator (TI) devices is essential for accessing edge and interface states, but conventional lithography and etching compromise the atomically clean surfaces required for scanning tunneling microscopy and spectroscopy (STM/STS). We demonstrate a fabrication strategy that combines ex situ lithographic patterning with in situ ultrahigh-vacuum cleaving and flipping, yielding atomically clean, nanopatterned TI-superconductor heterostructures suitable for STM/STS. In Design I, nanoribbons were defined by etching trenches into a TI film and capping with Nb. This enabled spectroscopy on large areas, although edge quality was limited by etch debris. In Design II, local thinning defined buried nanoribbons within a continuous TI film, producing pristine planar surfaces. STM/STS revealed well-developed superconducting gaps in the surrounding film, with suppressed gaps on the nanoribbons, consistent with vertical proximity coupling. This approach establishes a reproducible pathway for high-resolution STM/STS studies of proximitized nanostructures, providing a scalable platform for exploring topological superconductivity.

cond-mat.mtrl-sci↗

arXiv2024

Characterizing the chemical potential disorder in the topological insulator (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ thin films

We use scanning tunneling microscopy and spectroscopy under ultra-high vacuum and down to 1.7 K to study the local variations of the chemical potential on the surface of the topological insulator (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ thin films (thickness 7 - 30 nm) with varying Sb-concentration $x$, to gain insight into the charge puddles formed in thin films of a compensated topological insulator. We found that the amplitude of the potential fluctuations, $Γ$, is between 5 to 14 meV for quasi-bulk conducting films and about 30 - 40 meV for bulk-insulating films. The length scale of the fluctuations, $λ$, was found to span the range of 13 - 54 nm, with no clear correlation with $Γ$. Applying a magnetic field normal to the surface, we observe the condensation of the two-dimensional topological surface state into Landau levels and find a weak but positive correlation between $Γ$ and the spectral width of the Landau-level peaks, which suggests that quantum smearing from drift motion is the source of the Landau level broadening. Our systematic measurements give useful guidelines for realizing $(\mathrm{Bi}_{1-x}\mathrm{Sb}_x)_2\mathrm{Te}_3$ thin films with an acceptable level of potential fluctuations. In particular, we found that $x\approx 0.65$ realizes the situation where $Γ$ shows a comparatively small value of 14 meV and the Dirac point lies within $\sim$10 meV of the Fermi energy.

cond-mat.mes-hall↗

arXiv2023

Top-down fabrication of bulk-insulating topological insulator nanowires for quantum devices

In a nanowire (NW) of a three-dimensional topological insulator (TI), the quantum-confinement of topological surface states leads to a peculiar subband structure that is useful for generating Majorana bound states. Top-down fabrication of TINWs from a high-quality thin film would be a scalable technology with great design flexibility, but there has been no report on top-down-fabricated TINWs where the chemical potential can be tuned to the charge neutrality point (CNP). Here we present a top-down fabrication process for bulk-insulating TINWs etched from high-quality (Bi$_{1-x}$Sb$_{x}$)$_2$Te$_3$ thin films without degradation. We show that the chemical potential can be gate-tuned to the CNP and the resistance of the NW presents characteristic oscillations as functions of the gate voltage and the parallel magnetic field, manifesting the TI-subband physics. We further demonstrate the superconducting proximity effect in these TINWs, preparing the groundwork for future devices to investigate Majorana bound states.

cond-mat.mes-hall↗