arXiv subjects

Jechiel van Dijk

Publications and source records attributed to Jechiel van Dijk.

3 recordsLinked to original sources

arXiv2026

Crystalline Germanium Josephson Junctions

Conventional superconducting quantum electronics rely on well-established Josephson junctions made of Al/AlO$_{x}$ where the weak link AlO$_{x}$ is amorphous and is believed to host two-level systems that limit coherence. Crystalline Josephson junctions exhibit atomically ordered interface quality but remain constrained by complex fabrication and intrinsic asymmetry of epitaxial growth. Here, we demonstrate a fully epitaxial approach based on superconductivity in gallium-doped germanium, enabling the realization of Josephson junctions entirely grown in situ by molecular beam epitaxy. These devices feature atomically sharp interfaces and crystalline weak links, resulting in strong Josephson coupling in the ultra-short regime. We observe an unconventional enhancement of the switching current under applied magnetic field, which we attribute to quasiparticle-assisted thermalization processes from the Al contacts. This platform combines structural coherence, fabrication simplicity, and scalability, offering a promising route toward low-disorder, CMOS-compatible superconducting qubits in a merged element transmon architecture.

cond-mat.mes-hall↗

arXiv2024

Superconductivity in Substitutional Ga-Hyperdoped Ge Epitaxial Thin Films

Doping-induced superconductivity in group IV elements may enable quantum functionalities in material systems accessible with well-established semiconductor technologies. Non-equilibrium hyperdoping of group III atoms into C, Si, or Ge can yield superconductivity; however, its origin is obscured by structural disorder and dopant clustering. Here, we report the epitaxial growth of hyperdoped Ga:Ge films and trilayer heterostructures by molecular beam epitaxy with extreme hole concentrations ($n_\textup{h} = 4.15 \times 10^{21}$~cm$^{-3}$, ~17.9\% Ga substitution) that yield superconductivity with a critical temperature of $T_{\textup{c}} = 3.5$~K and an out-of-plane critical field of 1~T at 270~mK. Synchrotron-based X-ray absorption and scattering methods reveal that Ga dopants are substitutionally incorporated within the Ge lattice, introducing a tetragonal distortion to the crystal unit cell. Our findings, corroborated by first-principles calculations, suggest that the structural order of Ga dopants creates a narrow band for the emergence of superconductivity in Ge, establishing hyperdoped Ga:Ge as a low-disorder, epitaxial superconductor-semiconductor platform.

cond-mat.mes-hall↗

arXiv2024

Molecular beam epitaxy growth of superconducting tantalum germanide

Developing new material platforms for use in superconductor-semiconductor hybrid structures is desirable due to limitations caused by intrinsic microwave losses present in commonly used III/V material systems. With the recent reports on tantalum superconducting qubits that show improvements over the Nb and Al counterparts, exploring Ta as an alternative superconductor in hybrid material systems is promising. Here, we study the growth of Ta on semiconducting Ge (001) substrates grown via molecular beam epitaxy. We show that at a growth temperature of 400$^{\circ}$C the Ta diffuses into the Ge matrix in a self-limiting nature resulting in smooth and abrupt surfaces and interfaces with roughness on the order of 3-7 Å as measured by atomic force microscopy and x-ray reflectivity. The films are found to be a mixture of Ta$_{5}$Ge$_{3}$ and TaGe$_{2}$ binary alloys and form a native oxide that seems to form a sharp interface with the underlying film. These films are superconducting with a $T_{C}\sim 1.8-2$K and $H_{C}^{\perp} \sim 1.88T$, $H_{C}^{\parallel} \sim 5.1T$. These results show this tantalum germanide film to be promising for future superconducting quantum information platforms.

cond-mat.mes-hall↗