arXiv subjects

Martin Endres

Publications and source records attributed to Martin Endres.

5 recordsLinked to original sources

arXiv2024

Large tunable kinetic inductance in a twisted graphene superconductor

Twisted graphene based moir\'e heterostructures host a flat band at the magic angles where the kinetic energy of the charge carriers is quenched and interaction effects dominate. This results in emergent phases such as superconductors and correlated insulators that are electrostatically tunable. We investigate superconductivity in twisted trilayer graphene (TTG) by integrating it as the weak link in a superconducting quantum interference device (SQUID). The measured current phase relation (CPR) yields a large and tunable kinetic inductance, up to 150 nH per square, of the electron and hole type intrinsic superconductors. We further show that the specific kinetic inductance and the critical current density are universally related via the superconducting coherence length, and extract an upper bound of 200 nm for the coherence length. Our work opens avenues for using graphene-based superconductors as tunable elements in superconducting circuits.

cond-mat.supr-con↗

arXiv2022

Current-phase relation of a WTe2 Josephson junction

When a topological insulator is incorporated into a Josephson junction, the system is predicted to reveal the fractional Josephson effect with a 4$\pi$-periodic current-phase relation. Here, we report the measurement of a $4\pi$-periodic switching current through an asymmetric SQUID, formed by the higher-order topological insulator WTe$_2$. Contrary to the established opinion, we show that a high asymmetry in critical current and negligible loop inductance are not sufficient by themselves to reliably measure the current-phase relation. Instead, we find that our measurement is heavily influenced by additional inductances originating from the self-formed PdTe$_{\text{x}}$ inside the junction. We therefore develop a method to numerically recover the current-phase relation of the system and find the $1.5\,\mu \text{m}$ long junction to be best described in the short ballistic limit. Our results highlight the complexity of subtle inductance effects that can give rise to misleading topological signatures in transport measurements.

cond-mat.mes-hall↗

arXiv2022

Transparent Josephson Junctions in Higher-Order Topological Insulator WTe2 via Pd Diffusion

Highly transparent superconducting contacts to a topological insulator (TI) remain a persistent challenge on the route to engineer topological superconductivity. Recently, the higher-order TI WTe$_2$ was shown to turn superconducting when placed on palladium (Pd) bottom contacts, demonstrating a promising material system in pursuing this goal. Here, we report the diffusion of Pd into WTe$_2$ and the formation of superconducting PdTe$_x$ as the origin of observed superconductivity. We find an atomically sharp interface in vertical direction to the van der Waals layers between the diffusion crystal and its host crystal, forming state-of-the-art superconducting contacts to a TI. The diffusion is discovered to be non-uniform along the width of the WTe$_2$ crystal, with a greater extend along the edges compared to the bulk. The potential of this contacting method is highlighted in transport measurements on Josephson junctions by employing external superconducting leads.

cond-mat.mes-hall↗

arXiv2020

Superconductivity in type-II Weyl-semimetal WTe2 induced by a normal metal contact

WTe$_2$ is a material with rich topological properties: it is a 2D topological insulator as a monolayer and a Weyl-semimetal and higher-order topological insulator (HOTI) in the bulk form. Inducing superconductivity in topological materials is a way to obtain topological superconductivity, which lays at the foundation for many proposals of fault tolerant quantum computing. Here, we demonstrate the emergence of superconductivity at the interface between WTe$_2$ and the normal metal palladium. The superconductivity has a critical temperature of about 1.2 K. By studying the superconductivity in perpendicular magnetic field, we obtain the coherence length and the London penetration depth. These parameters correspond to a low Fermi velocity and a high density of states at the Fermi level. This hints to a possible origin of superconductivity due to the formation of flat bands. Furthermore, the critical in-plane magnetic field exceeds the Pauli limit, suggesting a non-trivial nature of the superconducting state.

cond-mat.mes-hall↗

arXiv1999

Ferromagnetic Semiconductor - Singlet (or Triplet) Superconductor - Ferromagnetic Semiconductor Systems as Possible Logic Circuits and Switches

We consider thin superconducting (S) films of thickness d $\ll ξ_{0}$, sandwiched between two ferromagnetic semiconducting insulators (FI) with differently orientated magnetizations - the FI-S-FI system. We calculate the dependence of the superconducting critical temperature on the orientation of the magnetization in the insulators and on the thickness of the superconducting film. The calculations are done for singlet as well as triplet superconductors. In the singlet case T_{c} depends on the relative orientation of the left and right magnetization only, while in the triplet case T_{c} depends on the absolute orientation of magnetization. The latter property can serve as a kind of spin-spectroscopy of triplet and unconventional superconductors, for instance in resolving the structure of the triplet order parameter in the recently discovered layered superconductor Sr_{2}RuO_{4}. The possibility of logic circuits and switches, which are based on the FI-S-FI systems with arbitrary orientation of magnetizations in FI films, is analyzed too.

cond-mat↗