arXiv subjects

Robert M. Klaes

Publications and source records attributed to Robert M. Klaes.

5 recordsLinked to original sources

arXiv2026

Rapid supercurrent decay in Mn$_5$Si$_3$ Josephson junctions

Theoretical work predicts that Josephson junctions containing metallic altermagnetic barriers should display $0$-$π$ transitions of the critical current as a function of both barrier thickness and temperature, with the decay and oscillation period of the supercurrent depending on the orientation of the crystal axes relative to the transport direction. Motivated by these predictions, and by reports of a compensated magnetic phase attributed to altermagnetism in epitaxial Mn$_5$Si$_3$ thin films, we fabricate and measure Nb/Pt/Mn$_5$Si$_3$/Pt/Nb Josephson junctions varying the thickness of the Mn$_5$Si$_3$ barrier. The critical current decays as a single exponential over more than four orders of magnitude with decay length $ξ_{\text{Mn}_5\text{Si}_3} = 0.31 \pm 0.03$ nm, shorter than reported for Josephson junctions containing the metallic antiferromagnets FeMn, Cr, and NiMn. The Mn$_5$Si$_3$ barrier has an estimated current-perpendicular-to-plane resistivity of $320 \pm 10 μΩ\,$cm. No $0$-$π$ transition is resolved at the sampled barrier thicknesses, and the temperature dependence of the critical current of a junction with a 1 nm barrier is smooth and monotonic. We discuss the absence of resolvable transitions in terms of the microstructure of the barrier, its uncertain magnetic phase, and the narrow thickness window imposed by the rapid decay, and identify barriers with well-defined crystalline orientation as the key requirement for future tests of altermagnetic Josephson physics.

cond-mat.supr-con↗

arXiv2026

Large critical current density Josephson $π$ junctions with PdNi barriers

We report large $π$-state critical current densities, $J_c(π)$, in Nb/Pd$_{89}$Ni$_{11}$/Nb Josephson junctions at Pd$_{89}$Ni$_{11}$ thicknesses near the first $π$-state. We observe oscillations in the critical current with ferromagnetic barrier thickness consistent with a $0$-$π$ transition. For a junction with a 9.4~nm Pd$_{89}$Ni$_{11}$ barrier, we obtain $J_c(π) = 410~\mathrm{kA/cm^{2}}$ at 4.2~K, exceeding values reported in prior PdNi-based studies. Magnetization measurements on continuous films, together with coercivity tests on patterned arrays, confirm that Pd$_{89}$Ni$_{11}$ exhibits perpendicular magnetic anisotropy, enabling zero-field operation without magnetic initialization. The combination of large $J_c(π)$ and intrinsic anisotropy establishes Pd$_{89}$Ni$_{11}$ as a promising barrier material for passive $π$-shifters in superconducting digital logic and qubit architectures.

cond-mat.supr-con↗

arXiv2025

Upper critical fields in normal metal-superconductor-normal metal trilayers

The role of spin orbit interaction in superconducting proximity effect is an area of intense research effort. Recent theoretical and experimental works investigate the possible role of spin-orbit interaction in generating spin-triplet pair correlations. In this work, we present an experimental survey of thin normal metal-superconductor-normal metal trilayers with Nb superconductor and Al, Ti, Cu, Pt, Ta, and Au normal metals, along with single layers of Nb as reference. We aim to probe the role of spin-orbit interaction and resistivity on the normal metal proximity effect through measurements of the upper critical field. We find that the upper critical fields of the trilayers are lower than that of a single layer Nb reference sample, and that the trilayers with higher resistivity metals, Ti, Pt, and Ta, behave as 2-dimensional superconductors. At low applied in-plane magnetic fields and temperatures close to the zero field transition temperature, we find a possible deviation from 2-dimensional to 3-dimensional behavior in the Ti and Pt trilayers. We also find that compared to single layer Nb films, all of our trilayers show a greater suppression of critical temperature during rotation from an in-plane to an out-of-plane applied magnetic field, with the greatest suppression observed in trilayers with Au or Al. This suppression of the critical temperature under field rotation might appear analogous to the colossal spin valve effect that can be achieved in systems with ferromagnetic materials; however, in our trilayers, only conventional orbital screening contributions to the suppression are present and the additional suppression is not present in the absence of applied magnetic field.

cond-mat.supr-con↗

arXiv2025

Comparison of NiFeCr and NiFe in ferromagnetic Josephson junctions

Josephson junctions containing ferromagnetic materials are under consideration for applications in digital superconducting logic and memory. Some memory applications rely on the ability to reverse the magnetization direction of a "soft" magnetic layer within the junction using a small local magnetic field generated on the chip. It is crucial, therefore, to find a suitable soft magnetic material with a low switching field and low switching energy. A popular magnetic material for such applications is Ni$_{80}$Fe$_{20}$, also known as Permalloy, however Permalloy has a rather large magnetization, leading to large magnetic switching energies. In this work we explore Cr-doped Permalloy, specifically Ni$_{73}$Fe$_{18}$Cr$_{9}$, which has a saturation magnetization just under two-thirds that of Permalloy. Josephson junctions containing this NiFeCr alloy undergo a 0-$π$ transition at a NiFeCr thickness of 2.3 nm, and the critical supercurrent decays in the alloy over a short characteristic length of 0.36 nm. Switching fields of a few millitesla are promising, but the short decay length and overall small values of the critical current in the Josephson junctions may preclude the use of NiFeCr in current cryogenic memory technologies.

cond-mat.supr-con↗

arXiv2022

Enhancement of supercurrent through ferromagnetic materials by interface engineering

Josephson junctions containing ferromagnetic materials exhibit interesting physics and show promise as circuit elements for superconducting logic and memory. For memory applications, the properties of the junction should be controllable by changing the magnetic configuration inside the junction. To achieve good magnetic switching properties, one should choose a soft magnetic material such as NiFe (permalloy); however, NiFe exhibits poor supercurrent transmission in Josephson junctions. In this work we put thin layers of Ni on either side of the NiFe and characterize the magnetic behavior and supercurrent transmission properties of the Ni/NiFe/Ni trilayers as a function of Ni and NiFe thicknesses. Using a Ni thickness of 0.4 nm, we find that the magnetic switching behavior of the trilayers is not severely degraded relative to plain NiFe, while the maximum supercurrent in the $π$-state of the trilayer Josephson junctions is increased by a factor of four relative to that of NiFe junctions. We speculate that the supercurrent enhancement is due to the different spin-dependent transport properties of the Cu/Ni and Cu/NiFe interfaces.

cond-mat.supr-con↗