Alongside this work, the archive contains studies of Josephson junctions, qubits, noise in quantum computers, and currents in graphene. All of them touch on how quantum correlations and disorder manifest in electrical current. This paper stands apart: it connects the physics of superconducting hybrids with noise spectroscopy, which is usually applied to normal metals or Josephson junctions with two superconductors. Until now, enhanced shot noise was considered the privilege of structures with two superconducting leads, where multiple Andreev reflections create an effective charge larger than two electrons. Here, however, noise is observed in a contact with a single superconductor, forcing a reconsideration of the role of spin-orbit coupling in the proximity effect.
The key technique was measuring shot noise in epitaxial V(40 nm)/MgO(2 nm)/Fe(10 nm) contacts at a temperature of 0.3 K. A cross-correlation scheme with two amplifiers was used to subtract uncorrelated electronics noise. Bias voltage was supplied by a low-noise current source, and the noise spectral density was measured over a frequency range up to 100 kHz. For comparison, control samples V/MgO/Au and V/MgO/V were fabricated. The theoretical part relied on the Bogoliubov-de Gennes equation with Rashba spin-orbit coupling at the interface, and shot noise was computed via a recursive expansion of the distribution function in the number of Andreev reflections.
At voltages below the superconducting gap of vanadium (about 1 meV), the Fano factor reached values above 100, and in one sample, nearly 200. This means the effective charge transferred in each tunneling event exceeds 100 elementary charges. For comparison, in the normal state the Fano factor is unity, and in a conventional superconducting tunnel junction it is two. The noise was practically independent of frequency from a few hertz to hundreds of kilohertz, ruling out contributions from vortices or other low-frequency processes. A magnetic field suppressed the noise anisotropically: most strongly when oriented out of plane, but also differing between the in-plane [100] and [110] directions. Such anisotropy is incompatible with conventional singlet superconductivity and points to triplet correlations.
This work differs from neighboring studies in that it applies noise spectroscopy to a single superconductor-ferromagnet contact for the first time and observes an effect previously associated only with Josephson junctions with two superconductors. Neighboring papers study noise in Josephson junctions or qubits but do not consider the role of orbital symmetry and spin-orbit coupling at the superconductor/ferromagnet interface. Here, it is shown that these factors turn a single superconductor into a source of giant noise, creating an induced triplet gap in the ferromagnet. This expands the understanding of the proximity effect and provides a new tool for probing hidden interfaces.
The topic could develop toward a more precise quantitative description: the current model underestimates the measured Fano factor even at maximum induced gap. Accounting for random scattering at the spin-orbit barrier and destructive interference may bring theory closer to experiment. Furthermore, noise spectroscopy could become a standard method for verifying the presence of triplet superconductivity in other hybrid systems, including structures with half-metals or topological insulators.
The results will impact superconducting spintronics and the search for Majorana states for topological quantum computing, where triplet superconductivity plays a key role.
The next step is to measure noise in contacts with different crystallographic orientations of iron and vanadium to test the model's predictions about the role of orbital symmetry. It is also important to investigate the temperature dependence of noise near the critical temperature.
In the physics of superconducting hybrids, an unresolved question remains: how to distinguish true triplet superconductivity from artifacts related to magnetic textures or resonant tunneling. This work shows that shot noise can serve as a clear criterion: a giant Fano factor is impossible without an induced triplet gap supporting multiple Andreev reflections.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
Shot noise was measured in epitaxial V/MgO/Fe contacts at 0.3 K. At voltages below the superconducting gap, the Fano factor exceeded 100, corresponding to an effective charge of more than 100 electrons. The cause is attributed to spin-triplet superconductivity induced in iron via spin-orbit coupling at the interface.
Where its strength is
This work is the first to demonstrate giant shot noise in a structure with a single superconductor, linking it to triplet correlations and spin-orbit interaction. It opens a path to noise spectroscopy of hidden interfaces.
Where it could go next
Worth trying
Why this may matter
If the method is confirmed, it will provide a simple way to detect triplet superconductivity and spin-orbit effects in hybrid structures, which is important for superconducting spintronics and topological devices.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.54, against a usual 0.64. It looks like few have drilled nearby.
The effective charge estimated from the noise exceeds 100 electrons, as if a whole "tangle" of a hundred charges passed through the barrier instead of a single electron, even though physically only Cooper pairs are transferred.
The idea of hidden quantum states that reveal themselves only through noise echoes Peter Watts' novel "Blindsight," where characters try to understand the nature of invisible entities through indirect signs.