Diode Effects in Current-Biased Josephson Junctions

Current-biased Josephson junctions exhibit hysteretic transitions between dissipative and superconducting states as characterized by switching and retrapping currents. Here, we develop a theory for diodelike effects in the switching and retrapping currents of weakly damped Josephson junctions. We find that while the diodelike behavior of switching currents is rooted in asymmetric current-phase relations, nonreciprocal retrapping currents originate in asymmetric quasiparticle currents. These different origins also imply distinctly different symmetry requirements. We illustrate our results by a microscopic model for junctions involving a single magnetic atom. Our theory provides significant guidance in identifying the microscopic origin of nonreciprocities in Josephson junctions

Direct observation of intrinsic surface magnetic disorder in amorphous superconducting films

The interplay between disorder and interactions can dramatically influence the physical properties of thin-film superconductors. In the most extreme case, strong disorder is able to suppress superconductivity as an insulating phase emerges. Due to the known pair-breaking potential of magnetic disorder on superconductors, the research focus is on the influence of nonmagnetic disorder. Here we provide direct evidence that magnetic disorder is also present at the surface of amorphous superconducting films. This magnetic disorder is present even in the absence of magnetic impurity atoms and is intimately related to the surface termination itself. While bulk superconductivity survives in sufficiently thick films, we suggest that magnetic disorder may crucially affect the superconductor-to-insulator transition in the thin-film limit

Diode effect in Josephson junctions with a single magnetic atom

Current flow in electronic devices can be asymmetric with bias direction, a phenomenon underlying the utility of diodes and known as non-reciprocal charge transport. The promise of dissipationless electronics has recently stimulated the quest for superconducting diodes, and non-reciprocal superconducting devices have been realized in various non-centrosymmetric systems. Probing the ultimate limits of miniaturization, we have created atomic-scale Pb--Pb Josephson junctions in a scanning tunneling microscope. Pristine junctions stabilized by a single Pb atom exhibit hysteretic behavior, confirming the high quality of the junctions, but no asymmetry between the bias directions. Non-reciprocal supercurrents emerge when inserting a single magnetic atom into the junction, with the preferred direction depending on the atomic species. Aided by theoretical modelling, we trace the non-reciprocity to quasiparticle currents flowing via Yu-Shiba-Rusinov (YSR) states inside the superconducting energy gap. Our results open new avenues for creating atomic-scale Josephson diodes and tuning their properties through single-atom manipulation

Diode effect in Josephson junctions with a single magnetic atom

Current flow in electronic devices can be asymmetric with bias direction, a phenomenon underlying the utility of diodes1 and known as non-reciprocal charge transport2. The promise of dissipationless electronics has recently stimulated the quest for superconducting diodes, and non-reciprocal superconducting devices have been realized in various non-centrosymmetric systems3,4,5,6,7,8,9,10. Here we investigate the ultimate limits of miniaturization by creating atomic-scale Pb–Pb Josephson junctions in a scanning tunnelling microscope. Pristine junctions stabilized by a single Pb atom exhibit hysteretic behaviour, confirming the high quality of the junctions, but no asymmetry between the bias directions. Non-reciprocal supercurrents emerge when inserting a single magnetic atom into the junction, with the preferred direction depending on the atomic species. Aided by theoretical modelling, we trace the non-reciprocity to quasiparticle currents flowing by means of electron–hole asymmetric Yu–Shiba–Rusinov states inside the superconducting energy gap and identify a new mechanism for diode behaviour in Josephson junctions. Our results open new avenues for creating atomic-scale Josephson diodes and tuning their properties through single-atom manipulation

Josephson Spektroskopie auf einzelnen Atomen in einem Rastertunnelmikroskop

Josephson junctions are formed by a non-superconducting weak link between two superconducting electrodes. The dissipation-less superconducting tunnel current flowing across the Josephson junction is carried by superconducting charge carriers which are called Cooper pairs. The tunnel junction is characterized by the overlap of the macroscopic wavefunctions of the two superconducting electrodes. Josephson junctions are highly sensitive to changes of the phase difference between the two wavefunctions. In this thesis Josephson junctions between a superconducting tip and superconducting sample were investigated in a scanning tunneling microscope (STM). The STM enables atomic resolution of the sample surface as well the formation and manipulation of atomic structures on the surface. Josephson spectroscopy in an STM allows the investigation of the influence of the junction composition on the phase coherence between the macroscopic wavefunctions of the superconducting electrodes on the atomic scale. Josephson junctions can be interpreted as an oscillating circuit with distinct damping properties. The damping behaviour due to energy losses caused by the interaction with the environment can be frequency dependent. Biasing the Josephson junction by either an applied current or voltage influences the frequency-dependent impedance that couples the system to the electromagnetic environment. Phase coherence is strongly influenced by the energy exchange of the junction with the environment. In this study, the Josephson junction is exposed to a high frequency (HF) electromagnetic field in order to further investigate the phase coherence of the system. Generally, tunneling charge carriers can absorb quantised energy from the photons of the HF field (photon-assisted tunneling). However, in Josephson contacts coherent absorption of energy from the external field is expected (Shapiro steps). The investigated junctions show resonant absorption at the expected energies in the presence of the HF field. Additionally the current-biased V(I)-curves show a hysteresis depending on the direction of the applied current. This hysteresis indicates phase coherence between tunneling Cooper pairs. However, other features observed in the V(I)-characteristics are correlated to dissipative processes. For that reason a clear identification of Shapiro steps is not possible in this setup. Furthermore, Josephson spectroscopy was performed on single magnetic adatoms on the Pb(111) surface. The magnetic moment of the atom’s unpaired electrons couples to the superconducting condensate and induces localized bound states within the energy gap of the superconductor (YSR states). The tunneling probability for electrons and holes into the YSR states is known to vary due to potential scattering on the surface (electron-hole asymmetry). In a Josephson junction magnetic adatoms were found to induce a diode-like behaviour, i.e., the transition from the resistive single-particle conductance into the Cooper-pair tunneling regime depends on the direction of the applied current. In collaboration with the theory group of Felix von Oppen at Freie Universität Berlin the observed non-reciprocity was correlated to the damping properties of the Josephson junction and explained by the electron-hole asymmetry of the induced YSR bound states.Ein Tunnelkontakt zwischen zwei Supraleitern durch den ein supraleitender Strom fließt nennt sich Josephson-Kontakt und die tunnelnden Ladungsträger werden als Cooper Paare bezeichnet. Josephson-Kontakte sind sehr empfindlich gegenüber Änderungen der Phasenrelation der makroskopischen Wellenfunktionen der beiden supraleitenden Elektroden. In dieser Arbeit wurden Josephson-Kontakte zwischen einer supraleitenden Spitze und einem supraleitendem Substrat (Pb(111)) in einem Rastertunnelmikroskop (RTM) untersucht. Mit dem RTM ist es möglich die Oberfläche atomar genau abzubilden und atomare Strukturen zu formen und zu manipulieren. Das eröffnet die Möglichkeit den Einfluss von Änderungen der Josephson-Kontakte auf atomarer Ebene auf die Phasenkorrelation der Wellenfunktionen zu untersuchen. Josephson-Kontakte können als eine Art Schwingkreis verstanden werden in denen die Dämpfung eine große Rolle spielt. Die Dämpfung kann frequenzabhängig sein und beschreibt den Verlust von Energie des Systems an die Umgebung. Die Frequenzabhängigkeit des untersuchten Systems unterscheidet sich abhängig davon, ob der Kontakt strom- oder spannungs- getrieben ist. Dieser Abhängigkeit wird auf Unterschiede der frequenzabhängigen Impedanz zurückgeführt, mit der das System an die elektro-magnetische Umgebung koppelt. Die Phasenkohärenz der Josephson-Kontakte ist stark beeinflusst von diesem Energieaustausch. Um die Phasenkohärenz genauer zu untersuchen, wird ein elektromagnetisches (em) Feld eingestrahlt. In Tunnelkontakten können individuell tunnelnde Ladungsträger die quantisierte Energie der Photonen des em Feldes absorbieren (photon-assistiertes Tunneln). In Josephson-Kontakten wird neben diesem Effekt auch die kohärente Absorption der Strahlung erwartet (Shapiro-Stufen). In den untersuchten Kontakten werden Stufen in den Kennlinien bei den erwarteten Energien beobachtet. Zusätzlich tritt in den Stromgetriebenen Josephson Kontakten eine Hysterese abhängig von der Richtung des angelegten Stroms auf. Diese Hysterese deutet auf einen phasenkohärenten Tunnelprozess der Cooper Paare hin. Da allerdings zeitgleich auch dissipative Prozesse erkennbar sind können Shapiro-Stufen nicht eindeutig festgestellt werden. Zusätzlich zu diesen Erkenntnissen wurde Josephson Spektroskopie auf einzelnen magnetischen Atomen durchgeführt. Der Spin der ungepaarten Ladungsträger der magnetischen Atome wechselwirkt mit dem supraleitenden Kondensat und induziert gebundene Einzelelektronenzustände (YSR-Zustände). Aufgrund von Potential-Streuung durch die Änderung der elektro-magnetischen Umgebung kann die Tunnelwahrscheinlichkeiten in diese Zustände für Elektronen und Löcher variieren (Elektron-Loch-Asymmetrie). In den untersuchten Josephson-Kontakten rufen diese Zustände ein dioden-artiges Verhalten der übergänge von dem normalleitenden zu dem supraleitenden Zustand des Kontakts hervor. In Zusammenarbeit mit der Forschungsgruppe von Felix von Oppen an der Freien Universität Berlin konnte herausgearbeitet werden, dass die Dämpfung der Josephson-Kontakte durch die Elektron-Loch-Asymmetrie der YSR-Zustände beeinflusst wird und für das nicht-reziproke Verhalten der Josephson-CPKontakte verantwortlich ist

Original experimental data and code for the Paper "Diode effect in Josephson junctions with a single magnetic atom"

Current flow in electronic devices can be asymmetric with bias direction, a phenomenon underlying the utility of diodes and known as non-reciprocal charge transport. The promise of dissipationless electronics has recently stimulated the quest for superconducting diodes, and non-reciprocal superconducting devices have been realized in various non-centrosymmetric systems. Here, we probe the ultimate limits of miniaturization by creating atomic-scale Pb--Pb Josephson junctions in a scanning tunneling microscope. Pristine junctions stabilized by a single Pb atom exhibit hysteretic behavior, confirming the high quality of the junctions, but no asymmetry between the bias directions. Non-reciprocal supercurrents emerge when inserting a single magnetic atom into the junction, with the preferred direction depending on the atomic species. Aided by theoretical modelling, we trace the non-reciprocity to quasiparticle currents flowing via electron-hole asymmetric Yu-Shiba-Rusinov (YSR) states inside the superconducting energy gap and identify a new mechanism for diode behavior in Josephson junctions. Our results open new avenues for creating atomic-scale Josephson diodes and tuning their properties through single-atom manipulation

The 170ms Response to Faces as Measured by MEG (M170) Is Consistently Altered in Congenital Prosopagnosia

<div><p>Modularity of face processing is still a controversial issue. Congenital prosopagnosia (cPA), a selective and lifelong impairment in familiar face recognition without evidence of an acquired cerebral lesion, offers a unique opportunity to support this fundamental hypothesis. However, in spite of the pronounced behavioural impairment, identification of a functionally relevant neural alteration in congenital prosopagnosia by electrophysiogical methods has not been achieved so far. Here we show that persons with congenital prosopagnosia can be distinguished as a group from unimpaired persons using magnetoencephalography. Early face-selective MEG-responses in the range of 140 to 200ms (the M170) showed prolonged latency and decreased amplitude whereas responses to another category (houses) were indistinguishable between subjects with congenital prosopagnosia and unimpaired controls. Latency and amplitude of face-selective EEG responses (the N170) which were simultaneously recorded were statistically indistinguishable between subjects with cPA and healthy controls which resolves heterogeneous and partly conflicting results from existing studies. The complementary analysis of categorical differences (evoked activity to faces minus evoked activity to houses) revealed that the early part of the 170ms response to faces is altered in subjects with cPA. This finding can be adequately explained in a common framework of holistic and part-based face processing. Whereas a significant brain-behaviour correlation of face recognition performance and the size of the M170 amplitude is found in controls a corresponding correlation is not seen in subjects with cPA. This indicates functional relevance of the alteration found for the 170ms response to faces in cPA and pinpoints the impairment of face processing to early perceptual stages.</p></div