arXiv subjects

F. S. Bergeret

Publications and source records attributed to F. S. Bergeret.

At least 55 records · Page 3Linked to original sources

arXiv2017

Revealing the magnetic proximity effect in EuS/Al bilayers through superconducting tunneling spectroscopy

A ferromagnetic insulator attached to a superconductor is known to induce an exchange splitting of the Bardeen-Cooper-Schrieffer (BCS) singularity by a magnitude proportional to the magnetization, and penetrating into the superconductor to a depth comparable with the superconducting coherence length. We study this long-range magnetic proximity effect in EuS/Al bilayers and find that the exchange splitting of the BCS peaks is present already in the unpolarized state of the ferromagnetic insulator (EuS), and is being further enhanced when magnetizing the sample by a magnetic field. The measurement data taken at the lowest temperatures feature a high contrast which has allowed us to relate the line shape of the split BCS conductance peaks to the characteristic magnetic domain structure of the EuS layer in the unpolarized state. These results pave the way to engineering triplet superconducting correlations at domain walls in EuS/Al bilayers. Furthermore, the hard gap and clear splitting observed in our tunneling spectroscopy measurements indicate that EuS/Al bilayers are excellent candidates for substituting strong magnetic fields in experiments studying Majorana bound states.

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arXiv2017

Anomalous current in diffusive ferromagnetic Josephson junctions

We demonstrate that in diffusive superconductor/ferromagnet/superconductor (S/F/S) junctions a finite, {\it anomalous}, Josephson current can flow even at zero phase difference between the S electrodes. The conditions for the observation of this effect are non-coplanar magnetization distribution and a broken magnetization inversion symmetry of the superconducting current. The latter symmetry is intrinsic for the widely used quasiclassical approximation and prevent previous works, based on this approximation, from obtaining the Josephson anomalous current. We show that this symmetry can be removed by introducing spin-dependent boundary conditions for the quasiclassical equations at the superconducting/ferromagnet interfaces in diffusive systems. Using this recipe we considered generic multilayer magnetic systems and determine the ideal experimental conditions in order to maximize the anomalous current.

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arXiv2016

Manifestation of extrinsic spin Hall effect in superconducting structures: Non-dissipative magnetoelectric effects

We present a comprehensive quasiclassical approach for studying transport properties of superconducting diffusive hybrid structures in the presence of extrinsic spin-orbit coupling. We derive a generalized Usadel equation and boundary conditions that in the normal state reduce to the drift-diffusion theory governing the spin-Hall effect in inversion symmetric materials. These equations predict the non-dissipative spin-galvanic effect, that is the generation of supercurrents by a spin-splitting field, and its inverse -- the creation of magnetic moment by a supercurrent. These effects can be seen as counterparts of the spin-Hall, anomalous Hall and their inverse effects in the superconducting state. Our theory opens numerous possibilities for using superconducting structures in magnetoelectronics.

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arXiv2016

Andreev spectrum of a Josephson junction with spin-split superconductors

The Andreev bound states and charge transport in a Josephson junction between two superconductors with intrinsic exchange fields are studied. We find that for a parallel configuration of the exchange fields in the superconductors the discrete spectrum consists of two pairs of spin-split states. The Josephson current in this case is mainly carried by bound states. In contrast, for the antiparallel configuration we find that there is no spin-splitting of the bound states and that for phase differences smaller than certain critical value there are no bound states at all. Hence the supercurrent is only carried by states in the continuous part of the spectrum. Our predictions can be tested by performing a tunneling spectroscopy of a weak link between two spin-split superconductors.

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arXiv2016

The $ω$-SQUIPT: phase-engineering of Josephson topological materials

Multi-terminal superconducting Josephson junctions based on the proximity effect offer the bright opportunity to tailor non trivial quantum states in nanoscale weak-links. These structures can realize exotic topologies in multidimensions as, for example, artificial topological superconductors able to support Majorana bound states, and pave the way to emerging quantum technologies and future quantum information schemes. Here, we report the first realization of a three-terminal Josephson interferometer based on a proximized nanosized weak-link. Our tunneling spectroscopy measurements reveal transitions between gapped (i.e., insulating) and gapless (i.e., conducting) states, those being controlled by the phase configuration of the three superconducting leads connected to the junction. We demonstrate the $topological$ nature of these transitions: a gapless state necessarily occurs between two gapped states of different topological index, very much like the interface between two insulators of different topology is necessarily conducting. The topological numbers characterizing such gapped states are given by superconducting phase windings over the two loops forming the Josephson interferometer. Since these gapped states cannot be transformed to one another continuously withouth passing through a gapless condition, these are topologically $protected$. Our observation of the gapless state is pivotal for enabling phase engineering of more sophisticated artificial topological materials realizing Weyl points or the anomalous Josephson effect.

cond-mat.mes-hall↗

arXiv2016

Magnetically-driven colossal supercurrent enhancement in InAs nanowire Josephson junctions

The Josephson effect is a fundamental quantum phenomenon consisting in the appearance of a dissipationless supercurrent in a weak link between two superconducting (S) electrodes. While the mechanism leading to the Josephson effect is quite general, i.e., Andreev reflections at the interface between the S electrodes and the weak link, the precise physical details and topology of the junction drastically modify the properties of the supercurrent. Specifically, a strong enhancement of the critical supercurrent $I_C$ is expected to occur when the topology of the junction allows the emergence of Majorana bound states. Here we report charge transport measurements in mesoscopic Josephson junctions formed by InAs nanowires and Ti/Al superconducting leads. Our main observation is a colossal enhancement of the critical supercurrent induced by an external magnetic field applied perpendicular to the substrate. This striking and anomalous supercurrent enhancement cannot be ascribed to any known conventional phenomenon existing in Josephson junctions including, for instance, Fraunhofer-like diffraction or a $π$-state behavior. We also investigate an unconventional model related to inhomogenous Zeeman field caused by magnetic focusing, and note that it can not account for the observed behaviour. Finally, we consider these results in the context of topological superconductivity, and show that the observed $I_C$ enhancement is compatible with a magnetic field-induced topological transition of the junction.

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arXiv2015

Stimulated quasiparticles in spin-split superconductors

In superconductors spin-split by an exchange field, thermal effects are coupled to spin transport. We show how an oscillating electromagnetic field in such systems creates spin imbalance, that can be detected with a spin-polarized probe. The sign and magnitude of the probe signal result from a competition between processes converting field induced spin energy imbalance to spin imbalance, dominant at low frequencies, and microwave-driven pair breaking at high frequencies. In the presence of spin-flip scattering, we show that ac excitation also leads to multistabilities in the superconducting state.

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arXiv2015

Ferromagnetic insulator-based superconducting junctions as sensitive electron thermometers

We present an exhaustive theoretical analysis of charge and thermoelectric transport in a normal metal-ferromagnetic insulator-superconductor (NFIS) junction, and explore the possibility of its use as a sensitive thermometer. We investigated the transfer functions and the intrinsic noise performance for different measurement configurations. A common feature of all configurations is that the best temperature noise performance is obtained in the non-linear temperature regime for a structure based on an europium chalcogenide ferromagnetic insulator in contact with a superconducting Al film structure. For an open-circuit configuration, although the maximal intrinsic temperature sensitivity can achieve $10$nKHz$^{-1/2}$, a realistic amplifying chain will reduce the sensitivity up to $10$$μ$KHz$^{-1/2}$. To overcome this limitation we propose a measurement scheme in a closed-circuit configuration based on state-of-art SQUID detection technology in an inductive setup. In such a case we show that temperature noise can be as low as $35$nKHz$^{-1/2}$. We also discuss a temperature-to-frequency converter where the obtained thermo-voltage developed over a Josephson junction operated in the dissipative regime is converted into a high-frequency signal. We predict that the structure can generate frequencies up to $\sim 120$GHz, and transfer functions up to $200$GHz/K at around $\sim 1$K. If operated as electron thermometer, the device may provide temperature noise lower than $35$nKHz$^{-1/2}$ thereby being potentially attractive for radiation sensing applications.

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arXiv2015

Pb/InAs nanowire Josephson junction with high critical current and magnetic flux focusing

We have studied mesoscopic Josephson junctions formed by highly $n$-doped InAs nanowires and superconducting Ti/Pb source and drain leads. The current-voltage properties of the system are investigated by varying temperature and external out-of-plane magnetic field. Superconductivity in the Pb electrodes persists up to $ \sim 7$ K and with magnetic field values up to 0.4 T. Josephson coupling at zero backgate voltage is observed up to 4.5 K and the critical current is measured to be as high as 615 nA. The supercurrent suppression as a function of the magnetic field reveals a diffraction pattern that is explained by a strong magnetic flux focusing provided by the superconducting electrodes forming the junction.

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arXiv2014

Spin Hanle effect in mesoscopic superconductors

We present a theoretical study of spin transport in a superconducting mesoscopic spin valve under the action of a magnetic field misaligned with respect to the injected spin. We demonstrate that superconductivity can either strongly enhance or suppress the coherent spin rotation depending on the type of spin relaxation mechanism being dominated either by spin-orbit coupling or spin-flip scattering at impurities. We also predict a hitherto unknown subgap contribution to the nonlocal conductance in multiterminal superconducting hybrid structures which completely eliminates the effect of spin rotation at sufficiently low temperatures.

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arXiv2014

Long-range spin and charge accumulation in mesoscopic superconductors with Zeeman splitting

We describe the far from equilibrium non-local transport in a diffusive superconducting wire with a Zeeman splitting, taking into account the different spin relaxation mechanisms. We demonstrate that due to the Zeeman splitting an injection of a current in a superconducting wire creates a spin accumulation that can only relax via thermalization. In addition the Zeeman splitting also causes a suppression of the spin-orbit and spin-flip scattering rates. These two effects lead to long-range spin and charge accumulations detectable in the non-local signal. Our model explains the main qualitative features of recent experimental results in terms of realistic parameters and predicts a strong dependence of the non-local signal on the orbital depairing effect from an induced magnetic field.

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arXiv2014

Very large thermophase in ferromagnetic Josephson junctions

The concept of thermophase refers to the appearance of a phase gradient inside a superconductor originating from the presence of an applied temperature bias across it. The resulting supercurrent flow may, in suitable conditions, fully counterbalance the temperature-bias-induced quasiparticle current therefore preventing the formation of any voltage drop, i.e., a thermovoltage, across the superconductor. Yet, the appearance of a thermophase is expected to occur in Josephson-coupled superconductors as well. Here we theoretically investigate the thermoelectric response of a thermally-biased Josephson junction based on a ferromagnetic insulator. In particular, we predict the occurrence of a very large thermophase which can reach $π/2$ across the contact for suitable temperatures and structure parameters, i.e., the quasiparticle thermal current can reach the critical current. Such a thermophase can be several orders of magnitude larger than that predicted to occur in conventional Josephson tunnel junctions. In order to assess experimentally the predicted very large thermophase we propose a realistic setup realizable with state-of-the-art nano fabrication techniques and well-established materials which is based on a superconducting quantum interference device. This effect could be of strong relevance in several low-temperature applications, for example, for revealing tiny temperature differences generated by coupling the electromagnetic radiation to one of the superconductors forming the junction.

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arXiv2014

Detection of small exchange fields in S/F structures

Ferromagnetic materials with exchange fields E_ex smaller or of the order of the superconducting gap Delta are important for applications of corresponding (s-wave) superconductor/ ferromagnet/ superconductor (SFS) junctions. Presently such materials are not known but there are several proposals how to create them. Small exchange fields are in principle difficult to detect. Based on our results we propose reliable detection methods of such small E_ex. For exchange fields smaller than the superconducting gap the subgap differential conductance of the normal metal - ferromagnet - insulator - superconductor (NFIS) junction shows a peak at the voltage bias equal to the exchange field of the ferromagnetic layer, eV=E_ex. Thus measuring the subgap conductance one can reliably determine small E_ex < Delta. In the opposite case E_ex > Delta one can determine the exchange field in scanning tunneling microscopy (STM) experiment. The density of states of the FS bilayer measured at the outer border of the ferromagnet shows a peak at the energy equal to the exchange field, E=E_ex. This peak can be only visible for small enough exchange fields of the order of few Delta.

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arXiv2014

Theory of diffusive ϕ0 Josephson junctions in the presence of spin-orbit coupling

We present a full microscopic theory based on the SU(2) covariant formulation of the quasiclassical formalism to describe the Josephson current through an extended superconductor-normal metal- superconductor (SNS) diffusive junction with an intrinsic spin-orbit coupling (SOC) in the presence of a spin-splitting field h. We demonstrate that the ground state of the junction corresponds to a finite intrinsic phase difference 0 < ϕ0 < 2π between the superconductor electrodes provided that both, h and the SOC-induced SU(2) Lorentz force are finite. In the particular case of a Rashba SOC we present analytic and numerical results for ϕ0 as a function of the strengths of the spin fields, the length of the junction, the temperature and the properties of SN interfaces.

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arXiv2014

Proximity nanovalve with large phase-tunable thermal conductance

We propose a phase-controlled heat-flux quantum valve based on the proximity effect driven by a superconducting quantum interference proximity transistor (SQUIPT). Its operation relies on the phase-dependent quasiparticle density of states in the Josephson weak-link of the SQUIPT which controls thermal transport across the device. In a realistic Al/Cu-based setup the structure can provide efficient control of thermal current inducing temperature swings exceeding $\sim100$~mK, and flux-to-temperature transfer coefficients up to $\sim 500$~mK/$Φ_0$ below 100~mK. The nanovalve performances improve by lowering the bath temperature, making the proposed structure a promising building-block for the implementation of coherent caloritronic devices operating below 1~K.

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arXiv2014

Proposal for a phase-coherent thermoelectric transistor

Identifying materials and devices which offer efficient thermoelectric effects at low temperature is a major obstacle for the development of thermal management strategies for low-temperature electronic systems. Superconductors cannot offer a solution since their near perfect electron-hole symmetry leads to a negligible thermoelectric response; however, here we demonstrate theoretically a superconducting thermoelectric transistor which offers unparalleled figures of merit of up to $\sim 45$ and Seebeck coefficients as large as a few mV/K at sub-Kelvin temperatures. The device is also phase-tunable meaning its thermoelectric response for power generation can be precisely controlled with a small magnetic field. Our concept is based on a superconductor-normal metal-superconductor interferometer in which the normal metal weak-link is tunnel coupled to a ferromagnetic insulator and a Zeeman split superconductor. Upon application of an external magnetic flux, the interferometer enables phase-coherent manipulation of thermoelectric properties whilst offering efficiencies which approach the Carnot limit.

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arXiv2014

Spin-orbit coupling as a source of long-range triplet proximity effect in superconductor-ferromagnet hybrid structures

We investigate the proximity effect in diffusive superconducting hybrid structures with a spin-orbit (SO) coupling. Our study is focused on the singlet-triplet conversion and the generation of long-range superconducting correlations in ferromagnetic elements. We derive the quasiclassical equations for the Green's functions including the SO coupling terms in form of a background SU(2) field. With the help of these equations, we first present a complete analogy between the spin diffusion process in normal metals and the generation of the triplet components of the condensate in a diffusive superconducting structure in the presence of SO coupling. From this analogy it turns out naturally that the SO coupling is an additional source of the long-range triplet component (LRTC) besides the magnetic inhomogeneities studied in the past. We demonstrate an explicit connection between an inhomogeneous exchange field and SO coupling mechanisms for the generation of the LRTC and establish the conditions for the appearance of the LRTC in different geometries. We also consider a S/F bilayer in contact with normal metal with SO coupling and show that the latter can be used as a source for the LRTC. Our work gives a global description of the singlet-triplet conversion in hybrids structures in terms of generic spin-fields and our results are particularly important for the understanding of the physics underlying spintronics devices with superconductor elements.

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arXiv2014

Scanning tunneling spectroscopy study of the proximity effect in a disordered two-dimensional metal

The proximity effect between a superconductor and a highly diffusive two-dimensional metal was revealed in a Scanning Tunneling Spectroscopy experiment. The in-situ elaborated samples consisted of superconducting single crystalline Pb islands interconnected by a non-superconducting atomically thin disordered Pb wetting layer. In the vicinity of each superconducting island the wetting layer acquires specific tunneling characteristics which reflect the interplay between the proximity-induced superconductivity and the inherent electron correlations of this ultimate diffusive two-dimensional metal. The observed spatial evolution of the tunneling spectra was accounted for theoretically by combining the Usadel equations with the theory of dynamical Coulomb blockade; the relevant length and energy scales were extracted and found in agreement with available experimental data.

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