Non-equilibrium superconductivity and spintronics

09:05-09:30 Robust superconductivity coexisting with ferromagnetism display unusual behaviour

Dr Jagadeesh S Moodera, Massachusetts Institute of Technology, USA

Abstract

Proximity coupling across superconductor-ferromagnet bilayers can give rise to the triplet component of the superconducting condensate. Superconductivity and ferromagnetism have been reported to coexist in a Ni/Bi bilayer system. Thus spin polarised triplet supercurrent in ferromagnetic-superconducting Josephson junctions can be expected. The Moodera group has investigated the complex and rich behaviour of this phenomenon in bilayers of Ni/Ga and Ni/Bi systems, including Josephson and quasiparticle tunnelling. Ni/Ga as well as Ni/Bi bilayer systems show unusual superconductivity, with high T_c that can co-exist with ferromagnetism. Tunnelling spectroscopy studies at low temperatures show the presence of three superconducting energy gaps in the bilayers, attributable to surface, interface and bulk states within the bilayers. Ni layers, ranging from 0.8 to 6nm thick, confirmed to be ferromagnetic by the magnetisation studies while spin polarised tunnelling studies revealed that the tunnelling electrons coming from the Ni surface were spin polarised and simultaneously displayed superconducting gap, supporting the co-existence of SC and FM. The interplay of SC and FM with the presence of spin polarised carriers in such bilayer system could be a strong case for triplet pairing. In addition, the observed Josephson current could be spin polarised. Interestingly, the superconductivity in the Ni/Bi bilayer is expected to be topological. The occurrence of zero bias conductance may reflect odd-frequency symmetry in the superconducting condensate, supporting the presence of a non-zero component associated with triplet pair superconductivity insensitive to disorder. Jagadeesh Moodera will present this ongoing work leaving it open for discussion. This work has been done in collaboration with Madison Sutula, Sebastian Bergeret, Jia Song, Valeria Lauter and Niladri Banerjee.

09:45-10:15 Transport properties of topological superconducting hybrid structures

Dr Cecilia Holmqvist, Linnaeus University, Sweden

Abstract

Dirac materials with strong spin-orbit interaction have been shown to generate large surface spin accumulations in response to applied currents. Such materials have, in addition, been demonstrated to exert spin-orbit torques on adjacent ferromagnetic structures. This magnetoelectric effect in these materials is strong due to the efficient spin-momentum locking. In heterostructures consisting of superconductors and three-dimensional superconductors, this spin-momentum locking leads to an induced unconventional superconductivity that may be useful for superconducting spintronics. Here, Cecilia Holmqvist investigates theoretically the quantum transport properties of a ballistic junction consisting of two topological superconductors coupled over a quantum dot that is coupled to a ferromagnet. The spin-orbit torques acting on the ferromagnet are examined and are shown to depend strongly on the magnetisation direction relative to the current direction.

10:30-11:00 Coffee

11:00-11:30 Magnetic moment manipulation by a superconducting current in Josephson junctions

Professor Alexander Buzdin, University of Bordeaux, France

Abstract

Recently several mechanisms realising the direct coupling between magnetic moment and Josephson current in S/F/S junctions have been proposed. In such junctions, the ac Josephson effect may generate a magnetic precession providing then a feedback to the current. Magnetic dynamics results in several anomalies of current-phase relations (second harmonic, dissipative current) which are strongly enhanced near the ferromagnetic resonance frequency. The simulations of magnetic moment dynamics show that by applying an electric current pulse, it may be possible to realise the full magnetisation reversal which is quite important for the elaboration of superconducting spintronic devices with low dissipation.

11:45-12:15 Superconducting field-effect transistors go metal

Professor Francesco Giazotto, National Enterprise for Nanoscience and Nanotechnology, Italy

Abstract

In their original formulation of superconductivity, the London brothers predicted the exponential suppression of an electrostatic field inside a superconductor over the London penetration depth. Despite a few experiments indicating hints of perturbation induced by electrostatic fields, no clue has been provided so far on the possibility to manipulate conventional superconductors via field-effect. In this talk, Francesco Giazotto will show the evidence of full field-effect control of the supercurrent in all-metallic transistors made of different BCS superconducting films. At a low temperature, the field-effect transistors (FETs) show a monotonic decay of the critical current under increasing electrostatic field up to total quenching for gate voltage values as large as ±40V in titanium-based devices. A similar behaviour, though less pronounced, was observed in aluminum FETs. In addition, Francesco will report on the realisation of Ti-based Dayem bridge Josephson field-effect transistors. The latter show full suppression of I_C for gate voltages as low as ±8V. Finally, Francesco will show the behaviour of mesoscopic superconductor-normal metal-superconductor Josephson field-effect transistors that will reveal the impact of electrostatic fields even on proximity metals thereby suggesting that the field effect is universal. Possible electronic and circuital schemes based on this all-metallic technology will be discussed.

13:30-14:00 Voltage-control over a high-field superconducting transition and the superconducting exchange interaction

Professor Jacob Linder, Norwegian University of Science and Technology, Norway

Abstract

Jacob Linder’s group predicts two interesting phenomena related to superconductivity in hybrid structures driven out of equilibrium by application of an electric voltage. Firstly, they theoretically demonstrate that superconductivity in thin films can be stabilised in high magnetic fields if the superconductor is driven out of equilibrium by a voltage bias. For realistic material parameters and temperatures, they show that superconductivity is restored in fields many times larger than the Chandrasekhar–Clogston limit. After motivating the effect analytically, they present rigorous numerical calculations to corroborate the findings, and discuss concrete experimental signatures. Secondly, Linder discusses how the magnetic exchange interaction in a spin-valve is influenced by non-equilibrium superconductivity. Linder shows that the sign of the exchange interaction in a spin-valve, determining whether a parallel or antiparallel magnetic configuration is favoured, can be controlled via an electric voltage. This occurs due to an interplay between a non-equilibrium quasiparticle distribution and the presence of spin-polarized Cooper pairs. These findings may be of relevance for spin-based superconducting devices which in practice most likely have to be operated precisely by non-equilibrium effects.

14:15-14:45 Comparison of the spin-transfer torque mechanisms in three terminal spin-torque-oscillators

Dr Emilie Jué, National Institute of Standards and Technology, USA

Abstract

The spin transfer torque is one of the most active field of spintronics due to its potential for use in memory and logic applications. This control can be achieved via a spin polarised current with the mechanism of spin-filtering torque (SFT) or through a pure spin current via the mechanism of spin-orbit torque (SOT). Over the past several years, SOT has gained increased attention due to the new possibilities that it offers for data storage applications. However, the quantification and comparison of both mechanisms’ efficiencies remains uncertain, due to the uncertainty in material parameters needed to quantify the torque. In this work, researchers at NIST compared for the first time the SFT and SOT efficiencies acting on the same nanomagnetic element. To do so, they created 3-terminal spin-torque oscillators (STO) composed of spin-valves (SV) patterned on top of Pt nanowires. The devices are excited either by SFT or by SOT depending on whether the current is applied through the SV or through the Pt wire. By comparing the magnetization dynamics obtained with the different STT mechanisms, they quantify the relative efficiencies of the SOT and SFT in the system as a function of the dimensions of the SV and Pt nanowires.

15:00-15:30 Tea

15:30-16:00 Ferromagnetic resonance studies of ferromagnets and superconductors

Dr Chiara Ciccarelli, University of Cambridge, UK

Abstract

Ferromagnetic resonance is a powerful method to extract information on the magnetic torques, spin damping and magnetic anisotropies. In Chiara Ciccarelli’s group ferromagnetic resonance methods are applied to quantitatively evaluate the spin torques in asymmetric ferromagnets, magnetic insulators and synthetic antiferromagnets. The focus of this talk will be on some recent work where spin pumping experiments are conducted on HM/SC/FM and SC/FM structures (SC=niobium, HM=platinum and FM=permalloy). Ferromagnetic resonance in the permalloy is excited via a waveguide and the damping measured through the superconducting transition temperature of niobium. These results show that while in the SC/FM structures spin pumping is suppressed when the niobium turns superconducting, in the HM/SC/FM structures the presence of platinum leads to an enhanced spin transfer through superconducting niobium with respect to its normal state and even with respect to bare platinum.

16:15-16:45 Anomalous Meissner effect in superconductor-ferromagnet proximity systems

Dr Machiel Flokstra, University of St Andrews, UK

Abstract

In mesoscopic superconducting systems the Meissner response of a superconducting film can be very different from its bulk behaviour. It was theoretically shown that adding a normal metal (N) to a mesoscopic superconductor (S) can lead to a greatly enhanced Meissner screening, opposite to the case of adding a homogeneous ferromagnet (F) which is expected to reduce the screening due to its strong pair-breaking effect. However, a recently developed theory shows that for the S/F case, spontaneous screening currents can appear, generated by the vector-potential of magnetisation near the S/F interface. Using low energy muon spin spectroscopy, which is an exquisite tool to probe the local flux inside a thin film, Michiel Flokstra presents experimental data on various N/S/F type systems and observe enhanced screening due to the addition of a normal metal, as well as clear signatures of the newly predicted electromagnetic proximity effect generated at the S/F interfaces.

17:00-18:00 Poster session

09:00-09:30 Universal correspondence between edge spin accumulation and equilibrium spin-currents in nanowires with spin-orbit coupling

Dr Sebastian Bergeret Material Physics Center (CFM-CSIC), Spain

Abstract

In 2003 Rashba himself demonstrated that any system described by the standard two-dimensional Rashba Hamiltonian may support spin currents even in thermodynamical equilibrium. However, he also concluded that such currents are nontransport currents in a customary sense and therefore not detectable. Since then it is a common belief that equilibrium currents cannot lead to a spin accumulation. In this talk Sebastian Bergeret demonstrates that this belief is not necessarily correct. Specifically the interplay between a Zeeman field and the spin-orbit coupling in nanowires leads to an equilibrium spin current that manifests itself in a spin accumulation at the wire ends with a polarisation perpendicular to both fields. This is a universal property that occurs in both the normal and superconducting state, independently of the degree of disorder. The edge spin polarisation transverse to the Zeeman field is strongly enhanced in the superconducting state when the Zeeman energy is of the order of the superconducting gap. This hitherto unknown transverse magnetic susceptibility can be much larger than the longitudinal one, and hence it drastically changes the paramagnetic response of the wire.

09:45-10:15 Thermoelectric effects in superconductor-ferromagnet structures

Professor Detlef Beckmann, Karlsruhe Institute of Technology, Germany

Abstract

Detlef Beckmann covers the experimental observation of spin-dependent thermoelectric effects in superconductor-ferromagnet tunnel junctions in high magnetic fields. The thermoelectric signals are due to a spin-dependent lifting of particle-hole symmetry on the energy scale of the superconducting gap. Due to the small energy scale, the thermoelectric effects can be quite large, and a maximum Seebeck coefficient of about 100 µV/K can be inferred from the data. The thermoelectric signals can be further enhanced by an exchange splitting induced by the proximity effect with a ferromagnetic insulator. The results directly prove the coupling of spin and heat transport in high-field superconductors, which also leads to nonlocal thermoelectric effects in multiterminal devices where several ferromagnetic wires are attached to a single superconducting wire via tunnel junctions. In these structures, heating one of the ferromagnetic wires leads to a nonlocal thermoelectric current in a remote wire, over distances of the order of 10 µm.

10:30-11:00 Coffee

11:00-11:30 Generation of pure superconducting spin current in magnetic heterostructures via non-locally induced magnetism

Dr Xavier Montiel, Royal Holloway, University of London, UK

Abstract

The researchers of RHUL propose a mechanism for the generation of pure superconducting spin-current carried by equal-spin triplet Cooper pairs in a superconductor (S) sandwiched between a ferromagnet (F) and a normal metal (Nso) with intrinsic spin-orbit coupling. They show that in the presence of Landau Fermi-liquid interactions the superconducting proximity effect can induce non-locally a ferromagnetic exchange field in the normal layer, which disappears above the superconducting transition temperature of the structure. The internal Landau Fermi-liquid exchange field leads to the onset of a spin supercurrent associated with the generation of long-range spin-triplet superconducting correlations in the trilayer. They demonstrate that the magnitude of the spin supercurrent as well as the induced magnetic order in the Nso layer depends critically on the superconducting proximity effect between the S layer and the F and Nso layers and the magnitude of the relevant Landau Fermi-liquid interaction parameter. Their results demonstrate the crucial role of Landau Fermi-liquid interaction in combination with spin-orbit coupling for the creation of spin supercurrent in superconducting spintronics.

11:45-12:15 Magnetoelectric effects in S/F hybrids and Josephson detection of magnetisation dynamics

Dr Irina Bobkova, Institute of Solid State Physics of RAS and Moscow Institute of Physics and Technology, Russia

Abstract

It is demonstrated that the hybrid structures consisting of superconductors and spin-textured ferromagnets exhibit variety of equilibrium magnetoelectric effects originating from coupling between conduction electron spin and supercurrent. It is discussed that in S/F/S Josephson junctions the magnetoelectric effect, which takes the form of the anomalous ground phase shift, is a manifestation of a generic supercurrent-mediated interaction between localised spins that breaks the global inversion symmetry of magnetic moments. The proposed interaction mechanism is capable of removing fundamental degeneracies between topologically distinct magnetic textures. The prospects of using the coupling between the superconducting condensate phase and the magnetisation for Josephson detection of magnetisation dynamics and topology are discussed. In a thin film S/F bilayer geometry as direct so as inverse magnetoelectric effects are predicted. The essence of the direct magnetoelectric effect is that a supercurrent generates spin polarisation in the superconducting film which is non-coplanar with the local ferromagnetic moment. The resulting dissipationless spin torques are discussed. The inverse magnetoelectric effect in such structures is shown to result in the spontaneous phase difference across the magnetic topological defects such as a domain wall and helical spin texture. The possibility to detect domain wall motion through this effect is discussed.

13:30-14:00 Electrically induced and detected Néel vector reversal in a collinear antiferromagnet

Dr Joerg Wunderlich, Hitachi Cambridge Lab, UK, and Institute of Physics of the Academy of Science, Czech Republic

Abstract

Antiferromagnets are enriching spintronics research by many favorable properties that include insensitivity to magnetic fields, neuromorphic memory characteristics, and ultra-fast spin dynamics. Designing memory devices with electrical writing and reading is one of the central topics of antiferromagnetic spintronics. So far, such a combined functionality has been demonstrated via 90° reorientations of the Néel vector generated by the current-induced spin-orbit torque and sensed by the linear-response anisotropic magnetoresistance. In this presentation Joerg Wunderlich will explain the group’s understanding of electrical switching and detection of antiferromagnetic states. He will also show that they can electrically control 180° Néel vector reversals by simply switching the polarity of the writing current. Finally, Joerg will explain how to distinguish two stable states with opposite Néel vector orientations in a collinear antiferromagnet with broken time reversal and spatial inversion symmetries by measuring a second-order magnetoresistance effect.

14:15-14:45 Phase-controllable Josephson junctions and cryogenic memory

Professor Norman Birge, Michigan State University, USA

Abstract

Josephson junctions containing ferromagnetic (F) materials are being developed for use in cryogenic random access memory. In principle, either the critical current amplitude or the ground-state phase difference across the junction could be used to store information. In the Northrop Grumman JMRAM memory architecture, the ferromagnetic junction acts as a passive phase shifter in a SQUID loop that also contains two SIS junctions. Phase control has now been demonstrated in two kinds of Josephson junctions: in a simple spin-valve device containing two ferromagnetic layers and in a more complex multi-layer device that carries spin-triplet supercurrent. In both types of devices, the ground-state phase difference can be controllably switched between 0 and π by reversing the magnetisation direction of one of the magnetic layers in the device. The physical mechanism by which the ground-state phase difference changes, however, is different in the two types of devices. Both types of devices require further optimisation to be suitable for large-scale superconducting memory arrays. This talk will review the physics underlying the behaviour of both types of ferromagnetic Josephson junctions, and discuss progress in optimising devices for use in cryogenic memory.

15:00-15:30 Tea

15:30-16:00 Surface superconducting property proved by pure spin current

Professor Takashi Kimura, Kyushu University, Japan

Abstract

The transport properties for the charge and spin currents in a normal metal/superconductor interface have been investigated by using a pure spin current generated in a specially-developed lateral spin valve. Owing to the efficient reduction of the Joule heating, Takashi Kimura’s group were able to observe the temperature and bias-current dependences of the spin absorption propertis in the Cu/Nb bilayer system. From the temperature dependence of the spin signal, the superconducting gap of the Nb was found to open gradually with decreasing the temperature. The group also demonstrated the spin transport through a Nb/Cu/Nb Josephson junction, indicating that Cooper pairs and spin accumulation coexist in the Cu channel.

16:15-16:45 Superconducting quantum-classical information processing systems

Dr Oleg Mukhanov, HYPRES Inc, USA

Abstract

Superconducting rapid single flux quantum (RSFQ) logic is now used in radio frequency signal digital receivers. Sophisticated superconducting SFQ-based digital processors and memories are being developed for the next generation energy efficient data centres. Recently, RSFQ and its energy-efficient successor logics were used to realize classical control for quantum processors. Traditionally, control operations are performed at room temperature leading to scalability limitations and long latency in quantum operations. For scalable low-latency qubit control, the classical electronics should located as close to quantum chips as possible. Superconducting SFQ circuits proximally located to the quantum processor can be the technology of choice due to its low power (10-21 Joule to per switching at 20 mK). They can be engineered to produce the minimal back action to qubits. The ability to operate at very high speed (tens of gigahertz clock) opens a way for digitizing and fast processing qubit output data for error correction and generation of qubit control signals. Furthermore, hybrid systems integrating together the quantum and classical processing hardware units are envisioned to enable various application algorithms which typically combine quantum and classical algorithmic modules. The implementation of scalable quantum-classical 3D integrated system extending across multiple temperature stages will be discussed.