2020 Abstracts Coherent Order

Chiara CICCARELLI

Unlike conventional spin-singlet Cooper pairs, spin-triplet pairs can carry spin. Triplet supercurrents were discovered in Josephson junctions with metallic ferromagnet spacers, where spin transport can occur only within the ferromagnet and in conjunction with a charge current. Ferromagnetic resonance injects a pure spin current from a precessing ferromagnet into adjacent non-magnetic materials. For spin-singlet pairing, the ferromagnetic resonance spin pumping efficiency decreases below the critical temperature (Tc) of a coupled superconductor. Here we present ferromagnetic resonance experiments in which spin sink layers with strong spin–orbit coupling are added to the superconductor. We show that the induced spin currents, rather than being suppressed, are substantially larger in the superconducting state compared with the normal state and show that this cannot be mediated by quasiparticles and is most likely a triplet pure spin supercurrent.

Bart van Wees

Graphene-based van der Waals heterostructures have shown to be an excellent choice for twodimensional (2D) spintronic devices as the superior spin and charge transport properties of graphene are enriched via the proximity to other 2D materials. By proximity effects, one can induce spin-orbit and magnetic exchange interactions in the graphene which provide strong coupling between charge and spin currents [1-4]. In particular, our recent spin transport measurements in graphene in proximity of a 2D interlayer antiferromagnet, Chromium Sulfide Bromide (CrSBr) have shown strong spin polarization of conductivity in graphene (~14%) that arises from a large induced exchange interaction. The strong spin-polarization of conductivity also results in the observation of the spindependent Seebeck effect in graphene. This is the first-time experimental realization of the active role of the magnetic graphene in the electrical and thermal generation of spin currents, addressing the most technologically relevant aspects of the magnetism in graphene. Also, the high sensitivity of the spin-transport in graphene to the magnetization of the outer-most layer of the CrSBr provides the tool for studying the magnetic behavior of a single magnetic sublattice. The spin-polarization of conductivity and spin-dependent Seebeck coefficient in magnetic graphene, together with its exceptional long-distance spin transport introduce the magnetic graphene as an ultimate building block for ultra-compact magnetic memory and sensory devices and provides substantial advances in 2D spintronic and caloritronic systems [4].

[1] Ghiasi, T.S., et al., Nano letters 17.12 (2017): 7528-7532
[2] Ghiasi, T.S., et al., Nano letters 19.9 (2019): 5959-5966
[3] Avsar, A., et al., Reviews of Modern Physics 92.2 (2020): 021003
[4] Ghiasi, T.S., et al., arXiv:2007.15597 (2020), submitted to Nature Nanotechnology

Andrew Mackenzie

I will describe the work of colleagues in my Institute and beyond on the discovery of twophase unconventional superconductivity in CeRh2As2. Using thermodynamic probes, we establish that the superconducting critical field of its high-field phase is as high as 14 T, remarkable in a material whose transition temperature is 0.26 K. Furthermore, a c-axis field drives a transition between two different superconducting phases. In spite of the fact that CeRh2As2 is globally centrosymmetric, we show that local inversion symmetry breaking at the Ce sites enables Rashba spin-orbit coupling to play a key role in the underlying physics.

Tomosato Hioki

In recent years, it was demonstrated that spin angular momentum transfer in magnetic insulators is enhanced or modulated by hybridizing magnons with phonons [1-3]. The hybridization refers to formation of new normal state as a result of interaction between two different excitations. By the hybridization with phonons, some characteristics of phonons are transferred to magnons such as long lifetime and mode degree of freedom.
In this talk, I will discuss the coherent interconversion between magnons and phonons, and demonstration of transfer of mode degree of freedom of phonons to magnons. We developed time-resolved magneto-optical imaging technique by combining conventional magneto-optical imaging and pump-and-probe spectroscopy, which enables us to obtain snapshots of spin-wave propagation dynamics in real space with sub nanoseconds temporal resolutions. In a Bi-doped magnetic garnet, Lu2Bi1Fe3.4Ga1.6O12, we observed coherent temporal oscillation between magnons and phonons as a result of hybridization, where magnons and phonons are coherently interconverted to each other during propagation. It is also found that the magnon-phonon hybridized wave exhibits abnormal reflection at the sample edge owing to the mode degree of freedom of phonons [4]. Since phonons have longitudinal and transverse modes, both modes may not be an eigenstate where translational symmetry is broken down, such as a sample edge. Owing to the mode degree of freedom, the hybridized wave may split into two reflected waves with the same frequency, which is not the case for pure magnon propagation. The experimental demonstration of these dynamics of magnon-phonon hybridized waves will be reported in the talk.

[1] T. Kikkawa, K. Shen, B. Flebus, R. A. Duine, K. Uchida, Z. Qiu, G. E. W. Bauer, and E. Saitoh, Phys. Rev. Lett, 117, 207203 (2016)
[2] J. Holanda, D. S. Maior, A. Azevedo, and S. M. Rezende, Nat. Phys. 14, 500-506 (2018)
[3] K. An, A. N. Litvinenko, R. Kohno, A. A. Fuad, V. V. Naletov, L. Vila, U. Ebels, G. de Loubens, H. Hurdequint, N. Beaulieu, J. Ben Youssef, N. Vukadinovic, G. E. W. Bauer, A. N. Slavin, V. S. Tiberkevich, and O. Klein, Phys. Rev. B, 101, 060407(R) (2020)
[4] T. Hioki, Y. Hashimoto, E. Saitoh, Commun. Phys. 3, 188 (2020)

Irina Bobkova

It has been commonly accepted that electromagnetic fields suppress superconductivity by inducing the ordered motion of Cooper pairs. We demonstrate a mechanism which instead provides generation of perconducting correlations by moving the superconducting condensate. This effect arises in superconductor/ferromagnet heterostructures in the presence of spin-orbit coupling. We predict the odd-frequency spin-triplet superconducting correlations called the Berezinskii order to be switched on in ferromagnets at large distances from the superconductor/ferromagnet interface by application of a static magnetic field or irradiation inducing condensate motion. In the last case the induced spin-triplet superconducting order is
dynamical. The effect is shown to result in the unusual dependence of Josephson current on the applied magnetic field and possibility of a photo-induced Josephson current.

Tero Heikkilä

In this tutorial I will describe how the proximity to magnets affects the nonequilibrium properties of superconductors [1,2]. I will in particular emphasize the peculiar superconducting state hosting a spin-splitting field that can be induced into thin superconducting films via in-plane magnetic fields or by their proximity to ferromagnets. Moderate spin splitting does not destroy superconductivity, but results in the presence of an odd-frequency pairing state that also affects the transport properties of the superconductor in a fundamental manner. These transport properties can be studied via tunnelling experiments. In case of local measurements, the spin splitting shows up as a giant thermoelectric effect, whereas the non-local case demonstrates a coupling of the different nonequilibrium modes in the superconductor. The thermoelectric effect can be used for example in new types of superconducting detectors [3]. The dynamic features of the superconducting state are also strongly modified by the spin-splitting field. If the time allows, I will exemplify this via the examples of thermoelectric torques and spin cooling [4], and of the dynamic coupling of magnetization precession and the Higgs amplitude mode in superconductors [5].

[1] F.S. Bergeret, M. Silaev, P. Virtanen, and T.T. Heikkilä, Rev. Mod. Phys. 90, 041001 (2018)
[2] T.T. Heikkilä, M. Silaev, P. Virtanen, and F.S. Bergeret, Prog. Surf. Sci. 94, 100540 (2019)
[3] T.T. Heikkilä, R. Ojajärvi, I.J. Maasilta, E. Strambini, F. Giazotto, and F. Bergeret, Phys. Rev. Applied 10, 034053 (2018)
[4] R. Ojajärvi, J. Manninen, T.T. Heikkilä, and P. Virtanen, Phys. Rev. B 101, 115406 (2020)
[5] M. Silaev, R. Ojajärvi, and T.T. Heikkilä, Phys. Rev. Research 2, 033416 (2020)

So Takei

We propose an experimental method utilizing a strongly spin-orbit coupled metal to quantum magnet bilayer that will probe quantum magnets lacking long-range magnetic order via examination of the voltage noise spectrum in the metal layer. The bilayer is held in thermal equilibrium, and spin fluctuations arising across the single interface are converted into voltage fluctuations in the metal as a result of the inverse spin Hall effect (ISHE). We elucidate the theoretical workings of the proposed bilayer system and provide precise predictions for the frequency characteristics of the enhancement to the AC electrical resistance measured in the metal layer for three candidate quantum spin liquid models: the quantum Heisenberg antiferromagnet on the kagomé lattice, fermionic spinons coupled to a U(1) gauge field, and the Kitaev model in the gapless spin liquid phase. The ISHE-facilitated spin noise probe is then applied to quantum spin systems hosting elementary bosonic excitations with topologically nontrivial band structures. We show how the method can be used to detect topological phase transitions in these systems by directly probing the topologically-protected fractional spin excitations localized at their edges.

Rembert Duine

In this talk I will review spin currents in various materials and how they may be carried by electronic quasiparticles, collective excitations, or non-linear dynamics of, for example, the magnetic order. Examples will be discussed of magnon, phonon, and other types of spin transport.

Burkard Hillebrands

A room-temperature magnon Bose-Einstein condensate (BEC) observed in magnetic insulators (single-crystal films of yttrium iron garnet, YIG) has a large potential in high-speed and low-power information processing and data transfer. At the same time, the miniaturization of magnon BEC-based magnonic devices constitutes an extraordinary challenge for their future applications.
We present a new and universal approach to enable Bose–Einstein condensation of magnons in confined systems [1]. The essential feature of this approach is the introduction of a disequilibrium of magnons with the phonon bath. After heating to an elevated temperature, a sudden decrease in the temperature of the phonons, which is approximately instant on the time scales of the magnon system, results in a large excess of incoherent magnons. The consequent spectral redistribution of these magnons triggers the Bose–Einstein condensation. We have observed this phenomenon by time-resolved Brillouin light scattering spectroscopy.
Moreover, we have studied by numerical simulations the formation of the magnon BEC in parametrically excited nanoscopic systems and proposed a new way to enhance condensate’s lifetime of by lateral confinement [2]. We revealed the role of dipolar interactions in the generation of a magnon BEC as a metastable state in YIG ultrathin film structures. We directly map out the nonlinear magnon scattering processes to show how fast quantized thermalization channels allow the BEC formation in confined structures.
Both our investigations greatly extends the freedom to study dynamics of magnon BEC in confined systems and to design integrated circuits for magnon BEC-based applications at room temperature.

[1] M. Schneider et al., Bose–Einstein condensation of quasiparticles by rapid cooling, Nat. Nanotechnol. 15, 457 (2020).
[2] M. Mohseni et al., Bose-Einstein condensation of nonequilibrium magnons in confined systems, New J. Phys. 22, 083080 (2020).

Yaroslav Tserkovnyak

We theoretically study the entanglement between two arbitrary spins in a magnetic material where magnons naturally form a general squeezed coherent state in the presence of an applied magnetic field and axial anisotropies. Employing concurrence as a measure of entanglement, we demonstrate that spins are generally entangled in thermodynamic equilibrium, with the amount of entanglement controlled by the external fields and anisotropies. As a result, the magnetic medium can serve as a resource to store and process quantum information. We furthermore show that the entanglement can jump discontinuously when decreasing the transverse magnetic field. This tunable entanglement can be potentially used as an efficient switch in quantum-information processing tasks.