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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - SuperMagnonics (Supercurrents of Magnon Condensates for Advanced Magnonics)

Teaser

Summary

With the fast growth in the volume of information being processed, researchers are charged with the primary task of finding new ways for fast and efficient processing and transfer of data. Dynamic eigen-excitations of a magnetically ordered bodyâ€”spin waves and their quanta magnonsâ€”open up a very promising branch of high-speed and low-power information processing. Magnons can be excited, manipulated and detected at room temperature. They operate in the microwave frequency regime with wavelengths reaching far into the sub-micrometer regime and with the potential to go even farther beyond.

Magnons are bosons, and thus they are able to form spontaneously a spatially extended, coherent macroscopic quantum state described by a single coherent wavefunctionâ€”magnon Bose-Einstein condensates (BEC). Magnon BEC can be established independently of the magnon excitation mechanism and, most importantly, can be realized at room temperature. The information transfer can be realized by means of magnon supercurrents, which constitute the transport of angular momentum, driven by a phase gradient in the magnon-condensate wave function.

Magnon condensates and supercurrents will offer unprecedented opportunities to address novel, emergent, fundamental perspectives for the investigation of macroscopic quantum phenomena and their potential applications. Nowadays, an extraordinary challenge is the use of macroscopic quantum phenomena such as the magnon BEC and supercurrents for the information transfer and processing at room temperature. Potential applications range from logic devices with ultra-low dissipation to ultra-sensitive magnetic field sensors, extending concepts, which have already been developed for superconducting devices.

The SuperMagnonics project aims at the realization and investigation of macroscopic quantum transport phenomena at room temperature as a novel approach for information processing technology. The key idea is to use magnon supercurrents, which are currents of angular momentum, carried by magnons, excitations in a magnetic medium. The material class of choice is magnetic insulators, with no movement of electrons, where Ohmic losses are avoided. SuperMagnonics will pioneer the generation, processing and detection of magnonic supercurrents. Therefore, physical concepts as well as a technological basis should be established on the current stage to achieve the ambitioned goals of magnonic macroscopic quantum transport and data processing in the future.

To reach these ambitioned goals the SuperMagnonics project specifically addresses several significant and strongly interlinked scientific objectives:

* Establishing the foundation of magnon supercurrent physics. The existing knowledge relating to magnon condensates will be expanded according to the needs of other scientific objectives.

* Investigation of magnon supercurrents induced by a phase gradient. It focuses on the realization of magnon supercurrents in different physical environments, which will allow for the ascending control of supercurrent formation, manipulation, and detection.

* Investigation of Josephson-like magnon supercurrents and the realization of magnonic Josephson junctions. It covers studies of connected magnonic macroscopic quantum states with a supercurrent driven by the phase difference, addressing AC/DC Josephson effects. Specifically, the direct creation of a constant phase difference via local microwave injection is addressed here for the DC Josephson effect and the coupling strength between the condensates will be controlled by tunneling barriers of different origin.

* Realization of magnon supercurrents controlled by an electric potential. For this, the magnon version of the Aharonov-Casher effect will be exploited, where the phase of a magnon condensate and, thus, a persistent supercurrent, is controlled by an electric field. This approach will allow for fundamentally new means of magnon control.

Work performed

The SuperMagnonics project is running successfully and close to the proposed plan. Work performed from the beginning of the project resulted in a number of scientific achievements. All of them were advertised on a large variety of conferences and published in a number of articles. The main results of the performed work are:

* Successful creation of magnon supercurrents:
The evolution of an overpopulated gas of magnons to a Bose-Einstein condensate and excitation of a magnon supercurrent, propelled by a phase gradient in the condensate wave function, was investigated at room temperature in an yttrium iron garnet film in a wide range of external magnetic fields. It was found that for the defined magnetic field regions opening the kinetic instability channel, which is a two-stage decay of pumped magnons, leads to the formation of a much denser magnon condensate and to a stronger magnon supercurrent compared to the cascade mechanism of magnon thermalization [1].

* Transport using magnon supercurrents and Boboliubov waves:
Using the spatially resolved probing of the dynamics of a magnon BEC, direct evidence of supercurrent-related motion of the condensate outwards from the heated laser spot is provided. The observed occurrence of the magnon BEC propagation outside of the temperature gradient is associated with the excitation of a new type of the second sound: magnon second soundâ€”a wave of the density of a magnon condensate. The direct experimental observation of a long-distance spin transport in such a system is presented [2]. The condensed magnons being pushed out from the potential well within the heated area form a density wave, which propagates through the BEC many hundreds of micrometers in the form of a specific second sound pulseâ€”Bogoliubov wavesâ€”and even is reflected from the sample edge.

* BEC formation by rapid cooling:
The fundamentally new phenomenon of microwave-free Bose-Einstein condensation of magnon quasiparticles by rapid cooling of the system is discovered in an individual magnetic nanostructure [3]. BEC of magnons has been achieved in a typical spintronic YIG/Pt structure by the application of low-power current pulses rather than by the usage of high-power microwave pumping. The critical point to 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.

* Creation of a space-time crystal:
The experimental realization of a space-time crystal with tunable periodicity in time and space in the magnon Bose-Einstein condensate is reported [4]. It is formed in a room-temperature magnon BEC having a well-defined energy and non-zero wavevector and prepared in a yttrium iron garnet (YIG) film by radio-frequency space-homogeneous magnetic field. It is demonstrated how the crystalline â€œdensityâ€ as well as the time and space textures of the resulting crystal may be tuned by varying the experimental parameters: external static magnetic field, temperature, thickness of the YIG film and power of the radio-frequency field. The proposed space-time crystal mechanism provides a new dimension for exploring dynamical phases of matter and can serve as a model nonlinear Floquet system, that brings in touch the rich fields of classical nonlinear waves, magnonics and periodically driven systems.

* Transverse spin currents in coherent magnon transport:
Traditionally, it was assumed, that a spin wave in a free magnetic film can transfer energy and angular momentum only along its propagation direction. Using the data of BLS spectroscopy in combination with an extended theory of dipole-exchange spin-wave spectra, it is demonstrated that in obliquely magnetized magnetic films the in

Final results

The successful execution of the project resulted in a number of fruitful discoveries that go beyond the state of the art of the research field of magnonics. For instance, the discovery of the long distance supercurrent transport in the form of Bogoliubov second sound waves, which are excited in the magnon condensate, further advances the frontier of the physics of quasiparticles and allows for the application of related transport phenomena for low-loss data transfer in perspective magnon spintronics devices. Also the discovery a novel way to create a magnon Bose-Einstein condensate by rapid cooling paves the way for the usage of macroscopic quantum magnon states in conventional spintronics and to on-chip solid-state quantum computing. It is important to stress that the observed way to BEC is genuine to any solid-state quasi-particles in exchange with the phonon bath. The injection mechanism is originally incoherent, which is in direct opposition to laser or microwave irradiation, and can be applied to other bosonic systems such as exciton-polaritons and photons in cavities. The proposed tunable magnon space-time crystal in the magnon Bose-Einstein condensate, realized by a periodically driven room-temperature YIG film, represents an example of a nonlinear Floquet system and therefore serves as a bridge between magnonics and classical nonlinear wave physics from one side and the Floquet time-crystal description of periodically driven systems from the other side. Joining these two perspectives may give birth to a new field of physical research, which is Floquet (or periodically driven) nonlinear wave physics. The novel magnonic STIRAP device is a first realization of a magnonic logic device consisting of two coherently coupled logic elements (here two magnonic directional couplers).

All these groundbreaking discoveries will help in the further realization of the SuperMagnonics project. They will help in the demonstration of the applicability of a new macroscopic quantum phenomenon â€“ magnon supercurrents â€“ in information processing technologies.