Nippon Telegraph and Telephone: Elucidation of Spin Wave Physics Using a Graphene Electron Interferometer


[Overview]

The Nippon Telegraph and Telephone Corporation (NTT), CEA-Saclay and the National Institute for Materials Science (NIMS) have successfully acquired knowledge about the process of spin wave generation and the electrical control of spin waves in a quantum Hall state of graphene* 1. A spin wave is a phenomenon in which a spin oscillation propagates in a magnetic material (Fig. 1). This process does not generate Joule heat, so spin waves show promise for application to low power devices. The technology that targets device applications by controlling spin waves is called “magnetic” (magnon: a quantized spin wave). In this regard, a quantum Hall state of graphene is an ideal research platform for the development of magnetism because it offers a simple and clean insulator, in which the spins are fully polarized and the crystalline disorders or defects are extremely small. In addition, the spin state can be controlled between the fully polarized and unpolarized state (Fig. 2). The joint research group succeeded in detecting the amplitude and fluctuation of spin waves in a quantum Hall state of graphene using an electronic interferometer and thus obtained a way to study the fundamental properties of the waves of spin. The feedback of knowledge from this system in search of applications will contribute to the development of magnificent devices. In addition, the knowledge acquired on the process of spin wave generation and their electrical control could contribute to the realization of magnon quantum devices and magnonic devices with new principle. This research was published online in Physics of natureon 12, 8, 2021.

1. Origins

In general, electronic devices perform information processing by controlling the flow of electrons (electric current). However, the resistance of metal wiring and semiconductor elements increases as devices get smaller, causing a problematic increase in thermal energy loss. While spintronic devices with new functions have been made using the flow of spins (spin current), most of them use the flow of electrons having spin, which does not solve the problem of the loss of spin. ‘thermal energy. In this context, the field of magnetics which uses spin waves attracts attention. A spin wave is a phenomenon by which spin current is transported without being accompanied by electric current. Therefore, the magnetics hold promise for application to ultra-low power devices without loss of thermal energy. Magnetic research is at the stage where a variety of functions in different materials are carried out as well as discussions on the advantages and disadvantages of magnetic. A quantum Hall state of graphene is an insulator having an extremely low amount of lattice defects and impurities with a spin-tunable state between fully polarized and unpolarized states. Thanks to these characteristics, the quantum Hall state of graphene is attracting attention as an ideal platform for fundamental research into the fundamental properties of spin waves.

2. Results

The joint research group detected spin waves with high sensitivity using an electronic interferometer made using graphene pn junction and succeeded in gaining knowledge about the process of generating spin waves and their electrical control.
Applying a magnetic field perpendicular to graphene having a pn junction results in the formation of a current channel which surrounds the pn junction. At this time, the input and output of the pn junction each function as an electron beam splitter, resulting in a Mach-Zehnder electronic interferometer (Fig. 3). A characteristic of the current measured with this interferometer is its very sensitive response to slight changes in the area of ​​the interferometer (an area change of about 1 m × 0.1 nm results in a current change of about 70%) . Most experiments are conducted at a Landau level fill factor* 2 ν = 1 state in which the spins are completely aligned in one direction. By generating spin waves in association with electron-hole pairs in this state ν = 1 and by feeding these spin waves with the interferometer, we studied the characteristics of the spin waves by measuring the resulting change in the amplitude and phase interference. It was found that the spin waves were generated stochastically following the Poisson distribution (Fig. 4). This result indicates that spin waves do not appear simply as waves but also as quanta (magnons) having the property of particles. We have also shown a change in the excitation energy of the spin wave when changing the spin state from the ν = 1 state by adjusting the electron density (Fig. 5). In principle, this property can be exploited to allow an electrical control by spin waves which has not been possible with conventional insulators, which should lead to the development of magnetic devices of new principle.

3. Technical points

We fabricated a pn junction and electrodes for electrical measurements by stratification and fine processing of graphene, hexagonal boron nitride (hBN) and metal electrodes (Fig. 6). In particular, by controlling the polarizations on the small grids fabricated at the entrance and exit of the pn junction, we controlled the transmittance and reflectance of the beam splitters to maximize the spin wave detection sensitivity of the electronic interferometer.

4. Future developments

We consider that a clean and simple system in the form of a quantum Hall state of graphene can facilitate the search for spin waves and clarify their basic properties such as spin wave speed, attenuation mechanism , etc. We are waiting for magnon-based quantum devices through which quantum information can be transmitted by magnons.

5. Publication details

Newspaper:

Physics of nature

Title:

“Unveiling of the excitonic properties of magnons in a quantum Hall effect ferromagnet”

Authors:

A. Assouline, M. Jo, P. Brasseur, K. Watanabe, T. Taniguchi, T. Jolicoeur, DC Glattli, N. Kumada, P. Roche, FD Parmentier and P. Roulleau

6. Glossary of terms

  1. State of the quantum hall
    Applying an intense magnetic field perpendicular to a low-temperature two-dimensional electron system results in an electronic state quantized as an integer value (integer quantum Hall effect) or a fractional value (fractional quantum Hall effect) in which the Hall conductance is a multiple or fraction, respectively, of e2/h(his Plank’s constant). Current flows along the edge of the two-dimensional system and the interior becomes an insulator.

  2. Fill factor at Landau level
    The ratio of electron density to a magnetic field (magnetic flux density). When the number of electrons and the number of quanta of magnetic flux are equal, = 1 and the spins are aligned in one direction. When the number of electrons is twice the number of quanta of magnetic flux, = 2 and the spins are completely unpolarized.

Fig. 1. Diagram of spin waves. Spin oscillations propagate like waves.

Fig. 2. Quantum Hall state of graphene. (left) The quantum Hall state ν = 1 in which the spins are completely aligned in one direction. (right) The spin state can be changed from a spin-polarized state to an unpolarized state by changing the density of carriers.

Fig. 3. (left) Mach-Zehnder optical interferometer. The change in phase as it passes through the sample alters the intensity of the transmitted light. (right) Mach-Zehnder electronic interferometer using a graphene pn junction. The magnitude of the current changes depending on the number of quanta of magnetic flux entering the interferometer. This makes it possible to detect with great sensitivity the magnetic field and the variations in the area of ​​the interferometer.

Fig. 4. (above) Diagram of the experimental set-up. Apply tension VEat the metal electrode (yellow) generates spin waves, which can be detected by measuring the current through the interferometer. (bottom left) Current flowing through the electronic interferometer. The interference pattern changes when VEexceeds minimum voltage EZ/efor exciting spin waves. (bottom right) Phase and visibility as a function of VE. Visibility decreases exponentially compared to VE. This indicates that the spin waves are excited stochastically following the Poisson distribution.

Fig. 5. Lowering the electron density decreases the voltage at which the intensity of the interference begins to change. This corresponds to a decrease in the excitation energy of the spin wave.

Fig. 6. (left) Micrograph of the sample used. Many metal electrodes are attached to graphene encapsulated by hexagonal boron nitride (hBN). (right) Cross-sectional diagram of the sample, which has a layered structure made up of graphene, hBN, and metal electrodes.


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