Condensed-matter physics

Spintronics, the atomic way

Ultracold atomic gases are excellent platforms for exploring phenomena in condensed-matter physics. They have now been used to engineer the spin Hall effect and to make the atomic counterpart of the spin transistor. See Letter p.201

The spin of elementary particles is a concept in quantum mechanics that has no counterpart in the classical world. Although the electron's spin was inferred from the 1922 Stern–Gerlach experiment on the deflection of particles, it has only recently been exploited in electronic devices — conventional electronic circuitry is based on the electron's charge. Atoms also carry a spin, and so could similarly be exploited for spin-based electronics. On page 201 of this issue, Beeler et al.1 demonstrate how ultracold atoms can be used to build the atomic analogue of an electronic switch that was proposed more than 20 years ago: the spin transistor2.

Spin-based electronics, or spintronics, has seen tremendous developments in the past decade3. At the heart of this field is the manipulation of the electron's spin, which in condensed-matter materials can be rather complicated owing to the interaction of this spin with its surroundings and the limited controllability of the materials. In this regard, an effect called spin–orbit coupling, which allows the manipulation of the electron's spin without the use of local magnetic fields, has been instrumental in simplifying the construction of spintronic devices.

During the past ten years, ultracold atomic gases have become the ideal playground in which to investigate fundamental phenomena in many fields of physics, particularly condensed-matter physics. The excellent manipulation of both the internal and the external degrees of freedom of ultracold atoms allows the study of complex physics in a controlled way. Atoms have the advantage that they carry spin but have no charge. Therefore, charge effects that would otherwise need to be considered can be excluded. Furthermore, atoms can have either a fermionic or a bosonic particle character (they have half-integer or integer spin, respectively), so the effect of different quantum-particle statistics on spin interactions can readily be tested. Finally, ultracold atomic gases confer more versatility on spin manipulation and detection than do electrons, yielding new ways to explore the richness of spintronics.

To make spintronic devices based on atomic spins, spin–orbit coupling needs to be addressed. In atomic physics, the effect couples the electron's spin with its orbital angular momentum through the electrical field of the nucleus, and leads to the fine structure (small splitting) of the internal states of the atom. In condensed-matter systems, however, the coupling is between the electron's spin and its linear motion, and is caused by the electrical field of the underlying atomic lattice. Researchers have recently proposed ways to create artificial electromagnetic fields (gauge fields) to induce such spin–motion interaction in atomic systems. These fields are produced by coupling internal states of atoms with laser beams at ultra-low temperatures. In their experiment, Beeler et al. used one such field to engineer spin–motion interaction in an atomic system and to observe a quantum effect known as the spin Hall effect.

The spin Hall effect is similar to the conventional Hall effect, in which the positively and negatively charged particles of an electrical conductor are separated by a transverse magnetic field, producing a voltage at a right angle to the current. In the spin Hall effect, the particles go their separate ways according to whether their spin points in one of two opposing directions. By using their gauge field, which was created by means of two laser beams, Beeler et al. convincingly show that atoms with opposite spin states that travel at right angles to the magnetic field produced by the gauge field move in opposite directions (Fig. 1). The researchers go on to show that their data agree well with theoretical calculations, indicating that the authors have a proper understanding of the mechanism behind the spin Hall effect in their system.

Figure 1: Going separate ways.
figure1

Beeler et al.1 have demonstrated that two laser beams (purple arrows) can be directed at a cloud of atoms to generate an artificial magnetic field (not shown) that makes atoms of two spin orientations (red and blue) move in opposite directions.

But Beeler and colleagues have taken their work one step further: they realized the atomic analogue of a spin transistor. Although previously proposed2 in 1990, the spin transistor has been made only within the past few years4. By using the displacement of the atoms in the gauge field as the transistor's voltage difference between the drain and source electrodes, and the strength of the two laser beams as the transistor's voltage of the gate electrode, Beeler et al. realized an atomic system that shows the characteristics of a typical spin transistor. The simplicity and robustness of the authors' transistor also makes it a good option for splitting atoms according to their spin in a device known as a Mach–Zehnder interferometric sensor.

Beeler and colleagues' experiment opens up many avenues in the field of ultracold atomic gases. The gauge field produced is of the 'Abelian' type; however, there are proposals to generate non-Abelian gauge fields5. These non-Abelian fields are more difficult to realize experimentally but allow a closer comparison with condensed matter, in which non-Abelian fields are usually responsible for the spin–motion interaction. Beeler et al. induce the spin Hall effect in their system by using spin–motion coupling, but in condensed matter the effect can also be induced by scattering of electrons by impurities. Although ultracold atomic gases are free of impurities, interactions between the atoms can be tuned to be made strong and yield exotic phenomena such as superfluidity.

Spin–orbit interactions can lead to topological insulators, which are insulating in their bulk but have topologically protected conducting states on their boundaries. Such states can easily be controlled and detected in ultracold atomic gases. The crossroads between ultracold atomic gases and condensed-matter physics provide fertile ground for research: the former focuses on fundamental knowledge obtained through the study of well-characterized systems under controllable conditions, whereas the latter applies such knowledge in information technologies. Many new phenomena can be expected to surface in these areas in the next few years.

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Correspondence to Peter van der Straten.

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van der Straten, P. Spintronics, the atomic way. Nature 498, 175–176 (2013). https://doi.org/10.1038/498175a

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