Electrons possess an intrinsic angular momentum called spin, enabling them to align with a magnetic field similar to a compass needle. Beyond their electric charge, which influences their behavior in electronic circuits, electron spin is increasingly utilized for data storage and processing. MRAM (magnetic random access memory) elements, available for purchase, use this principle by storing information in small, classical magnets containing numerous electron spins. MRAMs leverage currents of electrons with parallel spins to modify the magnetization at specific points in a material.

Figure 1. The Proposed Method. (Credit: ETH Zürich)

Figure 1 shows Left: Single pentacene molecules (yellow) on the insulating layer (blue). Right: Electrons with spins aligned in parallel (small arrows) tunnel from the tungsten tip (top) to the molecule (bottom).

Pietro Gambardella and his team at ETH Zurich have demonstrated that spin-polarized currents can also control the quantum states of single electron spins. Their findings, published in Science, have potential applications in technologies such as the control of quantum states in quantum bits (qubits).

“Traditionally, electron spins are manipulated using electromagnetic fields such as radio-frequency waves or microwaves,” explains Sebastian Stepanow, a Senior Scientist in Gambardella’s lab. This method, known as electron paramagnetic resonance, has been around since the mid-1940s and is used in fields like materials research, chemistry, and biophysics. “A few years ago, it was demonstrated that one can induce electron paramagnetic resonance in single atoms; however, so far the exact mechanism for this has been unclear,” Stepanow adds [1].

To investigate these quantum mechanical processes further, the researchers prepared pentacene (an aromatic hydrocarbon) molecules on a silver substrate with a thin insulating layer of magnesium oxide. This layer ensures that the electrons in the molecule behave similarly to those in free space.

Using a scanning tunneling microscope, the team characterized the electron clouds in the molecule by measuring the current generated when electrons tunnel quantum mechanically from the tungsten needle tip to the molecule. Classical physics would predict that electrons cannot cross the gap due to insufficient energy. However, quantum mechanics allows electrons to tunnel through the gap, resulting in a measurable current.

By adding a few iron atoms to the tungsten tip, creating a miniature magnet, the researchers could spin-polarize the tunnel current. This means the electron spins in the current align with the magnetization of the iron atoms.

Applying both a constant and a fast-oscillating voltage to the magnetized tungsten tip, the researchers measured the resulting tunnel current. By varying the voltages' strengths and the oscillating voltage's frequency, they observed characteristic resonances in the tunnel current. These resonances provided insights into the interactions between the tunneling electrons and those in the molecule.

From their data, the researchers found two main insights. First, the electron spins in the pentacene molecule responded to the electromagnetic field from the alternating voltage, similar to ordinary electron paramagnetic resonance. Second, the resonance shapes suggested an additional process influencing the molecule's electron spins.

“That process is the so-called spin transfer torque, for which the pentacene molecule is an ideal model system,” explains PhD student Stepan Kovarik. Spin transfer torque is an effect where a molecule's spin is altered by a spin-polarized current without direct electromagnetic field action. The ETH researchers showed that quantum mechanical superposition states of the molecular spin could be created this way, which are essential in quantum technologies.

“This spin control by spin-polarized currents at the quantum level opens up various possible applications,” says Kovarik. Unlike electromagnetic fields, spin-polarized currents act very locally and can be controlled with sub-nanometer precision. These currents could precisely address electronic circuit elements in quantum devices and control the quantum states of magnetic qubits.

Source: ETH Zurich

References:

1. https://www.eurekalert.org/news-releases/1050249

Cite this article:

Hana M (2024), Controlling Quantum States with Spin-Polarized Currents, AnaTechMaz, pp. 140