Quantum Study may Lead to New Generation of Logic and Memory Devices

When it comes to developing electronic devices, researchers generally search for ways to exploit and regulate the three rudimentary characteristics of electrons—their spin states, which result in magnetism; their charge; and the shapes of the indistinct clouds they create around the atoms’ nuclei, which are called orbitals.

These balloon-and-disk shapes represent an electron orbital—a fuzzy electron cloud around an atom’s nucleus—in two different orientations. Scientists hope to someday use variations in the orientations of orbitals as the 0s and 1s needed to make computations and store information in computer memories, a system known as orbitronics. A SLAC study shows it is possible to separate these orbital orientations from electron spin patterns, a key step for independently controlling them in a class of materials that is the cornerstone of modern information technology. Image Credit: Greg Stewart/SLAC National Accelerator Laboratory.

To date, electron orbitals and spins were believed to work in close association in a group of materials that are the cornerstone of contemporary information technology; one would not be able to change one property quickly without changing the other.

However, a new study carried out at the Department of Energy’s SLAC National Accelerator Laboratory has demonstrated that a pulse of laser light can considerably alter the spin state of one significant group of materials without changing its orbital state.

These outcomes could pave the way for developing next-generation memory and logic devices on the basis of “orbitronics,” stated Lingjia Shen, one of the lead investigators of the study and a research associate at SLAC National Accelerator Laboratory.

What we’re seeing in this system is the complete opposite of what people have seen in the past. It raises the possibility that we could control a material’s spin and orbital states separately, and use variations in the shapes of orbitals as the 0s and 1s needed to make computations and store information in computer memories.

Lingjia Shen, Study Lead and Research Associate, SLAC National Accelerator Laboratory

The international team of researchers has recently reported the results in the Physical Review B Rapid Communications journal. The team was headed by Joshua Turner, a SLAC investigator and staff scientist with the Stanford Institute for Materials and Energy Sciences (SIMES).

An Intriguing, Complex Material

Manganese oxide-based quantum material, called NSMO, was the material that was analyzed by the research team. Available in very thin crystalline layers, this material has been around for 30 years and is employed in devices where data is preserved by applying a magnetic field to change from one electron spin state to another—a technique referred to as spintronics.

NSMO is also regarded as a potential candidate for developing upcoming memory storage devices and computers based on skyrmions. Skyrmions are minute particle-like vortexes that are produced by the magnetic fields of rotating electrons.

However, this material is also known to be highly complex, stated Yoshinori Tokura, director of the RIKEN Center for Emergent Matter Science in Japan, who also took part in the research.

Unlike semiconductors and other familiar materials, NSMO is a quantum material whose electrons behave in a cooperative, or correlated, manner, rather than independently as they usually do. This makes it hard to control one aspect of the electrons’ behavior without affecting all the others.

Yoshinori Tokura, Director, RIKEN Center for Emergent Matter Science

One standard method to study this kind of material is to strike it with laser light to observe how its electronic states react to energy input. And that is exactly what the scientists did here. They watched the material’s reaction with X-ray laser pulses emitted from SLAC’s Linac Coherent Light Source (LCLS).

One Melts, the Other Doesn’t

The researchers anticipated that they would see complete chaos, or melting of the systematic patterns of electron orbitals and spins in the material, as they absorbed the pulses of near-infrared laser light.

But to their amazement, the orbital patterns remained intact and only the spin patterns melted, stated Turner. The usual coupling that occurs between the orbital and spin states had been fully broken, he added, which is a difficult thing to do in this kind of correlated material and had not been seen in the past.

Usually only a tiny application of photoexcitation destroys everything. Here, they were able to keep the electron state that is most important for future devices—the orbital state—undamaged. This is a nice new addition to the science of orbitronics and correlated electrons.

Yoshinori Tokura, Director, RIKEN Center for Emergent Matter Science

Just like how electron spin states are changed in spintronics, the orbital states of the electrons could be changed to offer an analogous function. Theoretically, such orbitronic devices may work 10,000 faster when compared to that of spintronic devices, added Shen.

Instead of using the magnetic fields being implemented at present, short bursts of terahertz radiation could be used to switch between a pair of orbital states. “Combining the two could achieve much better device performance for future applications,” he stated. The researchers are exploring ways to do just that.

Shen is currently a postdoctoral researcher at Lund University in Sweden with a joint position with SIMES at SLAC.

Researchers from the Swiss Light Source at the Paul Scherrer Institute in Sweden, the Advanced Light Source at DOE’s Lawrence Berkeley National Laboratory, the University of Chicago, the University of Tokyo, and the University of Tsukuba in Japan also contributed to this study.

Both the Advanced Light Source and LCLS are DOE Office of Science user facilities, and the research was mainly supported by the DOE Office of Science. Turner’s study was financially supported via the DOE Office of Science Early Career Research Program.

Journal Reference:

Shen, L., et al. (2020) Decoupling spin-orbital correlations in a layered manganite amidst ultrafast hybridized charge-transfer band excitation. Physical Review B: Rapid Communications. doi.org/10.1103/PhysRevB.101.201103.

Source: https://www6.slac.stanford.edu/

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