X-rays show how quasiparticles interact in solids

Today’s computer technology, based on silicon chips, is slowly reaching its physical limits. Components cannot be miniaturized indefinitely, and clock speeds cannot be increased indefinitely. At the same time, huge AI data centres are being built in many places, and these consume an immense amount of electricity. This is why scientists around the world are searching for novel materials that can replace or complement silicon technology and consume significantly less electricity. Using a combination of X-ray techniques, a research team at PSI has now been able to break down in detail the rapid interplay among different excitations in such a material – electronic, magnetic and lattice excitations. 

From electrons to spin waves

The computer technology of the future could become much more efficient and faster if we succeeded in effectively controlling the rapid flip-flop movements of electron spins in solids. This would make it possible to build ultrafast logic components such as transistors. In today’s computer chips, electrons flow through the material, which causes power losses and generates waste heat. If, instead, the electrons remained stationary and only their spins were manipulated, spin waves could be excited to carry information – much like a Mexican wave in a football stadium: It’s not the people who move through the stadium, but rather everyone is raising their arms in turn. This field is therefore also called spintronics – from “spin” and “electronics.” It makes use of tiny collective spin excitations known as magnons. The name derives from the fact that spins act like atomic compass needles and are linked to a magnetic field, through which they can be excited and read out.

“We have now investigated the dynamics of these excitations and, in doing so, have been able to reveal in unprecedented detail all important aspects that happen in the material with a microscopic view,” says Hiroki Ueda, who led the project at the Paul Scherrer Institute. Until now, it had been possible to study various aspects of how such spin waves behave in the material. However, it was not possible to obtain a coherent picture, since the various analytical techniques could only capture a portion of the microscopic phenomena at a time. “We are particularly interested in the interrelationship between the various excitations in the material,” explains Ueda.

In addition to magnons, the vibrations of atoms in the crystal lattice – known as phonons – also play an important role. Magnons and phonons are also called “quasiparticles” because, although they are not true particles (such as atoms, electrons, or photons), they can nevertheless possess a specific energy and momentum as individual entities – just like particles. This behaviour is deeply rooted in the laws of quantum physics.

Magnons live long in the quantum world 

To make these phenomena visible, the researchers first had to generate magnons. To do this, they fired an intense, short optical laser pulse at the material sample. “We chose copper oxide because it is a well-known magnetic material with strong interactions between magnetism and the lattice,” said publication first-author Arnau Romaguera of the PSI Center for Photon Science. The laser pulse first excited the electrons in the material, which in turn caused the initially disordered spins to flip in rows, thereby generating magnons. This happened in the blink of an eye, within femtoseconds (one quadrillionth of a second).

The optically excited magnons then scattered off each other and formed a state with incoherent waves in the material, like rough seas, over a period of picoseconds (one-trillionth of a second). Another three orders of magnitude later, in the range of nanoseconds (one-billionth of a second), the magnons interacted with the atoms in the crystal lattice and converted their energy into phonons – that is, quantised lattice vibrations. 

What is interesting about these results is that the magnons have a very long lifetime before they convert into phonons. By human standards, all these timescales are extremely short. In the quantum world, however, they span many orders of magnitude. Romaguera says, “Considering the strong coupling between magnetism and the lattice of copper oxide, we were surprised how long it then takes the generated magnons to pass their energy to the lattice.” The comparatively long lifetime of the magnons could thereby allow enough time to perform and control logical operations. “The magnons and phonons exhibit very different relationships between their momentum and energy values,” explains Ueda. “This modulates the efficiency in which magnons can transfer their energy to the lattice, leading to the long lifetime of the magnons and providing us with important insights into how to use specifically designed materials to adjust the lifetime of the magnons as desired.” 

Following the dynamics from femtoseconds to nanoseconds

The challenge in these experiments was to make all these different phenomena visible across such diverse time scales. “We used several X-ray techniques for this,” says Romaguera. “What they all have in common is that a highly brilliant X-ray pulse from a free-electron laser is directed at the sample shortly after the optical laser pulse that generates the magnons.” This is also known as the pump-probe technique. By gradually varying the time between the two pulses, the researchers can probe the material at different moments after excitation and thereby reconstruct a sequence of snapshots of the microscopic events. In addition to two established X-ray techniques, the research team employed a novel X-ray technique for the first time, known as “time-resolved resonant diffuse scattering.” These measurements were conducted at the Furka experimental station of SwissFEL (FEL for “free-electron laser”) at PSI. Other measurements were done at the ESRF synchrotron in Grenoble, France. The results were supplemented by quantum kinetic simulations to model the measured energy flow. 

“Our measurements are largely consistent with the theoretical calculations,” explains Ueda. This shows that the underlying theories are well understood, even if they need to be adapted to the structures in each material. While this is still basic research, it brings the electronics of the future – spintronics – one important step closer to practical application.

Text: Dirk Eidemüller

About PSI

The Paul Scherrer Institute PSI is the largest research institute for natural and engineering sciences in Switzerland. It is the central location of Switzerland’s large research facilities, available to both the scientific community and industry. Its research spans future technologies, energy and climate, health innovation and fundamentals of nature. The annual budget is CHF 450 million. PSI employs 2300 people; around a quarter of these are postdoctoral researchers, PhD students or apprentices. PSI is part of the ETH Domain, which also includes ETH Zurich and ETH Lausanne, as well as the other federal research institutes Empa, Eawag and WSL.