Quantum leaps in materials research

Quantum Research for new materials

100 years after the discovery of quantum mechanics, the International Quantum Year 2025 is being launched today in Paris. At the level of the smallest particles, conditions and laws prevail that seem to run counter to our intuitive understanding of physical processes. These include the phenomenon of superconductivity, where certain materials – cooled to very low temperatures – lose their electrical resistance. In the supraregional Collaborative Research Center ELASTO-Q-MAT, researchers are trying to produce materials with such “quantum properties”.  In “Forschung Frankfurt”, Roser Valentí and Cornelius Krellner reveal how theory and experiment lead to quantum materials with surprising properties. (Research Frankfurt 1/2023)

How crystals with surprising properties are produced in the laboratory and on the computer

Protected by noble gases, crystals are formed in the heart of this apparatus by means of the Czochralski method (large picture p. 28 and detail above): a metal rod with a seed crystal is immersed in a melt and then slowly pulled out again so that the material can crystallize just below the melting point.



That the microscopic world of atoms and molecules is governed by the laws of quantum physics has been known for a hundred years. For a long time, quantum phenomena were considered obscure and uncontrollable. Today, physicists are using quantum physical effects to develop materials with novel
properties.

Cornelius Krellner, a physics professor originally from Dresden, is based at the Institute of Physics on Riedberg Campus in the north of Frankfurt. He swivels his chair round and opens the door of a mustard-colored cabinet, smiling mischievously at his hidden treasure. “In this cabinet, we have virtually all chemical elements in their purest form,” says Krellner. It is a collection that you would tend to expect from a chemist. In this case, Krellner is using the chemical elements for his research in experimental physics. He produces crystals from them, that is, substances in which the atoms are arranged in a regular pattern. “Thanks to their ordered structure, crystals are particularly suitable for understanding the atomic structure of matter,” says Krellner.

Superconducting quantum materials

Cornelius Krellner is conducting research in the area of quantum materials. In a sense, every substance in the material world is a quantum one because every substance is made up of atoms, and their building blocks obey the laws of quantum physics. However, science defines “quantum material” more narrowly: here, the term refers to materials whose macroscopic properties are based on quantum effects. The most prominent example is superconductors. In these materials, electrical current flows without resistance – unlike in the copper conductors we are familiar with. Why this is the case can be explained by quantum physics: electrons move without resistance in superconductors because they join up to form agile pairs that are unstoppable.

Superconductors have fascinating potential because they could in the future enable a power supply without transmission losses. It is therefore no surprise that researchers are looking for other quantum materials that have similarly useful properties. Cornelius Krellner has already produced several such materials. Krellner gives an example of a quantum material whose magnetic properties are puzzling: the substance loses its magnetic capability when it is compressed. To crystallize the material, Krellner used three elements from his mustard-colored cabinet: europium, palladium and silicon. These elements form a crystal, a regular lattice of unit cells. Each unit cell consists of one europium atom, two palladium atoms and two silicon atoms. The chemical formula EuPd2Si2 is also the name of the quantum material: “europium palladium two silicon two”.

One electron makes all the difference

“The interesting physics is in the europium,” says Cornelius Krellner. To explain why this material loses its magnetism when compressed, he takes us on a journey into the microscopically small world of atoms. Europium is a rare earth metal. Sixty-three electrons orbit its atomic nucleus. They are distributed over six shells or 13 subshells. The subshell with one electron defines the distance to the atomic nucleus and its energy state. Of its 63 electrons, only one electron is responsible for the europium’s magnetism, known as an 4f electron because it is usually located on the subshell 4f. However, when the crystal is compressed, the electron jumps from 4f to the subshell 5d.

Although its energy state changes only imperceptibly, this quantum-physical process has two far-reaching consequences for the europium: it shrinks by about 20 percent – and it loses its magnetism. Through external pressure,the entire EuPd2Si2 crystal becomes non-magnetic. If the pressure is removed, the electrons from the subshell 5d return to their original position; the crystal becomes magnetic again. In the language of physics: through external manipulation, quantum materials perform a phase transition between two quantum states. For this effect to occur, the EuPd2Si2 crystal must be cooled to -100 °C.

IN A NUTSHELL
  • At low temperatures, quantum materials can conduct current without resistance (superconductors), for example, while others lose their magnetism under pressure.
  • Physicists are trying to grow crystals in such a way that as quantum materials they change their electrical or magnetic properties when they are deformed.
  • Theoretical physicists complement this work with mathematical models that show how new quantum materials might look.

The question is: Why?

Cornelius Krellner’s group comprises 15 researchers and students. They are currently studying a dozen quantum materials that change their magnetic properties, but also their electrical or other properties, when compressed, bent or stretched. “We typically look at a material for two to five years,” reports Krellner. “We are looking for materials that show such interesting properties that other research groups worldwide want to delve deeper into them.” In other words: the quantum materials from the physics laboratory in Frankfurt are not intended to enable a new generation of high-tech applications tomorrow. Rather, the research is aimed at creating the basic understanding required to commercially exploit quantum materials in the medium and long term.

For their experiments, the research group uses an armada of special equipment. They measure magnetic moments, electric currents, thermodynamic properties, the atomic structure of the crystal lattice, as well as special states that show the electrons on the surface of the material samples. But all these measurements do not reveal why quantum materials show these astonishing properties. It is a theoretical physicist who is answering this question. Like Cornelius Krellner, Roser Valentí, Professor of Theoretical Physics, has an office in the physics building in Frankfurt, but in a wing further away. In her office, there is a blackboard covered with mathematical symbols. “We need the blackboard for the discussions in my research group,” says Valentí.

Doctoral researcher Katharina Zoch places one of the tiny crystals grown in the laboratory (see screen) under an incident light microscope.

In the language of quantum physics

Originally from Spain, Professor Valentí supervises a dozen postdoctoral researchers, doctoral candidates, Master’s and Bachelor’s students. Unlike Cornelius Krellner’s team, which works with laboratory equipment, Valentí’s team works with equations. Their mathematical tools were initially introduced in the 1920s by physicists such as Erwin Schrödinger and Paul Dirac. They can be used to accurately describe electrons that play a key role in quantum materials, namely by means of wave functions. This works because, according to quantum physics, electrons are hybrids of particles and waves.

The research group led by Roser Valentí conducts part of its simulations on crystals that have been measured in Cornelius Krellner’s laboratories. The scientists translate the information from the measurements into equations, which they then solve. This helps them to understand why crystals behave the way they do, says Roser Valentí: “By solving the equations, we can deduce which interactions of the electrons within an atom or between neighboring atoms of the crystal lattice are responsible for the properties of the quantum material under study.” Solving such equations requires sophisticated mathematical methods and techniques that Roser Valentí has helped to develop. It also calls for a lot of computing power.

Model and crystal: at -100 °C, this crystal called “europium-palladium-two-silicon-two” (EuPd2Si2, lying on 1mm graph paper) loses its magnetic properties under pressure. The theoretical physicist Roser Valentí is studying how this change occurs. In this model of the crystal unit cell, the europium atoms are red, the palladium atoms are blue and the silicon atoms are white.

New materials from the theorists’ melting pot

Understanding quantum materials is one thing. The theoretical physicists go one step further. They make assumptions as to how a previously non-existent quantum material would have to be designed in order for it to manifest certain properties. They feed their ideas back to the experimental physicists, who try to produce the crystals and measure the predicted properties. For example, in EuPd2Si2, the silicon was partially replaced by germanium, which is related to it. It turned out that this material also loses its magnetism when subjected to pressure, but at a temperature that is 50 Kelvin lower.

The back and forth between theoretical and experimental physics is only possible because experimental physicists such as Cornelius Krellner have made great progress in the fabrication of crystals in recent years. The development of EuPd2Si2 illustrates the magnitude of these advances: the material has been known for forty years, but the growth of crystals that can be used for experiments only succeeded six years ago. The researchers in Frankfurt use the Czochralski method to grow crystals: in this process, the crystals are drawn from a melt, like in candle making. The mixture heated to 1400 °C is so aggressive that no container can withstand it. That is why the melt must be kept in suspension by electromagnetic forces in order for crystal growth to succeed.

ELASTO-Q-MAT research initiative

Despite all the difficulties with practical implementation, quantum phenomena are now very often understood and controllable, which brings them ever closer to technical applications. In 2022, the Nobel Prize in Physics was awarded to three researchers whose work has laid the foundation for quantum technology. They have created the conditions necessary for the use of quantum phenomena in computers, encryption technology or sensors. These are all possible areas of application that could also benefit from the work of quantum material researchers Roser Valentí and Cornelius Krellner.

The duo from Frankfurt has been working for two years within the transregional Collaborative Research Center ELASTO-Q-MAT, which is funded by the German Research Foundation. ELASTO-Q-MAT is studying quantum materials that change their properties when subjected to elastic deformation. Under the leadership of Goethe University Frankfurt with Roser Valentí as spokesperson, 18 groups from research institutions in Mainz, Karlsruhe and Dresden are participating in the project. If everything goes well, the scientists have another ten years to fully understand the interaction of electrons and crystal lattices in quantum materials. “What motivates us is the simple desire to discover new things,” says Cornelius Krellner. “We believe that if you understand more, you can make something useful out of it later.”

Photos: Uwe Dettmar, AG Cornelius Krellner

About

Cornelius Krellner, Cornelius Krellner, born in 1978, studied physics at TU Dresden and ETH Zurich. He was awarded the Otto Hahn Medal of the Max Planck Society for his dissertation. After a research stay in Cambridge, England, he was appointed Professor of Experimental Physics at Goethe University Frankfurt in 2012.

krellner@physik.uni-frankfurt.de

Roser Valentí, born in 1963, earned her doctoral degree at the University of Barcelona, was a researcher at the University of Florida and then appointed as Professor of Theoretical Solid State Physics at Goethe University Frankfurt in 2003, where she served as Vice President from 2009 to 2012. Valentí is the spokesperson for the Collaborative Research Center ELASTO-Q-MAT and thus the driving force behind this research initiative. Together with Luciano Rezzolla, Professor of Theoretical Astrophysics (see page 7), she is spokesperson of the profile area “Space, Time, Matter” at Goethe University Frankfurt.

valenti@itp.uni-frankfurt.de




The author

Dr. Benedikt Vogel, born in 1964 in Lucerne/Switzerland, works from Berlin as a freelance science
journalist. His areas of specialization are physics, energy and medicine. In addition, he advises universities, associations and authorities on strategic communication.

vogel-komm.ch


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