Karl Alex Müller, an IBM researcher and a Nobel Prize laureate, passed away January 9, 2023 at the age of 95.
Müller was born in Basel on April 20, 1927, and joined the Zurich lab in 1963, just a few years after the lab was opened. He started as a research assistant in the physics department. His leadership skills, together with a strong scientific intuition, brought him to lead the department in 1971. He excelled in that role, remaining the head of the department until 1985, and greatly contributed to the culture of innovation, growth and scientific excellence that still permeate the lab today. Müller was made an IBM Fellow in 1982.
Despite his managerial role, Müller still found time to do research. He started a project with the ambitious goal to synthesize new superconducting materials together with J. Georg Bednorz. In 1986, the two made the groundbreaking discovery of high-temperature superconductivity. They found that the resistance of a specific copper oxide vanishes at -228 ⁰C (35 K). Both researchers were awarded the 1987 Nobel Prize in Physics for the breakthrough.
Superconductivity was first discovered in 1911; the phenomenon can occur when a material is cooled to extremely low temperatures. Below a critical temperature, an electric current flows without resistance. This way, a current circulating in a loop of superconducting wire can persist indefinitely without any power source. Today, we use high temperature superconductivity for magnetic resonance imaging (MRI) machines. As for regular superconductivity, we rely on it, for example, for our quantum computing research today, with superconducting qubits mimicking the behavior of atoms for a much more powerful computational capabilities than classical machines.
Researchers had been trying for decades to find a material that would remain superconducting at higher temperatures. In 1973, scientists managed to produce an alloy that became superconducting at -250 °C (23 K), higher than 11 K required for the original superconducting materials discovered in 1911, but still very low. The breakthrough came when Bednorz and Müller used new, ceramic superconductors. Based on their seminal work, superconductors of the same material class were soon identified, with critical temperatures of 90 K or even above.
Müller and Bednorz used an oxide material that contained oxygen, copper and one or more of the rare earth metals. The idea was that the copper-oxide planes in such a material would transport the electrons. The researchers added barium to crystals of lanthanum-copper-oxide to create a ceramic material — the first successful high temperature superconductor. Soon thereafter, many labs around the world started experimenting with ceramic perovskites, in a bid to reach even higher superconducting temperatures, creating one of the most interesting and researched fields of investigation in physics.
Müller liked science from the young age, but while attending the Evangelical College in Schiers, Switzerland, he first became interested in radio and electronics. One of his teachers, though, thought that the young Müller could become a great physicist – and encouraged him to pursue the field. Having graduated in 1945 and after mandatory service in the Swiss military, Müller joined the Physics and Mathematics Department at the Eidgenössische Technische Hochschule (ETH) in Zurich. One of his professors happened to be Wolfgang Pauli, a Nobel laureate who postulated the existence of a fourth quantum number and developed the exclusion principle.
Müller graduated with the the equivalent of a master’s degree in 1952 and proceeded to research solid-state physics, focusing on perovskites when he earned a PhD in 1958. After a brief stint managing a magnetic resonance research group at the Battelle Memorial Institute in Geneva, he joined IBM Research in 1963 and continued his work on perovskites.
As the head of the physics department, Müller hired many talented researchers, among them Gerd Binnig. Together with his colleague Heinrich Rohrer, Binnig developed the scanning-tunneling microscope and in 1986 both scientists shared part of the Nobel Prize in Physics. Müller also hired Bednorz, then a recent ETH graduate, who was also interested in ceramic oxides.
Together, they created the first high-temperature superconducting oxide, a material that lost its electrical resistance at 35 K, the highest temperature at which a material loses its electrical resistance known at the time. The pair announced their discovery in 1986 in the German journal Zeitschrift für Physik, titled “Possible High [Transition] Temperature Superconductivity in the Ba-La-Cu-O System.” The finding was quickly confirmed by other research groups, triggering a global race to find perovskite superconductors with even higher transition temperatures. Soon, researchers found ceramics that had such high transition temperatures that they could be cooled with liquid nitrogen rather than the more expensive liquid helium.
Today, there are many different materials that become superconducting at even higher temperatures. The legacy of Karl Alex Müller lives on, along with the memories of a humble, incredibly talented researcher that left a lasting mark at IBM — and the world.