25.01.2023In memoriam Prof. Dr. Dr. h.c. mult. K. Alex Müller

K. Alex Müller’s scientific career path started with the studies of physics at ETH Zürich where he was intensely influenced and impressed by the lectures of Prof. Wolfgang Pauli. His diploma work was on the Hall effect in grey tin which was supervised by Prof. G. Busch, who was also the supervisor of his PhD thesis which dealt with the paramagnetic resonance in the newly synthesized double perovskite SrTiO₃.

After finalizing his PhD thesis K. Alex Müller started his professional life as head of the magnetic resonance group at the Battelle Memorial Institute in Geneva. Upon the recommendation of Prof. E. Brun he did his habilitation at the University of Zurich in 1962. In view of his high impact on the scientific research, the IBM Research Laboratory Zurich offered him in 1963 the position of a researcher where he was promoted to a group leader of the physics department in 1971, a position that he held until 1985. During this time his research focused on SrTiO3 and related perovskites with emphasis on their chemical binding, their ferroelectric and soft-mode properties, and later on their critical and multicritical phenomena at their phase transitions. In 1970 he was appointed as titular professor of the University of Zurich. A decisive moment in his career occurred in 1982 when he was nominated as IBM fellow. This enabled him to decide freely and independently about his further research areas – a milestone on his way to the Nobel prize.

J. Georg Bednorz visited the IBM Research Laboratory as a summer student in 1972. This visit marked his first encounter with K. Alex Müller. A year later he started his diploma thesis on the characterization and crystal growth of perovskites supervised by K. Alex Müller. He conducted also his PhD work at ETH Zurich mentored by Prof. H. Gränicher and K. Alex Müller from 1977 till 1982.

During a 18 months sabbatical leave starting in 1979 at IBM Yorktown Height (USA) K. Alex Müller – for the first time in his research career – paid attention to superconductivity and superconducting materials and gained profound knowledge in this field. Back to Zurich he even gave a lecture series on this topic. He was especially interested in oxide superconductors which were rare at that time. However, he observed that this material class has unusually high transition temperatures which suggested a novel pairing mechanism beyond the Bardeen–Cooper–Schrieffer (BCS) theory. Theoretical ideas as developed by his friend Prof. Harry Thomas and his group at the University of Basel gave him the impulse to concentrate on complex oxides with Jahn-Teller centers. Such ions provide a source for polaron and even bipolaron formation and thereby a new and very strong electron-phonon interaction. Together with J. Georg Bednorz he started in 1983 a new research project concentrating on superconductivity in oxide Jahn-Teller systems. In 1986 they achieved the breakthrough with the discovery of cuprate high-temperature superconductors (HTS), which only a year later in 1987 was honored with the Nobel prize in physics for the two scientists. In the same year, but before their nomination, K. Alex Müller was promoted to a full professor at the University of Zurich.

In the following years both scientists were honored with numerous prizes and awards including many honorary doctorates. Nothing, however, stopped them in their further scientific activities and both also remained modest and autochthonous.

During the years following the Nobel prize K. Alex Müller continued his work on cuprate HTSs by focusing on their pairing mechanism. Since his favorite view point was based on a polaronic or bipolaronic mechanism with specifically strong electron-lattice interaction, he founded a new research project with emphasis on isotope effects in cuprate HTSs. In collaboration with researchers from the University of Zurich, PSI, ETH Zurich, and other international institutions, novel and unexpected isotope effects on various fundamental properties of cuprate HTSs were discovered which confirmed his early intuitions that the charge carriers in these superconductors are strongly coupled to the lattice as is typical for polarons or bipolarons, indicating that local lattice effects are relevant for the appearance of high-temperature superconductivity in cuprates. This notion was in strong contrast to the widely accepted conviction that lattice effects are negligible, but that the paring mechanism in the cuprates is of purely electronic origin.

Furthermore, he concentrated on a very specific aspect of these superconductors, namely their intrinsic inhomogeneity which is a basic aspect of their ceramic character and their doping dependent properties. In particular, these properties are inconsistent with translational invariance which is for most theoretical attempts a very challenging problem beyond conventional theories. Instead of a single order parameter – typical for conventional superconductors – it enables a multiple order parameter coexistence and admits that characteristic superconducting properties deviate strongly from single order parameter expectations. K. Alex Müller was the first to suggest that HTSs have combined s+d order parameters which, in the following, was confirmed by numerous experimental results. An implicit further conclusion by him was that cuprates are intrinsically heterogeneous, meaning that local properties differ substantially from global ones. Since experiments typically test time and/or length scales, attention has to be paid to the fact that different experimental tools scrutinize specific regimes of both and may thus lead to seemingly contradictory results which when concentrating on the specific experiments indeed were no more controversial by complimentary. He dedicated several paper to this aspect, which achieved much attention.

He also came back to his roots, i.e. ferroelectrics and SrTiO₃ and other ferroelectric perovskite oxides, where he emphasized a frequently overlooked feature consistent with the above aspects of HTSs. In these materials long-range and short-range properties and time scales also result in distinctly different experimental observations. Here the case of BaTiO₃ attained special interest, since long range neutron scattering experiments classified the system as displacive, whereas results from e.g., NMR could only be understood in terms of an order/disorder mechanism of the phase transition. This apparent discrepancy was resolved by K. Alex Müller, by demonstrating that both phenomena coexist.

Besides of his ingenious scientific achievements and engagements he was also a dedicated and inspiring teacher with profound interest in the students and their life. K. Alex Müller promoted them not only with inspiring ideas, but also by being present for them also in human issues and needs.

Also financially he supported students through the K. Alex Müller Foundation. He attended the seminars at the Physik-Institut with deep scientific interest and was known for his perceptive and subtle questions and contributions. Up to an old age he stayed in close contact with the University of Zurich and vividly took part in the university and social life.

With the death of K. Alex Müller we do not only loose an excellent and very engaged scientist, but also a very good friend and colleague.

Katharina Mueller