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Properties of Perovskites and Other Oxides, by K. Alex Müller and Tom W. Kool

In 1986, Alex Müller and Georg Bednorz published a paper in the journal Zeitschrift für Physik B which reported the discovery of superconductivity at a temperature above 30 K in a lathanum barium copper oxide. This rather unassuming-looking paper in a low impact factor journal represented the discovery of high-temperature superconductivity and has been cited more than 8000 times at the time of writing. It set off a highly productive search for new superconductors in the copper oxides that would see the record transition temperature rise to well above 100 K. It also earned its authors the 1987 Nobel Prize in Physics. Some breakthroughs come out of nowhere, the result of some extraordinarily serendipitous discovery and the sharp eyes and prepared minds that have the skill to recognise the true significance of an unexpected result. The discovery of high temperature superconductivity may look like exactly this kind of surprising and chance development, but an examination of the scientific career of Alex Müller reveals how his fascination with novel and unusual oxides over many decades had led him on a scientific path that forms a coherent and unbroken thread all the way to the Nobel Prize winning work (conducted, it should be noted, when Müller was in his late fifties). This collection of reprints allows the reader to chart the course of Müller’s scientific development, from his early papers in the late 1950s all the way to high-temperature superconductivity in 1985 (though there the story stops, and Müller’s later work on superconductivity is not included – he has remained scientifically active even in nominal retirement). The reprinted papers are arranged thematically rather than chronologically, and comprise around 60 published works selected from more than 400 of Müller’s publications (though half a dozen are not Müller’s at all, but instead are by Tom Kool, the coauthor of this edition). A major theme is the use of electron paramagnetic resonance (EPR) to study the local properties of materials, in particular defect states and transition metal ion impurities in non-magnetic oxides such as SrTiO3. This sensitivity to local structure allows one to study oxygen ion coordination, photochromism and structural phase transitions. A particular highlight is Müller’s work on the Jahn– Teller effect in which a spontaneous distortion occurs in order to lift the orbital degeneracy of a central ion. Müller’s review of this effect, written in 1967 and appearing only as an article in a conference proceedings is a gem. Other treasures include the reviews on structural phase transitions (from 1981 and 1991, respectively) that are otherwise only obtainable in specialist books. Müller clearly became fascinated by the quantum paraelectric behaviour of SrTiO3. The dielectric constant becomes very large on cooling to low temperature but is roughly constant below 4 K, evidence that it enters a quantum-mechanical regime which stabilises large ferroelectric fluctuations in the paraelectric phase. Here the zero-point motion of the atoms prevents a transition to a ferroelectric phase. This quantum paraelectric state can be destroyed by applying stress or by doping. Once again EPR was employed to study this state. The focus throughout this collection of reprints is oxides. By studying these systems so intensively throughout his career, Müller was well aware of the rich physics that oxides exhibit and open to the possibilities that such materials offer. He realised that electron-phonon interactions could be strong in such systems, particularly due to polaron formation and he posited a link with the Jahn–Teller effect. This led him to narrow down a search for new superconductors in oxides containing the Jahn–Teller ion Ni3þ (an avenue that proved unfruitful). The next step was to employ instead the isoelectronic ion Cu2þ and the rest, as they say, is history. Even though the link between hightemperature superconductivity and the Jahn–Teller effect is now thought to be spurious, the clear lesson from this history is that a career-long focus from an outstanding scientist, even in a research area that perhaps during most of his career looked unpromising and decidedly unfashionable, can repay extraordinary dividends. What is more, the story of oxides is very definitely far from over.