From SrTiO3 to Cuprates and Back to SrTiO3: A Way Along Alex Müller’s Scientific Career

Abstract

K.A. Müller took a long route in science leaving many traces and imprints, which have been and are still today initiations for further research activities. We “walk” along this outstanding path but are certainly not able to provide a complete picture of it, since the way was not always straight, often marked by unintended detours, which had novel impact on the international research society.

Meeting and Working with K. Alex Müller: Personal Memories

“…(Photo HK). STO remained Alex's "favorite child" throughout his scientific career from his PhD thesis (see Figure 5) [2] in 1958 until old age [3,4]. In a recent highlight of Physical Review B [5] it was even argued that the Müller-Burkard paper [6] initiated the discovery of hightemperature superconductivity in cuprates [7].…”

Superconductivity and the Jahn–Teller Polaron

“…Müller was convinced early on that the isotope effect also plays a vital role in understanding the nature of high-temperature superconductity in the cuprates [35]. Consequently, in 1990, he initiated a new project, "Isotope Effects in Cupate Superconductors", at the University of Zurich [36][37][38][39]. The primary aim of this project was to perform a so-called site-selective oxygen isotope effect (SOIE) study on T c in optimally doped YBa 2 Cu 3 O 7−δ [40]: Which oxygen atoms (planar (p), apical (a) or chain (c) oxygens) in the lattice contribute most to the OIE shift of T c ?…”

“…(From [43]). Besides the OIE and SOIE on T c discussed above, doping-dependent unconventional OIEs were also observed on various physical quantities, such as the magnetic penetration depth λ, the spin-glass transition temperature T g , the spin-stripe ordering tempertaure T so and the pseudogap temperature T * [36][37][38][39][43][44][45]. In the following, we only discuss the OIEs on λ and T * , which both demonstrate the polaronic character of the supercarriers in the cuprates.…”

“…This implies that any OIE shift of the superfluid density ρ s arises from an OIE shift of n s and/or m * ab . OIE studies of the magnetic penetration were performed on a number of families of cuprates by means of different experimental techniques (magnetization measurements, torque magnetometry, muon-spin rotation (µSR)) and on various kinds of samples (finegrained powder samples, microcrystals, thin films) (see, e.g., [36][37][38][39]). All these studies show a clear OIE on the magnetic penetration depth, which increases with decreasing doping (decreasing T c ) for all investigated cuprates.…”

Is Reduced Strontium Titanate a Semiconductor or a Metal?

“…Redox processes in transition metal oxides have been under investigations for decades, given their substantial potential for applications in the fields of sensorics, information technology, superconductivity, and energy conversion [1][2][3][4][5]. SrTiO 3 has emerged as one of the most widely investigated model materials for metal oxides with a perovskite structure and its electronic properties have been intensively analyzed [6,7].…”