Superconductivity and  quantum  paraelectric fluctuations in SrTiO3

Alexander Balatsky
Los Alamos National Laboratory and Nordita


Following the pioneering discovery of the superconductivity in STO interfaces the field of  collective states     
at interfaces has been rapidly growing. Recently we come to realize that the key to a lot of puzzles in these     
devices lies in the complicated states of bulk STO. STO is one of the earliest examples of superconductivity in   
oxides. Superconducting state is STO exhibits the dome as a function of doping. STO is also a known quantum       
paraelectric.  Historically ferroelectric quantum criticality and superconducting dome in STO were assumed to     
be unrelated. We propose that they are in fact intimately connected.  We suggest that quantum criticality can     
induce superconductivity due to quantum ferroelectric fluctuations. I will present our results on investigation   
of  the origin of superconductivity in doped STO using a combination of density functional and strong coupling    
theory. Our approach suggests a model in which the soft mode fluctuations provide the pairing interaction for     
superconductivity carriers.  Based on this model I will illustrate our  prediction that the highest critical      
temperature will increase and shift to lower carrier doping with increasing 18O isotope substitution, a           
scenario that is experimentally verifiable.  I will also illustrate the ongoing debate on the multiband nature    
of the superconducting state in bulk STO and in related heterostructure devices.           
Haraldsen et al., Phys. Rev. B 84, 020103(R) (2011),  
Haraldsen et al., Phys. Rev. B 85, 134501 (2012),          
Fernandeset al., Phys. Rev. B 87, 014510 (2013) ,
J. Edge et al Phys. Rev. Lett. 115, 247002 (2015) .