The discovery and research progress of the potassium tantalate interface superconductors
Very recently,superconductivity with a transition temperature(Tc)up to 2.2 K was discovered at KTaO3(KTO)-based interfaces.Compared with the classic SrTiO3 interface superconductors,this new oxide interface superconductivity has a higher Tc,a stronger spin-orbit coupling,and a comparable tunability.These excellent properties make KTO interface superconductors an exciting platform to explore rich low-dimensional superconducting physics,as well as to push forward the frontier of oxide electronics.In this paper,we aim to give a brief review of this rapidly developing field of KTO interface superconductors,in order to instruct readers who are interested in,but not familiar with,it.We are not intended to include all the relevant published literatures thoroughly.Rather,we choose materials in a way that can provide readers a consistent story,and let them have a big picture in a short time.For those who are interested in deep details,please refer to the original research papers.The contents are arranged as follows.The first part provides the basic properties of KTO,including its crystalline structure,the dielectric constant,the spin-orbit coupling,orbitals,conduction,etc.The second part reviews the history of the discovery of the KTO interface superconductors.In the third part,we introduce the important physical properties of KTO interface superconductors:(1)The thickness of the interface conducting layer and the two-dimensional superconductivity.(2)The fruitful combinations of a variety of KTO-based interfaces.Here we note that the key role of the deposited films is to supply electrons to the KTO layer near the interface.It is not so crucial whether the deposited films are epitaxial or not.(3)The dependence of superconductivity on the orientations of KTO.(4)The extremely low superfluid density and the strong superconducting fluctuations.We believe that many interesting properties of KTO interface superconductors are related to them.(5)The tuning.We introduce a few methods in tuning the KTO interface superconductors.More attentions are paid to the back gating across KTO,the most useful technique in controlling this system.In the fifth part we discuss the conduction mechanism of the KTO interfaces.In the classic STO interfaces,the conduction mechanism has been a long-standing debate.Here we would like to point out that the conduction of KTO(as well as STO)interfaces can be well understood by taking into account of the effects of oxygen vacancies in films(in addition to oxygen vacancies in KTO and STO substrates).In the sixth part,we review and discuss the developments in understanding the mechanism of KTO interface superconductivity.At the present stage,the superconducting mechanism is still an open question.It is mostly likely that the pairing glue is still electron-phonon coupling.However,it is also rather clear that the superconductivity is unconventional.It is because that the Fermi energy is comparable or even lower than the Debye energy of optical phonons,which violates the conditions of applicability of the standard BCS theory.
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