Strain and ferroelectric soft-mode induced superconductivity in strontium titanate

Abstract

We investigate the effects of strain on superconductivity with particular reference to SrTiO3. Assuming that a ferroelectric mode that softens under tensile strain is responsible for the coupling, an increase in the critical temperature and range of carrier densities for superconductivity is predicted, while the peak of the superconducting dome shifts towards lower carrier densities. Using a Ginzburg-Landau approach in 2D, we find a linear dependence of the critical temperature on strain: if the couplings between the order parameter and strains in different directions differ while their sum is fixed, different behaviours under uniaxial and biaxial (uniform) strain can be understood.

Superconductivity mediated by polar modes in ferroelectric metals

“…repulsion as ε 0 increases, might be supplemented by an additional pairing mechanism to account for superconductivity or at least for a quantitative understanding of the magnitude of T c . A number of additional candidates have been proposed involving: (i) residual coupling to critical TO modes missing in the dynamical screening model 20,43,44 ; (ii) plasmons in the conduction electron system 21 , which must be included for minimal consistency of any proposed model; (iii) multi-valley transition processes 9,13 ; (iv) non-polar acoustic phonons (see, e.g., the Appendix in ref. 21 45 ; and other distinct models, e.g., refs.…”

A Local Maximum in the Superconducting Transition Temperature of Nb-Doped Strontium Titanate Under Uniaxial Compressive Stress

“…Even further increase in strain leads to a reduction of the critical temperature ( Figure 1), in agreement with transitioning to the polar phase which has been shown to result in the hardening of the relevant ferroelectric phonon modes (29). This inverse correlation between the critical temperature and the ferroelectric phonon energy is believed to occur in certain theoretical models proposing that these are the most relevant phonons for the electron-electron pairing (3,7).…”

“…The mechanism of superconducting pairing in doped strontium titanate (STO) is a major fundamental open question that has stimulated intense debate (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12). Several rather new experimental works have demonstrated the enhancement of superconductivity in STO via changes in the crystal composition (13-15) and uniaxial or epitaxial strain techniques (16,17), and have revealed the potential importance of the ferroelectric quantum phase transition to this material's mysterious electron pairing (3, 7, 18-24, 13, 14, 10, 25, 26).…”

Theoretical investigation of twin boundaries in WO3 : Structure, properties, and implications for superconductivity

“…Before leaving the topic of soft-mode superconductivity, we point out that the superconductivity in WO 3 is somewhat reminiscent of that in SrTiO 3 , for which a model of superconductivity mediated by fluctuations associated with the ferroelectric quantum critical point has been proposed [83]. The quantum criticality model had considerable success in reproducing the measured behavior, as well as in making rather bold predictions about strain and isotope effects that were subsequently verified experimentally [84][85][86][87]. An important difference is that SrTiO 3 has a superconducting dome as a function of doping, whereas in WO 3 an analogous picture would have the left side of the dome cut off due to the absence of superconductivity in the α 2 phase.…”