Superconductivity mediated by polar modes in ferroelectric metals

Abstract

The occurrence of superconductivity in doped SrTiO3 at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades. We report experimental results showing the pressure dependence of the superconducting transition temperature, Tc, near to optimal doping that sheds light on the nature of this interaction. We find that Tc increases dramatically when the energy gap of the ferroelectric critical modes is suppressed, i.e., as the ferroelectric quantum critical point is approached in a way reminiscent to behaviour observed in magnetic counterparts. However, in contrast to the latter, the coupling of the carriers to the critical modes in ferroelectrics is predicted to be small. We present a quantitative model involving the dynamical screening of the Coulomb interaction and show that an enhancement of Tc near to a ferroelectric quantum critical point can arise due to the virtual exchange of longitudinal hybrid-polar-modes, even in the absence of a strong coupling to the transverse critical modes.

Giant Grüneisen parameter in a superconducting quantum paraelectric

“…The recent debate about appropriate models [10][11][12][13][14][15][16][17][18][19][20][21]7] mostly swirls around the question of which phonons provide the superconducting pairing and how. Showing experimentally which phonons are the most relevant is not a trivial task either: Some recent experiments on approaching the quantum paraelectric to ferroelectric phase transition show correlations of the ferroelectric phase with superconductivity [22][23][24][25][26][27][28]. However, most of these observations are rather qualitative, and have mainly shown the mere correlations of the tuning parameter with the change in critical temperatures.…”

Isotope tuning of the superconducting dome of strontium titanate

“…1 Although the coupling to the LO1 mode at 22 meV is very small (λ = 0.003), 28,29 the main features of the phase diagram have been reproduced in a hybrid model of coupling to plasmons and the LO1 mode. 15,22,30,31 In this approach the electron-electron interaction is treated in the random phase approximation. Since this approximation has been shown to strongly overestimate the stability of the superconducting state, 32 further analysis of the hybrid phonon-plasmon model is required taking into account exchange and correlation terms.…”

“…Figures 2(a)-(d) show the normal-state resistivity as a function of T and T 2 of selected substituted samples. At low temperatures, ρ xx of samples with carrier density below a certain critical threshold n * deviates from 22 Negative 1/ 0 signifies that insulating samples (δ = 0) with same x or y are ferroelectric. Tc was normalized to the value obtained for y = 0, x = 0 or zero pressure.…”

“…Pressure experiments have shown that T c decreases linearly as a function of pressure 27 and that superconductivity is eventually destroyed at 5.5 kbar for SrTi 1.98 Nb 0.02 16 O 3 (n = 3.4 × 10 19 cm −3 ). 22 Figure 4(c) compares the data obtained for 18 O-and Ca-substituted STO together with the pressure studies as it plots normalized T c as a function of the inverse dielectric constant 1/ 0 taken at low temperature from the undoped systems.…”

Ferroelectricity, Superconductivity, and SrTiO3—Passions of K.A. Müller

“…In the course of the more than five decades of research on SrTiO 3 a variety of models have been proposed for the pairing mechanism: intervalley scattering [27,31], bipolarons [37], two-phonon exchange [38], longitudinal optical phonons [39], full dielectric function for longitudinal phonons and screened Coulomb [40,41], acoustic phonons [42]. A possible role of ferroelectricity was proposed by Bussmann-Holder [28], an idea that has gained momentum in recent years [43].…”