Superconductivity enhancement in polar metal regions of Sr0.95Ba0.05TiO3 and Sr0.985Ca0.015TiO3 revealed by systematic Nb doping

Abstract

Two different ferroelectric materials, Sr0.95Ba0.05TiO3 and Sr0.985Ca0.015TiO3, can be turned into polar metals with broken centrosymmetry via electron doping. Systematic substitution of Nb5+ for Ti4+ has revealed that these polar metals both commonly show a simple superconducting dome with a single convex shape. Interestingly, the superconducting transition temperature Tc is enhanced more strongly in these polar metals when compared with the nonpolar matrix Sr(Ti, Nb)O3. The maximum Tc reaches 0.75 K, which is the highest reported value among the SrTiO3-based families to date. However, the Tc enhancement is unexpectedly lower within the vicinity of the putative ferroelectric quantum critical point. The enhancement then becomes much more prominent at locations further inside the dilute carrier-density region, where the screening is less effective. These results suggest that centrosymmetry breaking, i.e., the ferroelectric nature, does not kill the superconductivity. Instead, it enhances the superconductivity directly, despite the absence of strong quantum fluctuations.

A doping threshold for polar metals

“…In Fig. 2C, n * is plotted as a function of n d for BTO 3−δ , which is compared with that of other polar metals: SCTO 3−δ (10 , 14 ) and SCTNO (15 ). n * is proportional to n d , even if n d varies by almost three orders of magnitude.…”

“…To address these questions, we present a study of P-NP transition in BaTiO 3−δ (BTO 3−δ ), in comparison with previous reports on Sr 1-x Ca x TiO 3−δ (SCTO 3−δ ) (14 ) and Sr 1-x Ca x Ti 1-y Nb y O 3 (SCTNO) (15 ). We find the polar phase terminates at a threshold doping, when one electron is met by about eight dipoles.…”

“…(C) n * scales with n d .Three black circles were collected from Sr1-xCaxTiO3 with x = 0.22%, 0.45% and 0.9% in a previous report(14 ). The red circle was collected from SCTNO(15 ). For SCTO and SCTNO, n d = x/a 3 STO , aSTO = 3.905 Å is the lattice constant of STO.…”

Low-temperature electron mobility in doped semiconductors with high dielectric constant

“…Thus, short-range potentials yield a mobility scaling as µ short (n) ∝ n −4/3 . None of these mechanisms is able to explain the data [1][2][3][4][5][6][7][8][9]. The common feature of all these doped semiconductors is high dielectric constant of the corresponding undoped material, which makes Coulomb scattering by charged impurities very weak.…”

“…A number of doped semiconductors is known to demonstrate low-temperature mobility µ(n) with a nearly power-law dependence on electron density, µ ∝ n −β , with the exponent β in the interval 1 2 < β < 1, see Ref. [1][2][3][4][5][6] for Strontium Titanate SrTiO 3 , Ref. [7] for Potassium Tantalate KTaO 3 , Ref.…”