Giant Grüneisen parameter in a superconducting quantum paraelectric

Abstract

Superconductivity and ferroelectricity are typically thought of as incompatible because the former needs free carriers, but the latter is usually suppressed by free carriers. This is unless the carrier concentration is sufficiently low to allow for polar distortions and mobile electrons to cooperate. In the case of strontium titanate with low carrier concentration, superconductivity and ferroelectricity have been shown to be correlated via various tuning methods, such as strain. Here, we report theoretically and experimentally evaluated Grüneisen parameters whose divergent giant values under tensile stress indicate that the dominant phonon mode which enhances the superconducting order is the ferroelectric transverse soft phonon mode. This finding puts strong constraints on other phonon modes as the main contributors to the enhanced superconductivity in strained strontium titanate. The methodology shown here can be applied to strain-tune and probe properties of other materials with polar distortions including topologically non-trivial ones.

Superconducting dome in ferroelectric-type materials from soft mode instability

“…In all the relevant experimental systems 10,14,15,34 the anharmonic phonon damping plays a key role 24 , which is reflected in gigantic values of the Grüneisen parameter as recently observed in Ref. 15 .…”

“…In all the relevant experimental systems 10,14,15,34 the anharmonic phonon damping plays a key role 24 , which is reflected in gigantic values of the Grüneisen parameter as recently observed in Ref. 15 . If the soft mode is induced by electron doping, the damping arises from the enhanced electron-phonon coupling; if, instead, it is induced by mechanical strain, the damping is increased by the sheer interatomic potential anharmonicity as the strain pushes atoms away from the harmonic part of the bonding wells.…”

“…52 . In ferroelectric-type SCs, n could be electron doping 14 or the mechanical strain 15 , n ≡ . In both cases, the variation of the control parameter n changes the phonon damping Γ, as mentioned above.…”

“…Even though the unusual conditions due to extreme dilution of electrons make the pairing mechanism in SrTiO 3 still a highly debated topic, a basic BCS framework can be applied to extract qualitative pairing trends. 12,13 More recently, enhanced superconductivity domes in the proximity of soft mode instabilities, induced by electron doping 14 and mechanical strain 15 , have been observed. The role of structural instability in promoting superconductivity is suggested by the recent finding that plastic mechanical deformations appear to further enhance superconductivity possibly due to selforganized dislocation networks 16 .…”

“…The role of structural instability in promoting superconductivity is suggested by the recent finding that plastic mechanical deformations appear to further enhance superconductivity possibly due to selforganized dislocation networks 16 . Various theoretical explanations 10,15,[17][18][19] have been proposed to understand such effects in these ferroelectric-type materials, although a complete picture of the mechanism of T c enhancement near the instabilities is unclear.…”