Strain-engineered interaction of quantum polar and superconducting phases

Abstract

Much of the focus of modern condensed matter physics concerns control of quantum phases with examples that include flat band superconductivity in graphene bilayers (1), the interplay of magnetism and ferroelectricity (2), and induction of topological transitions by strain (3). Here we report the first observation of a reproducible and strong enhancement of the superconducting critical temperature, Tc, in strontium titanate (SrTiO3) obtained through careful strain engineering of interacting superconducting phase and the polar quantum phase (quantum paraelectric). Our results show a nearly 50% increase in Tc with indications that the increase could become several hundred percent. We have thus discovered a means to control the interaction of two quantum phases through application of strain, which may be important for quantum information science. Further, our work elucidates the enigmatic pseudogap-like and preformed electron pairs phenomena in low dimensional strontium titanate (4, 5) as potentially resulting from the local strain of jammed tetragonal domains. Main text:Among the main goals of this work is to address the open question of the nature of the superconducting pairing mechanism in strontium titanate (STO) (6, 7) and to inspire searches for enhanced superconducting temperatures in materials not just with suppressed to zero Kelvin structural transitions, as in (CaxSr1−x)3Rh4Sn13 (8), MoTe2 (9) and Lu(Pt1−xPdx)2In ( 10), but with incipient quantum phase transitions, for example, ScF3 which has a structural quantum phase transition (11), and may become superconducting when doped (12). It has been predicted that superconducting doped strontium titanate with its peculiar phonon dynamics (13-17) is an example of a superconductivity arising near an incipient quantum polar (quantum ferroelectric) phase transition (4,7,(18)(19)(20)(21)(22)(23)(24)(25)(26)(27), but this has not been fully demonstrated experimentally, in part, due to the fact that existing results on isotope effect and Ca substitution (25, 28) may be explained by non-uniformity in the chemical composition, and the absolute enhancement of the critical temperature values have not been found.It is also unusual to find a pseudogap-like behavior in superconductors that cannot be explained by compositional inhomogeneities, as is the case in cuprates (29). A pseudogap-like behaviors, such as a tunneling gap and a 2e charge transport, occur in STO at temperatures up to about 0.9 K, almost twice the bulk superconducting transition temperature (4, 5).

Reply to “Comment on ‘Superconductivity at low density near a ferroelectric quantum critical point: Doped SrTiO3 ”'

“…As for the observed strong isotope effect, which may be expected to arise from a pair interaction contri-bution mediated by the soft TO phonon mode, we have to withdraw our earlier proposal of a deformation potential electron-phonon coupling. We do, however, take the present observations of the isotope effect at face value, noting that in the meantime new data on the effect of strain on T c also suggest a strong coupling of carriers to the soft mode 8 . Work on deriving a sufficiently strong alternative e-ph coupling is in progress.…”

Millikelvin-compatible apparatus for studies of quantum materials under uniaxial stress

“…The strain in the samples is measured with a miniature gauge directly attached to the sample. [2], [6], [7] The cell was designed to accommodate the changes in strontium titanate sample size upon cooling with accuracy of about 0.01% in residual strain. We verified the accuracy by comparing the gauge readings on clamped samples to the readings of the strain gauges mounted on freestanding samples (data not shown).…”

“…Therefore, as-cooled samples typically had very close to zero strain. [2] Thus, this design provides small strains and minimizes dislocation defects in the sample.…”

“…The changes in superconductivity are interpreted as being due to influence of strain on the phonon modes and the proximity to the ferroelectric quantum phase transition. [2], [6] Additional data from other samples and reproducibility of the results related to the sample mounting procedure are discussed elsewhere. [2], [6] Figure 3.…”

“…[2], [6] Additional data from other samples and reproducibility of the results related to the sample mounting procedure are discussed elsewhere. [2], [6] Figure 3. Simultaneously acquired optical images (a, b) as well as magnetic and resistive signatures (c) of the superconducting transition in an x=0.014 sample oriented with (110) The superconducting transition is strongly suppressed, and the microscopy allows us to determine the orientation of the antiferrodistortive axis which is essential to understand the effect.…”