Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session F37: Ferroelectricity and Superconductivity as Competing Orders in Strontium TitanateInvited
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Sponsoring Units: DCMP Chair: Peter Littlewood, University of Chicago Room: Room 233 |
Tuesday, March 7, 2023 8:00AM - 8:36AM |
F37.00001: Isotope tuning and the pairing mechanism of superconducting strontium titanate Invited Speaker: Dirk Van Der Marel Pure SrTiO3 is a para-electric insulator, in which ferro-electric order can be stabilized by substituting the (natural) 16O atoms with the heavier isotope 18O. Weak doping with electrons renders SrTiO3 superconducting. We have determined the influence of 18O substitution on the superconducting critical temperature and observed that the effect of substituting 16O with the heavier 18O isotope is that Tc increases - opposite to the BCS prediction. We furthermore observe a downward shift of the optimal carrier density. The unusually large size of the observed isotope effect supports a scenario where superconducting pairing is mediated by fluctuations of the ferroelectric soft phonon, possibly by pairs of these phonons. |
Tuesday, March 7, 2023 8:36AM - 9:12AM |
F37.00002: Mesoscopic fluctuating domains in strontium titanate Invited Speaker: Benoit Fauque Spatial correlations between atoms can generate a depletion in the energy dispersion of acoustic phonons. Two well-known examples are rotons in superfluid helium and the Kohn anomaly in metals. Here I will present the observation of a large softening of the transverse acoustic mode in quantum paraelectric SrTiO3 by means of inelastic neutron scattering. In contrast to other known cases, this softening occurs at a tiny wave vector implying spatial correlation extending over a distance as long as 40 lattice parameters. We attribute this to the formation of mesoscopic fluctuating domains due to the coupling between local strain and ferroelectric fluctuations. Thus, a hallmark of the ground state of insulating SrTiO3 is the emergence of hybridized optical-acoustic phonons. I will discuss how these mesoscopic fluctuating domains may play a role in quantum tunneling, which impedes the emergence of a finite macroscopic polarization and change our understanding of the ground state of quantum paralectrics and their remarkable transport properties. |
Tuesday, March 7, 2023 9:12AM - 9:48AM |
F37.00003: Superconductivity enhancement in polar metals Nb:(Sr,Ba)TiO3 and Nb:(Sr,Ca)TiO3. Invited Speaker: Isao Inoue Substitution of Nb5+ for Ti4+ in SrTiO3 (denoted as Nb:SrTiO3) is an excellent electron doping method with reduced disorder, superior mobility, and higher Tc, compared with the electron doping by creating the oxygen defects denoted as SrTiO3-δ. We applied the Nb doping to the two different ferroelectric materials, Sr0.95Ba0.05TiO3 and Sr0.985Ca0.015TiO3, confirmed, by the x-ray structural analyses, they turned into polar metals with broken centrosymmetry. We can define a particular carrier density n* for the polar metals. When the carrier density n < n*, the resistance shows an upturn below the temperature TK. The values of TK increase monotonically as the carrier density decreases. Interestingly, they coincide with the ferroelectric Curie temperature at the zero carrier density limit. Although the values of n* for Nb:Sr0.985Ca0.015TiO3 and Nb:Sr0.95Ba0.05TiO3 are different by one order of magnitude, both materials show typical superconducting domes with single peaks commonly located around 1020 cm−3. Compared with the nonpolar matrix Nb:SrTiO3, the values of Tc are enhanced and reach 0.75 K for Nb:Sr0.95Ba0.05TiO3. However, the Tc enhancement was insignificant (less than 0.1 K) at around n*. The enhancement becomes much more prominent 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. Our results call for a reconsideration of the existing microscopic models of superconductivity in SrTiO3. |
Tuesday, March 7, 2023 9:48AM - 10:24AM |
F37.00004: Oxygen vacancies in strontium titanate: a DFT+DMFT perspective Invited Speaker: Claude Ederer We address the long-standing question of the nature of oxygen vacancies in strontium titanate, using a combi- |
Tuesday, March 7, 2023 10:24AM - 11:00AM |
F37.00005: Lamellar fluctuations melt ferroelectricity Invited Speaker: Gian G Guzman-Verri We consider a Ginzburg-Landau theory for a ferroelectric phase transition whose primary order parameter (electrical polarization) is coupled to gradients of elastic strain (flexoelectricity). At the harmonic level, such couplings lead to a hybridization of acoustic and optic phonon modes and can lead to phases with modulated lattice structures that precede the symmetry broken state due to level repulsion. Here, we show that at the mean field plus Gaussian approximation, the fluctuations of polarization of the parent phase tend to diverge at the onset of the transition and therefore the long-range-ordered modulated phase is avoided. We discuss the implications for the nearly ferroelectric SrTiO3 and KTaO3, and propose that these systems are melted versions of an underlying modulated state which is dominated by non-zero momentum thermal fluctuations except at the very lowest temperatures. |
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