Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V05: Pairing in the Most Dilute SuperconductorInvited
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Sponsoring Units: DCMP Chair: Harold Hwang, Stanford Univ Room: LACC 152 |
Thursday, March 8, 2018 2:30PM - 3:06PM |
V05.00001: A ferroelectric quantum phase transition inside a superconducting dome Invited Speaker: Joachim Hemberger The quantum paraelectric SrTiO3 becomes a metal with a superconducting instability after removal of an extremely small number of oxygen atoms [1]. Furthermore, it turns into a ferroelectric upon substitution of a tiny fraction of strontium atoms with calcium [2]. These two orders seem to be more intimately connected than intuitively anticipated at first glance. We show (by means of magnetic, transport, thermal expansion, ultrasound, dielectric, and spectroscopic measurements) that in Sr1-xCaxTiO3-δ (0.002 < x < 0.009, δ < 0.001) the "ferroelectric" order, in the sense of a screened but longrange polar lattice distortion coexists with dilute metallicity and its superconducting instability in a finite window of doping. At a critical carrier density, which scales with the Ca content, a quantum phase transition destroys the ferroelectric order. An upturn in the normal-state scattering and a significant modification of the superconducting dome can be detected in the vicinity of this quantum phase transition. The enhancement of the superconducting transition temperature with calcium substitution documents the role played by "ferroelectric" polar lattice distortions for the precocious emergence of superconductivity in this system, restricting possible theoretical scenarios for pairing. |
Thursday, March 8, 2018 3:06PM - 3:42PM |
V05.00002: Insights on dilute superconductivity in SrTiO3 from electron tunneling spectroscopy Invited Speaker: Adrian Swartz The origin of superconductivity in the oxide semiconductor SrTiO3 has remained an open question for more than 50 years. Tunneling spectroscopy, which directly probes the electronic structure of the material, is a powerful technique to study superconductivity and pairing interactions. We have developed an approach using atomically designed interfaces to realize high-quality tunneling junctions, enabling the first high-resolution examination of the systematic carrier density evolution of the intrinsic electronic structure in SrTiO3. I will discuss our experimental results examining the strength of the electron-phonon coupling, the structure of the superconducting gap, and the role of intrinsic spin-orbit coupling. These results provide insight on the nature of the highly mobile polaronic carriers and the superconducting condensate which they form. |
Thursday, March 8, 2018 3:42PM - 4:18PM |
V05.00003: Superconductivity and quantum paraelectric fluctuations in STO Invited Speaker: Alexander Balatsky
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Thursday, March 8, 2018 4:18PM - 4:54PM |
V05.00004: Superconductivity at ultra low-densities Invited Speaker: Jonathan Ruhman Superconductivity at an anomalously low density of charge carriers has been measured in doped SrTiO3 a long time ago. Only recently though, a number of new materials exhibiting superconductivity at similar densities were discovered, making the anomaly into a phenomenon. In the extremely dilute limit, the screening of Coulomb repulsion is poor, and moreover, the conventional phonon mechanism for superconductivity is completely irrelevant. This raises the question: how can these materials be superconducting? I will propose two different mechanisms for superconductivity in low-density metals, based on dynamically screened Coulomb interactions and fluctuations near a structural quantum critical point. I will then specifically focus on the former mechanism to explain superconductivity in the two lowest density examples: SrTiO3 and elemental bismuth. |
Thursday, March 8, 2018 4:54PM - 5:30PM |
V05.00005: Discovery of superconductivity in a low carrier density system: Bismuth Invited Speaker: Om Prakash Bismuth(Bi) has played a very important role in uncovering many interesting physical properties in condensed matter research, and still continues to draw enormous scientific interests due to its anomalous electronic properties. Many important phenomena such as Seebeck effect, Nernst effect, Shubnikov-de Haas effect, de Haas-van Alphen (dHvA) effect etc. were first discovered in Bi. Determination of the Fermi surface (FS) in Bi using dHvA measurements provided the basis to determine the Fermi surface of other elements\compounds. The layered structure of Bi plays a crucial role in observing many quantum phenomena rather easily. However, bulk rhombohedral Bi at ambient pressure is a compensated semimetal and it remains in the normal state down to 10 mK. The superconductivity (SC) in bulk Bi is thought to be very unlikely due to extremely low carrier density (ne=np=3x1017/cm3 at 4.2 K) . The question of SC in Bi has remained unsolved both theoretically and experimentally even today. In this talk, I will describe the first-ever observation of bulk SC in highly pure Bi single crystals (99.9999%) below 0.53 mK under ambient pressure with an estimated critical magnetic field of 5.2 µT at 0 K 1. The conventional Bardeen-Cooper-Schrieffer (BCS) theory or its extensions cannot explain the observed SC in Bi, since the adiabatic approximation used in the theories, ωD/EF<< 1 (where ωD is the Debye frequency and EF is the Fermi energy), does not hold true for Bi. One needs new theoretical ideas to understand SC in such a low carrier systems with unusual band structure in the non-adiabatic limit, ωD/EF≥1. |
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