Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A08: Low Carrier Density Superconductors |
Hide Abstracts |
Sponsoring Units: DCMP DMP Chair: Kamran Behnia, ESPCI ParisTech Room: BCEC 150 |
Monday, March 4, 2019 8:00AM - 8:12AM |
A08.00001: Electron-phonon coupling and superconductivity in SrTiO3 Dirk Van Der Marel Doped STO is a superconductor with a maximal Tc rising from zero at zero doping to 400 mK around 0.005 electrons per Ti atom, and dropping to zero above 0.02 electrons per Ti atom. Substituting the heavier O-18 isotope for the natural O-16 in the samples results in a strong increase of Tc with an isotope coefficient some 20 times larger and of opposite sign as predicted by the BCS model. This effect was theoretically anticipated based on a model where the superconducting pairing in STO is mediated by the optical phonons which are associated to the para- to ferro-electric phase transition in in the undoped insulating compound, the so-called TO1 mode. Here we demonstrate from experimental data that the spectral weight of the TO1 mode is anomalously large, providing a strong indication for the so-called "charged phonon" effect predicted by MJ Rice. The experimentally observed spectral weight of mode TO1 is used to determine the nature and strength of its' coupling to the conduction electrons, and implications of these numbers are presented for the corresponding channel of superconducting pairing. |
Monday, March 4, 2019 8:12AM - 8:24AM |
A08.00002: Low density superconductivity of SrTiO3 bounded by the adiabatic condition Hyeok Yoon, Adrian G Swartz, Hisashi Inoue, Yasuyuki Hikita, Harold Hwang The dilute oxide semiconductor SrTiO3 exhibits superconductivity over a wide range of carrier densities (1017-1021 cm-3) lying outside of the Bardeen-Cooper-Schrieffer (BCS) and Migdal-Eliashberg theories for conventional superconductors. Across this range, the Fermi level traverses a variety of vibrational modes in the system and therefore spans the widest range of adiabatic parameter (ω/EF) of any superconductor, making it an ideal choice to study the physics of dilute superconductivity. Recently, we have developed an approach using atomically controlled oxide heterostructures to spectroscopically probe the electronic structure of SrTiO3 using planar tunneling junctions [1, 2]. Using this technique, we have examined the superconductivity of bulk doped SrTiO3 across the superconducting dome and found that superconducting state is bounded by the adiabatic condition, maintaining the thermodynamic relationship of BCS weak-coupling theory (2Δ/kBTc = 3.53). |
Monday, March 4, 2019 8:24AM - 8:36AM |
A08.00003: Plasmon-Polaron Superconductivity in Strontium Titanate Alexander Edelman, Peter B Littlewood Strontium titanate (STO) is a bulk insulator that becomes a semiconducting superconductor at remarkably low carrier densities - below 1017 cm-3 - with a characteristic superconducting dome as a function of doping which peaks at Tc~300mK, all in very close proximity to a ferroelectric quantum critical point. Photoemission[1] and tunneling[2] have revealed multiple phonon replica bands, and the challenge to theory is to reconcile these observations of a very strongly coupled normal state with the apparently conventional weakly-coupled BCS superconducting state[3]. We propose a simple model that extends an Engelsberg-Schrieffer theory of electrons coupled to a single longitudinal optic (LO) phonon mode to self-consistently include the effects of electronic interactions. For the carrier densities of interest, we find that the plasmon hybridizes strongly with LO mode with one of the resulting coupled modes inherting the low energy scales of the softening ferroelectric mode. We calculate spectral signatures of our model and the superconducting phase diagram, including vertex corrections crucial in this strongly-coupled regime, and compare to experiment. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A08.00004: Gate voltage tunable superconductivity in low-carrier-density SrTiO3 Terence Bretz-Sullivan, J Samuel Jiang, Alexey Suslov, JOHN E. PEARSON, Alex Martinson, Anand Bhattacharya We demonstrate gate voltage control of the critical current of a surface channel of the low-carrier-density, bulk-superconducting oxide system SrTiO3. The devices are in a conventional field effect transistor geometry with a solid gate dielectric and superconducting leads. We can modulate the critical current by a factor of 2-3 in our devices as a function of gate voltage. Additionally, we study the magnetic field dependence of the differential resistance of these devices to understand the role the superconducting leads play on the channel properties. A device such as this could serve as a new qubit architecture when integrated into a microwave resonator circuit. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A08.00005: Enhancement of the superconducting Tc upon isotopic substitution in Strontium Titanate Willem Rischau, Dorota Pulmannova, Gernot Scheerer, Enrico Giannini, Dirk Van Der Marel SrTiO3, a quantum paraelectric, becomes a metal with a superconducting instability after removal of an extremely small number of oxygen atoms. It turns into a ferroelectric upon isotopic 18O substitution exceeding a quantum critical point (QCP) located at a substitution level of around 33 at% 18O. The exceptionally dilute superconducting and the ferroelectric order may be accidental neighbors or intimately connected, as in the picture of quantum critical ferroelectricity. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A08.00006: WITHDRAWN ABSTRACT
|
Monday, March 4, 2019 9:12AM - 9:24AM |
A08.00007: Pairbreaking Induced by Magnetic Impurity Doping Shrinks the Giant Magnetoresistance Peak of the Cooper Pair Insulator James Valles, Xue Zhang, James C Joy, Chunshu Wu, Jimmy Xu Films near the critical point of a boson fluctuation dominated superconductor to insulator transition exhibit a giant magnetoresistance peak that grows exponentially with decreasing temperature. Experiments indicate that Cooper pair transport dominates the low magnetic field side of the MR peak so that the resistance rise indicates increasing Cooper pair localization. To probe whether the peak arises from magnetic field induced orbital dephasing of the Cooper pair states or Cooper pair breaking, our group investigated its response to a pure pairbreaking perturbation: magnetic impurity doping. We quench deposited a-Bi films onto nanoporous anodized aluminum oxide substrates and tuned their thickness to produce an insulating state of Cooper pairs. Gd impurity doping 1) reduces the size of the MR peak while pushing it to lower magnetic fields and 2) causes changes in the zero magnetic field resistance that are much smaller than the MR peak. How this behavior suggests that orbital dephasing effects are primarily responsible for the MR peak will be discussed. |
Monday, March 4, 2019 9:24AM - 9:36AM |
A08.00008: Electron-phonon mediated superconductivity in dilute systems Maria Navarro Gastiasoro, Andrey Chubukov, Rafael M Fernandes
|
Monday, March 4, 2019 9:36AM - 9:48AM |
A08.00009: Superconductivity at low carrier density beyond leading order in the coupling constant Dan Phan, Andrey Chubukov We present results for the onset pairing instability temperature for superconductivity mediated by an Einstein phonon in a 2D electron gas with a parabolic dispersion, in the regime of low carrier density. We go beyond the leading order in the electron-phonon coupling and include contributions to the pairing kernel from fermionic self-energy and from Kohn-Luttinger processes. We then compute the phase stiffness and obtain the actual critical temperature Tc. We analyze thermodynamic properties of our system, such as the specific heat jump across Tc. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A08.00010: Strain-engineered interaction of quantum polar and superconducting phases Chloe Herrera, Jonah Cerbin, Kirsty Dunnett, Alexander Balatsky, Ilya Sochnikov Much of the focus of modern condensed matter physics concerns control of quantum phases with examples that include flat band superconductivity in graphene bilayers, the interplay of magnetism and ferroelectricity, and induction of topological transitions by strain. 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 as potentially resulting from the local strain of jammed tetragonal domains. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A08.00011: Superconductivity in semimetals with strong Coulomb repulsion Sergueï Tchoumakov, William Witczak-Krempa We investigate the superconductivity of compounds with a small carrier density, such as SrTiO3, bismuth and YPtBi. In these materials, the pairing instability is often described as a consequence of charge fluctuations either due to longitudinal phonons or through the dynamic screening of the Coulomb repulsion. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A08.00012: A sign of anticorrelation between the orbital related order and the superconductivity in LiTi2O4-δ system Wei Hu Previously studies show that for the Li1+xTi2-xO4-δ(x<1/3) samples grown below a certain oxygen pressure, Tc0 remains the same whereas the orbital related order become weaker as the oxygen pressure increasing. It seems that the superconductivity of those samples isn’t obviously related to the orbital related order. However, it is recently founded that, for those samples, there is a small linear region exists in temperature dependent resistance curve (R-T curve) above the superconducting state, and the temperature that the R-T curve begin to deviate from this linear region at the low temperature side enhances with the increase of the oxygen pressure. The suppressive effect of current on superconductivity shows that this temperature is the beginning of the superconductivity. Therefore, it indicates that the superconductivity is actually strengthened as the oxygen pressure increasing, contrary to the effect of the oxygen pressure on the orbital related order. Moreover, a new superconducting film similar to LiTi2O4-δ has been synthesized by PLD, which shows more directly that the superconductivity can be weakened by the orbital order. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A08.00013: Unconventional full-gap superconductivity in Kondo lattice with semi-metallic conduction bands Shoma Iimura, Motoaki Hirayama, Shintaro Hoshino Non-local and anisotropic Cooper pairings are favored in heavy-electron materials with nearly localized f-electrons. On the other hand, the full-gap nature of the superconducting states in CeCu2Si2 and UBe13 have been revealed by the specific heat measurements in a rotating magnetic-field. Therefore, it is desirable to propose a new mechanism of superconductivity that is specific to heavy-electron materials. |
Monday, March 4, 2019 10:36AM - 10:48AM |
A08.00014: Dynamical effects on BCS-BEC crossover in the Holstein model Tae-Ho Park, Han-Yong Choi We present a study of the half-filled Holstein model with superconducting order employing the dynamical mean-field theory in combination with the numerical renormalization group. Here, we investigate the dynamical effects on the crossover from the BCS to Bose-Einstein condensation (BEC) regimes as the on site electron-phonon coupling g is varied for both adiabatic (t/ω0 >>1) and antiadiabatic (t/ω0 <1) phonons, where t is the hopping amplitude and ω0 is the phonon frequency. It turns out that the maximum superconducting transition temperature Tc universally coincides with the critical electron-phonon coupling gc1 where the bipolaron instability takes place in the normal state. However, in the case of the adiabatic phonon, the pairing amplitude ΔP is suppressed with increasing the electron-phonon coupling in the BEC regime, while it increases with increasing g for the antiadiabatic phonon. Further, the calculated superfluid stiffness DS for the adiabatic phonon decreases rapidly as increasing g together with the suppression of the coherence peak in the BEC regime. We also calculated the spectral intensities to identify BCS and BEC regimes and those results are comparable to the angle resolved photoemission spectroscopy (ARPES) experiment. |
Monday, March 4, 2019 10:48AM - 11:00AM |
A08.00015: Intermolecular Coupling and Superconductivity in Chevrel Phase Compounds Jia Chen, Andrew Millis, David Reichman To understand superconductivity in Chevrel phase compounds and guide the search for interesting properties in materials created with Chevrel phase molecules as building blocks, we use ab-initio methods to study the properties of single Mo6X8 molecules with X=S, Se, Te as well as the bulk solid PbMo6S8. In bulk PbMo6S8, the different energy scales from strong to weak are: the band kinetic energy, the intra-molecular Coulomb interaction, the on-molecule Jahn-Teller energy and the Hund's exchange coupling. The metallic state is stable with respect to Mott and polaronic insulating states. The bulk compound is characterized by a strong electron-phonon interaction with the largest coupling involving phonon modes with energies about 12 meV and with a strong inter-molecule (Peierls) character. A two-band Eliashberg equation analysis shows that the superconductivity has different gaps on the two Fermi surface sheets. A Bergman-Rainer analysis reveals that the Peierls modes provide the most important contribution to the superconductivity. This work illustrates the importance of inter-molecular coupling for collective phenomena in molecular solids. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700