Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session C47: MgB2, Complex Compounds, Organics, and Other SuperconductorsLive
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Sponsoring Units: DCMP Chair: John Wei, Univ of Toronto |
Monday, March 15, 2021 3:00PM - 3:12PM Live |
C47.00001: Plasmon-Phonon 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. The normal state is perhaps even more enigmatic. I will give a biased introduction to this fascinating material, then I will investigate a scenario of superconductivity mediated by coupling to a strongly anti-adiabatic longitudinal optic phonon, by extending a simple Engelsberg-Schrieffer theory of electron-phonon coupling to include the effects of electronic Coulomb interactions. For the carrier densities of interest, there is a "cavity enhancement" of superconductivity by the material's open plasmons, which hybridize strongly with LO mode. Working within the cumulant expansion, I calculate the spectral signatures of this unusual regime to compare to photoemission1 and tunneling2 experiments, as well as the superconducting phase diagram. |
Monday, March 15, 2021 3:12PM - 3:24PM Live |
C47.00002: Restoration of superconductivity in high magnetic fields in UTe2 Andrei Lebed It was theoretically predicted more that 20 years ago [see A.G. Lebed and K. Yamaji, Phys. Rev. Lett., v. 80, 2697 (1998)] that a |
Monday, March 15, 2021 3:24PM - 3:36PM Live |
C47.00003: Prediction of phonon-mediated high-TC superconductivity in monolayer Mg2B4C2 Sobhit Singh, Aldo H. Romero, José D. Mella, Vitalie Eremeev, Enrique Munoz, Anastassia N. Alexandrova, Karin M Rabe, David Vanderbilt, Francisco Muñoz We theoretically design a novel two-dimensional material - Mg2B4C2, which is predicted to exhibit superconductivity with critical temperature TC estimated in the 67-84 K range (predicted using the isotropic Migdal-Eliashberg theory) without any tuning of external parameters such as doping, strain, or substrate-induced effects. This 2D material is stable and it belongs to the family of the conventional high-TC bulk superconductor MgB2. It is obtained after replacing the chemically active boron layers in MgB2 with chemically inactive boron-carbon layers. The key feature in 2D Mg2B4C2 is the fact that, unlike in bulk MgB2, more than just two E2g phonon modes strongly couple to the electronic states near the Fermi level, thus resulting in a substantially large electron-phonon coupling (λ = 1.75), as compared to the bulk MgB2 (λbulk = 0.6-0.7). This material also features a topologically nontrivial electronic bandstructure (Dirac metal). Interestingly, we find that the key features of this material remain essentially unchanged when its thickness is increased by modestly increasing the number of inner MgB2 layers. |
Monday, March 15, 2021 3:36PM - 3:48PM Live |
C47.00004: Defect Structures and properties of MgB2 wires and bulks with O2 and Dy2O3 doping Mike D Sumption MgB2 wires and bulks were fabricated via both reactive diffusion process of in-situ Mg+B powders processed above the melting temperature of Mg (bulks), and by gas-solid reaction of B+Mg vapor in an powder compact gas-diffusion couple (wires). SnO2 powder was added to the reactive diffusion route powders, and Dy2O3 powder was added to the B+Mg vapor route processed materials (in some cases to materials with underlying C-doping). Bc2 and Birr were measured resistively as a function of temperature up to 14 T. In both cases, the additions tended to increase the irreversibility field at moderate temperatures. The SnO2 decomposed and led to nanoscale MgO precipitates, the best BC2 for SnO2 doping is comparable to the best values (15 T at around 20 K) achieved by others in C-doped MgB2 bulks. In the case of Dy2O3, DyB4 precepitates were formed. TEM was used to quantify the preciptate sizes and distrubutions. It was seen that the additions tended to increase Birr but do not reduce Tc. Comparisons of C+Dy2O3 doping and C only doping showed that a larger Birr was acheivable with co-doping than by single element doping. |
Monday, March 15, 2021 3:48PM - 4:00PM Live |
C47.00005: Surface s-wave superconductivity for oxide-terminated infinite-layer nickelates Xianxin Wu, Kun Jiang, Domenico Di Sante, Werner R Hanke, Andreas P Schnyder, Jiangping Hu, Ronny Thomale We analyze the electronic structure of different surface terminations for infinite-layer nickelates. Surface NiO2 layers are found to be buckled, in contrast to planar bulk layers. While the rare-earth terminated surface fermiology is similar to the bulk limit of the nickelates, the NiO2 terminated surface band structure is significantly altered, originating from the effect of absence of rare-earth atoms on the crystal field splitting. Contrary to the bulk Fermi surfaces, there are two Ni-3d Fermi pockets, giving rise to enhanced spectral weight around the $\bar{M}$ point in the surface Brillouin zone. From a strong-coupling analysis, we obtain dominant extended s-wave superconductivity for the surface layer, as opposed to d-wave for the bulk. This finding distinguishes the nickelates from isostructural cuprates, where the analogous surface pairing mechanism is less pronounced. Our results are consistent with region-dependent gap structures revealed in recent STM measurements and provide an ansatz to interpret experimental data of surface-sensitive measurements on the infinite-layer nickelates. |
Monday, March 15, 2021 4:00PM - 4:12PM Live |
C47.00006: The influence of magnetic impurities on superconductivity in the high entropy alloys [TaNb]1-x(TiZrHf)x William Nelson, Ryan Baumbach High entropy alloys are an unusual class of materials where random mixtures of selected elements are distributed on a simple crystalline lattice (e.g., BCC). The alloys [TaNb]1-x(TiZrHf)x are damage resistant refractory metals that also exhibit superconductivity with Tc<8K. Here we report results for examples where Gd is substituted into the lattice in small amounts. X-ray diffraction measurements show that, like the parent compounds, the substituted specimens form in the BCC structure. Chemical analysis measurements also confirm uniform mixing of the components. Magnetization measurements reveal ferromagnetic ordering near room temperature, likely due to localized Gd f-moments. At low temperatures, electrical resistivity, magnetization, and heat capacity measurements show that Tc is modestly suppressed. These results will be compared to earlier studies of matrix-impurity superconductors, where the evolution of Tc and the specific heat jump ΔC at Tc provide insights about the solute spin and the nature of the superconducting pairing. |
Monday, March 15, 2021 4:12PM - 4:24PM Live |
C47.00007: Intrinsic Superconductivity in Quasiperiodic Systems Chris Matsumura, Gautam Rai, Stephan Wolfgang Haas, Anuradha Jagannathan The discovery of superconductivity in an Al-Zn-Mg quasicrystal by Kamiya et al. in 2018 has resulted in renewed efforts by the quasicrystal community to characterize the properties of superconducting phases in quasiperiodic materials. As a first step in that direction, we apply the Bogoliubov-de Gennes approach to the simplest quasicrystal, the Fibonacci Chain. We consider the hopping model in which the amplitudes can take two possible values tA and tB. We find that increasing the strength of the quasiperiodic modulation stabilizes the SC phase, in that the critical temperature Tc becomes larger with the difference of hopping amplitudes. We will show results for the local densities of states. We will report on how the superconducting gap and the resonant peaks evolve as a function of increasing interaction strength V. We will also discuss the spatial dependence of the local order parameters, as plotted in real space. As is well-known, the quasicrystal can be derived from a higher dimensional periodic system. We will use this property to show that the order parameter depends on the local environments in a simple and systematic way. |
Monday, March 15, 2021 4:24PM - 4:36PM Live |
C47.00008: Unusual Sn State in the Superconducting Disordered Selenide Ag1-xSn1+xSe2 Mohamed Oudah, Jose Lado, Shinji Kitao, Graham McNally, Kathrin Küster, Makoto Seto, Douglas A. Bonn, Bernhard Keimer, Manfred W Sigrist, George Albert Sawatzky, Hidenori Takagi We investigate the NaCl-type superconducting series Ag1-xSn1+xSe2 by means of XPS and Mössbauer spectroscopy. By varying the Ag/Sn ratio we observe a change in the ratio of 2+ and 4+ state of Sn as probed by XPS, with increasing 2+ state as the Sn amount is increased. In contrast, we detect only one state for Sn in the Mössbauer spectra, which corresponds to the unusual 3+ state for Ag:Sn of 1:1. We study the Mössbauer spectra at low temperature to elucidate the charge fluctuations scenario for Sn. Furthermore, we calculate the band structure of AgSnSe2, which reveal contributions from all three elements near the fermi level. We discuss the experimentally measured state of Sn in light of our calculations. |
Monday, March 15, 2021 4:36PM - 4:48PM Live |
C47.00009: Multimodal sub-kelvin synchrotron diffraction as a probe of the origin of nematic pinning in the candidate topological superconductor Sr0.1Bi2Se3 Matthew Smylie, Zahirul Islam, Stephan Rosenkranz, Jared Dans, Genda Gu, Wai-Kwong Kwok, Ulrich Welp In the superconducting doped topological insulator SrxBi2Se3 (Tc ~ 3.1 K), a pronounced two-fold in-plane symmetry is observed in electronic properties below Tc, despite the three-fold symmetry of the R-3m space group. The axis of symmetry is always pinned to one of three rotational equivalent directions; crystallographic strain has been proposed to be the origin of this pinning. Here, we present multimodal synchrotron diffraction measurements down to ~0.7 K on a single crystal of Sr0.1Bi2Se3 combining in-situ resistivity measurements and XRD. Magnetic fields up to 4.5 T were also applied. We find no crystallographic distortions from the normal-state structure to 1 part in 104 indicating that the two-fold anisotropy is not caused by structural distortions but is electronic in origin. |
Monday, March 15, 2021 4:48PM - 5:00PM Live |
C47.00010: Ion Beam Induced Damage in MgB2 Thin Films Leila Kasaei, Hussein Hijazi, Raj K Sah, Jay R Paudel, Hongbin Yang, Weibing Yang, Torgny Gustafsson, Ke Chen, Xiaoxing Xi, Philip Edward Batson, Alexander Gray, Leonard C Feldman We report new studies that illuminate the physics of the superconducting (S) to normal (N) transition associated with the ion bombardment of MgB2. The investigation is motivated by our recent reports of successful Josephson Junction (JJ) arrays with exceptional uniformity, where weak link (N-state) barrier line is formed by ion bombardment using the Rutgers He+ Ion Microscope (HeIM) (30 keV, <0.5nm diameter beam spot size), in a patterned, epitaxial MgB2 film [1]. |
Monday, March 15, 2021 5:00PM - 5:12PM Live |
C47.00011: Predicted Nearly Room Temperature Superconductivity in Binary Metal Hydride Systems Tianran Chen, Taner Yildirim Due to the low atomic mass and high electron-phonon coupling strength |
Monday, March 15, 2021 5:12PM - 5:24PM Live |
C47.00012: Pressure-induced superconductivity in novel polymorphic phase of BaCu2As2 (Z=10) Zheng Wu, Liangzi Deng, Shuyuan Huyan, Paul C. W. Chu, Hanlin Wu, Sheng Li, Bing Lv High-pressure electrical transport studies up to 60 GPa have been conducted on a completely new polymorphic phase of BaCu2As2 (β-BaCu2As2), in which the number of formula units Z is 10. A superconducting phase was detected at pressure above 19 GPa with the highest Tc of 7.2 K observed at 24 GPa. Two additional superconducting phases were also observed at a lower pressure range, but their respective pressure dependences are very similar to those of superconducting phases in elemental Sn, which served as the flux for growing the single-crystal β-BaCu2As2. Our high-pressure electrical transport measurements of Sn show that no new superconducting phase exists at pressure above 19 GPa and other known possible superconducting impurities are excluded as well. |
Monday, March 15, 2021 5:24PM - 5:36PM Live |
C47.00013: Kondo effect in sputtered Nb films Hansong Zeng, Dan Zhou, Guoqing Liang, Rujun Tang, Zhi Hong Hang, Zhiwei Hu, Zixi Pei, Xinsheng Ling We report a transport anomaly in sputtered niobium films after annealing in nitrogen or argon. It was found that the as-sputtered samples exhibit a single superconducting transition near 9K. However, after annealing in nitrogen and argon gases for an hour, the superconducting transition is suppressed down below 2 K. A resistance minimum appears around 20 K. This resistance minimum persists up to 8.5 T at which the superconductivity is completely suppressed. We attribute this anomaly to the Kondo effect and it is likely that iron impurities have been incorporated into the granular films during the annealing. |
Monday, March 15, 2021 5:36PM - 5:48PM On Demand |
C47.00014: Giant Grüneisen parameter in a superconducting quantum paraelectric Jacob D Franklin, Bochao Xu, Aditi Mahabir, Alexander Balatsky, Ulrich Aschauer, Ilya Sochnikov 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. |
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