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 Y22: Superconductivity:Nikelates & others |
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Sponsoring Units: DCMP Chair: Hongyi Xie, University of Oklahoma Room: Room 214 |
Friday, March 10, 2023 8:00AM - 8:12AM |
Y22.00001: Charge ordering as the driving mechanism for superconductivity in rare-earth nickel oxides Julien Varignon, Manuel Bibes, Wilfrid Prellier, Sébastien Petit, Lucia Iglesias, Alvaro Adrian Carrasco Alvarez Superconductivity is one of the most intriguing properties of matter characterized by zero electrical resistance and described on the basis of electrons bound into Cooper pairs by an attractive interaction. The most widely studied superconductors are the cuprates which have the highest critical temperature (Tc) at ambient conditions. In the search for compounds analogous to cuprates, nickel oxides were long proposed to show superconductivity [PRB 59, 7901 (1999); PRL 100, 016404 (2008)]. This research crystalized in 2019 with its discovery in infinite layer [Nature 572, 624 (2019)] and reduced Ruddlesden-Popper [Nature Mat. 21, 160 (2021)] nickelate thin films (Tc=9-15 K), albeit without clarifying the underpinning mechanism. |
Friday, March 10, 2023 8:12AM - 8:24AM |
Y22.00002: Superconducting Instabilities in Strongly-Correlated Infinite-Layer Nickelates Andreas Kreisel, Brian M Andersen, Astrid T Rømer, Ilya Eremin, Frank Lechermann The discovery of superconductivity in infinite-layer nickelates has added a new family of materials to the fascinating growing class of unconventional superconductors. By incorporating the strongly correlated multi-orbital nature of the low-energy electronic degrees of freedom, we compute the leading superconducting instability from magnetic fluctuations relevant for infinite-layer nickelates. Specifically, by properly including the doping dependence of the Ni dx2-y2 and dz2 orbitals as well as the self-doping band, we uncover a transition from d-wave pairing symmetry to nodal s+- superconductivity, driven by strong fluctuations in the dz2-dominated orbital states. We discuss the properties of the resulting superconducting condensates in light of recent tunneling and penetration depth experiments probing the detailed superconducting gap structure of these materials. |
Friday, March 10, 2023 8:24AM - 8:36AM |
Y22.00003: Two-gap superconductivity and decisive role of rare-earth d electrons in infinite-layer nickelates Zhenglu Li, Steven G Louie In 2019, superconductivity was discovered in infinite-layer nickelates, which has a latest experimental Tc up to 23 K. The conventional mechanism for superconductivity – phonon-mediated pairing – was presumably ruled out because density functional theory (DFT) calculations reported a very weak electron-phonon (e-ph) coupling. Here, by performing ab initio GW calculations for the electronic structure and the e-ph coupling, we discover that infinite-layer Nd0.8Sr0.2NiO2 is a dominantly two-gap phonon-mediated superconductor. We show electron correlations (self-energy effects going beyond DFT) alter dramatically the orbital character of its multi-band Fermi surface and also strongly enhance the e-ph coupling, leading to a large Tc in good agreement with experiment. The computed e-ph coupling constant λ is enhanced by an unprecedented factor of 5.5 as compared to DFT. Solutions of the anisotropic Eliashberg equations yield two dominant s-wave gaps – a large gap on states of rare-earth Nd d-electron and interstitial orbital characters but a small gap on those of transition-metal Ni d-electron character. The predicted two-gap s-wave superconductivity explains well tunneling experiments. |
Friday, March 10, 2023 8:36AM - 8:48AM |
Y22.00004: Ba137 and Ba135 NMR measurements of the high-Tc superconductor Ba1-xKxBiO3 Sylvia L Griffitt, Ana Najev, Marin Spai?, Dayu Zhai, Martin Greven, Damjan Pelc Potassium-doped barium bismuthate, Ba1-xKxBiO3, is a high-Tc superconductor that exhibits local inversion-breaking distortions despite being globally centrosymmetric [1]. Ba1-xKxBiO3 is exceptionally difficult to measure with NMR due to its long relaxation time, spin-spin diffusion, and broad spectrum, so many spin-echo pulses must be acquired to achieve a reasonable signal. In spite of these experimental difficulties, here we present Ba137 and Ba135 NMR measurements for two single-crystal samples, one metallic and one insulating. Both samples exhibit very broad spectra of several MHz as well as frequency shifts that can be replicated by assuming quadrupolar relaxation as the primary relaxation mechanism. The spectra exhibit no angle dependence, which is indicative of a highly anisotropic Ba environment that lacks the expected axial symmetry along [111]. This observation supports the picture that Ba1-xKxBiO3 is a locally non-centrosymmetric system. |
Friday, March 10, 2023 8:48AM - 9:00AM |
Y22.00005: First molecular-beam-epitaxy-grown superconducting BaPb1-xBixO3 (BPBO) thin films and discovery of a new growth mode: element-exchange epitaxy Xiaoyu Yuan, Gaurab Rimal, Seongshik Oh BaPb1-xBixO3 (BPBO) is one of the very first tunable high temperature (Tc hovering over 10 K) oxide superconductors, which preluded the famous high Tc cuprate superconductor family by over a decade. This system has a unique standing with many of its own interesting properties bridging the big temperature gap between the two famous doped oxide superconductors, the cuprate superconductors with ambient Tc well over 100 K and the doped SrTiO3 superconductor with Tc below ~0.5 K. In particular, BPBO has the strongest spin-orbit coupling strength among all known oxide superconductors, so it is one of the strong candidates for topological superconductor. Also, despite this system being known as a BCS superconductor, whether and how its strong spin-orbit coupling and the proximity to the charge-ordered BaBiO3 system affects the superconductivity is little known. As such, despite the long history of the BPBO system and its potential importance, this system is under-investigated partly due to lack of quality thin film systems. |
Friday, March 10, 2023 9:00AM - 9:12AM |
Y22.00006: Superconductivity and local structural correlations in Ba1-xKxBiO3 Dayu Zhai, Sylvia L Griffitt, Marin Spai?, Zachary W Anderson, Siddharth Gorregattu, Xing He, Joseph Joe, Matthew J Krogstad, Raymond Osborn, Damjan Pelc, Martin Greven The doped perovskite BaBiO3 exhibits a maximum superconducting transition temperature (Tc) of 34 K and was the first high-Tc oxide to be discovered, yet decades after its discovery the pairing mechanism is still debated, with strong electron-phonon coupling and bismuth valence/bond disproportionation possibly playing a role. Our recent diffuse x-ray scattering experiments and Monte Carlo modeling across its insulator-metal boundary revealed no evidence for either long- or short-range disproportionation, which resolves a major conundrum, as disproportionation and the related polaronic effects are likely not relevant for the metallic and superconducting states [1]. Instead, this work revealed nanoscale structural correlations that break inversion symmetry, which has far-reaching implications for the electronic physics, including the pairing mechanism. Here I will report our efforts to build on this work through diffuse scattering experiments at additional doping levels as well as charge transport and nonlinear magnetic response measurements of the normal and superconducting states. |
Friday, March 10, 2023 9:12AM - 9:24AM |
Y22.00007: Strain-induced superconductivity in RuO2 ? Neha Wadehra, Benjamin Z Gregory, Jacob Ruff, Hanjong Paik, Nathaniel J Schreiber, Hari P Nair, Kyle M Shen, Andrej Singer, Darrell G Schlom Ruthenates are an intensively investigated class of materials because of their wide variety of ground states which include superconductivity in Sr2RuO4, metamagnetism in Sr3Ru2O7, ferromagnetism in SrRuO3 and antiferromagnetism in Ca2RuO4, etc. [1-2]. Epitaxial strain has always been a key knob to tune these ground states and realize new functionalities in the Ruddlesden-Popper ruthenates when grown in thin-film form [3]. Recently, RuO2, the parent binary oxide of these ruthenates has attracted attention because of the discoveries of antiferromagnetism [4,5], and superconductivity [6] in this material. Here, we employ molecular-beam epitaxy to grow thin films of RuO2 on isostructural (100)-oriented TiO2 substrates and realize superconductivity at about 1 K. We show that the superconducting transition temperature (Tc) and the nature of the superconducting dip changes as a function of the thickness of the films. In contrast to the strain-induced superconductivity of RuO2 grown on (110)-oriented TiO2 [6], our relaxed, thicker films of RuO2/(100) TiO2 have higher Tc than the coherently strained thinner RuO2/(100) TiO2 films. |
Friday, March 10, 2023 9:24AM - 9:36AM |
Y22.00008: Constraints on a split superconducting transition under uniaxial strain in Sr2RuO4 from scanning SQUID microscopy Eli Mueller, Yusuke Iguchi, Clifford W Hicks, Christopher Watson, Yoshiteru Maeno, Kathryn A Moler Whether the superconducting order parameter in Sr2RuO4 is a single component or has two degenerate components is still an unresolved question. Under a degenerate two-component scenario, uniaxial strain is a useful tool because it is expected to split the degeneracy, resulting in two superconducting transitions: a primary transition at temperature Tc,1 and a secondary transition at a lower temperature Tc,2. To test for a transition splitting, we performed scanning SQUID microscopy on Sr2RuO4 under uniaxial strain and we will present the results of this measurement in this talk. By measuring the temperature dependence of the local susceptibility under non-zero strain, we place limits on the possibility of a second transition. |
Friday, March 10, 2023 9:36AM - 9:48AM |
Y22.00009: FLEX modeling of the superconducting Tc vs. strain in Sr2RuO4 John Deisz The fluctuation exchange approximation (FLEX) is used to model Tc versus strain in Sr2RuO4. Previously we showed that using first-principles parameters for band-structure, spin-orbit coupling and the orbital dependence of an atomically local electron-electron interaction, FLEX produces a state dominated by singlet, dx2-y2 pairing with smaller, spin-orbit-induced contributions in other channels (DOI: 10.1103/PhysRevLett.107.277003). While models based on triplet pairing were favored at that time, strong evidence has emerged for large or even dominant contributions from singlet pairing. Thus, FLEX, an approximation where electron-electron correlations are treated in a fully self-consistent manner, remains a useful tool for model studies to elucidate the nature of pairing in this compound. Indeed, our results show that FLEX accounts for the strain dependence of Tc observed for Sr2RuO4. Namely, for a broad range of coupling strengths, we find that Tc plummets when strain pushes a γ-band van Hove singularity through and below EF. At larger electon-electron coupling strengths, Tc vs strain results are nearly flat from zero strain to the strain-induced dropoff. However, at small coupling strengths, consistent with Tc < 10K, FLEX reproduces the experimentally observed enhancement in Tc with strain with a large peak in Tc before the dropoff. These results argue that the strain dependence is fully consistent with a pairing state dominated by singlet dx2-y2 contributions. |
Friday, March 10, 2023 9:48AM - 10:00AM |
Y22.00010: Reconciling π phase shift in Josephson junction experiments with even-parity superconductivity in Sr2RuO4 Austin W Lindquist, Hae-Young Kee The superconducting state of Sr2RuO4 was once thought to be a leading candidate for p-wave superconductivity. A constant Knight shift below the transition temperature provided evidence for spin-triplet pairing, and a π phase shift observed in Josephson junction tunneling experiments suggested odd-parity pairing [1], both of which are described by p-wave states. However, with recent experiments observing a significant decrease in the Knight shift below the transition temperature, signifying a spin-singlet state, the odd-parity results are left to be reconciled. In this work, we show that an even-parity pseudospin-singlet state originating from interorbital pairing via spin-orbit coupling can explain what has been assumed to be evidence for an odd-parity state [2]. Further, we discuss how these experiments may be used to differentiate the proposed pairing states of Sr2RuO4. |
Friday, March 10, 2023 10:00AM - 10:12AM |
Y22.00011: Phenomenological Model for Specific Heat in Multi-Orbital Sr2RuO4 Marc-Antoine Gauthier, Olivier Gingras, André-Marie S Tremblay The superconducting gap of the Sr2RuO4 is still uncertain [1]. The most recent striking inconsistency is observed under uniaxial pressure. While high-sensitivity specific heat measurements detect only one transition temperature [2], muon spin relaxation experiments suggest two [3]. The possible superconducting gap functions are extremely intricate and complex due to the multiple orbitals involved and coupled by spin-orbit interaction [4]. We compute the electronic structure, free energy and specific heat obtained for different superconducting order parameters by using the Bogoliubov-de Gennes formalism. We focus on the non-trivial effects of spin-orbit coupling, relative phases between different spin-orbital sectors, and coexisting components. |
Friday, March 10, 2023 10:12AM - 10:24AM |
Y22.00012: Electron-phonon coupling and superconducting pairing mechanism in doped SrTiO3 Nikolaus M Kandolf, Roxana Margine, Feliciano Giustino The phase diagram of n-doped SrTiO3 (STO) combines intriguing many-body effects observed in many highly researched quantum materials. The STO ground state is close to a quantum critical point, and the strong quantum fluctuations in the material are connected to structural instabilities. Moreover, its normal-state transport properties are heavily altered by strong electron correlations and polaron formation. Central to the STO phase diagram is the superconducting dome, which is insensitive to magnetic impurities and shares many features with unconventional high-$T_C$ superconductors (e.g. cuprates, iron pnictides and chalcogenides). The nature of the pairing interaction is controversial, even though interactions with various bosons have been studied; they include longitudinal optical and soft transverse phonons, plasmons, and ferroelectric fluctuations. |
Friday, March 10, 2023 10:24AM - 10:36AM Author not Attending |
Y22.00013: Superconductivity in Strontium Titanate Alexander Edelman, Peter Littlewood Strontium titanate 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. Experimentally, a series of replica bands in photoemission[1] and tunneling[2] experiments reveal very prominent coupling of the electrons to a longitudinal optic phonon. A straightforward theory coupling this phonon to an electron gas produces a superconducting phase diagram with a dome but is not well-controlled due to adiabaticity, and relies on coupled plasmon-phonon modes that are not experimentally observed in the material. In this work we examine how the inclusion of interactions beyond leading order can address these discrepancies. |
Friday, March 10, 2023 10:36AM - 10:48AM |
Y22.00014: Criterion for vestigial order above two-component superconductors from Ginzburg-Landau theory Pye Ton How, Sungkit Yip MxBi2Se3 displays nematic superconductivity, where the two-component superconducting order parameter breaks both the U(1) symmetry and the lattice rotational symmetry. More recent experiments [1, 2] seem to have shown that the rotational symmetry breaking occurs at a temperature slightly higher than the superconducting Tc. In the so-called "vestigial order" [3] scenario, the rotational symmetry breaking is driven by the fluctuation of the same superconducting order parameter. By studying the Ginzburg Landau theory with fluctuation, we rule out this scenario for a very large portion of the parameter space: the existence of this vestigial order phase requires some extreme choice of the parameters, that seems exceedingly unlikely for the case of MxBi2Se3. The same GL theory can be easily generalized to describe a p-wave chiral superconductor. For the chiral case, the possibility of a vestigial phase, where the time-reversal symmetry breaking occurs in the normal state above TC, has been discussed theoretically [4] but found little experimental support in Sr2RuO4 [5], the most prominent candidate material. Our result also helps to clarify the non-observation of the vestigial phase there. The present result has implication on any two-component superconductors. |
Friday, March 10, 2023 10:48AM - 11:00AM |
Y22.00015: Superfluid density for underdoped cuprates PATRICIA SALAS CASALES, Miguel A Solís-Atala Within the extended Layered Boson-Fermion Model of superconductivity for underdoped cuprates [1, 2], we obtain the superfluid density ρ(T) combining: the difference between superconducting and normal isobaric specific heat ?Cp(T) = Csp(T) -Cnp(T); the thermodynamic critical field Hc(T), both as functions of temperature; and the improved Rutgers Relation after including the second derivative of Hc(T). In this way, we obtain a curve for the penetration length λ(T) more suitable for underdoped cuprates YBa2Cu3O6+x , which is needed for the calculation of ρ(T)/ρ(0) = (λ(0)/λ(T))2. |
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