Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S67: Recent Experimental Surprises in Strongly Correlated SuperconductorsInvited
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Sponsoring Units: DCMP Chair: Richard Greene, University of Maryland, College Park Room: Four Seasons 2-3 |
Thursday, March 5, 2020 11:15AM - 11:51AM |
S67.00001: Unconventional superconductivity in Sr2RuO4 probed under stressed conditions Invited Speaker: Stuart Brown The stoichiometric transition metal oxide Sr2RuO4 is widely considered a model unconventional superconductor, owing to experimental evidence for strong correlations, and to the emergence of an odd-parity superconducting ground state with transition temperature near 1 K. With additional evidence for time-reversal symmetry breaking came the proposal for a two-component chiral p-wave order parameter, for which there is an expectation for a split superconducting transition when subjected to in-plane uniaxial stress. Recent studies instead revealed a factor 2.5 increase in the transition temperature of stressed samples, motivating us to probe the evolution of the normal and superconducting states using 17O NMR. Under stressed conditions, the normal state Knight shifts are consistent with tuning the Fermi surface through a van Hove singularity, along with an associated Stoner factor enhancement [1]. Over a broad temperature range, the magnetic response implies a dominant influence of the small energy difference between EF and that of the van Hove singularity, which can be controlled by strain, and independently by the Zeeman interaction. A reduced superconducting state spin polarization for in-plane magnetic elds, for all strain values studied, rules out the long-considered chiral p-wave state [2]. Although the helical state is an unlikely possibility under stressed conditions, further tightening of constraints on the superconducting state is a priority. |
Thursday, March 5, 2020 11:51AM - 12:27PM |
S67.00002: Re-measurement of Knight-shift on Superconducting Sr2RuO4 Invited Speaker: Kenji Ishida NMR experiments have been done in various unconventional superconductors, and gave information about superconducting (SC) pairing state and SC gap structure. We reported that the Knight shift at the Ru and O sites in Sr2RuO4 is invariant on passing through Tc.[1,2]. Quite recently, Pustogow and Luo et al. reported a reduction of the Knight shift at the O site measured with a small-energy RF pulse[3]. We reexamined 17O-NMR on our sample with similar small-energy RF pulses and reproduced their results[4]. Furthermore, we measured the temperature variation of the Knight shift by a standard spin-echo method with small-power RF pulses, and found that the spin susceptibility decreases in the SC state. We conclude that our previous results of the invariance of the Knight shift in the SC state were due to instantaneous destruction of superconductivity by theRF pulses. The heat-up effect was characterized by the temperature variation of the Knight shift under various measurement conditions. In the presentation, we will also present new results obtained near Hc2 along the ab plane. |
Thursday, March 5, 2020 12:27PM - 1:03PM |
S67.00003: A laser-sharp view of electron correlations in Sr2RuO4 Invited Speaker: Anna Tamai We used laser-based angle-resolved photoemission spectroscopy to explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquid Sr2RuO4. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about 2 due to electronic correlations. The self-energies for the β and γ sheets are found to display significant angular dependence. We demonstrate that this behavior does not imply momentum-dependent many-body effects, but arises from a substantial orbital mixing induced by spin-orbit coupling. A comparison to single-site dynamical mean-field theory further supports the notion of dominantly local orbital self-energies and provides strong evidence for an electronic origin of the observed nonlinear frequency dependence of the self-energies, leading to “kinks” in the quasiparticle dispersion of Sr2RuO4. |
Thursday, March 5, 2020 1:03PM - 1:39PM |
S67.00004: Magnetic excitations and their possible role in the superconducting pairing in Sr2RuO4 Invited Speaker: Markus Braden The magnetic excitations in the unconventional superconductor Sr2RuO4 consist of several contributions, an incommensurate signal arising from nesting and quasiferromagnetic fluctuations. We were able to follow the nesting signal of the quasi-one-dimensional bands across the superconducting transition down to very low energies. Even at E=0.325 meV, which lies well below the superconducting gap 2D values reported from tunneling experiments or deduced from BCS theory, there is no change in the magnetic response [1], which seems incompatible with the picture of a large gap on these Fermi-surface sheets. The quantitative analysis of the quasiferromagnetic fluctuations in Sr2RuO4 is hampered by the smaller amplitude and the little structure in Q space of this signal. Only by use of polarized neutron scattering we can determine the strength and characteristics of the ferromagnetic response, which agrees with reports of specific heat, magnetic susceptibility and NMR. Incorporating this ferromagnetic response into the gap equation, however, does not stabilize a triplet pairing state [2]. Furthermore, the quasiferromagnetic response in Sr2RuO4 does not resemble the paramagnon scattering expected for a nearly ferromagnetic material, but it seems to arise from broad instabilities at low propagation vectors. In contrast recent INS on the ferromagnetic perovskite SrRuO3 find the typical ferromagnetic magnon and paramagnon scattering below and above the Curie temperature [3]. The magnon stiffness and gap in SrRuO3 is found to anomalously soften upon cooling well below the Curie temperature, which can be attributed to the impact of Weyl points [3]. |
Thursday, March 5, 2020 1:39PM - 2:15PM |
S67.00005: Ferromagnetic order beyond the superconducting dome in a cuprate superconductor Invited Speaker: Tarapada Sarkar The cuprate high-Tc superconductors have been the subject of intense study for more than 30 years with no consensus yet on the underlying |
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