Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E04: Open Questions in Unconventional SuperconductivityInvited
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Sponsoring Units: DCMP Chair: Steven Kivelson, Stanford University Room: LACC 151 |
Tuesday, March 6, 2018 8:00AM - 8:36AM |
E04.00001: What’s Up with the Cuprates? Invited Speaker: Michael Norman It has been over thirty years since the discovery of high temperature cuprate superconductivity, and yet, despite much work, many open questions remain [1]. What is the pseudogap phase? Preformed pairs? Nematic order? Loop current order? And is the strange metal phase really a strange metal? Does it exhibit quantum critical scaling? Planckian physics? And ultimately, what is the origin of high temperature superconductivity? Can one get away with a weak coupling spin fluctuation picture, or is a liquid of singlets a more appropriate way of thinking about it? Finally, do cuprate analogues exist [2], and if so, what can they tell us that we already do not know? |
Tuesday, March 6, 2018 8:36AM - 9:12AM |
E04.00002: Heavy fermion superconductors: immediate family and other relatives Invited Speaker: Ilya Vekhter Heavy fermion superconductors were the earliest studied electronic systems where correlations lead to unconventional pairing, and remain the broadest family of such compounds. Yet the very breadth of the observed behaviors makes it critical to decide what features are idiosyncrasies of a specific family member, and which ones have common origin across the entire family and possibly connect with more distant relatives in the unconventional superconductor family such as cuprates, iron, or organics. What is the role of traditional Kondo and Mott physics, frustration, quantum critical fluctuations, hidden orders, composite and multicomponents order parameters, electronic nematicity, in determining the superconducting phases? Do we understand what aspects play the dominant role for superconductivity in different families and why? Is there a broader consensus on what the important questions and answers are? I will review the current status of the field emphasizing the common trends and open questions within several families of heavy fermion compounds, and make connection with other systems. |
Tuesday, March 6, 2018 9:12AM - 9:48AM |
E04.00003: The odd case of superconductivity in strontium ruthenate Invited Speaker: Daniel Agterberg Discovered 24 years ago, strontium ruthenate was rapidly established to be an unconventional superconductor [1]. Further early experiments strongly suggesting a spin-triplet, odd-parity chiral p-wave state – providing a platform for non-Abelian Majorana modes. However, more recent experiments have revealed flaws in this interpretation [2], re-opening key questions: What is the origin of superconductivity? What is the pairing symmetry? These questions are all the more pertinent given that the normal state electronic structure is well understood, so the theory of the superconducting state should be within reach. Fortunately, recent strain experiments provide a new experimental tool that should provide essential insights into this vexing problem [3]. |
Tuesday, March 6, 2018 9:48AM - 10:24AM |
E04.00004: Superconductivity in Molecular Solids Invited Speaker: Stuart Brown The notion of synthetic metals designed to optimize the conditions for phonon-mediated pairing is often traced back to Little’s ideas related to conducting polymers. However, the ultimate realization of organic metals and superconductors took a different form, that of charge transfer salts such as (TMTSF)2X, (BEDT-TTF)2X. Exploration of that system and related materials revealed a rich interplay of correlation effects and dimensionality, and the emergence of superconductivity proximate to magnetic phases. Apparently irreconcilable data point to different descriptions of the order parameters, and therefore leaves open whether pairing originates with magnetic fluctuations, has some charge-fluctuation character, or is more mundane. Experimental limitations have left a number of research directions and questions unexplored: what circumstances prevent the fairly limited set of applied spectroscopies from being expanded to regularly include what is widely used in oxides and other materials, such as STM, neutron scattering, and ARPES? By what routes can the carrier density be controlled, and would parallels to cuprate physics emerge under those circumstances? Can the observations of high field superconductivity be ascribed to a transition to a Larkin-Ovchinnikov phase, or is a singlet to triplet transition the explanation for superconductivity exceeding the paramagnetic limiting field? How does proximity to a gapless spin liquid phase influence the phase diagram? Finally, what’s behind reports of a superconducting transition temperature of 120 K? |
Tuesday, March 6, 2018 10:24AM - 11:00AM |
E04.00005: Current challenges in Fe-based superconductors Invited Speaker: I.I. Mazin Fe-based superconductors (FeBS) are different from cuprates in many aspects, including time placement and social climate in the superconducting community. Unlike cuprates, where it took 20 years for the researchers to arrive at a reasonably broad consensus regarding their pairing symmetry and mechanism, in case of FeBS the opinions converged to the spin-fluctuation driven $s_\pm$ pairing within a few years. Unfortunately, the feeling that we have gained a fundamental understanding of FeBS turned out to be fleeting. With the broadening of superconducting materials in this class, this simple scenario has been increasingly questioned, and some of the unsolved problems of the early stage have been re-evaluated and found to be more important than it had seemed. Some parent compounds are magnetic, but it is not clear why they do not order at much higher temperatures. Others show no long range order, despite the fact that frustration and reduced dimensionality are relatively mild. Are the local $\pi,0$ correlations that survive up to the highest measurable temperatures relevant? Second, what pushes apart electron and hole pockets? A whole block of questions arises in intercalated or monolayered FeSe, with Tc up to 40 K in the bulk and at least 80 at some interfaces. Neither $d-$wave, not ``old'' $s_\pm$, not a constant-sign $s$ are apparently consistent with the experiments, and why does removing the hole pocket enhance $T_c$, in apparent contradiction to the original-consensus spin-fluctuation theory? Moreover, these FeBS feature some very small $E_F$ but no indications of BEC and pre-formed pairs. What other important effects may be triggered by a small $E_F$? What is the role of phonons, in particular the forward scattering process ascribed to the monolayers, proposed to enhance Tc over that possible by electronic excitations? |
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