Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z40: Invited Session: Unconvential Superconducting Pairing in Heavy Fermion Materials |
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Sponsoring Units: DCMP Chair: Qimiao Si, Rice University Room: Mile High Ballroom 2B-3B |
Friday, March 7, 2014 11:15AM - 11:51AM |
Z40.00001: Visualizing nodal superconductivity and heavy fermion formation in CeCoIn$_{5}$ Invited Speaker: Brian Zhou In solids containing elements with $f-$orbitals, the interaction between $f$-electron spins and those of itinerant electrons leads to the development of low-energy excitations with heavy effective mass. Previously, we used the scanning tunneling microscope (STM) to visualize the scattering of quasiparticles and detect their mass enhancement with the lowering of temperature in the prototypical Ce-115 heavy fermion family. Tunneling into different surface terminations revealed the composite nature of these heavy excitations, arising from the entanglement of conduction and $f$ electrons [1]. Here, by extending our techniques to milli-Kelvin temperature and high magnetic field, we first observe a spectroscopic pseudogap in the tunneling density of states of the heavy quasiparticles both prior to superconductivity and also above the critical field, indicating the development of further correlations from which the unconventional superconducting state arises. Quasiparticle interference (QPI) measurements in the superconducting and normal states demonstrate the onset of strong particle-hole asymmetry in the superconducting state, dissimilar from previous STM QPI studies of gap symmetry. Nevertheless, we can directly pinpoint the d-wave nature of Cooper pairing through visualizing the spatial symmetry of quasi-particle bound states in the vicinity of atomic scale defects [2]. Work done in the collaboration with R. E. Baumbach, J. D. Thompson, E. D. Bauer, and A. Yazdani. Primary financial support from DOE-BES.\\[4pt] [1] P. Aynajian, E. da Silva Neto, et al., Nature, 486, 201 (2012).\\[0pt] [2] B. B. Zhou, S. Misra, et al. Nature Phys., 9, 474 (2013). [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:27PM |
Z40.00002: f-electron mediated Cooper Pairing in CeCoIn$_5$ Invited Speaker: Dirk Morr Recent experimental breakthroughs in scanning tunneling spectroscopy have made it possible to probe how the complex electronic structure of the heavy fermion compound CeCoIn$_5$ evolves with decreasing temperature, eventually leading to the emergence of an unconventional superconducting state [1,2]. Using a recently developed theoretical model for quasi-particle interference (QPI) spectroscopy in heavy fermion materials [3,4], we demonstrate that the experimental QPI data are consistent with a superconducting order parameter of $d_{x^2 y^2}$-symmetry, possessing a complex, multi-band momentum space structure [1]. Furthermore, we show that the unprecedented insight into the complex electronic structure of CeCoIn$_5$ above $T_c$ opens a new path for identifying quantitatively the superconducting pairing potential, arising from the strong antiferromagnetic correlations in the heavy $f$-band [5]. Using this pairing potential to solve the multi-band superconducting gap equations provides us with a series of quantitative predictions for the critical temperature, the momentum space structure of the superconducting gaps, the phase sensitive QPI signature of the $d_{x^2 y^2}$ pairing symmetry, the spin-lattice relaxation rate, and the form of the magnetic ``spinresonance.'' The quantitative agreement between these predictions and the measured properties of superconducting CeCoIn$_5$ provides strong evidence for Cooper pairing being mediated by $f$-electron magnetism. \\[4pt] [1] M. P. Allan, F. Massee, D. K. Morr, J. van Dyke, A.W. Rost, A. P. Mackenzie, C. Petrovic and J. C. Davis, Nature Physics 9, 468 (2013).\\[0pt] [2] B.B. Zhou, S. Misra, E.H. da Silva Neto, P. Aynajian, R.E. Baumbach, J.D. Thompson, E.D. Bauer, and A. Yazdani, Nature Physics, 9, 474 (2013)\\[0pt] [3] F. Parisen~Toldin, J. Figgins, S. Kirchner, and D.K. Morr, Phys. Rev. B 88, 081101(R) (2013) \\[0pt] [4] T. Yuan, J.Figgins, and D.K. Morr, Phys. Rev. B 86, 035129 (2012). \\[0pt] [5] J. van Dyke F. Massee, M. P. Allan, J. C. Davis, C. Petrovic and D. K. Morr, submitted (2013). [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 1:03PM |
Z40.00003: Slicing a Kondo lattice: the quest for exotic superconductivity in artificially engineered Ce-based superlattices Invited Speaker: Yuji Matsuda Condensed matter systems that are both low-dimensional and strongly interacting often exhibit unusual electronic properties, with the high-$T_c$ superconductivity in cuprates and iron pnictides as the most prominent example. A metallic state with the strongest electron correlation is realized in heavy fermion compounds, whose electronic structure is essentially 3D. Recently, by fabricating epitaxial superlattices built of alternating layers of Ce-based heavy-fermion and La- or Yb-based conventional nonmagnetic metals, we have succeeded in confining heavy fermions to two dimensions, resulting in slices of 2D Kondo lattice. In CeIn$_3$/LaIn$_3$ superlattices, 2D heavy fermions display striking deviations from the standard Fermi liquid properties, and these are associated with the dimensional tuning of quantum criticality [1]. Moreover, superconductivity is observed in CeCoIn$_5$/YbCoIn$_5$ superlatttices even in the superlattice with only one-unit-cell-thick CeCoIn$_5$ layers [2]. These superconducting superlattices with atomic layer thickness exhibit highly unusual behaviors, including striking enhancement and highly unusual angular dependence of $H_{c2}$ [3]. We discuss these phenomena in terms of extremely strong coupling superconducting nature as a result of two-dimensionalization, and the entanglement of Pauli paramagnetism and Rashba interaction associated with the local inversion symmetry breaking at the heavy fermion interface. The heavy fermion superlattices offer a new playground for exploring exotic superconducting phases.\\[4pt] In collaboration with M. Shimozawa, S.K. Goh, H. Shishido, Y. Mizukami, T. Watashige, R. Endo, R. Kobayashi, T. Shibauchi and T. Terashima (Kyoto).\\[4pt] [1] H. Shishido {\it et al}. Science {\bf 327}, 980 (2010).\\[0pt] [2] Y. Mizukami {\it et al}. Nature Physics {\bf 7}, 849 (2011).\\[0pt] [3] S. K. Goh {\it et al}. Phys. Rev. Lett. {\bf 109}, 157006 (2012). [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:39PM |
Z40.00004: Nodal quasiparticle dynamics in the heavy fermion superconductor CeCoIn$_5$ revealed by precision microwave spectroscopy Invited Speaker: David Broun CeCoIn$_5$ is a heavy fermion superconductor with strong similarities to the high-$T_c$ cuprates, including quasi-two-dimensionality, proximity to antiferromagnetism and probable $d$-wave pairing arising from a non-Fermi-liquid normal state. Experiments allowing detailed comparisons of the electronic properties of these two types of superconductor are of particular interest, but in most cases are difficult to realize, due to their very different transition temperatures. Here we use low-temperature microwave spectroscopy to study the charge dynamics of the CeCoIn$_5$ superconducting state. The similarities to cuprates, in particular to ultra-clean YBa$_2$Cu$_3$O$_y$, are striking: the frequency and temperature dependence of the quasiparticle conductivity are instantly recognizable, a consequence of rapid suppression of quasiparticle scattering below $T_c$; and penetration-depth data, when properly treated, reveal a clean, linear temperature dependence of the quasiparticle contribution to superfluid density. The measurements also expose key differences, including prominent multiband effects and a temperature-dependent renormalization of the quasiparticle mass. C.J.S. Truncik et al., Nat. Comm. 4, 2477 (2013). [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 2:15PM |
Z40.00005: Charge Aspects of Composite Pair Superconductivity Invited Speaker: Rebecca Flint Conventional Cooper pairs form from well-defined electronic quasiparticles, making the internal structure of the pair irrelevant. However, in the 115 family of superconductors [1-3], the heavy electrons are forming as they pair and the internal pair structure becomes as important as the pairing mechanism. Conventional spin fluctuation mediated pairing cannot capture the direct transition from incoherent local moments to heavy fermion superconductivity, but the formation of composite pairs favored by the two channel Kondo effect can [4]. These composite pairs are local d-wave pairs formed by two conduction electrons in orthogonal Kondo channels screening the same local moment. Composite pairing shares the same symmetries as magnetically mediated pairing, however, only composite pairing necessarily involves a redistribution of charge within the unit cell originating from the internal pair structure, both as a monopole (valence change) and a quadrupole effect [5]. This redistribution will onset sharply at the superconducting transition temperature. A smoking gun test for composite pairing is therefore a sharp signature at Tc - for example, a cusp in the Mossbauer isomer shift in NpPd$_5$Al$_2$ or in the NQR shift in (Ce,Pu)CoIn$_5$. \\[4pt] [1] J. L.Sarrao and J.D. Thompson, JPSJ 76, 051013(2007).\\[0pt] [2] E. D. Bauer et al. J. Phys Cond. Mat. 24, 052206 (2012).\\[0pt] [3] D. Aoki et al, JPSJ 76, 063701 (2008).\\[0pt] [4] R. Flint, M. Dzero and P. Coleman, Nat. Phys. 4, 643 (2008).\\[0pt] [5] R. Flint, A. Nevidomskyy and P. Coleman, PRB 84, 064514 (2011). [Preview Abstract] |
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