Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session B3: Symmetry Breaking in Unconventional SuperconductorsInvited
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Sponsoring Units: DCMP Chair: Sauls James, Northwestern University Room: Ballroom III |
Monday, March 14, 2016 11:15AM - 11:51AM |
B3.00001: Signatures of time reversal symmetry breaking in multiband superconductors Invited Speaker: Saurabh Maiti Multiband superconductors serve as natural host to several possible gound states that compete with each other. At the boundaries of such competing phases, the system usually compromises and settles for `mixed' phases that can show intriguing properties like co-existence of magnetism and superconductiivty or even co-existence of different superconducting phases. The latter is particularly interesting as it can lead to non-magnetic ground states that spontaneously break Time-Reversal symmetry. While the experimental verification of such states has proved to been challenging, the theoretical investigations have provided exciting new insights into the nature of the ground state and its excitations all of which have experimental consequences of some sort. These include extrinsic properties like spontaneous currents around impurity sites, and intrinsic properties in the form of collective excitations. These collective modes bear a unique signature and should provide clear evidence for time reversal symmetry broken state. While the results are general, in light of recent Raman scattering experiments, its direct relevance to extremely hole doped Ba_(1-x)K_x(FeAs)_2 will be presented where a strong competition of s-wave and d-wave ground state is expected. [Preview Abstract] |
Monday, March 14, 2016 11:51AM - 12:27PM |
B3.00002: Strain-tuning through a possible van Hove singularity in Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ Invited Speaker: Clifford Hicks The superconducting transition temperature $T_{\mathrm{c}}$ of the tetragonal compound Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ was recently shown to be strongly sensitive to \textless 100\textgreater orthorhombic distortion: $T_{\mathrm{c}}$ increases strongly both when Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ is tensioned and compressed along a \textless 100\textgreater direction. This sensitivity was tentatively attributed to the proximity of one of its Fermi surface sheets to van Hove singularities (vHS): the sections of this sheet that pass closest to the Brillouin zone boundaries are also those perturbed most strongly by \textless 100\textgreater orthorhombic distortion. By increasing the applied orthorhombic distortion to 0.5--1.0{\%} --- a uniaxial pressure almost certainly above 1 GPa --- we have now been able to tune through a sharp peak in both $T_{\mathrm{c}}$ and the upper critical field, $H_{\mathrm{c2}}$. At the peak, $T_{\mathrm{c}}$ more than doubles. The peak is at a strain value consistent with predictions for when the Fermi surface contacts the zone boundary, $i.e.$ the van Hove singularity, although concrete verification will require further measurement. The strength of the enhancement of $H_{\mathrm{c2}}$ may have bearing on the symmetry of the order parameter. The large change in properties at this peak mean that it can almost be considered as a new, unexplored material, opening avenues for future research. [Preview Abstract] |
Monday, March 14, 2016 12:27PM - 1:03PM |
B3.00003: Is Sr2RuO4 a chiral p-wave superconductor? Insights from edge currents and uniaxial strain Invited Speaker: Thomas Scaffidi The prevailing candidate for the superconducting order parameter in Sr2RuO4 is chiral p-wave and signatures of this phase have been looked for experimentally. In this work, we discuss two of these experiments at the light of theoretical results obtained from a weak coupling RG calculation. First, we show that the most favored chiral superconducting order parameter in Sr2RuO4 has Chern number |C|=7 in the weak coupling limit, owing to a dominant longer range pairing. Since it was shown that the edge currents of a |C|>1 superconductor vanish exactly in the continuum limit, and can be strongly reduced on the lattice, this form of order parameter could help resolve the conflict between experimental observation of time-reversal symmetry breaking and yet the absence of observed edge currents in Sr2RuO4. Second, the p-wave order parameter obtained from the RG calculation exhibits a large Tc enhancement under uniaxial strain along 100. This enhancement is symmetric for tensile and compressive strain, and shows no measurable cusp at zero strain, in agreement with experiments. The absence of such a cusp is therefore not incompatible with a chiral p-wave state. Finally, we make predictions about the evolution of the superconducting state as a Van Hove singularity is crossed at larger strain. [Preview Abstract] |
Monday, March 14, 2016 1:03PM - 1:39PM |
B3.00004: Study of the Topological-insulator-based Topological Superconductors Invited Speaker: Dong Qian Three-dimensional topological insulators possess nontrivial spin-momentum locked surface states under the protection of time-reversal symmetry. The interplay between topological order and superconductivity can lead to topological superconducting state. In this talk, I will discuss our recent progress in topological-insulator-based topological superconductors. Using molecular beam epitaxy (MBE) method, we succeeded in fabricating very high quality TI/s-wave superconductor heterostructure by growing topological insulator thin films on the conventional superconductor niobium diselenide (NbSe$_{\mathrm{2}})$ substrate. Using low temperature scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoemission spectroscopy (ARPES), we systematically studied its electronic structure and superconducting behavior. Through superconducting proximity effect, coexistence of Cooper pairs and topological surface states on the surface of topological insulator film was realized. By exploring the superconducting vortex core state as the function of film thickness, existing of nontrivial superconducting state on the TI's surface was proposed. Our topological insulator/superconductor heterostructure may host single zero-energy Majorana mode in the vortex core. In addition, I will also discuss STM and ARPES studies on the recently discovered superconducting Sr-doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ bulk crystals. Our results suggest that Sr-doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ could be an excellent candidate for exploring topological superconducting states. [Preview Abstract] |
Monday, March 14, 2016 1:39PM - 2:15PM |
B3.00005: Proximity-induced Superconductivity in Topological Insulator/Superconductor Heterostructures Invited Speaker: Nitin Samarth The exploration of superconductivity in helical Dirac fermions is strongly motivated by scenarios for realizing exotic quantum states in condensed matter. Examples include Majorana modes [1] and supersymmetry [2]. Motivated by such proposals, we are pursuing the development of epitaxially grown heterostructures wherein we attempt to induce superconductivity in the surface states of a three dimensional topological insulator thin film when interfaced with both conventional [3] and unconventional [4] superconductors. This talk will provide an overview of the materials synthesis challenges in this context and discuss the picture of induced superconductivity (or lack thereof) that emerges from angle-resolved photoemission spectroscopy [3,4], point contact Andreev reflection spectroscopy [5] and tunneling spectroscopy.\\ Work done in collaboration with A. Richardella, S.-Y. Xu, M. Z. Hasan, M. Gilbert, F.-C. Chou, G. Gu, W.-Q. Dai and Qi Li.\\ 1. X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. {\bf 83} 1057 (2011).\\ 2. T. Grover, D. N. Sheng, A. Vishwanath, Science {\bf 344}, 280 (2014).\\ 3. S.-Y. Xu {\it et al.}, Nature Physics {\bf 10}, 943 (2014).\\ 4. S.-Y. Xu {\it et al.}, Phys. Rev. B {\bf 90}, 085128 (2014).\\ 5. W.-Q. Dai {\it et al.}, in preparation. [Preview Abstract] |
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