Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K13: Sr2RuO4 and Chiral Topological SuperconductivityFocus Session
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Sponsoring Units: DMP Chair: Timo Hyart, Univ of Jyvaskyla Room: LACC 304A |
Wednesday, March 7, 2018 8:00AM - 8:36AM |
K13.00001: Demystifying the growth of superconducting Sr2RuO4 thin films Invited Speaker: Hari Nair Sr2RuO4 is an unconventional superconductor with potentially a spin-triplet, odd-parity superconducting ground state. There are many reports of high purity single crystals of Sr2RuO4 with a Tc onset of up to 1.5 K. Furthermore, recent studies have shown that the Tc can be further increased up to 3.5 K using uniaxial strain. To date, however, there are only two published reports of superconducting Sr2RuO4 thin films. This relative paucity of superconducting thin films is likely due to the extreme sensitivity of the odd-parity superconducting ground state in Sr2RuO4 to disorder, with the dominant defect in films being out-of-phase boundaries. According to recent theoretical predictions biaxially strained epitaxial thin films with isotropic in-plane strain can potentially maintain the topologically nontrivial px ± ipy superconducting ground state while simultaneously enhancing Tc by tuning the Fermi level towards a van Hove singularity. Thin films also provide a pathway for scalability, which is critical for potential practical applications of spin-triplet superconductors such as qubits for ground-state quantum computing. In this talk I outline and demonstrate a thermodynamic growth window to achieve repeatable growth of superconducting Sr2RuO4 thin films using molecular-beam epitaxy. I hope that identifying this growth window and demystifying the growth process will enable superconducting Sr2RuO4 thin films to be routinely grown by many groups. This would enable widespread studies into this extremely interesting unconventional superconductor. |
Wednesday, March 7, 2018 8:36AM - 8:48AM |
K13.00002: Half-quantum fluxoid features in the magnetotransport of Sr2RuO4 micro rings Yuuki Yasui, Kaveh Lahabi, Muhammad Shahbaz Anwar, Shingo Yonezawa, Takahito Terashima, Jan Aarts, Yoshiteru Maeno Sr2RuO4 is a leading candidate for spin-triplet superconductors with an equal-spin pairing state. For such a state, an unusual fluxoid state, the half-quantum fluxoid (HQF), can be realized because the Cooper pairs have an additional degree of freedom in its superconducting order parameter. Although the observation of HQF in Sr2RuO4 micro rings using magnetic torque has been reported [1], detection with other experimental technique is still desired [2]. |
Wednesday, March 7, 2018 8:48AM - 9:00AM |
K13.00003: Resonant Ultrasound Spectroscopy on Sr2RuO4 Sayak Ghosh, Brad Ramshaw Sr2RuO4 is believed to be a possible candidate for a spin-triplet superconductor. The superconducting order parameter of Sr2RuO4 has been the subject of extensive research for quite some time, although reaching a consensus has been primarily limited by the difficulty to do bulk sensitive measurements of the order parameter symmetry. We performed resonant ultrasound measurements on Sr2RuO4 to study the evolution of all its elastic moduli down to low temperatures and through TC. Since different symmetry order parameters couple differently to the various shear and compressional modes, measurement of all the moduli can reveal the symmetry of the order parameter. Our experiment thus gives insight on the possible pairing symmetry of the Cooper pairs in this material. |
Wednesday, March 7, 2018 9:00AM - 9:12AM |
K13.00004: Molecular beam epitaxy and characterization of superconducting Sr2RuO4 films Masaki Uchida, Motoharu Ide, Hirosuke Watanabe, Kei Takahashi, Yoshinori Tokura, Masashi Kawasaki Growth of superconducting Sr2RuO4 thin films has been extremely challenging [1], while it has long been desired in order to realize junction and microfabricated device experiments. In this talk, we report molecular beam epitaxy (MBE) growth and characterization of superconducting Sr2RuO4 films [2]. Careful tuning of the Ru flux with an electron beam evaporator enables us to optimize growth conditions including the Ru/Sr flux ratio and also to investigate stoichiometry effects on the structural, defect, and transport properties of the superconducting films. The highest onset transition temperature of 1.1 K is observed for a slightly Ru-rich flux condition. The realization of superconducting Sr2RuO4 films via oxide MBE opens up a new route to study the exotic superconductivity of this material. [1] Y. Krockenberger et al., Appl. Phys. Lett. 97, 082502 (2010). [2] M. Uchida et al., APL Mater. 5, 106108 (2017). |
Wednesday, March 7, 2018 9:12AM - 9:24AM |
K13.00005: Impurity-induced states in superconducting heterostructures Dong Liu, Enrico Rossi, Roman Lutchyn Different heterostructures allow the realization of novel electronic states. Exemplary is the case of quasi 1D heterostructures formed by a superconductor and a semiconductor with spin-orbit coupling in which Majorana zero-energy modes can be realized. The presence of disorder can strongly affect these states and, in general, the properties of the heterostructures. We study the effect of a single impurity on the spectrum of superconducting heterostructures. We find that the coupling between the superconductor and the semiconductor can strongly affect the impurity-induced states and may induce additional sub-gap bound states that are not present in isolated superconductors. For the case of quasi 1D superconductor/semiconductor heterostructures we obtain the conditions for which the low-energy impurity-induced bound states appear. |
Wednesday, March 7, 2018 9:24AM - 9:36AM |
K13.00006: Cooper pair spin current in SrRuO3 / Sr2RuO4 heterostructure Suk Bum Chung, Se Kwon Kim, Ki Hoon Lee, Yaroslav Tserkovnyak It has been recognized that the condensation of spin-triplet Cooper pairs requires not only the broken gauge symmetry but also the spin ordering as well. One consequence of this is the possibility of the Cooper pair spin current analogous to the magnon spin current in magnetic insulators, the analogy extending to the existence of the Gilbert damping. The recently fabricated heterostructure of the thin film of the itinerant ferromagnet SrRuO3 on the bulk SrR2uO4, the best-known candidate material for the spin-triplet superconductor, offers a promising platform for generating such spin current. We will show how such heterostructure allows us to not only realize the long-range spin valve but also electrically drive the collective spin mode of the spin-triplet order parameter. Our proposal represents both a new realization of the spin superfluidity and a transport signature of the spin-triplet superconductivity. |
Wednesday, March 7, 2018 9:36AM - 9:48AM |
K13.00007: Nematic and chiral superconductivity induced by odd-parity fluctuations Ivar Martin, Fengcheng Wu Recent experiments indicate that superconductivity in Bi2Se3 intercalated with Cu, Nb, or Sr is nematic with rotational symmetry breaking. Motivated by this observation, we present a model study of nematic and chiral superconductivity induced by odd-parity fluctuations. We show that odd-parity fluctuations in the two-component Eu representation of D3d crystal point group can generate attractive interaction in both the even-parity s-wave and odd-parity Eu pairing channels, but repulsive interaction in other odd-parity pairing channels. Coulomb repulsion can suppress s-wave pairing relative to Eu pairing, and thus the latter can have a higher critical temperature. Eu pairing has two distinct phases: a nematic phase and a chiral phase, both of which can be realized in our model. When s-wave and Eu pairings have similar instability temperature, we find an intermediate phase in which both types of pairing coexist. |
Wednesday, March 7, 2018 9:48AM - 10:00AM |
K13.00008: Chiral p-wave superconductivity induced by Berry phase effect in doped monolayer Pb3Bi on Ge (111) Wei Qin, Leiqiang Li, Shang Ren, Zhenyu Zhang Unconventional superconductivity stems from strong anisotropic electron correlation in the momentum space. For systems with nonvanishing Berry curvature, the electron-electron scatterings in the reciprocal space will acquire additional geometric (Berry) phases, thus enhancing the degrees of freedom for accommodating the anisotropy of electron correlation. Here, we explore the exotic superconducting phases in doped monolayer Pb3Bi grown on Ge (111). First, based on first-principles calculations, we reveal a large Rashba-type spin-orbit splitting of the band structure and a type-II van Hove singularity of the density of states below the Fermi level. Next, a phenomenological low-energy model is developed to capture the essential physics at the van Hove filling. Using renormalization group method, we find that a superconducting instability with p-wave pairing symmetry can be induced by Berry phase effect within repulsive electron-electron interaction, and dominates over other competing orders in the weak interaction regime. Furthermore, condensation energy calculations show that the superconducting state prefers fully-gapped chiral px+ipy pairing symmetry. These results demonstrate that monolayer Pb3Bi is a promising candidate for realizing intrinsic topological superconductivity. |
Wednesday, March 7, 2018 10:00AM - 10:12AM |
K13.00009: Topology and symmetry in multiple superconducting phases of U1-xThxBe13 Takeshi Mizushima, Muneto Nitta We theoretically investigate the topological aspect of the multiple superconducting phases in the heavy-fermion compound U1-xThxBe13. The gap symmetry and the multiple phase transitions in this superconductor (SC) have been longstanding unsolved issues. Recent measurements with high-quality single crystals suggests the odd-parity Eu state as a possible scenario [Y. Shimizu et al., arXiv:1709.00128]. In this scenario, the multiple superconducting phases are composed of uniaxial nematic, biaxial nematic, and nonunitary cyclic phases, corresponding to the DIII topological SC, Dirac SC, and Weyl SC, respectively. In this talk, we show that all the phases are accompanied by different types of topological quasiparticles: surface helical Majorana fermions in nematic phases and itinerant Majorana fermions in the cyclic phase. In particular, in the cyclic phase, surface states evoluve from a Majorana cone to Majorana arcs under rotation of surface orientation. The cone is protected solely by an accidental spin rotation symmetry and fragile against spin-orbit coupling, while the arcs are protected by the first Chern number and one-dimensional winding number. Lastly, we discuss how topological quasiparticles can be captured from tunneling conductance spectra in U1-xThxBe13. |
Wednesday, March 7, 2018 10:12AM - 10:24AM |
K13.00010: Spontaneous surface magnetization and chiral Majorana mode in p±is superconductors Wang Yang, Chao Xu, Congjun Wu Majorana fermion is usually realized by first singling out a nondegenerate Fermi surface with only one component of the electrons, |
Wednesday, March 7, 2018 10:24AM - 10:36AM |
K13.00011: A mechanism of 1/2 e^2/h conductance plateau without 1D chiral Majorana fermions Wenjie Ji, Xiao-Gang Wen We address the question about the origin of the 1/2 e^2/h |
Wednesday, March 7, 2018 10:36AM - 10:48AM |
K13.00012: Pair-Breaking Study of High-Spin Topological Superconductor YPtBi Connor Roncaioli, Prathum Saraf, Hyunsoo Kim, Johnpierre Paglione YPtBi has been a promising candidate for exploring the interplay of topological band structure and superconductivity. With a superconducting transition at 0.7 K, the extremely low carrier density semimetal YPtBi hosts an extremely unusual superconducting state with posited high-spin (j=3/2) pairing states that explain a nodal order parameter structure. Here we describe experiments to probe the pair-breaking effect on superconductivity in YPtBi as a function of controlled disorder in order to gain insight into the nature of the superconducting state. |
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