Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H09: Superconductivity: Sr2RuO4 and other Triplet Candidates |
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Sponsoring Units: DCOMP Chair: Liuyan Zhao, University of Michigan Room: BCEC 151A |
Tuesday, March 5, 2019 2:30PM - 2:42PM |
H09.00001: Investigating the Pairing Symmetry in Uniaxially Strained Sr_{2}RuO_{4} by ^{17}O NMR Shifts Andrej Pustogow, Yongkang Luo, Aaron Chronister, Yue-Shun Su, Naoki Kikugawa, Dmitry Sokolov, Fabian Jerzembeck, Andrew Mackenzie, Clifford Hicks, Eric Bauer, Stuart E Brown The unconventional superconductor and Fermi liquid Sr_{2}RuO_{4} exhibits clear effects of electronic correlations associated with Hund’s Rule coupling. In the superconducting (SC) state, several experiments produced evidence for time reversal symmetry breaking, and nuclear magnetic resonance (NMR) Knight shift measurements were consistent with an equal-spin pairing triplet state. To that end, Sr_{2}RuO_{4} is widely considered as a potential candidate for a chiral p-wave pairing with an order parameter of p_{x} ± ip_{y} symmetry - a solid state analog to ^{3}He-A. |
Tuesday, March 5, 2019 2:42PM - 2:54PM |
H09.00002: Microscopic model of the Knight shift in an anisotropic type-II superconductor Richard Klemm, Aiying Zhao, Jingchuan Zhang, Qiang Gu We have simplified our model of the Knight shift in an anisotropic metal and superconductor by forcing the Hamiltonian for an electron in the Landau orbits for an ellipsoidally anisotropic single electron Hamiltonian and the Zeeman interaction to be relativistically consistent. The resulting Hamiltonian with three effective masses in the three directions of an orthorhombic crystal is invariant under the most general proper Lorentz transformation involving general rotations about all three crystal axes and general boosts in all three directions. The non-relativistic limit then has the following general properties: there is no Zeeman interaction for a one-dimensional metal, and for a two-dimensional metal, the Zeeman interaction is only present for the magnetic field applied perpendicular to the conducting plane. These exact results show that for the field parallel to the layers of a highly layered superconductor or for any field direction in a quasi-one-dimensional superconductor, the temperature dependence of the Knight shift in the superconducting state should either vanish or be very weak. Hence, many Knight shift measurements on such superconducting materials, while correct, have been misinterpreted in the literature. |
Tuesday, March 5, 2019 2:54PM - 3:06PM |
H09.00003: Possible three-dimensional nematic odd-parity pairing in Sr$_2$RuO$_4$ evidenced by uniaxial strain measurements Wen Huang, Yi Zhou, Hong Yao In the presence of a nontrivial three-dimensional (3D) spin-orbital entanglement, the $E_u$ superconducting pairing in Sr$_2$RuO$_4$ is inherently 3D, consisting both in-plane and out-of-plane channels: $(k_x,k_y)\hat{z}$ and $(k_z\hat{x},k_z\hat{y})$. When the latter dominates, the system may develop a time-reversal invariant nematic pairing. We show that, as the out-of-plane pairing lacks a linear-order coupling to certain symmetry-lowering perturbations such as in-plane uniaxial strains, the nematic state may observe a superconducting $T_c$ enhancement indistinguishable from a quadratic function of the strain. Further, in contrast with the chiral pairing, the nematic pairing shall exhibit only a single phase transition or a comparatively less robust secondary transition under the stated perturbations. In combine these may be consistent with several recent experiments on uniaxially-strained samples of Sr$_2$RuO$_4$, thereby providing stronger support for the 3D nematic $p$-wave pairing in this material. |
Tuesday, March 5, 2019 3:06PM - 3:18PM |
H09.00004: Upper critical field of Sr_{2}RuO_{4} under uniaxial stress Fabian Jerzembeck, Alexander Steppke, YOU-SHENG LI, Dmitry Sokolov, Naoki Kikugawa, Andrew Mackenzie, Clifford Hicks Through application of in-plane uniaxial stress, the unconventional superconductor Sr_{2}RuO_{4} can be driven through a Lifshitz transition and associated Van Hove singularity (VHS) in the density of states. At this point, its Tc is enhanced by a factor of about 2.5, and H_{c2} by about twenty. Here, we present data on H_{c2} at intermediate strains, and observe that the peak in H_{c2} is much sharper than that in T_{c}. The ratio H_{c2}/T_{c}^{2} increases in the close vicinity to the VHS, providing information on the evolution of the superconducting gap as the VHS is approached. |
Tuesday, March 5, 2019 3:18PM - 3:30PM |
H09.00005: Superconducting penetration depth measurement and zero field muon spin relaxation experiments on Sr_{2}RuO_{4 }under uniaxial strain Shreenanda Ghosh, Vadim Grinenko, Rajib Sarkar, Felix Brückner, Jean-Christophe Orain, Hubertus Luetkens, Nikitin Artem, Matthias Elender, Joonbum Park, Mark E Barber, Naoki Kikugawa, Dmitry Sokolov, Andrew Mackenzie, Clifford Hicks, Yoshiteru Maeno, Hans-Henning Klauss To probe its superconducting order parameter, we have performed muon spin relaxation (muSR) measurements on samples of Sr_{2}RuO_{4} placed under uniaxial stress. Previous studies on unstressed Sr_{2}RuO_{4} have revealed enhanced relaxation in the superconducting state, which is interpreted as evidence for a chiral px ± ipy order parameter. With this order parameter, uniaxial stress is expected to induce a splitting between Tc and the onset of chirality. muSR requires large samples, so to perform these measurements, we have developed piezoelectric-based apparatus capable of applying forces of up to ~ 500 N. |
Tuesday, March 5, 2019 3:30PM - 3:42PM |
H09.00006: Electronic correlations and enhanced spin-orbit coupling in Sr_{2}RuO_{4} determined from high-resolution laser-based ARPES. Anna Tamai, Manuel Zingl, Minjae Kim, Antoine Georges, Felix Baumberger We combine laser-based angle-resolved photoemission and dynamical mean-field theory calculations to study the interplay of electron-electron correlations and spin-orbit coupling (SOC) in the model Fermi liquid Sr_{2}RuO_{4}. Analyzing the experimental Fermi surface, we show that correlations enhance SOC by a factor of ∼2 over the bare value. We further reveal that the real part of the self-energy of the β and γ sheet is momentum dependent and strongly non-linear down to low energies, in contrast to widely held believes about the phenomenology of Fermi liquids. Introducing a new method to determine orbital self-energies from quasiparticle states with multi-orbital composition, we demonstrate that the anisotropy of the self-energy does not imply momentum dependent many-body interactions. The non-linearity of the self-energy is reproduced by single-site dynamical mean field theory, which provides strong evidence for a dominantly electronic origin of ‘kinks’ in the quasiparticle dispersion of Sr_{2}RuO_{4}. |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H09.00007: Sr_{2}RuO_{4} Josephson junctions built in epitaxial films Masaki Uchida, Ikkei Sakuraba, Minoru Kawamura, Motoharu Ide, Kei Takahashi, Yoshinori Tokura, Masashi Kawasaki A layered-perovskite superconductor Sr_{2}RuO_{4} has attracted continuing interest as a leading candidate with chiral p-wave symmetry, which is one of topological superconductors potentially hosting Majorana fermions. For further investigation and possible applications of the unique state, the use of Sr_{2}RuO_{4} thin films for junctions has been increasingly demanded. While growth of the superconducting Sr_{2}RuO_{4} films had been extremely difficult over the past decades [1,2], the reproducible and controllable growth has been recently achieved by refining molecular beam epitaxy techniques [3]. In this talk, we report fundamental superconducting properties of Sr_{2}RuO_{4} thin films and also Josephson junctions as revealed by systematic ultralow-temperature transport measurement. The Sr_{2}RuO_{4}-Sr_{2}RuO_{4} Josephson junctions built in the epitaxial films have some advantages in investigating the possible multicomponent order parameter. The observed I_{c}R_{n} product is as high as 2 μeV and its temperature dependence shows clear deviations from the conventional Ambegaokar-Baratov model. |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H09.00008: Spontaneous emergence of Josephson junctions in homogeneous rings of single-crystal Sr_{2}RuO_{4} Yuuki Yasui, Kaveh Lahabi, Victor Fernandez Becerra, Muhammed Shahbaz Anwar, Shingo Yonezawa, Takahito Terashima, Milorad Milosevic, Yoshiteru Maeno, Jan Aarts Sr_{2}RuO_{4 }is expected to be a spin-triplet chiral p-wave superconductor, where the chirality means that the ground state is two-fold degenerate, with two different directions of the Cooper-pair orbital angular momentum. We fabricated Sr_{2}RuO_{4} microrings and performed resistance (R) and critical-current (I_{c}) measurements in an axial magnetic field. In some such rings, Little-Parks magnetoresistance oscillations are observed close to the transition temperature T_{c} as recently reported [1]. In other rings, however, we find that I_{c} oscillates with a period corresponding to the fluxoid quantization down to temperatures far below T_{c}. This behavior resembles that of a superconducting quantum interference device (SQUID) and suggests that a pair of weak links of an intrinsic origin is spontaneously formed in the arms of the ring. Such weak links are most naturally attributable to domain walls separating domains with different chirality, which is also the outcome of order parameter calculations. We believe this to be strong new evidence for the chiral superconducting state in Sr_{2}RuO_{4} |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H09.00009: Signature of Unconventional Superconductivity in a Metal-Organic Framework with a Perfect Kagome Structure Takaaki Takenaka, Kota Ishihara, Yijie Miao, Xing Huang, Wei Xu, Daoben Zhu, Na Su, Jinguang Cheng, Takasada Shibauchi Recently, the superconductivity in a Metal-organic framework (MOF) has been discovered for the first time in copper(II) benzenehexathiolate ([Cu_{3}(C_{6}S_{6})]_{n}, Cu-BHT). A theoretical study predicts that the electron-phonon coupling constant of 0.51 for bulk Cu-BHT can lead to superconductivity at T_{c} ∼ 1.58 K, thus conventional s-wave superconducting state has been proposed as a candidate of the pairing state. Meanwhile, recent measurements revealed strong quantum spin fluctuation possibly related to the 2D Kagome lattice of Cu atoms with S = 1/2 spin. Quantum spin fluctuations can promote unconventional superconducting pairing states, and it is therefore important to experimentally determine whether the superconductivity in Cu-BHT has conventional or unconventional nature. |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H09.00010: Spin Polarized Triplet Supercurrent in Ferromagnetic-Superconducting Josephson Junctions Madison Sutula, Mirko Rocci, Niladri Banerjee, Cui-Zu Chang, F. Sebastian Bergeret, Jagadeesh Moodera Proximity coupling across superconductor-ferromagnet bilayers can give rise to the triplet component of the superconducting condensate [1]. Superconductivity and ferromagnetism have been reported to coexist in a Ni/Bi bilayer system [2]. We investigated Josephson and quasiparticle tunneling across such ferromagnetic-superconducting layers in Bi/Ni/Insulator/Ni/Bi junctions. The superconductivity in the Ni/Bi bilayer is expected to be topological [3]. Moreover, the observed Josephson current could be spin polarized, as superconducting quasiparticles originating from Ni in the Ni/Bi bilayer have been shown to also exhibit spin polarization [2]. The occurrence of conductance at zero bias in these junctions points to odd-frequency symmetry in the superconducting condensate, supporting the presence of a non-zero component associated with a triplet pair superconductivity insensitive to disorder. |
Tuesday, March 5, 2019 4:30PM - 4:42PM |
H09.00011: Unconventional superconductivity near frustrated orders in quasi-one-dimensional CrAs based systems Keith Taddei, Guangzong Xing, Jifeng Sun, Yuhao Fu, Yuwei Li, Qiang Zheng, Athena S. Sefat, David Singh, Clarina Dela Cruz Even in the absence of a microscopic theory, a generalized recipe has emerged for unconventional superconductivity. In the common scenario, some ordered phase in a strongly-ish correlated material is suppressed giving rise to quantum fluctuations which can act as a possible pairing mechanism. The nature of these fluctuations – whether they are of the ordered phase, resultant of a quantum critical point or some more exotic pair density wave – is still hotly debated. Therefore, it is useful to find new unconventional superconductors which can provide further insight into the most relevant components of this recipe. The recently discovered family of A_{x}TM_{3}As_{3} (A = Alkali, x = 1,2 and TM = Cr, Mo) superconductors provides such an opportunity in a system with a novel quasi-one-dimensional structure. In this presentation the latest results of joint ab initio and neutron scattering studies will be discussed which reveal a frustrated structural instability in K_{2}Cr_{3}As_{3}. Together with our previous reports of spin-fluctuations, this observation indicates a rich phase diagram for this system analogous to the traditional unconventional superconductors. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H09.00012: Spontaneous magnetisation in the superconducting state of LaNiGa_{2} Gabor Csire, Sudeep Ghosh, Philip Whittlesea, Jorge Quintanilla, Balazs Ujfalussy, Kazumasa Miyake, James Annett Muon spin relaxation shows spontaneous magnetism in the superconducting state of LaNiGa2. Symmetry analyses imply nonunitary triplet pairing with line nodes and predict a sub-dominant magnetisation consistent with SQUID measurements. In contrast, there is evidence from penetration depth and specific heat for two-gap, nodeless superconductivity. It was proposed to reconcile this by assuming equal-spin inter-orbital pairing. Here we show within a mean-field framework, that this gives rise to a nodeless, two-gap spectrum. We probe the state's stability in the presence of finite inter-orbital energy splitting and derive an analytical expression for the resulting spontaneous magnetisation. Furthermore, we present a detailed calculation combining a realistic first-principles band structure with a pheonmenological pairing interaction. We find that equal-spin pairing between certain Nickel d-orbitals can describe the specific heat quantitatively. We predict the two-gap structure in differential tunnelling conductance and the size of the spontaneous magnetic moment. |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H09.00013: Nodeless bulk superconductivity in the time-reversal symmetry breaking Bi/Ni bilayer system Prashant Chauhan, Fahad Mahmood, Di Yue, Pengchao Xu, Xiaofeng Jin, Peter Armitage Epitaxial bilayer films of Bi(110) and Ni host a time-reversal symmetry (TRS) breaking superconducting order with an unexpectedly high transition temperature Tc = 4.1K. Using time-domain THz spectroscopy, we measure the low energy electrodynamic response of a Bi/Ni bilayer thin film from 0.2THz to 2THz as a function of temperature and magnetic field. We analyze the data in the context of a BCS-like superconductor with a finite normal-state scattering rate. In zero magnetic field, all states in the film become fully gapped, providing important constraints into possible pairing symmetries. Our data appears to rule out the odd-frequency pairing that is natural for many ferromagnetic-superconductor interfaces. By analyzing the magnetic field-dependent response in terms of a pair-breaking parameter, we determine that superconductivity develops over the entire bilayer sample which may point to the p-wave like nature of unconventional superconductivity. |
Tuesday, March 5, 2019 5:06PM - 5:18PM |
H09.00014: Nontrivial effects of point-like disorder on critical fields and pinning in s_{±} superconductors Ruslan Prozorov, Marcin Konczykowski, Vladimir G. Kogan, Makariy Tanatar In most superconductors non-magnetic point-like defects enhance pinning and upper critical field. However, in s_{±} superconductors, for example iron-based compounds, point-like disorder influences not only intra-band potential scattering, but also inter-band pair-breaking scattering rates. The latter leads to suppression of T_{c}, H_{c2} and the condensation energy. The balance between these opposite effects leads to an increase of (dH_{c2}/dT)_{T=Tc} slope with the decrease of T_{c}, opposite to what is observed as a function of doping. Even more unusual, it may also lead to a suppression of strong pinning believed to be responsible for a sharp peak in magnetization near H=0. In our experiments controlled disorder was induced by 2.5 MeV electron irradiation. |
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