Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L16: Superconductivity: Spin Triplet |
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Sponsoring Units: DMP DCMP Chair: Maria Gastiasoro, Niels Bohr Institute Room: 275 |
Wednesday, March 15, 2017 11:15AM - 11:27AM |
L16.00001: Scanning SQUID Microscopy of Sr$_2$RuO$_4$ Under Uniaxial Pressure Christopher A. Watson, Hilary Noad, Alexandra Gibbs, Andrew P. Mackenzie, Clifford W. Hicks, Kathryn A. Moler The superconducting order parameter of Sr$_2$RuO$_4$ is thought to be $p_x \pm ip_y$ in the absence of a symmetry breaking field, owing to a degeneracy of the $p_x$ and $p_y$ components that results from tetragonal lattice symmetry. This order parameter would manifest as a cusp in the superconducting critical temperature, T$_c$, as a function of orthorhombic lattice distortion, applied through uniaxial pressure. T$_c$ has been found to respond sensitively to uniaxial pressure; however, the strain resolution of bulk measurements so far appears to be limited by inhomogeneity of the applied strain field and/or intrinsic sample inhomogeneity, such that the expected cusp might not have been observable. Here, we use scanning SQUID microscopy to resolve the low pressure response of T$_c$ on micron length scales, rendering the measurement insensitive to longer range spatial inhomogeneity. Furthermore, we report the dependence of T$_c$ on uniaxial strain to temperature and strain resolution sufficient to resolve the predicted cusp, thereby demonstrating the extent to which the symmetry protected degeneracy is lifted locally. [Preview Abstract] |
Wednesday, March 15, 2017 11:27AM - 11:39AM |
L16.00002: Vertical line nodes in the superconducting gap structure of Sr$_2$RuO$_4$ Louis Taillefer, Patrick Bourgeois-Hope, Samuel Rene de Cotret, Gael Grissonnanche, Nicolas Doiron-Leyraud, Elena Hassinger, Haruka Taniguchi, Muhammad Anwar, Yoshiteru Maeno |
Wednesday, March 15, 2017 11:39AM - 11:51AM |
L16.00003: Abstract Withdrawn Under various conditions of the growth process, when the presumably unconventional superconductor Sr$_2$RuO$_4$ (SRO) contains micro-inclusions of Ru metal, the superconducting critical temperature increases significantly. An STEM study shows a sharp interface geometry between tetragonal SRO and hcp Ru-metal with mediating interface columns of ruthenium between them, which allows perfect crystals of SRO and of Ru-metal to grow by means of formation of a superlattice structure of the Ru-metal interface commensurate with the SRO interface. We investigate the atomic structure and electronic properties of this interface using \textit{ab initio} density functional theory (DFT) calculations. Our work indicates that it is energetically favored over other types of Ru-metal/SRO interfaces and shows that a $t_{2g}$-$e_g$ orbital mixing occurs at the interface with significantly enhanced magnetic moments. |
Wednesday, March 15, 2017 11:51AM - 12:03PM |
L16.00004: Heat Capacity Measurements of Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ Under Uni-axial Stress You-Sheng Li, Alexandra Gibbs, Andrew Mackenzie, Clifford Hicks, Michael Nicklas One of the most-discussed possible pairing symmetries of the superconductor Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ is p$_{\mathrm{x}}+$/-ip$_{\mathrm{y}}$. By applying in-plane uniaxial stress, the degeneracy of the p$_{\mathrm{x}}$ and p$_{\mathrm{y}}$ components should be lifted, yielding two critical temperatures (T$_{\mathrm{c}})$. Hicks et al. observed an increase of T$_{\mathrm{c}}$ of Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ under both compressive and tensile stress [1], and did not find evidence for splitting of transition. However, that result was based on magnetic susceptibility measurements, which would be sensitive only to the upper transition. For a direct test of possible splitting, we measure the heat capacity of Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ under uniaxial stress. To do so, we have developed an approach to measure heat capacity under non-adiabatic conditions. We have observed the increase in T$_{\mathrm{c}}$ under compressive strain, providing the first thermodynamic evidence for the strain-induced increase in T$_{\mathrm{c}}$ of Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$, and also resolve strong strain-induced changes in the normal-state heat capacity. [1] Clifford W. Hicks et al., Science \textbf{344}, 283 (2014). [Preview Abstract] |
Wednesday, March 15, 2017 12:03PM - 12:15PM |
L16.00005: Structure of spin-fluctuations and symmetry of the order parameter in unconventional Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ superconductor. S. Khmelevskyi, I. I. Mazin, Bongjae Kim, D. F. Agterberg, C. Franchini The superconductivity (SC) in the Sr2RuO4 has attracted a considerable interest comparable to that in cuprates and iron pnictides due to the presumed triplet character of the SC order parameter (OP) and spin-fluctuations (SF) mechanism of the electron pairing. Early NMR experiments suggested a triplet chiral OP, while recent probes of strained crystals point toward singlet pairing. Interpretation of the NMR data relies upon the idea that the order parameter vector rotates in an external field of \textasciitilde 200 Oe. In this work we explore the paramagnetic SF in the Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ from first principles by calculating effective exchange interactions in disordered local moment state. We find that long-range interactions lead to the competition of several magnetic ground states with wave vectors close to the experimentally observed. The degeneration is removed only by SOC and a very special collinear modulated magnetic structure stabilizes. We derive the isotropic and anisotropic exchange terms and a single-site magnetic anisotropy, and show that these terms provide an energy penalty for rotating the order parameter that is several orders of magnitude too large, thus rendering the NMR experiment completely inexplicable in terms of the conventional theory. Instead, we write down an effective anisotropic double-exchange model based on our calculated parameters. [Preview Abstract] |
Wednesday, March 15, 2017 12:15PM - 12:27PM |
L16.00006: Edge Josephson junction detection of topological chiral edge current in spin-triplet superconductor Sr$_2$RuO$_4$ with a calibrated sensitivity Brian Zakrzewski, Alex Ying, Jian-Jian Ge, Zhiqiang Mao, Ying Liu Sr$_2$RuO$_4$, a spin-triplet, odd-parity superconductor, is predicted to possess a chiral $p$-wave order parameter. A major consequence of this doubly degenerate state is the presence of chiral edge currents, domains, and domain walls. Direct evidence for these effects is still lacking. Using a Josephson junction formed between Al and the edge of a thin Sr$_2$RuO$_4$ crystal prepared by mechanical exfoliation in conjunction with a bulk supercurrent that generates a field along the same direction of that generated by the chiral edge current, we are able to probe the local magnetic field at the edge of the crystal with a sensitivity that is improved by two orders of magnitude over work previously reported. The calibration is done using a custom-built magnet. We found a small non-zero offset in the magnetic flux response when the external field is zero, signaling the existence of an intrinsic field at the edge. We compare our results to those predicted in various theoretical models, in particular, the recent theoretical work on scenarios in which the chiral edge current in Sr$_2$RuO$_4$ first calculated by Matsumoto and Sigrist can be suppressed. [Preview Abstract] |
Wednesday, March 15, 2017 12:27PM - 12:39PM |
L16.00007: Observation of a mixed pairing state in a mesoscopic-sized Ru island embedded in Sr$_2$RuO$_4$: Interplay between the condensation and the Josephson coupling energies Libin Wen, Shun Wang, Hui Xing, Alex Ying, Brian Zakrzewski, Jian-Jian Ge, Zhiqiang Mao, Ying Liu A bulk crystal possessing an inversion symmetry can only host either an even-parity, spin-singlet, or odd-parity, spin-triplet superconducting pairing state. As the size of the superconductor is reduced and the inversion symmetry is lost because of the boundary effects, however, a mixed pairing state featuring both odd- and even-parity pairings is allowed by symmetry considerations and can be realized if a proper mechanism exists to facilitate the condensations in both pairing channels. In this talk we report the observation of a mixed pairing state in a mesoscopic-sized Ru island embedded in odd-parity, spin-triplet superconductor Sr$_2$RuO$_4$ found in the so-called eutectic phase of Sr$_2$RuO$_4$-Ru. Even though the bulk Ru features only spin-singlet pairing, evidence for a superconducting condensate in the spin-triplet channel was found in the current system from magneto electrical transport measurement through a single Ru island. We argue that the occurrence of the mixed pairing state arises from the interplay between the condensation and the Josephson coupling energies. [Preview Abstract] |
Wednesday, March 15, 2017 12:39PM - 12:51PM |
L16.00008: Odd-frequency superconductivity in superconducting strontium ruthenate Lucia Komendova, Annica Black-Schaffer We predict the existence of odd-frequency superconductivity in the strontium ruthenate Sr$_2$RuO$_4$ superconductor, concentrated where its three bands are closest to each other in k-space. We investigate the effect in a realistic three-band model [1], as well as in a simplified two-band model [2], which both have been shown to lead to the appearance of a polar Kerr effect signal. In the two-band model it is also clear that the experimentally confirmed presence of the Kerr effect [3] guarantees the necessary condition for the appearance of the odd-frequency superconductivity, namely the presence of either single-particle hybridization between bands, and/or interband Cooper pairing. [1] M. Gradhand, K.I. Wysokinski, J.F. Annett, and B.L. Gyorffy, Phys. Rev. B 88, 094504 (2013). [2] E. Taylor and C. Kallin, Phys. Rev. Lett. 108, 157001 (2012). [3] J. Xia, Y. Maeno, P. T. Beyersdorf, M. M. Fejer, and A. Kapitulnik, Phys. Rev. Lett. 97, 167002 (2006). [Preview Abstract] |
Wednesday, March 15, 2017 12:51PM - 1:03PM |
L16.00009: Field and Temperature Dependence of the Superconducting Anisotropy in Sr$_2$RuO$_4$ Stephen Kuhn, W. Morgenlander, E. R. De Waard, M.R. Eskildsen, C.D. Dewhurst, J. Gavilano, Y. Maeno The superconductor Sr$_2$RuO$_4$ (SRO) has been well-studied for triplet pairing of electrons and an odd, p-wave order parameter symmetry. However, because of its complexity, questions remain about the superconducting state. To study this we measured the superconducting anisotropy ($\Gamma_{ac}$). Direct imaging of the vortex lattice (VL) anisotropy in SRO is possible with small-angle neutron scattering (SANS), with $H$ applied close to the basal plane. The VL anisotropy reflects the intrinsic anisotropy of $\Gamma_{ac}$, and may differ significantly from the $H_{c2}$ anisotropy ($\Gamma_{H_{c2}}$) as recently seen for intermediate fields [C. Rastovski $et$ $al.$, Phys. Rev. Lett. {\bf 111}, 087003 (2013)]. For $H \rightarrow H_{c2}$, the VL anisotropy is found to exceed earlier results at intermediate fields. Our data, combined with new low field measurements, indicate a field dependent superconducting anisotropy in SRO, increasing as $H$ approaches $H_{c2}$. This suggests a varying contribution of the different Fermi Surface bands on the $\Gamma_{ac}$. In contrast, the VL anisotropy is found to remain constant as the temperature is increased toward $T_{c2}$, while the $\Gamma_{H_{c2}}$ increases with increasing temperature. [Preview Abstract] |
Wednesday, March 15, 2017 1:03PM - 1:15PM |
L16.00010: Superconducting gap symmetry of strained Sr$_2$RuO$_4$ Olivier Gingras, Reza Nourafkan, Andr\'e-Marie Tremblay, Michel C\^ot\'e The superconducting gap symmetry of Sr$_2$RuO$_4$ is still under debate. Even though many experimental evidences suggest a chiral p-wave symmetry, others would rather support a d-wave symmetry\footnote{E. Hassinger \textit{and al}, arXiv:1606.04936}. Recently, it was shown that the uniaxial or biaxial strain can be employed to induce a Lifshitz transition in this compound. The ARPES results of the strained Sr$_2$RuO$_4$ have shown that the Fermi level could be brought very close to van Hove singularities, leading to a change in the Fermi surface topology and strong variations of the quasi-particule mass enhancement\footnote{Y. Hsu \textit{and al}, Phys. Rev. B \textbf{94}, 045118 (2016)}$^{,}$\footnote{B. Burganov \textit{and al}, Phys. Rev. Lett. \textbf{116}, 197003 (2016)}. Such a drastic change in the electronic structure of the system should influence the competition between different pairing symmetries. In our study, we used DFT+DMFT calculation to investigate the effect of strain on the spin excitation spectra of Sr$_2$RuO$_4$ and in turn the spin fluctuation mediated pairing interaction. [Preview Abstract] |
Wednesday, March 15, 2017 1:15PM - 1:27PM |
L16.00011: Impurity Induced Anomalous Thermal Hall Effect in Chiral Superconductors Vudtiwat Ngampruetikorn, J. A. Sauls We report theoretical results for the electronic contribution to thermal transport for chiral superconductors belonging to even- or odd-parity E$_1$ and E$_2$ representations of the tetragonal and hexagonal point groups. Chiral superconductors exhibit novel transport properties that depend on the topology of the order parameter, topology of the Fermi surface, the spectrum of bulk Fermionic excitations, and - as we highlight - the structure of the impurity potential. The anomalous thermal Hall effect is shown to be sensitive to the structure of the electron-impurity t-matrix, as well as the winding number, $\nu$, of the chiral order parameter, $\Delta(p)=|\Delta(p)|\,e^{i\nu\phi_p}$. For heat transport in a chiral superconductor with isotropic impurity scattering, i.e. point-like impurities, a transverse heat current is obtained for $\nu=\pm1$, but vanishes for $|\nu|>1$. This is not a universal result. For finite-size impurities with radii of order or greater than the Fermi wavelength, $R\ge\hbar/p_f$, the thermal Hall conductivity is finite for chiral order with $|\nu|\ge2$. Our results also provide quantitative formulae for interpreting heat transport experiments for superconductors predicted to exhibit broken time-reversal and mirror symmetries. [Preview Abstract] |
Wednesday, March 15, 2017 1:27PM - 1:39PM |
L16.00012: Specific Heat Characterization of UPt$_3$ with Varied Anisotropic Quasiparticle Scattering Jan Kycia, David Pomaranski, Christopher Mitchelitis, William Halperin UPt$_3$ is an unconventional superconductor with multiple superconducting phases. It is believed to be a chiral triplet f-wave superconductor. Previous work, through UHV float zone refining and annealing at a range of temperatures, created a set of high quality UPt$_3$ crystals. Through electrical transport measurements, the upper transition temperature was found to be related to the level of structural defects and that the scattering was anisotropic.\footnote{J.B. Kycia, J.I. Hong, M.J. Graf, J.A. Sauls, D.N. Seidman, W.P. Halperin, Phy. Rev. B \textbf{58}, R603 (1998).} In this work, we will present specific heat characterization of a set of these samples (with RRR of 420, 720, and 1460). Through this we can identify the sensitivity of the A and B superconducting phases to anisotropic scattering. [Preview Abstract] |
Wednesday, March 15, 2017 1:39PM - 1:51PM |
L16.00013: Fragile surface zero-energy flat band in three-dimensional chiral superconductors Shingo Kobayashi, Yukio Tanaka, Masatoshi Sato Gapless phases of matter have received enormous attention in recent years. In the context of unconventional superconductors (SCs), such gapless phases manifest as nodal excitations in superconducting gaps and have a surface zero-energy flat band due to nontrivial topology of bulk superconducting states. For heavy-fermion SCs such as UPt3 and URu2Si2, time-reversal-breaking gap functions with both line and point nodes have been proposed and this type of gap functions also host surface zero-energy flat bands in spite of the absence of time-reversal symmetry [1]. In this presentation, we show that the surface flat bands in three-dimensional chiral SCs, which include UPt3 and URu2Si2, are fragile against (i) the surface misorientation and (ii) the surface Rashba spin-orbit interaction. The fragility of (i) is specific to chiral SCs, whereas that of (ii) happens for general odd-parity SCs. We demonstrate that these flat-band instabilities vanish or suppress a zero-bias conductance peak in a normal/insulator/superconductor junction, which behavior is clearly different from high-T$_{\mathrm{c}}$ and noncentrosymmetric SCs. By calculating the angle-resolved conductance, we also discuss a topological surface state associated with the coexistence of line and point nodes. [1] P. Goswami and L. Balicas, arXiv:1312.3632; P. Goswami and A. H. Nevidomskyy, Phys. Rev. B 92, 214504 (2015). [2] S. Kobayashi, Y. Tanaka, and M. Sato, Phys. Rev. B 92, 214514 (2015). [Preview Abstract] |
Wednesday, March 15, 2017 1:51PM - 2:03PM |
L16.00014: Geometric effects in 2d chiral superconductors Thomas Kvorning, Hans Hansson, Anton Quelle We show that the 2d chiral superconductors in general produce spontaneous magnetic fields in presence of geometric curvature. Apart from this being interesting in its own right, we will propose how this could be used to get a direct experimental signature of chiral superconductivity. [Preview Abstract] |
Wednesday, March 15, 2017 2:03PM - 2:15PM |
L16.00015: Anomalous magnetic moments as evidence of chiral superconductivity in Bi/Ni bilayer Li Lu, Junhua Wang, Xinxin Gong, Guang Yang, Zhaozheng Lyu, Yuan Pang, Guangtong Liu, Zhongqing Ji, Jie Fan, Xiunian Jing, Changli Yang, Fanming Qu, Xiaofeng Jin There have been continuous efforts in searching for unconventional superconductivity over the past five decades. Compared to the well-established d-wave superconductivity in cuprates, the existence of superconductivity with other high-angular-momentum pairing symmetries is less conclusive. Bi/Ni epitaxial bilayer is a potential unconventional superconductor with broken time reversal symmetry (TRS), for that it demonstrates superconductivity and ferromagnetism simultaneously at low temperatures. We employ a specially designed superconducting quantum interference device (SQUID) to detect, on the Bi/Ni bilayer, the orbital magnetic moment which is expected if the TRS is broken. An anomalous hysteretic magnetic response has been observed in the superconducting state, providing the evidence for the existence of chiral superconducting domains in the material. [1] Junhua Wang et al., arXiv: 1611.02946v1 [Preview Abstract] |
Wednesday, March 15, 2017 2:15PM - 2:27PM |
L16.00016: $f$-wave triplet superconductivity in a twisted triangular Hubbard tube as a model of A$_2$Cr$_3$As$_3$ Sahinur Reja, Satoshi Nishimoto Triplet superconductivity (SC) recently has been one of the active research topics partly due to its intrinsic connection to quantum computations. In this contex, we study the ground state properties of a twisted triangular Hubbard tube using the perturbation theory and density-matrix renormalization group method. We show that two electrons in an odd-site Hubbard ring always form a spin-triplet pair, and subsequently a polarized ferromagnetic (FM) order is stabilized in a wide range of electron filling ($n$) when these rings are weakly coupled. By calculating the binding energy and spin gap, we confirm the presence of the spin-triplet SC after melting of the FM order with increasing the inter-triangle couplings ($t_2$). We show that triplet SC pair correlations are consistent with the $f$-wave channel. We present a detailed $n$-$t_2$ phase diagram which features also singlet SC at $n\sim1$ and $t_2\sim1$. Finally we argue that this model has possible relevance to the $f$-wave SC observed in alkali chromium arsenides A$_2$Cr$_3$As$_3$ (A=K,Rb,Cs). [Preview Abstract] |
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