Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z47: Superconductivity: Pseudogap, Fluctuations, Anisotropy |
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Chair: Werner Hanke, University of Wuerzburg Room: Mile High Ballroom 4F |
Friday, March 7, 2014 11:15AM - 11:27AM |
Z47.00001: Superconductivity in Sr$_2$RuO$_4$: Two-Dimensional versus One-Dimensional Origin of Pairing Werner Hanke, Christian Platt, Ronny Thomale There is growing experimental evidence that Sr$_2$RuO$_4$ displays an unconventional superconducting (SC) state with a chiral p-wave, i.e. ``p+ip'' symmetry. The continuing strong interest in this superconductor has recently been fuelled by speculations of the possible realization of topologically protected edge modes. However, there exists also a variety of challenges for the chiral p-wave state, such as a power-law behavior in the specific heat, no clear observation of edge currents and conflicting pictures for the pairing mechanism (2D versus 1D). In this contribution, we discuss our recent theory of SC in a 3-orbital model of Sr$_2$RuO$_4$ [1], and extensions thereof, in light of these challenges, in particular, of recent tunneling spectroscopy data, which were interpreted as evidence for a quasi-1D origin for SC [2].\\[4pt] [1] Q. H. Wang, C. Platt, Y. Yang, C. Honerkamp, F. C. Zhang, W. Hanke, T. M. Rice, R. Thomale, Eur. Phys. Lett. 104, 17013 (2013).\\[0pt] [2] I.A. Firmo, S. Lederer, C. Lupien, A. P. Mackenzie, J. C. Davis, and S. A. Kivelson, Phys. Rev. B 88, 134521 (2013) [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z47.00002: RG Approach to Superconductivity in Sr$_2$RuO$_4$ : Multi-Band and Spin-Orbit Effects Thomas Scaffidi, Jesper Romers, Steven Simon We study the superconductivity pairing mechanism in Sr$_2$RuO$_4$ in the limit of small interaction by extending a renormalization group (RG) calculation developed by Raghu et al (Phys Rev B.81.224505) to include multiband and spin-orbit coupling (SOC) effects. We show these effects to be crucial to discriminate between the possible order parameters. In contrast to the usual theory of an ``active'' gamma band with a large superconducting gap and ``passive'' alpha and beta bands with smaller gaps induced by a proximity effect, we obtain pseudo-spin triplet gaps of the same order of magnitude on all three bands for a large range of interaction parameters. The inclusion of SOC in the microscopic model allows us to study ab initio the breaking of degeneracy between the different d vector orientations. Implications for experiments will be discussed. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z47.00003: Fluctuation exchange analysis of quasiparticle properties of strontium ruthenate John Deisz, Tim Kidd We utilize a weak-coupling approximation, FLEX, to analyze quasiparticle properties of strontium ruthenate. Utilizing first-principles-derived parameters for the band structure, spin-orbit interaction and Coulomb and exchange interactions, we quantitatively reproduce the band-structure renormalization and linewidths observed in photoemission. However, the electronic specific heat coefficient is underestimated by a factor of two and superconducting $T_c$ is overestimated by 30 K. Analysis of of the band and momentum-resolved self-energy reveals an essentially momentum-independent, Fermi-liquid-like self-energy with the exception of the $\gamma$ band along the $\Gamma$-M cut for which non-Fermi-liquid behavior is observed. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z47.00004: Manipulating superconducting tendency in ruthenates through Fermi surface engineering Yi-Ting Hsu, Jian-Huang She, Bulat Burganov, Carolina Adamo, Darrell Schlom, Kyle Shen, Eun-Ah Kim $\rm{Sr_2RuO_4}$ is the leading material candidate for topological triplet superconductivity yet its low transition temperature ($T_c$) limits experimental investigation of the system. One of the leading proposals for the mechanism of the observed superconductivity is the one dimensional band driven superconductivity mediated by antiferromagnetic fluctuations. Within this proposal a perturbative RG approach on a microscopic model with purely repulsive interactions yielded dominant triplet pairing tendency. In this approach the fermiology plays a key role in tilting the balance among different pairing possibilities and the superconducting $T_c$. This implies one can manipulate superconductivity through Fermi surface engineering. Motivated by the recent experimental advance in the growth of $\rm{Ba_2RuO_4}$ films where an isovalent substitution of $\rm{Sr^{2+}}$ by $\rm{Ba^{2+}}$ produces negative chemical pressure, we investigate how the resulting changes in Fermi surface affect superconducting instability using the perturbative RG approach. We take the band structure fitted to Fermi surface measured using angle resolved photoemission spectroscopy as our input. We then compare the results to known effects of hydrostatic pressure. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z47.00005: The collective mode spectrum and electromagnetic response of spin-triplet models for the superconducting Sr$_2$RuO$_4$ Hao Wu, Suk-Bum Chung, James Sauls Collective modes in unconventional superconductors provide spectroscopic signatures of the broken symmetries of the superconducting ground state. We consider the effects of Fermi surface and pairing anisotropy on the mode energies and selection rules for the coupling of the order parameter collective modes to an electromagnetic field for spin-triplet pairing models of the superconducting phase of Sr$_2$RuO$_4$. We show that the well known doubly degenerate clapping modes with frequency $\sqrt{2} \Delta$ are split by anisotropy. We report calculations of the mode frequencies and their damping by pairbreaking based on our microscopic pairing model for Sr$_2$RuO$_4$. We report results for the current response to in plane electromagnetic field from both the single particle and collective mode excitations. The clapping modes give strong absorption peaks at frequencies below the maximal pair-breaking energy of $2\Delta$. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z47.00006: Evidence from tunneling spectroscopy for a quasi-one-dimensional origin of superconductivity in Sr$_2$RuO$_4$ S. Lederer, I.A. Firmo, C. Lupien, A.P. Mackenzie, J.C. Davis, S.A. Kivelson To establish the mechanism of unconventional superconductivity in Sr$_2$RuO$_4$, a prerequisite is direct information concering the momentum-space structure of the energy gaps $\Delta_i(k)$, and in particular whether the pairing strength is stronger (``dominant'') on the quasi-one-dimensional ($\alpha$ and $\beta$) or on the quasi-two-dimensional ($\gamma$) Fermi surfaces. We present scanning tunneling spectroscopy measurements of the density of states spectra in the superconducting state of Sr$_2$RuO$_4$ for $0.1 T_c |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z47.00007: Nodal Behavior of the Superconducting State in Sr${_2}$RuO${_4}$ S.J. Kuhn, M.R. Eskildsen, C. Rastovski, C.D. Dewhurst, J. Gavilano, Y. Maeno Multiple experimental and theoretical studies provide compelling support for triplet pairing of electrons and an odd, $p$-wave order parameter symmetry in superconducting Sr$_2$RuO$_4$. However, seemingly contradictory experimental results have left important questions concerning the detailed structure and coupling of the orbital and spin parts of the order parameter in this compound unresolved. The nodal behavior constrains the possibilities for the order parameter. We have used small-angle neutron scattering (SANS) to study the scattering intensity of the vortex lattice (VL) in Sr$_2$RuO$_4$, with $H$ along the [110] crystalline direction. Because the VL anisotropy in Sr$_2$RuO$_4$ is large [C. Rastovski $et$ $al.$, Phys. Rev. Lett. {\bf 111}, 087003 (2013)], this measurement effectively determines the current along the field direction, and hence the penetration depth along [1$\bar{1}$0]. The intensity vs. temperature curve is linear for $T/T{_c}$ of 0.25-0.75, but flattens out as $T$ approaches 0. This is consistent with a gap mode in the [110] direction, coupled with non-local effects. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z47.00008: Competition Between Antiferromagnetism and Ferromagnetism in Sr$_{2}$RuO$_{4}$ Probed by Mn and Co Doping Zhiqiang Mao, John Ortmann, Jinyu Liu, Jin Hu, M. Zhu, Jin Peng, M. Matsuda, Xianglin Ke Spin-triplet superconductivity in Sr$_{2}$RuO$_{4}$ has attracted enormous interest. Like other unconventional superconductors, superconductivity in Sr$_{2}$RuO$_{4}$ is in close proximity to magnetic instability. Undoped Sr$_{2}$RuO$_{4}$ exhibits incommensurate antiferromagnetic (AFM) fluctuations, which can evolve into static, short-range AFM order via Ti doping. Moreover, weak ferromagnetic (FM) coupling in Sr$_{2}$RuO$_{4}$ has also been suggested by NMR/neutron scattering experiments and studies on Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ and Sr$_{2-y}$La$_{y}$RuO$_{4}$. In this talk, we will report bulk static, short-range FM order in Sr$_{2}$RuO$_{4}$ triggered by \textless 2{\%} Co doping, showing superconductivity in Sr$_{2}$RuO$_{4}$ is much closer to FM instability than previously reported in Ca$_{2-x}$Sr$_{x}$RuO$_{4}$. We also find Mn doping can effectively establish incommensurate AFM order, with $T_{N}$ $\sim$ 50 K for 3{\%} Mn doping. These new results highlight the important role of competing magnetic fluctuations in determining superconducting properties of Sr$_{2}$RuO$_{4}$[1]. \\[4pt] [1] Ortmann et al., Scientific Report 3, 2950 (2013). [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z47.00009: Anomalous Knight Shift properties in a variety of Superconductors Bianca Hall, Richard Klemm Anomalous Knight shift properties have been found in a variety of superconductors below the superconducting transition temperature. In most instances this anomaly is an observed Knight shift along one axis but no observed Knight shift perpendicular to this axis (usually the c-axis). Materials that show this anomaly include LiFeAs, UPt3, CeTIn5 (T$=$Co and Ir), Tl2Ba2CuO6$+$y, and YBa2Cu3O7-$\delta $. Another anomaly occurs in the material Na0.35CoO2$\cdot$1.3H2O, where NMR data for the Knight shift done on 59Co and 23Na do not agree. Additionally, NMR data of FeSe show no shift, which is contrary to the material's s-wave pairing superconductivity. Finally, Sr2RuO4 has a constant Knight shift, which is contrary to upper critical field measurements and superconducting gap measurements. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z47.00010: Observation of unconventional Little-Parks resistance oscillations in mesoscopic rings of Sr$_2$RuO$_4$ Xinxin Cai, Yiqun Ying, David Fobes, Tijiang Liu, Zhiqiang Mao, Ying Liu Spin-triplet superconductor Sr$_2$RuO$_4$ may host half-flux-quantum (h/4e) states, which carry Majorana modes useful for topological quantum computing. Recent cantilever magnetometry measurements on micron-size samples of Sr$_2$RuO$_4$ in the presence of an in-plane magnetic field revealed unusual magnetization steps suggesting the presence of both integer and half-integer flux quanta. To observe h/4e resistance oscillations that will not only provide independent confirmation of the existence of the half-flux-quantum states but also provide insights into the physical origin of this novel phenomenon, we fabricated mesoscopic superconducting rings of Sr$_2$RuO$_4$ and carried out Little-Parks resistance oscillation measurements. Without the application of an in-plane field, resistance oscillations with a full-flux period (h/2e) and a large amplitude were observed. A pronounced second set of resistance peaks was found in one sample when the in-plane field and the measurement current were sufficiently large. This sample featured a large critical current density of $\sim 10^5\mathrm{A/cm^2}$, which we believe is of significance. Preparation and measurements of more samples are currently underway. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z47.00011: Unconventional Magnetoresistance Oscillations in Sr$_2$RuO$_4$ Kevin Roberts, Victor Vakaryuk Unconventional quantum oscillations have been detected in Sr$_2$RuO$_4$ which cannot be explained by the traditional Little-Parks effect [1]. To gain insight into the problem we use the theory of oscillatory magnetoresistance induced by thermally excited vortex transitions [2]. We numerically obtain energy barriers for vortex entry, calculate the resulting magnetoresistance, and compare our results with experimental data.$\\ \\$ [1] X. Cai, Y.A. Ying, N.E. Staley, Y. Xin, D. Fobes, T.J. Liu, Z.Q. Mao, and Y. Liu, Phys. Rev. B, $\textbf{87}$, 081104(R) (2013)$\\$ [2] I. Sochnikov, A. Shaulov, Y. Yeshurun, G. Logvenov, and I. Bozovic, Phys. Rev. B, $\textbf{82}$, 094513 (2010) [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z47.00012: Non-topological nature of the edge current in a chiral p-wave superconductor Wen Huang, Edward Taylor, Samuel Lederer, Srinivas Raghu, Catherine Kallin Influential work by Volovik and others (see e.g., [1] and [2]) has tried to establish an analog of the quantum Hall effect in chiral superconductors subject to a spatially varying chemical potential. It is further argued that this quantized static nonlocal Hall conductivity (taking the frequency to zero before the momentum) leads to a topological edge current insofar as the edge can be thought of as a spatially varying chemical potential. Using Bogoliubov-de Gennes calculations as well as diagrammatic calculations of the Hall conductivity using exact Ward identities, we find this analogy breaks down in two respects. First, we show that for a rapidly varying chemical potential, such as would arise at the edge, there can be significant deviations from what is expected from a linear response formulation of the problem. Second, even for a slowly varying chemical potential, we show that the quantization of the Hall current is violated for non-Galilean invariant systems. In contrast to the quantum Hall effect in topological insulators, U(1) symmetry breaking in topological superconductors eliminates the possibility of a topological current response. [1] G. E. Volovik, Sov. Phys. JETP 67, 1804 (1988). [2] J. Goryo and K. Ishikawa, Phys. Lett. A 246, 549 (1998). [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z47.00013: Parametric suppression of edge currents in Sr$_2$RuO$_4$ due to surface roughness Srinivas Raghu, Samuel Lederer, Wen Huang, Edward Taylor, Catherine Kallin The unconventional superconductor Sr$_2$RuO$_4$ is widely believed to have chiral p-wave symmetry, which requires spontaneous charge currents (of some magnitude) at sample edges. However, scanning magnetometry experiments[1], have set an upper bound on currents at least two orders of magnitude smaller than original theoretical predictions[2]. We propose that the currents are suppressed due to surface roughness, which quenches superconductivity within approximately a coherence length of the edge. The system can then be modeled as a normal metal in contact with a chiral p-wave superconductor. We justify this model with spectroscopic evidence from tunneling conductance[3], and calculate the edge current in the model. The current is suppressed by a factor on the order of $\Delta_0/E_F$, which suffices to resolve the seeming contradiction between chiral p-wave pairing and undetectable edge currents. \\ \\ 1. C. W. Hicks, et al. Phys. Rev. B 81, 214501 (2010). \\ 2. M. Matsumoto and M. Sigrist, J. Phys. Soc. Jpn. 68, 994 (1999). \\ 3. S. Kashiwaya et al. Phys. Rev. Lett. 107, 077003 (2011). [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z47.00014: Strongly correlated Dirac electrons and f-wave superconductivity in Ga- herbertsmithite Igor Mazin, Harald Jeschke, Frank Lechermann, Hunpyo Lee, Mario Fink, Ronny Thomale, Roser Valent\'I Herbertsmithite ZnCu$_3$(OH)$_6$C$_{12}$ is essentially the only real-world realization of the ideal single-orbital Kagome model. Being half-doping, it is a Mott insulator. In the nn $p-d$ TB model, it maps exactly onto a single $s$-orbital Kagome Hamiltonian, in particular, exhibits topologically protected Dirac points (DP) at the 4/3 doping. We propose to achieve this doping by substituting Ga for Zn. Such Ga-herbertsmithite (GHS) would be a rare example of a material with strongly correlated Dirac electrons at symmetry-protected locations in the Brillouin zone. We have investigated GHS by means of DFT, TB-DCA and the Slave Bosons approaches and searched for Mott and/or charge order instabilities, and found that it remains metallic and uniform, retaining the DPs. Such a metal with strongly correlated DP electrons would have rather unique topological, magnetic and transport properties. In particular, we show analytically and using fRG that when back-doped with Zn, GHS would harbor unconventional spin-fluctuation driven superconductivity which by symmetry must be $f$-wave of the $+-+-+-$ type. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z47.00015: Physical Properties of Modified Compositions of Strontium Ruthenates Armen Gulian, Vahan Nikoghosyan We performed systematic research on ceramic materials Sr2RuO4 with Sulfur, Selenium or Tellurium added, in combination with other dopants such as: Au, Pt, Al, Zn, Mn, Ba, Na, Ca, Os, Co, Ni, Fe, and Ir. Data on resistive, magnetic, structural, compositional, morphological and other physical properties are obtained, and the most interesting results are presented, as well as corresponding synthesis conditions. [Preview Abstract] |
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