Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L41: Theory of Superconductivity 
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Sponsoring Units: DCMP Chair: Khadijeh (Sona) Najafi, Georgetown University Room: 388 
Wednesday, March 15, 2017 11:15AM  11:27AM 
L41.00001: Anomalies in the FFLO transition of ultrathin superconducting films with Rashba Spinorbit interaction Gertrud Zwicknagl, Simon Jahns, Peter Fulde We calculate the inplane upper critical fields of ultrathin superconducting films with Rashba spinorbit (SO) interaction. Thereby we cover the range from small to large SO interactions compared with the superconducting gap parameter of the bulk reference system. The competition between intra and interband pairing is reflected in the variation with temperature of the magnetic field of the second order transition to the normal state. We find a regime of SO coupling parameters where the corresponding centerofmass momentum of the Cooper pairs varies nonmonotonically with temperature. [Preview Abstract] 
Wednesday, March 15, 2017 11:27AM  11:39AM 
L41.00002: Superfluid density and gauge invariance in LarkinOvchinnikovFuldeFerrell (LOFF) superfluid phases Rufus Boyack, Brandon Anderson, Kathryn Levin The recent focus on finite wavevector ordering in the high temperature superconductors and ultracold Fermi superfluids has led to renewed interest in LOFF and related pairdensity wave phases. A central feature in describing a superfluid is the superfluid density, which in BCS theory is relatively straightforward to compute in a fully gauge invariant fashion. What distinguishes the LOFF phases are broken symmetries associated with the special wavevector $\textbf{Q}$. As a result the electromagnetic response, and in particular the superfluid density tensor, must be computed in a more sophisticated fashion. In particular, the collective mode contribution to the superfluid density, which is usually ignored in the literature, was included by Larkin and Ovchinnikov. Here we discuss these issues and show how to compute a proper gauge invariant electromagnetic response for these novel superfluid phases. [Preview Abstract] 
Wednesday, March 15, 2017 11:39AM  11:51AM 
L41.00003: Comparison of functional renormalization group and dynamic cluster quantum Monte Carlo results for the 2D Hubbard model Thomas Maier, Carsten Honerkamp, Daniel Rohe, Douglas Scalapino The Functional Renormalization Group (FRG) provides a method for determining the evolution of the 4point scattering vertex of the Hubbard model as the temperature is lowered. Although it is usually implemented using perturbation theory, the general belief is that if the starting values of the coupling constants are chosen judiciously and the renormalization flow stopped at an appropriate point, the resulting vertex can provide unbiased information about the scattering processes that determine the low temperature properties of the system. Here, for a halffilled 2D Hubbard model, we compare a 1loop FRG calculation with a Dynamic Cluster quantum Monte Carlo Approximation (DCA) calculation with the goal of examining this belief. [Preview Abstract] 
Wednesday, March 15, 2017 11:51AM  12:03PM 
L41.00004: FunctionalRenormalizationGroup Analysis on Electron Nematic State in Cuprate Superconductors Masahisa Tsuchiizu, Kouki Kawaguchi, Youichi Yamakawa, Hiroshi Kontani To elucidate the nematic phase transition recently observed at the pseudogap temperature $T^*$ in cuprate superconductors, we study the charge susceptibilities by utilizing the improved functionalrenormalizationgroup method [1] to the $d$$p$ Hubbard model. We reveal that the most dominant charge fluctuation is the uniform ($q=0$) charge modulation on the $p_x$ and $p_y$ orbitals with antiphase ($d$symmetry) form factor. The spontaneous symmetry breaking with respect to the occupation of $p_x$ and $p_y$ orbitals with the wavevector $q=0$ accounts for the electronic nematic phase transition at $T^*$ in cuprates. In addition, we find that the $p$orbital density wave instability at the wavevector $Q_{\rm{a}} \approx (0.3\pi, 0)$ is further enhanced by the presence of the $q=0$ nematic ordering, consistently with experimental observations of the densitywave states inside the pseudogap region. \\ \ [1] M. Tsuchiizu, Y. Yamakawa, H. Kontani, Phys. Rev. B \textbf{93}, 155148 (2016). [Preview Abstract] 
Wednesday, March 15, 2017 12:03PM  12:15PM 
L41.00005: A Functional Renormalization Group Study of Hund's Rule Coupling in Multiband Hubbard Models Nahom Yirga, David Campbell Twoband Hubbard models are the simplest systems that capture the interplay between magnetism and superconductivity, as seen in many of the Pnictides [1]. They have also been crucial in understanding the material dependence of the critical temperature in the Cuprates [2]. We consider the role of Hund's Rule coupling in a generalized twoband Hubbard Hamiltonian within the framework of the Functional Renormalization Group. We derive the phase diagram for the model and discuss the effects of a strong Hund's Rule coupling on the predicted critical temperature. Finally, to fully address the interplay between the bands and interactions in the Pnictides and the Cuprates, we expand our model to include the effects of bands away from the Fermi surface. [1] Masahisa Tsuchiizu, Yusuke Ohno, Seiichiro Onari, and Hiroshi Kon tani. Orbital nematic instability in the twoorbital hubbard model: Renormalizationgroup + constrained rpa analysis. Phys. Rev. Lett., 111:057003, Jul 2013. [2] Hirofumi Sakakibara, Hidetomo Usui, Kazuhiko Kuroki, Ryotaro Arita, and Hideo Aoki. Origin of the material dependence of t c in the singlelayered cuprates. Physical Review B, 85(6):064501, 2012. [Preview Abstract] 
Wednesday, March 15, 2017 12:15PM  12:27PM 
L41.00006: A possible new family of unconventional high temperature superconductors Jiangping Hu, congcong le We suggest a new family of Co/Nibased materials that may host unconventional high temperature superconductivity (highT$_c$). These materials carry layered square lattices with each layer being formed by vertexshared transition metal tetrahedra cationanion complexes. The electronic physics in these materials is determined by the two dimensional layer and is fully attributed to the three near degenerated $t_{2g}$ dorbitals close to a $d^7$ filling configuration in the dshell of Co/Ni atoms . The electronic structure meets the necessary criteria for unconventional high T$_c$ materials proposed recently by us to unify the two known highT$_c$ families, cuprates and ironbased superconductors. We predict that they host superconducting states with a dwave pairing symmetry with T$_c$ potentially higher than those of ironbased superconductors. These materials, if realized, can be a fertile new ground to study strongly correlated electronic physics and provide decisive evidence for superconducting pairing mechanism. [Preview Abstract] 
(Author Not Attending)

L41.00007: Two types of superconducting domes in unconventional superconductors Tanmoy Das, Christos Panagopoulos We will present a comprehensive analysis of the superconducting (SC) properties and phase diagrams across several families of unconventional superconductors, including the cuprates, heavyfermions, organics and the recently discovered pnictides, chalcogenides, and oxybismuthides. We find that all these families possess two types of SC domes, both unconventional but with distinct SC and normal state properties. The lower Tc dome arises with or without a quantum critical point (QCP), but not always associated with a nonFermi liquid (NFL) background. On the contrary, the higherTc dome stems from a NFL or strange metal phase without an intervening QCP. Our analysis suggests that NFL physics may be a generic route to higherTc superconductivity. [1] T. Das, C. Panagopoulos, New J. Phys. 18, 103033 (2016); arXiv:1512.08186. [Preview Abstract] 
Wednesday, March 15, 2017 12:39PM  12:51PM 
L41.00008: Strong impurities freezing nematicity in underdoped dwave highâ€“temperature superconductors Hong Yi Chen, Yu Yo Chen, Yuanuuan Zhao, Ching Yu Huang, Chung Yu Mou The physical properties of high$T_c$ superconductors are affected by spatial inhomogeneities introduced by impurities. In addition, superconductivity, smectic and electronic nematicity seem intertwined in these materials. Due to a connection between dislocation positions and the position of dopants which represent external disorder, we examine effects of multiple nonmagnetic impurities on the stripe phase induced by antiferromagnetic ordering in underdoped region. The calculation is based on selfconsistent Bogoliubovde Gennesâ€™ (BdG) equations derived from a meanfield $tt'UV$ Hamiltonian. Our results indicate that the quasi onedimensional stripe is pinned by nonmagnetic impurities in the system. In addition, as the impurity concentration increases, the system undergoes phase transitions from the stripe phase to a smectic phase and then from the smectic phase to a nematic phase. We further examine the spatial distribution of order parameters and compute disorder effects on the density of states and the superfluid density for the pure dwave superconducting phase and the stripe phase. [Preview Abstract] 
Wednesday, March 15, 2017 12:51PM  1:03PM 
L41.00009: Prediction of Properties of Pseudo(Symmetric) State of High Temperature Superconductors Ram R. Sharma Recently the dipolon theory [14] has predicted [5] new very low energy excitations in HTSCs due to transition of quasiparticles (QPs) from antisymmetric ("as") to symmetric ("s") state (or vice versa) which creates (annihilates) the quantum ("asson") of energy $ \hbar \omega_{a} (\vec{q}_a) = E^{s} (\vec{k'}) E^{as} (\vec{k"})$; "a" stands for "asson" and $E^{s} (\vec{k'}) $ and $E^{as} (\vec{k"}) $are QP energies in "s" and "as" states, respectively. Here we point out that if the QPs acquire energy equal to or greater than asson energy, they get transited from "s" to "as" state reducing the population of "s" state thereby making its observable properties vague. This is evident if the temperature of the system increases above $T_a$ where $(3/2)k_BT_a=\hbar \omega_{a}$ and then one finds that the energy gap and $I^*$ become vague consistent with experiments. Since the "asson" energy is about 10 meV, $T_a $ is about 77 K. \\ \\$(1)$ R. R. Sharma, Phy. Rev. {\bf B 63}, 054506 (2001). (2) R. R. Sharma, Physica {\bf C 439}, 47 (2006).(3) R. R. Sharma, Physica {\bf C 468}, 190 (2008)(4) R. R. Sharma, "Dipolon Theory of Kink Structure ...", in "Superconducting ...", Ed. K. N. Courtlandt, P. 81100, Nova Sc, Pub., Inc., New York, 2009.(5) R. R. Sharma, http://meetings.aps.org/lnk/BAPS.2016.MAR.D9.15. [Preview Abstract] 
Wednesday, March 15, 2017 1:03PM  1:15PM 
L41.00010: A computational approach to the inverse problem of unconventional superconductivity Eli Chertkov, Bryan Clark Condensed matter physics is concerned with the relationship between microscopic interactions, encoded by the Hamiltonian, and macroscopic emergent properties, encoded by the ground state. In this work, we tackle the following inverse problem: given a desired macroscopic property, what microscopic interactions produce that property? As a first step towards tackling this problem, we developed a new computational approach we call inverse variational Monte Carlo (IVMC). IVMC takes as input a desired wave function and outputs the interaction parameters of a variational Hamiltonian that produces the target wave function as a ground state. We apply the IVMC method to study unconventional superconductivity in the cuprates by searching for the Hamiltonian parameters that most closely produce a superconducting wave function as the ground state. [Preview Abstract] 
Wednesday, March 15, 2017 1:15PM  1:27PM 
L41.00011: Size, Shape and Impurity Effects on Superconducting critical temperature. Masaki Umeda, Masaru Kato, Osamu Sato Bulk superconductors have their own critical temperatures Tc. However, for a nanostructured superconductor, Tc depends on size and shape of the superconductor [1]. Nishizaki showed that the high pressure torsion on bulks of Nb makes Tc higher, because the torsion makes many nanosized fine grains in the bulks [2]. However the high pressure torsion on bulks of V makes Tc lower, and Nishizaki discussed that the decrease of Tc is caused by impurities in the bulks of V. We studied size, shape, and impurity effects on Tc, by solving the Gor'kov equations, using the finite element method. We found that smaller and narrower superconductors show higher Tc. We found how size and shape affects Tc by studying spacial order parameter distributions and quasiparticle eigenenergies [3]. Also we studied the impurity effects on Tc, and found that Tc decreases with increase of scattering rate by impurities. [1] H. Suematsu, M. Kato and T. Ishida, J. Phys.: Conf. Ser. 150 (2009) 052250. [2] T. Nishizakiet al., Physica C 493 (2013) 132. [3] M. Umeda, M. Kato, O.Sato IEEE Trans. Appl. Supercond. 26 (2016) 8600104. [Preview Abstract] 
Wednesday, March 15, 2017 1:27PM  1:39PM 
L41.00012: Reduction of superconducting transition temperature by spin fluctuations in elemental transition metals: firstprinciples study Kentaro Tsutsumi, Mitsuaki Kawamura, Ryosuke Akashi, Shinji Tsuneyuki Effects of spin fluctuations(SF) in superconductors are mainly discussed in the context of the origin of a possible pairing interaction in unconventional superconductors. However, the SF effect is also crucial in conventional phonondriven superconductors. Ferromagnetic fluctuations driven by the exchange effect are relevant in many metals in which electrons are nearly homogeneous and they can suppress the singlet pairing. So far, there are some efforts to include this SF effect in the Eliashberg theory in case of transition metals, e.g. Nb and V(H. Rietschel and H. Winter, Phys. Rev. Lett. 43, 1256(1979)). On the other hand, nonempirical calculations on this effect are lacking. We used the recently developed extended version of density functional theory for superconductors(M. Lueders $et$ $al$., Phys. Rev. B 72, 024545 (2005)) including the SF effect(F. Essenberger $et$ $al$., Phys. Rev. B 90, 214504 (2014)) and explored the SF effect on $T_c$ of V, Nb and Al from firstprinciples. We show that the SF effect commonly reduces the $T_c$ of transition metals and the amount of its reduction is correlated with the magnitude of electronic localization. [Preview Abstract] 
Wednesday, March 15, 2017 1:39PM  1:51PM 
L41.00013: The central question in superconductivity J. E. Hirsch I will argue that the most basic and fundamental question in superconductivity is: when a superconductor in a magnetic field goes normal, how does the supercurrent stop? The supercurrent has to stop before the material becomes resistive because the transition is reversible in an ideal situation, with no Joule heat dissipated. I will argue that the conventional BCSLondon theory of superconductivity cannot answer this question. I will propose an answer to this question that requires that there is flow and counterflow of charge across the normalsuperconductor phase boundary, and requires that the normal state current carriers have holelike character [1]. The conventional BCSLondon theory of superconductivity does not have these physical elements, the theory of hole superconductivity does. [1] J. E. Hirsch, EPL 115, 57001 (2016). [Preview Abstract] 
Wednesday, March 15, 2017 1:51PM  2:03PM 
L41.00014: Crossover From Strong to Weak Pairing States in $tJU$ Model Studied by A Slave Spin Method WeiCheng Lee We investigate the superconductivity in the $tJU$ model within a slavespin method. We show that the BCS meanfield theory implemented with the slave spin formalism naturally predicts two distinct gaps which are the pairing gap of the spinons $\Delta_{\mathrm{f\thinspace }}$and the Cooper pairing gap of the electrons $\Delta_{\mathrm{SC\thinspace }}=$ Z$\Delta _{\mathrm{f\thinspace }}$, where Z is the quasiparticle weight. If U exceeds the critical value for the Mott insulating state at half filling, Z develops a strong doping dependence, leading to a dopingdriven crossover from strong to weak pairing states. In the strong pairing state, while $\Delta_{\mathrm{f\thinspace }}$is enhanced as x $\to $ 0, $\Delta _{\mathrm{SC\thinspace }}\sim $ x due to the renormalization of Z. In the weak pairing state, Z does not change with x significantly. Therefore, $\Delta_{\mathrm{SC\thinspace }}$is mainly controlled by $\Delta _{\mathrm{f\thinspace }}$, and both of them go to zero at larger doping. The crossover from strong to weak pairing states is well captured by the slave spin formalism within reasonable range of parameters just at the meanfield level, indicating the slave spin formalism is a powerful tool to study correlated materials. [Preview Abstract] 
Wednesday, March 15, 2017 2:03PM  2:15PM 
L41.00015: Variational MonteCarlo study for the pairing mechanism and symmetry of superconducting copperoxide monolayer films on Bicuprates Fan Yang, Chen Lu, Tao Xiang The recent STM experiments suggest a Ushape pairing gap for the superconducting copperoxide monolayer films on the Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$[Science Bulletin 2016,61(16):12391247]. To explore the physical origin of this Ushape gap, we performed a variational MonteCarlo (VMC) study for the possible pairing symmetry on the tt'J model over a wide doping region and parameter range. Our VMC results suggest that in a physically reasonable doping region and parameter range for the model, the standard dwave pairing is robustly the leading pairing symmetry, with the possibilities of swave and the d+is wave pairing symmetries ruled out. Our results excluded the possibility of swave or swave component driven by antiferromagnetism in the material. Other possibilities of the origin of the Ushape gap in the system will also be discussed. [Preview Abstract] 
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