Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session Y25: Superconductivity: Theory III 
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Sponsoring Units: DCMP Chair: Khadijeh Najafi, Georgetown Room: 324 
Friday, March 18, 2016 11:15AM  11:27AM 
Y25.00001: Valence Bond Theory of CorrelatedElectron Superconductivity Tirthankar Dutta, Sumit Mazumdar, Torsten Clay Whether or not the weakly doped MottHubbard semiconductor is superconducting remains controversial. We present a new valence bond theory of correlatedelectron superconductivity, that has overlap with the original RVB approach, and yet is substantively different. Superconductivity within the theory emerges from a correlatedelectron state in which there is a strong tendency to spinsinglet formation, and where the bandwidth due to pairtunneling is very large. We show that such a situation occurs far away from the 1/2filled band, at or near banfilling of 1/4. In the presence of electronphonon interactions the 1/4filled band can form a spinpaired CDW state that we have called a pairedelectron crystal, and that is a Wigner crystal of pairs. In the presence of frustration the spinpaired bonds become mobile to give a pairedelectron liquid, which is a precursor to superconductivity. The superconducting state here is reached from a cooperative effect between electronelectron and electronphonon interactions, and the theory thus has overlap also with the bipolaron theory of superconductivity. We will present exact numerical calculations on a 4x4 lattice using the valence bond basis to substantiate our conjectures. [Preview Abstract] 
Friday, March 18, 2016 11:27AM  11:39AM 
Y25.00002: Retarded VS instantaneous : not so conflicting views on the pairing dynamics of the extended Hubbard model A. Reymbaut, M. Fellous Asiani, L. Fratino, M. Charlebois, S. Verret, G. Sordi, D.. S\'en\'echal, A.M. S. Tremblay While most experimental and theoretical clues lean towards a magnetic origin for the pairing mechanism of high temperature superconductors, the question of its degree of retardation in the strong correlation regime remains highly controversial [15]. Part of the answer to this question lies in the frequency dependence of the anomalous spectral function of doped Mott insulators. That spectral function is associated with the Gorkov function and can be extracted at finite temperature using the MaxEntAux method for analytic continuation [6]. Using Cellular Dynamical MeanField Theory for the Hubbard model with nearestneighbor repulsion, we show that the retarded contribution coming from the anomalous spectral function is accompanied by a contribution to the real part of the anomalous selfenergy at infinite frequency. This contribution suggests the emergence of a "mixed" pairing mechanism, mostly retarded, slightly instantaneous. [1] P. W. Anderson, Science 316 (5832), 1705 (2007) [2] D. J. Scalapino, Eletter resp. to Science 316, 1705 (2007) [3] T. A. Maier, et al., PRL 100, 237001 (2008) [4] D. S\'en\'echal, et al., PRB 87, 075123 (2013) [5] E. Gull and A. Millis, PRB 90, 041110(R) (2014) [6] A. Reymbaut, et al., PRB 92, 060509(R) (2015) [Preview Abstract] 
Friday, March 18, 2016 11:39AM  11:51AM 
Y25.00003: Crucial Role of Internal Collective Modes in Underdoped Cuprates Aabhaas V. Mallik, Umesh K. Yadav, Amal Medhi, H. R. Krishnamurthy, Vijay B. Shenoy The enigmatic cuprate superconductors have attracted resurgent interest with several recent reports and discussions of competing orders in the underdoped side. Motivated by this, here we address the natural question of frailty of the $d$wave superconducting state in underdoped cuprates. Using a combination of theoretical approaches we study a $tJ$ like model. We report an  as yet unexplored  instability that is brought about by an ``internal'' fluctuation (antisymmetric mode) of the $d$wave state. This new theoretical result helps in understanding recent ARPES and STM studies. We also suggest further experiments to uncover this physics. [Preview Abstract] 
Friday, March 18, 2016 11:51AM  12:03PM 
Y25.00004: Competition between antiferromagnetism and superconductivity: a quantum Monte Carlo study Tianxing Ma, Da Wang, Congjun Wu Among correlated materials, the vicinity between various magnetic orders and superconductivity is one of the most notorious issues, which is present in high temperature superconductors like doped cuprates and ironpnictides, as well as in organic superconductors. In a multiband Hubbard model, or, equivalently, a largespin Hubbard model, it has been shown that the sign problem of quantum Monte Carlo simulations can be removed at arbitrary fillings in a wide parameter regime, which offers the unique opportunity to perform a detailed unbiased analysis of the competition between antiferromagnetism and superconductivity. Within this framework, we performed QMC simulations to investigate the phase diagram as doping and interaction strength, and our nonbiased numerical results reveal that the antiferromagnetic longrange order can be realized around doping 0\textasciitilde 0.06, and the maximal superconducting pairing correlation is brought by doping, which occurs around the optimal doping 0.30, and then decreases in both the underdoped and overdoped regimes. [Preview Abstract] 
Friday, March 18, 2016 12:03PM  12:15PM 
Y25.00005: Superconducting Fluctuations in the Normal State of the TwoDimensional Hubbard Model Xi Chen, James LeBlanc, Emanuel Gull The dynamical mean field theory and its cluster extensions, such as the dynamical cluster approximation, are effective and accurate methods for solving the 2D Hubbard model. Progress is limited by exponential scaling, especially for quantities relevant to superconducting correlations. In this work, we demonstrate how the vertex contribution to the pairing susceptibility can be used as an indicator of the proximity to the superconducting transition temperature. This allows us to analyze a wider region of parameter space at a higher (numerically accessible) temperature in the normal state. The optimal interaction strength, doping and nextnearest hopping for $d_{x_2y_2}$ superconductivity are located. We conclude that optimal transition temperature occurs at intermediate coupling strength, electrondoped side. This approach is systematically extended to other superconducting symmetries as well. A change in sign of the vertex contribution to $d_{xy}$ superconductivity from repulsive near half filling to attractive at large doping is discovered. [Preview Abstract] 
Friday, March 18, 2016 12:15PM  12:27PM 
Y25.00006: Strongly enhanced superconductivity in coupled $t\!\!\!\!J$ segments Sahinur Reja, Jeroen van den Brink, Satoshi Nishimoto The $t\!\!\!\!J$ Hamiltonian is one of the cornerstones in the theoretical study of strongly correlated copperoxide based materials. Using the density matrix renormalization group method we calculate the phase diagram of the onedimensional (1D) $t\!\!\!\!J$ chain in the presence of a periodic hopping modulation, as a prototype of coupledsegment models. While in the uniform 1D $t\!\!\!\!J$ model near halffilling superconducting (SC) state dominates only at unphysically large values of the exchange coupling constant $J/t>3$, we show that a small hopping and exchange modulation very strongly reduces the critical coupling to be as low as $J/t\sim 1/3$  well within the physical regime. The phase diagram as a function of the electron filling also exhibits metallic, insulating line phases and regions of phase separation. We suggest that a SC state is easily stabilized if $t\!\!\!\!J$ segments creating local spinsinglet pairing are coupled to each other  another example is ladder system. [Preview Abstract] 
Friday, March 18, 2016 12:27PM  12:39PM 
Y25.00007: Cluster dynamic meanfield study on the superconductivity in doped honeycomb lattice Hubbard model Xiao Yan Xu, Hung T. Dang, Stefen Wessel, Zi Yang Meng The issue of superconductivities emerging from doped honeycomb lattice Mott insulator remains inconclusive. Existing proposals, such as p+ip triplet pairing driven by ferromagnetic fluctuations, d+id singlet pairing driven by antiferromagnetic fluctuations or van Hove singularities in the band structure, are not compatible. This is mainly due to the limitation of various approximated techniques employed in addressing such question with inherent strongly correlated nature. Trying to clarify the situation, we perform largescale cluster dynamic meanfield simulations to explore the superconductivity instabilities in the doped honeycomb lattice Hubbard model, from medium to strong coupling. To benchmark, we make use of both interaction and hybridizationexpansion continuous time quantum Monte Carlo methods to exactly solve the quantum cluster embedded in selfconsistently determined meanfield bath. Temperature dependence of various superconducting susceptibilities are calculated, hence, we provide the least biased results of the competition of the superconductivity in different channels in the phase diagram spanned by doping and electronic interaction. [Preview Abstract] 
Friday, March 18, 2016 12:39PM  12:51PM 
Y25.00008: Charge dynamics in doped cuprates Maciej Maska, Marcin Mierzejewski, Evgeny Kochetov, Janez Bonca It has recently been suggested that contrary to common belief, the quantum spin fluctuations of the antiferromagnetic background may not be crucial in explaining the dynamical properties of quasiparticles in strongly correlated systems near halffilling (see H. Ebrahimnejad, {\em et al.} Nature Physics {\bf 10}, 951 (2014)). In accordance with this suggestion, we demonstrate that the $t$$J$ model even without the transverse spin components reproduces many of the ARPES results, provided that the threesite term, usually neglected in calculations, is properly taken into account. The dynamical properties of doped charges are calculated with the help of the Monte Carlo method combined with exact diagonalization. The validity of neglecting the spinflip processes in the Ising version of the $t$$J$ model is checked by a comparison with results of a fully quantum approach based on exact diagonalization in the limited functional space (EDLFS). Our method allows us to show how the spectral properties of doped holes change for a wide range of the doping level. We also demonstrate that the effective model reveals a tendency towards formation of charge density waves. [Preview Abstract] 
Friday, March 18, 2016 12:51PM  1:03PM 
Y25.00009: Controllable electron interactions in quantum dots coupled to nanowires Alexandre Tacla, Guanglei Cheng, Michelle Tomczyk, Jeremy Levy, Andrew Daley, David Pekker We theoretically study transport properties in quantum dot devices proximity coupled to superconducting nanowires. In particular, we investigate the controllable transition from resonant pair tunneling to Andreev bound states, which has been recently observed in nanodevices fabricated at the interface of the oxide heterostructure LaAlO$_3$/SrTiO$_3$. We show that such a transition in transport features can signify a Lifshitz transition, at which electron interactions change from attractive to repulsive. We also discuss an alternate description in terms of magnetic impurities. [Preview Abstract] 
Friday, March 18, 2016 1:03PM  1:15PM 
Y25.00010: Spontaneously broken timereversal symmetry in hightemperature cuprate superconductors. Mikael Fogelstrom, Mikael Hakansson, Tomas Lofwander Conventional superconductors are strong diamagnets that through the Meissner effect expel magnetic fields. It would therefore be surprising if a superconducting ground state would support spontaneous magnetics fields. Such timereversal symmetry broken states have been proposed for the hightemperature superconductors, but their identification remains experimentally controversial. Here we show a route to a lowtemperature superconducting state with broken timereversal symmetry that may accommodate currently conflicting experiments. This state is characterised by an unusual vortex pattern in the form of a necklace of fractional vortices around the perimeter of the material, where neighbouring vortices have opposite current circulation. This vortex pattern is a result of a spectral rearrangement of current carrying states near the edges. Ref. M. H{\aa}kansson, T. L\"{o}fwander, and M. Fogelstr\"{o}m, Nature Phys.11 755, (2015) [Preview Abstract] 
Friday, March 18, 2016 1:15PM  1:27PM 
Y25.00011: Searching for stripe order in the Hubbard model Edwin Huang, Christian Mendl, Hongchen Jiang, Shenxiu Liu, Yvonne Kung, Brian Moritz, Steven Johnston, Thomas Devereaux The existence of stripe ordering in doped cuprates is wellestablished experimentally, but the microscopic mechanisms of their formation and their relation to superconductivity remain open questions. Previous densitymatrix renormalization group (DMRG) studies on tJ and Hubbard ladders, in the parameter regimes relevant to cuprates, have suggested the presence of charge and spin stripes in the ground states of these microscopic models. To further investigate these tendencies towards stripe formation, we perform determinant quantum Monte Carlo (DQMC) and DMRG simulations for the singleband and threeband Hubbard models on identical rectangular lattice geometries. For both methods, upon hole doping, we find the spontaneous formation of domain walls in the spin correlation function, characterized by a piphase shift of the antiferromagnetic ordering upon traversing a domain wall. We compare and contrast the results from the singleband and threeband Hubbard models using both techniques. [Preview Abstract] 
Friday, March 18, 2016 1:27PM  1:39PM 
Y25.00012: Theory of dirty Rashba superconductivity in ultrathin Pb films Hua Chen, Hyoungdo Nam, ChihKang Shih, Allan MacDonald Pb is a typical swave superconductor and also has strong atomic spinorbit coupling. In Pb thin films inversion symmetry breaking combined with the large atomic spinorbit coupling will split the otherwise spindegenerate bands of Pb, an effect which can be roughly accounted for by Rashba spinorbit coupling. Motivated by a recent experiment, we used a 2D model involving Rashba spinorbit coupling and swave pairing to study the influence of the former on the superfluid density, parallel critical field, and the resistive transition to the superconducting phase. We found that in both the clean and the dirty limits, Rashba spinorbit coupling has little influence on the superfluid density and the KosterlitzThouless transition temperature. However, in the dirty limit the Rashba spinorbit coupling can significantly enhance the parallel critical field, making the ClogstonChandrasekhar limit inapplicable. The strong suppression of the spin pairbreaking effect of a parallel magnetic field by the Rashba spinorbit coupling and impurity scattering can make the orbital pairbreaking effect dominant again even in ultrathin films. Finally, we propose and examine a number of mechanisms that can lead to a smeared resistive transition under parallel magnetic fields. [Preview Abstract] 

Y25.00013: ABSTRACT WITHDRAWN 
Friday, March 18, 2016 1:51PM  2:03PM 
Y25.00014: Current Correlations in a Majorana Beam Splitter Erez Berg, Arbel Haim, Felix von Oppen, Yuval Oreg We study current correlations in a $T$junction composed of a grounded topological superconductor and of two normalmetal leads which are biased at a voltage $V$. We show that the existence of an isolated Majorana zero mode in the junction dictates a universal behavior for the cross correlation of the currents through the two normalmetal leads of the junction. The cross correlation is negative and approaches zero at high bias voltages as $1/V$. This behavior is robust in the presence of disorder and multiple transverse channels, and persists at finite temperatures. In contrast, an accidental lowenergy Andreev bound state gives rise to nonuniversal behavior of the cross correlation. We employ numerical transport simulations to corroborate our conclusions. [Preview Abstract] 
Friday, March 18, 2016 2:03PM  2:15PM 
Y25.00015: Oddparity superconductors with twocomponent order parameters: nematic and chiral, gapped and nodal Jörn W F Venderbos, Vladyslav Kozii, Liang Fu Motivated by growing experimental evidence that superconductivity in the doped topological insulator CuxBi2Se3 has an oddparity pairing with broken rotational symmetry, we study the general class of oddparity superconductors with twocomponent order parameters. We address the energetics and physical properties of different superconducting phases, with special emphasis on the role of spinorbit coupling, which is generally strong in topological insulator and related materials. We show that within the weakcoupling BCS theory, in the absence of spinorbit coupling, isotropic superconductors are favored, which are analogs of the A or Bphase of $^3$He. In the presence of spinorbit coupling, however, we find that a nematic superconducting phase, which spontaneously breaks rotational symmetry are favored. We determine the superconducting gap structures and find, in addition to fully gapped oddparity superconductors, nodal superconductors with Dirac and Weyl quasiparticles, which are markedly different from superfluid phases of $^3$He. [Preview Abstract] 
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