Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P30: Superconductivity Theory: Pairing Interactions 
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Sponsoring Units: DCMP Chair: Andreas Kreisel, university of Leipzig Room: LACC 406B 
Wednesday, March 7, 2018 2:30PM  2:42PM 
P30.00001: Impact of Electronphonon Coupling on Superconductivity of Strontium Titanate Maria Navarro Gastiasoro, Andrey Chubukov, Rafael Fernandes Recent experimental work has led to new effort to understand the mechanism behind this very dilute superconductor, and highlights the potential role of ferroelectricity. As an incipient ferroelectric, STO shows a softening of a zone center optical phonon accompanied by a huge dielectric constant. This transverse optical phonon does not couple to the electrons, but the associated longitudinal optical phonon can provide an effective attraction. However, the energy of the longitudinal phonon considerably exceeds the Fermi energy, challenging the screened pseudopotential mechanism by which the electronphonon mediated attraction can overcome Coulomb repulsion. In this work we consider an effective electronelectron interaction that includes the bare electronic Coulomb repulsion and an optical phonon mediated interaction, renormalized by electronic screening in the random phase approximation. We solve numerically the superconducting gap equation and explore how the dynamics of the optical phonons and the Fermi energy size of the conduction electrons affect the instability in the superconducting channel. 
Wednesday, March 7, 2018 2:42PM  2:54PM 
P30.00002: Superconductivity and Strain Effects in Quantum Paraelectric STO Kirsty Dunnett, Awadhesh Narayan, Nicola Spaldin, Alexander Balatsky We will present our results on investigation of the origin of superconductivity in doped STO using a combination of density functional and strong coupling theories[1]. Our approach suggests a model in which the ferroelectric soft mode fluctuations provide the pairing interaction for superconducting carriers. Based on this model, we made a prediction that superconducting Tc will increase with increasing 18O isotope substitution, a scenario that is experimentally verifiable. We now discuss a proposal to use strain as another tool to control the paraelectric fluctuations and thus control superconductivity in STO[2]. 
Wednesday, March 7, 2018 2:54PM  3:06PM 
P30.00003: Theory of Electron Pairing Mediated by Ferroelastic Distortions Anthony TylanTyler, David Pekker, Jeremy Levy The pairing mechanism in LaAlO_{3}/SrTiO_{3} heterointerfaces has remained elusive despite a concentrated experimental and theoretical efforts. Examining the critical current through a series of nanostructures, it was found that the critical current was largely independent of the width of the nanostructure but could be increased by increasing the number of nanowires [Y.Y. Pai arXiv:1707.01488]. In these nanostructures, a carrier density dependent uplift of the surface has also been observed at room temperature [F. Bi, J. App. Phys. 119, 025309 (2016).], suggesting that the electron doping seeds Zoriented tetragonal domains while the epitaxial strain favors X/Yoriented tetragonal domains. From these observations, we propose that electron pairing may be induced at domain walls between these ferrodistortive domains and present the results of a minimal model which couples the structural degrees of freedom to the electron gas. 
Wednesday, March 7, 2018 3:06PM  3:18PM 
P30.00004: Light bipolarons stabilized by Peierls electronphonon coupling John Sous, Monodeep Chakraborty, Roman Krems, Mona Berciu It is widely accepted that phononmediated hightemperature superconductivity is impossible at ambient pressure, because of the extremely large effective masses of the polarons/bipolarons at strong electronphonon coupling. Here we challenge this belief by showing that strongly bound yet very light bipolarons appear for strong Peierls/SuSchriefferHeeger coupling in onedimensional systems. Apart from being light, these bipolarons show a host of unconventional properties, for example at strong coupling there are two bipolaron bands which are stable against strong onsite Hubbard repulsion. Using numerical simulations and analytical arguments, we show that these properties result from the unusual form of the phononmediated interactions, which are of “pair hopping” type instead of regular densitydensity interactions. These unusual properties of the bipolarons should have nontrivial consequences on the superconducting state expected to arise at finite carrier concentrations, in particular on its critical temperature. 
Wednesday, March 7, 2018 3:18PM  3:30PM 
P30.00005: The role of phonons in antiferromagnetic spinfluctuation mediated superconductors within FLEX approximations. Ken Nakatsukasa, Yan Wang, Steven Johnston The physics of phonon mediated conventional superconductors is well explained by the Bardeen 
Wednesday, March 7, 2018 3:30PM  3:42PM 
P30.00006: Strong ElectronPhonon and Band Structure Effects in the Optical Properties of Superconducting Hydrogen Miguel Borinaga, Julen IbañezAzpiroz, Aitor Bergara, Ion Errea Early in this year, Dias and Silvera [Science 355, 715 (2017)] reported the first ever laboratoryproduced sample of metallic hydrogen at 495 GPa. As most breakthroughs in science, this claim came along with tremendous controversy, being the pressure calibration and sample characterization the main issues put under question. 
Wednesday, March 7, 2018 3:42PM  3:54PM 
P30.00007: Mechanism of plain swave superconductivity in heavy fermion compounds near magnetic criticality: Impact of vertex corrections for gap equation Rina Tazai, Youichi Yamakawa, Hiroshi Kontani We study the orbitalfluctuationmediated pairing mechanism in heavy fermion systems, motivated by the recent experimental repots of emerging the plane swave state without signreversal in CeCu2Si2. For this purpose, 
Wednesday, March 7, 2018 3:54PM  4:06PM 
P30.00008: Superconductivity from valence fluctuations Priyo Adhikary, Tanmoy Das

(Author Not Attending)

P30.00009: Relevance of atomic multiplet structure to models of cuprate layers Mi Jiang, Mona Berciu, George Sawatzky We calculate the spectra of two holes doped in a CuO$_{2}$ layer with Cud$^{10}$ and O2p$^{6}$ including the full multiplet structure for both atoms. Distinct from previous studies that treated Cu as an impurity within a featureless O2p band, we dealt with the lattice of Cu and employed the tight binding band structure to describe the O2p band. We claim that the combination of the full Cu3d multiplets and realistic O2p band structure is important to understand the correlated properties of cuprates. 
Wednesday, March 7, 2018 4:18PM  4:30PM 
P30.00010: Signatures of Pairing in the Magnetic Excitation Spectrum of Strongly Correlated twoleg Ladders Alberto Nocera, Nirav Patel, Elbio Dagotto, Gonzalo Alvarez Understanding the superconducting mechanism in cuprates and pnictides remains elusive, mainly due to the lack of wellcontrolled theoretical methods to calculate the doping evolution of the magnetic excitations in two dimensions. However, twoleg ladder systems have provided considerable insights in the physics of cuprates, and recent discoveries support the idea that they could also be much helpful in pnictides. In this talk I will address the following question: can we identify signatures of pairing in the magnetic excitation spectrum of twoleg ladders? I will use the density matrix renormalization group to obtain the dynamical spin structure factor of a generalized t–U–J Hubbard model^{1}. This model includes an exchange correlation strength J independent of U, enhancing pairing tendencies that otherwise would be weak. Motivated by recent neutron scattering results^{2}, I will present a systematic study of the magnetic excitations as a function of hole doping, Hubbard repulsion, and magnetic exchange interactions, and I will discuss the implications of our study for RIXS and neutron scattering experiments. 
Wednesday, March 7, 2018 4:30PM  4:42PM 
P30.00011: DC conductivity, superfluid stiffness, and phase diagrams of quasi 2d organic Mott insulators (κBEDT) CharlesDavid Hébert, Louis Bourassa, A.M. Tremblay Homes law reveals a universal scaling behavior in unconventional superconductors, relating superfluid stiffness, DC conductivity and the superconducting critical temperature. Organic superconductors are modeled by a oneband Hubbard model on a slightly different lattice than cuprates, but they are nevertheless similar. The Mott transition is pressureinduced in organics whereas it is dopinginduced in cuprates. Homes law applies to both classes. Hence, reproducing this behavior is important to assess the validity of theories of superconductivity in strongly correlated materials. Using continuoustime Quantum MonteCarlo in the hybridization expansion, we verify Homes Law in organics. We also reproduce key features of the phase diagrams of these compounds, including highly nonBCS behavior such as increasing superfluid stiffness concomitant with increasing effective mass at constant density. This shows that the interplay between the Mott transition, superconductivity, magnetic order and frustration are key ingredients for the understanding of stronglycorrelated superconductors, including the cuprates. 
Wednesday, March 7, 2018 4:42PM  4:54PM 
P30.00012: Paring From Strong Repulsion in Triangular Lattice Hubbard Model ShangShun Zhang, Wei Zhu, Cristian Batista We propose a paring mechanism between holes in the dilute limit of doped frustrated Mott insulators. Hole pairing arises from a holeholemagnon threebody bound state. This pairing mechanism has its roots on singlehole kinetic energy frustration, which favors antiferromagnetic (AFM) correlations around the hole. We demonstrate that the AFM polaron (holemagnon bound state) produced by a singlehole propagating on a fieldinduced polarized background is strong enough to bind a a second hole. The effective interaction between these threebody bound states is repulsive, implying that this pairing mechanism is relevant for superconductivity. 
Wednesday, March 7, 2018 4:54PM  5:06PM 
P30.00013: Dynamic properties of superconductors: from BCS to BEC regime Dima Mozyrsky, Andrey Chubukov We analyze the evolution of the dynamics of neutral superconductors between BCS and BEC regimes. We consider 2D case, when BCSBEC crossover occurs already at weak coupling and is governed by the ratio of the two scales  the Fermi energy E_F and the bound state energy for two fermions in a vacuum, E_0. BCS and BEC regimes correspond to E_F >> E_0 and E_F << E_0, respectively. We compute the spectrum of lowenergy bosonic excitations and show that the velocity of phase fluctuations remains v_F/\sqrt{2} through BCSBEC crossover. We also derive the prefactor of the topological $A {\dot \phi}$ term in the effective action and argue that A = N_0 E_0 for E_F > E_0, and A = N_0 E_F for E_0 > E_F (N_0 is the density of states per spin). We apply our approach to a propagating vortex in a superconductor and compute the Magnus force acting on a vortex and the effective mass of a vortex. Our results partly agree and partly disagree with earlier studies. 
Wednesday, March 7, 2018 5:06PM  5:18PM 
P30.00014: Weak threedimensional mediators of twodimensional triplet pairing Shanwen Tsai, Shane Kelly Recent experiments demonstrate the ability to construct cold atom mixtures with species selective optical lattices. This allows for the possibility of a mixeddimension system, where one fermionic atomic species is confined to a two dimensional lattice, while another species is confined to a three dimensional lattice that contains the twodimensional one. We show that by tuning the density of an arbitrary number of threedimensional atomic species, we can engineer an arbitrary, rotationallysymmetric, densitydensity, effective interaction for the twodimensional particles. This possibility allows for an effective interaction that favours triplet pairing for twodimensional, SU(2) symmetric particles. Using a functional renormalizationgroup analysis for the twodimensional particles, we derive and numerically confirm that the critical temperature for triplet pairing depends exponentially on the effective interaction strength. We then analyse how the stability of this phase is affected by the particle densities and the fine tuning of interaction parameters. 
Wednesday, March 7, 2018 5:18PM  5:30PM 
P30.00015: Structural and electronic properties of α, β(BEDTTTF)_{2}I_{3} and κ(BEDTTTF)_{2}X_{3} (X=I,F,Br,Cl) organic charge transfer salts Gayanath Fernando, Benjamin Commeau, Matthias Geilhufe, Alexander Balatsky Studies of organic crystalline materials have become an active area of research over the past few decades. Physical properties of some of these materials can be tuned with relative ease utilizing minor modifications of their structures by applying pressure or chemical substitution. One class of such compounds is the BEDTTTFbased charge transfer salts that are packed in a quasitwodimensional structure. Using first principles calculations, we have studied α, β and κphases of (BEDTTTF)_{2}I_{3} as well as chemical substitutions of other Halogens in the κphase, which has a nonsymmorphic space group (1). The κphase, under the anion substitution I> F, shows a topological transformation in its band structure near the Fermi level. In addition, we are evaluating the superconducting order parameter and its symmetry properties of some of these phases through a BetheSalpeter approach. 
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