Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F41: Mainly Theory of Topological Effects in Superconductors |
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Sponsoring Units: DCMP Chair: Martin Claassen, Stanford University Room: 388 |
Tuesday, March 14, 2017 11:15AM - 11:27AM |
F41.00001: Charge-4e superconductors: a Majorana quantum Monte Carlo study Yi-Fan Jiang, Zi-Xiang Li, Steven Kivelson, Hong Yao Many features of charge-4e superconductors remain unknown because even the ``mean-field Hamiltonian'' describing them is an interacting model. Here we introduce an interacting model to describe a charge-4e superconductor (SC) deep in the superconducting phase, and explore its properties using quantum Monte Carlo (QMC) simulations. The QMC is sign-problem-free, but only when a Majorana representation is employed. As a function of the chemical potential we observe two sharply-distinct behaviors: a ``strong'' quarteting phase in which charge-4e quartets are tightly bound (like molecules) so that charge-2e pairing does not occur even in the temperature $T\to 0$ limit, and a ``weak'' quarteting phase in which a further transition to a charge-2e superconducting phase occurs at a lower critical temperature. Analogous issues arise in a putative Z4 spin-liquid with a pseudo-Fermi surface and other interacting models with composite order parameters. Under certain circumstances, we also identified a stable $T=0$ charge-4e SC phase with gapless nodal quasiparticles. We further discuss possible relevance of our results to various experimental observations in 1/8-doped LBCO. (ref: Y.-F. Jiang, Z.-X. Li, S. A. Kivelson, H. Yao, arXiv:1607.01770) [Preview Abstract] |
Tuesday, March 14, 2017 11:27AM - 11:39AM |
F41.00002: Dynamical Chern-Simons Theory in the Brillouin Zone Biao Lian, Cumrun Vafa, Farzan Vafa, Shou-Cheng Zhang Berry connection is conventionally defined as a static gauge field in the Brillouin zone. Here we show that for three-dimensional (3d) time-reversal invariant superconductors, a generalized Berry gauge field behaves as a dynamical fluctuating field of a Chern-Simons gauge theory. The gapless nodal lines in the momentum space play the role of Wilson loop observables, while their linking and knot invariants modify the gravitational theta angle. This angle induces a topological gravitomagnetoelectric effect where a temperature gradient induces a rotational energy flow. We also show how topological strings may be realized in the 6 dimensional phase space, where the physical space defects play the role of topological D-branes. [Preview Abstract] |
Tuesday, March 14, 2017 11:39AM - 11:51AM |
F41.00003: Superfluidity and geometry of Bloch bands Sebastiano Peotta In flat Bloch bands the critical temperature for superconductivity is linear in the coupling constant rather than exponentially suppressed as in conventional BCS theory. I will present our ongoing work on flat-band superconductivity and superfluidity. We show that in the flat-band limit the superfluid weight $D_{\rm s}$ is not controlled by the effective mass, but by a geometric invariant, the quantum metric [2], namely the gauge-invariant part of the Marzari-Vanderbilt localization functional for Wannier orbitals. The quantum metric is related to the Berry curvature, as a consequence we obtain the inequality $D_{\rm s} \geq |C|$ between superfluid weight and Chern number $C$ [2]. This effect is important in a number of lattice models relevant for material science and ultracold gases [2-5]. References: 1) N. B. Kopnin, T. T. Heikkil\"a, and G. E. Volovik, Phys. Rev. B 83, 220503(R) (2011); 2) SP and P. T\"orm\"a, Nature Communications 6, 8944 (2015); 3) A. Julku, SP, T. Vanhala, D.-H. Kim, P. T\"orm\"a, Phys. Rev. Lett. 117, 045303 (2016); 4) M. Tovmasyan, SP, P. T\"orm\"a, S. D. Huber, arXiv:1608.00976; 5) L. Liang, T. I. Vanhala, SP, T. Siro, A. Harju, and P. T\"orm\"a, in preparation. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:03PM |
F41.00004: Photoemission on holographic fermi arcs Bikash Padhi, Garrett Vanacore, Philip Phillips We study the coexistence of Fermi arcs, seen in the pseudogap regime of the cuprates, and superconductivity. We use holographic methods, where the fermions live in an asymptotically anti-de Sitter (AdS) space-time with a black hole at the origin. By devising a suitable coupling, the fermions are allowed to interact with a superconducting condensate formed by the scalar field (hair) emanating from this black hole. Additionally, the black hole is assumed to be charged, so that it's electric field can be used to probe the Fermi surfaces. This fermion-electric field coupling is assumed to be non-minimal. Computing the dual two-point fermion correlator we study the interplay of these two interactions on the boundary fermion spectrum. [Preview Abstract] |
Tuesday, March 14, 2017 12:03PM - 12:15PM |
F41.00005: Odd-frequency superconductivity at the edge of a two-dimensional topological insulator without ferromagnetic insulators Jorge Cayao, Annica Black-Schaffer We investigate the emergence and consequences of odd-frequency spin-triplet superconductivity in a hybrid normal-superconductor (NS) junction at the edge of a two-dimensional topological insulator. In particular, we consider proximity-induced conventional spin-singlet s-wave superconducting pairing with a finite in-plane gradient around the NS interface. We perform analytical and numerical calculations and show that odd-frequency spin-triplet superconductivity does not require the presence of ferromagnetic insulators but instead it naturally arises due to the nature of the induced pairing. We also consider SNS and NSN junctions and investigate potential transport measurements in order to identify such exotic superconducting state. [Preview Abstract] |
(Author Not Attending)
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F41.00006: Thermal Hall conductivity of a nodal chiral superconductor. Sungkit Yip Motivated by the suggestion that Sr2RuO4 is a chiral superconductor and the experimental observation of universal thermal conductivity at low temperature (indicating line nodes or nearly line nodes in the gap) for this system, we evaluate the zero field thermal Hall conductivity of a chiral nodal superconductor. We show that this thermal Hall conductivity (in contrast to the diagonal component) is not universal in the low temperature limit but depends on impurity concentration and phase shift characterizing the impurities. This zero-field Hall thermal conductivity vanishes when the phase shifts are multiple of $\pi $/2. However, under general circumstances, it is smaller than the universal diagonal thermal conductivity only by the factor ln(2$\Delta $/$\gamma )$, where $\Delta $ is the maximum superconducting gap and $\gamma $ is the impurity band width. Numerically this is roughly 0.1-0.2 for available samples. Hence this value of the thermal Hall conductivity is quite large. In particular it is much larger than the expected edge state contributions. Measurement of this zero-field thermal Hall conductivity would be an unambiguous indication that Sr2RuO4 is a chiral superconductor. [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 12:39PM |
F41.00007: Nodeless and topological superconducting phases of d-wave superconductors in proximity to antiferromagnets Guang-Ming Zhang, Guo-Yi Zhu, Ziqiang Wang Motivated by the recent observations of nodeless superconductivity in high-T$_{\mathrm{c}}$ cuprates, we study the superconducting (SC) phase of doped Mott insulators described by the two-dimensional t-J model in proximity to an antiferromagnetic insulator. We found that (i) the nodal d-wave SC state can be driven via a continuous transition into a nodeless (fully gapped) d-wave pairing state by the proximity induced staggered magnetic field. (ii) The energetically favorable pairing states in the strong field regime have extended s-wave symmetry and can be nodal or nodeless. (iii) Between the pure d-wave and s-wave paired phases, there emerge two topologically distinct SC phases with (s$+$id) symmetry, the weak and strong pairing phases, and the weak pairing phase represents a valley symmetry protected Z$_{\mathrm{2}}$ topological superconductor, supporting robust gapless non-chiral edge modes. [Preview Abstract] |
Tuesday, March 14, 2017 12:39PM - 12:51PM |
F41.00008: Superconductivity near a quantum-critical point --- special role of the first Matsubara frequency Artem Abanov, Yuxuan Wang, Emil Yuzbashyan, Boris Altshuler, Andrey Chubukov Near a quantum-critical point in a metal strong fermion-fermion interaction mediated by a soft collective boson gives rise to incoherent, non-Fermi liquid behavior. It also often gives rise to superconductivity which masks the non-Fermi liquid behavior. We analyze the interplay between the tendency to pairing and fermionic incoherence for a set of quantum-critical models with effective dynamical interaction between low-energy fermions. We argue that superconducting $T_c$ is non-zero even for strong incoherence and/or weak interaction due to the fact that the self-energy from dynamic critical fluctuations vanishes for the two lowest fermionic Matsubara frequencies $\omega_m = \pm \pi T$. We obtain the analytic formula for $T_c$ which reproduces well earlier numerical results for the electron-phonon model at vanishing Debye frequency. [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F41.00009: Incomplete condensation in Konh-Luttinger superconductors Da Wang In BCS superconductors, all conduction band electrons are bound together by exchanging phonons to form Cooper pairs at zero temperature. Therefore, all conduction band electrons contribute to superfluid density, regardless of the value of Tc. However, such a picture may fail in intrinsic superconductors (termed as Konh-Luttinger superconductors) mediated by Coulomb interaction between conduction electrons themselves. In Konh-Luttinger superconductors, I propose that one part of electrons has to remain normal in order to mediate pairing interactions for the superconducting electrons. This picture is hopeful to explain the experimentally observed relation between superfluid density and Tc in cuprates. Both a microscopic model calculation and a generalized phenomenological two fluid model are discussed to support to my proposal. [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:15PM |
F41.00010: Study of a one-dimensional three-orbital Hubbard model and effect of spin orbit coupling using the Density Matrix Renormalization Group method. Nitin Kaushal, Shaozhi Li, Yan Wang, Y Tang, Gonzalo Alvarez, Alberto Nocero, Thomas Maier, Steven Johnston, Elbio Dagotto We performed an extensive study of a three-orbital Hubbard model in one dimension using the Density Matrix Renormalization Group technique [1]. The importance of pair-hopping and spin-flip interactions was analyzed. We also calculated the orbital-resolved density of states in the ferromagnetic phase using Dynamical DMRG [2]. The presence of a charge gap was confirmed by performing finite size scaling. The effect of spin-orbit coupling (SOC) on the same model has also been studied, and the Hubbard U versus SOC strength phase diagram was constructed; this analysis was carried out considering the potential role of SOC in iridates [3] and also in iron superconductors [4]. [1] Guangkun Liu et al., Phys. Rev. E~93, 063313 (2016). [2] Shaozhi Li et al., ArXiv:1608.05297 (2016). [3] H. Okabe et al., Phys. Rev. B 83, 155118 (2011). [4] S. V. Borisenko et al., Nat. Phys. 12, 311 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 1:15PM - 1:27PM |
F41.00011: Higgs modes in a pair density wave superconducting state Rodrigo Soto Garrido, Yuxuan Wang, Eduardo Fradkin The pair density wave (PDW) superconducting state has been proposed to explain the layer-decoupling effect observed in the La$_{2-x}$Ba$_x$CuO$_4$ compound at $x=1/8$ (Phys. Rev. Lett. 99, 127003). In this state the superconducting order parameter is spatially modulated, in contrast with the usual superconducting (SC) state where the order parameter is uniform. In this work, we study the properties of the amplitude (Higgs) modes in a PDW state coupled to a Fermi surface. Despite that in the PDW state the Fermi surface largely remains gapless, we found that the damping of the Higgs mode is much weaker than that for a uniform SC, in which case the Fermi surface is {\it completely} gapped. We show that this suppression of damping in the PDW state is due to kinematics. In addition, motivated by the experimental phase diagram, we discuss of the mixing of Higgs modes in the coexistence regime of PDW and uniform SC. These results could be observed directly in a Raman spectrum experiment, providing evidence of the PDW states. [Preview Abstract] |
Tuesday, March 14, 2017 1:27PM - 1:39PM |
F41.00012: Depairing Current in the Pair-Density Wave State Jonatan W\aa rdh, Mats Granath A frustrated pair-density wave (PDW) state has been proposed as a mechanism for decoupling layers at $1/8$ doping in LBCO, giving rise to an effective 2D SC. We investigate the destruction of the PDW-order in the presence of a current. We consider a BCS-like calculation with stripe-like spatially modulated pair-hopping. Without any external driven current this model has two states: a time-reversal invariant PDW state of Larkin Ovchinnikov-type, and a state with broken time-reversal symmetry of Fulde Ferrell-type. We observe a highly anisotropic critical current $J_{\parallel}/J_{\perp} \sim 25$, where $J_{\parallel}$ runs along the PDW ordering vector. In the PDW state, driving a current along the ordering vector induces a first order transition to a current carrying homogeneous SC-state. Further we discuss experimental signatures. [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 1:51PM |
F41.00013: BCS theory of a striped superconductor. Mats Granath, Jonatan Wårdh We consider the BCS theory of a system with periodically modulated pair-hopping as a caricature of a a two dimensional striped superconductor. In a parameter regime relevant for the stripe ordered superconductor LBCO we find two types of near degenerate ordered states, with or without spontaneously broken time reversal symmetry. Including a weak interlayer single particle hopping in a model with staggered interlayer stripe order we estimate the 3D transition temperature of the system and calculate the resulting single particle spectral function. We explore the consequences of these states in light of the phenomenology of the putative pair density wave state in LBCO. [Preview Abstract] |
Tuesday, March 14, 2017 1:51PM - 2:03PM |
F41.00014: Phase diagram and quantum criticality of disordered Majorana-Weyl fermions Justin Wilson, Jed Pixley, Pallab Goswami A three-dimensional $p_x+i p_y$ superconductor hosts gapless Bogoliubov-de Gennes (BdG) quasiparticles which provide an intriguing example of a thermal Hall semimetal (ThSM) phase of Majorana-Weyl fermions. We study the effect of quenched disorder on such a topological phase with both numerical and analytical methods. Using the kernel polynomial method, we compute the average and typical density of states for the BdG quasiparticles; based on this, we construct the disordered phase diagram. We show for infinitesimal disorder, the ThSM is converted into a diffusive thermal Hall metal (ThDM) due to rare statistical fluctuations. Consequently, the phase diagram of the disordered model only consists of ThDM and thermal insulating phases. Nonetheless, there is a cross-over at finite energies from a ThSM regime to a ThDM regime, and we establish the scaling properties of the avoided quantum critical point which marks this cross-over. Additionally, we show the existence of two types of thermal insulators: (i) a trivial thermal band insulator (ThBI), and (ii) a thermal Anderson insulator (AI). We also discuss the experimental relevance of our results for three-dimensional, time reversal symmetry breaking, triplet superconducting states. [Preview Abstract] |
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