Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H30: Superconductivity Theory: Intertwined Orders |
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Sponsoring Units: DCMP Chair: Peter Hirschfeld, University of Florida - Gainesville Room: LACC 406B |
Tuesday, March 6, 2018 2:30PM - 2:42PM |
H30.00001: Breakdown of Migdal-Eliashberg theory; a Quantum Monte Carlo study Ilya Esterlis , Benjamin Nosarzewski , Edwin Huang , Brian Moritz , Thomas Devereaux , Steven Kivelson , Douglas Scalapino The superconducting (SC) and charge-density-wave (CDW) susceptibilities of the two dimensional Holstein model are computed using Determinental Quantum Monte Carlo (DQMC), and compared with results computed using the Migdal-Eliashberg (ME) approach. We access temperatures as low as 20 times less than the Fermi energy, $E_F$, which are still above the SC transition. We find that the SC susceptibility at low $T$ agrees quantitatively with the ME theory up to a dimensionless electron-phonon coupling $\lambda_0 \approx 0.4$ but deviates dramatically for larger $\lambda_0$. We find that for large $\lambda_0$ and small phonon frequency $\omega_0 \ll E_F$ CDW ordering is favored and the preferred CDW ordering vector is uncorrelated with any obvious feature of the Fermi surface. |
Tuesday, March 6, 2018 2:42PM - 2:54PM |
H30.00002: Fermi surface reconstruction in the nematic state in underdopded d-wave superconductors Hong-Yi Chen , Yu-Yo Chen , Yuanyuan Zhao , Ching-Yu Huang , Chung-Yu Mou We study the nematic state generated by strong impurities by mean-field model in a two-dimensional square lattice. The impurity concentration is considered as a control parameters. As the impurity concentration increases, the appearance of the smectic and nematic states indicates that the spin-density-wave order and the superconducting order are intertwined each other. The density-of-states show that around the Fermi energy, the sub-gap induced by the SDW order is smeared out gradually and the impurity states emerge in the nematic state. Furthermore, the plateau in the superfluid density also indicates the partial existences of the SDW order and the sub-gap in the smetic state. As the impurity state emerges, the superfluid density decreases due to the impurities destroyed superconductivity associated the enhanced magnetization. These results provide an evidence to explain the experimental results of the Fermi surface reconstruction in the nematic state. |
Tuesday, March 6, 2018 2:54PM - 3:06PM |
H30.00003: Role of Interlayer Coupling on the Competition Between Bond-Density-Wave Order and Superconductivity Zachary Raines , Victor Galitski Over the past few years, several exciting experiments in the cuprates have seen evidence of a transient superconducting state upon optical excitation polarized along the c-axis. The competition between d-form-factor density wave order and superconductivity in these materials has been proposed as an important factor in the observed enhancement of superconductivity. Central to this effect is the structure of the bond-density-wave along the c-axis, in particular the c-axis component of the ordering vector. Motivated by the fact that the bond-density-wave order empirically shows a broad peak in c-axis momentum, we consider a model of randomly oriented charge ordering domains and study how interlayer coupling affects the competition of this order with superconductivity. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H30.00004: Robust spectra of different pair density wave states Wei-Lin Tu , Ting-Kuo Lee Pair density wave states have been in the central stage of high-Tc cuprate since its direct observation[1]. The nodal pair density wave(nPDW) state first proposed by Tu and Lee[2] comes from the anti-phase charge density wave state but with a non-zero constant pairing, which is generated from its (quasi-)incommesurate nature. This accords with previous experimental data[3] that within the superconducting dome, the modulations of Cu-O surface observed are incommensurate. The concept of discommensuration[4] can be also seen from STM experiment[5]. Here we show that within the strong correlation regime, it is possible to obtain all above states with similar energies. Some of their robust properties such as the particle-hole asymmetry in the anti-nodal region and the two-gap spectra, which are measured by ARPES, imply the fact that they are of the same genre. However, physical traits in detail can be different, relating to various experimental detections along with the change of parameters such as temperature, doping, etc. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H30.00005: Pair density wave driven charge order in cuprates Peayush Choubey , Peter Hirschfeld Recently, a pair density wave (PDW) state has been argued to exist in underdoped cuprate compound BSCCO based on Josephson tunneling spectroscopy [1]. We investigate the possibility that charge order, observed in most cuprate materials, is driven by PDW correlations. In particular, we study unidirectional, commensurate PDW states with a periodicity of eight lattice constants coexisting with uniform d-wave superconductivity within a renormalized mean-field theory treatment of the extended t - J model [2]. We find that the PDW induces a d-form factor charge order with the same periodicity. In this state, the local density of states (LDOS) maps and intra-unit cell form factors at various biases, computed using a recently developed Wannier function based method, compare well with the corresponding scanning tunneling spectroscopy (STS) results. We discuss the effects of discommensuration, finite correlation lengths, and disorder on such PDW driven charge ordered states. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H30.00006: Hund's Coupling Stabilized Superconductivity in the Presence of Spin Orbit Interactions Alfred Cheung , Daniel Agterberg Local repulsive Coulomb interactions cannot lead to attractive superconducting pairing states, except through the Kohn-Luttinger mechanism. The situation may change however when we include additional local interactions such as the interorbital repulsion $U^\prime$ and Hund's interactions $J$. Including these local interactions, we study the nature of the superconducting pairs in a system including the $d_{xz},d_{yz}$, and $d_{xy}$ orbitals within octahedral and tetragonal crystal environments. It is found that in the presence of spin orbit interactions, attractive pairing states can be stabilized that contain spin triplet, orbital singlet character. The energy and structure of these states change as the degree of tetragonal distortion is tuned away from the cubic limit. Our results extend the work of earlier studies which were restricted to two orbital systems in two dimensions and support the notion that local atomic interactions, in tandem with spin orbit interactions, can lead to novel spin triplet superconducting states. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H30.00007: Cooperation of correlated hopping and spin-orbit coupling in superconductors Joel Hutchinson , Jorge Hirsch , Frank Marsiglio In two-dimensional crystals, crystalline inversion asymmetry or Fermi surface instabilities can lead to the the electronic spin degeneracy being lifted by Rashba spin-orbit coupling. This has unique implications for superconductivity, where spin splitting ensures that Cooper pairs form in mixed singlet-triplet states. In this talk we explore a two-dimensional tight-binding model on a square lattice with an on-site interaction, Rashba spin-orbit coupling as well as a correlated hopping term arising from an off-diagonal part of the electron-electron interaction. It is known that correlated hopping leads to superconductivity when the Fermi level is near the top of the band, even in the presence of strong on-site repulsion. Here we show that the addition of Rashba spin-orbit coupling enhances the critical temperature and tunnelling asymmetry of this model. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H30.00008: Effects of Spin Orbit Coupling on the NMR Response of Multiband Superconductors Yue YU , Alfred Cheung , Daniel Agterberg , Srinivas Raghu Nuclear magnetic resonance (NMR) and Knight shift measurements are critical tools in the identification of spin-triplet superconductors. We discuss the effects of spin-orbit coupling on the Knight shift and susceptiblities for a variety of spin triplet multi-orbital gap functions with orbital singlet character and compare their responses to "traditional" single band spin-triplet ($p_x+ip_y$) superconductors. Surprisingly, we find no decrease in the spin susceptibility and Knight shift in any direction within the superconducting phase under weak spin-orbit coupling. We interpret this as a consequence of the dominance of inter-band pairing. |
Tuesday, March 6, 2018 4:06PM - 4:18PM |
H30.00009: Singlet-Quintet Mixing in Spin-Orbit Coupled Superconductors with Spin-3/2 Fermions Jiabin Yu , Chao-Xing Liu In this work we explored the mixing between spin-singlet pairing and spin-quintet pairing in a spin-orbit coupled superconductors with spin-3/2 fermions. We demonstrate the importance of this new pairing mixing by revealing the emergence of nodal line and the enhancement of critical temperature due to this mechanism. Our theory can be applied to half-Heusler compounds with superconductivity. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H30.00010: Superfluid-Insulator Transition and BEC-BCS Crossover in a Rashba Moat Band Hassan Allami , Dmytro Pesin , Oleg Starykh We study the superconducting transition in a two-dimensional electron gas with strong Rashba spin-orbit coupling. We assume low electron density, such that only the majority spin band participates in the transition. The BEC side of the superfluid-insulator transition, pertaining to the case of chemical potential being below the bottom of the majority band, corresponds to the appearance of a dilute gas of tightly bound Cooper pairs in the system. It can be described in terms of a single-pair wave-function. We show that this transition can be strongly affected by the electrostatic effects of the Coulomb repulsion, which suppresses superfluidity in the system. We observe that in the case of strong screening, provided by a gate on top, the perturbatively small repulsion leads to renormalization of the critical parameters of the transition but does not change its nature. However, when the screening is very weak we show that there is a range of parameters for which the nature of transition changes dramatically and it becomes of the first order type. |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H30.00011: The FFLO State and Charge Density Wave States in Organic Superconductors Charles Agosta , Raju Ghimire , Camille Bales We have strong evidence for inhomogeneous superconductivity (the FFLO state) in organic superconductors. Our data show high sensitivity of the Pauli paramagnetic limit (Clogston-Chandrasekhar limit) to impurities, in contrast to the FFLO-normal state phase line, which is robust to the addition of impurities. Using fits to specific heat data and the alpha-model we show how the FFLO state can be predicted and parameters such as spin-orbit coupling extracted from the data. We will discuss the FFLO phase diagram in view of the more general superconducting phase diagram for organic and unconventional superconductors where the superconducting state is found adjacent to an ordered insulating magnetic state as a function of carrier density. As an example, given the proximity to a charge density wave state (CDW), some organic conductors have a CDW coexisting with a metallic state at ambient pressure. We will discuss early results and proposed experiments to measure the q-vector of a CDW in organic conductors with x-rays. We will show that these experiments will help understand the FFLO state. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H30.00012: Landau levels from neutral Bogoliubov particles in two-dimensional nodal superconductors under
strain and doping gradients Emilian Nica , Marcel Franz Motivated by recent work on strain-induced pseudo-magnetic fields in Dirac and Weyl semimetals, we analyze |
Tuesday, March 6, 2018 4:54PM - 5:06PM |
H30.00013: Superconducting properties of high-T_{c} cuprates from power-law liquid Zhidong Leong , Kridsanaphong Limtragool , Chandan Setty , Philip Phillips Recent photoemission spectroscopy measurements (arXiv:1509.01611) on cuprate superconductors have inferred that over a wide doping range, the imaginary part of the electron self-energy scales as Σ''~(ω^{2} + π^{2}T^{2})^{α}, with α = 1 in the overdoped Fermi-liquid state and α ≤ 0.5 in the optimal to underdoped regime. We show that this power-law self-energy leads to an enhancement of superconductivity at α = ½. By evaluating the BCS gap equation, we find that the minimal coupling needed for pairing instability behaves as |α - ½| near α = ½, where the non-analyticity at α = ½ stems from a logarithmic divergence in the gap equation. In addition, we find that the enhancement of superconductivity at α = ½ is amplified by including the effects of impurity scattering. This qualitatively reproduces the superconducting dome in the cuprates, suggesting that a power-law self-energy could provide a natural framework for studying these systems. Within this framework, we find that a pseudogap suppresses superconductivity by lifting the logarithmic divergence at α = ½. |
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