Enhanced superconductivity in systems with nano-scale charge modulation

Recent photoemission and transport measurements have shown that superconductivity in some respect is optimized in the striped state at 1/8 doping in LaBaCuO. In this state, stripe-like regions with large hole-density are separated by antiferromagnetically correlated spin regions with low hole density. To understand this phenomenon, we have used dynamic cluster quantum Monte Carlo simulations to study the superconducting behavior of a 1/8 doped two-dimensional Hubbard with imposed uni-directional stripe-like charge modulation. Consistent with experiments, we have found a significant increase of the pairing correlations and critical temperature relative to the homogeneous system when the modulation length-scale is sufficiently large. Using a separable form of the pairing interaction, we have found that a delicate balance between the modulation enhanced pairing inter- action, and a concommittent degradation of quasiparticles results in optimized superconductivity for moderate modulation strength.

References:

• T.A. Maier, G. Alvarez, M. Summers, T.C. Schulthess, Phys. Rev. Lett. 104, 247001 (2010).
• S. Okamoto and T.A. Maier, Phys. Rev. B 81, 214525 (2010).

Spin-fluctuation pairing in iron-pnictide superconductors

In both the electron- and hole-doped iron-pnictide compounds, superconductivity appears in proximity to or in coexistence with the antiferromagnetic spin density wave order. It is, therefore, natural to consider the possibility that spin fluctuations provide the pairing mechanism. We have performed enstensive calculations based on a random phase approximation for multi-orbital Hubbard models of the iron-pnictide superconductors to determine the signatures of a spin-fluctuation mechanism on the momentum structure of the superconducting gap. Since the spin-fluctuation pairing interaction is peaked at large momentum transfer, an overall sign-changing structure of the gap between hole- and electron Fermi pockets is usually favored. However, we have found that due to the near-nesting of many distinct Fermi surface sheets, the local Coulomb interaction and the orbital character of the active bands, the gap has significant anisotropy which usually results in nodes on the electron sheets. These calculations have also predicted that not only the Fermi surface topology and shape, but also the orbital character of the Fermi surface play an important role in the strength of the pairing and hence the magnitude of the crtitical temperature.

References:

• S. Graser, T.A. Maier, P.J. Hirschfeld, D.J. Scalapino, New J. Phys. 11, 025016 (2009).
• T. Maier, S. Graser, D. J. Scalapino, and P. J. Hirschfeld, Phys. Rev. B 79, 224510 (2009).
• S. Graser, A. F. Kemper, T. A. Maier, H.-P. Cheng, P. J. Hirschfeld, D. J. Scalapino, preprint arXiv:1003.0133v1 (2010).
• A. F. Kemper, T. A. Maier, S. Graser, H.-P. Cheng, P. J. Hirschfeld, D. J. Scalapino, preprint arXiv:1003.2777v1 (2010).

Nature of the cuprate pairing mechanism

The question of what is causing strongly repulsive electrons to pair up into Cooper pairs in the cuprate high- temperature superconductors is one of the most important questions at the forefront of science. In search of theoretical answers to this important question, we have performed systematic dy- namic cluster quantum Monte Carlo simulations of Hubbard models with parameters relevant to the cuprates. These simulations have allowed us to develop, for the first time, a fundamental understanding of the pairing mechanism responsible for superconductivity in the Hubbard model: The effective pairing interaction is attractive between nearest neigbor, spin singlet electron pairs and is carried by the spin S=1 particle-hole excitations. Its dynamics reflects the spin fluctuation spectrum. A detailed analysis of the frequency scales that contribute to the pairing interaction have allowed us to reconcile two of the most promiment pictures for the pairing mechanism: The dominant contribution comes from low frequencies and is provided by spin fluctuations, while an additional smaller contribution at much higher frequencies is provided by an instantaneous, resonating-valence-bond exchange mechanis. This work motivated a successful experimental validation of the phenomenological spin fluctuation model, which analyzed angle-resolved photoemission and neutron scattering measurements on the same material, in order to show that spin fluctuations have sufficient strength to mediate high-temperature superconductivity.

References:

• T.A. Maier, M.S. Jarrell, and D.J. Scalapino, Phys. Rev. Lett. 96, 047005 (2006)
• T.A. Maier, M. Jarrell, and D.J. Scalapino, Phys. Rev. B 74, 094513 (2006)
• T.A. Maier, M. Jarrell, and D.J. Scalapino, Physica C 460-462, 13-19 (2007)
• T.A. Maier, M. Jarrell, and D.J. Scalapino, Phys. Rev. B, 75, 134519 (2007).
• T.A. Maier, A. Macridin, M. Jarrell and D.J. Scalapino, Phys. Rev. B 76, 144516 (2007).
• T.A. Maier, D. Poilblanc and D.J. Scalapino, Phys. Rev. Lett. 100, 237001 (2008).