2006 Fiscal Year Final Research Report Summary
Optimization variational Monte Carlo studies of pairing symmetry in cobaltate superconductors
| Project/Area Number |
16540306
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Condensed matter physics II
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| Research Institution | Tohoku University |
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Principal Investigator |
YOKOYAMA Hisatoshi Tohoku University, Graduate School of Science, Research Associate, 大学院理学研究科, 助手 (60212304)
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| Co-Investigator(Kenkyū-buntansha) |
OGATA Masao Tohoku University, University of Tokyo, Graduate School of Science, Associate Professor, 大学院理学系研究科, 助教授 (60185501)
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| Project Period (FY) |
2004 – 2006
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| Keywords | cobaltate / pairing symmetry / triangular lattice / variational Monte Carlo method / frustration / Mott transition / non-monotonic d-wave gap / doublon-holon binding |
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| Research Abstract |
In the last fiscal year, using an optimization variational Monte Carlo (VMC) method, we studied ground-state and low-energy properties of single-band Hubbard models and its strongly correlated version t-J models on anisotropic triangular lattices, which is considered to be plausible models of the newly-found superconducting cobaltates.
As a continuation of these studies for the single-band models, in this year, we mainly studied ground-state properties of a two-band Hubbard model, in particular, a stable condition of superconductivity and its pairing symmetry, because it is probable that the degeneracy of d orbitals are important in the cobaltate. Assuming a simple one-electron structure which roughly fits the band structure near Fermi surface of the cobaltate, we performed VMC calculations with changing the strength of electron correlation and electron density. As a result, we found that superconducting states with s-, d-and p-waves and some mixed waves like a d+id-wave become stable only when an isolated pocket-type Fermi surface overlaps with van-Hove-singularity points near half filling. Among these symmetries, the d+id is the most stable. This stabilization of the superconducting states originates in the gain in interaction energy, which is induced by the enhancement of antiferromagnetic (AF) correlation, as in the cuprate superconductors.
As another topic, we also studied Mott transitions and existence of superconductivity near the transition in a Hubbard model on frustrated square lattices with diagonal hoppings. The main result we obtained is that, for doped cases, the sign of diagonal hopping is crucial; for realistic correlation strength,.a metallic AF order appears for the positive sign (corresponding to electron-doped cases), whereas AF states lose intrinsic stability and phase separate to the regions of an AF state with the half-filled density and of a d-wave superconducting state with a high doping ratio.
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