46 research outputs found
Nematicity-enhanced superconductivity in systems with a non-Fermi liquid behavior
We explore the interplay between nematicity~(spontaneous breaking of the
sixfold rotational symmetry), superconductivity, and non-Fermi liquid behavior
in partially flat-band models on the triangular lattice. A key result is that
the nematicity (Pomeranchuk instability), which is driven by many-body effect
and stronger in flat-band systems, enhances superconducting transition
temperature in a systematic manner on the dome. There, a
-wave symmetry, in place of the
conventional -wave, governs the nematicity-enhanced pairing with a
sharp rise in the dome on the filling axis. When the sixfold
symmetry is spontaneously broken, the pairing becomes more compact in real
space than in the case when the symmetry is enforced. These are accompanied by
a non-Fermi character of electrons in the partially flat bands with many-body
interactions.Comment: 6+18 pages, 5+26 figure
動的平均場理論のダイアグラマティックな拡張による高温超伝導の研究
学位の種別: 課程博士審査委員会委員 : (主査)東京大学教授 川島 直輝, 東京大学教授 藤森 淳, 東京大学准教授 羽田野 直道, 東京大学准教授 加藤 雄介, 東京大学准教授 杉野 修University of Tokyo(東京大学
Nickelate superconductors -- a renaissance of the one-band Hubbard model
Following the discovery of superconductivity in the cuprates and the seminal
work by Anderson, the theoretical efforts to understand high-temperature
superconductivity have been focusing to a large extent on a simple model: the
one-band Hubbard model. However, superconducting cuprates need to be doped, and
the doped holes go into the oxygen orbitals. This requires a more elaborate
multi-band model such as the three-orbital Emery model. The recently discovered
nickelate superconductors appear, at first glance, to be even more complicated
multi-orbital systems. Here, we analyse this multi-orbital system and find that
it is instead the nickelates which can be described by a one-band Hubbard
model, albeit with an additional electron reservoir and only around the
superconducting regime. Our calculations of the critical temperature Tc are in
good agreement with experiment, and show that optimal doping is slightly below
the 20% Sr-doping of Ref. 11. Even more promising than 3d nickelates are 4d
palladates.Comment: 6+10 pages. In the second (arXiv) version we also compare to the
experimental phase diagram [arXiv:2003.08506] in the Supplementary
Information Section S.6; in the final (npj) version also the experimental
phase diagram [arXiv:2004.11281] is mentione
Optimizing superconductivity: from cuprates via nickelates to palladates
Motivated by cuprate and nickelate superconductors, we perform a
comprehensive study of the superconducting instability in the single-band
Hubbard model. We calculate the spectrum and superconducting transition
temperature as a function of filling and Coulomb interaction for a
range of hopping parameters, combining first principles calculations with the
dynamical vertex approximation. We find the sweet spot for high to
be at intermediate coupling, moderate Fermi surface warping, and low hole
doping. Neither nickelates nor cuprates are close to this optimum. Instead, we
identify some palladates, notably RbSrPdO and
PdOCl (=BaLa), to be
virtually optimal, while others, such as NdPdO, are too weakly correlated.Comment: 7+10 pages, 5+11 figure
Phase diagram of nickelate superconductors calculated by dynamical vertex approximation
We review the electronic structure of nickelate superconductors with and without effects of electronic correlations. As a minimal model, we identify the one-band Hubbard model for the Ni 3dx2−y2 orbital plus a pocket around the A-momentum. The latter, however, merely acts as a decoupled electron reservoir. This reservoir makes a careful translation from nominal Sr-doping to the doping of the one-band Hubbard model mandatory. Our dynamical mean-field theory calculations, in part already supported by the experiment, indicate that the Γ pocket, Nd 4f orbitals, oxygen 2p, and the other Ni 3d orbitals are not relevant in the superconducting doping regime. The physics is completely different if topotactic hydrogen is present or the oxygen reduction is incomplete. Then, a two-band physics hosted by the Ni 3dx2−y2 and 3d3z2−r2orbitals emerges. Based on our minimal modeling, we calculated the superconducting Tc vs. Sr-doping x phase diagram prior to the experiment using the dynamical vertex approximation. For such a notoriously difficult to determine quantity as Tc, the agreement with the experiment is astonishingly good. The prediction that Tc is enhanced with pressure or compressive strain has been confirmed experimentally as well. This supports that the one-band Hubbard model plus an electron reservoir is the appropriate minimal model
Self-consistent ladder DA approach
We present and implement a self-consistent DA approach for
multi-orbital models and ab initio materials calculations. It is applied to the
one-band Hubbard model at various interaction strengths with and without
doping, to the two-band Hubbard model with two largely different bandwidths,
and to SrVO. The self-energy feedback reduces critical temperatures
compared to dynamical mean-field theory, even to zero temperature in
two-dimensions. Compared to a one-shot, non-self-consistent calculation the
non-local correlations are significantly reduced when they are strong. In case
non-local correlations are weak to moderate as for SrVO, one-shot
calculations are sufficient.Comment: 21 Pages, 20 Figure