Superconductivity in repulsive Fermi-systems at low density
M. Yu. Kagan, D. V. Efremov, M. S. Mar'enko, V. V. Val'kov
备注：
12 pages 11 figures, accepted for publication in Journal of Superconductivity and Novel Magnetism, based on invited talks on Ginzburg conference in Zvenigorod and Vortex VII conference in Rhodes, 2011
In the large variety of models such as 3D and 2D Fermi-gas model with hard-core repulsion, 3D and 2D Hubbard model, and Shubin-Vonsovsky model we demonstrate the possibility of triplrt p-wave pairing at low electron density. We show that the critical temperature of the p-wave pairing can be strongly increased in a spin-polarized case or in a two-band situation already at low density and reach experimentally observable values of (1-5)K. We also discuss briefly d-wave pairing and high-Tc superconductivity with Tc~100K which arises in the t-J model in the range of parameters realistic for cuprates.
We introduce a new and general mechanism for superconductivity in Fermi systems with strong repulsive interaction. Because kinetic terms are small compared to the bare repulsion, the dynamic of charge carriers is constrained by the the presence of other nearby carriers. By treating kinetic terms as a perturbation around the atomic limit, we show that pairing can be induced by correlated multi-particle tunneling processes that favor two itinerant carriers to be close together. Our analytically-controlled theory provides a quantitative formula relating $T_c$ to microscopic parameters, with maxim…
We review recent theoretical progress in understanding spatially uniform $s$-wave superconductivity which arises from a fermion-fermion interaction, which is repulsive on the Matsubara axis, where it is real, but does depend on the transferred frequency. Such a situation holds, e.g., for systems with a screened Coulomb and retarded electron-phonon interaction. We show that despite repulsion, superconductivity is possible in a certain range of system parameters. However, at $T=0$ the gap function on the Matsubara axis, $Δ(ω_m)$, must pass through zero and change sign at least once. These zeros …
p-wave superconductivity in weakly repulsive 2D Hubbard model with Zeeman splitting and weak Rashba spin-orbit coupling
本文研究了具有弱排斥作用的二维正方晶格哈伯德模型在塞曼场和弱Rashba自旋轨道相互作用下的超导序。对角化的非相互作用的哈密顿量导致两个单独的带，并通过推导一个有效的低能相互作用，我们找到了平均场带隙方程的超导序参量的带。在低于临界温度时，通过求解带隙方程，我们发现超导电性是由Kohn-Luttinger型相互作用引起的，而自旋-轨道耦合间接影响了能带的对对称性. Kohn-Luttinger项的主要吸引动量通道依赖于系统的填充分数$n$，因此可以通过调节$n$来改变序参量的动量依赖性。此外，$n$还确定哪个带具有最高的临界温度。旋转磁场将动量依赖性从小动量的状态改变为平面外场的手征$p_x \pm i p_y$型状态，到纯平面内场的节p波型状态。
Superconductivity from purely repulsive interactions in the strong coupling approach : Application of the SU(2) slave-rotor theory to the Hubbard model
The discovery of superconductivity in layered vanadium-based kagome metals $A$V$_3$Sb$_5$ ($A$: K, Rb, Cs) has added a new family of materials to the growing class of possible unconventional superconductors. However, the nature of the superconducting pairing in these materials remains elusive. We present a microscopic theoretical study of the leading superconducting instabilities on the kagome lattice based on spin- and charge-fluctuation mediated Cooper pairing. The applied methodology includes effects of both on-site and nearest-neighbor repulsive Coulomb interactions. Near the upper van Hov…
Perturbation Theory for a Repulsive Hubbard Model in Quasi-One-Dimensional Superconductors
研究了准一维排斥哈伯德模型的对对称性和相变温度。我们用三阶微扰展开法求解了Eliashberg方程，并考虑了在位排斥$U$ .我们发现，当电子数密度从半充满的，过渡到非常规超导预期。当一维性较弱时，自旋单重态是有利的。相比之下，当一维性强且电子数密度远离半填充时，自旋三重态稳定。最后，我们讨论了在$β$ -Na $_{0.33}$ V $_2$ O $_5$中由在位库仑排斥引起的非常规超导电性的可能性。
Specific heat of Ca$_{0.32}$Na$_{0.68}$Fe$_2$As$_2$ single crystals: unconventional s$_\pm$ multi-band superconductivity with intermediate repulsive interband coupling and sizable attractive intraband couplings
报道了高质量重空穴掺杂超导体Ca $_{0.32}$ Na $_{0.68}$ Fe $_2$ As $_2$单晶的低温比热研究。该化合物表现出体超导性，转变温度为$T_c \approx 34$ \，K，这是明显的从磁化，运输，和比热测量。零场数据表明，在正常状态下的索末菲系数$γ\approx 53$ mJ/mol K $^{2}$的显着的电子比热。使用多带Eliashberg分析，我们证明了在超导状态下的零场比热的依赖性是很好地描述了一个三带模型与非常规的s $_\pm$配对对称性和间隙幅度$Δ_i$约为2.35，7.48，和-7.50 meV。我们的分析表明，一个不可忽视的吸引力带内耦合，这有助于显着相对较高的$T_c$值。费米表面平均的排斥和吸引耦合强度是可比的大小和外部的强耦合限制经常采用描述高$T_c$铁磷属元素化物超导体。我们进一步推断总质量重整化的顺序为5，包括相关性和电子玻色子相互作用的影响。
We study the emergence of superconductivity in rhombohedral trilayer graphene due purely to the long-range Coulomb repulsion. This repulsive-interaction-driven phase in rhombohedral trilayer graphene is significantly different from those found in twisted bilayer and trilayer graphenes. In the latter case, the nontrivial momentum-space geometry of the Bloch wavefunctions leads to an effective attractive electron-electron interaction; this allows for less modulated order parameters and for spin-singlet pairing. In rhombohedral trilayer graphene, we instead find spin-triplet superconductivity wit…
Fractal Hofstadter bands have become widely accessible with the advent of moiré superlattices, opening the door to studies of the effect of interactions in these systems. In this work we employ a renormalization group (RG) analysis to demonstrate that the combination of repulsive interactions with the presence of a tunable manifold of Van Hove singularities provides a new mechanism for driving unconventional superconductivity in Hofstadter bands. Specifically, the number of Van Hove singularities at the Fermi energy can be controlled by varying the flux per unit cell and the electronic filling…
We discuss superconducting pairing in a narrow conduction band sandwiched between unoccupied and occupied bands, an arrangement that enables an unconventional pairing mechanism governed by Coulomb repulsion. Pairing interaction originates from repulsion-assisted scattering between far-out pair states in the higher-energy bands and those at the Fermi level. Optimizing the bandstructure design and carrier density in order to bring plasma frequency below the bandgap renders the repulsion unscreened for the processes with a large frequency transfer. This allows the pairing to fully benefit from th…
Fermion systems with flat bands can boost superconductivity by enhancing the density of states at the Fermi level. We use quasiexact numerical methods to show that repulsive interactions between spinless fermions in a one-dimensional (1D) flat-band system, the Creutz ladder, give a finite pairing energy that increases with repulsion, though charge quasi-order (QO) remains dominant. Adding an attractive component shifts the balance in favor of superconductivity and the interplay of two flat bands further yields a remarkable enhancement of superconductivity, well outside of known paradigms for 1…
We discuss a pairing mechanism in interacting two-dimensional multipartite lattices that intrinsically leads to a second order topological superconducting state with a spatially modulated gap. When the chemical potential is close to Dirac points, oppositely moving electrons on the Fermi surface undergo an interference phenomenon in which the Berry phase converts a repulsive electron-electron interaction into an effective attraction. The topology of the superconducting phase manifests as gapped edge modes in the quasiparticle spectrum and Majorana Kramers pairs at the corners. We present symmet…
Topological Odd-Parity Superconductivity at Type-II 2D Van Hove Singularities
我们研究了范霍夫奇点（VHS）处二维电子系统中弱排斥相互作用引起的非常规超导电性，其中态密度呈对数发散。我们定义了两种类型的VHS。对于I型VHS的系统，弱排斥相互作用一般诱导非常规单重态配对。然而，更有趣的是，对于II型VHS，重整化群（RG）分析表明，当费米面没有充分嵌套时，弱排斥相互作用有利于三重态配对（例如p波）。对于遵守四对称性的II型VHS系统，拓扑超导性（手性p+ip配对或时间反演不变的$Z_2$ p+ip配对）通常发生。我们还将讨论我们的研究的相关性材料，包括最近发现的超导体LaO $_{1-x}$ F $_x$ BiS $_2$，可以调整到II型VHS掺杂。
Hund's pairing refers to Cooper pairing generated by onsite interactions that become attractive due to large Hund's exchange $J$. This is possible in multiorbital systems even when all local bare interactions are repulsive, since attraction in specific channels are given by certain linear combinations of interaction parameters. On the other hand, pairing processes such as the exchange of spin fluctuations, are also present. We compare these two mechanisms on an equal footing using electronic bands appropriate for different classes of multiorbital systems over a wide range of interaction parame…
Superconductivity in the repulsive Hubbard model on different geometries induced by density-assisted hopping
University of Bonn（波恩大学）；ENSL, CNRS, Laboratoire de Physique（里昂高等师范学院，法国国家科学研究中心，物理实验室）；Physics Department “E. R. Caianiello” and CNR-SPIN, Universitá degli Studi di Salerno, INFN, Gruppo Collegato di Salerno（萨莱诺大学“E.R.卡亚涅洛”物理系及CNR-SPIN，意大利国家核物理研究所萨莱诺联合小组）；Universitá degli Studi di Salerno（萨莱诺大学）
