Superconductivity is studied for a fermionic system with attractive four-body interaction. Applying a Green function approach, the gap equation is derived.
From the solution, the transition temperature is calculated. Under the condition that the respective coupling constants are comparable, the transition temperature of four-fermion complexes is considerably larger than the corresponding BCS value.
We explore a spin-fermion model with fermion-spin-quadrupolar interaction. In a nematic phase, this interaction reduces to a four-fermion interaction that is a basis of superconductivity. When the coupling constant is positive the superconductivity is p-wave with spin-parallel paired fermions. When it is negative the superconductivity is p-wave and fermions are spin-antiparallel paired. For a system with zero chemical potential, even a very small coupling can bind fermions into bound state that leads to the superconductivity. When the chemical potential is non-zero the system possesses quantum…
Charge density waves and superconductivity in the electron-positive fermion gas using a simple intuitive model. Part II: Collective modes, effective interactions, superconductivity, and transport
On the role of chalcogen vapor annealing in inducing bulk superconductivity in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$
最近的研究表明，Fe $_{1+y}$ Te $_{1-x}$ Se $_{x}$可以通过在Se和O蒸汽中退火而成为超导体。目前的知识是，这些硫属元素蒸气通过去除磁性过剩的Fe原子而诱导超导性。为了研究这一现象，我们进行了磁化率，比热和运输测量的组合，连同扫描隧道显微镜和光谱学和密度泛函理论计算的Fe $_{1+y}$ Te $_{1-x}$ Se $_{x}$与Te蒸汽处理。我们的结论是，Te蒸汽的主要作用是通过形成FeTe $_{m}$（m $\geq$ 1）配合物淬灭多余Fe原子的磁矩。我们发现，剩余的FeTe $_{m}$配合物仍然破坏超导性，因此，它们的去除可能会进一步提高这些化合物的超导性能。
We study three-dimensional time-reversal-invariant topological superconductivity in noncentrosymmetric materials such as RhSi, CoSi, and AlPt which host coupled multifold nodes energetically split by the spin-orbit coupling at the same time-reversal-invariant momentum (TRIM). The topological superconductivity arises from the $s_{+} \oplus s_{-}$ gap function, which is $\boldsymbol{k}$ independent, but with opposite signs for the two nodes split at the same TRIM. We consider various electron-electron interactions in the tight-binding model for RhSi and find that the topological superconducting …
Topological crystalline metals/semimetals (TCMs) have stimulated a great research interest, which broaden the classification of topological phases and provide a valuable platform to explore topological superconductivity. Here, we report the discovery of superconductivity and topological features in Pb-intercalated transition-metal dichalcogenide Pb$_{1/3}$TaS$_2$. Systematic measurements indicate that Pb$_{1/3}$TaS$_2$ is a quasi-two-dimensional (q-2D) type-II superconductor ({\em T}$_c \approx$ 2.8 K) with a significantly enhanced anisotropy of upper critical field ($γ_{H_{c2}}$ = $H_{c2}^{ab…
Investigation of fermionic pairing on two- dimensional tight binding lattice under phonon and electronic mechanisms within a simple Fermi liquid like scenario and beyond- possible consequences for superconductivity in overdoped cuprates
We describe boundary effects in superconducting systems with Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting instability, using Bogoliubov-de-Gennes and Ginzburg-Landau (GL) formalisms. First, we show that in dimensions larger than one the standard GL functional formalism for FFLO superconductors is unbounded from below. This is demonstrated by finding solutions with zero Laplacian terms near boundaries. We generalize the GL formalism for these systems by retaining higher order terms. Next, we demonstrate that a cuboid sample of a superconductor with imbalanced fermions at a mean-field…
Helical Majorana fermions in d_{x^2-y^2} + i d_{xy}-wave topological superconductivity of doped correlated quantum spin Hall insulators
利用重整化平均场论研究了锯齿形带状蜂窝晶格Kane-Mele模型的大Hubbard U极限。基态表现出时间反演对称性（TRS）破缺d + i d '波超导电性。在大自旋轨道耦合下，纯Kane-Mele模型中具有非平凡自旋Chern数的Z2相在TRS破缺态（称为自旋Chern相）中持续存在，并且在边缘处具有两对反向传播的螺旋Majorana模.随着自旋-轨道耦合的降低，系统经历了从自旋-陈到手征超导态的拓扑量子相变。我们的研究结果可能的相关性adatom掺杂的石墨烯和irridate化合物进行了讨论。
The heavy fermion CeMIn5 family with M = Co, Rh, Ir provide a prototypical example of strange superconductors with unconventional d-wave pairing and strange metal normal state, emerged near an antiferromagnetic quantum critical point. The microscopic origin of strange superconductor and its link to antiferromagnetic quantum criticality and strange metal state are still open issues. We propose a microscopic mechanism for strange superconductor, based on the coexistence and competition between the Kondo correlation and the quasi-2d short-ranged antiferromagnetic resonating-valence-bond spin-liqu…
Charge density waves and superconductivity in the electron-positive fermion gas using a simple intuitive model. Part I: The model, instabilities, and phase diagram
In heavy fermion systems, higher-rank multipole operators are active thanks to the strong spin-orbit interaction (SOI), and the role of diverse multipole fluctuations on the pairing mechanism attracts a lot of attention. Here, we study a mechanism of superconductivity in heavy fermion systems, by focusing on the impact of vertex corrections (VCs) for the pairing interaction going beyond the Migdal approximation. In heavy fermion systems, strong interference between multipole fluctuations cause significant VCs, that represent many-body effects beyond mean-field-type approximations. Especially, …
We introduce a microscopic model aimed at describing the behavior of fermionic excitations in the background of a magnetic texture called "spin-vortex checkerboard". This texture was proposed previously as a possible alternative to stripes to interpret the experimental phenomenology of spin and charge modulations in 1/8-doped lanthanum cuprates. The model involves two kinds of interacting fermionic excitations residing in spin-rich and spin-poor regions of the modulated structure. It is a generalization of another model developed earlier for the so-called "grid checkerboard". The principal ter…
Influence of the temperature dependent chemical potential on the condensation energy from a ternary Boson-Fermion model of superconductivity is reported, it consist of unbound electrons/holes which are fermions plus two-electron and two-hole Cooper pairs which are bosons. When solving simultaneously the set of equations of the mixture (two gap-like equations, one for electron pairs and another one for hole pairs, plus the particle number conservation equation) within the weak-coupling (BCS regime), the resulting superconducting chemical potential shows a shift from its normal state counterpart…
Motivated by a scarcity of simple and analytically tractable models of superconductivity from strong repulsive interactions, we introduce a simple tight-binding lattice model of fermions with repulsive interactions that exhibits unconventional superconductivity (beyond BCS theory). The model resembles an idealized conductor-dielectric-conductor trilayer. The Cooper pair consists of electrons on opposite sides of the dielectric, which mediates the attraction. In the strong coupling limit, we use degenerate perturbation theory to show that the model reduces to a superconducting hard-core Bose-Hu…
We use the spin-fermion model to describe the CuO$_2$ planes of the high-Tc superconductors. Using a large wavelength approach, we show that the ferromagnetic component of the Cu spin fluctuations couple to the oxygen holes producing a pairing interaction that leads to a superconducting gap whose symmetry is determined by the anisotropy of the Kondo interaction. We calculate Tc as a function of the hole concentration in a mean-field approximation and our numerical results are in good agreement with the experiments.
Heavy fermion superconductivity: X-boson treatment
我们用X玻色子方法研究了重费米子超导性，在$ U = \infty $极限下，扩展了周期性Anderson模型，在局域$ f $ -电子之间存在最近邻吸引相互作用，我们表明，较高的杂化参数值意味着与超导临界温度最大值相关的较低的杂化参数值$ T_{c} $，这表明能带之间的真实电荷转移倾向于使库珀配对不稳定。此外，我们表明，超导性被限制在附近的职业范围内，在费米能级$ ρ_{f} (μ) $的$ f $ -带态密度有足够高的值，虽然超导性被发现的配置，系统目前的中间价和重费米子的行为，实验观察到。最后，随着总占有率的提高和杂交参数$ V $的增加，存在绝缘体-超导体转变，这与存在无法通过从玻色子方法获得的杂交间隙有关。
