Within a gauge-invariant microscopic kinetic theory, we study the electromagnetic response in the superconducting states. Both superfluid and normal-fluid dynamics are involved. We predict that the normal fluid is present only when the excited superconducting velocity $v_s$ is larger than a threshold $v_L=|Δ|/k_F$. Interestingly, with the normal fluid, we find that there exists friction between the normal-fluid and superfluid currents. Due to this friction, part of the superfluid becomes viscous. Therefore, a three-fluid model: normal fluid, non-viscous and viscous superfluids, is proposed. For the stationary magnetic response, at $v_s<v_L$ with only the non-viscous superfluid, the Meissner supercurrent is excited and the gap equation can reduce to Ginzburg-Landau equation. At $v_s{\ge}v_L$, with the normal fluid, non-viscous and viscous superfluids, in addition to the directly excited Meissner supercurrent in the superfluid, normal-fluid current is also induced through the friction drag with the viscous superfluid current. Due to the normal-fluid and viscous superfluid currents, the penetration depth is influenced by the scattering effect. In addition, a modified Ginzburg-Landau equation is proposed. We predict an exotic phase in which both the resistivity and superconducting gap are {\em finite}. As for the optical response, the excited ${v_s}$ oscillates with time. When $v_s<v_L$, only the non-viscous superfluid is present whereas at $v_s{\ge}v_L$, normal fluid, non-viscous and viscous superfluids are present. We show that the excited normal-fluid current exhibits the Drude-model behavior while the superfluid current consists of the Meissner supercurrent and Bogoliubov quasiparticle current. Due to the friction between the superfluid and normal-fluid currents, the optical conductivity is captured by the three-fluid model. ......
A broad review of theoretical research work involving different types of microscopic mechanism in various classes of superconductors, carried out in our research group over a decade or so, is presented. These mechanisms include both conventional as well as exotic ones. Special emphasis is placed on the possible applications to the experimental situations. Moreover, comparison of our works with various theoretical proposals made by various other researchers, with regard to high temperature superconductivity in particular, is made. The crucial importance and special significance of our results a…
It is widely believed that, in contrast to its electron doped counterparts, the hole doped compound Ba1-xKxFe2As2 exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume sensitive muon spin rotation study of underdoped Ba1-xKxFe2As2 (0 \leq x \leq 0.25) showing that this paradigm is wrong. Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the …
Microscopic derivation of the Ginzburg-Landau equations for the periodic Anderson model in the coexistence phase of superconductivity and antiferromagnetism
Using the standard Bardeen-Cooper-Schrieffer (BCS) theory, we revise microscopic derivation of the superconductor-insulator boundary conditions for the Ginzburg-Landau (GL) model. We obtain a negative contribution to free energy in the form of surface integral. Boundary conditions for the conventional superconductor have the form $\textbf{n} \cdot \nabla ψ= \text{const} ψ$. These are shown to follow from considering the order parameter reflected in the boundary. The boundary conditions are also derived for more general GL models with higher-order derivatives and pair-density-wave states. It sh…
Microscopic gauge-invariant theory of the c-axis infrared response of bilayer cuprate superconductors and the origin of the superconductivity induced absorption bands
Microscopic coexistence of superconductivity and charge order in organic superconductor $β''$-(BEDT-TTF)$_{4}$[(H$_3$O)Ga(C$_2$O$_4$)$_3$]$\cdot$C$_6$H$_5$NO$_2$
对有机超导体$β''$ -（BEDT-TTF）$_{4}$ [（H $_3$ O）Ga（C $_2$ O $_4$）$_3$ ] $\cdot$ C $_6$ H $_5$ NO $_2$的电子顺磁共振研究表明，在一种材料中，超导性与电荷有序态均匀共存。在电荷有序状态下，面间自旋交换是带隙的，而面内电导率没有显着修改。这种各向异性的行为解释了外来的电荷有序状态，其中分子位置选择性的载流子本地化共存于其他分子上的导电载流子。研究了这种导电电荷有序态与超导电性的关系。
Shortly after the Gor'kov microscopic derivation of the Ginzburg-Landau (GL) model via a small order parameter expansion in Bardeen-Cooper-Schrieffer theory of superconductivity, the derivation was carried to next-to-leading order in that parameter and its spatial derivatives. The aim was to obtain a generalized GL free energy that approximates the microscopic model better. Since 1960s, multiple works have claimed or implicitly assumed that this extended GL model corresponds to the free energy and has solutions in the form of local minima describing superconductivity, such as vortex solutions.…
The standing wave model describes the well-known phenomenon of superconductivity in a new way [1]. Starting from a new definition of superconductivity, a microscopic London relation is derived from first principles. The relation between the macroscopic electric current and the field is derived from this microscopic relation. The coherence length as function of the superfluid density is derived from the microscopic theory. The critical temperature as function of the superfluid density is thus obtained. These results are compared to the experimental work of Bozovic et. al. [2].
The recent Comment by Vorontsov [arXiv:2007.13696] claims that surface pair-density-wave superconductivity with critical temperature higher than the bulk FFLO critical temperature is not supported by microscopic theory. The conclusion is reached by using an approximate semi-microscopic quasiclassical approach. Here we show that a fully microscopic approach unambiguously demonstrates the existence of surface pair-density-wave superconductivity.
The phenomenon of oxygen incorporation induced superconductivity in iron telluride (Fe1+yTe, with antiferromagnetic (AFM) orders) is intriguing and quite different from the case of FeSe. Until now, the microscopic origin of the induced superconductivity and the role of oxygen are far from clear. Here, by combining in-situ scanning tunneling microscopy/spectroscopy (STM/STS) and x-ray photoemission spectroscopy (XPS) on oxygenated FeTe, we found physically adsorbed O2 molecules crystallized into c(2/3x2) structure as an oxygen overlayer at low temperature, which was vital for superconductivity.…
Rock-salt type niobium nitride ($δ$-NbN) is a well-known superconductor having superconducting transition temperature (Tc) $\approx$ 18\,K and a large superconducting gap $\approx$3\,meV. The Tc of $δ$-NbN thin film exhibits a large scattering irrespective of the growth conditions and lattice parameter. In this work, we investigate the atomic origin of suppression of Tc in $δ$-NbN thin film by employing combined methods of experiments and ab-initio simulations. Sputtered $δ$-NbN thin films with different disorder were analyzed through electrical resistivity and x-ray absorption spectroscopy. A…
The recent successful experimental observation of quantum anomalous Hall effect in graphene under laser irradiation demonstrates the feasibility of controlling single particle band structure by lasers. Here we study superconductivity in a Hubbard honeycomb model in the presence of an electromagnetic drive. We start with Hubbard honeycomb model in the presence of an electromagnetic field drive, both circularly and linearly polarized light and map it onto a Floquet $t$-$J$ model. We explore conditions on the drive under which one can induce superconductivity (SC) in the system. We study the Floq…
Superconductivity driven helical magnetic structure in EuRbFe$_4$As$_4$ ferromagnetic superconductor
最近在易磁化$ab$平面铁磁超导体EuRbFe $_4$ As $_4$中发现了沿$c$轴以四个晶格参数为周期调制的螺旋磁结构的证据[K. Iida等人，物理修订版B 100，014506（2019）]。我们认为，这种结构可能会出现由于超导性的存在。在EuRbFe $_4$ As $_4$中，尽管作用在超导电子上的交换场很小，但超导与铁磁相互作用的交换机制可能优于电磁机制，这种情况有利于短周期磁结构（周期小于超导相干长度）的出现。这种情况不同于类似化合物P掺杂EuFe $_2$ As $_2$中的情况，其中电磁机制占主导地位并导致具有显著更大周期（伦敦穿透深度的数量级）的磁性结构。我们还分析了外部磁场对调制磁结构起始温度的影响。
Motivated by the observation of nematic superconductivity in several systems, we revisit the problem of the leading pairing instability of two-component unconventional superconductors on the triangular lattice -- such as $(p_{x},\,p_{y})$-wave and $(d_{x^{2}-y^{2}},\,d_{xy})$-wave. Such a system has two possible superconducting states: the chiral state (e.g. $p+ip$ or $d+id$), which breaks time-reversal symmetry, and the nematic state (e.g. $p+p$ or $d+d$), which breaks the threefold rotational symmetry of the lattice. Weak-coupling calculations generally favor the chiral over the nematic supe…
