Microscopic Mechanism Of Superconductivity Research Articles

Critical magnetic fields and electron pairing in magic-angle twisted bilayer graphene

The microscopic mechanism of superconductivity in magic-angle twisted bilayer graphene (MATBG) is currently under active debate. The authors propose here that valuable constraints are implied by the band-filling dependences of the critical temperature and the critical magnetic field, which together define an average Fermi velocity. They show that a robust inverse correlation between the critical temperature and the average Fermi velocity holds in models with band-filling independent pairing interactions. The absence of such an association in experiments suggests that the pairing glue in MATBG is strongly band-filling dependent.

Exploring high transition temperature topological superconductivity in (Li1−xCoxOH)CoSb superlattices

Among the FeSe-based superconductors, $({\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}\mathrm{OH})\mathrm{FeSe}$ has been discovered as an air-stable superlattice structure harboring both high transition temperature (high-${T}_{\mathrm{c}}$) superconductivity and intriguing topological superconducting properties. Here, we use first-principles approaches to identify a chemically isovalent and structurally identical counterpart of $({\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}\mathrm{OH})\mathrm{FeSe}$, namely, $({\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{OH})\mathrm{CoSb}$, which not only is an attractive candidate to harbor high-${T}_{\mathrm{c}}$ superconductivity, but also exhibits two distinct features surrounding topology and magnetism. The rationale of this predictive design is based on the recent identification of CoSb as a layered high-${T}_{\mathrm{c}}$ superconductor stabilized on ${\mathrm{SrTiO}}_{3}$. We first show that the superlattice structures of $({\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{OH})\mathrm{CoSb}$ $(x=0 \mathrm{or} 0.25)$ are dynamically and thermodynamically stable. Next, we demonstrate that $({\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{OH})\mathrm{CoSb}$ possesses superior superconducting properties to $({\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}\mathrm{OH})\mathrm{FeSe}$, offering an appealing platform for realizing high-${T}_{\mathrm{c}}$ superconductivity beyond the Cu- and Fe-based superconducting systems. More strikingly, we find that (LiOH)CoSb is already topologically nontrivial even without extrinsic doping in the spacer layers, thereby offering a cleaner and more ideal candidate system for realizing topological superconductivity. Furthermore, $({\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{OH})\mathrm{CoSb}$ exhibits weaker elemental magnetic moments than $({\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}\mathrm{OH})\mathrm{FeSe}$, which may provide a different angle for elucidating the microscopic mechanisms of superconductivity in these and related systems.

Measurement of magnetic penetration depth in superconducting films by two-coil mutual inductance technique

The magnetic penetration depth (<i>λ</i>) of a superconductor is an important parameter which connects the macroscopic electrodynamics with the microscopic mechanism of superconductivity. High-accuracy measurement of <i>λ</i> is of great significance for revealing the pairing mechanism of superconductivity and exploring the applications of superconductors. Among various methods used to measure <i>λ</i> of superconducting films, the two-coil mutual inductance (MI) technique has been widely adopted due to its high precision and simplicity. In this paper, we start with introducing the principle of MI technique and pointing out that its accuracy is mainly limited by the uncertainties in the geometric parameters (e.g. the distance between two coils) and the leakage flux around the film edge. On this basis, we build a homemade <i>transmission</i>-type MI device with a delicate design to achieve high-accuracy. Two coils are fixed by a single-crystal sapphire block machined with high precisions to minimize the uncertainty in geometry. As a result, the reproducibility in induced voltage measured with sample remounted is better than 4%. Besides, the flux leakage around the film edge is accurately determined by measuring a thick Nb film and Nb foils. The voltage induced by leakage flux is only around 1% of that measured in the normal state. Therefore, the absolute value of <i>λ</i> can be accurately extracted after flux leakage subtraction and normalization. It is shown that the error of the measured <i>λ</i> is less than 10% for a typical superconducting film with a thickness of 100 nm and a penetration depth of 150 nm. Furthermore, the performance of our apparatus is tested on epitaxial NbN films with thickness of 6.5 nm. The results show that the low temperature variation of superfluid density is well described by the dirty s-wave BCS theory, and at temperatures close to <i>T<sub>c</sub></i>, the superfluid density decrease drastically, owing to the Berezinski-Kosterlitz-Thouless transition transition. Moreover, the zero-temperature magnetic penetration depth and the superconducting energy gap extracted from the fitting parameters are both consistent with the reported values. Our device provides an ideal platform for carrying out detailed studies of the dependence of <i>λ</i> on temperature, chemical composition and epitaxial strain, etc. It could also be utilized to characterize other parameters of superconductor such as the critical current density, and when combined with the ionic liquid gating technique, our device offers an efficient route for revealing the microscopic mechanism of superconductivity.

Large Magnon Contributions to Thermal Conductance in Quasi-One-Dimensional Fe-Based Ladder Compounds BaFe_{2}(S_{1-x}Se_{x})_{3}.

We investigate the magnetic phase diagram of a solid solution BaFe_{2}(S_{1-x}Se_{x})_{3}, which is categorized into a new family of Fe-based superconductors hosting a quasi-one-dimensional ladder lattice. We show that the antiferromagnetic order is largely suppressed due to bicritical fluctuations in the vicinity of the magnetic transition at x=0.23 from the stripe-type to block-type ordering. Moreover, we observed large magnon contributions in the thermal conductance in a wide x range. These results indicate the quasi-one-dimensional nature of spin systems and give a solid foundation for modeling the material toward unraveling the microscopic mechanism of superconductivity.

An Overview on the Local Atomic Displacements and Electronic Structures in BiS2/BiSe2-Based Superconductors

Superconductivity with layered structure has attracted intensive research interests due to their exotic superconducting properties. In 2012, a new family of BiS2-based superconductor with their desired layered structure has been discovered. Recently, the isostructural superconductor with the substitution of Se for all S ions in BiS2-based compounds is recognized as the BiSe2-based superconductors. To date, enormous efforts were dedicated to explore the physical properties of this new family. Among them, synchrotron radiation-based X-ray absorption spectroscopy (XAS) contributed significantly to the understanding of local atomic structure and the electronic structure of BiS2/BiSe2-based systems. This article reviewed the progresses in the study of the new BiS2/BiSe2-based materials revealed by the unique technique XAS, including the determination of valence state, local atomic displacements, as well as the different structural distortion between BiS2-based and BiSe2-based systems. The superconductivity of BiS2- and BiSe2-based system is indeed intimately related to valence states and local atomic displacements. Further development of microscopic mechanism of superconductivity in BiS2/BiSe2-based materials requires synergetic endeavors integrating new experimental, numerical, and theoretical techniques.

Electronic structure of FeSe monolayer superconductors

We review a variety of theoretical and experimental results concerning electronic band structure of superconducting materials based on FeSe monolayers. Three type of systems are analyzed: intercalated FeSe systems AxFe2Se2–xSx and [Li1–xFexOH]FeSe as well as the single FeSe layer films on SrTiO3 substrate. We present the results of detailed first principle electronic band structure calculations for these systems together with comparison with some experimental ARPES data. The electronic structure of these systems is rather different from that of typical FeAs superconductors, which is quite significant for possible microscopic mechanism of superconductivity. This is reflected in the absence of hole pockets of the Fermi surface at T-point in Brillouin zone, so that there are no “nesting” properties of different Fermi surface pockets. LDA + DMFT calculations show that correlation effects on Fe-3d states in the single FeSe layer are not that strong as in most of FeAs systems. As a result, at present there is no theoretical understanding of the formation of rather “shallow” electronic bands at M-points. LDA calculations show that the main difference in electronic structure of FeSe monolayer on SrTiO3 substrate from isolated FeSe layer is the presence of the band of O-2p surface states of TiO2 layer on the Fermi level together with Fe-3d states, which may be important for understanding the enhanced Tc values in this system. We briefly discuss the implications of our results for microscopic models of superconductivity.