We measured the Fermi surface (FS), band dispersion and superconducting gap in LuNi2B2C using Angle Resolved Photoemission Spectroscopy. Experimental data were compared with the tight-binding version of the Linear Muffin-Tin Orbital (LMTO) method and Linearized Augmented Plane-Wave (LAPW) calculations. We found reasonable agreement between the two calculations and experimental data. The measured FS exhibits large parallel regions with a nesting vector that agrees with a previous positron annihilation study and calculations of the generalized susceptibility. The measured dispersion curves also agree reasonably well with the TB-LMTO calculations, albeit with some differences in the strength of the hybridization. In addition, the spectrum in the superconducting state revealed a 2meV superconducting gap. The data also clearly shows the presence of a coherent peak above the chemical potential, that originates from thermally excited electrons above the energy of 2 delta. This feature was not previously observed in the Lu-based material.
Effect of Fermi surface evolution on superconducting gap in superconducting topological insulator
超导拓扑绝缘体是一种很有希望的拓扑超导体。最近的实验表明，三维费米面演化为二维费米面。我们发现费米面上的超导能隙结构随着这种演化而系统地变化。阐明了自旋晶格弛豫速率和比热等体电子性质与费米面的形状和能隙函数的类型有关。这些结果可作为确定Cu $_x$ Bi $_2$ Se $_3$的配对对称性的指导。
Fermi surface effect on spontaneous breaking of time-reversal symmetry in unconventional superconducting films
我们提出了一种有助于稳定具有破缺时间反转对称性的超导态的机制，该机制被预测在d波超导膜中实现[A。B. Vorontsov，Phys. Rev. Lett. 102，177001（2009）]。在此超导相，时间反演对称性破缺伴随着沿膜表面的平移对称性的自发破缺。我们研究如何正常超导相边界的厚度-温度相图中的薄膜被修改取决于费米面形状。特别地，当费米面满足嵌套条件时，发现非均匀超导相基本上延伸到相图中的较小厚度区域。我们证明了这种费米表面效应使用正方形晶格紧束缚模型。
Iron selenide (FeSe) is an iron-based superconductor which shows unique properties, including strongly anisotropic superconducting gap, paramagnetism in undoped compound and extremely small Fermi pocket size. In this work, we demonstrate that the sizes of electron and hole pockets in FeSe monolayer become much smaller than those in bulk. The Fermi energy is in the order of a few meV and can be fine-tuned by the thickness of graphene layers underneath. Despite the low carrier density, the FeSe monolayers grown on trilayer or multi-layer graphene are superconducting. The superconducting gap size…
Cooper's formalism for fermionic pairing has been revisited considering upto 3rd neighbour hopping terms, firstly with a Fermi liquid like background on a square lattice keeping in mind the overdopped cuprates. Then the whole scheme is repeated with a Marginal Fermi liquid- like background, taking into account the self energy correction of the mobile electrons to include a more realitic density of states in the calculation. Detailed comparison of our theoretical results with those from experiments strongly supports the Marginal Fermi liquid- like character of the normal phase with exciton medi…
We propose a mechanism for the generation of pure superconducting spin-current carried by equal-spin triplet Cooper pairs in a superconductor (S) sandwiched between a ferromagnet (F) and a normal metal (N$_{\rm so}$) with intrinsic spin-orbit coupling. We show that in the presence of Landau Fermi-liquid interactions the superconducting proximity effect can induce non-locally a ferromagnetic exchange field in the normal layer, which disappears above the superconducting transition temperature of the structure. The internal Landau Fermi-liquid exchange field leads to the onset of a spin supercurr…
We report on the Fermi surfaces and superconducting parameters of CaSb$_2$ single crystals (superconducting below $T_{\text{c}}\sim 1.8~\text{K}$) grown by the self-flux method. The frequency of de-Haas-van-Alphen and Shubnikov-de-Haas oscillations evidences a quasi-two-dimensional (quasi-2D) Fermi surface, consistent with one of the Fermi surfaces forming Dirac lines predicted by first-principles calculations. Measurements in the superconducting state reveal that CaSb$_2$ is close to a type-I superconductor with the Ginzburg-Landau (GL) parameter of around unity. The temperature dependence of…
Dirac-Fermion-Induced Parity Mixing in Superconducting Topological Insulators
我们自洽地研究了超导拓扑绝缘体的表面态。我们澄清，如果一个拓扑平凡的散装s-波配对对称性实现，宇称混合的对潜在的表面附近是有害的增强表面狄拉克费米子，打开一个额外的表面间隙大于散装。与经典的s波超导体相比，所得到的表面态密度在诱导能隙处除了在体能隙处的常规峰之外，还具有额外的相干峰。我们还发现，没有这样的额外的峰出现奇宇称超导体与圆柱费米面。计算结果表明，Cu $_x$ Bi $_2$ Se $_3$中的简单U形扫描隧道显微镜谱不是源于s波超导电性，而是可以用具有圆柱费米面的奇宇称超导电性来解释.
The van-der-Waals gap of iron chalcogenide superconductors can be intercalated with a variety of inorganic and organic compounds that modify the electron doping level of the iron layers. In Lix(C3N2H10)0.37FeSe, a dome in the superconducting transition temperature Tc has been reported to occur in the doping range of x=0.06 to x=0.68. We use a combination of density functional theory and spin fluctuation theory to capture the evolution of superconducting transition temperatures theoretically. We clearly demonstrate how the changing electronic structure supports an increasing superconducting Tc.…
Unconventional Pairing in Doped Band Insulators on a Honeycomb Lattice : the Role of the Disconnected Fermi Surface and a Possible Application to Superconducting $β$-MNCl (M=Hf,Zr)
High-temperature superconductivity occurs near antiferromagnetic instabilities and nematic state. Debate remains on the origin of nematic order in FeSe and its relation with superconductivity. Here, we use transport, neutron scatter- ing and Fermi surface measurements to demonstrate that hydro-thermo grown superconducting FeS, an isostructure of FeSe, is a tetragonal paramagnet without nematic order and with a quasiparticle mass significantly reduced from that of FeSe. Only stripe-type spin excitation is observed up to 100 meV. No direct coupling between spin excitation and superconductivity i…
Effects of Fermi surface and superconducting gap structure in the field-rotational experiments: A possible explanation of the cusp-like singularity in YNi$_2$B$_2$C
Fermi surfaces and orbital polarization in superconducting CeO$_{0.5}$F$_{0.5}$BiS$_{2}$ revealed by angle-resolved photoemission spectroscopy
利用偏振相关角分辨光电子能谱（ARPES）研究了BiSPDMATHABMM基CeO $_{0.5}$ F $_{0.5}$ BiS $_2$超导体的电子结构，并成功地阐明了费米面上的轨道特征.在X点周围的矩形费米口袋中，平行于$k_y$方向的直线部分由Bi $6p_x$特征主导。轨道极化表明BiS $_2$系统的准一维电子结构。此外，四边形排列的Bi上的扭曲可以引起带Jahn-Teller效应。
Since its discovery more than a century ago, superconductivity has been at the epicentre of condensed matter physics research. The electron phonon coupling in conventional superconductors, which obeys BCS theory, causes an attractive interaction, resulting in a unique isotropic and fixed sign pairing symmetry ground state. Exotic pairing symmetries are hard to come by in this favourable interaction. While unconventional superconductivity is still a mystery, the potential for novel and exotic coupling symmetries due to the interplay of structural symmetries and Fermi surface (FS) topology makes…
We consider a non-Hermitian superconducting system by coupling a conventional superconductor to a ferromagnet lead and demonstrate the emergence of exceptional points when an external Zeeman field is applied. We discover that, depending on the non-Hermiticity and the Zeeman field, the exceptional points mark the ends of lines with zero real energy, thus giving rise to topologically protected and highly tunable bulk Fermi arcs, which we coin bulk Bogoliubov Fermi arcs due to their superconducting nature. We show that these bulk Bogoliubov Fermi arcs are the non-Hermitian counterparts of the Her…
The order parameter and pairing mechanism for superconductivity in heavy fermion compounds are still poorly understood. Scanning tunneling microscopy and spectroscopy at ultra-low temperatures can yield important information about the superconducting order parameter and the gap structure. Here, we study the first heavy fermion superconductor, CeCu_2Si_2. Our data show the superconducting gap which is not fully formed and exhibits features that point to a multi-gap order parameter. Spatial mapping of the zero bias conductance in magnetic field reveals the vortex lattice, which allows us to uneq…
A spin-imbalanced Fermi gas with an attractive contact interaction forms a superconducting state whose underlying components are superpositions of Cooper pairs that share minority-spin fermions. This superconducting state includes correlations between all available fermions, making it energetically favorable to the Fulde--Ferrell--Larkin--Ovchinnikov superconducting state. The ratio of the number of up- and down-spin fermions in the instability is set by the ratio of the up- and down-spin density of states in momentum at the Fermi surfaces, to fully utilize the accessible fermions. We present …
In this work we investigate the description of superconducting systems with multiple Fermi surfaces. For the case of one Fermi surface we re-obtain the result that the superconductor is more precisely described as a topological state of matter. Studying the case of more than one Fermi surface, we obtain the effective theory describing a time reversal symmetric topological superconductor. These results are obtained by employing a general procedure to construct effective low energy actions describing states of electromagnetic systems interacting with charges and defects. The procedure consists i…
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…
