We consider a multiband metal with deep primary bands and a shallow secondary one. In the normal state the system undergoes Lifshitz transition when the bottom of the shallow band crosses the Fermi level. In the superconducting state Cooper pairing in the shallow band is induced by the deep ones. As a result, the density of electrons in the shallow band remains finite even when the bottom of the band is above the Fermi level. We study the density of states in the system and find qualitatively different behaviors on the two sides of the Lifshitz transition. On one side of the transition the density of states diverges at the energy equal to the induced gap, whereas on the other side it vanishes. We argue that this physical picture describes the recently measured gap structure in shallow bands of iron pnictides and selenides.
Classification of Lifshitz invariant in multiband superconductors: an application to Leggett modes in the linear response regime in Kagome lattice models
Type II Lifshitz invariant and optically active Higgs mode in time-reversal symmetry broken superconductors
Institute for Theory of Condensed Matter, Karlsruhe Institute of Technology（卡尔斯鲁厄理工学院凝聚态理论研究所）；Department of Physics, University of Tokyo（东京大学物理学系）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所新兴物质科学中心）；Trans-Scale Quantum Science Institute, University of Tokyo（东京大学跨尺度量子科学研究所）
Recent chain-like structured materials have shown a robust superconducting phase. These materials exhibit the presence of quasi-one-dimensional bands (q1D) coupled to conventional higher-dimensional bands. On the mean-field level such systems have a high critical temperature when the chemical potential is close to the edge of a q1D band and the related Lifshitz transition is approached. However, the impact of the pair fluctuations compromises the mean-field results. Recently it has been demonstrated that these fluctuations can be suppressed (screened) by a specific multiband mechanism based on…
The effect of hydrostatic pressure on the electronic structures of iron pnictide superconducting compounds LiFeAs and NaFeAs through density functional theory based first principles studies are presented, using the predicted crystal structures at very high pressure by Zhang et al. [1]. The orbital selective pressure induced modifications in the partial density of states and electronic band structures reveal mixed multi-band multi-orbital nature of these compounds, with energetically degenerate dxz/dyz orbitals at ambient pressure. Due to larger hydrostatic pressures some of the electron/hole b…
The rise and fall of the superconducting transition temperature $T_c$ upon tuning carrier density or external parameters, such as pressure or magnetic field, is ubiquitously observed in a wide range of quantum materials. In order to investigate such domes of $T_c$, we go beyond the prototypical attractive Hubbard model, and consider a lattice model of electrons coupled via instantaneous, spatially extended, attractive interactions. By numerically solving the mean-field equations, as well as going beyond mean field theory using a functional renormalization group approach, we find that for a cha…
No anomalous spin fluctuation in optimally doped and overdoped iron-based oxypnictide superconductors LaFeAsO$_{1-x}$F$_x$ probed by muon spin relaxation
在最佳掺杂和过掺杂超导体LaFeAsO $_{1-x}$ F $_x$中进行了零场和纵向场μ子自旋弛豫测量，以研究在较宽的温度和纵向场范围内的磁涨落.在μ子的时间窗（10 $^{-5}\sim$ 10 $^{-9}$ s）内，我们没有观察到磁涨落随温度变化的迹象.考虑到目前的结果和其他结果，即，通过中子散射观察到的自旋涨落、核磁共振和光电子能谱观察到的类赝能隙行为，表明不仅自旋涨落，而且费米表面具有几种不同轨道贡献的多带特征可能在LaFeAsO $_{1-x}$ F $_x$的超导配对机制中发挥重要作用。
Induced superconductivity in noncuprate layers of the Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ high-temperature superconductor: Modeling of scanning tunneling spectra
我们分析了在铜氧化物高温超导体的扫描隧道光谱（STS）中观察到的相干峰如何从铜氧化物层转移到STS显微镜尖端附近的氧化物层。为此，我们对Bi $_2$ Sr $_2$ CaCu $_2$ O $_{8+δ}$（Bi 2212）超导态进行了多带计算，假设短程d波对相互作用局限于最近邻Cu $d_{x^2-y^2}$轨道.由此产生的异常矩阵元素的格林函数，使我们能够监测如何配对，然后诱导不仅在铜酸盐双层内，但也内和跨其他层和网站。描述了各种反常矩阵元的对称性和相关的选择规则。
Study of Nb$_{0.18}$Re$_{0.82}$ non-centrosymmetric superconductor in the normal and superconducting states
我们研究了非中心对称超导体Nb $_{0.18}$ Re $_{0.82}$的多带超导性和非s波配对的证据，使用电输运，磁化强度和比热测量。在正常状态下，电阻率随温度和磁场的演化都支持多带图像。在超导状态下，Werthamer，Helfand和Hohenberg（WHH）模型不能充分描述上临界场，$H_{c2}(T)$，在整个温度范围内测量的温度依赖性。此外，观察到的$H_{c2}(0)$超过泡利极限，表明非s波配对。有趣的是，Kadowaki-Woods比和Uemura图揭示了Nb $_{0.18}$ Re $_{0.82}$中的行为，这与非常规超导体中发现的行为相似。低临界场的温度依赖性，$H_{c1}(T)$，遵循一个异常的$T^3$行为和导出的归一化超流密度（$ρ_s$）很好地解释了使用无节点的两个间隙描述。对可逆磁化数据进行的相位波动分析揭示了与平均场常规s波行为的显著偏差。这种趋势被解释在配对对称的非s波自旋三重态组件，可以预期在一个非中心对称的超导体，反对称自旋轨道耦合起着主导作用。
The study of subtle effects on transport in semiconductors requires high-quality epitaxial structures with low defect density. Using hybrid molecular beam epitaxy (MBE), SrTiO$_3$ films with low-temperature mobility exceeding 42,000 cm$^2$V$^{-1}$s$^{-1}$ at low carrier density of 3 x 10$^{17}$ cm$^{-3}$ were achieved. A sudden and sharp decrease in residual resistivity accompanied by an enhancement in the superconducting transition temperature were observed across the second Lifshitz transition (LT) where the third band becomes occupied, revealing dominant intra-band scattering. These films f…
