We study the finite-temperature vestigial superconducting phases of a two-dimensional system of fluctuating spin-triplet pairing and spin magnetism. Denoting the respective primary order parameters by $\mathbf{d}$ and $\mathbf{N}$, which are not long-range ordered at finite temperature, the composite fields $ϕ_{dd} = \mathbf{d}\cdot\mathbf{d}$ and $ϕ_{dN} = \mathbf{d}\cdot\mathbf{N}$ are spin-rotation invariant and can condense at finite temperature. Using a large-$N$ approach that respects the Mermin-Wagner theorem, we here derive the phase diagram which features two vestigial superconductors: $(A)$ a charge-$4e$ superconductor with $ϕ_{dd}\neq 0$ and $ϕ_{dN} =0$ and $(B)$ a charge-$2e$ state with $ϕ_{dN} ,ϕ_{dd}\neq 0$. We analyze the temperature and coupling-constant dependent properties of these two superconductors using a perturbative approach and a variational Hartree-Fock study. This reveals non-trivial spectra in the superconductors, which result from the fundamental building blocks being distinct from the usual Cooper pairs--in phase $(A)$, the elementary bosons are bound states of four electrons and, in phase $(B)$, of three electrons and a hole. This work complements the previous study [Nat. Commun. 15, 1713 (2024), arXiv:2301.01344], which focused on the properties of phase $(B)$.
We report results of a muon spin relaxation study of slow magnetic fluctuations in the pseudogap phase of underdoped single-crystalline YBa$_{2}$Cu$_{3}$O$_{y}$, $y = 6.77$ and 6.83. The dependence of the dynamic muon spin relaxation rate on applied magnetic field yields the rms magnitude~$B\mathrm{_{loc}^{rms}}$ and correlation time~$τ_c$ of fluctuating local fields at muon sites. The observed relaxation rates do not decrease with decreasing temperature~$T$ below the pseudogap onset at $T^\ast$, as would be expected for a conventional magnetic transition; both $B\mathrm{_{loc}^{rms}}$ and $τ_…
We report an x-ray emission spectroscopy (XES) study of the local fluctuating magnetic moment ($μ_{bare}$) in $\mathrm{NaFe_{1-x}Co_{x}As}$ and $\mathrm{NaFe_{1-x}Cu_{x}As}$. In NaFeAs, the reduced height of the As ions induces a local magnetic moment higher than $\mathrm{Ba_2As_2}$, despite lower T$_N$ and ordered magnetic moment. As NaFeAs is doped with Co $μ_{bare}$ is slightly reduced, whereas Cu doping leaves it unaffected, indicating a different doping mechanism: based on electron counting for Co whereas impurity scattering dominates in the case of Cu. Finally, we observe an increase of …
Fluctuating superconductivity - vestigial Cooper pairing in the resistive state of a material - is usually associated with low dimensionality, strong disorder or low carrier density. Here, we report single particle spectroscopic, thermodynamic and magnetic evidence for persistent superconducting fluctuations in heavily hole-doped cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ ($T_c$ = 66~K) despite the high carrier density. With a sign-problem free quantum Monte Carlo calculation, we show how a partially flat band at ($π$,0) can help enhance superconducting phase fluctuations. Finally, w…
Nature of magnetic excitations in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$
自从铁-磷属元素化物超导体中发现金属反铁磁（AF）基态接近超导性以来，一个中心问题是这些材料中的磁性是否来自弱相关电子，如纯铬中的自旋密度波，需要强电子相关，或者甚至可以用局域电子来描述，如铜氧化物的AF绝缘态。在这里，我们使用非弹性中子散射，以确定整个布里渊区的磁激发的绝对强度，在电子掺杂超导BaFe $_{1.9}$ Ni $_{0.1}$ As $_{2}$（$T_c=20$ K），这使我们能够获得的波动磁矩$<m^2>$的大小，和它的能量分布。我们发现超导BaFe $_{1.9}$ Ni $_{0.1}$ As $_{2}$和AF BaFe $_2$ As $_2$都具有涨落磁矩$<m^2>\approx3.2\ μ_B^2$/Fe（Ni），这与AF绝缘铜氧化物中的磁矩相似。这两类高温超导体的共同主题是磁激发具有部分局域化的特征，从而显示了强关联对高温超导性的重要性。
We determine the phase diagram of LaFe$_{1-x}$Co$_x$AsO single crystals by using nuclear magnetic resonance (NMR). Up to a nominal doping of $x=0.03$, it follows the phase diagram for F-doped polycrystals. Above $x=0.03$, the F-doped samples become superconducting, whereas for Co-doping the structural and magnetic transitions can be observed up to $x=0.042$, and superconductivity occurs only for higher doping levels and with reduced transition temperatures. For dopings up to $x=0.056$, we find evidence for short-range magnetic order. By means of relaxation-rate measurements, we map out the mag…
We have performed inelastic neutron scattering measurements on optimally-doped Fe$_{0.98}$Te$_{0.5}$Se$_{0.5}$ and 10% Cu-doped Fe$_{0.88}$Cu$_{0.1}$Te$_{0.5}$Se$_{0.5}$ to investigate the substitution effects on the spin excitations in the whole energy range up to 300 meV. It is found that substitution of Cu for Fe enhances the low-energy spin excitations ($\le$ 100 meV), especially around the (0.5, 0.5) point, and leaves the high-energy magnetic excitations intact. In contrast to the expectation that Cu with spin 1/2 will dilute the magnetic moments contributed by Fe with a larger spin, we f…
Bulk FeSe is superconducting with a critical temperature $T_c$ $\cong$ 8 K and SrTiO$_3$ is insulating in nature, yet high-temperature superconductivity has been reported at the interface between a single-layer FeSe and SrTiO$_3$. Angle resolved photoemission spectroscopy and scanning tunneling microscopy measurements observe a gap opening at the Fermi surface below $\approx$ 60 K. Elucidating the microscopic properties and understanding the pairing mechanism of single-layer FeSe is of utmost importance as it is a basic building block of iron-based superconductors. Here, we use the low-energy …
