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Register a new userIntense paramagnon excitations in a large family of high-temperature superconductors
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In the search for the mechanism of high-temperature superconductivity, intense research has been focused on the evolution of the spin excitation spectrum upon doping from the antiferromagnetic insulating to the superconducting states of the cuprates. Because of technical limitations, the experimental investigation of doped cuprates has been largely focused on low-energy excitations in a small range of momentum space. Here we use resonant inelastic x-ray scattering to show that a large family of superconductors, encompassing underdoped YBa$_2$Cu$_4$O$_8$ and overdoped YBa$_2$Cu$_3$O$_{7}$, exhibits damped spin excitations (paramagnons) with dispersions and spectral weights closely similar to those of magnons in undoped cuprates. %The results are in excellent agreement with the spin excitations obtained by exact diagonalization of the $bf t-J$ Hamiltonian on finite-sized clusters. The comprehensive experimental description of this surprisingly simple spectrum permits quantitative tests of magnetic Cooper pairing models. A numerical solution of the Eliashberg equations for the magnetic spectrum of YBa$_2$Cu$_3$O$_{7}$ reproduces its superconducting transition temperature within a factor of two, a level of agreement comparable to Eliashberg theories of conventional superconductors.
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We suggest that a family of Ni-based compounds, which contain [Ni$_2$M$_2$O]$^{2-}$(M=chalcogen) layers with an antiperovskite structure constructed by mixed-anion Ni complexes, NiM$_4$O$_2$, can be potential high temperature superconductors upon doping or applying pressure. The layer structures have been formed in many other transitional metal compounds such as La$_2$B$_2$Se$_2$O$_3$(B=Mn, Fe,Co). For the Ni-based compounds, we predict that the parental compounds host collinear antiferromagnetic states similar to those in the iron-based high temperature superconductors. The electronic physics near Fermi energy is controlled by two e$_{g}$ d-orbitals with completely independent in-plane kinematics. We predict that the superconductivity in this family is characterized by strong competition between extended s-wave and d-wave pairing symmetries.
The observation of an unusual spin resonant excitation in the superconducting state of various High-Tc ~copper oxides by inelastic neutron scattering measurements is reviewed. This magnetic mode % (that does not exist in conventional superconductors) is discussed in light of a few theoretical models and likely corresponds to a spin-1 collective mode.
The symmetry operations of the crystal groups relevant for the high temperature superconductors HgBa2CuO4+x (Hg1201), YBa2Cu3O7-x (YBCO), and Bi2Sr2CaCu2O8+x (BSCCO) are elucidated. The allowable combinations of the superconducting order parameter (OP) components are presented for both the angular momentum and lattice representations. For tetragonal Hg1201, the spin singlet OP components are composed from four sets of compatible basis functions, which combine to give the generalized s-, dx2-y2-, dxy-, and gxy(x2-y2)- wave OPs. In YBCO, elements of s- and dx2-y2- wave sets are compatible, but in BSCCO, elements of s- and dxy- wave sets are compatible. The Josephson critical current density JcJ across c-axis twist junctions in the vicinity of Tc is then evaluated as a function of the twist angle phi0, for each allowable OP combination, for both coherent and incoherent tunneling. Recent experiments of Li et al. demonstrated the independence of JcJ(phi0)/JcS upon phi0 at and below Tc, where JcS is the critical current density of a constituent single crystal. These experiments are shown to be consistent with an OP containing an s-wave component, but inconsistent with an OP containing the purported dx2-y2-wave component. In addition, they demonstrate that the interlayer tunneling across untwisted layers in single crystal BSCCO is entirely incoherent. We propose a new type of tricrystal experiment using single crystal c-axis twist junctions, that does not employ substrate grain boundaries.
A magnetic field relaxation at the center of a pulse-magnetized single-domain Y-Ba-Cu-O superconductor at 78K has been studied. In case of a weak magnetization, the magnetic flux density increases logarithmically and normalized relaxation rate defined as S=-d(lnB)/d(lnt) is negative (S=-0.037). When an external magnetic field magnitude increases, the relaxation rate first decreases in absolute value, then changes sign (becomes positive, S>0) and after reaching some maximum finally reduces to a very small value. Non-monotonous dependence of S vs Ha is explained by a non-homogeneous local temperature distribution during a pulse magnetization.
The spectral energy gap is an important signature that defines states of quantum matter: insulators, density waves, and superconductors have very different gap structures. The momentum resolved nature of angle-resolved photoemission spectroscopy (ARPES) makes it a powerful tool to characterize spectral gaps. ARPES has been instrumental in establishing the anisotropic d-wave structure of the superconducting gap in high-transition temperature (Tc) cuprates, which is different from the conventional isotropic s-wave superconducting gap. Shortly afterwards, ARPES demonstrated that an anomalous gap above Tc, often termed the pseudogap, follows a similar anisotropy. The nature of this poorly understood pseudogap and its relationship with superconductivity has since become the focal point of research in the field. To address this issue, the momentum, temperature, doping, and materials dependence of spectral gaps have been extensively examined with significantly improved instrumentation and carefully matched experiments in recent years. This article overviews the current understanding and unresolved issues of the basic phenomenology of gap hierarchy. We show how ARPES has been sensitive to phase transitions, has distinguished between orders having distinct broken electronic symmetries, and has uncovered rich momentum and temperature dependent fingerprints reflecting an intertwined & competing relationship between the ordered states and superconductivity that results in multiple phenomenologically-distinct ground states inside the superconducting dome. These results provide us with microscopic insights into the cuprate phase diagram.
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