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Register a new userEffect of Spinless Impurities on Reduction of $T_c$ in High $T_c$ Superconductors
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In-Ho Lee
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The notion of a finite pairing interaction energy range suggested by Nam, results in some states at the Fermi level not participating in pairings when there are scattering centers such as impurities. The fact that not all states at the Fermi level participate in pairing is shown to suppress $T_c$ in an isotropic superconductor and destroy superconductivity. We have presented quantitative calculations of $T_c$ reduced via spinless impurities, in good agreements with data of Zn-doped YBCO and LSCO, respectively. It is not necessary to have the anisotropic order parameter, to account for the destruction of superconductivity via non-magnetic impurities.
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We analyse fluctuations about $T_c$ in the specific heat of (Y,Ca)Ba$_2$Cu$_3$O$_{7-delta}$, YBa$_2$Cu$_4$O$_8$ and Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$. The mean-field transition temperature, $T_c^{mf}$, in the absence of fluctuations lies well above $T_c$ especially at low doping where it reaches as high as 150K. We show that phase and amplitude fluctuations set in simultaneously and $T_c^{mf}$ scales with the gap, $Delta_0$, such that $2Delta_0/k_BT_c^{mf}$ is comparable to the BCS weak-coupling value, 4.3, for d-wave superconductivity. We also show that $T_c^{mf}$ is unrelated to the pseudogap temperature, $T^*$.
An inelastic neutron scattering experiment has been performed in the high-temperature superconductor $rm YBa_2Cu_3O_{6.89}$ to search for an oxygen-isotope shift of the well-known magnetic resonance mode at 41 meV. Contrary to a recent prediction (I. Eremin, {it et al.}, Phys. Rev. B {bf 69}, 094517 (2004)), a negligible shift (at best $leq$ +0.2 meV) of the resonance energy is observed upon oxygen isotope substitution ($^{16}$O$to^{18}$O). This suggests a negligible spin-phonon interaction in the high-$T_c$ cuprates at optimal doping.
We theoretically investigate the vortex state of the cuprate high-temperature superconductors in the presence of magnetic fields. Assuming the recently derived nonlinear $sigma$-model for fluctuations in the pseudogap phase, we find that the vortex cores consist of two crossed regions of elliptic shape, in which a static charge order emerges. Charge density wave order manifests itself as satellites to the ordinary Bragg peaks directed along the axes of the reciprocal copper lattice. Quadrupole density wave (bond order) satellites, if seen, are predicted to be along the diagonals. The intensity of the satellites should grow linearly with the magnetic field, in agreement with the result of recent experiments.
We address the origin of the Cooper pairs in high-$T_c$ cuprates and the unique nature of the superconducting (SC) condensate. Itinerant holes in an antiferromagnetic background form pairs spontaneously, without any `glue, defining a new quantum object the `pairon. In the incoherent pseudogap phase, above $T_c$ or within the vortex core, the pairon binding energies are distributed statistically, forming a `Cooper-pair glass. Contrary to conventional SC, it is the mutual pair-pair interaction that is responsable for the condensation. We give a natural explanation for the {it ergodic rigidity} of the excitation gap, being uniquely determined by the carrier concentration $p$ and $J$. The phase diagram can be understood, without spin fluctuations, in terms of a single energy scale $sim J$, the exchange energy at the metal-insulator transition.
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.
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