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Register a new userVan Hove features in BSCCO(2212) and effective parameters for Ni impurities inferred from STM spectra
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We present a detailed quantitative comparison between theoretical calculations of the local density of states and recent experimental measurements of scanning tunneling spectra around Ni impurities in BSCCO(2212). A double-peak structure on the hole side of the spectrum at the Ni site is identified as the spin-split van Hove singularity in the band structure. The Ni atom induces local changes in hopping matrix elements comparable in size to the induced on-site spin-dependent potential. We find evidence from the measurements of order parameter suppression in the vicinity of the Ni impurity. These extracted impurity parameters can be of use in quantitative calculations of macroscopic response properties, such as the AC conductivity.
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We report measurements of anamolously large dissipative conductivities in BiSrCaCuO(2212) at low temperatures. We have measured the complex conductivity of BSCCO thin films at 100-600 GHz as a function of doping from the underdoped to the overdoped state. At low temperatures there exists a residual dissipative conductivity which scales with the T=0 superfluid density as the doping is varied. This residual dissipative conductivity is larger than the possible contribution from a thermal population of quasiparticles at the d-wave gap nodes.
We have measured the complex conductivity of a BSCCO(2212) thin film between 0.2 and 1.0 THz. We find the conductivity in the superconducting state to be well described as the sum of contributions from quasiparticles, the condensate, and order parameter fluctuations which draw 30% of the spectral weight from the condensate. An analysis based on this decomposition yields a quasiparticle scattering rate on the order of k_(B)*T/(hbar) for temperatures below Tc.
Microscopy (STM). At all dopings, the low energy density-of-states modulations are analyzed according to a simple model of quasiparticle interference and found to be consistent with Fermi-arc superconductivity. The superconducting coherence-peaks, ubiquitous in near-optimal tunneling spectra, are destroyed with strong underdoping and a new spectral type appears. Exclusively in regions exhibiting this new spectrum, we find local `checkerboard charge-order with wavevector Q=(2pi/4.5a,0);(0,2pi/4.5a)+15%. Surprisingly, this order coexists harmoniously with the the low energy
The cuprate material BSCCO-2212 is believed to be doped by a combination of cation switching and excess oxygen. The interstitial oxygen dopants are of particular interest because scanning tunnelling microscopy (STM) experiments have shown that they are positively correlated with the local value of the superconducting gap, and calculations suggest that the fundamental attraction between electrons is modulated locally. In this work, we use density functional theory to try to ascertain which locations in the crystal are energetically most favorable for the O dopant atoms, and how the surrounding cage of atoms deforms. Our results provide support for the identification of STM resonances at -1eV with dopant interstitial O atoms, and show how the local electronic structure is modified nearby.
One of interesting open questions for the high transition temperature (Tc) superconductivity in sulfur hydrides is why high pressure phases of H3S have extremely high Tcs. Recently, it has been pointed out that the presence of the van Hove singularities (vHs) around the Fermi level is crucial. However, while there have been quantitative estimates of Tc based on the Migdal-Eliashberg theory, the energy dependence of the density of states (DOS) has been neglected to simplify the Eliashberg equation. In this study, we go beyond the constant DOS approximation and explicitly consider the electronic structure over 40eV around the Fermi level. In contrast with the previous conventional calculations, this approach with a sufficiently large number of Matsubara frequencies enables us to calculate Tc without introducing the empirical pseudo Coulomb potential. We show that while H3S has much higher Tc than H2S for which the vHs is absent, the constant DOS approximation employed so far seriously overestimates (underestimates) Tc by ~ 60K (~ 10K) for H3S (H2S). We then discuss the impact of the strong electron-phonon coupling on the electronic structure with and without the vHs and how it affects the superconductivity. Especially, we focus on (1) the feedback effect in the self-consistent calculation of the self-energy, (2) the effect of the energy shift due to the zero-point motion, and (3) the effect of the changes in the phonon frequencies due to strong anharmonicity. We show that the effect of (1)-(3) on Tc is about 10-30K for both H3S and H2S. Eventually, Tc is estimated to be 181K for H3S at 250GPa and 34K for H2S at 140GPa, which explains the pressure dependence of Tc observed in the experiment. In addition, we evaluate the lowest order vertex correction beyond the Migdal-Eliashberg theory and discuss the validity of the Migdal approximation for sulfur hydrides.
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