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Register a new userEvidence of strong correlations at the van Hove singularity in the scanning tunneling spectra of superconducting Bi_{2}Sr_{2}CaCu_{2}O_{8+delta} single crystals
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We present realistic multiband calculations of scanning tunneling spectra in Bi_{2}Sr_{2}CaCu_{2} O_{8+delta} over a wide doping range. Our modeling incorporates effects of a competing pseudogap and pairing gap as well as effects of strong electronic correlations, which are included by introducing self-energy corrections in the one-particle propagators. The calculations provide a good description of the two-gap features seen in experiments at low energies and the evolution of the Van Hove singularity (VHS) with doping, and suggest a possible quantum critical point near the point where the VHS crosses the Fermi level.
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A complete knowledge of its excitation spectrum could greatly benefit efforts to understand the unusual form of superconductivity occurring in the lightly hole-doped copper-oxides. Here we use tunnelling spectroscopy to measure the Tto 0 spectrum of electronic excitations N(E) over a wide range of hole-density p in superconducting Bi_{2}Sr_{2}CaCu_{2}O_{8+/delta}. We introduce a parameterization for N(E) based upon an anisotropic energy-gap /Delta (vec k)=/Delta_{1}(Cos(k_{x})-Cos(k_{y}))/2 plus an effective scattering rate which varies linearly with energy /Gamma_{2}(E) . We demonstrate that this form of N(E) allows successful fitting of differential tunnelling conductance spectra throughout much of the Bi_{2}Sr_{2}CaCu_{2}O_{8+/delta} phase diagram. The resulting average /Delta_{1} values rise with falling p along the familiar trajectory of excitations to the pseudogap energy, while the key scattering rate /Gamma_{2}^{*}=/Gamma_{2}(E=/Delta_{1}) increases from below ~1meV to a value approaching 25meV as the system is underdoped from p~16% to p<10%. Thus, a single, particle-hole symmetric, anisotropic energy-gap, in combination with a strongly energy and doping dependent effective scattering rate, can describe the spectra without recourse to another ordered state. Nevertheless we also observe two distinct and diverging energy scales in the system: the energy-gap maximum /Delta_{1} and a lower energy scale /Delta_{0} separating the spatially homogeneous and heterogeneous electronic structures.
Using high energy resolution angle resolved photoemission spectroscopy, we have resolved the bilayer splitting effect in a wide range of dopings of the bilayer cuprate $Bi_{2}Sr_{2}CaCu_{2}O_{8+delta}$. This bilayer splitting is due to a nonvanishing intracell coupling $t_{perp}$, and contrary to expectations, it is not reduced in the underdoped materials. This has implications for understanding the increased c-axis confinement in underdoped materials.
Measurements of non-local in-plane resistance originating from transverse vortex-vortex correlations have been performed on a Bi_{2}Sr_{2}CaCu_{2}O_{8+delta} high-T_c superconductor in a magnetic field up to 9 T applied along the crystal c-axis. Our results demonstrate that a rigid vortex lattice does exist over a broad portion of the magnetic field -- temperature (H-T) phase diagram, well above the first-order transition boundary H_{FOT}(T). The results also provide evidence for the vortex lattice melting and vortex liquid decoupling phase transitions, occurring above the H_{FOT}(T).
Pure electronic Raman spectra with no phonon structures superimposed to the electronic continuum, are reported, in optimally doped HgBa_{2}CaCu_{2}O_{6+delta } single crystals (T_{c }=126 K). As a consequence, the spectra in the A_{1g }, B_{1g } and B_{2g } symmetries, including the crucial low energy frequency dependence of the electronic scattering, are directly and reliably measured. The B_{2g } and, most strikingly, the B_{1g } spectra exhibit a strong intrinsic linear term, which suggests that the nodes are shifted from the [110] and [1bar{1}0] directions, a result inconsistent with a pure d_{x^{2}-y^{2}} model.
Pure electronic Raman spectra with no phonon structures superimposed to the electronic continuum, are reported for the first time, in optimally doped $HgBa_{2}CaCu_{2}O_{6+delta } $ single crystals $(T_{c }=126 $ K). Our low temperature spectra (15 K) for the $A_{1g}$, $B_{1g} $ and $B_{2g} $ symmetries exhibit striking differences with previous data in $Bi_{2}Sr_{2}CaCu_{2}O_{8+delta }. $ The shape of the spectra for the various symmetries cannot be fitted by Devereauxs $d_{x^{2}-y^{2}} $ calculations, but strongly suggests a $d_{x y } $ gap, with its minima in the [100] and [010] directions.
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