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Register a new userUnusual dependence of vortex core states on the superconducting gap in Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$
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We present a scanning tunneling spectroscopy study on quasiparticle states in vortex cores in Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$. The energy of the observed vortex core states shows an approximately linear scaling with the superconducting gap in the region just outside the core. This clearly distinguishes them from conventional localized core states, and is a signature of the mechanism responsible for their discrete appearance in high-temperature superconductors. The energy scaling of the vortex core states also suggests a common nature of vortex cores in Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$ and YBa$_2$Cu$_3$O$_{7-delta}$. Finally, the observed vortex core states do not show any dependence on the applied magnetic field in the range from 1 to 6 T.
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A magnetic field applied to type-II superconductors introduces quantized vortices that locally quench superconductivity, providing a unique opportunity to investigate electronic orders that may compete with superconductivity. This is especially true in cuprate superconductors in which mutual relationships among superconductivity, pseudogap, and broken-spatial-symmetry states have attracted much attention. Here we observe energy and momentum dependent bipartite electronic superstructures in the vortex core of Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$ using spectroscopic-imaging scanning tunneling microscopy (SI-STM). In the low-energy range where the nodal Bogoliubov quasiparticles are well-defined, we show that the quasiparticle scattering off vortices generates the electronic superstructure known as vortex checkerboard. In the high-energy region where the pseudogap develops, vortices amplify the broken-spatial-symmetry patterns that preexist in zero field. These data reveal canonical d-wave superconductivity near the node, yet competition between superconductivity and broken-spatial-symmetry states near the antinode.
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$_{8+delta}$ ($T_c$ = 66~K) despite the high carrier density. With a sign-problem free quantum Monte Carlo calculation, we show how a partially flat band at ($pi$,0) can help enhance superconducting phase fluctuations. Finally, we discuss the implications of an anisotropic band structure on the phase-coherence-limited superconductivity in overdoped cuprates and other superconductors.
In the present work, we report the new findings on the doping level dependence of the phase coherence between superconducting Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$ (Bi-2212) grains. The experimental results from the strongly underdoped and overdoped regimes deviated from the expectation based on the doping level dependence of the superfluid density at $T$ = 0 K. These findings appear to be governed by interplay between competing orders inside the superconducting dome of cuprate superconductors. Two quantum critical points are likely to exist at the underdoped and overdoped regimes beneath the superconducting dome.
We report time and angle resolved spectroscopic measurements in optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$. The spectral function is monitored as a function of temperature, photoexcitation density and delay time from the pump pulse. According to our data, the superconducting gap becomes slightly stiffer when moving off the nodal direction. The nodal quasiparticles develop a faster dynamics when pumping the superconductor with a fluence that is large enough to induce the total collapse of the gap. We discuss the observed relaxation in terms of a dynamical reformation of Cooper pairs.
In cuprate superconductors, the doping of carriers into the parent Mott insulator induces superconductivity and various other phases whose characteristic temperatures are typically plotted versus the doping level $p$. In most materials, $p$ cannot be determined from the chemical composition, but it is derived from the superconducting transition temperature, $T_mathrm{c}$, using the assumption that $T_mathrm{c}$ dependence on doping is universal. Here, we present angle-resolved photoemission studies of Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$, cleaved and annealed in vacuum or in ozone to reduce or increase the doping from the initial value corresponding to $T_mathrm{c}=91$ K. We show that $p$ can be determined from the underlying Fermi surfaces and that $in-situ$ annealing allows mapping of a wide doping regime, covering the superconducting dome and the non-superconducting phase on the overdoped side. Our results show a surprisingly smooth dependence of the inferred Fermi surface with doping. In the highly overdoped regime, the superconducting gap approaches the value of $2Delta_0=(4pm1)k_mathrm{B}T_mathrm{c}$
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