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Register a new userOrigin of positive out-of-plane magnetoconductivity in overdoped Bi$_{1.6}$Pb$_{0.4}$Sr$_{2}$CaCu$_{1.96}$Fe$_{0.04}$O$_{8+delta}$
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To elucidate the pseudogap phase diagram including the overdoped state of high transition temperature (high-$T_c$) cuprates, we must understand the origin of the positive out-of-plane magnetoconductivity (MC) observed in these compounds. For this purpose, the out-of-plane resistivity $rho_c(T,H)$ of an overdoped Bi$_{1.6}$Pb$_{0.4}$Sr$_{2}$CaCu$_{1.96}$Fe$_{0.04}$O$_{8+delta}$ (Bi-2212) single crystal is measured under pulsed magnetic fields up to 60 T. We show that the superconductive density-of-states (DOS) depletion effect, in addition to the pseudogap effect, clearly appears below the superconductive fluctuation regime, and the contribution becomes dominant in the superconducting state.
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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.
We report intrinsic tunnelling data for mesa structures fabricated on three over- and optimally-doped $rm{Bi_{2.15}Sr_{1.85}CaCu_{2}O_{8+delta}}$ crystals with transition temperatures of 86-78~K and 0.16-0.19~holes per CuO$_2$ unit, for a wide range of temperature ($T$) and applied magnetic field ($H$), primarily focusing on one over-doped crystal(OD80). The differential conductance above the gap edge shows clear dip structure which is highly suggestive of strong coupling to a narrow boson mode. Data below the gap edge suggest that tunnelling is weaker near the nodes of the d-wave gap and give clear evidence for strong $T$-dependent pair breaking. These findings could help theorists make a detailed Eliashberg analysis and thereby contribute towards understanding the pairing mechanism. We show that for our OD80 crystal the gap above $T_c$ although large, is reasonably consistent with the theory of superconducting fluctuations.
We report an ARPES investigation of the circular dichroism in the first Brillouin zone (BZ) of under- and overdoped Pb-Bi2212 samples. We show that the dichroism has opposite signs for bonding and antibonding components of the bilayer-split CuO-band and is antisymmetric with respect to reflections in both mirror planes parallel to the c-axis. Using this property of the energy and momentum intensity distributions we prove the existence of the bilayer splitting in the normal state of the underdoped compound and compare its value with the splitting in overdoped sample. In agreement with previous studies the magnitude of the interlayer coupling does not depend significantly on doping. We also discuss possible origins of the observed dichroism.
Mixing of topological states with superconductivity could result in topological superconductivity with the elusive Majorana fermions potentially applicable in fault-tolerant quantum computing. One possible candidate considered for realization of topological superconductivity is thin bismuth films on Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$ (Bi2212). Here, we present angle-resolved and core-level photoemission spectroscopy studies of thin Bi films grown {it in-situ} on as-grown Bi2212 that show the absence of proximity effect. We find that the electron transfer from the film to the substrate and the resulting severe underdoping of Bi2212 at the interface is a likely origin for the absence of proximity effect. We also propose a possible way of preventing a total loss of proximity effect in this system. Our results offer a better and more universal understanding of the film/cuprate interface and resolve many issues related to the proximity effect.
In high-temperature cuprate superconductors, superconductivity is accompanied by a plethora of orders, and phenomena that may compete, or cooperate with superconductivity, but which certainly complicate our understanding of origins of superconductivity in these materials. While prominent in the underdoped regime, these orders are known to significantly weaken or completely vanish with overdoping. Here, we approach the superconducting phase from the more conventional highly overdoped side. We present angle-resolved photoemission spectroscopy (ARPES) studies of Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$ (Bi2212) single crystals cleaved and annealed in ozone to increase the doping all the way to the metallic, non-superconducting phase. We show that the mass renormalization in the antinodal region of the Fermi surface, associated with the structure in the quasiparticle self-energy, that possibly reflects the pairing interaction, monotonically weakens with increasing doping and completely disappears precisely where superconductivity disappears. This is the direct evidence that in the overdoped regime, superconductivity is determined by the coupling strength. A strong doping dependence and an abrupt disappearance above the transition temperature ($T_{mathrm c}$) eliminate the conventional phononic mechanism of the observed mass renormalization and identify the onset of spin-fluctuations as its likely origin.
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