Continuous doping of a cuprate surface: Insights from in situ angle-resolved photoemission

Abstract

The cuprate superconductors distinguish themselves from the conventional superconductors in that a small variation in the carrier doping can significantly change the superconducting transition temperature ( ), giving rise to a superconducting dome where a pseudogap 1,2 emerges in the underdoped region and a Fermi liquid appears in the overdoped region. Thus a systematic study of the properties over a wide doping range is critical for understanding the superconducting mechanism. Here, we report a new technique to continuously dope the surface of Bi 2 Sr 2 CaCu 2 O 8+x through an ozone/vacuum annealing method. Using in-situ ARPES, we obtain precise quantities of energy gaps and the coherent spectral weight over a wide range of doping. We discover that the d-wave component of the quasiparticle gap is linearly proportional to the Nernst temperature that is the onset of superconducting vortices 3 , strongly suggesting that the emergence of superconducting pairing is concomitant with the onset of free vortices, with direct implications for the onset of superconducting phase coherence at and the nature of the pseudogap phenomena.Bi 2 Sr 2 CaCu 2 O 8+x (Bi2212) single crystals have been extensively studied by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS) 4,5 , two of the major experimental techniques for probing the cuprates. However, high-quality Bi2212 crystals can be only obtained within a narrow doping range. Moreover, surface cleaving, necessary for surface techniques such as ARPES and STS, posses a serious problem for quantitative comparisons from sample to sample due to variation of surface conditions. Realizing that the doping level in this material is solely controlled by the excess oxygen concentration, we use ozone/vacuum annealing to continuously change the doping level of the surface layers, which are subsequently measured by in-situ ARPES (Figs. 1a-c).

Low-energy antiferromagnetic spin fluctuations limit the coherent superconducting gap in cuprates

“…In YBa 2 Cu 3 O 6+x , the effective mass diverges as p is reduced towards 0.085 [91], which correlates with the closing of the spin gap [74,82,92]. Recent ARPES results covering a broad range of doping in Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) appear to be quite consistent with our analysis [93,95,96]. For under-and optimally-doped samples, the quasiparticle peak near the antinodal (AN) wave vector is broad and has an energy of 30 meV or greater.…”

“…Recent ARPES results covering a broad range of doping in Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) appear to be quite consistent with our analysis [93,95,96]. For under-and optimally-doped samples, the quasiparticle peak near the antinodal (AN) wave vector is broad and has an energy of 30 meV or greater.…”

Generalization of Anderson's theorem for disordered superconductors

“…Moreover, this correlates with the obser-vation that T c is typically determined by the superfluid stiffness, rather than the pairing scale. [33][34][35] On the other hand, the above mentioned results primarily concern the underdoped cuprates, where even the "normal" state above T c deviates dramatically from the usual Fermi liquid metal phase on which BCS theory is based. Below the critical temperature, however, mean-field d-wave pairing inhomogeneity can still reproduce a relatively homogeneous LDOS at sufficiently low energy 36 such that the proposed granularity above the critical temperature does not directly conflict with local spectroscopy in BSCCO observing a homogeneous superconducting state.…”

Electronic inhomogeneity and band structure on superstructural CuO2 planes of infinite-layer Sr0.94La0.06CuO2+y films

“…2(e). While the UHV annealing reduces the oxygen intake and shifts E F upwards [25], the electronic inhomogeneity becomes even more prominent: the brighter the STM contrast is, the smaller the energy separation between E F and CTB is. Similar to the previous report [19], we determine the onset energies of CTB and UHB, as well as the separation ∆ CT between them [Fig.…”