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Register a new userSuppressed antinodal coherence with a single d-wave superconducting gap leads to two energy scales in underdoped cuprates
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Conventional superconductors are characterized by a single energy scale, the superconducting gap, which is proportional to the critical temperature Tc . In hole-doped high-Tc copper oxide superconductors, previous experiments have established the existence of two distinct energy scales for doping levels below the optimal one. The origin and significance of these two scales are largely unexplained, although they have often been viewed as evidence for two gaps, possibly of distinct physical origins. By measuring the temperature dependence of the electronic Raman response of Bi2Sr2CaCu2O8+d (Bi-2212) and HgBa2CuO4+d (Hg-1201) crystals with different doping levels, we establish that these two scales are associated with coherent excitations of the superconducting state which disappears at Tc. Using a simple model, we show that these two scales do not require the existence of two gaps. Rather, a single d-wave superconducting gap with a loss of Bogoliubov quasiparticle spectral weight in the antinodal region is shown to reconcile spectroscopic and transport measurements.
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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 momentum dependence of the superconducting gap in the cuprates has been debated, with most experiments reporting a deviation from a simple $d_{x^2-y^2}$ form in the underdoped regime and a few experiments claiming that a simple $d_{x^2-y^2}$ form persists down to the lowest dopings. We affirm that the superconducting gap function in sufficiently underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$ (Bi-2212) deviates from a simple textit{d}-wave form near the antinode. This is observed in samples where doping is controlled only by oxygen annealing, in contrast to claims that this effect is only seen in cation-substituted samples. Moreover, a quasiparticle peak is present at the antinode down to p$=$0.08, refuting claims that a deviation from a simple textit{d}-wave form is a data analysis artifact stemming from difficulty in assessing a gap in the absence of a quasiparticle.
The cuprate high temperature superconductors develop spontaneous charge density wave (CDW) order below a temperature $T_{CDW}$ and over a wide range of hole doping (p). An outstanding challenge in the field is to understand whether this modulated phase is related to the more exhaustively studied pseudogap and superconducting phases. To address this issue it is important to extract the energy scale $Delta_{CDW}$ associated with the charge modulations, and to compare it with the pseudogap (PG) $Delta_{PG}$ and the superconducting gap $Delta_{SC}$. However, while $T_{CDW}$ is well-characterized from earlier works little has been known about $Delta_{CDW}$ until now. Here, we report the extraction of $Delta_{CDW}$ for several cuprates using electronic Raman spectroscopy. Crucially, we find that, upon approaching the parent Mott state by lowering $p$, $Delta_{CDW}$ increases in a manner similar to the doping dependence of $Delta_{PG}$ and $Delta_{SC}$. This shows that CDW is an unconventional order, and that the above three phases are controlled by the same electronic correlations. In addition, we find that $Delta_{CDW} approx Delta_{SC}$ over a substantial doping range, which is suggestive of an approximate emergent symmetry connecting the charge modulated phase with superconductivity.
Recent angle-resolved photoemission electron spectroscopy (ARPES) experiments demonstrate that the momentum dependence of the spectral gap in underdoped cuprates does not follow a pure $d$-wave form [H. Anzai et a., Nat. Comm. {bf 4}, 1815 (2013)]. This deviation is highly controversial. It has often been interpretated as a proof of the non-superconducting origin of the antinodal gap in the underdoped regime. In this article, we show that the measured angular dependence of the spectral gap can be explained by the basic nature of pairs in high-T$_c$ cuprates. Hole pairs, or {it pairons}, form as a result of the local antiferromagnetic environment on the scale $xi_{AF}$, the magnetic coherence length. The spatial extension of the pairon wavefunction beyond first nearest neighbours gives rise to the anomalous angular dependence of the gap, in quantitative agreement with experiments. This simple interpretation strongly indicates a common origin of the nodal and antinodal gaps.
Recent excperiments (ARPES, Raman) suggest the presence of two distinct energy gaps in high-Tc superconductors (HTSC), exhibiting different doping dependences. Results of a variational cluster approach to the superconducting state of the two-dimensional Hubbard model are presented which show that this model qualitatively describes this gap dichotomy: One gap (antinodal) increases with less doping, a behavior long considered as reflecting the general gap behavior of the HTSC. On the other hand, the near-nodal gap does even slightly decrease with underdoping. An explanation of this unexpected behavior is given which emphasizes the crucial role of spin fluctuations in the pairing mechanism.
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