Session V50: Pairing in the High-Tc Normal State: Evidence and Theoretical Models

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In most superconductors with a metallic normal state, zero-resistance state appears when Cooper pairs form below T_c. However in many optimally hole-doped high-temperature cuprate superconductors, superconducting T_c is much suppressed from the Cooper-pair formation temperature. This is usually considered a result of low dimensionality and/or low carrier density, which is further complicated by the pseudogap. Conventionally, superconducting T_c is expected to restore to the pairing temperature in sufficiently overdoped systems. Surprisingly, recent optical, magnetic and photoemission measurements show evidence of deviation from this conventional wisdom. We present direct thermodynamic and spectroscopic evidence that such superconducting fluctuation persists in heavily hole-doped cuprate Bi₂Sr₂CaCu₂O_8+d (Bi-2212 T_c =66K), which has a hole density as high as 10²² cm^-3. Supported by a sign-problem-free Quantum Monte Carlo calculation, the flat, shallow dispersion near the Brillouin boundary is found to be capable of promoting this unexpected suppression of superconducting phase coherence. Last but not least, we show evidence of weak-coupling superconductivity in this doping regime, and suggest overdoped Bi-2212 as a promising platform to advance quantitative understanding of the cuprate high-T_c mechanism.

[1] He, Chen, Li et al, arxiv:2009.10932
[2] Chen, Hashimoto et al, Science 366, 1099 (2019)
[3] He, Hashimoto et al, Science 362, 62 (2018)
[4] Kondo et al., Nat Commun 6, 7699 (2015)   

Live

Electronic correlation effects are the driving forces behind much of complex matter, but are typically difficult or impossible to directly access via experiment. We have made major strides in developing ARPES into a true spectroscopy of self-energies, here discussing how it is applied to the study of cuprate high Tc superconductors. One major topic includes the “strange metal” normal state with “Planckian” scattering and non-Fermi-liquid non-quasiparticle correlations or self-energies [1]. Another major topic is how these self-energies transform as the system enters the superconducting state – something that can now be directly and precisely determined with ARPES [2]. In particular, we show how the normal state strange-metal self-energies that are incoherent in nature are dramatically converted into highly coherent self-energy effects in the normal state that lead to stronger superconducting state renormalization. This conversion begins well above T_C at the onset of superconducting fluctuations and it greatly increases the number of states that can pair. Therefore, there is positive feedback––the superconductive pairing creates the conversion between incoherent and coherent correlations that in turn strengthens the pairing.

1. T. J. Reber et al. A unified form of low-energy nodal electronic interactions in hole-doped cuprate superconductors. Nat. Commun. 10, 5737 (2019)
2. Haoxiang Li et al. Coherent organization of electronic correlations as a mechanism to enhance and stabilize high temperature cuprate superconductivity, Nature Communications 9, 26 (2018)   

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Shot noise provides a means of inferring the effective charge of carriers. A direct determination of the charge of carriers as a function of temperature and bias could help resolve among alternatives scenarios for ordering in the pseudogap. Here we report measurements of the shot noise of tunnelling current in high-quality La_2−xSr_xCuO₄/La₂CuO₄/La_2−xSr_xCuO₄ (LSCO/LCO/LSCO) heterostructures fabricated using atomic layer-by-layer molecular beam epitaxy at several doping levels. The data delineate three distinct regions in the bias voltage–temperature space. Well outside the superconducting gap region, the shot noise agrees quantitatively with independent tunnelling of individual charge carriers. Deep within the superconducting gap, shot noise is greatly enhanced, reminiscent of multiple Andreev reflections. Above the critical temperature and extending to biases much larger than the superconducting gap, there is a broad region in which the noise substantially exceeds theoretical expectations for single-charge tunnelling, indicating pairing of charge carriers. These pairs are detectable deep into the pseudogap region of temperature and bias. The presence of these pairs constrains current models of the pseudogap and broken symmetry states, while phase fluctuations limit the domain of superconductivity.   

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The idea that preformed Cooper pairs could exist in a superconductor above its zero-resistance state has been explored for unconventional, interface, and disordered superconductors, yet direct experimental evidence is lacking. In this talk, I will present scanning tunneling noise spectroscopy experiments [1,2] that unambiguously show that preformed Cooper pairs exist up to temperatures much higher than the zero-resistance critical temperature T_c in the disordered superconductor titanium nitride, by observing a clear enhancement in the shot noise that is equivalent to a change of the effective charge from 1 to 2 electron charges [3]. Our data further reveals that the spectroscopic gap fills up rather than closes when increasing temperature. Our results thus demonstrate the existence of a novel state above T_c that, much like an ordinary metal, has no (pseudo)gap, but carries charge via paired electrons. Finally, I will discuss the relation of our work to the pseudogap state of the cuprates.

[1] Amplifier for scanning tunneling microscopy at MHz frequencies, KM Bastiaans, et al. Rev. Sci. Instrum. 89, 093709 (2018)
[2] Imaging doubled shot noise in a Josephson scanning tunneling microscope, KM Bastiaans, et al., Phys. Rev. B 100, 104506 (2019)
[3] KM Bastiaans, et al. (2020)   

Live

The apparently missing electron density and the 'incomplete Fermi surface' are big mysteries of the pseudogap phase. Their relation with charge density waves, which are ubiquitous at lower temperatures in the pseudogap region, especially with an applied magnetic field, is also unclear. We provide a new perspective using quantum fluctuating pair density waves to unify the phenomenology of charge density waves and the electron spectrum. The resulting theory contains both gapped Cooper pairs made of antinodal electrons, and a Fermi pocket of nodal electrons. The gapped Cooper pair offers a potential solution to numerous experimental puzzles, including the ARPES spectrum, the 'incoherent' optical conductivity, and the photo-induced superconductivity.