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Electron pairing in the pseudogap state revealed by shot noise in copper oxide junctions

Zhou, Panpan and Chen, Liyang and Liu, Yue and Sochnikov, Ilya and Bollinger, Anthony T. and Han, Myung-Geun and Zhu, Yimei and He, Xi and Boz̆ović, Ivan and Natelson, Douglas (2019) Electron pairing in the pseudogap state revealed by shot noise in copper oxide junctions. Nature, 572 (7770). pp. 493-496. ISSN 0028-0836. https://resolver.caltech.edu/CaltechAUTHORS:20190903-130240658

[img] PDF (Extended Data Fig. 1: Device fabrication process) - Supplemental Material
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[img] PDF (Extended Data Fig. 2: Transport in LSCO/LCO/LSCO (x = 0.15) film and tunnel junction properties) - Supplemental Material
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[img] PDF (Extended Data Fig. 3: Bias-dependent noise and differential conductance) - Supplemental Material
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[img] PDF (Extended Data Fig. 4: Electrical circuit diagrams for the shot-noise measurement setup) - Supplemental Material
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[img] PDF (Extended Data Fig. 5: R_SC_P model fitting, noise PSD calibration and example spectra of an LSCO tunnel junction) - Supplemental Material
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[img] PDF (Extended Data Fig. 6: Noise ratios as a function of temperature and bias) - Supplemental Material
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[img] PDF (Extended Data Fig. 7: The variance in PSD with different averaging times) - Supplemental Material
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[img] PDF (Extended Data Fig. 8: Shot noise in a Nb tunnel junction) - Supplemental Material
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[img] PDF (Extended Data Fig. 9: Noise as a function of current, and comparison with Andreev reflection) - Supplemental Material
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Abstract

In the quest to understand high-temperature superconductivity in copper oxides, debate has been focused on the pseudogap—a partial energy gap that opens over portions of the Fermi surface in the ‘normal’ state above the bulk critical temperature. The pseudogap has been attributed to precursor superconductivity, to the existence of preformed pairs and to competing orders such as charge-density waves. A direct determination of the charge of carriers as a function of temperature and bias could help resolve among these alternatives. Here we report measurements of the shot noise of tunnelling current in high-quality La_(2−x)Sr)xCuO)4/La)2CuO)4/La_(2−x)Sr)xCuO)4 (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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https://doi.org/10.1038/s41586-019-1486-7DOIArticle
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Additional Information:© 2019 Springer Nature Publishing AG. Received: 16 January 2019; Accepted: 18 June 2019; Published online 21 August 2019. Y. Zhang helped with TEM sample preparation and S. Yang with STEM EDX spectroscopy data acquisition. We are also grateful to P. Lee, S. Kivelson, D. Scalapino, J. Kono, M. Foster, T. C. Wu, J. C. Cuevas, P. Samuelsson, A. Gozar and I. Drozdov for their comments and questions. The research at Brookhaven National Laboratory, including heterostructure synthesis and characterization and device fabrication, was supported by the US Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. X.H. was supported by the Gordon and Betty Moore Foundation’s EPiQS Initiative through grant GBMF4410. The work at the University of Connecticut was supported by the US state of Connecticut. The research at Rice University was supported by the US Department of Energy, Basic Energy Sciences, Experimental Condensed Matter Physics award DE-FG02-06ER46337. Some of the Rice noise measurement hardware were acquired through National Science Foundation award DMR-1704264. Data availability: The data used to produce the figures in the main text as well as in the Extended Data are provided with the paper. Data are also available online at https://doi.org/10.6084/m9.figshare.8247140 through the Springer Nature Research Data Support Service. Author Contributions: X.H. and I.B. synthesized the high-temperature-superconductor heterostructures. A.T.B. and P.Z. fabricated the tunnelling and Hall bar devices. P.Z., L.C. and Y.L. built the noise measurement system. P.Z. performed and analysed the noise measurements. M.-G.H. and Y.Z. conducted the TEM studies. D.N. and I.B. designed the experiments. I.S. performed differential conductance measurements down to dilution refrigerator temperatures. All authors contributed to writing the manuscript. The authors declare no competing interests.
Funders:
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG02-06ER46337
Gordon and Betty Moore FoundationGBMF4410
State of ConnecticutUNSPECIFIED
NSFDMR-1704264
Issue or Number:7770
Record Number:CaltechAUTHORS:20190903-130240658
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20190903-130240658
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:98393
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:04 Sep 2019 14:51
Last Modified:03 Oct 2019 21:40

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