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Title: Quantum phase transition inside the superconducting dome of Ba(Fe 1− x Co x ) 2 As 2 from diamond-based optical magnetometry

Abstract

Unconventional superconductivity often emerges in close proximity to a magnetic instability. Upon suppressing the magnetic transition down to zero temperature by tuning the carrier concentration, pressure, or disorder, the superconducting transition temperature $$T_c$$ acquires its maximum value. A major challenge is the elucidation of the relationship between the superconducting phase and the strong quantum fluctuations expected near a quantum phase transition (QPT) that is either second order (i.e. a quantum critical point) or weakly first order. While unusual normal state properties, such as non-Fermi liquid behavior of the resistivity, are commonly associated with strong quantum fluctuations, evidence for its presence inside the superconducting dome are much scarcer. In this paper, we use sensitive and minimally invasive optical magnetometry based on NV-centers in diamond to probe the doping evolution of the $T=0$ penetration depth in the electron-doped iron-based superconductor Ba(Fe$$_{1-x}$$Co$$_x$$)$$_2$$As$$_2$$. A non-monotonic evolution with a pronounced peak in the vicinity of the putative magnetic QPT is found. This behavior is reminiscent to that previously seen in isovalently-substituted BaFe$$_2$$(As$$_{1-x}$$P$$_x$$)$$_2$$ compounds, despite the notable differences between these two systems. Whereas the latter is a very clean system that displays nodal superconductivity and a single simultaneous first-order nematic-magnetic transition, the former is a charge-doped and significantly dirtier system with fully gapped superconductivity and split second-order nematic and magnetic transitions. Thus, our observation of a sharp peak in $$\lambda (x) $$ near optimal doping, combined with the theoretical result that a QPT alone does not mandate the appearance of such peak, unveils a puzzling and seemingly universal manifestation of magnetic quantum fluctuations in iron-based superconductors and unusually robust quantum phase transition under the dome of superconductivity.

Authors:
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Publication Date:
Research Org.:
Ames Lab., Ames, IA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Materials Sciences & Engineering Division
OSTI Identifier:
1631336
Alternate Identifier(s):
OSTI ID: 1609155
Report Number(s):
IS-J-10,185
Journal ID: ISSN 1367-2630
Grant/Contract Number:  
AC02-07CH11358; SC0012336; SC0017888; SC0020313
Resource Type:
Published Article
Journal Name:
New Journal of Physics
Additional Journal Information:
Journal Name: New Journal of Physics Journal Volume: 22 Journal Issue: 5; Journal ID: ISSN 1367-2630
Publisher:
IOP Publishing
Country of Publication:
United Kingdom
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY

Citation Formats

Joshi, K. R., Nusran, N. M., Tanatar, M. A., Cho, K., Bud’ko, S. L., Canfield, P. C., Fernandes, R. M., Levchenko, A., and Prozorov, R. Quantum phase transition inside the superconducting dome of Ba(Fe 1− x Co x ) 2 As 2 from diamond-based optical magnetometry. United Kingdom: N. p., 2020. Web. doi:10.1088/1367-2630/ab85a9.
Joshi, K. R., Nusran, N. M., Tanatar, M. A., Cho, K., Bud’ko, S. L., Canfield, P. C., Fernandes, R. M., Levchenko, A., & Prozorov, R. Quantum phase transition inside the superconducting dome of Ba(Fe 1− x Co x ) 2 As 2 from diamond-based optical magnetometry. United Kingdom. https://doi.org/10.1088/1367-2630/ab85a9
Joshi, K. R., Nusran, N. M., Tanatar, M. A., Cho, K., Bud’ko, S. L., Canfield, P. C., Fernandes, R. M., Levchenko, A., and Prozorov, R. Fri . "Quantum phase transition inside the superconducting dome of Ba(Fe 1− x Co x ) 2 As 2 from diamond-based optical magnetometry". United Kingdom. https://doi.org/10.1088/1367-2630/ab85a9.
@article{osti_1631336,
title = {Quantum phase transition inside the superconducting dome of Ba(Fe 1− x Co x ) 2 As 2 from diamond-based optical magnetometry},
author = {Joshi, K. R. and Nusran, N. M. and Tanatar, M. A. and Cho, K. and Bud’ko, S. L. and Canfield, P. C. and Fernandes, R. M. and Levchenko, A. and Prozorov, R.},
abstractNote = {Unconventional superconductivity often emerges in close proximity to a magnetic instability. Upon suppressing the magnetic transition down to zero temperature by tuning the carrier concentration, pressure, or disorder, the superconducting transition temperature $T_c$ acquires its maximum value. A major challenge is the elucidation of the relationship between the superconducting phase and the strong quantum fluctuations expected near a quantum phase transition (QPT) that is either second order (i.e. a quantum critical point) or weakly first order. While unusual normal state properties, such as non-Fermi liquid behavior of the resistivity, are commonly associated with strong quantum fluctuations, evidence for its presence inside the superconducting dome are much scarcer. In this paper, we use sensitive and minimally invasive optical magnetometry based on NV-centers in diamond to probe the doping evolution of the $T=0$ penetration depth in the electron-doped iron-based superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. A non-monotonic evolution with a pronounced peak in the vicinity of the putative magnetic QPT is found. This behavior is reminiscent to that previously seen in isovalently-substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$ compounds, despite the notable differences between these two systems. Whereas the latter is a very clean system that displays nodal superconductivity and a single simultaneous first-order nematic-magnetic transition, the former is a charge-doped and significantly dirtier system with fully gapped superconductivity and split second-order nematic and magnetic transitions. Thus, our observation of a sharp peak in $\lambda (x) $ near optimal doping, combined with the theoretical result that a QPT alone does not mandate the appearance of such peak, unveils a puzzling and seemingly universal manifestation of magnetic quantum fluctuations in iron-based superconductors and unusually robust quantum phase transition under the dome of superconductivity.},
doi = {10.1088/1367-2630/ab85a9},
journal = {New Journal of Physics},
number = 5,
volume = 22,
place = {United Kingdom},
year = {2020},
month = {5}
}

Journal Article:
Free Publicly Available Full Text
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https://doi.org/10.1088/1367-2630/ab85a9

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