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State with spontaneously broken time-reversal symmetry above the superconducting phase transition
KTH, School of Engineering Sciences (SCI), Physics.ORCID iD: 0000-0002-5342-1170
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2021 (English)In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 17, no 11, p. 1254-1259Article in journal (Refereed) Published
Abstract [en]

The most well-known example of an ordered quantum state—superconductivity—is caused by the formation and condensation of pairs of electrons. Fundamentally, what distinguishes a superconducting state from a normal state is a spontaneously broken symmetry corresponding to the long-range coherence of pairs of electrons, leading to zero resistivity and diamagnetism. Here we report a set of experimental observations in hole-doped Ba1−xKxFe2As2. Our specific-heat measurements indicate the formation of fermionic bound states when the temperature is lowered from the normal state. However, when the doping level is x ≈ 0.8, instead of the characteristic onset of diamagnetic screening and zero resistance expected below the superconducting phase transition, we observe the opposite effect: the generation of self-induced magnetic fields in the resistive state, measured by spontaneous Nernst effect and muon spin rotation experiments. This combined evidence indicates the existence of a bosonic metal state in which Cooper pairs of electrons lack coherence, but the system spontaneously breaks time-reversal symmetry. The observations are consistent with the theory of a state with fermionic quadrupling, in which long-range order exists not between Cooper pairs but only between pairs of pairs.

Place, publisher, year, edition, pages
Springer Nature , 2021. Vol. 17, no 11, p. 1254-1259
Keywords [en]
Diamagnetism, Specific heat, Thermal variables measurement, Bound-states, Broken symmetry, Broken time-reversal symmetry, Cooper pair, Normal state, Quantum state, Specific heat measurement, Superconducting phase transitions, Superconducting state, Zero resistivity, Quantum theory
National Category
Subatomic Physics
Identifiers
URN: urn:nbn:se:kth:diva-312311DOI: 10.1038/s41567-021-01350-9ISI: 000708404600002Scopus ID: 2-s2.0-85117195368OAI: oai:DiVA.org:kth-312311DiVA, id: diva2:1660241
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QC 20220523

Available from: 2022-05-23 Created: 2022-05-23 Last updated: 2022-06-25Bibliographically approved

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Weston, DanielMaccari, Ilaria

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